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Heavy metal adsorption properties of a submerged aquaticplant ( Ceratophyllum demersum )

O. Keskinkan a , M.Z.L. Goksu b , M. Basibuyuk a , C.F. Forster c, *a Department of Environmental Engineering, Faculty of Engineering, Cukurova University, 01330 Balcali-Adana, Turkey

b Fisheries Faculty, Cukurova University, 01330 Balcali-Adana, Turkeyc Department of Civil Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

Received 25 April 2003; received in revised form 23 June 2003; accepted 27 July 2003

Abstract

Heavy metals can be adsorbed by living or non-living biomass. Submerged aquatic plants can be used for the removal of heavymetals. In this paper, lead, zinc, and copper adsorption properties of Ceratophyllum demersum (Coontail or hornwort) were in-vestigated and results were compared with other aquatic submerged plants. Data obtained from the initial adsorption studies in-dicated that C. demersum was capable of removing lead, zinc, and copper from solution. The metal biosorption was fast andequilibrium was attained within 20 min. Data obtained from further batch studies conformed well to the Langmuir Model.Maximum adsorption capacities ( qmax ) onto C. demersum were 6.17 mg/g for Cu(II), 13.98 mg/g for Zn(II) and 44.8 mg/g for Pb(II).Kinetics of adsorption of zinc, lead and copper were analysed and rate constants were derived for each metal. It was found that theoverall adsorption process was best described by pseudo second-order kinetics. The results showed that this submerged aquatic plantC. demersum can be successfully used for heavy metal removal under dilute metal concentration. 2003 Elsevier Ltd. All rights reserved.

Keywords: Aquatic plants; Biosorption; Heavy metals; Langmuir model; Pseudo second-order

1. Introduction

Heavy metals pose serious environmental risks and,therefore, their removal from wastewaters, particularly,has been examined extensively. A range of methods hasbeen used for heavy metal removal and one of the mostcommon methods is adsorption, with activated carbonbeing the most widely used adsorbent for this purpose.However, this can be expensive and there has beenconsiderable interest in the use of other adsorbent ma-terials, particularly biosorbents (Wase and Forster,1997; Ajmal et al., 2000). It has long been known thataquatic plants, both living and dead, are heavy metalaccumulators and, therefore, the use of aquatic plantsfor the removal of heavy metals from wastewater hasgained high interest (Kuyucak and Volesky, 1989).Some freshwater macrophytes including Potamogetonlucens , Salvinia herzogoi , Eichhornia crassipes , Myrio-

phyllum brasillensis , Myriophyllum spicatum , Cabombasp., Ceratophyllum demersum have been investigated forthe removal of heavy metals (Wang et al., 1996;Schneider and Rubio, 1999; Schneider et al., 1999; Ke-skinkan et al., 2003).

The removal of metals by biosorption and themechanisms of biosorption have been discussed previ-ously and it has been reported that biosorption may beclassied as being: extracellular accumulation/precipi-tation, cell surface sorption/precipitation, and intracel-lular accumulation (Veglio and Beolchini, 1997) and canoccur by complexation, co-ordination, chelation of metals, ion exchange, adsorption and micro precipita-tion (Wang et al., 1996). The bioremoval process usingaquatic plants contains two uptake processes:

an initial fast, reversible, metal-binding processes(biosorption);

a slow, irreversible, ion-sequestration step (bioaccu-mulation).

Schneider et al. (1999) have studied the biosorption of metals onto plant biomass in order to determine whether

* Corresponding author. Tel.: +44-121-414-5049; fax: +44-121-414-3675.

E-mail address: c.f.forster@bham.ac.uk (C.F. Forster).

0960-8524/$ - see front matter 2003 Elsevier Ltd. All rights reserved.doi:10.1016/j.biortech.2003.07.011

Bioresource Technology 92 (2004) 197200

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it was exchange adsorption or surface precipitation. Itwas found that the sorption was a function of pH andwas greatest at a pH value which was slightly moreacidic than the pH at which there was bulk precipitationof the metal hydroxide. These workers have also studiedthe t, or approximate t, of the sorption data to theLangmuir isotherm. This was considered to be evidencethat sorption stopped at one monolayer, consistent withspecic and strong sorption onto specic sites.

C. demersum (Coontail or hornwort) is a completelysubmersed plant and commonly seen in ponds, lakes,ditches, and quiet streams with moderate to high nu-trient levels (Johnson et al., 1995). It does not produceroots, instead it absorbs all the nutrients it requires fromthe surrounding water. If it is growing near the lakebottom, it will form modied leaves, which it uses toanchor to the sediment. However, it can oat free in thewater column and sometimes forms dense mats justbelow the surface. The main purpose of this study,therefore, is to investigate the adsorption characteristicsof zinc, copper, and lead onto C. demersum .

2. Methods

The plant biomass was collected locally and consistedof the oating part of the plant. After it had been har-vested, it was washed with dilute hydrochloric acid (3%)and distilled water before being used. Analytical gradecopper(II) sulphate, zinc(II) sulphate and lead(II) nitratewere used as the metal sources and stock solutions of

these metal ions were prepared in deionised water. Thesorption tests were conducted at 25 C in conical asks(250 ml) using an orbital shaker in a constant roomtemperature. The initial pH values were between 5 and 6during the batch experiments and no pH adjustment wasmade. Therefore, all the sorption experiments werecarried out at pH values of

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the adsorption of heavy metals by Spirulina sp. hasshown that the chemical composition of the cells didaffect the degree of adsorption which occurred, withCr(VI) being less readily adsorbed when there was ahigh concentration of polysaccharides in the cells(Hernandez and Olguin, 2002). It is recognised thatSpirulina is an algal species, not an aquatic plant, but itis a worthwhile comparison and points to an area whichneeds evaluation.

3.2. Kinetics of adsorption

If the movement of the solute from the bulk liquidlm surrounding the adsorbent is ignored, the adsorp-tion process for porous solids can be separated intothree stages:

mass transfer (boundary layer diffusion), sorption of ions onto sites, intraparticle diffusion.

External mass transfer is characterised by the initialrate of solute adsorption. This can be calculated fromthe initial slope of the C t =C 0 time curves. These slopescan be derived by assuming that the relationship waslinear over the rst 510 min (McKay et al., 1982). Thisgave initial rates of 0.094, 0.0715 and 0.0652 min 1 forlead, zinc and copper respectively.

In many cases, there is the possibility that intrapar-ticle diffusion will be the rate limiting step and this isnormally determined by using the equation described byWeber and Morris (1963):

k p q

t 1=2where q is the amount of the metal adsorbed at time t ,(mg/g); k p the intraparticle rate constant (mg/g min 0:5).

Fig. 2 shows that the relationships for C. demersumand copper zinc, lead systems are not linear over theentire time range, indicating that more than one processis affecting the adsorption. This type of non-linearity hasbeen reported previously by various authors (Keskin-kan et al., 2003; McKay et al., 1980; Ho et al., 1996;Fernandez et al., 1996) and has been interpreted asshowing that both boundary layer diffusion, the initialcurved portion, and intraparticle diffusion, the nal

linear portion, are occurring. The slope of the nal lin-ear portion can be used to derive a rate parameter, k p ,for intraparticle diffusion. It is also possible to derive aninitial rate parameter, j , by linear regression between

t 0 and t lim which is the rst breakpoint in the rela-tionship (Ho et al., 1996). Values for these parametersare given in Table 2. These rate parameters have units of mg/gmin 0:5 and, as such, are not a direct quanticationof the rates. Nevertheless, they can be interpreted inrelative terms. According to the theoretical equationsfor diffusion, when intraparticle diffusion is the onlyrate-determining step, the rate parameter is directly re-lated to the square root of the initial concentration (Hoet al., 1996). That is to say; j C 0

n where n 0:5.However, values for n were found to be 0.043 for zinc,0.0098 for copper and 0.177 for lead. This conrms thatintraparticle diffusion was not the only operativemechanism and not the rate-determining step for thelead, copper, zinc and C. demersum systems.

Table 1A comparison of the heavy metal uptake capacities ( qmax , mg/g) of various macrophytes

Adsorbent Lead Zinc Copper

Ceratophyllum demersum (this study) 44.8 ( R2 0:97) 13.98 ( R2 0:97) 6.17 ( R2 0:97)Myriophyllum spicatum (Keskinkan et al., 2003) 46.69 15.59 10.37Myriophyllum spicatum (Wang et al., 1996) 55.6 13.5 12.9Potamogeton lucens (Schneider et al., 1999) 141 32.4 40.8

Salvinia herzegoi (Schneider et al., 1999) 18.1 19.7Eichhornia crassipes (Schneider et al., 1999) 19.2 23.1

00.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 2 4 6 8 10 12 14Time 0.5 (min 0.5 )

S p e c

i f i c a

d s o r p

t i o n

( m g

/ g )

Fig. 2. Plots for the intraparticle diffusion for zinc ( r ), lead ( N ) andcopper ( j ) onto C. demersum .

Table 2Rate constants for each metal (mg/gmin 0:5)

Metal Initial rateconstant, k p(mg/gmin 0:5)

R2 Pseudo constants,second-order(g/mgmin) rate

R2

Zinc 1.103 0.995 0.0147 0.999Copper 1.023 0.986 0.1834 0.999Lead 1.506 0.970 0.0137 0.999

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In many cases, the kinetics of adsorption by any bio-logical material have been only tested for the rst-orderexpression given by Lagergren. However, it has alsobeen shown that a pseudo second-order approach cansometimes provide a better description of the adsorptionkinetics (Cheung et al., 1997). The rst-order Lagergrenequation is:

lnqe qt lnqe Kt

The pseudo second-order equation is:

t =qt b1=2 K 0 q2ec t =qe

where qe is the mass of metal adsorbed at equilibrium(mg/g), qt the mass of metal adsorbed at time t (mg/g), K is the rst-order reaction rate constant of adsorption(min 1), K 0 the pseudo second-order rate constant of adsorption (g/mg min).

It was found that, although the rst-order equation

was suitable for some of the data, it was not applicablefor all the results. Therefore, no further considerationwas given to it. The pseudo second-order reactionmodel, however, was applicable to all the data andvalues of the reaction rate constants and correlationcoefficients for each metal are listed in Table 2.

4. Conclusions

From the work presented here, the aquatic sub-merged plant C. demersum can be an effective biosorbentfor zinc, lead copper removal under dilute metal con-ditions. Batch adsorption studies showed that C. de-mersum would adsorb zinc, lead and copper and that,based on the Langmuir coefficients, the maximum ad-sorption capacity was 13.98 mg/g for zinc, 44.80 mg/gfor lead and 6.17 mg/g for copper.

The initial part of the adsorption was governed by arst degree lm diffusion process. The data also conrmthat intraparticle diffusion was not the rate-determiningstep. The overall adsorption rate showed that the zinc,copper and lead and C.demersum system was best de-scribed by the pseudo second-order model.

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