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Biosorption of mercury(II), cadmium(II) and lead(II) ions from aqueous system by microalgae Chlamydomonas reinhardtii immobilized in alginate beads

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Biosorption of mercury(II), cadmium(II) and lead(II) ions from aqueous system by microalgae Chlamydomonas reinhardtii immobilized in alginate beads
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  Biosorption of mercury(II), cadmium(II) and lead(II) ions fromaqueous system by microalgae  Chlamydomonas reinhardtii immobilized in alginate beads Gülay Bayramo ğ lu  a  , Ilhami Tuzun  b , Gokce Celik  a,b ,Meltem Yilmaz  b , M. Yakup Arica  a,b, ⁎ a   Biochemical Processing and Biomaterial Research Laboratory, Faculty of Science, K  ı r  ı kkale University, 71450 Yah  ş ihan-K  ı r  ı kkale, Turkey  b  Department of Biology, Faculty of Science, K  ı r  ı kkale University, 71450 Yah  ş ihan-K  ı r  ı kkale, Turkey Received 2 December 2005; received in revised form 13 June 2006; accepted 16 June 2006Available online 26 July 2006 Abstract The potential use of the immobilized microalgae (in Ca-alginate) of   Chlamydomonas reinhardtii  to remove Hg(II), Cd(II) andPb(II) ions from aqueous solutions was evaluated using bare Ca-alginate bead as a control system. Ca-alginate beads containingimmobilized microalgae were incubated for the uniform growth at 22 °C for 5 days. Effects of pH, temperature, initialconcentration of metal ions and biosorbent dosages on the adsorption of Hg(II), Cd(II) and Pb(II) ions were studied. Adsorption of Hg(II), Cd(II) and Pb(II) ions on the immobilized microalgae showed highest values at around pH 5.0 to 6.0. The adsorptionequilibrium was represented with Langmuir and Freundlich adsorption isotherms. The adsorption of these ions on the immobilizedmicroalgae followed second-order kinetics and equilibrium was established in about 60 min. The temperature change in the rangeof 5 – 40 °C did not affect the adsorption capacities of the immobilized microalgae. The immobilized-algal systems can beregenerated using 2 M NaCl for Hg(II), Cd(II) and Pb(II) ions.© 2006 Elsevier B.V. All rights reserved.  Keywords:  Heavy metal; Alginate; Adsorption; Biosorption; Adsorption kinetic; Microalgae 1. Introduction Theremovalofheavymetalionsbybiosorptionusing biological materials have been widely studied in the last decade due to its potential, particularly in wastewater treatment. Compared to some microbial biomasses suchas fungi (Bayramo ğ lu et al., 2003; Arica et al., 2004; Al-Qunaibitetal.,2005;Baiketal.,2002),bacteria(Trevorset al., 1986; Saygideger et al., 2005; Tunali et al., 2006b)andyeast(Lamelasetal.,2005),heavymetalbiosorptioncapacity of algae proved to be the highest because of thealgal cell wall, which is composed of a fiber-likestructure and an amorphous embedding matrix of var-ious polysaccharides (Ozdemir et al., 2005; Gekeler et al., 1998). There are several functional chemicalgroups on algal cell surface that can attract and sequester the heavy metal ions such amino, amido, sulfate andcarboxyl (Schiewer and Volesky, 2000). Biosorption Int. J. Miner. Process. 81 (2006) 35 – 43www.elsevier.com/locate/ijminpro ⁎ Corresponding author. Biochemical Processing and BiomaterialResearch Laboratory, Faculty of Science, K  ı r  ı kkale University, 71450Yah ş ihan-K  ı r  ı kkale, Turkey. Tel.: +90 318 357 2477; fax: +90 318 3572329.  E-mail address:  yakuparica@kku.edu.tr  (M.Y. Arica).0301-7516/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.minpro.2006.06.002  mechanisms include ionic interactions and formation of complexes between metal ions and the functional groupsof the cell wall components. The binding characteristicsof metal ions on the biosorbents can partly be explained by Lewis' hard and soft acid and base theory and byIrving – Williams' series (Schiewer and Volesky, 2000;Crist etal., 1981).Forexample,Pb(II), Cd(II) and Hg(II)ions have been effectively removed using a variety of algal groups including green ( Chlorella vulgaris ,  Sce-nedesmus  sp.,  Chlorococcum  sp. and  Fischerella  sp.)and blue green algae (  Lyngbya spiralis ,  Tolypothrixtenuis ,  Stigonema  sp. and  Phormidium molle ) (Inthornet al., 2002). Pb(II) and Cd(II) have also been effectivelysegregated from aqueous solutions by the dried biomassof brown marine algae (Wild and Benemann, 1993).The immobilization of biomass might also provideseveral advantages such as facility to reuse andseparation of solid biomass from the bulk liquid. The process will become cost effective by reusing the bio-mass after regeneration (Bayramo ğ lu et al., 2003; Aricaet al., 2005; Prakasham et al., 1999). Immobilization of algal biomass is usually obtained by the entrapment of the cells into a matrix of the natural polymers such asalginate, chitosan, chitin and cellulose derivatives(Bajpai et al., 2004; Bai and Abraham, 2003).Polysaccharide gel immobilized microorganisms can be used to remove heavy metal ions from aqueoussolutions, providing an alternative to physico-chemicaltechnologies for wastewater treatment (Bayramo ğ luet al., 2003; Veglio et al., 2002). Alginic acid is aheteropolysaccharide made of  α - L -guluronic acid and β - D -manuromic acid and is found in many algal speciesespecially inside the brown algae. This carboxylic poly-electrolyte is soluble in water and precipitates in theform of a coacervate in the presence of multivalent metalions like Ca(II), Co(II), Fe(II), Fe(III) and Al(III)(Bayramo ğ lu et al., 2003). The immobilization methodis easy and can be performed under very mild conditionswithout damaging the living fungal cells. Althoughnumerous studies are present, using particularly theentrapped green algae species,  Chlorella  sp. (Crist et al.,1981; Jalali et al., 2002)  Chlamydomonas reinhardtii has rarely been utilized for testing the heavy metalremoval efficiency from aqueous solutions (Cai et al.,1995; Roesijadi, 1992). The unicellular alga,  C.reinhardtii , has recently gained interest in bioremedia-tion studies (Macfie and Welbourn, 2000; Wetzel andLikens, 1991; Adhiya et al., 2002).Heavy metals in wastewater coming from batterymanufacturing, painting, printing, mining activities,alloy industries and the utilization of fossil fuels are just a few examples. Metal ions such as cadmium,copper, lead, mercury, chromium, etc., are hazardous tothe environment. Their presence in the aquatic ecosys-tem poses human health risks and causes harmful effectsto living organism in water and also to the consumers of them(Vilaretal.,2005).Variousmethodsofheavymetalremoval from wastewaters have been proposed such as precipitation,membranefiltration,chemicaloxidationor reduction, ion exchange and adsorption. These techni-ques are often ineffective or expensive, especially whenconcentrations are in the order of 1 – 100 mg L − 1 . Newtechnologies are required that can reduce heavy metalconcentrations to environmentally acceptable levels at reasonable cost. Biosorption has the potential to greatlycontributetotheachievementofthisgoal.Thismethodis based on metal sequestering properties of certain naturalmaterials of biological srcin. These materials investi-gated for heavy metals removal include fungi, bacteriaand algae (Baik et al., 2002; Al-Qunaibit et al., 2005).They are abundant in nature, or/and in by products or waste materials from industrial process.In this study, a wild type of   C. reinhardtii , isolatedfrom a polluted site of K  ı z ı l ı rmak River was cultured toachieve the most probable removal efficiency becausethe species grown in polluted areas were known to bemore resistant, and thus having more capability of accumulating heavy metals. The immobilized  C. rein-hardtii  was utilized for the removal of Hg(II), Cd(II) andPb(II)fromaqueoussolution.Theeffectsofcontacttime,solid/liquid ratio, and initial concentration of metal ions,and pH on the adsorption of Hg(II), Cd(II) and Pb(II)ionshavebeeninvestigated.AdsorptionofHg(II),Cd(II)and Pb(II) ions from aqueous solutions on the immobi-lized  C. reinhardtii  under different kinetic and equilib-rium conditions are scrutinized in some details. Finally,elution-reuse of the free and immobilized  C. reinhardtii was evaluated. 2. Materials and methods 2.1. Microorganism and media Individuals of   C. reinhardtii  were isolated from thefresh water samples obtained from K  ı z ı l ı rmak River inTurkey. The sampling site selected on the K  ı z ı l ı rmakRiver was located 1 km away downstream to thedischarge point of an oil refinery. Cell culture wasgrown in minimal base medium adjusted to pH 7.0,maintained at 22±1 °C with 16:8 h of light  – dark cycleusing 4000 lx light intensity of cool-white fluorescent.Algal cells at the middle of the logarithmic phase, whichwas reached in 15th day, were harvested by centrifuga-tion at 2000 rpm for 10 min. 36  G. Bayramo  ğ  lu et al. / Int. J. Miner. Process. 81 (2006) 35  –  43  2.2. Immobilization of microalga C. reinhardtii intoCa-alginate The immobilization of   C. reinhardtii  via entrapment was carried out as follows: Na-alginate (2.0 g; from  Macrosytia pyrifera , high viscosity, Sigma Chem. Co.,USA)wasdissolvedindistilledwater(50mL)anditwasthen mixed with thealgal suspension (50 mL, containing1.0 g algal cells). The mixture was introduced into asolution containing (0.1 M CaCl 2 ) with a burette and thesolution was stirred to prevent aggregation of the algalcell entrapped Ca-alginate beads. The algal cellsentrapped beads ( ∼ 2 mm) were cured in this solutionfor 30 min and then washed twice with 200 mL steriledistilled water. The beads with immobilized algal cellswere then transferred to the algal growth medium(200 mL) in 500-mL flask and were incubated at 22 °Cfor 5 days as described above. After 5 days incubation,the Ca-alginate beads with immobilized algal cells wereremoved from the medium by filtration and washedtwice with saline solution (0.85%). It was then stored at 4 °C until use in 5 mM CaCl 2  solution. 2.3. Adsorption studies Adsorption of Hg(II), Cd(II) and Pb(II), on plain Ca-alginate beads and on the immobilized  C. reinhardtii was investigated in batch adsorption-equilibrium experi-ments. The stock solutions of metal ions (i.e., Hg(II), Cd(II) and Pb(II): 1.0 g L − 1 ) were prepared using nitratesalts in double distilled water. A range of metal ionsconcentration was prepared from stock solution. Theeffect of pH on the biosorption rate was investigated inthe pH range 3.0 – 7.0 (which was adjusted with HCl or  NaOH at the beginning of the experiment and not controlled afterwards) at 25 °C. The effect of temper-ature on the adsorption capacity of the biosorbent wasexamined at four different temperatures (i.e., 5, 15, 25and 40 °C). Solution containing 100 mg L − 1 of each Hg(II), Cd(II) and Pb(II) ions and biosorbents were stirredat 150 rpm. The effect of the initial metal ions (i.e., Hg(II), Cd(II) and Pb(II)) concentration on adsorption wasstudied at pH 5.0 as noted above except that theconcentration of metal ions in the adsorption mediumvaried between 25 and 500 mg L − 1 . 2.4. Analytical procedure Adsorption of metal ions from aqueous solutions wasstudied in batch systems. After the desired incubation period (about 120 min) the aqueous phases wereseparated from the biosorbents and the concentrationof Hg(II), Cd(II) and Pb(II) ions in these phases weremeasured. An atomic absorption spectrophotometer (AAS) was used for the determination of metal ions.Deuterium background correction was used and thespectral slit width was 0.5 nm. The working current/ wavelength values for Cd(II) and Pb(II) were 8.0 mA/ 228.8 nm and 10 mA/283.3 nm, respectively. For mercury measurement, the instrument was equippedwith a Mercury Vapor Unit (MVU-1A). The workingcurrent and wavelength values for Hg(II) were 6.0 mAand 253.6 nm, respectively.The instrument response was periodically checkedwith metal ion standard solutions. For each set of datareported, standard statistical methods were used todetermine the mean values and standard deviations.Confidence intervals of 95% were calculated for eachset of samples in order to determine the margin of error.The amount of metal ions adsorbed per unit plain Ca-alginate and microalgae immobilized preparations(mg metal ions g − 1 dry beads) was obtained by usingthe following expression: q ex  ¼ ½ð C  0 − C  Þ  xV   =  M   ð 1 Þ where  q ex  is the amount of metal ions adsorbed onto theunit mass of the adsorbent (mg g − 1 ),  C  0  and  C   are theconcentrations of the metal ions before and after adsorption (mg L − 1 ),  V   is the volume of the aqueous phase (L), and  M   is the amount of the adsorbent (g). 2.5. Desorption In order to determine the reusability of the immobi-lized algal preparations consecutive adsorption – desorp-tion cycles were repeated 8 times by using the sameimmobilized algal preparations. Desorption of Hg(II),Cd(II) and Pb(II) ions was performed by 2 M NaClsolutions. The biosorbent loaded with heavy metal ionswere placed in desorption medium and stirred at 200 rpm for 2 h at 25 °C. After each cycle of adsorption – desorption, immobilized algal preparationwas washed with buffer solution and reconditioned for adsorption in the succeeding cycle. Desorption ratio wascalculated from the amount of metal ions adsorbed onthe algal biomass and the final heavy metal ionsconcentration in the adsorption medium. 2.6. Theoretical approach2.6.1. Adsorption isotherms Adsorption isotherms express the relation betweenthe amount of adsorbed metal ions per unit mass of  37 G. Bayramo  ğ  lu et al. / Int. J. Miner. Process. 81 (2006) 35  –  43   biosorbent ( q eq ) and the metal concentration in solution( C  eq ) at equilibrium. The Langmuir adsorption model is based on the assumption of surface homogeneity such asequally available adsorption sites, monolayer surfacecoverage, and no interaction between adsorbed species(Arica and Bayramoglu, 2005; Sheng et al., 2004). Themathematical description of this model is q eq  ¼  q m C  eq = ð k  d  þ  C  eq Þ ð 2 Þ where  C  eq  and  q eq  also show the residual metalconcentration and the amount of metal adsorbed onthe adsorbent at equilibrium, respectively,  k  d = k  2 /  k  1  isthe Langmuir constant of the system.The Freundlich equation is the empirical relationshipwhereby it is assumed that the adsorption energy of a protein binding to a site on an adsorbent depends onwhether or not the adjacent sites are already occupied.One limitation of the Freundlich model is that theamount of adsorbed solute increases indefinitely withthe concentration of solute in the solution. Thisempirical equation takes the form: q eq  ¼  K  F ð C  eq Þ 1 = n ð 3 Þ where  K  F  and  n  are the Freundlich constants, thecharacteristics of the system.  K  F  and  n  are the indicator of the adsorption capacity and adsorption intensity,respectively. 2.6.2. Biosorption kinetic modeling  The large number and different chemical groups onthe cell wall of the algal cells (e.g.,  – COOH,  –  NH 2 , f   NH,  – SH,  – OH) imply that there are many types of algal cell – metal ions interactions. The first-order rateequation of Lagergren is one of the most widely used for the sorption of solute from a liquid solution (Arica andBayramoglu, 2005; Tunali et al., 2006a,b). It may berepresented as follows: dq t  = dt   ¼  k  1 ð q eq − q t  Þ ð 4 Þ where  k  1  is the rate constant of first-order biosorption(min − 1 ) and  q eq  and  q t   denote the amounts of  biosorption at equilibrium and at time  t   (mg g − 1 ),respectively.The essential assumption of the Ritchie second-order model was that an adsorbate was adsorbed onto twosurface sites. Therefore, the chemical equation becomes:2M ð  solid  Þ  þ  Cd 2 þð aqueous Þ  Y M 2 Cd 2 þð adsorbed phase Þ  ð 5 Þ The sorption rate equation can be established based onthe adsorption mechanism as shown in Eq. (5): d  h = dt   ¼  k  ð 1  −  h  Þ 2 ð 6 Þ where  θ  is the fraction of surface available site for adsorption of solute;  n  is the number of surface sitesoccupied by each molecule of adsorbed solute; and  k   isthe rate constant. If metal ion adsorption in theadsorption medium is considered to be a second order reaction, then Eq. (6) becomes: q m = ð q m − q t  Þ ¼  kt   þ  1  ð 7 Þ 3. Results and discussion 3.1. Properties of biosorbent systems Alginic acid or alginate, the salt of alginic acid, is thecommon name given to the family of linear poly-saccharides containing 1,4-linked  β - D -manuronic (M)and  α - L -guluronic (G) acid residues arranged in a non-regular, block wise order along the chain (Roesijadi,1992; Draget et al., 1994; Feng and Aldrich, 2004; Aricaet al., 2005; Genç et al., 2003). Alginates were preferredover other materials because of their various advantagessuch as biodegradability, hydrophilic properties, pres-ence of carboxylic groups, and natural srcin. It could be preferred over other materials because of its variousadvantages such as biodegradability, hydrophilicity andnatural origin. These are very important because polymers of petroleum srcin are non-degradable,making them a major cause of pollution. In the present work, Ca-alginate in the bead form was used as anadsorbent and a support material for entrapment of amicroalgae  C. reinhardtii  and used for the removal of Hg(II), Cd(II) and Pb(II) ions from aqueous solution.Alginate beads were prepared by cross-linking withdivalent calcium ions, and alginate droplets were precipitated in the bead form (diameter about 2 mm)in calcium chloride solution. The water content of alginate beads was 245%. The alginate beads werestable over the experimental pH range of 3.0 – 7.0. 3.2. Adsorption rate The adsorption rates of Hg(II), Cd(II) and Pb(II) ionson the Ca-alginate and immobilized-algal preparationswere obtained by following the decrease of theconcentration of metal ions within the adsorptionmedium with time. As can be seen from Fig. 1, the Hg 38  G. Bayramo  ğ  lu et al. / Int. J. Miner. Process. 81 (2006) 35  –  43  (II), Cd(II) and Pb(II) adsorption rate was high at the beginning of adsorption and saturation levels werecompletely reached at about 60 min for Hg(II), Cd(II)and Pb(II) ions. After this equilibrium period, theamount of adsorbed metal ions on the biosorbents didnot significantly change with time. This trend in bindingof metal ions suggests that the binding may be throughinteractions with functional groups located on thesurface of the biosorbents. Feng and Aldrich studiedCu(II), Pb(II) and Cd(II) biosorption on marine algae  Ecklonia maxima  and the biosorption equilibrium wasestablished about 60 min (Feng and Aldrich, 2004). In astudy of Ni(II) biosorption onto free and immobilizedalgal cells and the biosorption equilibrium was reachedat 120 min (Abu Al-Rub et al., 2004). Note that there areseveral parameters, which determine the adsorption ratesuch as stirring rate of the aqueous phase, structural properties of both the support and the biosorbent. It could be said that relatively rapid biosorption rates wereobtained using the Ca-alginate and immobilized-algal preparation in this study. 3.3. Effect of pH and temperature on the biosorptioncapacity The dependence of metal biosorption on pH is relatedto both the surface functional groups on the cell walls of the biosorbent and the metal chemistry in solution(Sheng et al., 2004; Bayramo ğ lu and Ar  ı ca, 2005). Themedium pH affects the solubility of metals and theionization state of the functional groups (i.e., carboxyl-ate, phosphate, and amino groups) on the algal cell wall.The carboxylate and phosphate groups carry negativecharges that allow the microbial cells to be potent scavengers of cations. The experimental results are presented in Fig. 2. In all cases, the maximum heavymetal ions biosorption occurred between pH 5.0 and 6.0.The amount of adsorbed heavy metal ions (Hg(II), Cd(II) and Pb(II) at 100 mg L − 1 ) on the immobilized algal- preparation at pH 5.0 were 89.5, 66.5 and 253.6 mg g − 1 dry biosorbent, and the corresponding values for wereCa-alginate were found to be 32.4, 27.9 and173.9 mg g − 1 , respectively. There was an increase inmetal ions adsorption per unit weight of biosorbent withincreasing pH from 3.0 to 5.0. It seemed to level off at  pH greater than 6.0. At acidic pH (pH ≈ 3), protonationof the cell wall component adversely affect the biosorption capacity of the algal preparation, but itseffect becomes minor with increasing pH in the medium(Bayramo ğ lu et al., 2003). Maximum adsorption wasobtained in the pH range of 5.0 – 6.0 and the interactionof the heavy metal ions with the algal preparation could be primarily with the carboxylate and phosphate groupsof the cell wall component. During the biosorption-equilibrium experiments with Ca-alginate and immobi-lized-algae no significant changes in pH of the mediumwere observed. Several researchers have investigatedthe effect of pH on biosorption of heavy metals by usingdifferent kind microbial biomass. For example, the biosorption of Cu(II) and Pb(II) by marine algae  E.maxima  was pH-dependent and maximum biosorptionwas obtained in the pH range of 5.8 – 8.5 (Feng andAldrich, 2004). The biosorption of Cd(II) and Cu(II) onthe algal surface was pH-dependent and the pH valuesfor both heavy metals were of the same order of magnitude (Xue et al., 1988). Fig. 1. Biosorption rates of Hg(II), Cd(II) and Pb(II) ions on Ca-alginate and immobilized-algal preparations: Biosorption conditions:initial concentration of metal ions=100 mg L − 1 ; pH=6.0;temperature=25 °C.Fig.2. Effect of pHon the biosorption capacitiesof the Ca-alginateandimmobilized-algal preparations: Biosorption conditions: initial con-centration of metal ions=100 mg L − 1 ; pH=6.0; temperature=25 °Cfor Hg(II), Cd(II) and Pb(II) ions.39 G. Bayramo  ğ  lu et al. / Int. J. Miner. Process. 81 (2006) 35  –  43
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