35622965

Radiation synthesis of kappa-carrageenan/acrylamidegraft copolymers as superabsorbentsand their possible applications

H. L. Abd El-Mohdy & H. A. Abd El-Rehim

Received: 25 December 2007 /Accepted: 18 April 2008 / Published online: 24 May 2008# Springer Science + Business Media B.V. 2008

Abstract Kappa-carrageenan (C) was modified via graftcopolymerization of acrylamide (AAm) using -irradiationand followed by alkaline hydrolysis to achieve a novelsuperabsorbent hydrogel. The effect of grafting variablesand alkaline hydrolysis conditions such as C/AAmcompositions and irradiation doses, NaOH concentration,hydrolysis time and temperature were systematically opti-mized to achieve a hydrogel with swelling capacity as highas possible. Swelling properties and FT-IR of untreated C-g-PAAm and hydrolyzed C-g-PAAm (H-C-g-PAAm)were determined. The swelling capacity of hydrolyzedC-g-PAAm was about 10 times in distilled water and threetimes in NaCl solution higher than that for C-g-PAAm.The swelling of superabsorbing hydrogels was examined inbuffer solutions. The swelling capacity of hydrogels wasalso measured in various salt solutions (LiCl, NaCl, KCl,CaCl2, and FeCl3). Results indicated that the swelling ratiosdecreased with an increase in the ion radius, valence andionic strength of the salt solutions. Due to their highswelling ability in salt solutions, the hydrogels may bereferred to as anti-salt superabsorbent polymers. Thus, thesalinity of water treated with H-C-g-PAAm hydrogels wasdetermined. It was found that the H-C-g-PAAm hydrogelshave a capability to absorb and retain the fresh water fromthe saline solution. The results obtained suggested thepossible used of such prepared superabsorbent in agricul-tural purposes as a material for sodic soil remediation.

Keywords Kappa-carrageenan . Hydrogels .

Superabsorbent . Irradiation . Graft . Swelling . Salinity

Introduction

Superabsorbent hydrogels (SAPs) are three-dimensionalhydrophilic crosslinked networks, which are able to absorband retain many times their weight of water, saline orbiological fluids, without dissolution [12]. Synthesis andcharacterization of these attractive materials have receivedsignificant attention because of their excellent applicationsin many fields. Hydrogels specially are used in disposablediapers, hygienic napkins, and agricultural applicationsand in medicine as drug delivery carriers[25]. Nowadays,the worldwide production of synthetic SAPs is more thanone million tons per year and this may cases damaged toenvironment. Natural-based superabsorbent hydrogelshave attracted much interest from the viewpoint ofimproving the tissue tolerance of synthetic polymers andthe mechanical properties of natural polymers. Because oftheir bio-compatibility, biodegradability and non-toxicity,polysaccharides are the main part of the natural-basedsuperabsorbent hydrogels.

Free radical graft copolymerization of vinyl monomersonto poly-saccharide back-bones followed by crosslinkingof their chains is a well-known method for the synthesis ofthese biopolymer-based networks [67]. Since the devel-opment of the first superabsorbent hydrogel, the hydrolyzedstarch-g-polyacrylonitrile, in the late 1960s, monomerssuch as acrylic acid and acrylamide have been graftcopolymerized onto polysaccharides such as starch, cellu-lose and their derivatives [67] to prepare superabsorbenthydrogels. Carrageenans are relatively new polysaccharidesin the synthesis of natural-based superabsorbent polymers.

J Polym Res (2009) 16:6372DOI 10.1007/s10965-008-9203-5

H. L. Abd El-Mohdy (*) :H. A. Abd El-RehimPolymer Chemistry Department,National Center for Radiation Research and Technology,3 Ahmad El-Zomor Street, P. O. Box No. 29,Nasr City, Cairo, Egypte-mail: [email protected]

These biopolymers are linear sulfate polysaccharides that areobtained from certain species of red seaweeds [8]. Thepresence of hydrophilic sulfate groups with high ionizationtendency and less sensitivity to salt solution was the mainconcept for synthesis of carrageenan-based superabsorbenthydrogels. Recently, kappa-carrageenan was found to en-hance the properties of synthetic hydrogels by incorporationinto the water-soluble polymer systems such as polyethyleneoxide and poly N-vinyl pyrrolidone (PVP) etc [913].

Hydrogels likely reduce the effects of salts in the soilmatrix, even though they limit water carrying capacity.Salinity in soils affects plant growth by osmotic inhibitionof water, nutritional imbalance caused by excessive salts,and toxic effects by high concentrations of salts in thesoil. In one study, the incorporation of hydrogels into agrowing media containing various salt concentrationsactually increased plant vitality. Under the control in theexperiments conducted by tomato and cucumber died inpure sand with concentrations of salt less than 8,000 ppmand lettuce died at salt concentrations of less than4,000 ppm. When the hydrogel was incorporated into thesand mix all three species were able to live with saltconcentrations of 32,000 ppm and actually performed betterthan in pure sand substrates with no salt. Although theplants were stunted, they generally had increased pigments,photosynthesis activity, amino acids, proline, and proteincontents [14].

In this work, superabsorbent hydrogel of kappa-carrageenan-graft-polyacrylamide (C-g-PAAm) were synthesized by asimultaneous irradiation technique using -rays. The optimi-zation of synthetic conditions to achieve maximum swelling(H-C-g-PAAm) was performed. The ability for such preparedsuperabsorbents to absorb the fresh water from the salinesolution was investigated.

Experimental

Materials

Kappa-carrageenan purchased fromCondinson Co., Denmark.Acrylamide (Merck, Darmstadt, Germany) was used asreceived. The chemical structures and formula of kappa-carrageenan and acrylamide were shown in Fig. 1. The otherchemicals, such as solvents and inorganic salts, were reagentgrade and used without further purification.

Preparation of the C/PAAm graft copolymers

Kappa-carrageenan were dissolved in distilled water at 80Cfor 2 h. The hot C solution was mixed with AAm, and thenpoured into a glass tube of diameter 15 mm. The finalconcentrations in the mixture of C and AAm were 20%.

The samples were cooled and then irradiated at roomtemperature by a 60Co- source.

Gel fraction

To extract the insoluble parts of the hydrogels (i.e., thegelled part), the prepared hydrogels were soaked in waterfor 48 h at 100C. Then, they were taken out and washedwith hot water to remove the soluble part, dried, andweighed. The gel percent in the hydrogel was determinedfrom Eq. 1:

Gel % We=Wg 100 1where We and Wg are dry hydrogel weights after andbefore extraction, respectively.

Swelling measurement

The dried hydrogels of known weights were immersed indistilled water or saline solution at 25C until the swellingequilibrium was reached (almost 24 h). The gel wasremoved, blotted quickly with absorbent paper, and thenweighed. For accuracy, the experiment was repeated twicefor each sample. The following equation was used todetermined water uptake:

Swelling WsWg =Wg 2Where Ws and Wg are the weights of wet and dry gel,respectively.

Alkaline hydrolysis

The graft copolymer (0.50 g) was saponified using 20.0 mlof NaOH (0.53.5 N) in a two-neck flask fitted with amechanical stirrer and a reflux condenser. The hydrolysistemperatures of 30100C; and hydrolysis times of 10100 min were studied. The mixture was allowed to cool toroom temperature and neutralized to pH 8.0 by addition of

H2C CH

C

O

NH2

OO

O

OHOSO3

OH

OH

O

O

Chemical Formula: C14H23O12S-

Acrylamide

Kappa-carrageenan

Chemical Formula: C3H5NO

Fig. 1 Chemical structures of kappa-carrageenan and acrylamide

64 H.L. Abd El-Mohdy, H.A. Abd El-Rehim

10 wt% aqueous acetic acid solution. Then the product waspoured into methanol (200 ml) to dewater for 10 h. Thehardened particles were filtered off, dried in an oven (50C,5 h), and kept in a dry and cool place.

Swelling rate

In order to study the absorbency rate of the hydrogels,certain amount of samples (0.5 g) was poured into numbersof weighed tea bags and immersed in distilled water(200 ml). At consecutive time intervals, the equilibriumswelling capacity of the hydrogels was measured accordingto the earlier mentioned method.

Measurement of hydrophilic groups

In some extent, RCOONa is a weak alkali, COONagroup amount in the hydrogel can be measured by titratingwith stronger acid HCl. A hydrogel sample (0.5 g) wasweighed and put into a NaCl saturation solution (to ensureRCOONa ionizes completely [15], then drips HClstandard solution into the sample solution slowly, and thetitration end point was judged by potentiometer. The molarpercentage of COONa in the sample was calculated byfollowing equation.

COONa mol% CHCI VHCI 71Wsample100% 3

The COOH group in the composite can be neutralizedby NaOH and its amount can be measured by electrochem-istry method. A sample (0.5 g) was immersed in overweightNaOH solution for 24 h to allow COOH group transfer toCOONa group completely. The residual NaOH wastitrated by using HCl standard solution and the end pointof the neutralization reaction was judged by potentiometer.The molar percentage of COOH in the sample wascalculated according to the Eq. 4.

COOH mol% CNaOH VNaOH CHCI VHCI 71Wsample 100% 4

It is difficult to directlymeasure the percentage of CONH2group in the hydrogel, CONH2 group can be measured bytransferring CONH2 group to COONa group according tothe reaction 5. A sample (0.5 g) was put into an overweightNaOH solution and was heated up to 95C for 4 h, then thesolution is boiled to ensure the following reaction takes placeand the NH3 was removed completely.

R CONH2 NaOH 95C; 4h ! R COONa NH3 " 5

The boiled solution was cooled and titrated withstandard HCl solution. Meanwhile, a blank test (withoutheating 95C and boiling) was done. The molar percentageof CONH2 group in the sample was calculated accordingto the Eq. 6:

COOH mol% CNaOH VNaOH CHCI VHCI Cblank Vblank 71Wsample 100%

6

FT-IR spectroscopy

The samples were powdered and mixed with KBr to makepellets. Analysis by Infra Red Spectroscopy was carried outby using Mattson 1000,Unicam, Cambridge, England in therange from 4004,000 cm1

pH measurements

The pH of solutions was determined by using Jenway 3310pH Meter.

Salinity assay

One gram of each hydrogel sample was soaked in 100 ml of0.9% NaCl solution, the salinity of saline solutions weredetected by using Conductivity Meter, WTW, LF340,Germany.

Results and discussion

Preparation of a new type of water superabsorbent (H-C-g-PAAm) is performed by crosslinking graft copolymeriza-tion of AAm onto C backbones followed by hydrolysiswith NaOH solution (H-C-g-PAAm). The effect ofgrafting variables and alkaline hydrolysis conditions suchas C/AAm compositions and irradiation doses, NaOHconcentration, hydrolysis time and temperature were sys-tematically optimized to achieve a hydrogel with swellingcapacity as high as possible.

Gel content

Gel content and crosslinking network density of preparedpolymer have a great influence on its swelling character.There are many factors affecting the polymer gel contentamong them, polymer compositions and irradiation doses.The gel content of C/PAAm of different compositionsprepared at various irradiation doses was investigated and

Radiation synthesis of -carrageenan/acrylamide graft copolymers 65

shown in Fig. (2). It is clear that the gel content of C-g-PAAm hydrogel increases with increasing the amount ofAAm in the copolymer feed solutions as well as irradiationdose. The maximum gel content obtained for pure PAAmgels at different irradiation doses. PAAm is well known as acrosslinked polymer by radiation. Consequently, as thepolyacrylamide increases in the graft copolymer, the gelcontent increases.

Swelling of C-g-PAAm

The swelling of C-g-PAAm hydrogel of different compo-sitions was investigated and shown in Fig. (3). It revealsthat the swelling ratio continuously decreases with increas-ing the irradiation dose and acrylamide content in the graftcopolymer. The results can be explained by the fact that theincreasing of AAm content in the feed solution resulted indense crosslinked polyacrylamide chains in the hydrogelnetwork which decreases swelling and restricted the waterdiffusion into prepared copolymers. The phenomena of gelswelling has been the subject of numerous studies inpolymer physics. It has been demonstrated that minutechanges in external conditions such as temperature, solventcomposition, and ionic strength can induce drastic changesin state of the swollen network [16]. In this connection, it isan important to understand the osmotic and structuralchanges of polymer gels induced by addition of some salts.Therefore, the effect of NaCl solution on the swellingbehavior of C-g-PAAm hydrogels prepared by -irradiationis investigated and shown in Fig. (4). In general, the swellingof hydrogels in 0.9 wt. % NaCl solution is much lower thanthat obtained in distilled water.

Hydrolysis of C-g-PAAm

Practically, most of acrylamide-based polymers are actuallypartially hydrolyzed to introduce some carboxylic groupsinto the chain. The presence of these charged unitsimproves water solubility and increase hydrodynamicvolume of the chain due to the mutual repulsion of thenegative charges [1719].

Concentration of NaOH

The alkaline hydrolysis of the graft copolymer, C-g-PAAm, was carried out using different concentrations ofNaOH (0.53.5 N) and their swelling capacity wasdetermined. As shown in Fig. (5), swelling capacity isincreased with increasing the NaOH concentration, to reach

Fig. 2 Effect of various C/AAm compositions on the gel (%) of C-g-PAAm hydrogel at different irradiation doses. Polymer conc., 20%(wt/vol)

Fig. 3 Effect of various C/AAm compositions on the swelling ofC-g-PAAm hydrogel at different irradiation doses. Polymer conc.,20% (wt/vol)

Fig. 4 Effect of various C/AAm compositions on the swelling ofC-g-PAAm hydrogel in 0.9% NaCl at different irradiation doses.Polymer conc., 20% (wt/vol)


a maximum at 3.0 N NaOH concentration thereafter, itdecreases with increasing NaOH concentration. Alkalinedegradation of the polysaccharide backbone is the reasonfor the swelling decrease in highly concentrated alkalinehydrolytic media [2021].

Temperature of hydrolysis

The relationship between the hydrolysis temperature of C-g-PAAm and swelling capacity of H-C-g-PAM wasstudied (Fig. 6). Swelling capacity is increased withincreasing the temperature, to reach a maximum at 80Cthereafter, it decreases as the temperature increases. Byincreasing the hydrolysis temperature up to 80C, thekinetics of alkaline hydrolysis increased which, in turn,results in carboxylate anion increment and consequently,

absorbency enhancement. Decreasing the swelling capacitymay be attributed to alkaline degradation of the C part ofthe hydrogel over this temperature.

Hydrolysis time

In this series of experiments, swelling capacity of the finalhydrogel, H-C-g-PAAm, was studied as a function of thehydrolysis time of C-g-PAAm (Fig. 7). Swelling capacityis increased with increasing the time, to reach a maximumat 60 min thereafter; it decreases as the time increases. Byincreasing the hydrolysis time, the kinetics of alkalinehydrolysis increased which, in turn, results in carboxylateanion increment and consequently, absorbency enhance-ment. Decreasing the swelling capacity may be attributed toalkaline degradation of the C part of the hydrogel.

FT-IR spectroscopy

Infrared spectroscopy was carried out to confirm the chemicalstructure of the prepared graft copolymers. Figure (8)shows the FT-IR spectra of C, C-g-PAAm, andH-C-g-PAAm. The bands observed at 842, 913, 1,080,1,222, and 3,2003,400 cm1 can be attributed to D-galactose-4-sulfate, 3,6-anhydro-D-galactose, glycosidiclinkage, ester sulfate stretching, and stretching of OHgroups of non-modified C, respectively (Fig. 8a). Thegraft copolymer, C-g-PAAm, comprises a C-backbonewith side chains that carry carboxamide functional groupsthat are evidenced by a new peak at 1,660 cm1 (Fig. 8b).This peak attributed to C=O stretching in carboxamidefunctional groups of PAAm. The stretching band of NHoverlapped with the OH stretching band of the C portionof the copolymer. After alkaline hydrolysis, the new

Fig. 5 Influence of NaOH concentration on the swelling of H-C-g-PAAm superabsorbent hydrogel. Polymer conc., 20% (wt/vol), polym.Composition; 10:90 (C/AAm wt/wt), irradiation dose; 10 kGy.Alkaline hydrolysis conditions: 80C and 60 min

Fig. 6 Influence of hydrolysis temperature on the swelling of H-C-g-PAAm superabsorbent hydrogel. Polymer conc., 20% (wt/vol),polym. composition; 10:90 (C/AAm wt/wt), irradiation dose;10 kGy. Alkaline hydrolysis conditions: 60 min, 3 N

Fig. 7 Influence of hydrolysis time on the swelling of H-C-g-PAAmsuperabsorbent hydrogel. Polymer conc., 20% (wt/vol), polym.composition; 10:90 (C/AAm wt/wt), irradiation dose; 10 kGy.Alkaline hydrolysis conditions: 80C, 3 N


absorption modes at 1,459 and 1,557 cm1 can be attributedto symmetric and asymmetric stretching modes of carboxyl-ate groups, respectively (Fig. 8c) [22]. As shown in Fig. (8c),after partially alkaline hydrolysis of the C-g-PAAm, someof the amide groups are converted to carboxylate anions.

H-C-g-PAAm swelling behavior

The swelling capacity of both C-g-PAAm and H-C-g-PAAm which prepared with optimum grafting and hydro-lysis conditions at different C/AAm compositions indistilled water and in 0.9% NaCl solution was investigatedand shown in Fig. (9). It is clear that the NaOH treated C-g-PAAm copolymers water absorbency is much higher thanthat of untreated C-g-PAAm copolymers in distilled waterand in 0.9% NaCl solution. The maximum swelling value(950, 70 g/g in distilled water and 0.9% NaCl solutionrespectively) was observed for H-C-g-PAAm copolymersof C/AAm composition; (20/80 wt%). Improvement ofhydrogel water absorbency can be attributed to theelectrostatic repulsion of its network ionic charges. Increasingof charged carboxylate groups in H-C-g-PAAm enhances itsaffinity towards water; the higher the ionic charge in thecopolymer, the higher the water absorbency was obtained[2324]. The formed carboxylic and charged carboxylategroups which enhance the swelling of H-C-g-PAAmsuperabsorbent hydrogels as a result of the partial hydrolysisof acrylamide groups are shown in Scheme 1. The relationbetween the molar percentage of CONH2, COONa and COOH group and the amount of NaOH in the hydrolysisreaction is shown in Table 1. It can be found that the molarpercentage of CONH2 group decreases and that of COOHor COONa group increase with the increase of the amountof NaOH in the hydrolysis reaction, i.e. CONH2 group isconverted to COOH or COONa group in the hydrolysis

process according to the Eq. 5. Noticeable, owing to thereaction equilibrium and other compounds in the solution,under the condition of preparation, the conversion cannottake place quantificationally. For example, when the con-

Fig. 8 FT-IR spectra of a C, b C-g-PAAm, c H-C-g-PAAm

Fig. 9 Swelling of various compositions of C-g-PAAm and H-C-g-PAAm superabsorbent hydrogels in distilled water (a) and 0.9% NaClsolution (b). Polymer conc., 20% (wt/vol) irradiation dose; 10 kGy

Scheme 1 Partial hydrolysis of amide groups at polyacrylamidechains


centration of NaOH is 1, only 80% of CONH2 is conversed,instead of 100%.

Swelling in various salt solutions

It is an important to understand the osmotic and structuralchanges of polymer gels induced by addition of salts have

different atomic radii, valences and ionic strengths. TheC-g-PAAm and H-C-g-PAAm hydrogels belong to ananionic type absorbent, thereby making it worthwhile toinvestigate the effect of counter ions (cations) and theirionic strength on swelling behavior of the C-g-PAAm andH-C-g-PAAm which have (SO4

2) and (SO42 and

COO) anionic groups, respectively. Therefore, the effect ofvarious salt solutions of different valences on the swellingproperties of gel is investigated.

Effect of valence of cations on H-C-g-PAAm swelling

The swelling behavior of H-C-g-PAAm hydrogels im-mersed in various salt solutions of different ionic valences(Na+, Ca2+ and Fe3+) at various ionic strengths is shown inFig. (10). Under a given ionic strength, the swellingdecreases as the ionic valence of the salt ions increases,the minimum swelling capacity obtained when in trivalentions (Fe3+) was used and maximum one observed whenmonovalent ions (Na+) was used. Alkali metal ions canfreely move all over the entire network. Addition ofdivalent or trivalent counter ions which leads to a volume

Table 1 Variation of the amount of hydrophilic groups with differentNaOH concentrations in hydrolysis for C-g-PAAm hydrogels underthe conditions of 80C and 60 min

NaOH concentration (N) Group amount (%)

CO=NH2 COOH COONa

0.5 60 5 101.0 51 10 201.5 42 15 302.0 31 20 402.5 22 25 503.0 18 31 583.5 14 36 64

Composition; 10:90 (C/AAm wt/wt) and irradiation dose; 10 kGy

Fig. 10 Swelling capacity of the hydrogels, a C-g-PAAm and b H-C-g-PAAm, in various concentrations of different salt solutions.Polymer conc., 20% (wt/vol), polym. composition; 20/80 (C/AAmwt/wt), irradiation dose; 10 kGy

Fig. 11 Swelling capacity of the hydrogels, a C-g-PAAm and b H-C-g-PAAm, in various univalent cation solutions. Polymer conc.,20% (wt/vol), polym. composition; 20/80 (C/AAm wt/wt), irradia-tion dose; 10 kGy


transition in H-C-g-PAAm gels, governed by the inter-actions between the polyion and the counter ions.

Alkaline earth-metal ions bind strongly with doublecarboxylate groups, and its mobility through the polymernetwork become limited. However, H-C-g-PAAm networkexhibits strong chelating ability with heavy metal ions (Fe3+),and as a result, the ion mobility is restricted. It was reportedthat the amount of gel water retained is affected by chemicalsor divalent cations present in the water. These cationsdevelop strong interactions with the polymer gels and areable to displace water molecules trapped within the gel [2527]. Even though monovalent cations such as Na+ canreplace water molecules, this effect is not as pronounced aswith the divalent counterparts. Furthermore, the process isfully reversible by repeated soaking with distilled water.

Effect of ionic radius of cations on H-C-g-PAAm swelling

To investigate the salt-sensitivity of the C-g-PAAm andH-C-g-PAAm, the equilibrium swelling was measured inchloride salt solutions of monovalent cations, Li+, Na+, andK+ with various ionic strengths (Fig. 11). The waterabsorbency of the H-C-g-PAAm hydrogels decrease withincreasing ionic strengths. Also, the water absorbency ofthe H-C-g-PAAm hydrogels increases with decreasingatomic radius of monovalent cation solutions. The resultsobtained can be explained taking into account that theradius of the small cation is surrounded and shielded by alarge number of water molecules. Therefore, the cationiccharge density becomes low and its bonding ability to thecarboxylate group is weak. However, the cation with a largeradius tends to enter the network and easily binds to thehydrogel carboxylate groups, resulting in decrease of itsswelling capacity. The swelling behavior of H-C-g-PAAm

Fig. 12 Effect of pH on swelling capacity of H-C-g-PAAmsuperabsorbent hydrogels. Polymer conc., 20% (wt/vol), polym.composition; 10:90 (C/AAm wt/wt), irradiation dose; 10 kGy

Fig. 13 Swelling rate of C-g-PAAm and H-C-g-PAAm superab-sorbent hydrogels. Polymer conc., 20% (wt/vol), polym. composition;10:90 (C/AAm wt/wt), irradiation dose; 10 kGy

Fig. 14 Salinity of various compositions of H-C-g-PAAm superab-sorbent hydrogels. Polymer conc., 20% (wt/vol) irradiation dose;10 kGy

Fig. 15 Bean plants cultivated in a gel free soil and irrigated withbrackish water (control), b gel free soil with saline water and c H-C-g-PAAm gel treated soil with saline water


is also significantly lowered by enhancing ionic strength ofsalt solutions. The decrease in swelling ratio of hydrogelswith increasing salt concentration in the external solution isdue to a decrease in the expansion of the hydrogel networkin salt solutions. This is not only due to the decrease in theosmotic pressure difference between the gel and theexternal solution but also due to the screening of the ioniccharges bound to the hydrogel network [27].

Swelling variation with pH

Since the H-C-g-PAAm hydrogels comprise of anionicsulfate and carboxylate groups, they may exhibit sharpswelling changes at a wide range of pH values. Therefore,the equilibrium swelling of H-C-g-PAAm hydrogel wasmeasured at various buffer solutions with pH ranging from1 to 13 (Fig. 12). Under acidic pH values, most of thecarboxylate anions are protonated, so the main anionanionrepulsive forces are eliminated and consequently swellingvalues are decreased. At higher pH values (58), some ofthe carboxylate groups are ionized and the electrostaticrepulsion betweenCOO groups causes an enhancement ofthe swelling capacity. Again, a charge screening effect ofthe counter ions (cations) limit the swelling at higher basicpH values (pH>8) [27].

Swelling rate

Figure (13) shows swelling rate of the hydrogels, C-g-PAAm and H-C-g-PAAm, as a function of swelling timein distilled water at room temperature. According toFig. 13, the swelling rate sharply increases with increasingtime and then begins to level off. There are a similar insaturation time for the two types of hydrogels, nearly;180 min.

Application of the prepared H-C-g-PAAm hydrogels

H-C-g-PAAm hydrogels of different compositions wereimmersed in saline solution (0.9% NaCl). The salinity ofresidual solutions was measured. The relation between theresidual salinity and H-C-g-PAAm hydrogels composi-tions was determined and is shown in Fig. (14). It wasfound that the salinity of 0.9% NaCl solution increased withincreasing C content in C-g-PAAm gel to maximumvalue (23 mS/m) at 20/80 C/AAm. This means that thegels have ability to absorb fresh water from saline solutioni.e the gel possess the ability to desalinate brackish water.Thus the sodic soil can be reclaimed by using suchhydrogels [14].

The possible use of grafted carrageenan copolymerssuper-absorbed hydrogels to adsorb fresh water from salinewater was investigated. Field evaluation of the H-C-g-

PAAm hydrogels for its possible uses in agriculture as a soilconditioner adsorb fresh water from saline water wasinvestigated in a range of garden plants using bean as amodel plant. As shown in Fig. (15), it is clear that the growthof bean plants cultivated in the soil treated with H-C-g-PAAm hydrogels and irrigated with saline water (Fig. 15c)is greater than those cultivated in gel-free soil and irrigatedwith saline water (Fig. 15b). The results suggested that thehydrogel particles around the bean root can absorb largequantities of fresh water; thus, soil water retention wasimproved, and the available pure water for the plantincreases, resulting in growth promotion of the bean plant.

Conclusion

A superabsorbent hydrogel, H-C-g-PAAm, was preparedvia graft copoly-merization of AAm onto C backbonesfollowed by alkaline hydrolysis of the C-g-PAAmcopolymer. The swelling capacity of the hydrogels wasrecognized to be affected by the grafting and alkalinehydrolysis variables. The swelling ratio continuouslydecreases with increasing irradiation dose and acrylamideconcentration in the feed mixture. The maximum waterabsorbency (704 g/g) was achieved under optimum hydro-lysis conditions (NaOH concentration of 3N, hydrolysistime 60 min, and hydrolysis temperature 80C). Due to thepolyelectrolyte nature of the prepared hydrogels, they weresensitive to the pH and ionic strength of the medium. Itexhibited high swelling capacity at basic pH values as wellas a reversible pH-responsiveness property. Swellingmeasurements of the synthesized hydrogels in different saltsolutions showed appreciable swelling capacity, especiallyin LiCl, NaCl, and KCl solutions. The results suggest thatthe H-C-g-PAAm hydrogel can absorb large quantities offresh water from saline one. This suggested the possible useof H-C-g-PAAm hydrogels in agriculture as a soilconditioner adsorb fresh water from saline water.

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