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PREPARATION, CHARACTERIZATION OF CHITOSAN DERIVATIVES AND
APPLICATION IN REMOVAL OF HEAVY METAL IONS FROM WATER
A THESIS SUBMITTED FOR THE DONGRUN-YAU SCIENCE
AWARD (CHEMISTRY),2016
By
Group Members: LI MINZHANG
YANG YANRU
WANG ZEKAI
Supervisor: WANG CHANGLI
QINGDAO NO.2 MIDDLE SCHOOL
QINGDAO, CHINA
DECEMBER 9, 2016
论文题目:PREPARATION, CHARACTERIZATION OF
CHITOSAN DERIVATIVES AND APPLICATION IN
REMOVAL OF HEAVY METAL IONS FROM WATER
参赛队员姓名: 李敏章 杨燕如 王泽凯
中学: 山东省青岛第二中学
省份: 山东省
国家/地区: 中 国
指导教师姓名: 王昌丽
2016 年 12 月
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ABSTRACT
The pollution caused by growth in industrialization and urbanization, is becoming more and
more severe, and heavy metal pollution has become a global problem. Heavy metal
contamination in water is a potential health hazard due to their possible reactivity, mobility and
toxicity. Heavy metals may enter the food chain by plant uptake, permeation into the water bodies
or be uptaken by aquatic organisms that serve as food.
Adsorption, an effective separation technology based on the affinity of adsorbents, is widely
used in water treatment. This inspired many workers to seek more economic and efficient
adsorbents.
Chitosan, a derivative of chitin is a versatile biopolymer with metal uptake capabilities. Chitin
is the second abundant natural biopolymer after cellulose and is distributed in the shells of
crustacean, which is waste product from marine food processing. Huge amounts of crab and
shrimp shells have been abandoned as wastes by worldwide seafood companies. Thus, the
utilization of chitosan results in alleviating the solid waste problem by converting the dumped
crustacean shell into an invaluable asset.
Chitosan is recognized excellent metal ligand, forming stable complexes with many metal ions.
In particular, chitosan is considered one of the best natural environment-friendly chelators for
heavy metal ions. However, the fatal defect of chitosan is that chitosan solids can graduately
dissolve in strong acid solutions.
By chemical modification, it prevents chitosan solids from dissolution in acidic media,
improving mechanical strength, and increasing the porosity and surface area.
Considering the above, epichlorohydrin-crosslinked chitosan resin (ECHC), epichlorohydrin-
crosslinked carboxymethyl-chitosan resin (ECHCMC), EDTA-modified epichlorohydrin-
crosslinked chitosan resin (EDTAEC), and thiourea-modified O-carboxymethyl-chitosan resin
(TUCMC) were designed and synthesized. FTIR-ATR and SEM were used to identify the
structures and characteristics of the resins. Adsorption experiments were utilized to testify
adsorption capacity of the synthesized resins for heavy metals ions (Pb2+, Cd2+). ICP-OES was
employed to determine the concentration of metal ions in solution.
Experimental data showed that EDTA-modified epichlorohydrin-crosslinked chitosan resin has
better adsorption capacity for Pb2+ and the maximum adsorption capacity for Pb2+ was 0.95
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mmol/g, and ECHCMC resin has relatively strong adsorption capacity for both Pb2+ and Cd2+
with the maximum adsorption capacity of 0.77 and 0.40 mmol/g, respectively.
The kinetic parameter of the Pb2+ adsorption process of EDTAEC was obtained, and the results
indicated that adsorption process for Pb2+ followed the Lagergren pseudo second order model. In
the reusability experiments, the EDTAEC resin showed that the adsorption capacity was not
significantly changed up to three cycles. Therefore, the resin could be easily regenerated and
efficiently reused.
In the present study, sorption performance of four cross-linked chitosan derivatives in multi-
component system was investigated in order to evaluate the uptake ability for the metal ions.
Infra-red spectrometry has been applied to study adsorption mechanisms that may be helpful
for synthesizing better adsorption property of modified chitosan.
The results revealed that the novel resin EDTAEC has outstanding performance on adsorption
of Pb2+, Cd2+, Cu2+, Ni2+, and Cr3+ from aqueous solutions. Order of metal chelation for 1mmol/g
was as follows: Cu2+> Pb2+> Ni2+> Cd2+> Cr3+. Metal chelating ability of EDTAEC for Cu2+ and
Pb2+ was higher than that of the other three resins. TUCMC adsorbed heavy metal ions in the
following order: Cu2+ > Ni2+ > Cd2+. ECHCMC indicated that it has binding capacities of 0.19
mmol/g for Cu2+ and ca. 0.05mmol/g Pb2+, Cd2+, Ni2+ and Cr3+. ECHC, as expressed by the order
of its affinity, was in the order of Cu2+>Cr3+> Ni2+> Pb2+> Cd2+. ECHC is more efficient in
scavenging Cr3+ from metal mixture solution as compared to other resins.
The novel chitosan chelating resin (EDTAEC) that we synthesized in this project showed great
potential in the field of removal of heavy metals from water.
Our experimental results approved that grafting of specific functional groups onto the chitosan
backbone allows sorption performances to be improved due to the appearance of new sorbing
functions. Hence, an efficient way to get chelating resin is to modify chitosan structure, and the
resins synthesized have been testified that they can efficiently adsorb heavy metals in aqueous
medium, which has drawn more and more attention.
Keywords: Chitosan resin; Chemical modification; Epichlorohydrin-crosslinked chitosan;
Heavy metal ions; Chemical adsorption
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DECLARATION
The research work embodied in the entitled, “PREPARATION,
CHARACTERIZATION OF CHITOSAN DERIVATIVES AND APPLICATION
IN REMOVAL OF HEAVY METAL IONS FROM WATER” is an original work
carried out under the supervision of the instructor. The work has not been submitted
in part or full for publication. The extent of information derived from existing
literature has been indicated in the thesis at appropriate places, giving the source of
information. If there is any inaccuracy, this team is accountable for all liabilities.
Signature:
Date:
iv
ACKNOWLEDGEMENT
We wish to express our sincere gratitude to our tutor WANG Changli for providing
patient guidance throughout my research project.
We thank Dr. LI Xiancui of the Institute of Oceanology, Chinese Academy of
Sciences (IOCAS), who co-supervised this work and provided the laboratory for the
study. He was an inspiration to us throughout this study.
We also wish to extend our thanks to Mr. LIU Wei, Institute of Oceanology, Chinese
Academy of Sciences, for assisting us with Scanning Electron Microscopy. We are also
grateful to Dr. YU Ying, Testing and Analysis Center, Institute of Oceanology, for
assisting us with FTIR-ATR and ICP-OES analysis.
We would also like to express my grateful thanks to Associate Professor, V.
Thiyagarajan, The Swire Institute of Marine Science and School of Biological Sciences,
The University of Hong Kong, for his constructive criticism, suggestions and positive
thought.
Special thanks are due to SUN Xianliang, the President of Qingdao No. 2 Middle
School for his support.
We wish to express our deepest appreciation to our loving parents for their blessing
and encouragement.
v
TABLE OF CONTENTS
Abstract iDeclaration iiiAcknowledgement ivTable of Contents vAbbreviation viList of Tables viiList of Figures viiiChapter I Introduction 1Chapter II Literature review 4 Motivation for research 12Chapter III Synthesis and characterization of epichlorohydrin-crosslinked
chitosan resin 13
3.1 Introduction 13 3.2. Materials and methods 13 3.3 Results and discussion 14Chapter IV Preparation and characterization of epichlorohydrin-crosslinked
carboxymethyl-chitosan 19
4.1 Introduction 19 4.2. Materials and methods 19 4.3 Results and discussion 20Chapter V Preparation and characterization of a novel bioadsorbent EDTA-
modified epichlorohydrin-crosslinked chitosan 26
5.1 Introduction 26 5.2. Materials and methods 26 5.3 Results and discussion 27Chapter VI Synthesis of thiourea-modified O-carboxymethyl -chitosan 32 6.1 Introduction 32 6.2. Materials and methods 33 6.3 Results and discussion 34Chapter VII Physicochemical characterization of four crosslinked chitosan
derivatives 38
7.1 Introduction 38 7.2. Materials and methods 40 7.3 Results and discussion 42Chapter VIII Evaluation of sorption performance and adsorption mechanism of
four crosslinked chitosan derivatives 44
8.1 Introduction 44 8.2. Materials and methods 45 8.3 Results and discussion 47Summary 58References 60
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ABBREVIATION
ATR Attenuated Total Reflectance
CM-chitosan Carboxymethyl-chitosan
DD Degree of Deacetylation
ECH Epichlorohydrin
ECHC Epichlorohydrin-crosslinked chitosan
ECHCMC Epichlorohydrin crosslinked carboxymethyl-chitosan
EDTA Ethylenediamine Tetraacetic Acid
EDTAEC EDTA-modified Epichlorohydrin-crosslinked chitosan
FTIR Fourier Transform Infrared Spectroscopy
ICP Inductively Coupled Plasma
OES Optical Emission Spectrometry
SEM Scanning Electron Microscopy
TUCMC Thiourea-modified O-carboxymethyl-chitosan
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LIST OF TABLES
Table 2.1 Approximate chitin content in various living species 5
Table 2.2 Applications of Chitin and its derivatives 10
Table 3.1 Peaks of FTIR-ATR spectra of Chitosan and ECHC and their assignment 17
Table 4.1 Peaks of FTIR-ATR spectra of Chitosan and ECHCMC and their assignment 24
Table 5.1 Peaks of FTIR-ATR spectra of Chitosan and EDTAEC and their assignment 30
Table 6.1 Peaks of FTIR-ATR spectra of Chitosan and TUCMC and their assignment 37
Table 7.1 Comparison of physical capacities of synthesized chitosan chelating resins 42
Table 7.2 Comparison of adsorption capacities of synthesized chitosan resins for metal ions 43
Table 8.1 Co-adsorption data of four resins in multiple-metals aqueous solution 48
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LIST OF FIGURES
Figure 2.1 Scheme for chitin and chitosan production 6
Figure 2.2 Structural comparisons of functional groups on chitin, chitosan and cellulose 8
Figure 3.1 Flowchart of synthesis of the epichlorohydrin-crosslinked chitosan 15
Figure 3.2 Synthetic pathway of epichlorohydrin crosslinked chitosan resin 15
Figure 3.3 SEM of chitosan microspheres, modified with epichlorohydrin 16
Figure 3.4 FTIR-ATR spectra of chitosan (A) and ECHC (B) 18
Figure 4.1 Flowchart of synthesis of epichlorohydrin crosslinked carboxymethyl-
chitosan
21
Figure 4.2 Synthetic pathways of epichlorohydrin crosslinked carboxymethyl-chitosan 22
Figure 4.3 Scanning electron micrograph of epichlorohydrin-crosslinked carboxymethyl
chitosan
23
Figure 4.4 FTIR-ATR spectra of chitosan (A), ECHC (B) and ECHCMC (C) 25
Figure 5.1 Flowchart of synthesis of EDTA-modified epichlorohydrin-crosslinked
chitosan
28
Figure 5.2 Synthetic routes of EDTA-modified epichlorohydrin-crosslinked Chitosan 28
Figure 5.3 Scanning electron micrograph of EDTAEC 29
Figure 5.4 FTIR-ATR spectra of chitosan (A), ECHC (B) and EDTAEC (C) 30
Figure 6.1 Flowchart of synthesis of thiourea-modified O-carboxymethyl-chitosan 34
Figure 6.2 Synthetic pathway of thiourea-modified O-carboxymethyl-chitosan 34
Figure 6.3 Scanning electron micrograph of thiourea-modified O-carboxymethyl-chitosan 36
Figure 6.4 FT-IR spectra of chitosan (A), CM-Chitosan (B) and TUCMC (C) 36
Figure 7.1 Chemical structures of four crosslinked-chitosan derivatives 39
Figure 7.2 The synthesized crosslinked chitosan derivatives 39
Figure 8.1 Adsorption performance of four resins in multiple-metals aqueous solution 47
Figure 8.2 Sorption of Pb2+, Cd2+, Cu2+, Cr3+ and Ni2+ from metal ion mixtures on four
resins
48
Figure 8.3 Effect of contact time on the uptake of Pb2+ with initial concentration 0.01 M by
EDTAEC
51
Figure 8.4 Pseudo-second-order kinetic plots for the adsorption of Pb2+ by EDTAEC 51
Figure 8.5 The air-dried spent cross-linked chitosan beads after sorption of multi-metal ions 52
Figure 8.6 FTIR spectra of ECHC beads and ECHC-metal complexation 53
Figure 8.7 FTIR spectra of ECHCMC beads and ECHCMC-metal complexation 54
Figure 8.8 FTIR spectra of EDTAEC beads and EDTAEC-metal complexation 55
Figure 8.9 FTIR spectra of TUCMC beads and TUCMC-metal complexation 56
1
Chapter I INTRODUCTION
Water is one of the most important necessities for the sustenance and continuation of life in
plants and animals. Of all the water available on earth, only less than 2.5% is fresh water
required for human activities. So it is important to have a supply of good quality water for
living beings to perform various activities.
With the rapid development of economy and the modern industry, water pollution caused
by oil spillage and industrial wastewater discharge (Mark, 2011; Dvoˇrák et al., 2013) has
drawn extensive attention, including heavy metal ions, organic dyes and other toxic pollutants.
The harmful contaminants inflict great threat on environment and human being (Govers et al.,
2014; Li et al., 2016). Disposal of water contamination has always been a major
environmental issue all over the world.
In recent years, the heavy metal ions concentration besides other pollutants has increased to
a dangerous level in water resources. Trace amounts of contaminates will result in high
volumes of contaminated water which threaten human health and other living organisms.
Heavy metals have a number of industrial applications due to their technological importance
but the wastewater released from these industries creates a permanent toxic effect on
environment and human beings (Xiong et al., 2009). The source of heavy metal contamination
is from various industrial activities, such as mining operations, metal plating, electric devices
manufacturing units, abandoned waste disposal sites and others includes natural weathering
processes, waste emissions, atmospheric depositions (Bhattacharyya, 2006). Thus, the
treatment of wastewater is an issue for paramount importance. The removal of heavy metal
ions from wastewater is always a challenging task for environmentalists, due to their trace
quantities, and they form complexes with natural organic matter (Wana et al., 2010).
Discharge of heavy metals like Pb (II), Hg (II), Cr (VI), Cd (II), Ni(II) and Cu (II) accumulate
in living organisms, causing disorder and cannot be degraded or destroyed by the organisms
(Bailey, 1999). Wastewater released from industries often contains a considerable amount of
toxic metals that would endanger public health and environment if discharged without
adequate treatment.
Lead is present in several minerals principally in galena, PbS, the main source for lead
production. Lead is one of the most commonly used metal, suitable for batteries due to the
resistance to corrosion and the reversible reaction between lead oxide and sulfuric acid, which
can be recycled. Other uses of the metals are for radiation shielding, ammunition, cable
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sheathing and pipework etc. Lead compounds are used as pigments in paints and ceramics,
catalysts, antibacterial substances and wood preservatives. The major reason for lead
pollution in the environment is due to anthropogenic factor of industrial applications such as,
in electroplating etc. (Ngah, 2002). Lead in the atmosphere comes mainly from the oxidation
of gasoline in internal combustion engines.
Lead is well known to be accumulative poison through water intake or food chains and can
cause brain damage and dysfunction of the kidneys, the liver and the central nervous system
in human beings, especially in children (Ng, 2003). Due to its toxicity, the upper limit for lead
in drinking water recommended by WHO is 0.05mg/l (Harikishore, 2010).
Cadmium exists in nature principally as sulphide ore greenockite, CdS. Many types of
soluble forms of cadmium exist in water. Cadmium enters the environment from wastewater
released from industries such as surface treatment, cement production, mining and from metal
processing units (Jha et al., 1988).
Anthropogenic sources of cadmium emission to the air are mainly from uncontrolled
burning of waste on dumpsites, recovery of metal, steel and cement production etc. Aquatic
inputs of cadmium are mostly from iron and steel production and to the soil. The contribution
of Ni-Cd batteries also plays a big role to cadmium emission from municipal solid waste.
Cadmium in the effluents is absorbed and accumulated by micro-organism. Eventually
cadmium will be transferred to human beings via the food chain and once the cadmium
ingested in the body its expulsion is very difficult. It causes damage even at very low
concentration (Izadiyar, 2010). Cadmium toxicity has a lot of adverse effects on human
beings, which causes renal dysfunction, lung diseases, bone degeneration & lesions, increased
blood pressure, and several types of cancers (Sankararamakrishanan, 2007). Due to its
toxicity, cadmium removal from aqueous effluents has been classified as a priority in the last
decade.
Disposal of water contamination has always been a major environmental issue all over the
world. Treatment methods have been continuously exploring for decade years, such as
precipitation (Al-Harahsheh et al., 2014; Mbamba et al., 2015), flotation (Saththasivam et al.,
2016), membrane technologies (Lin et al., 2015; Neoh et al., 2015), oxidation-reduction (Li et
al., 2014), photocatalytic degradation (Murgolo et al.,2015), adsorption (Bi et al., 2013), etc.
Chemical precipitation is the most traditional, and simple method for water purification
(Grimshaw, 2011). It involves addition of chemicals to facilitate their removal by
sedimentation. However, this method is inappropriate for large solution volumes with very
low concentrations of metal ions. In spite of its advantages, chemical precipitation requires a
3
large amount of chemicals to reduce metals to an acceptable level for discharge. The
precipitation method produces a large amount of sludge, which transforms an aquatic
pollution problem to a solid waste problem. In addition, chemical precipitation is usually
inefficient to deal with low concentration of heavy metals.
Coagulation-flocculation processes are widely used techniques can be employed to deal
with wastewater laden with heavy metals. In spite of its merits, coagulation-flocculation has
limitations such as high operational cost due to chemical consumption. The increased volume
of sludge generated from coagulation flocculation may hinder its adoption as a global strategy
for wastewater treatment.
Among the methods, adsorption has been wildly concerned by researchers in virtue of its
uncomplicated operation, high removal rate, less secondary pollution, as well as low-cost.
Various absorbents were studied and applied in water treatment. Activated carbon has proved
to be most popular and widely used adsorbent for the removal of heavy metals and other
pollutants from wastewater, because of their great capacity to adsorb pollutants. However,
activated carbon presents several disadvantages. It is non-selective, quite expensive, and the
higher the quality, the greater the cost. The regeneration of saturated carbon by thermal and
chemical procedure is also expensive. This forced many workers to search for more economic
and efficient adsorbents.
Due to the problems mentioned above, research interest into the production of alternative
sorbents to replace the costly activated carbon has intensified in recent years. A sorbent can
be regarded as low cost if it requires little processing, is abundant in nature, or is a by-product
or waste material from another industry.
Chitosan, produced by alkaline deacetylation of chitin, is considered one of the most
abundant polysaccharides on the earth especially in coastal regions and well-known for
renewable, nontoxic, biocompatible and degradable (Bhatnagar et al., 2009). As the only
natural alkaline and cationic polysaccharide, chitosan has great potentials in wastewater
treatment, because its amine and hydroxyl groups act as active sites for heavy metal and
anionic organic pollutants (Crini et al., 2008).
4
Chapter II LITERATURE REVIEW
Chitin is a non-toxic and biodegradable polymer of N-acetyl-glucosamine and
glucosamine.
2.1 Discovery of Chitin and Chitosan
Chitin is the most abundant natural biopolymer and the most abundant nitrogen-bearing
biopolymer and is second only to cellulose on earth (Singh and Ray, 2000). It is mainly found
as a component of the exoskeletons of crustaceans and insects and also from cell walls of
fungi (Wu, 2005). Chitin was first described by the French scientist, Braconnot (1811), when
he isolated it from mushrooms using diluted alkali, he called it “fungine”. Later, Odier (1823)
isolated the same substance from insects and called it chitin, using the Greek word for “tunic
envelope”.
Rouget first reportedly discovered Chitosan in 1859 when he boiled chitin in a very
concentrated potassium hydroxide solution making it soluble in organic acid (Muzzarelli,
1977). It was not until 1894 that this substance was named ‘Chitosan’ by Hoppe-Seyler.
During 1930’s and 1940’s these biopolymers gained much interest within the oriental world,
mainly in applications in the field of medicine and water purification. During 1970’s the
interest in these bio-macromolecules renewed at a brisk pace. Pioneering work of Muzzarelli
during 1980’s has greatly advanced our understanding of these materials. Today we know that
chitin and chitosan are found in abundance in nature and are renewable sources and this has
attracted much interest in developing new applications from these simple substances.
2.2 Sources of chitin and chitosan
Considering the annually total production, chitosan and chitin is second ubiquitous natural
polymer after cellulose. Chitin is the most abundant nitrogen-containing biopolymer found on
earth. Traditionally, chitin is produced from crustaceans, even though the largest source of
chitin is fungi (Muzzarelli, 1977). At least 10 gigatons (1 × 1013 kg) of chitin are synthesized
and degraded each year in the biosphere (Jollès and Muzzarelli, 1999). Chitin can be extracted
from different sources, such as from crustacean shells (crabs, cuttlefish, shrimp and crayfish)
and can also be prepared from squid pens. Another source is from the exoskeleton of insects,
bacteria, and some fungi.
Chitin forms a part of the supporting tissue and exoskeleton of arthropoda and is an
essential cell wall component of some plants and most fungi (Muzzarelli, 1977). Chitin in
nature is usually associated with protein (animal) or polysaccharides (yeast, fungi)
5
(Muzzarelli, 1977). In the crustacean exoskeleton, it is bound to polypeptides (proteins) and
calcium carbonate, which function as inorganic fillers. Marine benthic animals are also rich
sources of chitin. Chitosan is converted from chitin, which is a structural polysaccharide
found in the skeleton of marine invertebrates, insects and some algae.
The aquatic species that are rich in chitinous material (10-55 % on a dry weight basis)
include squids, crabs, shrimps, cuttlefish and oysters. It has been found that shrimp and crab
processing waste contains 14-27% and 13-15% on a total mass and dry weight basis,
respectively (No et al., 1989).
Table 2.1 Approximate chitin content in various living species
Species Weight % chitin by
dry body weight
Fungi 5-20%
Worms 20-38%
Squids/Octopus 3-20%
Scorpions 30%
Spiders 38%
Cockroaches 35%
Water Beetle 37%
Silk Worm 44%
Hermit Crab 69%
Edible Crab 70%
Chitosan is one of the most available polysaccharides with positive charges found in nature
(Xing et al., 2005, Shepherd et al., 1997; Krisana et al., 2004). The annual biosynthesis of
chitin has been estimated from 109 to 1011 tons. The traditional and commercial sources of
chitin are the shells of crab, shrimp and krill, all of which are waste product from marine food
processing (Devlieghere et al., 2004). Chitin content in various sources is given in Table 2.1
(Allan et al., 1978). The worldwide annual production of crustacean shells has estimated to be
1.2×106 tons and the production of chitin from this waste can be considered as a major
additional source of commercial income (Knorr, 1991; Roberts,1992). It accounts for
approximately one third of the dry weight of the waste shells. Amongst several sources, the
exoskeleton of crustaceans consists of 15-20% chitin, protein (15-40%), and calcium
carbonate (35-55%) by dry weight.
2.3 Preparation of chitin and chitosan
Commercially, chitin is extracted from the exoskeleton of crustaceans (Muzzarelli et al.,
1986). The industrial development of chitin and chitosan was brought about in the mid-80s.
6
Due to its insolubility in common solvents, the uses of chitin are limited (Kumar, 2000).
Chitosan is obtained by partially deacetylating chitin, removing acetyl groups from the
polysaccharide leaving free amine groups (Khan et al., 2002). The production of chitosan is a
multi-step process including the grinding, deproteinization, demineralization, discoloration
and deacetylation as follow:
Figure 2.1 Scheme for chitin and chitosan production
2.3.1 Production of chitin
Chitin can be extracted from many natural sources, however the main commercial source of
chitin is shrimp, crab and prawn waste. Extraction of chitin from crustacean shells is a time
consuming process that involves extensive demineralization and deproteinization treatments.
Many processes have been implemented with various treatment times, temperatures,
concentrations of acid and alkali solvents, and solid-to-solvent ratios. Crustacean shells are
first crushed into a pulverous powder to make a greater surface area available for the
heterogeneous processes to follow. An initial treatment of the shell with 5% sodium
hydroxide dissolves various proteins, leaving behind chitin, lipids and calcium salts (mainly
as CaCO3). Treatment with 5% hydrochloric acid hydrolyzes lipids; dissolves calcium salts
(demineralization) and other minor inorganic constituents. This is performed at or below
ambient temperatures for 2-3 hours (Wiles, 2000). Chitin thus obtained can be hydrolyzed
using 50% sodium hydroxide at high temperature to convert the amide functionality into the
amino group to provide chitosan. Alternatively, if isolation of chitin is not desired, the
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acid-base sequence may be reversed to directly produce chitosan.
2.3.2 Chemical Conversion of Chitin to Chitosan
Chitosan is commonly prepared by deacetylating-chitin using 40-50% aqueous alkali such
as sodium and potassium hydroxide under heterogeneous conditions at 100-160°C for a few
hours (Campbell, 2003). This can give chitosan with a degree of deacetylation, which
determines the content of free amino groups in the polysaccharides, between 0.70 and 0.95.
For complete deacetylation, the alkaline treatment can be repeated. When deacetylation is
conducted with dilute alkali (20 or 30% sodium hydroxide) at gentle refluxing, DD levels of
33 and 45% can be obtained.
2.4 Chemical structures of chitin and chitosan
The structure of chitin, chitosan, and cellulose is shown in Figure 2.1. Both chitin and
chitosan have a similar chemical structure to cellulose with linear glycosidic backbone
connected by -(1-4) glycosidic bonds.
Chitin consists of linear repeating primary units of 2-acetamido-2-deoxy-D-glucopyranose
with a molecular weight ranging from about 10,000 to 2 million Daltons. These units are
combined by 1-4 glycosidic linkages, forming a long chain linear polymer without side chains.
Chitin is chemically identical backbone with cellulose, except that the secondary hydroxyl
group on the alpha carbon atom of the cellulose molecule is substituted with acetamide
groups.
Chitosan is a modified natural polymer derived from chitin. Chitosan is a
heteropolysaccharide composed of β-(1-4)-2-deoxy-2-amino-D-glucopyranose units (75-85%),
and of β-(1-4)-2-deoxy- 2-acetamido -D-glucopyranose units (Yasser and Ahmed, 2002).
Chitosan and cellulose differ at carbon-2, where the hydroxyl group of cellulose is replaced
by an amino group in chitosan (Muzzarelli, 1985). The high percentage of nitrogen on
chitosan gives it greater reactivity and therefore, greater commercial appeal (Muzzarelli,
1973).
Chitin and chitosan are similar in their chemical structure. The main difference is the
presence of an N-acetyl group attached at the C2 location in chitin. Either an acetamide group
(-NHCOCH3) or an amino group (-NH2) is attached to the C-2 carbon of the glucopyran ring.
When more than 70% of the C-2 attachment is amine group, the material is termed chitosan.
Removal of most of the acetyl groups of chitin by treatment with strong alkali yields chitosan
(Peniston and Johnson, 1980). The N-acetyl group becomes a NH2 amine group in chitosan. In
reality, the range of deacetylation is commonly 70 to 99%. The repeating units of the chitosan
8
backbone are glucosamine and N-acetylated glucosamine (2-acetylamino-2-deoxy-D
-glucopyranose).
(a)
O
HONH2
NHHO
OO
O
HONH2
O
CH3O
OH
OH
NH2HO
OO
H
OH
1
23
4
5
OH
6
n (b)
(c)
Figure 2.2 Structural comparisons of functional groups on chitin (a), chitosan (b), and cellulose (c)
2.5 Physicochemical characteristics of chitosan
Chitosan occurs as odorless substance. It is an amorphous solid and off-white in color. The
properties of chitosan vary considerably depending on the source and production process.
Most of the commercial polysaccharides like cellulose, pectin, alginic acid etc. are neutral
or acidic. But chitosan is an abundant basic polysaccharide. Its pH comes around 8 and this
basic nature makes it unique for different applications (Austin et al., 1981).
Among these physical and chemical characters of chitosan, its solubility and chemical
activity are of most concern because they are the main factors impacting its various
applications.
2.5.1 Physicochemical properties
Chitosan can be characterized in terms of its quality, intrinsic properties such as purity,
molecular weight, viscosity, and degree of deacetylation and physical forms.
2.5.1.1 Solubility of chitosan
The solubility of chitosan is very important for its utilization, such as for chemical
modification. Neither chitin nor chitosan are soluble in neutral water.
Chitosan readily dissolves in dilute mineral or organic acids by protonation of free amino
groups at pH below about 6.5. This cationic nature is the basis of a number of applications of
chitosan. Acetic and formic acids are most widely used in research and applications of
9
chitosan. Generally, the solubility of chitosan decreases with an increase in molecular weight.
Oligomers of chitosan with a degree of polymerization (DP) of 8 or less are water-soluble
regardless of pH.
2.5.1.2 Degree of deacetylation (DD)
The DD is the proportion of glucosamine monomer residues in chitin. It has a striking
effect on the solubility and solution properties of chitin. A number of methods have been
employed to measure the DD, such as IR spectroscopy, UV spectroscopy and so on. However,
one of the most frequently used methods is infrared spectroscopy because of its simplicity.
2.5.1.3 Molecular weight
Molecular weight of the chitosan obtained at the end of the production process depends on
process parameters such as time, temperature and concentration of HCl and NaOH used. The
MW determination of chitosan samples can be performed by various techniques such as
viscometer and gel permeation chromatography. Average molecular weight of chitosan is
around 1.2×105 Daltons.
2.5.1.4 Surface activity of chitosan:
Pure chitosan has low surface activity which can be described by the chemical structure.
The chitosan polysaccharide having cationic amine group (-NH3) and an alcoholic hydroxyl
group (-OH). The chitosan self-aggregates could be formed in acetate buffer solutions by
intra- and inter-molecular hydrophobic interactions.
2.5.1.5 Viscosity
Viscosity is an important characteristic of chitosan. Viscosity of chitosan is highly
dependent on the degree of deacetylation, molecular weight, concentration of solution, ionic
strength, pH, and temperature.
The processes involved in the extraction of chitosan also affect the viscosity of chitosan.
For instance, chitosan viscosity decreases with an increased time of demineralization. Another
characteristic of chitosan is its high viscosity in an acidic environment.
2.5.2 Chemical properties
Chitosan has three reactive groups, including primary and secondary hydroxyl groups at the
C-2, C-3 and C-6 positions (Furusaki et al., 1996) on each repeat unit, and the amino group on
each deacetylated unit. Chitosan undergoes the typical reactions of amines, of which
N-acetylation and Schiff reaction are the most important. Chitosan forms aldimines and
ketimines respectively with aldehydes and ketones at room temperature. Chitosan can also be
modified by either cross-linking or graft copolymerization. A number of chemically modified
chitosan derivatives are listed in the literature (Muzzarelli, 1985).
10
2.5.2.1 Complex formation of chitosan
A significant unique reaction involving amine groups is complex formation. To be specific,
chitosan is known to have good ability of interacting with metals in various environments.
Researches indicated that chelate process is related to the physical state of chitosan, degree of
deacetylation, pH, metal content and distribution of amino groups.
2.5.2 Derivatives of chitosan
Chitosan is a linear polyamine and it can be widely derivatized because a large amount of
the amine groups on the C-2 position and hydroxide groups on the C-3 and C-6 positions.
Alkylation: Alkylated chitosan is an outstanding surfactant and can absorb hydrophobic
molecules.
Carboxymethylchitosan: The carboxymethylchitosan (CM-chitosan) is a highly developed
chitosan derivatives. The carboxymethylation can be carried on C-2, C-3 and C-6.
Graft copolymerization: PEG can be grafted on chitosan and bring the good water
solubility to the copolymer.
Table 2.2 Applications of Chitin and its derivatives (Khor, 2001; Majeti and Kumar, 2000)
Application Areas Specific Use Health Care Burn and Wound dressing
Tissue Engineering Drug and gene delivery
Food and Beverages Preservative agent Food additive and natural thickener Food processing (e.g. sugar)
Agriculture Seed coating Fertilizer Antimicrobial agent
Waste and Water Treatment Removal of metal ions Flocculating agent for polluted water Treating food waste
Cosmetic and Diet-aids Oral health care Dietary aid (fat binding properties) Cosmetic component
Product Separation Membrane separation Chromatographic columns Encapsulating adsorbents
2.6 Uses of chitosan
In the past decades chitosan products have been largely manufactured and applied in
diverse fields ranging from waste management, agriculture to biotechnology.
By changing the degree of acetylation of chitin and the level of viscosity, a wide range of
chitosan forms can be produced. Chitosan is a versatile polymer with applications in waste
treatment (Savant and Torres, 2000), food processing (Ahmed and Pyle, 1999), medical and
11
pharmaceutical industries (Lee and others, 2001), and agriculture (Peter, 1995).
MOTIVATION FOR RESEARCH
1. Research background
Environmental pollution has become more and more serious, especially regarding heavy
metal ions. Heavy metals, mainly from industrial activities such as metal plating facilities,
mining operations, fertilizer and electronic device manufactures are a serious threat to human
beings and the environment, due to their highly toxicity and persistence after being released
into the natural environment. The amount of heavy metals produced from metal industries,
agricultural activities, and waste disposal has increased dramatically. They must be removed
from the polluted water in order to meet increasingly stringent environmental quality
standards.
Many methods including chemical precipitation, electrodeposition, ion exchange,
membrane separation, and adsorption have been used to treat waste water. Among these
techniques, adsorption has been recognized as one of the most popular methods due to its
simplicity of operation, cost effectiveness, high efficiency, easy recovery, regeneration
capacity and sludge-free operation. Many conventional adsorbents are used for removal of
toxic metals, such as activated carbon (Kalpakli et al., 2007), and cellulose.
However, most absorbents are relatively expensive, low adsorption capacity and poor
reusability which have restricted their applications. Likewise, the defects of synthetic organic
absorbents (such as non-biodegradability and non-renewability) are still to be solved before
their widespread application.
Therefore, fungible and effective adsorbents are required for the disposal of heavy
metal-contaminated water.
As effective biosorbent, chitosan has drawn much attention due to its low cost compared to
activated carbon and its high contents of amino and hydroxyl functional groups on chitosan
chains serve as coordination sites. This biopolymer represents an attractive alternative to other
biomaterials because of its physico-chemical characteristics, chemical stability, high reactivity,
excellent chelation behavior and high selectivity toward pollutants.
As mentioned before, the polycationic nature of chitosan makes it available for chelation
with metal. Chitosan acts as a powerful chelating agent owing to the presence of amino
groups within its backbone. Chitosan has been reported as an excellent chelator of many
harmful metals ions (copper, nickel, chromium, cadmium, manganese, cobalt, lead, mercury,
zinc, uranium and silver) from wastewater. The amino groups of chitosan are readily available
12
for chemical reactions with acids. In solution, the chitosan amine groups are protonated
resulting in a positively charged polymer which readily reacts with negatively charged
colloids such as alginate, carrageenan and pectin by electrostatic interactions between -COO-
or -SO3- in the polyanion and forming large polymeric complexes (Mireles et al., 1992).
2. Project significance and values
The project significance and values are embodied in following aspects:
a. Natural chitosan has been modified by several methods in order to enhance the
adsorption capacity for various types of pollutants. The grafting of specific functional groups
onto a chitosan backbone allows sorption performances to be improved due to the appearance
of new sorbing functions. This simple technique of modification of the structure of the
chitosan allows the uptake capacity and sorption kinetics to be increased.
b. Huge amounts of crab and shrimp shells have been abandoned as wastes. Thus, the
utilization of chitosan results in alleviating the solid waste problem by converting the
otherwise dumped crustacean shell into an invaluable asset.
c. The advantages of chemical modification are
(i) It prevents chitosan solids from dissolution in acid solutions
(ii) Mechanical strength of chitosan solids can be improved which provides resistance to
chemical degradation.
(iii) It may increase the porosity and surface area of chitosan solids.
13
Chapter III Synthesis and Characterization of Epichlorohydrin-
Crosslinked Chitosan Resin
3.1 INTRODUCTION
Increasing industrialization worldwide had caused serious pollution all around the world,
especially in the aquatic environment. Wastewaters produced by humans are frequently laden
with toxic heavy metals such as lead, cadmium, mercury, etc. The soluble form of these heavy
metals is very dangerous because it is easily transported to plants and animals. Hence, to
remove toxic heavy metals from wastewaters has become increasingly focused.
Adsorption using low-cost adsorbents is recognized as an emerging technique and a large
variety of adsorbents have been developed and tested. Chitosan, as one of the most frequently
reported biosorbents, has been investigated by many researchers for removal of heavy metals
from aqueous solution. Modifications of chitosan can make it more selective and effective for
several metal ions, especially heavy metal ions. Homogeneous cross linked materials are easy
to prepare with relatively inexpensive reagents and are available in a variety of structures with
a variety of properties.
(1) They are insoluble in acidic and alkaline mediums as well as organic solvents. Cross
linked gels are very stable hydrophilic polymers. They become more resistant to shear, high
temperature and low pH compared to their parent polysaccharide.
(2) After crosslinking, they maintain their properties, original characteristics and strength in
acidic and basic solutions. These characteristics are important for an adsorbent so that it can
be used in a lower pH environment.
(3) After adsorption, the crosslinked materials can also be easily regenerated by washes
using a solvent or by solvent extraction.
In this work, a crosslinked chitosan resin with epichlorohydrin was synthesized and
characterized by Fourier transform infrared (FT-IR) spectroscopy and a scanning electron
microscope.
3.2. MATERIALS AND METHODS
3.2.1 Instrumentation
Fourier transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR) was
used to identify the functional groups in the synthesized resin and the original chitosan. The
morphological characterization of chitosan bead was performed by images acquired using a
scanning electron microscope (S-3400N, Hitachi, Japan), operated 5.00 kV after coated with
14
Au to make the samples conductive.
3.2.2 Chemicals
Chitosan with 80 meshes, 96% degree of deacetylation and average-molecular weight of
1.5×105 was purchased from Qingdao Baicheng Biochemical Corp. (China). Formaldehyde,
epichlorohydrin and other chemicals and reagents were from Sigma Chemicals Co. All the
other reagents were analytical grade and distilled water was used to prepare all the solution.
3.2.3 Synthesis of the epichlorohydrin crosslinked chitosan
A 5% chitosan solution was prepared by immersing 10g chitosan in 190 ml 2% aqueous
acetic acid solution to swell for 24 h. The mixture was poured into the dispersion medium,
consisting of 167 mL paraffin liquid and 2.0 mL Span80. During this process, the dispersion
medium was vigorously stirred at 60oC.
After 30 min, a 9.74-mL formaldehyde solution was added to the dispersion medium and
then stirred for 1 h to protect the amino group. At the end of this period, 5% NaOH was added
drop-wise to keep pH 10 of the mixture, and then added 7.3 ml of crosslinking reagent
epichlorohydrin. The reaction was carried out at 70°C for 2 h under gentle agitation.
Chitosan beads were filtered and washed with water to give the crosslinked microspheres,
which were suspended in 1 M HCl for 9 h to remove the protective group. After filtration and
washing with water, the microspheres were immersed in 1 M NaOH aqueous solution for 5 h
for conversion to base chitosan resin.
Finally, the crosslinked chitosan beads were collected and washed consecutively with water
and ethanol. The product was dried in vacuum and kept under vacuum for further analysis.
3.3 RESULTS AND DISCUSSION
3.3.1 Synthesis of the epichlorohydrin crosslinked chitosan
Crosslinked chitosan beads were prepared by using an organic suspension medium and
crosslinking technique. The process of synthesis of epichlorohydrin crosslinked chitosan resin
is shown in Figure 3.1.
15
Figure 3.1 Flowchart of synthesis of the epichlorohydrin-crosslinked chitosan
Figure 3.2 Synthetic pathway of epichlorohydrin crosslinked chitosan resin (ECHC)
16
It is well known that chitosan is soluble in an acidic solution. So in most cases, it is applied
with crosslinking products. The methods for producing spherical particles with crosslinking
agent, such as glutaraldehyde, were reported. Aldehyde reacts with amino groups of chitosan
to form the Schiff base. Glutaraldehyde makes chitosan resin more hydrophobic and would
cause the properties of chitosan change.
Among the crosslinking agents, the most popular is epichlorohydrin. It can form stable
chemical bonds with amino groups or hydroxyl groups under a certain condition. The results
from SEM showed that the tensile strength of the crosslinked chitosan was considerably
improved. Then it is reasonable to believe that chitosan beads crosslinking with
epichlorohydrin have superior properties to those with glutaraldehyde.
Epichlorohydrin is widely used in chemical industries as intermediates for synthesis of
many products. One bifunctional molecule, which contains two functional groups, is highly
reactive with hydroxyl groups.
One of the advantage of epichlorohydrin is that it does not eliminate the cationic amine
function of chitosan, which is the major adsorption site attracting the pollutant during
adsorption.
In the reaction, the amino group was protected by forming chemical unstable Schiff base
with formaldehyde first, because the crosslinking reagent, epichlorohydrin, can form stable
chemical bonds with amino groups.
Finally the Schiff base was hydrolyzed under low pH solution and converted into the amino
groups.
3.3.2 Morphology
Figure 3.3 SEM of chitosan microspheres, modified with epichlorohydrin Morphology of epichlorohydrin crosslinked chitosan microspheres was examined by
scanning electron microscopy (SEM). A sample SEM micrograph is given in Figure 3.3.
Analysis reveals some interesting features about the texture and morphology of ECHC.
17
ECHC beads have a spherical shape, smooth structure and nonporous appearance.
Microsphere size was evaluated, and the related results show that the mean diameter of the
particles was ca. 220µm.
3.3.3 FTIR-ATR spectra of chitosan and ECHC
The FTIR spectra of ECHC and chitosan are shown in Figure 3.4. FTIR analysis shows that
the adsorption band around 3300cm-1 in all spectra, revealed the stretching vibration of –NH2
and -OH groups in chitosan and ECHC. Bands near 1634 and 1554cm-1 in the spectrum of the
resin ECHC are assigned to C=N of Schiff base moiety and N-H of ECHC moiety,
respectively. Compared with chitosan, the differences in the spectra of ECHC were observed.
Figure 3.4 exhibits the strengthened characteristic peaks of C-H at 2923, 2876cm-1, which
indicates that more methylene groups appeared in ECHC resin. It could be appreciated that a
specific band appears at 1320 cm-1 for N-acetylglucosamine. The reaction of chitosan with
epichlorohydrin can occur at the more reactive hydroxyl group at C6 or the amino group at C2
of chitosan. However, N-substitution is reported to be preferable to O-substitution. The fact
that the intensity of the adsorption band around 1057 cm-1 was substantially increased in the
spectrum of ECHC, and the intensity of characteristic peak of C–N at 1375cm-1 is similar,
indicates that epichlorohydrin has reacted with the hydroxyl group of chitosan at C6.
Table 3.1 Peaks of FTIR-ATR spectra of Chitosan and ECHC and their assignment
-OH C-H Amide II N-H C-H amide II C-N C-O-C C-O C-O
Chitosan 3352 2867 1650 1589 1416 1374 1315 1150 1024
ECHC 3284 2923
2876
1634 1554 1414 1375 1316 1150 1057
Above all, chitosan has crosslinked with epichlorohydrin and the crosslinking takes place
only between hydroxyl groups of chitosan molecules at C6 and epichlorohydrin, due to the
amino group protected by forming a Schiff base with formaldehyde.
19
Chapter IV Preparation and characterization of Epichlorohydrin crosslinked carboxymethyl-chitosan
4.1 INTRODUCTION
Chitosan, as one of the most frequently reported biosorbents, has been investigated by
many researchers for removal of heavy metals from aqueous solution. Modifications of
chitosan can make it more selective and effective for several metal ions, especially heavy
metal ions. In addition, grafting new functional groups such as carboxymethyl group onto
cross-linked chitosan backbone was regarded as a simple and efficient way to facilitate the
adsorption capacity of chitosan for many heavy metals.
The grafting of specific functional groups onto a chitosan backbone allows sorption
performances to be improved due to the appearance of new sorbing functions. This simple
technique of modification of the structure of the chitosan allows the uptake capacity and
sorption kinetics to be increased.
In this work, Epichlorohydrin crosslinked carboxymethyl-chitosan resin was synthesized
and characterized by Fourier transform infrared (FT-IR) spectroscopy and a scanning electron
microscope.
4.2 MATERIALS AND METHODS
4.2.1. Instrumentation
Fourier transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR) was
used to identify the functional groups in the synthesized resin and the original chitosan. The
morphological characterization of chitosan bead was performed by images acquired using a
scanning electron microscope (S-3400N, Hitachi, Japan), operated 5.00 kV after coated with
Au to make the samples conductive.
4.2.2. Chemicals
Chitosan with 80 meshes, 96% degree of deacetylation and average-molecular weight of
1.5×105 was purchased from Qingdao Baicheng Biochemical Corp. (China). Formaldehyde,
epichlorohydrin, monochloroacetic acid and other chemicals and reagents were from Sigma
Chemicals Co. All the other reagents were analytical grade and distilled water was used to
prepare all the solution.
20
4.2.3 Synthesis procedures:
Preparation of crosslinked carboxymethyl-chitosan resin is shown in Figure 4.1.
A 5% chitosan solution was prepared by immersing 10g chitosan in 190 ml 2% aqueous
acetic acid solution to swell for 24 h. The mixture was poured into the dispersion medium,
consisting of 167 mL of paraffin liquid and 2.0 mL Span80. During this process, the
dispersion medium was vigorously stirred at 60oC.
After 30 min, a 9.74 mL formaldehyde solution was added to the dispersion medium and
then stirred for 1 h to protect the amino group. At the end of this period, 5% NaOH was added
drop-wise to keep pH 10 of the mixture, and then added 7.3 ml of crosslinking reagent
epichlorohydrin. The reaction was carried out at 70°C for 2 h under gentle agitation.
Chitosan beads were filtered and washed with water to give the crosslinked microspheres,
which were suspended in 1 M HCl for 9 h to remove the protective group. After filtration and
washing with water, the microspheres were immersed in 1 M NaOH aqueous solution for 5 h,
and then was filtered and washed with deionized water to neutral.
72.5g epichlorohydrin crosslinked chitosan beads were immersed in a solution consisting of
100 ml ethanol and 4g NaOH to alkalize for 3 h. 7.2 grams of monochloroacetic acid was
dissolved in 25 ml of isopropanol, and then added drop-wise to the flask containing the
alkalized chitosan. The reaction continued 4 h at room temperature and then the mixture was
filtered to remove the solvent. The product was washed twice with 75% ethanol and the resin
was dried at 60oC for 3 h and 7.3g of ECHCMC was obtained.
4.3. RESULTS AND DISCUSSION
4.3.1 Synthesis of Epichlorohydrin crosslinked carboxymethyl-chitosan (ECHCMC)
22
Figure 4.2 Synthetic pathways of epichlorohydrin crosslinked carboxymethyl-chitosan
The procedure and synthesis mechanism of Epichlorohydrin crosslinked carboxymethyl
-chitosan (ECHCMC) was provided in Figure 4.1 and 4.2. The synthesis of ECHCMC was
based on carboxymethylation of epichlorohydrin-crosslinked chitosan resin (ECHC).
Concerning carboxymethylation of ECHC, the accessible reaction site was amino group,
due to C6 OH group crosslinked with epichlorohydrin. In the mildly alkaline medium, only
the amine groups will be activated and N-substitution will take place most probably.
4.3.2 Morphology
Morphology of epichlorohydrin crosslinked carboxymethyl-chitosan (ECHCMC)
microspheres was examined by scanning electron microscopy (SEM). The SEM micrograph
is given in Figure 4.3.
SEM micrograph shows some interesting features about the texture and morphology of
ECHCMC, compared with ECHC. The beads have a spherical shape, rough and uneven
surface structure and porous internal structure. The sample contained heterogeneously
23
distributed pores, and obviously different pore sizes
The microsphere size was evaluated, and the related results show that the mean diameter of
the particles was ca. 230µm.
Figure 4.3 Scanning electron micrograph of
epichlorohydrin-crosslinked
carboxymethylchitosan
4.3.3 FTIR-ATR spectra of chitosan and ECHCMC
FTIR-ATR spectra of ECHCMC and the attribution of characteristic peaks are shown in
Figure 4.4 and Table 4.1.
Figure 4.4 shows the basic characteristics of chitosan at 3352 cm-1 (O-H stretching and
N-H stretching), 2867 cm-1 (C–H stretching in methylene), 1650 cm−1 (C=O of NH C=O
stretching), 1589 cm-1 (N–H bending), 1154 cm-1 (bridge-O-stretch), and 1024 cm-1 (C-OH
stretching). The spectrum of ECHCMC beads displays a number of absorption peaks, an
indication of different types of functional groups present in the crosslinked beads. The broad
and strong band ranging from 3200 to 3600 cm-1 indicates the presence of OH and N-H
groups, which is consistent with the peak at 1055 cm-1 assigned to C-OH and C-N stretching
vibration. The peaks at 2923cm-1 can be assigned to asymmetric and symmetric CH2 groups.
Compared with the peaks of chitosan, the bands at 1597 cm-1 and 1404 cm-1 corresponding
24
to the carboxyl group (which overlaps with N–H bend) and -CH2COOH group, respectively
are intense in spectrum of ECHCMC indicating carboxymethylation on both the amino and
hydroxyl groups of chitosan. The peaks at the 1055 cm-1 (C–O stretch) also increase. When
–COOH becomes –COONa, its absorption peak will shift to 1598 cm-1, no bands at 1730 cm-1
for –COOH will be observed in the spectrum.
Table 4.1 Peaks of FTIR-ATR spectra of Chitosan and ECHCMC and their assignment
-OH C-H C=N
C=O
N-H C-H amide II C-N C-O-C C-O C-O
Chitosan 3352 2867 1650 1589 1416 1374 1315 1150 1024
ECHC 3284 2923
2876
1634 1554 1414 1375 1316 1150 1057
ECHCMC 3260 2923 - 1574 1404 - 1320 - 1055
Comparing with the spectra of ECHC, the difference in spectra of ECHCMC was observed.
Figure 4.4 exhibits the strengthened characteristic peaks of C-H at 2923, 2876cm-1, which
indicates that more methylene groups appeared in ECHCMC resin. The carboxymethylation
reaction of chitosan can occur at the more reactive hydroxyl group at C6 or the amino group
at C2 of chitosan. However, hydroxyl group at C6 is crosslinked with epichlorohydrin already.
The sharper band at 1574, 1404 and 1320 cm-1, corresponding to the bending vibration of
aliphatic secondary amine, confirmed the introduction of carboxymethyl group into the ECHC
backbone at –NH2 group of chitosan, namely the NH2 group of chitosan was involved in the
reaction.
25
Wavenumbers (cm-1)
Fig.4.4 FTIR-ATR spectra of chitosan (A), ECHC (B) and ECHCMC (C)
The FTIR-ATR analysis results of the derivatives show that a large amount of the
characteristic group (carboxyl group) exists in the product, and the carboxymethylation
reaction takes place mainly between amino groups of chitosan molecules at C2 and
monochloroactic acid.
The information above indicates the success of carboxymethylation process of the
epichlorohydrin crosslinked chitosan derivative.
26
Chapter V Preparation and characterization of a novel bioadsorbent EDTA-modified Epichlorohydrin crosslinked Chitosan
5.1. INTRODUCTION
Heavy metal ions in wastewater can bring harmful effects to human beings, as well as to
animals and plants. As a result, efficient removal of heavy metal ions from various water
resources has been a crucial issue by employing appropriate adsorbents.
Many researchers demonstrated that chitosan is an excellent natural adsorbent for metal
ions with much higher selectivity than usual commercial chelating resins and with a high
loading capacity (Inoue et al., 1988; 1993; 1994).
The large number of primary amine groups and hydroxyl groups at the sixth position with
high reactivity enables a variety of chemical modification on the backbone of chitosan (Kurita,
1986). Hence, it is very desirable to modify chitosan by grafting new functional groups on the
cross-linked chitosan to preserve or enhance the adsorption capacity.
Because a number of organic ligands containing amino-acetic acid groups (-NHCH2COOH)
are known to form stable complexes with a variety of metal ions, carboxylic acids (such as
ethylenediaminetetraacetic acid) represent promising candidates for the preparation of
environmental friendly adsorbents. Supposedly, the introduction of EDTA residues into
chitosan could significantly enhance the adsorption ability. Chemical modification of chitosan
with chelating agents such as ethylenediaminetetraacetic acid (EDTA), which form very
strong chelates with metal ions, may produce adsorbents with excellent metal binding
properties.
To date, no report has been identified on grafting EDTA onto epichlorohydrin crosslinked
chitosan resin.
In the present work, we prepared epichlorohydrin crosslinked chitosan chemically modified
with functional group ethylenediamine-N,N,N’,N’-tetraacetic acid (EDTA). The novel
biosorbent was synthesized and characterized by Fourier transform infrared (FT-IR)
spectroscopy and a scanning electron microscope.
5.2. MATERIALS AND METHODS
5.2.1. Instrumentation
Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, Thermo-Fisher)
was used to determine the concentration of metal ions. Fourier transform infrared
27
spectroscopy with attenuated total reflectance (FTIR-ATR) was used to identify the functional
groups in the synthesized resin and the original chitosan.
5.2.2. Chemicals
Chitosan with 80 mesh, 96% degree of deacetylation and average-molecular weight of
1.5×105 was purchased from Qingdao Baicheng Biochemical Corp. (China). Formaldehyde,
epichlorohydrin and other chemicals and reagents were from Sigma Chemicals Co. All the
other reagents were analytical grade and distilled water was used to prepare all the solution.
5.2.3. Synthesis of EDTA-modified Epichlorohydrin crosslinked Chitosan
Preparation of EDTA dianhydride
EDTA anhydride was synthesized according to Tülü et al. (1999). 10.0g of EDTA (34mmol)
was suspended in 16ml of pyridine; then 14.0g of acetic anhydride (0.14mmol) was added and
the mixture was stirred vigorously at 65oC for 24h. The product was filtered, washed with
acetic anhydride and diethyl ether, and dried in vacuum for 24h.
Synthesis of EDTA-modified Epichlorohydrin crosslinked Chitosan
To improve its reactivity, epichlorohydrin crosslinked chitosan was functionalized with
EDTA. The epichlorohydrin crosslinked chitosan (ECHC) was chemically modified with
EDTA anhydride according to according to Nagib et al. (Figure 5.1), the reaction showed in
Fig. 1 for EDTAEC. A total of 6.4g of ECHC was suspended in 100 ml of 10 vol% aqueous
acetic acid solution. The mixture was diluted to four times with methanol. Afterwards,
approximately 4g of EDTA anhydride suspended in methanol was added to this solution and
stirred for about 24 h at room temperature to allow the reaction with ECHC to proceed.
After filtration, the precipitate was mixed with ethanol and stirred for a further 12h. After
filtering again, the precipitate was mixed with a dilute sodium hydroxide solution at pH 11
and stirred for 12 h to remove the unreacted EDTA. The precipitate was further washed
several times with deionized water. The repeated washing was followed by decantation using
a centrifuge until the solution became neutral. It was then mixed with 0.1 M hydrochloric acid
solution and washed with deionized water. After stirring in ethanol followed by filtration, the
final product was dried in vacuum to obtain 6.7g white microbeads and stored in a desiccator.
5.3. RESULTS AND DISCUSSION
5.3.1. Synthesis of EDTA-modified Epichlorohydrin crosslinked Chitosan
The flowchart and the mechanisms of synthesis of EDTA-modified epichlorohydrin
crosslinked Chitosan are shown in Figure 5.1 and Figure 5.2, respectively.
28
Figure 5.1 Flowchart of synthesis of EDTA-modified epichlorohydrin-crosslinked Chitosan
(EDTAEC)
OHN
HO
O
NHO
O OH
O
O
+ H3C O CH3
O O
2pyridine
N2, 65oC, 24hO
NN
O
O
O
O
O
EDTA EDTA anhydride
+ CH3COOH4
ON
N
O
O
O
O
O
EDTA anhydride
+room temp
24h
O
OOH
O
N
OHO
N
HO
O
H
O
HO
HH
CH2H
NH2
O
H
O
HO
O
H
O
HO
HH
CH2 H
NH2
O
H
O
O
H
O
OH
HH
CH2H
H2NH
O
OH
O
H
O
OH
HH
CH2H
H2NH
O
n
+
O
H
O
HO
HH
CH2H
O
H
O
HO
O
H
O
HO
HH
CH2 H
NHO
H
O
O
H
O
OH
HH
CH2H
H
O
OH
O
H
O
OH
HH
CH2H
HNH
O
nNH
O
OOH
O
N
OHO
N
HO
O
OHO
O
N
OOH
N
OH
HN
O
OHO
O
N
OOH
N
OH
Figure 5.2 Synthetic routes of EDTA-modified Epichlorohydrin-crosslinked chitosan
(EDTAEC)
The procedure and synthesis mechanism of EDTAEC were shown in Figure 5.1 and 5.2,
29
respectively. The synthesis of EDTAEC began with epichlorohydrin-crosslinked Chitosan
Resin (ECHC). Concerning structure of ECHC, the accessible reaction site was amino group
due to C6 OH group already crosslinked with epichlorohydrin. In the mildly alkaline medium,
only the amine groups will be activated and N-substitution will take place.
5.3.2 Morphology
Morphology of EDTA-modified Epichlorohydrin-crosslinked Chitosan (EDTAEC)
microspheres was examined by scanning electron microscopy (SEM). The SEM micrograph
is given in Figure 5.3.
SEM analysis reveals some features about the fine structure and morphology of EDTAEC.
The EDTAEC beads have a spherical shape, relatively smooth surface structure and
nonporous appearance.
After magnification, some interesting structure appeared on the surface of EDTAEC
microsphere.
The grafting of EDTA on the backbone of ECHC resulted in the remarkable change of the
surface (see Figure 5.3). Many fissures on the surface EDTAEC microsphere appeared.
The size of microsphere was evaluated, and the related results show that the mean diameter
of the particles was ca. 200µm.
Figure 5.3 Scanning electron micrograph of EDTAEC
30
5.3.3 FTIR-ATR spectra of chitosan and EDTAEC
The FTIR-ATR spectra of EDTAEC and chitosan are shown in Figure 5.4.
Wavenumbers (cm-1)
Figure 5.4 FTIR-ATR spectra of chitosan (A), ECHC (B) and EDTAEC (C)
Table 5.1 Peaks of FTIR-ATR spectra of Chitosan and EDTAEC and their assignment
-OH C-H Amide II N-H C-H amide II C-N C-O-C C-O C-O
Chitosan 3352 2867 1650 1589 1416 1374 1315 1150 1024
ECHC 3284 2923
2876
1634 1554 1414 1375 1316 1150 1057
EDTAEC 3267 2927 1621 1377 1318 1152 1057
FTIR analysis shows that the basic characteristics of chitosan at: 3352 cm-1 (O-H stretching
and N-H stretching), 2867 cm-1 (C–H stretching in methylene), 1650 cm−1 (C=O of NH C=O
stretching), 1589 cm-1 (N–H bending), 1154 cm-1 (bridge-O-stretch), and 1024 cm-1 (C-OH
stretching). Compared with chitosan, the spectrum of ECHCMC beads displays remarkable
difference among the three spectra. The broad and strong band at 3267 cm-1 indicates the
presence of OH and N-H groups, which is consistent with the peak at 1057 cm-1 assigned to
31
C-OH and C-N stretching vibration. The peak at 2927 was assigned to asymmetric and
symmetric CH2 groups.
Compared with the peaks of chitosan and ECHC, the bands at 1554 cm-1 and 1414 cm-1
corresponding to the carboxyl group (which overlaps with N–H bend) and -CH2COOH group,
respectively are disappeared in spectrum of EDTAEC, while the intensity of the peaks at the
1621 cm-1 (C –O stretch) and 1377cm-1 (C-N) increase considerably. The reason is that when
–COOH becomes –COONa, its absorption peak will shift to 1621 cm-1, and no bands at 1730
cm-1 for –COOH will be observed in the spectrum.
The sharper bands at 1621, and 1377 cm-1 confirmed the introduction of EDTA group into
the ECHC backbone at –NH2 group of chitosan.
Results from IR spectrograph suggest that EDTA was grafted on to the molecular skeleton
of ECHC successfully. And so far the EDTAEC resin has not been reported after information
retrieval. It is most probably a novel bioabsorbent.
32
Chapter VI Synthesis of Thiourea-Modified O-Carboxymethyl-Chitosan
6.1. INTODUCTION
Wastewaters produced by humans are frequently laden with toxic heavy metals such as lead,
copper, cadmium, etc. The soluble form of these heavy metals is very dangerous because it is
easily transported and more readily available to plants and animals. Hence, to remove toxic
heavy metals from wastewaters has become increasingly focused.
Chitosan, as one of the most frequently reported biosorbents, has been investigated by
many researchers for recovery of heavy metals from aqueous solution. The high nitrogen
content of this abundant natural biopolymer exceeds a 7% (w/w) proportion which explains
the ability of the biosorbent to remove metal ions from dilute solutions. Free electronic
doublet of nitrogen is responsible for the sorption of many cations: copper, lead and cadmium.
Chitosan is also a versatile material which can be easily modified by grafting new specific
functional groups on the backbone of the polymer. Transformations occur on several
functional groups of chitosan but more specifically on –CHOH and –NH2 groups depending
on the substitution mechanism.
Modifications of chitosan can make it more selective and effective for several metal ions,
especially heavy metal ions. The treatment induces new linkages between the chitosan chains
allowing the polymer to be highly resistant to dissolution even in harsh solutions such as
hydrochloric molar solutions.
Carboxymethylation was regarded as a simple and efficient way to facilitate the adsorption
capacity of chitosan for many heavy metals. In addition, grafting new functional groups such
as thiourea onto cross-linked chitosan backbone can also improve its selectivity and
adsorption ability. The grafting of new chelation groups on chitosan backbone is assumed to
increase sorption capacities owing to the coordination chemistry of grafted functional groups:
sulfur compounds are well-known for their affinity for heavy metals.
Sorbents with donor N and more especially S atoms in their functional groups are thus
performing resins. Muzzarelli and Tanfani showed dithiocarbamate chitosan is effective at
removing metal ions even in acid solutions. Guibal et al. had prepared a thiourea derivative of
chitosan which shows a greater selectivity for platinum.
In this work, we modified chitosan by both carboxymethylation and grafting sulfur groups
(thiourea). Thiourea-modified chitosan resin was synthesized and characterized by Fourier
transform infrared (FT-IR) spectroscopy.
33
6.2. MATERIALS AND METHODS
6.2.1. Instrumentation
Fourier transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR) was
used to identify the functional groups in the synthesized resin and the original chitosan. The
morphological characterization of chitosan bead was performed by images acquired using a
scanning electron microscope (S-3400N, Hitachi, Japan), operated 5.00 kV after coated with
Au to make the samples conductive.
6.2.2. Chemicals
Chitosan with 80 meshes, 96% degree of deacetylation and average-molecular weight of
1.5×105 was purchased from Qingdao Baicheng Biochemical Corp. (China). Glutaraldehyde,
thiourea, monochloroacetic acid and other chemicals and reagents were from Sigma
Chemicals Co. All the other reagents were analytical grade and deionized water was used to
prepare all the solution.
6.2.3 Synthesis procedures:
Thiourea-modified O-carboxymethyl-chitosan was prepared through two steps as showed
in Figure 6.1:
1) O-carboxymethyl-chitosan was prepared according to Zhu et al. 2g chitosan was
immersed in 25 ml of 50 wt% NaOH solution to swell and alkalize for 24 h. Five grams of
monochloroacetic acid was dissolved in 25 ml of isopropanol, and then added drop-wise to
the flask containing the alkalized chitosan after filtration for 20 min. The reaction continued 8
h at room temperature and then the mixture was filtered to remove the solvent.
The filtrate obtained was dissolved in 100 ml of water, and 2.5M HCl was added to it to
adjust its pH to 7. After this solution was centrifuged to remove the precipitate, 400 ml of
anhydrous ethanol was added. The precipitate was washed with absolute ethanol and dried in
vacuum. 2.3g of O-carboxymethyl-chitosan was obtained and kept under vacuum for further
application.
2) 3.0g of thiourea was dissolved in 60 ml distilled water,and then 17 ml of (50%)
glutaraldehyde solution was added to thiourea solution in a round flask. The mixture was
heated on a water bath for 3h at 50oC.
After reaction, 1.36g of carboxymethyl-chitosan was dissolved in 30 ml distilled water, and
then added to the mixture to the flask and heated for 8h at 70oC. Thiourea-modified chitosan
resin was formed, and washed several times with dilute sodium hydroxide, deionized water
and acetone respectively, and then the resin was dried at 70oC for 3h.
34
Finally, 3.2g of the thiourea-modified carboxymethyl chitosan resin was obtained.
6.3. RESULTS AND DISCUSSIONS
6.3.1 Synthesis of Thiourea-Modified O-Carboxymethyl-Chitosan Derivative
The process of synthesis of Thiourea-Modified O-Carboxymethyl-Chitosan is shown in
Figure 6.1.
Figure 6.1 Flowchart of synthesis of Thiourea-modified O-carboxymethyl-chitosan
Figure 6.2 Synthetic pathway of thiourea-modified O-carboxymethyl-chitosan
Thiourea-modified O-carboxymethyl-chitosan (TUCMC) was synthesized through two
steps as showed in Figure 6.1. The synthesis mechanism of TUCMC was provided in Figure
6.2. The O-carboxymethylation of chitosan is the key step to succeed in preparation of
TUCMC. The high degree of O-carboxymethylation was obtained by using a strong base
medium, pointing out the importance of alkaline conditions for O-carboxymethylation. A
35
sufficiently strong base is needed to allow chloroacetate penetration on the whole TMC chain,
avoiding side reactions between NaOH and chloroacetate (Barros et al., 2013).
Concerning carboxymethylation, isopropanol as reaction medium led to better results,
achieving degrees of O-carboxymethylation to the extent of 85%. This might be related to a
better conformation of chitosan in isopropanol offering higher accessibility to the reaction
sites.
The reaction conditions are responsible to attain the N versus O selectivity of
carboxyalkylation and degree of substitution (DS). To proceed with the reaction of
carboxymethylation of chitosan in the solvent as water/isopropyl alcohol, chitosan is first
activated by soaking it in the alkaline solution. At high alkali concentrations (more than 25%
aqueous NaOH), alkylation with monochloroacetic acid gives mixed N- and O-alkyl
chitosan derivatives with substitution at the C6 and C3 OH groups and also some substitution
on the C2- NH2 groups. The ease of substitution is in the order OH- 6 > OH-3 > NH2-2.
Yields of carboxymethyl-chitosan prepared in the mixed solvents were higher than in water
alone or in isopropanol alone. The highest yields were close to 100% at water/isopropanol
ratios between 1/4 and 1/1 at 50 °C. The carboxymethyl groups were mostly substituted on
the –OH groups, with a small amount on the –NH2 groups. The 6-OH group had the highest
degree of substitution.
6.3.2 Morphology
Morphology of thiourea-modified O-carboxymethyl-chitosan resin was examined by
scanning electron microscopy (SEM). The SEM micrograph is shown in Figure 6.3.
Some interesting features about the texture and morphology of TUCMC were revealed by
SEM micrograph analysis. The produced particles have an irregular shape, network structure
and porous internal structure and it can also be observed that TUCMC particles have irregular
surface structure. The particle size was evaluated, and the related results show that the mean
diameter of the particles was ca. 80µm.
As showed by SEM of the cross-sectional morphologies of TUCMC in Figure 6.3, the
sample contained interconnected and heterogeneously distributed pores, and obviously
different pore sizes.
36
Figure 6.3 Scanning electron micrograph of Thiourea-modified O-carboxymethyl-chitosan
6.3.3 FTIR-ATR spectra of chitosan and TUCMC
The FTIR-ART spectra of TUCMC and the attribution of characteristic peaks were shown
in Figure 6.4 and Table 6.1.
Wavenumbers (cm-1)
Figure 6.4 FT-IR spectra of chitosan (A), CM-Chitosan (B) and TUCMC (C)
37
Table 6.1 Peaks of FTIR-ATR spectra of chitosan and TUCMC and their assignment
-OH C-H C=N
C=O
N-H C-H amide II C-N C-O-C C-O C-O-O
Chitosan 3352 2867 1650 1589 1416 1374 1315 1150 1024
CM-chitosan 3285 2880 1581 1409 1379 1317 1148 1061
TUCMC 3320 2933
2870
1633 1555 1397 1369 1317 1198 1028
As seen from Figure 6.4 and Table 6.1, the basic characteristic peaks of chitosan are at
3352 cm-1(O–H stretch), 2923-2867 cm-1 (C–H stretch), 1589-1600cm-1 (N–H bend), 1154
cm-1 (bridge-O stretch), and 1024cm-1 (C–O stretch) (Brugnerotto et al., 2001 and Shigemasa
et al., 1996). Compared with the peaks of chitosan, the bands at 1597–1650 cm-1 and
1414-1401 cm-1 corresponding to the carboxyl group and S=C (which overlaps with N–H
bend) and -CH2COOH group, are intense in spectrum of TUCMC indicating
carboxymethylation on hydroxyl groups of chitosan. The peaks at the 1154–1029 cm-1 (C–O
stretch) also increase, and it is one of the characteristics of O-carboxymethyl-chitosan.
In Figure 6.4, comparing the spectrum of TUCMC with raw chitosan, the new band near
1397cm-1 corresponds to COO– stretching vibration. The band near 1033cm-1 is weakened,
and the band near 1155cm-1 disappears, which means that the carboxymethylation process
has happened on the –C–OH. Bands near 1633 and 1555cm-1 are assigned to v-C=N of
Schiff’s base moiety and -C–N of thiourea moiety, respectively. And the broad peak of
1633cm-1 is a result of the overlapping of peaks of C=N, –COOH and –COO−. In addition,
the spectrum shows no characteristic bands related to free aldehydic group near 1720cm-1 for
glutaraldehyde. The information above indicates the success of the modification process of
the thiourea-modified chitosan derivative.
38
Chapter VII PHYSICOCHEMICAL CHARACTERIZATION OF FOUR CROSSLINKED CHITOSAN DERIVATIVES
7.1. INTRODUCTION
In past decades, it is confirmed that chitosan is an excellent natural adsorbent for metal ions
with much higher selectivity and a high loading capacity than usual commercial chelating
resins. The grafting of specific functional groups onto a chitosan backbone allows sorption
performances to be improved due to the appearance of new sorbing functions and by an
improvement in diffusion properties. The simple technique of modification of the structure of
the chitosan allows the uptake capacity and sorption kinetics to be increased (Guibal et al.
2002). The advantages of chemical modification are:
(a) It prevents chitosan solids from dissolution in strong acid solutions
(b)Mechanical strength of chitosan solids can be improved which provides resistance to
chemical degradation.
(c) It may increase the porosity and surface area of chitosan solids.
Based on chitosan four bioadsorbents have been synthesized and characterized. The
structures of four crosslinked chitosan derivatives are shown in Figure 7.1.
O
H
O
HO
HH
CH2H
NH2
O
H
O
HO
O
H
O
HO
HH
CH2 H
NH2
O
H
O
O
H
O
OH
HH
CH2H
H2NH
O
OH
O
H
O
OH
HH
CH2H
H2NH
O
n
ECHC
O
H
O
HO
HH
CH2H
NH
O
H
O
HO
O
H
O
HO
HH
CH2 H
NH
O
H
O
O
H
O
OH
HH
CH2H
HNH
O
OH
O
H
O
OH
HH
CH2H
HNH
O
O
HO
O
OH
O
OH
O
HO
n
ECHCMC
39
EDTAEC
TUCMC
Figure 7.1 Chemical structures of four crosslinked-chitosan derivatives
A
B
C
D
(A) Epichlorohydrin-crosslinked chitosan resin (ECHC); (B) Epichlorohydrin-crosslinked carboxymethyl-chitosan resin (ECHCMC); (C) EDTA-modified epichlorohydrin-crosslinked chitosan resin (EDTAEC); (D) Thiourea-modified O-carboxymethyl-chitosan resin (TUCMC)
Figure 7.2 The synthesized crosslinked chitosan derivatives
40
Physiochemical characteristics of resin will affect remarkably on the absorption capacity.
Therefore, the aim of this study was to investigate the physicochemical properties and
adsorption properties of these promising materials in more detail.
7.2. MATERIALS AND METHODS
7.2.1. Instrumentation
Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, Thermo-Fisher)
was used to determine the concentration of metal ions. pH meter (PHS-25).
7.2.2. Chemicals
Chemicals and reagents were purchased from Sigma Chemicals Co. All the reagents were
analytical grade and deionized water was used to prepare all the solution. Standard solutions
of Lead and Cadmium for ICP-OES were obtained from Beijing NCS Analytical Instruments
Co. Ltd. Pb(NO3)2, and Cd(NO3)2 were used and 0.1M HNO3 and 0.1M NaOH were used for
pH adjustment.
7.2.3 Swelling Test of Cross-linked Chitosan Beads
Cross-linked chitosan derivatives were tested with regard to their solubility in distilled
water. 0.1g of cross-linked chitosan derivatives were taken in a 10 ml graduated cylinder and
dipped in 8 ml distilled water for 5 hours in a closed environment until the volume did change
anymore. The expansion coefficient of these bioabsorbents was calculated from the following:
where K is the expansion coefficient; V0 is the volume of dry beads; V is the volume of the
swelling crosslinked chitosan derivatives.
7.2.4 Determination of bulk density
Bulk density is determined by measuring the volume of a known mass of the powder
sample that has been passed through a screen into a graduated cylinder.
The resins were sieved where the particle size fraction 0.1–0.4mm was kept.
After about 2 ml of dry resin was transferred carefully to graduated cylinder, the volume
was measured accurately without compacting. Calculate the bulk density, in g per mL, by the
formula:
Where is the bulk density g/ml; W is the weight of resin; V is the volume of the
resin.
41
Generally replicate determinations are desirable for the determination of this property.
7.2.5 Determination of skeletal density
The skeletal density was measure by the following procedure:
Add 5 ml of n-heptane to a 10 ml graduated cylinder accurately and then weigh to get the
mass (W1). Add exactly 0.2g of dry resin (W) and 3 ml n-heptane. After 2 hours, add more
n-heptane to the level of 5 ml and weigh it (W2), Calculate the skeletal density, in g per mL,
by the formula:
(dt=0.6830 the density of n-heptane g/cm3)
Wheres is the skeletal density.
7.2.6. Determination of water uptake of modified Chitosan derivatives
Water uptake of the crosslinked Chitosan derivatives was determined at 37oC after
equilibration in distilled water. The preliminary tests lead to the equilibrium time required for
complete swelling and was shorter than 60 h. Therefore, the experiments were carried out
considering the water uptake calculation after equilibration in distilled water for 72 h. The
weight of swollen samples was determined after removal of the surface liquid with lint-free
tissue paper. Water uptake was then calculated according to the following Equation:
Where, Wu, Wf and W0 are water uptake, final weight and the initial weight of the sample,
respectively.
7.2.7 Evaluation of the porosity
The porosity (P) was expressed in the following formula:
Where s is the skeletal density (g/ml), H is water uptake of the resins (%).
7.2.8 Determination of absorption capacity
A sample of 0.1g dry resin was swelled in a 250 mL Erlemeyer flask containing 50mL of
deionized water for 1 hour. 50mL of 0.02 M Pb(NO3)2 and Cd(NO3)2 solutions, was
separately added, where the concentration of the metal ion became 0.01M. The mixture of the
flask was equilibrated for 12 h on a rotary shaker at 200rpm and 25◦C. The metal ion
42
concentration has been determined before and after the treatment using ICP-OES. The
difference in the amount of metal ion was considered as a function of metal ion uptake of the
investigated resin.
The following equation was used to calculate the polymer absorption capacity in mmol/g
polymer.
Where Qe is the maximum metal uptake capacity (mmol/g); Ci, the initial metal
concentration (mmol/L); Cf the final metal concentration (mmol/L); V the volume of solution
(L); M, the dry resin loading (g).
7.3. RESULTS AND DISCUSSION
7.3.1 Comparison of physical capacities of four resins
The results of the expansion coefficient, bulk density, skeletal density, water uptake, and
the porosity of the four synthesized resins are available in Table 7.1.
Table 7.1 Comparison of physical capacities of synthesized chitosan chelating resins
Expansion
Coefficient( K)
Bulk Density
(g/cm3)
Skeletal Density
(g/cm3)
Water Uptake
(%)
Porosity
(%)
ECHC 1.857 0.4585 0.5452 53.133 38.19
ECHCMC 2.000 0.6270 1.1562 59.200 62.65
EDTAEC 2.143 0.4979 0.7747 67.984 62.19
TUCMC 1.286 0.3730 1.5929 40.723 52.25
The expansion coefficient is one of acting factors of sorption capacity. The swelling can
make the gap between the macromolecular skeleton increases, and small molecules can easily
diffuse into the interior. It will improve the utilization rate of chelating ligands.
The expansion coefficient, bulk density and porosity of the chitosan were depicted in Table
7.1. Compared with ECHC, the bulk density of ECHCMC and EDTAEC increased obviously
from 0.4585 to 0.627g/cm3, while the porosity increased from 38.19% to 62.65%, and
expansion coefficient increased from 1.857 to 2.143. It is indicated that physical properties of
chitosan have changed remarkably after grafting with carboxymethyl and EDTA group.
Among the four resins, TUCMC has the lowest bulk density and water uptake, and the highest
skeletal density. As to the new resin EDTAEC, it has the highest expansion coefficient, water
uptake and porosity. The presence of hydrophilic amino and amide groups is known to
43
increase the hydrophilicity of the system and consequently they increase the equilibrium
swelling values of the samples in aqueous medium (Kandile et al., 2009).This means that the
novel resin has the characteristics of porous and high water absorption and swelling.
7.3.2 Comparison of adsorption capacities of four resins for metal ions
Table 7.2 Comparison of adsorption capacities of synthesized chitosan resins for metal ions
ECHC ECHCMC EDTAEC TUCMC
Pb2+ (mmol/g) 0.42 0.77 0.95 0.32
Cd2+(mmol/g) 0.20 0.40 0.16 0.18
Table 7.2 showed the adsorption capacity of four synthesized crosslinked derivatives for
Pb2+ and Cd2+. By comparative analysis of the maximum adsorption capacity of resins for
Pb(II) and Cd(II), among the four resins, ECHCMC is very effective at removing Cadmium
with sorption capacity exceeding 0.40mmol/g from aqueous solutions. Additionally, excellent
uptaking capacity of the novel resin EDTAEC for Pb(II) was 0.95mmol/g. ECHCMC resin
has the relatively strong adsorption capacity for both Pb2+ and Cd2+ with the maximum
adsorption capacity of 0.77 and 0.40 mmol/g, respectively. In addition, EDTAEC could be
used for the selective absorption of Pb from aqueous solution by analysis of the data in Table
7.2. In the following study, EDTAEC was singled out for further research.
44
Chapter VIII EVALUATION Of SORPTION PERFORMANCE AND ADSORPTION MECHANISM OF FOUR CROSSLINKED CHITOSAN
DERIVATIVES
8.1. INTRODUCTION
Heavy metal contamination in aquatic systems is one of the most critical environmental
issues today because these natural resources may impact human health through the food chain.
Heavy metals are released into the aqueous environment through a variety of sources such as
microelectronics and usage in fertilizers paints, pigments, batteries, and the like. These would
endanger public health and the environment if discharged improperly. The heavy metals play
a dual role as essential nutrients and toxic chemicals in plant production and human health.
Trace metals such as Cd, Cr, Cu, Hg, Pb and Ni are mainly regarded as responsible for water
pollution from the viewpoints of phytotoxicity of undesirable entrance into the food chain.
Toxic heavy metals are of concern due to their harmful effects and long-term persistence in
the environment. The hazards associated with the pollution of water bodies caused by heavy
metals have led to the development of various wastewater reclamation technologies such as
chemical precipitation, membrane separation, advanced oxidation process, electrochemical
technique, biological treatment, and adsorption. Among all treatments that are proposed,
adsorption is recognized as an effective and economical method for the removal of pollutants
from wastewaters. However, the removal of heavy metal ions from wastewater is always a
challenging task for environmentalists, so some researchers have attempted to develop
effective performance adsorbents for the disposal of heavy metal ions.
Chitosan have received great attention over the past few decades due to its outstanding
adsorption behavior toward various toxic heavy metals from aqueous solutions because they
possess a number of different functional groups such as hydroxyls and amines to which metal
ions can bind either by chemical or by physical adsorption. Nevertheless, chitosan has some
defects (i.e. low acid stability, inadequate mechanical strength and low thermal stability)
which restrict its application.
Chemical modification of chitosan can prevent chitosan solids from dissolution in acidic
media, improving mechanical strength, and increasing the porosity and surface area and
45
promoting its applications for heavy metal contaminate removal.
In the present study, epichlorohydrin-crosslinked chitosan resin (ECHC),
epichlorohydrin-crosslinked carboxymethyl-chitosan resin (ECHCMC), EDTA-modified
epichlorohydrin-crosslinked chitosan resin (EDTAEC), and thiourea-modified
O-carboxymethyl-chitosan resin (TUCMC) were designed and synthesized.
Moreover the effects of process variables, adsorption kinetics in single-component system
and sorption performance of four cross-linked chitosan derivatives in multi-component
system were investigated.
Finally, infra-red spectrometry has been applied to investigate adsorption mechanisms.
The main objective is to provide information about the most important features of chitosan-
based adsorbents that may be helpful for synthesizing better adsorption property of modified
chitosan.
8.2. MATERIALS AND METHODS
8.2.1. Instrumentation
Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, Thermo-Fisher)
was used to determine the concentration of metal ions. pH meter (PHS-25). Fourier transform
infrared spectroscopy with attenuated total reflectance (FTIR-ATR) was used to identify the
functional groups in cross-linked bead and cross-linked chitosan-metal complex. These
figures show the chemical modifications of cross-linked chitosan and the modifications due to
adsorption of metal.
8.2.2. Chemicals
Chemicals and reagents were purchased from Sigma Chemicals Co. All the reagents were
analytical grade and deionized water was used to prepare all the solution. Standard solutions
of Lead and Cadmium for ICP-OES were obtained from Beijing NCS Analytical Instruments
Co. Ltd. Pb(NO3)2, Cd(NO3)2, Cu(NO3)2, Ni(NO3)2 and Cr(NO3)3 were used and 0.1M HNO3
and 0.1M NaOH were used for pH adjustment.
8.2.3 Sorption experiments in multiple metals aqueous solution
1mmol of each of Pb2+, Cd2+, Cu2+, Cr3+ and Ni2+ metal ion mixture was dissolved in 500
ml deionized water.
A sample of 0.1g dry resin was swelled in a 250 mL Erlemeyer flask containing 50mL of
deionized water for 1 hour. 50mL of 0.002 M multiple-metals solution, was separately added,
46
where the concentration of the metal ion became 0.001M. The mixture of the flask was
equilibrated for 12 h on a rotary shaker at 200rpm and 25◦C.
After shaking, the sample was filtered through Whatman No. 2 filter. The filtrate was
gathered in a plastic bottle and stored until analysis, while the sorbent loaded with metal was
washed thoroughly with deionized water to remove the traces of unreacted metal ions. These
beads were air dried. Dried spent sorbent was collected for FTIR analysis. Metal solution
without sorbent was also shaken for 12 h, filtered and analyzed to take one accurate measure
of the initial metal concentration.
The metal ion concentration has been determined before and after the treatment using
ICP-OES. The difference in the amount of metal ion was regarded as a function of metal ion
uptake of the investigated resin.
The following equation was used to calculate the polymer chelation capacity in mmol/g
polymer.
Where Qe is the maximum metal uptake capacity (mmol/g); Ci, the initial metal
concentration (mmol/L); Cf the final metal concentration (mmol/L); V the volume of solution
(L); M, the dry resin loading (g).
8.2.4 Kinetic studies on EDTAEC
For the kinetics study, a single metal concentration (0.002mmol/L) was used and the
mixture was shaken for 60, 120, 180, 240, 300, 330 and 360 min.
The experimental method and calculation were described in section 8.2.3.
8.2.5. Desorption and reusability
Metal ions-loaded modified chitosan resin was collected and washed with deionized water
to remove any unabsorbed metal ions. Then batch desorption experiments were carried out
using various concentrations of HCl solutions. The resin after desorption was rinsed with
deionized water, dried and then reused in an adsorption experiment. The process was
replicated for five times.
8.2.6 Infrared Spectroscopy for Characterization
Experiment was on cross-linked chitosan derivatives and newly formed resin-metal
complex beads. Adsorption is a surface phenomenon; this technique is suitable to observe the
47
chemical changes occurring on the surface induced by either chemical modification or by
heavy metal adsorption. FTIR characterization of chitosan powder and chitosan metal
complex beads was characterized with Thermo Fisher Nicolet iS50 FT-IR spectrophotometer
Spectrum instrument with attenuated total reflectance (ATR).
8.2.6. Statistical analysis
Triplicate measurements were carried out and metal free were used as controls. The results
were reported as mean±standard deviation. The data were analyzed using statistic software
Sigmaplot 12.5.
8.3. RESULTS AND DISCUSSION
8.3.1 Evaluation of sorption performance in multiple-metals aqueous solution
The amounts of sorption of heavy metals from water on cross-linked chitosan derivatives
were measured. Experiments were performed at an initial concentration of 1 mmol/L of each
metal in multiple-metal solution.
The co-sorption data of Pb2+, Cd2+, Cu2+, Cr3+ and Ni2+ from multiple-metal system on four
resins were presented in table 8.1, Figure 8.1 and Figure 8.2.
Figure 8.1 Adsorption performance of four resins in multiple-metals aqueous solution
48
Table 8.1 Co-adsorption data of four resins in multiple-metals aqueous solution (mmol/g)
(±RSD%, relative standard deviation)
ECHC ECHCMC EDTAEC TUCMC
Pb2+ 0.0222±0.43 0.0632±0.60 0.1704±0.58 0.0048±0.21
Cd2+ 0.0205±0.07 0.0507±0.86 0.0569±0.51 0.0311±0.60
Cr3+ 0.0964±0.26 0.0400±0.20 0.0533±0.57 0.0000±0.48
Cu2+ 0.1253±0.30 0.1907±0.56 0.3692±0.68 0.2724±0.44
Ni2+ 0.0230±0.28 0.0460±0.87 0.0731±1.09 0.0579±0.23
Figure 8.2 Sorption of Pb2+, Cd2+, Cu2+, Cr3+ and Ni2+ from metal ion mixtures on four resins
The sorption of heavy metals including Pb2+, Cd2+, Cu2+, Ni2+ and Cr3+ from aqueous
streams by raw and chemically modified chitosan has been widely studied. Most of these
studies either obtained the sorption isotherms or purely compared the selectivity series based
on the results of single-metal systems or binary-component systems.
Considering metal ion mixtures, the novel resin EDTAEC has outstanding performance on
adsorption of Pb2+, Cd2+, Cu2+, Ni2+, and Cr3+ from aqueous metal ion solutions (Table 8.1 and
Figure 8.2). Order of metal chelation for 1mmol/g was as follows: Cu2+> Pb2+> Ni2+> Cd2+>
49
Cr3+. Metal chelating ability of EDTAEC for Cu2+ and Pb2+ was higher than that of the other
three resins.
TUCMC has binding capacities of more than 0.057 mmol/g for Ni2+ and 0.031mmol/g Cd2+,
with the exception of chromium, and is more efficient in scavenging Cu2+ from metal mixture
solution as compared to other resins. TUCMC beads adsorbed heavy metal ions in the
following order: Cu2+ > Ni2+ > Cd2+.
The result of metal ion sorption of ECHCMC indicated that it has binding capacities of
0.19 mmol/g for Cu2+ and ca. 0.05mmol/g Pb2+, Cd2+, Ni2+ and Cr3+.
Comparing the uptake capacity for the metal ions, the performance of ECHC, as expressed
by the order of its affinity, was in the order of Cu2+>Cr3+> Ni2+> Pb2+> Cd2+. Table 8.1 and
Figure 8.2 showed that ECHC is more efficient in scavenging Cr3+ from metal mixture
solution as compared to other resins.
By comparison of sorption performance of four resins, a conclusion could be reached that
the resins based on chitosan exhibit selective binding towards of Cu(II), Cd(II), Ni(II), Cr(III)
and Pb(II) ions in aqueous medium.
The lone pair electrons present on the amino nitrogen can establish dative bonds with
transition metal ions. Some hydroxyl groups in chitosan derivatives may function as second
donors; hence, hydroxyl groups can be involved in coordination with metal ions.
Table 8.1 shows remarkably that EDTAEC is of higher capacity for sorption of Pb(II) than
the others from multi-component metals solutions, and the adsorption performance of
EDTAEC for Pb(II) is more attractive to study.
8.3.2 Adsorption performance of EDTAEC for Pb2+
8.3.2.1 Reusability of EDTAEC
After adsorption of Pb (II) by EDTAEC, the loaded sorbent was resuspended in 20 mL of
0.1 M HCl. After this suspension was shaken for 6 h at room temperature, sorbents were
separated by filtration and washed with distilled water until neutralization and then dried.
Then, the sorption process was repeated by using the regenerated adsorbent. It was observed
that after three cycles there is no change in Pb (II) sorption capacity of EDTAEC. In
desorption experiments, it has been observed that almost total recovery of Pb (II) occurs,
suggesting that the EDTAEC is regenerable and can be used several times.
50
8.3.2.2 Kinetic studies on EDTAEC for Pb2+
The adsorption behavior of Pb (II) on the studied resin at initial concentration 0.01 M, pH
6.5 and 25◦C as a function of contact time is shown in Figure 8.3. It can be seen that the
maximum uptake of Pb (II) could be achieved within 6 h and maximum uptake capacity
reached 0.95 mmol/g. The good adsorption capacity of Pb (II) is mostly attributed to graft
EDTA in the structure of the epichlorohydrin-crosslinked chitosan resin.
The kinetic of adsorption is an important characteristic that defines the efficiency of
sorption. In order to evaluate the kinetic mechanism that controls Pb (II) adsorption process of
EDTAEC, the pseudo-first- and pseudo-second-order-kinetic models were applied.
The Lagergren pseudo-first-order model (Lagergren, 1898) is expressed as:
where k1 is the pseudo-first-order rate constant (h−1) of adsorption; and qe and qt (mmol/g)
are the amounts of metal ion adsorbed at equilibrium and at time t (h), respectively. Straight
line plots of log(qe-qt) against t were used to determine the rate constant, k1 and correlation
coefficient R2 values for Pb (II) under different concentration range conditions. On the other
hand, the pseudo-second-order equation (Deng et al., 2007) is expressed as:
where k2 is the pseudo-second-order rate constant of adsorption (g/(mmol h−1)). The kinetic
parameters for pseudo-second-order model were determined from the linear plots of t/qt
versus t. The validity of each model is checked by comparing the R2 values. The adsorption of
Pb (II) on the resin perfectly fits pseudo-second-order model (see Figure 8.4). The first-order
kinetic process has been used for reversible reaction with an equilibrium being established
between liquid and solid phases (Low, 2000).
Whereas, the pseudo-second-order kinetic model assumes that the rate-limiting step may
be chemical adsorption (Crini, 2007). In this case, the adsorption behavior of Pb (II) implies
the adsorption rate may be controlled by intraparticle diffusion.
51
1 2 3 4 5 6 70.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Upt
ake
(mm
ol/g
)
Time (h)
Figure 8.3 Effect of contact time on the uptake of Pb (II) with initial concentration 0.01 M by EDTAEC at 25◦C.
0 1 2 3 4 5 60
1
2
3
4
5
6
7
t/qt (
h,g
/mm
ol)
t (h)
Equation y = a + b*x
Weight No Weighting
Residual Sum of Squares
0.28964
Pearson's r 0.98888
Adj. R-Square 0.97346
Value Standard Error
C Intercept 1.74404 0.21896
C Slope 0.7822 0.05261
Figure 8.4 Pseudo-second-order kinetic plots for the adsorption of Pb2+ by EDTAEC
8.3.3 Infrared Spectroscopy for characterization and mechanism of adsorption
FTIR spectra have widely been used as a tool to detect the presence of certain functional
groups or chemical bonds on a solid surface in material modifications because each specific
chemical bond often shows a unique energy adsorption band.
In this study, Infra-red (IR) techniques are used to identify the active sites in chelate
52
formation. After filtration, the sorbent loaded with metal was washed thoroughly with
deionized water to remove the traces of unreacted metal ions. These air-dried spent sorbents
were collected for FTIR analysis (Figure 8.5).
(1) ECHCMC (2) TUCMC (3) EDTAEC (4) ECHC
Figure 8.5 The air-dried spent cross-linked chitosan beads after sorption of multi-metal ions
8.3.3.1 Infrared Spectroscopy for characterization of ECHC-metal complexation
FTIR studies were made on ECHC beads before and after being in contact with multi-metal
ions of solution. FTIR spectra (Figure 8.6) show that there is a remarkable difference between
two samples.
Infrared spectrum analysis reveals that the band at 3290cm-1 is due to the elongation of
N−H and O−H bonds; therefore it can be assigned to several functional groups present in the
sample as −NH2 primary amines, R2NH secondary amines and alcoholic group. The band at
2922cm-1 is due to C−H (−CH3, >CH2) bond elongation. The band at 1642cm-1 is
characteristic of the >C=O bond of an amide which was expected, since the chitosan was
prepared from chitin by partial deacetylation. The bands at 1375 cm-1 (signal of C-N bond)
and at 1554 cm-1 (signal of N-H bond) changed very much in intensity and the signal of C-OH
at 1025 cm-1 increased remarkably.
The difference between the IR spectra of ECHC before and after the heavy metal ions
were adsorbed that the wide band at 3290 cm-1 assigned to amines changed very much in
intensity due to vibration of the N−H bond is modified while forming a bond between the
nitrogen (by its free pair of electrons) and the metal. The band at 1642 cm-1 also assigned to
N−H bond decreased considerably in intensity and was displaced to 1531 cm-1.
53
Wavenumbers (cm-1)
A: ECHC beads B: ECHC-metal complexation
Figure 8.6 FTIR spectra of ECHC beads and ECHC-metal complexation
There are several bands in the IR spectrum that neither present a reduction in their intensity
nor change position this indicates that the metal does not associate with the functional groups
that generate these bands. Results from IR spectrograph suggest that a coordination complex
is established between the chitosan and the metal with the participation of the amino and
secondary alcohol functional group. It suggests that the metal predominantly associates with
−NH2 and −OH of secondary alcohols. There is no participation of −CH2OH groups observed
since they are involved in the crosslinking.
8.3.3.2 Infrared Spectroscopy for characterization of ECHCMC-metal complexation
FTIR studies performed on association complex between metal ions and ECH-crosslinked
NO-carboxymethyl chitosan (Figure 8.7). Compared with the FTIR spectrum of ECHCMC,
the remarkable changes were that the band at 1404 cm-1 disappeared and considerably in
intensity and the band at 1055cm-1 was displaced to 1028 cm-1. FTIR spectra revealed that the
complexing sites are the carbonyl groups, and the hydroxyl and amino groups probably do not
participate in the formation of complex. The chelation sites mainly occurred at carboxyl
groups as indicated by FTIR spectra.
54
Wavenumbers (cm-1)
A: ECHCMC beads B: ECHCMC-metal complexation
Figure 8.7 FTIR spectra of ECHCMC beads and ECHCMC-metal complexation
8.3.3.3 Infrared Spectroscopy for characterization of EDTAEC-metal complexation
FTIR spectra of EDTAEC-metal commplexation show (Figure 8.8) strong band at 1377,
1318 and 1027cm-1 in comparison with EDTAEC. The band in EDTAEC bead when chelated
with lead metals shows a remarkable change in the spectra. Band at 3267 cm-1 is similar, but
the EDTAEC bead with metal show intensive band at 2922cm-1. the band of EDTAEC with
metals at 1596 cm-1 shows a sharp band of NH bending. Band at 1621cm-1 in EDTAEC
disappeared after forming associated complexation with metals. EDTAEC with metals
1308cm-1 assigned to metal binding at carbonyl groups. EDTAEC with lead metal show
intensive band at 1373 cm-1 attributed to binding of metal at C-N of EDTA unit.
Coordination of EDTAEC with metal ions showed the involvement of tertiary amine,
carbonyl and secondary hydroxyl group in chelate formation. Also shifts in the 1620cm-1 band
(-NH and C=O) to 1597 cm-1, and shift in 1062 cm-1 band (sec OH stretching) to 1027 cm-1
supported the above data. There was no shift in the band at 1030 cm-1 (primary OH), which
55
suggested that primary hydroxyl was not involved in chelate formation because they are
involved in the crosslinking.
FTIR indicated that the complexing main sites are the carbonyl, amines and secondary
alcohol functional groups, since the nitrogen of the amino group and the oxygen have a pair
of electrons that can add themselves to a cation by coordinated covalent bonds
Wavenumbers (cm-1)
A: EDTAEC beads B: EDTAEC-metal complexation
Figure 8.8 FTIR spectra of EDTAEC beads and EDTAEC-metal complexation
8.3.3.4 Infrared Spectroscopy for characterization of TUCMC-metal complexation
Experiment was performed on TUCMC bead and TUCMC-metal complex. FTIR analysis
was carried out to confirm the binding mechanism of metal. The FTIR spectra of TUCMC
before and after metal ions uptake is shown in Figure 8.9. The spectrum modification of
TUCMC is caused by adsorption of metal. From Figure 8.9, the wide band at 3309 cm-1
assigned to the stretching vibration of -OH, and the extension vibration of N-H.
A change in the intensity of the C-N stretches at 1319 and 1369 cm-1 was observed, while
the band at 1369 cm-1 decreased significantly. By comparison with the spectra, the signals of
carbonyl group and C=S group from TUCMC backbone increased significantly after forming
complex with metals. FTIR analysis revealed that carbonyl and C=S group take part in
complexation.
56
Wavenumbers (cm-1)
A: TUCMC beads B: TUCMC-metal complexation
Figure 8.9 FTIR spectra of TUCMC beads and TUCMC-metal complexation
8.3.4 Adsorption mechanism
The adsorption mechanism is of crucial importance for further understanding the process of
heavy metal ion removal onto different adsorbents and providing an orientation for the design
of the desorption strategy, but it is a very tedious and complicated work to identify the
adsorption mechanism of heavy metal ions. This aspect has not been adequately studied and
there is very little literature focusing on this topic.
Metal ions can be bound to the surface of a sorbent by several mechanisms including
complexation, ion exchange, chelation, adsorption and co-ordination. However, complexation
have been regarded as major mechanisms for metal ion sorption by sorbents containing
functional groups such as –C=O, -NH2 and -OH. It is accepted that amine sites in chitosan are
the principal reactive groups for metal ions, though hydroxyl groups (especially in the C-3
position) may contribute to sorption.
FTIR spectra of four cross-linked chitosan derivatives after metal ions sorption showed
similar major changes implying similar binding mechanism.
Based on the electron donating nature of the N, O and S containing groups in cross-linked
57
chitosan derivatives and the electron-accepting nature of the metal ions, makes the
resin–metal complex form in the surface of the adsorbent. The involvement of N and O atoms
in binding metal from heavy metal solution was evident in four resins by FTIR analysis.
Cd(II) adsorption onto cross-linked chitosan beads can be explained by complexation,
which interacted through the electron pair sharing between Cd2+ and N and O atoms of the
functional groups. Similarly, an adsorption mechanism for the binding of Pb(II), Cd(II), Cr(III)
and Cu(II) microspheres can also explain by complexation.
A lone pair of electrons of the nitrogen atom was donated to the shared bond between the
N atom and metals, resulting in the decreasing of the electron cloud density of the nitrogen
atom and the increasing of binding energy.
The attraction of the electron pair to the atom's nucleus was stronger in oxygen, and
nitrogen had a greater tendency to donate its pair of electrons to a metal ion to form a
complex through a coordinated covalent bond.
All four resins were effective in the removal of metal ions Cd(II), Pb(II), Ni(II), Cu(II) and
Cr(III) from multi-component solution at pH 7. EDTAEC was demonstrated favorable
efficiency in adsorption for heavy metal ion. Therefore, the EDTAEC is regarded as a
potential candidate in the industrial wastewater treatment.
58
SUMMARY
Environmental pollution has become more and more serious, especially regarding heavy
metal ions. Heavy metals are a serious threat to human beings and the environment, due to
their toxicity and persistence after being released into the natural environment. The amount of
heavy metals produced from metal industries, agricultural activities, and waste disposal has
increased considerably.
Disposal of water contamination has always been a major environmental issue all over the
world. Treatment methods have been continuously exploring for decade years, such as
precipitation flotation, membrane technologies, oxidation-reduction, photocatalytic
degradation, adsorption, etc. Among these methods, adsorption has been wildly concerned by
researchers in virtue of its simple operation, high removal rate, less secondary pollution, as
well as low cost. Various absorbents were studied and applied in water treatment. However,
most of inorganic absorbents remain in laboratories because of their expensive cost, low
adsorption capacity and poor reusability. Likewise, defects of synthetic organic absorbents are
still to be solved before their widespread application.
Chitosan is one of the most abundant polysaccharides on the earth especially in coastal
regions and well known for renewable, nontoxic, biocompatible and degradable. Chitosan has
great potentials in wastewater treatment, because its amine and hydroxyl groups act as active
sites for heavy metal and anionic organic pollutants.
However, the fatal defect of chitosan is that chitosan solids can graduately dissolve in acid
solutions.
By chemical modification, it prevents chitosan solids from dissolution in strong acidic
solutions, improving mechanical strength, and increasing the porosity and surface area.
In this project, epichlorohydrin-crosslinked chitosan resin (ECHC),
epichlorohydrin-crosslinked carboxymethyl-chitosan resin (ECHCMC), EDTA-modified
epichlorohydrin-crosslinked chitosan resin (EDTAEC), and thiourea-modified
O-carboxymethyl-chitosan resin (TUCMC) were designed and synthesized successfully.
FTIR-ATR, SEM were used to identify the structures and characteristics of the resins.
59
Adsorption experiments were used to testify of the adsorption capacity of the synthesized
resins for heavy metals ions Pb2+ and Cd2+. ICP-OES was utilized to determine the
concentration of metal ions in solution in this study.
Experimental data showed that EDTAEC has better adsorption capacity for Pb2+ and the
maximum adsorption capacity for Pb2+ was 0.95 mmol/g, and ECHCMC resin has relatively
strong adsorption capacity for both Pb2+ and Cd2+ with the maximum adsorption capacity of
0.77 and 0.40 mmol/g, respectively.
The kinetic parameter of the Pb2+ adsorption process of EDTAEC was obtained, and the
results indicated that the adsorption process for Pb2+ followed pseudo second order model. In
the reusability experiments, the EDTAEC resin showed that the adsorption capacity was not
significantly changed up to three cycles. Therefore, the resin could be easily regenerated and
efficiently reused.
In the present study, sorption performance of four cross-linked chitosan derivatives in
multi-component system was investigated in order to evaluate the uptake ability for the metal
ions. FTIR spectrometry has been applied to study adsorption mechanisms that may be
helpful for synthesizing better adsorption property of modified chitosan.
The results revealed that the novel resin EDTAEC has outstanding performance on
adsorption of Pb2+, Cd2+, Cu2+, Ni2+, and Cr3+ from aqueous solutions. Order of metal
chelation was as follows: Cu2+> Pb2+> Ni2+> Cd2+> Cr3+. Metal chelating ability of EDTAEC
for Cu2+ and Pb2+ was higher than that of the other three resins. TUCMC adsorbed metal ions
in the following order: Cu2+ > Ni2+ > Cd2+. ECHCMC indicated that it has binding capacities
of 0.19 mmol/g for Cu2+ and ca. 0.05mmol/g Pb2+, Cd2+, Ni2+ and Cr3+. ECHC, as expressed
by the order of its affinity, was in the order of Cu2+>Cr3+> Ni2+> Pb2+> Cd2+.
The novel chitosan resin (EDTAEC) that we synthesized in this project showed potential in
the field of removal of heavy metals from water. It is worthy of further research on the
adsorption mechanism, adsorption kinetic studies, optimizing the conditions to synthesize
highly efficient adsorption capacity of EDTAEC.
From this study, it can be concluded that the modification of chitosan’s structure is an
efficient way to discover new resin that can efficiently adsorb heavy metals in aqueous
medium.
60
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