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Journal of Environmental Management 92 (2011) 2666e2674

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Journal of Environmental Management

journal homepage: www.elsevier .com/locate/ jenvman

Sorption and desorption of Cd, Cu and Pb using biomass from an eutrophizedhabitat in monometallic and bimetallic systems

J.M. Lezcano*, F. González, A. Ballester, M.L. Blázquez, J.A. Muñoz, C. García-BalboaDepartamento de Ciencia de los Materiales e Ingeniería Metalúrgica, Facultad de Ciencias Químicas, Universidad Complutense, Madrid, Spain

a r t i c l e i n f o

Article history:Received 12 July 2010Received in revised form12 May 2011Accepted 3 June 2011Available online 1 July 2011

Keywords:BiosorptionDesorptionHeavy metalsIsothermsResidual biomass

* Corresponding author.E-mail address: fgonzalezg@quim.ucm.es (J.M. Lez

0301-4797/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.jenvman.2011.06.004

a b s t r a c t

This work examines the sorption capacity of a natural biomass collected from an irrigation pond. Thebiomass mainly consisted of a mixture of chlorophyte algae with caducipholic plants. Biosorptionexperiments were performed in monometallic and bimetallic solutions containing different metalscommonly found in industrial effluents (Cd, Cu and Pb). The biosorption process was slightly slower inthe binary system comparing with monometallic system which was related to competition phenomenabetween metal cations in solution. The biosorbent behaviour was quantified by the sorption isothermsfitting the experimental data to mathematical models. In monometallic systems, the Langmuir modelshowed a better fit with the following sorption order: Cu w Pb > Cd; and biomass-metal affinity order:Pb > Cd w Cu. In bimetallic systems, the binary-type Langmuir model was used and the sorption orderobtained was: Pb w Cu > Cd. In addition, the effectiveness of the biomass was investigated in severalsorptionedesorption cycles using HCl and NaHCO3. The recovery of metal was higher with HCl than withNaHCO3, though the sorption uptake of the biomass was sensitively affected by the former desorptionagent in subsequent sorption cycles.

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1. Introduction

The fast development experienced by industry in the lastcentury has had a dramatic effect on the natural environment.Pollutants change the natural quality of the environment in phys-ical, chemical, or biological characteristics (Gupta et al., 2009).Certainly that is the case of pollution of aquatic systems by heavymetals (Capó Martí, 2002).

Separation methods based on physical or chemical properties ofsolutions have been usually applied in the treatment of industrialeffluents contaminated with heavy metals. The main drawbacks ofthose methods are the generation of large amounts of sludges andthe fact that they cannot lower metal concentrations below100 mg/L, despite that some metals are still extremely toxic at suchconcentration.

The potential use of biological materials (biomass) for thetreatment of contaminated sites has been proposed, since severaldecades ago, as a substitute to physicochemical decontaminationmethods. Among biological methods, biosorption has the uniqueadvantage that can be performed in the absence of microbial

cano).

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metabolism which allows the use of dead biomass with thesubsequent economic savings.

In recent years, there has been an increasing interest in devel-oping the sorption uptake of many industrial by-products such as:fertilizers, used for the treatment of hexavalent chromium solu-tions (Gupta et al., 2010); sugar, for the treatment of heavy metalsincluding lead, nickel and copper (Gupta and Ali, 2004; Gupta andAli, 2000; Gupta et al., 2003); and even different kind of residuessuch as red mud, an aluminium industry waste, for the sorption ofcadmium and zinc (Gupta and Sharma, 2002). In addition, algaehave shown good sorbent properties (Gupta and Rastogi, 2008a,b,c;Gupta et al., 2006) and, in general, low cost adsorbents (Ali andGupta, 2007).

However, the feasibility of biosorption versus traditionalmethods has been questioned on the base of a series of limitations:the relatively low metal uptake, the high brittleness of the biomassand the inherent poor reproducibility. Those limitations may beovercome, at least in part, whether recovering the metal biosorbedor reusing the biomass in a new process. In this way, the scale up ofbisorption technology involves studies to minimize both costs andthe environmental impact caused by waste generation (Lister andLine, 2001; Jalai-Rad et al., 2004; Vijayaraghavan et al., 2005a,b).

On the other hand, in an attempt to approach to actual condi-tions of water pollution by heavy metals, where is very improbable

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J.M. Lezcano et al. / Journal of Environmental Management 92 (2011) 2666e2674 2667

to have just one metal species in solution, research studies havebeen extended to systems concerning two or more metals (Chongand Volesky, 1996; Yu and Kaewsarn, 1999; Gardea-Torresdeyet al., 2004; Pagnanelli et al., 2004a,b; Mehta and Gaur, 2005).Those studies have focused on the effect of a given metal on thesorption uptake of another one by a certain biomass and on therelative affinity of the biomass for each metal. Nevertheless, thenumber of biosorption studies published on multimetallic systemsis comparatively less than those related to monometallic systems(Yu and Kaewsarn, 1999; Romera et al., 2006). In this way, theevaluation, interpretation and representation of multimetallicsystems is more complex than formonometallic systems because ofcompetition phenomena between metals in solution for thebiomass active sites (Lee and Volesky, 1999; Hammaini et al., 2003;Volesky, 2003). These competition phenomena, in turn, depend onfactors such as metal speciation, solution pH, nature of bond sites,metal concentration or selectivity of the biomass to metal species.

This work examines the metal sorption uptake of a biomasscollected in an eutrophized habitat, both in monometallic andbimetallic systems. The recovery of the adsorbed metals usingNaHCO3 and HCl as eluents and the reuse of the biomass insubsequent biosorption cycles. The use of residual biomassprovides two important advantages: low cost of the biosorbentmaterial and recycling of a natural residue with an environmentalimpact. Metal biosorption was evaluated for three different metals(Cd, Cu and Pb) frequently found in industrial effluents and witha high level of toxicity. The study was performed both in mono-metallic and in bimetallic systems and the biomass sorption uptakewas quantified from the isotherms plot and the fit of experimentaldata to different mathematical models.

2. Materials and methods

2.1. Biomass

The residual biomass tested was collected from an eutrophizedecosystem, an irrigation pond located at the Forestry ResearchCentre of Madrid, Spain (CIFOR). The composition of the biomass,shown in Table 1, indicates a wide compositional variety of severalspecies of clorophite algae (61.41%) and caducipholic plants

Table 1Composition of the biomass collected at CIFOR.

Species Amount (g) in 100 g of biomass

Oedogonium sp.a 26.93Eichhornia crassipesb 25.00Cladophora sp.a 20.18Spirogyra sp.a 13.52Aesculus hippocastanumb 6.94Platanus orientalisb 3.80Acer negundob 1.27Diatomeac 0.47Scenedesmus sp.a 0.47Cyanobacteriad 0.31Micrasterias sp.a 0.31Hedera elixb 0.20Euglena sp.c 0.16Ulmus sp.b 0.12Ligustrum sp.b 0.11Vinca sp.b 0.10Bambusia sp.b 0.08Ilex sp.b 0.03

a Chlorophyta algae.b Deciduous plants.c Protistes.d Photosynthetic bacteria.

(37.65%) and, in less extent, photosynthetic bacteria (0.31%) andprotistes (0.63%).

Previous to biosorption experiments, the as-received biomasswas dried to a constant weight in a stove at 60 �C then ground ina Fritsch Pulverisette grinding mill model 6 and screened toa particle size of

0,00

0,08

0,16

0 20 40 60 80 100 120

Ce(m

m/L

)

Time (min.)

C0=10 mg/L

Cd Cu Pb

Fig. 1. Metal concentration versus time in monometallic systems.

6

7

pH

Pb

10 mg/L 25 mg/L 50 mg/L100 mg/L 150 mg/L

J.M. Lezcano et al. / Journal of Environmental Management 92 (2011) 2666e26742668

where V is the liquid volume (L), C0 the initial metal concentration(mg or mmol/L), Ce the metal concentration at equilibrium (mg ormmol/L) and B the biomass concentration (g/L).

For bimetallic systems, the sorption isotherms plot was obtainedby contacting the biomass with metallic solutions of differentconcentration (0, 10, 25, 50, 100 and 150 mg/L). In these tests, theconcentration of the first metal was set while the concentration ofthe second metal varied within such range.

The competition between metals for the sorption sites of thebiomass was quantified by fitting the experimental data to thebinary-type Langmuir model. This model assumes the equilibriumbetween two metals in solution (M1 and M2) with the speciesresulting from their sorption (BM1 and BM2) by the biomass (B)(Sánchez et al., 1999; Hammaini et al., 2003; Volesky, 2003):

BþM1%BM1;K1 ¼k�1k1

¼ ½B�½M1�½BM1�; b1 ¼

1K1

(2)

BþM2%BM2;K2 ¼k�2k2

¼ ½B�½M2�½BM2�; b2 ¼

1K2

(3)

The binary Langmuir model as a function of parameter K, or theinverse of the equilibrium constant, is given by the followingexpressions:

qeðM1Þ ¼qmaxK1

CeðM1Þ

1þ 1K1

CeðM1Þ þ1K2

CeðM2Þ(4)

qeðM2Þ ¼qmaxK2

CeðM2Þ

1þ 1K1

CeðM1Þ þ1K2

CeðM2Þ(5)

Thus, parameter K is inversely proportional to the affinity of thebiomass for one of the two metals in solution. Then, a high value ofK for metal M2 (K2) versus M1 (K1) means that the biosorbentpresents a higher affinity for the latter than for the former metalsince a high value of K is associated to a high metal desorbed/metaladsorbed ratio. Those constants together with the maximumbiomass sorption uptake were determined with the MATLAB� 5.1software (MATLAB, 1997).

2.5. Recovery of the metal adsorbed

The reuse of the biomass was considered in consecutive sorp-tionedesorption cycles. After sorption, the biomass was recoveredby filtration and dried in a stove at 60 �C for 12 h. In that way,desorption tests were performed using the same biomass concen-tration as in sorption tests. Then, the biomass loaded with metalwas treated with an eluent (HCl at pH 3 or 0.1 N NaHCO3) andsamples of 5 mL were removed at different times to evaluatedesorption kinetics. After 120 min, tests were centrifuged and pHand metal concentration determined in the supernatant solution.This process was repeated two times.

4

5

0 50 100 150Time (min.)

Fig. 2. pH versus time in monometallic systems at different initial Pb concentrations.

3. Results and discussion

3.1. Sorption kinetics

There is a double interest in sorption kinetics: 1) to gatherinformation on the biosorption process at different pH and metalconcentrations and 2) to estimate the time required to completethe process.

Furthermore, the sorption rate of the process will determine thetype of reactor to be used and contribute final costs. In general, thesorption rate depends on several factors (Veglio et al., 2002; Jalai-Rad et al., 2004; Rangsayatorn et al., 2004; Diniz and Volesky,2005): 1) the biosorbent or type of biomass (chemical compositionand number of active sites) and its physiological state (active orinactive, free or immobilized) and structural properties (surfacearea, morphology); 2) the metal, its diffusion in the biosorbent,concentration, and competition with other metal ions by activesites; and 3) experimental conditions, especially stirring.

Fig.1 depicts the evolution of metal concentration versus time inmonometallic systems for an initial concentration of 10 mg/L. Thebiosorption process was very fast and around 80% of metal wasrecovered in the first 8 min while equilibrium was reached at30 min. Similar results were obtained for the rest of initial metalconcentrations tested in agreement with other studies that haveestablished biosorption times shorter than 15 min after bio-massemetal solution contact (Reddad et al., 2002; Deng and Bai,2004; Martins et al., 2004; Chojnacka et al., 2005; Goksunguret al., 2005; Seki et al., 2005).

Fig. 2 shows the variation of pH versus time for tests atdifferent initial Pb concentrations. pH values increased withdecreasing the concentration of this metal in solution. This wasexpected since the binding mechanism of protons to activecentres of the biomass is similar to that of metal cations insolution (Matheickal et al., 1999; Yu and Kaewsarn, 1999; Yuet al., 2001; Gulnaz et al., 2005; Han et al., 2005; Lin and Lin,2005; Saeed et al., 2005). For biomass with free active sites,

J.M. Lezcano et al. / Journal of Environmental Management 92 (2011) 2666e2674 2669

the simultaneous uptake of metal cations and protons increasedthe solution pH.

The fast biosorption rate would be related to two facts: theprocess takes placemainly on the biosorbent surface (Herrero et al.,2005; Kamala et al., 2005) and metal binding is accomplished byfast reversible reactions independent of metabolism (Cruz et al.,2004; Pagnanelli et al., 2004a,b).

In general, biosorption in bimetallic systems was slightly slowerthan for monometallic systems, and this is in agreement with otherexperimental studies (Ozdemir et al., 2005). Those differencescould be due to the absence of competition phenomena in mono-metallic systems.

3.2. Sorption isotherms in monometallic systems

The sorption process was quantified from the equilibriumparameters obtained by fitting the experimental data to the twomathematical models usually employed in literature: Langmuirand Freundlich (Chojnacka, 2005; Goksungur et al., 2005; Gulnazet al., 2005; Han et al., 2005; Herrero et al., 2005; Kamala et al.,2005; Lodeiro et al., 2005; Mehta and Gaur, 2005; Saeed et al.,2005).

Table 2 collects the values of the relative constants for bothmodels and the linear regression coefficient (R2). The latter valuesindicate a better fitting of experimental data to the Langmuir thanto the Freundlich model. According to the former model, the bio-sorption process: 1) takes place in a monolayer; 2) there is homo-geneity with respect to the type and affinity of metal by the activesites; 3) the adsorbed ions do not affect the sorption of other ionsby neighbour active centres; and 4) metal can saturate the biomass(Puranik and Paknikar,1997; Satiroglu et al., 2002; Goksungur et al.,2005; Saeed et al., 2005).

A similar conclusion can be reached from the analysis ofconstants for each metal derived from both models: values of qmaxfor the Langmuir model and Ke for the Freundlich model gave thefollowing sorption order: Pb w Cu > Cd. This sequence is inagreement with the electronegativity values of the different metalcations tested (Sheng et al., 2007): the higher the electronegativityof the ion the stronger is the attraction for the negatively chargedligands on the biomass. Of the three metals tested, Pb and Cu, withthe highest electronegativity (2.33 and 1.91, respectively), wereadsorbed in a greater extent. Cadmium, with a lower electronega-tivity (1.69) and a larger volume, presented the lowest metaluptake.

The metal ion affinity order of the biomass, deduced from thevalues of K from the Langmuir model, was: Pb > Cd w Cu. In spitethat the affinity for cadmiumwas high, the metal uptakewas low asindicated by the value of qmax. This behaviour could be related tothe presence of few active sites, but with low activation energy, onthe cell wall to bind this metal. In such case, Cd would be easilybioadsorbed at low concentrations but saturation is rapidly reached(Hashim and Chu, 2004).

Table 2Langmuir and Freundlich constants.

Metal Langmuir Freundlich

qmax (mmol/gbiomass)

K (mmol/L) R2 Ke (mmol/gbiomass)

1/n R2

Cd 0.290 0.055 0.993 0.576 0.188 0.955Cu 0.508 0.067 0.996 0.719 0.196 0.897Pb 0.388 0.003 1.000 0.707 0.125 0.986

3.3. Sorption isotherms in bimetallic systems

The analysis of bimetallic systems is based on competitionbetween metal species in solution and can be treated in threedifferent ways (Chong and Volesky, 1995; Loaec et al., 1997; Lee andVolesky, 1999; Hammaini et al., 2002; Volesky, 2003; Mehta andGaur, 2005):

(i) Fitting experimental data to bidimensional isotherms, as formonometallic systems. Thus, each isotherm represents thedifferent sorption uptakes of the first metal, as a function of itsequilibrium concentration, for each given concentration of thesecond metal. This method allows a qualitative but notquantitative analysis of the sorption influence of one metalover the other one.

(ii) Fitting experimental data to 3-D surfaces, using three axes: Xand Y axes represent the equilibrium concentrations of bothmetals and Z axis the sorption uptake of one of the two metalsor the sum of both. This method is a better plot than theformer but, as main drawback, sometimes shows irregularsurfaces that are far from being the real behaviour of thebimetallic system. In addition, it only provides qualitativeinformation.

(iii) Fitting experimental data to a mathematical model. Thismethod, unlike the others, provides quantitative informationof the biosorption process in bimetallic systems. Among themathematical models described in the literature for bimetallicsystems, the binary-type Langmuir model has been the mostapplied since, besides being the most simple, it allows thedetermination of the equilibrium constant of the bio-massemetal interaction given whether by the value of b or byits inverse (K). This method was adopted in the present work.

The values of the Langmuir constants corresponding to the threesystems studied and obtained using the MATLAB 5.1 software aregiven in Table 3. The maximum sorption uptake of the biomass,qmax, remained practically constant for all systems around0.5 mmol of cation adsorbed/g of biomass. That would be an indi-cation that the number of active sites in the biomass available formetal binding is fixed and independent of the working solution(Romera et al., 2008).

Considering the values of the affinity parameters, K1 and K2, forthe CueCd and PbeCd bimetallic systems, the affinity of thebiomass was five times higher for Cu or Pb than for Cd. In contrast,the biomass presented a high and similar affinity for bothmetals forthe PbeCu system.

Similar results were obtained by Sheng et al. (2007), in binarysystems and using the marine alga Sargassum sp. Moreover, ourresults agree with those recorded in monometallic systems, espe-cially for Pb, the metal with the lowest value of K (Table 2).However, unlike monometallic systems, the biomass presenteda different affinity for Cu and Cd in bimetallic systems. A similarbiosorption behaviour was observed using Fucus spiralis as bio-sorbent (Romera et al., 2008). In this case, the different electro-negativity of both ions (higher for Cu) and the different ion radius(bigger for Cd) could be responsible for such results.

Table 3Values of Langmuir constants for the three bimetallic systems studied.

System K1 (mmol/L) K2 (mmol/L) qmax (mmol/g)

CudCd Cu: 0.066 Cd: 0.340 0.474PbdCu Pb: 0.001 Cu: 0.004 0.452CdePb Cd: 0.340 Pb: 0.066 0.474

J.M. Lezcano et al. / Journal of Environmental Management 92 (2011) 2666e26742670

Let us consider each bimetallic system separately:

3.3.1. CdeCu systemThe competition between Cd and Cu for the active sites of the

biomass is depicted in Fig. 3. The surface isotherms showa decreaseof cadmium uptake in the presence of increasing amounts of Cu(Fig. 3b). In contrast, Cu uptake was kept high even at high Cdconcentrations (Fig. 3a). The higher affinity of the biomass for Cuagrees with the previous values of K. The sorption for the simul-taneous uptake of both metals reached a constant value

Fig. 3. Sorption isotherms for the CueCd system.

independently of each metal concentration, as expected from thevalues of qmax obtained (Fig. 3c).

3.3.2. CuePb systemThe isotherm for this system (Fig. 4) shows that the influence of

Pb on the Cu sorption uptake was similar to its reciprocal, inagreement with the low values of K obtained for bothmetals. In thisbimetallic system, the biomass reached saturation faster than in theprevious system which is a clear indication of the high affinity ofthe biomass for both metals (Fig. 4c).

Fig. 4. Sorption isotherms for the CuePb system.

J.M. Lezcano et al. / Journal of Environmental Management 92 (2011) 2666e2674 2671

3.3.3. PbeCd systemFig. 5 shows that Pb had a negative effect on the uptake of Cd,

especially at high concentrations of the former metal. Conversely,the effect of Cd on the Pb sorption uptake was practically negligible(Fig. 5a). This behaviour is in agreement with the previous quan-titative analysis according to which the affinity of biomass is pref-erentially towards Pb than Cd. Also, the simultaneous sorptionuptake of both metals reached a constant value independently ofmetal concentration (Fig. 5c).

Let us consider all bimetallic systems globally:In all cases, an increase of concentration of a given metal led to

a lower sorption uptake of the other metal present in solution since

Fig. 5. Sorption isotherms for the PbeCd system.

both metals compete for the same active centres of the bio-adsorbent (Chong and Volesky, 1995; Loaec et al., 1997).

The affinity order of the biomass agrees with that obtained formonometallic systems: Pb w Cu > Cd. A higher affinity of a givenmetal with respect to the other one can be due whether to loweractivation energy, to a stronger binding strength towards the activecentre, or to a different binding mechanism. Some authors havepointed out that the affinity can be related to specific properties ofeach metal, such as (Chong and Volesky, 1996): ionic radius (thesmaller the radius the higher the affinity) or cation charge density(the higher the charge density the higher the affinity becausea higher electrostatic attraction).

In the three cases studied, the total amount of metal adsorbed,obtained from the sorption uptakes of both metals in solution,tends to a maximum and constant value around 0.5 mmol/g. Thiswould be in agreement with Langmuir’s hypothesis that thenumber of active centres available on the biomass is fixed andremains constant over the whole surface and that each metal ioncould bind any of them (Chong and Volesky, 1995; Loaec et al.,1997).

3.4. Recovery of the metal adsorbed

The desorption process for the recovery of metals is based onseveral mechanisms. Basically, there are three desorption mecha-nisms (Huang et al., 1998; Kapoor et al., 1999; Gardea-Torresdeyet al., 2004):

(i) Precipitation of the metal adsorbed by formation of insolublecompounds with the desorbent agent, e.g. H2S.

(ii) Complexation of the metal adsorbed by reaction withcompounds which have pairs of electrons available to sharewith the metal cation. Desorbent agents such as NaHCO3,Na2CO3 and EDTA follow this mechanism.

(iii) Ion exchange. This mechanism gives good yields byexchanging a cation from the desorbent agent for the metaladsorbed. Themost common desorbent agents in this categoryare: HCl, H2SO4 and NaOH.

There is a large number of desorbent agents that can be used butacids, inorganic or organic, have the highest metal desorptioncapacity (Huang et al., 1998; Davis et al., 2000). The desorbent agentshould fulfil four basic requirements: 1) a high elution efficiency; 2)low damage of the biomass in order to be reused in subsequentcycles; 3) low degree of contamination; and 4) low cost (Davis et al.,2000; Gardea-Torresdey et al., 2004; Chojnacka et al., 2005; Mehtaand Gaur, 2005).

In the present study, two different eluent agents were selected:one acidic (HCl) and another complexing (NaHCO3), both witha relatively low cost.

3.4.1. HCl as desorbentFig. 6 shows the elution kinetics with HCl quantified as a func-

tion of the amount of metal release. Like biosorption, the desorp-tion process took place rapidly andmore than 80% of metal retainedon the biomass was released within the first 5 min. In addition, theincrease of pH during desorption (Fig. 6) would indicate anexchange between metal cations adsorbed and protons supplied bythe desorbent agent, in agreement with other authors (Huang et al.,1998; Gardea-Torresdey et al., 2004; Mehta and Gaur, 2005).

Fig. 7 shows the amount of metal retained by the biomass aftereach biosorption or desorption step. HCl was markedly efficient asdesorbent agent (Desorption I) with metal recoveries of 85.7% forCd, 66.7% for Cu and 63.2% for Pb. Thus, the desorption sequencewas inverse to the sorption order recorded. These results agreewith

0,00

0,05

0,10

0,15

0 20 40 60 80 100 120 140

Time (min.)

Metal released

(m

mo

l/L

)

7

7,5

8

8,5

9

pH

Cd released Cu released Pb releasedCd pH Cu pH Pb pH

Fig. 8. Desorption kinetics using NaHCO3.

0,00

0,05

0,10

0,15

0,20

0 20 40 60 80 100 120 140

Time (min.)

Metal released

(m

mo

l/L

)

3

3,2

3,4

3,6

3,8

4

pH

Cd released Cu released Pb releasedCd pH Cu pH Pb pH

Fig. 6. Desorption kinetics using HCl.

J.M. Lezcano et al. / Journal of Environmental Management 92 (2011) 2666e26742672

those obtained by several authors (Mattuschka et al., 1993; Puranikand Paknikar, 1997; Davis et al., 2000; Lin and Lin, 2005; Saeedet al., 2005).

However, once the biomass was reused in a new biosorption test(Biosorption II) its efficiency significantly decreased with respect tothe first sorption cycle (Biosorption I).

This could be due to several factors, such as: structural damagesof the active centres provoked by the desorbent agent, or blockageof those sites due to the inefficiency of the eluant leaving less activesites available for the a new sorption cycle (Puranik and Paknikar,1997; Davis et al., 2000; Rangsayatorn et al., 2004; Sekhar et al.,2004; Lin and Lin, 2005; Vijayaraghavan et al., 2005a,b).

3.4.2. NaHCO3 as desorbentThe desorption kinetics with NaHCO3 (Fig. 8) showed that

despite being a process relatively fast, recoveries higher than 80%can be reached in the first 30 min, had a worse performance thanHCl. The desorption mechanism can be explained by the existenceof bicarbonate anions with pairs of electrons available that canremove by complexation the metal cations uptaked by the biomass(Puranik and Paknikar, 1997; Gardea-Torresdey et al., 2004).

As in the previous case, an increase of solution pH took placesimultaneously to metal desorption (Fig. 8). In this case, however, itshould not be due to ion exchange since it is a weak acid salt and itsdissociation in the aqueous medium provides alkalinity due to thefollowing equilibria:

0

0,1

0,2

0,3

0,4

0,5

0,6

Biosorption I Desorption I Biosorption II

Am

ou

nt o

f m

etal r

etain

ed

(m

mo

l/g

of b

io

mass)

Cd Cu Pb

Fig. 7. Metal uptake by the biomass after different sorptionedesoprtion cycles withHCl.

NaHCO3%Naþ þ HCO�3 (6)

HCO�3 þ H2O%H2CO3 þ OH� (7)Thus, this would explain the slight increase of pH shown in

Fig. 8.Fig. 9 depicts the amount of metal at the end of each sorption or

desorption step. The efficiency of NaHCO3 as desorbent agent wasrelatively low, except for Cu, with a recovery of 76.7%, the recoveryfor the rest of metals was: 11.4% for Pb and 1.4% for Cd.

In spite that in the first desorption process (Desorption I) metalrecovery was low, the reuse of the remaining biomass (BiosorptionII) in a second biosorption cycle improved significantly both itssorption uptake and metal recovery that was even higher than inthe first cycle (Biosorption I). This would be related to chemicalchanges in the biomass produced by NaHCO3. Yan andViraraghavan (2000) have reported similar effects of NaHCO3,after a second cycle under identical conditions, onMucor rouxii andthe increase of the sorption uptake observed was attributed toseveral facts: removal of impurities on the biomass surface,breakdown of the cell wall or generation of new sorption activecentres.

After a second desorption (Desorption II) in identical conditionsto the first one, the desorption yield was even lower than thatrecorded in the first cycle (Fig. 9). The percentages of metal des-orbed, with respect to the total amount of biomass, after the secondbiosorption cycle, were: 53.8% for Cu, 3.0% for Pb and 0.3% for Cd.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

Biosorption I Desorption I Biosorption II Desorption II

Am

ou

nt o

f m

etal retain

ed

(m

mo

l/g

of b

iom

ass)

Cd Cu Pb

Fig. 9. Metal uptake by the biomass after different sorptionedesorption cycles withNaHCO3.

J.M. Lezcano et al. / Journal of Environmental Management 92 (2011) 2666e2674 2673

Clearly, metal desorption after the first cycle was higher usingHCl than NaHCO3. A plausible explanation is the differentperformance mechanism of each eluant: ion exchange for HCland complexation for NaHCO3 (Gardea-Torresdey et al., 2004).Nevertheless, HCl, a strong acid, could provoke a serious damageon the biomass affecting its sorption capacity. On the contrary,NaHCO3 significantly improved the sorption uptake of thebiomass.

Therefore, the significant increase of sorption of the biomasstreated with NaHCO3 is an indication that this reagent is able toimprove its metal sorption uptake.

4. Conclusions

- The eutrophized biomass can be used effectively for decon-taminating effluents contaminated with heavy metals.

- The sorption kinetics in monometallic systems was very fastand equilibrium was reached after 30 min. The pH valuesincreased with decreasing metal concentration because ofcompetence phenomena between protons and metal cationsfor the same active sites of the biomass.

- The sorption kinetics in bimetallic systems was slower than inmonometallic systems and the equilibrium was reached after120 min. That was related to the strong competence betweenmetal cations for the same active sites of the biomass.

- The sorption order inmonometallic systemswas: Cuw Pb> Cdand the biomass-metal affinity: Pb > Cd w Cu.

- The affinity order in bimetallic systems was: Pb w Cu > Cd. Inaddition, the maximum total amount of metal adsorbed for thethree systems studied was around 0.5 mmol/g.

- The desorption process was fast and released more than 80% ofthe metal adsorbed within the first 5 min with HCl and in thefirst 30 min with NaHCO3.

- The percentages of metal recovery obtained with HCl in thefirst cycle ranged between 63 and 100%. The lower sorptionmetal uptake with reused biomass was related to damage orblockage of its active centres.

- The percentages of metal recovery obtained with NaHCO3 inthe first cycle ranged between 1 and 77%. The sorption metaluptake increased for the reused biomass but the yield of thesecond desorption cycle was lower than for the first one.

Acknowledgements

The authors wish to express their gratitude to the SpanishMinistry of Science and Technology for funding this work.

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Sorption and desorption of Cd, Cu and Pb using biomass from an eutrophized habitat in monometallic and bimetallic systems1 Introduction2 Materials and methods2.1 Biomass2.2 Metal solutions2.3 Kinetic study: biosorption tests2.4 Sorption isotherms2.5 Recovery of the metal adsorbed

3 Results and discussion3.1 Sorption kinetics3.2 Sorption isotherms in monometallic systems3.3 Sorption isotherms in bimetallic systems3.3.1 Cd–Cu system3.3.2 Cu–Pb system3.3.3 Pb–Cd system

3.4 Recovery of the metal adsorbed3.4.1 HCl as desorbent3.4.2 NaHCO3 as desorbent

4 Conclusions Acknowledgements References