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Biosorption of Cr(VI) using low cost sorbents

Biosorption of Cr(VI) using low cost sorbents
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  Environ Chem Lett (2003) 1:135–139DOI 10.1007/s10311-003-0027-6 ORIGINAL PAPER N. Fiol · I. Villaescusa · M. Martnez · N. Miralles ·J. Poch · J. Serarols Biosorption of Cr(VI) using low cost sorbents Accepted: 7 April 2003 / Published online: 20 May 2003 Springer-Verlag 2003 Abstract  Waste products from industrial operations,such as yohimbe bark, grape stalks, cork and olive stoneswere investigated for the removal of Cr(VI) from aqueoussolutions. Equilibrium batch experiments at room tem-perature were performed. Metal uptake showed a pH-dependent profile and optimum uptake at initial pHbetween 2.0–3.0. Slight influence of NaCl on metaluptake was observed. The sorption data fitted well to theLangmuir model within the concentration range studied.Grape stalks proved to be the most efficient sorbentfollowed by yohimbe bark, cork and olive stones. Keywords  Yohimbe bark wastes · Cork wastes ·Grape stalk wastes · Olive stone wastes · Chromium ·Metal removal Introduction Chromium is a toxic metal of widespread use in manyindustries such as metal plating facilities, mining opera-tions and tanneries. Cr(III) and Cr(VI) are the chromiumoxidation states usually encountered in the environment.The hexavalent form is of particular concern because of its greater toxicity.Cr(VI) is typically present as an anion and its directprecipitation is not a usual practice. Instead, the anionicspecies are usually reduced to trivalent state form andthen precipitated as chromic hydroxide using lime.However, this method is only effective at high Crconcentrations and also has several disadvantages suchas a significant sludge production, an ever-increasing costof landfill disposal, and most importantly, long-termenvironmental consequences. Although ion exchange andactivated carbon adsorption are other available treatmentmethods, which can be used for low concentrations(Huang and Wu 1975), they suffer from the need for highcapital investment.Therefore, there is a need for the development of lowcost, easily available materials, which could adsorbhexavalent chromium. Natural materials that are availablein large quantities, or waste products from industrial oragricultural operations, may have potential as inexpensivesorbents. Biosorption is a process that uses biomass rawmaterials, which are either abundant in nature or wastefrom industrial operations to sequester toxic heavy metalsand it is particularly useful for the removal of contam-inants from industrial effluents. Biosorption is assumed toarise from two basic mechanisms: an initial rapid metalion uptake due to physical adsorption and a subsequentslow uptake due to chemisorption. Indeed, it has beenreported that most metal biosorption of divalent metalsoccurs in a matter of 5–15 min after solid–liquid contact(Volesky 2001). In this context, most current research isoriented towards removal of heavy metal cations (Vil-laescusa et al. 2000).However, in the 2.0–6.0 pH range, the dominantCr(VI) species are the oxyanions HCrO 4– and Cr 2 O 72 .Because apart from hexavalent chromium, other highlytoxic elements occur in nature in anionic forms (As, Se,Mo, Sb, etc.), studies of anion biosorption remain an openand relevant challenge. In other words, the interest inspecific toxic metals, such as Cr(VI), may also be basedon how representative their behaviour may be in terms of eventual generalisation of results for further studies onbiosorption of metals in their anionic form. In the N. Fiol is the recipient of the 2002 ACE Environmental ChemistryAwardN. Fiol ( ) ) · I. VillaescusaChemical Engineering Dept.,Universitat de Girona,Avda Lluis Santal, 17003 Girona, Spaine-mail: nuria.fiol@udg.esM. Martnez · N. MirallesChemical Engineering Dept., E.T.S.E.I.B.,Universitat Politcnica de Catalunya,Avda Diagonal 647, 08028 Barcelona, SpainJ. Poch · J. SerarolsApplied Mathematics Dept.,Universitat de Girona,Avda Lluis Santal, 17003 Girona, Spain  literature, the removal of Cr(VI) was achieved bydifferent biomasses and the optimum pH for removalwas reported to be in the 2.0–3.0 range (Bai and Abraham2001, Sharma and Forter 1994, Tan et al. 1993).The most convenient means of determining metaluptake abilities is through a batch process in which aquantity of the biomass is placed into contact with asolution containing the ions of interest.In this work, yohimbe bark, grape stalk, cork, and olivestone wastes were used as alternative low-cost sorbentsfor the removal of hexavalent chromium from aqueoussolutions. Equilibrium batch experiments at room tem-perature were designed to study the influence of pH, NaCland metal concentration on the sorption process. TheLangmuir sorption isotherm was used to obtain sorptionequilibrium data. Some desorption experiments were alsocarried out. Experimental Materials and solutionsYohimbe bark wastes kindly supplied by the companyBoehringer Ingelheim, Malgrat de Mar, Barcelona, Spain;grape stalk wastes supplied by a wine manufacturer of theEmpord-Costa Brava region, Girona, Spain; cork wasteskindly supplied by the company Oller S.A., Cass de laSelva, Girona, Spain; and olive stone wastes supplied byan oil manufacturer of the Jaen region, Spain were rinsedthree times with distilled water, dried in an oven at 110 Cuntil constant weight, cut and sieved for a particle size of 1.0–1.5 mm.Chromium solutions were prepared by dissolvingappropriate amounts of sodium dichromate (Na 2 Cr 2 O 7 )in distilled water. NaOH and HCl solutions were used forpH adjustment and for metal desorption experiments.NaCl was used as ionic medium. These reagents wereanalytical grade and were purchased from Panreac(Barcelona, Spain). Chromium standard 1,000 mg L 1 solutions, purchased from Carlo Erba (Milano, Italy),were used for atomic absorption calibrations.ProcedureBatch experiments were carried out at 25 C in stopperedglass tubes by shaking a fixed mass of 0.1 g of dry grapestalk, yohimbe bark or cork or 0.2 g of olive stones, with15 mL of chromium solution at 25 rpm (rotary mixerCenco Instrument) until equilibrium was reached. Aftersolid particles were removed by filtration through a 0.45- m m cellulose filter paper (Millipore Corporation), themetal concentration in filtrates as well as in the initialsolution was determined by flame atomic absorptionspectrometry using a Varian Spectrometer (Model 1275).The amount of metal removed by the biomass wascalculated by a mass balance. Initial metal concentrationwas 10 mg L 1 when the influence of pH and NaClconcentration was investigated. When studying the effectof pH, initial pH of the metal solutions was varied withinthe 1.0–8.0 range. Once the initial pH was adjusted to thedesired value, no efforts were made to maintain thesolution pH while the sorption process was on. The effectof ionic strength on metal removal was studied by varyinginitial NaCl concentration within the 0.1–1.0 mol L 1 range. To study the effect of the total concentration of Cr(VI) on metal uptake and to obtain the Langmuirisotherms, initial metal concentration was varied withinthe range 10–1,000 mg L 1 and pH values were adjusted atpH 3.0 for cork and grape stalks and at pH 2.0 foryohimbe bark and olive stone experiments. In all theexperiments the initial and equilibrium pH were measuredby using a Crison Model Digilab 517 pHmeter. For thedesorption study the biomaterials were previously loadedwith a solution of 1,000 mg L 1 Cr(VI) adjusted at theoptimum pH conditions found for each material. Desorp-tion experiments were carried out by treating 0.1 g of thedifferent Cr(VI)-loaded biomaterials (after gentle washingwith distilled water) with 15 mL of either HCl or NaOH atdifferent concentrations for 24 h. This operation wasrepeated four times. In all sets of experiments each testwas carried out in duplicate and the average results arepresented in this work. Results and discussion Effect of contact timeWe studied the sorption of Cr(IV) on yohimbe bark, grapestalk, cork and olive stone in aqueous solutions. Prelim-inary experiments were carried out in order to evaluatethe contact time needed by the systems to reach theequilibrium. These experiments consisted in putting intocontact 0.1 g of dry solid with 15 mL of metal solution indifferent tubes. Samples were drawn at predeterminedintervals of time for analysis. For an initial Cr(VI)concentration of 10 mg L 1 the results showed thatchromium removal seems to occur in two phases. Thefirst phase involved rapid metal uptake (50–60% removalefficiency) within the first 3 h of contact and wasfollowed by the subsequent removal of the metal whichcontinued for a longer period of time. After 16 h themajor sorption process was completed and the curvebecame flattened. Based on these results a shaking time of 24 h was assumed suitable for subsequent sorptionexperiments. Similar contact times were used by Sharmaand Foster, 1994 and Huang and Wu, 1977 when studyingCr(VI) removal by leaf mould and activated carbon,respectively. Thus, it seems that chromium biosorption isa slower phenomena than divalent metals biosorption.This could be due to the anionic form of chromium sinceall the experiments in the present study were carried out atpH 2.0–3.0 and low Cr(VI) concentrations and underthese conditions only one anionic species (HCrO 4 ) ispresent in solution. 136  Effect of pHpH is the major important parameter controlling thechromium sorption process (Huang and Wu 1977). Tostudy the influence of this parameter on chromiumsorption by the biomaterials under study, pH was variedfrom 1.0–8.0 pH units. In Fig. 1, the amount of Cr(VI)sorbed by the different biomaterials as a function of theequilibrium pH has been plotted. The percentage of Cr(VI) species percentage present in solution as afunction of the pH is superimposed on the same figure.As observed, metal uptake is in all cases pH-dependentshowing a maximum at equilibrium pH values between2.0–4.7, depending on the biomaterial, that correspond toinitial pH values of 2.0 units for yohimbe and olive stonesand 3.0 units for grape stalk and cork. Similar results werefound for the removal of Cr(VI) from aqueous solutionsby coconut husk and palm pressed fibres (Tan et al. 1993),and by  Rhizopus nigricans  (Bai and Abraham 2001). Inaddition, pH was found to increase up to neutral pHvalues, except in the case of the lowest initial pH tested.In order to investigate the reason for the pH increase,preliminary experiments performed with each of thebiomaterials in distilled water under the same conditionswere carried out (1.0–8.0 pH range and 24 h contacttime). Initial pHs exhibited a continuous increase ap-proaching neutral limit value. This increase was notobserved at the lowest pH values. The increase in pHcould be due to the presence of oxo groups (C x O andC x O 2 ) on the surface of the biomaterials. These groupshydrolyse water molecules, thus provoking the release of OH  ions into the liquid phase and the formation of positively charged groups on the solid surface (Sharmaand Foster 1994).Different mechanisms, such as electrostatic forces, ionexchange, chemical complexation, must be taken intoaccount when examining the effect of pH on Cr(VI)sorption. One of the common proposed mechanisms iselectrostatic attraction/repulsion between sorbent andsorbate. Thus, the increase of Cr(VI) sorption at acidicpH should be due to the electrostatic attraction betweenpositively charged groups of biomaterial surface and theHCrO 4- anion, which is the dominant species at low pH.Moreover, the decrease of the sorption with increasing pHcould be due to the decrease of electrostatic attraction andto the competitiveness between the chromium anionicspecies (HCrO 4 and CrO 42 ) and OH  ions in the bulk forthe adsorption on active sites of the sorbent. From theseassumptions it can be suggested that Cr(VI) removal takesplace by physical adsorption.Sorption Langmuir IsothermEquilibrium batch sorption experiments resulted in pointsof the sorption isotherm which was approximated by thenon-competitive Langmuir model:q ¼  q max bC eq   =  1 þ bC eq    ð 1 Þ where q max  is the maximum sorbate uptake and b is theLangmuir constant related to energy of sorption, whichquantitatively reflects the affinity between the sorbent andthe sorbate. The evaluation of the specific uptake q wasperformed according to:q ¼ V C i  C eq   w  1 ð 2 Þ where V is the volume, C i  and C eq  are the initial andunsorbed concentration of the metal ion at equilibrium,respectively, and w is the dry weight of the biomaterial.The experimental chromium (VI) sorption equilibriumpoints for grape stalks, yohimbe bark, cork and olivestones wastes are plotted in Fig. 2.The Langmuir parameters were obtained by fitting theexperimental data to the linearized equation derived fromEq. (1):1 = q ¼ 1 = q max þ 1 =  C eq  q max b    ð 3 Þ Fig. 1  Chromium uptake as a function of equilibrium pH ( sym-bols ). Initial metal concentration was 10 mg L 1 .The speciesdistribution diagram of Cr(VI) referred to 2.10 4 mol L 1 totalchromium concentration is superimposed ( lines ) ( Fig. 2  Fitting of the Langmuir isotherm equation ( lines ) to the datagathered from the experiments ( symbols ). Solution pH was 2.0(yohimbe and olive stones) or 3.0 (grape stalks and cork)137  The Langmuir parameters as well as the correlationcoefficient are listed in Table 1.From the values obtained for these parameters thetheoretical Langmuir curves were calculated and plottedin Fig.2.It can be observed that experimental data fit isothermadequately. Grape stalks proved to be the most efficientsorbent followed by yohimbe, cork and olive stones. Theq max  values obtained for grape stalks and yohimbe wastesare similar to that of other comparable materials such asleaf mould and spagnum moss peat giving 43.0 and43.9 mg/g, respectively, at pH 2.0 (Sharma and Forster1994, 1995), but greater than others such as  Chlorellavulgaris  giving 27.3 mg/g at pH 2.0 (Aksu et al. 1997),coconut husk and palm pressed fibers (29.0 and 14.0 mg/gat pH 2.0, respectively; Tan et al. 1993) or activated cowdung carbon (10.1 mg/g at pH 3.4; Das et al. 2000). Theapplicability of the Langmuir model to the experimentaldata indicates a monolayer coverage on the biomaterialssurface by each of chromium(VI) ions.Effect of sodium chloride concentrationTo study the effect of sodium chloride concentration onmetal uptake, different NaCl concentrations in the 0–1.0 mol L 1 range were investigated. The obtained resultsshowed that the presence of 0.1 mol L 1 NaCl provoked adecrease in sorption efficiency of 78.9–76.3%, 71.5–60.8% and 25.1–19.8% when grape stalk, cork and olivestones were used as sorbent materials, respectively.However an increase in NaCl concentration up to1.0 mol L 1 did not have any additional effect on metaluptake. In the case of yohimbe bark a concentration of 1.0 mol L 1 NaCl was necessary to observe a decrease of sorption efficiency from 83.7–76.7%. From these results,it seems that chloride anion fairly competes with chro-mium oxyanions and therefore anionic exchange couldnot be a significant mechanism for Cr(VI) removal.Desorption experimentsFrom results shown in Fig. 1, it can be suggested that achange of pH could desorb the metal from the biomate-rials, notably in basic pH conditions. Thus, 0.1, 0.5 and1.0 mol L 1 NaOH or HCl solutions were tested to removemetal from the solids. Desorption experiments broughtevidence that after four contacts neither HCl nor NaOHsolutions were able to desorb chromium completely.When NaOH solutions were tested, the material seemednot to be damaged only in the case of olive stones. In spiteof this, desorption from olive stones reached only 15.7%when the most concentrated NaOH solution was used.Grape stalks and olive stones were not damaged by thecontact with the highest HCl concentration, and after fourcontacts 41.1 and 13.9% of metal was recovered respec-tively. However, in the case of yohimbe and cork, a valueno higher than 0.5 mol L 1 HCl could be used and 13.0and 29.4% of metal removal was obtained, respectively.Little information about biosorption mechanisms canbe obtained from these desorption experiments since, ingeneral, biomaterials are decomposed under either basicand/or high acidic conditions.ConclusionsGrape stalk wastes prove to be the most efficient sorbentof Cr(VI) followed by yohimbe bark, cork and olive stonewastes. Metal sorption is pH-dependent and maximumsorption is found at initial pH 2.0 for yohimbe and olivestones and initial pH 3.0 for grape stalks and cork. Underthe experimental conditions used in this work, only aslight influence of NaCl on metal uptake was observed.From the results obtained in this work hexavalentchromium removal could be attributed to physicaladsorption mechanism especially at low pH. Furtherresearch is oriented towards the characterisation of surface functional groups involved in chromium removal.Experimental data fit the Langmuir equation adequate-ly for all biomaterials tested. The four biomaterials showeffective capacities of hexavalent chromium removal.These capacities are found to be similar to that of othercomparable raw cellulosic materials. Thus, these bioma-terials can be used as low cost sorbents for Cr(VI)removal from aqueous solutions. Acknowledgements  Thanks are due to Mrs Rosmi Munt andKatrien D’Hooghe for their help in the experimental work. Thiswork has been supported by Ministerio de Ciencia y TecnologiaSpain, project PPQ2002-04131-C02-01 and PPQ2002-04131-C02-02. References Aksu Z, Aikel U, Kutsal T (1997) Application of multicomponentadsorption isotherms to simultaneous biosorption of iron(III)and chromium(VI) on  C. vulgari s. J Chem Tech Biotechnol70:368–378Bai RS, Abraham TE (2001) Biosorption of Cr(VI) from aqueoussolution by  Rhizopus nigricans  Bioresource Technology 79:73–81Das DD, Mahapatra R, Pradhan J, Das SN, Thakur RS (2000)Removal of Cr(VI) from aqueous solution using activated cowdung carbon. J Colloid Interface Sci 232:235–240Huang CP, Wu MH (1975) Chromium removal by carbonadsorption. J Water Pollut Control Fed 47:2437–2445 Table 1  Langmuir parameters for Cr(VI) uptake, initial pH 2.0 foryohimbe and olive stones and initial pH 3.0 for grape stalks andcork q=(q max bC eq )/(1+bC eq )q max(mg/g)  b (L/mg) r 2 Grape stalks 59.8 0.004 0.98Yohimbe 42.5 0.019 0.99Cork 17.0 0.022 0.99Olive stones 9.0 0.012 0.98138  Huang CP, Wu MH (1977) The removal of chromium from diluteaqueous solution by activated carbon. Water Res 11:673–679Sharma DC, Forster CF (1994) The treatment of chromiumwastewaters using the sorptive potential of leaf mould.Bioresour Tech 49:31–40.Sharma DC, Forster CF (1995) Continuous adsorption and desorp-tion of chromium ions by spagnum moss peat. Process Biochem30: 293–298Tan WT, Ooi ST, Lee CK (1993) Removal of chromium(VI) fromsolution by coconut husk and palm pressed fibres. EnvironTechnol 14:277–283Villaescusa I, Martnez M, Miralles N (2000) Heavy metal uptakefrom aqueous solution by cork and yohimbe bark wastes. JChem Technol Biotechnol 75:812–816Volesky B (2001) Detoxification of metal-bearing effluents:biosorption for the next century. Hydrometallurgy 59:203–216.139
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