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Biosorption of lead from aqueous solution by seed powder of Strychnos potatorum L

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Biosorption of lead from aqueous solution by seed powder of Strychnos potatorum L
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  Colloids and Surfaces B: Biointerfaces 71 (2009) 248–254 Contents lists available at ScienceDirect Colloids and Surfaces B: Biointerfaces  journal homepage: www.elsevier.com/locate/colsurfb Biosorption of lead from aqueous solution by seed powder of  Strychnos potatorum  L. K. Jayaram a , I.Y.L.N. Murthy b , H. Lalhruaitluanga a , M.N.V. Prasad a , ∗ a Department of Plant Sciences, University of Hyderabad , Hyderabad-500046, Andhra Pradesh, India b Directorate of Oilseeds Research, Rajendranagar, Hyderabad-500030, Andhra Pradesh, India a r t i c l e i n f o  Article history: Received 22 July 2008Received in revised form 15 February 2009Accepted 16 February 2009Available online 3 March 2009 Keywords: AdsorptionFourier transform infrared anaysisIsothermsLead(II) Strychnos potatorum Seed powder a b s t r a c t In the present study, Pb(II) removal efficiency of   Strychnos potatorum  seed powder (SPSP) from aqueoussolutionhasbeeninvestigated.BatchmodeadsorptionexperimentshavebeenconductedbyvaryingpH,contacttime,adsorbentdoseandPb(II)concentration.Pb(II)removalwaspHdependentandfoundtobemaximum at pH 5.0. The maximum removal of Pb(II) was achieved within 360min. The Lagergren first-order model was less applicable than pseudo-second-order reaction model. The equilibrium adsorptiondatawasfittedtoLangmuirandFreundlichadsorptionisothermmodelstoevaluatethemodelparameters.Both models represented the experimental data satisfactorily. The monolayer adsorption capacities of SPSP as obtained from Langmuir isotherm was found to be 16.420mg/g. The FTIR study revealed thepresence of various functional groups which are responsible for the adsorption process.© 2009 Elsevier B.V. All rights reserved. 1. Introduction Heavy metal pollution is an environmental problem of globalconcern. The discharge of metals is increasing continuously as aresultofindustrialactivitiesandtechnologicaldevelopment,posingthreat to the environment and public health because of their tox-icity, accumulation through food chain and persistence. Lead (Pb)has been classified as a toxic heavy metal that can cause seriousdamage to the liver, brain, kidney, reproductive and nervous sys-tem. Severe exposure to Pb(II) has been associated with sterility,stillbirths,abortionandneonataldeaths[1–3].Themajorsourceof  Pb(II) pollution in natural waters is due to discharge of waste fromacid battery manufacturing, metal plating and finishing, printing,metallurgical alloying, lead mining, ceramics and glass industries[4,5].Thepresenceofleadindrinkingwaterevenatbelowpermis-sible concentration may cause anaemia, encephalopathy, hepatitisand nephritic syndrome [6]. The permissible limit (mg/l) for Pb(II) in wastewater given by Environmental Protection Agency (EPA) is0.05mg/landbyBureauofIndianStandards(BIS)is0.1mg/l[7].The healthhazardsduetothepresenceofPb(II)inwaterareofextremeconcerntothepublic,governmentandindustry[8].Theremovalof  Pb(II) from wastewaters by traditional processes includes its pre-cipitation with lime or alkali hydroxide, coagulation, electrolytic ∗ Corresponding author. Tel.: +91 40 23011604; fax: +91 40 23010120/145. E-mail addresses:  mnvsl@uohyd.ernet.in, prasad mnv@yahoo.com(M.N.V. Prasad). deposition, reverse osmosis and ion exchange. These methods areexpensiveandhavesignificantdisadvantagessuchasgenerationof metal bearing sludge or wastes, incomplete metal removal and thedisposal of secondary waste.Recently attention has been drawn to the development of alter-native methods like biosorption which uses organic materials asbiosorbents. Since the last decade, biosorption or sorption of con-taminants by sorbents of natural srcin has gained importantcredibility due to its good performance and low cost of thesecomplexing materials. Due to high uptake capacity and very cost-effective source of raw materials, biosorption is a progressiontowards a perspective method. Various plant materials viz. wastetea leaves [9], sphagnum moss peat [10], sago waste [11],  Medicagosativa  [12], peat [13],  Quercus ilex  leaf, stem and root phytomass[14], sawdust [15], rice polish [16],  Azadirachta indica  leaf powder[8], C aladiumbicolor  biomass[17], Oryzasativa husk[18],maizebran [19],palmshellactivatedcarbon[20],olivepomace[21],maizeleaf  [22], saw dust [23], coconut and seed hull [24], have been studied for Pb removal from aqueous system. Strychnos potatorum  L. (Loganiaceae) is a moderate sized treefound in Southern and central parts of India, Srilanka and Burma.SeedsarewidelyusedinAyurvedicandtraditionalmedicine.Apartfrom its medicinal properties the seed powder is being used forclearing muddy water by the rural community. They are reportedto be very effective as coagulant aids. This property is attributedbecause of the presence of polyelectrolyte, proteins, lipids, carbo-hydratesandalkaloidscontainingthe–COOHandfree–OHsurfacegroups in the seed [25–28]. Having established the coagulating 0927-7765/$ – see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.colsurfb.2009.02.016  K. Jayaram et al. / Colloids and Surfaces B: Biointerfaces 71 (2009) 248–254  249 propertiesofthe S.potatorum seedstherehasbeenarecentinterestinthemetalbindingproperty[28,29].Theaimofthepresentstudy istodeterminetheefficiencyof  S.potatorum seedpowder(SPSP)inremoval of heavy metal Pb(II). The influence of pH, biomass dose,contacttimeandinitialPb(II)concentrationonbiosorptionofPb(II)ions were studied in aqueous solutions. 2. Materials and methods  2.1. Preparation of Pb(II) stock solution All the chemicals used in the experiments were of analyticalgrade.StockPb(II)solution(1000mg/l)waspreparedbydissolving1.5984g of Pb(NO 3 ) 2  (Qualigens Fine Chemicals, Mumbai; mini-mumassay99%)in100mlofMilli-Q(Millipore)waterandthefinalvolume was made quantitatively to 1000ml using Milli-Q water.Pb(II) solutions of different concentrations were prepared by ade-quate dilution of the stock solution with Milli-Q water. pH of thesolutions was adjusted with 1N HNO 3  or 1N NaOH. All the glass-ware and polypropylene flasks used were washed with 10% (v/v)HNO 3  and rinsed several times with deionized distilled water.  2.2. Plant materialS. potatorum  L. seeds (SPSP) were collected from forests of Andhra Pradesh, India and were dried at 40 ◦ C for 2 days in hotair oven. Seeds were made into powder in Clotech 1093 samplemill, sieved to give a fraction of 100 mesh screen and used as abiosorbent.  2.3. Batch experiments Batch mode experiments were conducted at 28 ◦ C temperaturebyshaking0.100gofadsorbent(SPSP)in100mlofPb(II)solutionof desiredconcentrationin250mlglassconicalflasks.Theflaskswereagitatedonarotaryshakerat150rpmfor6htoensureequilibrium.The influence of pH (2.0, 3.0, 4.0, 5.0 and 6.0), contact time (15, 30,60,120,240and360min.),biomass(0.100,0.150,0.200and0.250g)and initial Pb(II) concentration (20, 30, 40, 50 and 60mg/l) wereevaluated during the present study. At the end of the experiment,the conical flasks were removed from the shaker and the solutionswereseparatedfromthebiomassbycentrifugationat10,000 × rpmfor5min.Metalconcentrationsweremeasuredusingflameatomicabsorptionspectrometer(GBC932plus,Australia).Thewavelengthused for the analysis of the metal in this study was 283.3nm. Theinstrumentwascalibratedwithinthelinearrangeofanalysisandacorrelationcoefficient( R  2 )of0.995–1.000wasobtainedforthecali-brationcurve.Theinstrumentwasperiodicallycheckedthroughouttheanalysiswithknownstandards.Tocheckforthereproducibility,alltheexperimentswererepeatedthriceandeachindividualexper-imentinturnwascarriedoutintriplicatesandfordataanalysistheaverage value was computed which is mean ± SD value.  2.4. Desorption of the adsorbed Pb(II) After saturation of the SPSP with 20mg/l of Pb(II), in order toremove the bound metal ions from the SPSP was washed severaltimeswithdeionizeddistilledwatertoremoveanyexcessofPb(II).Itwastreatedwith100mlof0.1MHClandequilibratedbyshakingfor 2h and then centrifuged at 5min for 10,000rpm. Supernatantswere collected and metal analysis was carried out.  2.5. Fourier transform infrared (FTIR) analysis In order to determine the functional groups responsible formetal uptake and to detect vibration frequency changes in thebiosorbent the untreated SPSP and pretreated with 20mg/l Pb(II) Fig. 1.  Effect of pH on Pb(II) removal by SPSP (initial Pb(II) concentration=20mg/l;adsorbentdose=0.100g/100ml,temperature=28 ◦ C,contacttime=360min,dataisthe mean ± S.D. of the three independent experiments). were analyzed using a Fourier transform infrared spectrometer(NICOLET 5700 – FTIR). 3. Results and discussion  3.1. Effect of pH on metal ion binding  The pH of the aqueous solution is an important controllingparameterintheadsorptionprocess[30].Inordertostudytheeffect ofpHonPb(II)adsorptionontoSPSP,pHofsolutionwasvariedfrom2.0 to 6.0. From Fig. 1, it is observed that the adsorption of Pb(II) varies with pH and there is a gradual increase in Pb(II) uptake asthe pH value increases from 2.0 to 6.0. The maximum uptake of these ions was obtained at pH 5.0 (40.21%) at 6h. At pH below 4.0,an uptake of Pb(II) was less, probably because the H 3 O + ions maycompete with the metal ions for the exchange sites in the sorbent.One of the reasons for the metal ions adsorption behavior of thebiosorbent is that the SPSP contains a large number of active func-tional groups [25–28] as well as on the nature of the metal ions in solution. When pH is increased (above pH 6.5), the Pb(II) ionsget precipitated due to hydroxide anion forming a lead hydroxideprecipitate. Similar results were reported for adsorption of Pb(II)on  A. indica  leaf powder [8]. For this reason, the optimum pH was selected to be 5.0 for further experiments.  3.2. Effect of biomass Biomassisasignificantfactortobeconsideredforeffectivemetalsorption. When Pb(II) removal at different adsorbent doses (0.100to 0.250g/l) was studied at pH 5.0 while keeping the volume andconcentrationofthemetalsolutionconstant.TheresultshavebeenpresentedinFig.2.ItisevidentthatpercentageadsorptionofPb(II) ion increased with increase in adsorbent dose. The resulting effectcan be easily explained by an increase in surface area (more avail-ability of active adsorption sites) with the increase in biosorbentmass. Similar behavior for the effect of sorbent concentrations onmetal sorption capacity was observed and discussed in the litera-ture for a variety of sorbents and metals [31–34].  3.3. Effect of contact time The effect of contact time on the adsorption of Pb(II) at 20mg/lis shown in Fig. 3. The rate of adsorption is very fast initially  250  K. Jayaram et al. / Colloids and Surfaces B: Biointerfaces 71 (2009) 248–254 Fig. 2.  Effect of adsorbent dose on Pb(II) removal by SPSP (initial Pb(II) con-centration=60mg/l; adsorbent dose=0.100, 0.150, 0.200 and 0.250g/100ml,temperature=28 ◦ C, contact time=360min, pH 5.0, data is the mean ± S.D. of thethree independent experiments). and maximum removal of Pb(II) occurs at 360min. The initial fastsorption may be explained as uptake of Pb(II) through physicaladsorption since adsorption phenomenon characteristically tendstoattaininstantaneousequilibrium[35].Thenumberofactivesites in the system is fixed and each active site can adsorb only one ionin a monolayer therefore metal uptake by the sorbent surface israpid initially and then decreases as the availability of active sitesdecreasesthusslowingdownthetransferofmetalionfromsolutionto adsorbent surface. The rate of metal removal is of great signifi-cance for developing adsorbent based water technology [36]. The ability of SPSP to adsorb maximum amount of Pb(II) at 360minindicates that it is an effective biosorbent for the removal of Pb(II)from wastewater.  3.4. Adsorption kinetics In order to investigate the mechanism of biosorption of Pb(II)by SPSP and the potential rate-controlling steps, such as masstransport and chemical reactions, kinetic models were used to test Fig.3.  EffectoftimeonPb(II)removalbySPSP(initialPb(II)concentration=20mg/l;adsorbent dose=0.100g/100ml, temperature=28 ◦ C, contact time=15, 30, 60, 120,240 and 360min, pH 5.0, data is the mean ± S.D. of the three independent experi-ments). Fig. 4.  Pseudo-first-order (Lagergren model) sorption kinetics plot of Pb(II) ontoSPSP. experimental data. In this study two different kinetic models wereused to adjust the experimental data of Pb(II) biosorption on SPSP.In general, the pseudo-first-order model derived by Lagergren [37]has found wide application.The pseudo-first-order Lagergren model is expressed as Eq. (1):log( q e − q t ) =  − k 1 ,ads 2 . 303  t  + log q e  (1)Where q e  (mg/g)and q t  aretheamountsofadsorbedmetalionsonthebiosorbentattheequilibriumandatanytime t  ,respectively; k 1 ,  ads  (min − 1 ) is the Lagergren rate constant of the first-orderbiosorption. q e  and k 1 ,ads canbecalculatedfromtheslopesandtheinterceptoftheplotlog( q e − q t )versus t  (Fig.4).TheLagergrenfirst- orderrateconstant k 1  and q e  determinedfromthemodelindicatesthat this model has failed to estimate  q e  since the experimentalvalue of   q e  differs from estimated one.Severalauthors[8,38,39]haveshownthatpseudo-second-order kinetics can also very well describe these interactions in certainspecific cases. The pseudo-second-order model is based on theassumption that biosorption follows a second-order mechanism.So, the rate of occupation of adsorption sites is proportional to thesquareofthenumberofunoccupiedsites.Thesecond-ordermodelis expressed as Eq. (2): t q t =   1 q e  t  + 1 k 2 ,ads · q 2e (2)Where  k 2 , ads  is the rate constant of second-order biosorption(g/mg/min).  q e  and  k 2 ,  ads  can be calculated from the slope andthe intercept of the plot  t  / q t  versus  t   (Fig. 5). It is important to notice that it is not necessary to estimate the experimental valueof   q e  for the application of such a model. The coefficient of correla-tion for second-order kinetic model was 0.9991 and the estimatedvalue of   q e  also agreed with the experimental data. Both factorssuggest that the sorption of Pb(II) ions followed the second-orderkineticmodel,indicatingthattherate-limitingstepwasachemicalbiosorption process between Pb(II) and SPSP. Similar conclusionswere found by Ho and McKay [40] and they reported that most of  the sorption systems follow a pseudo-second-order kinetic model.The values of regression coefficient, rate constants of pseudo-first-order Lagergren model and pseudo-second-order parameters aregiven in Table 1.  K. Jayaram et al. / Colloids and Surfaces B: Biointerfaces 71 (2009) 248–254  251  Table 1 Comparisonbetweenadsorptionrateconstant, q e  estimatedandcoefficientofcorrelationassociatedtotheLagergrenpseudo-first-orderandtopseudo-second-orderkineticmodels.Biosorbent Experimental value First-order kinetic model Second-order kinetic model q e  (mg/g)  q e , cal. (mg/g)  K  1 ads  (1/min)  R 2 q e , cal. (mg/g)  K  2  (g/mg/min)  R 2 SPSP 7.628 2.218 0.0251 0.8734 7.7041 0.0288 0.9991 Fig. 5.  Pseudo-second-order sorption kinetics plot of Pb(II) onto SPSP.  3.5. Effect of metal concentration Therateofadsorptionisafunctionoftheinitialconcentrationof metalions,whichmakesitanimportantfactortobeconsideredforeffectivebiosorption[41].ThepercentageremovalofPb(II)atdiffer- entmetalconcentrationsusingSPSPispresentedinFig.6.Whenthe initial Pb(II) concentrations were increased from 20–60mg/l, thepercentage of adsorption slightly decreased (38.14–21.08%). Thismay be due to saturation of active adsorption sites on SPSP. Thusthe adsorbent can be utilized effectively for the removal of Pb(II)from waste water at low concentrations of lead. Fig. 6.  Effect of different concentrations of Pb(II) (Pb(II) concentration=20, 30,40, 50 and 60mg/l; adsorbent dose=0.100g/100ml, temperature=28 ◦ C, contacttime=360min, pH 5.0, data is the mean ± S.D. of the three independent experi-ments).  3.6. Adsorption isotherms ThePb(II)uptakecapacityoftheSPSPatdifferentconcentrations(20–60mg/l) on a fixed amount of adsorbent (0.100g) at pH 5.0has been evaluated using the Langmuir and Freundlich adsorptionisotherms.The Freundlich isotherm is represented by the following Eq. (3)[42]:log q e  = log K  f  +  1 n  log C  e  (3)Where  C  e  is the equilibrium concentration (mg/l);  q e  is theamounts adsorbed per specified amount of adsorbent (mg/g) atequilibrium,  K  f   and  n  are constants which are adsorption capac-ity (mg/g) and intensity of adsorption, respectively. Linear plots of log q e  versus log C  e  (Fig. 7) show that the adsorption followed Fre- undlich model.  K  f   and  n  were calculated from the intercept andslope of the plots. The values of constants are given in Table 2.According to Kadirvelu and Namasivayam [43], ‘ n ’ values between1 and 10 represent beneficial adsorption.Langmuir model is commonly used for liquid phase adsorptionwhich assumes that the uptake of metal ions occurs on a homo-geneous surface by monolayer adsorption without any interactionbetweenadsorbedions.TheLangmuirisothermisexpressedasEq.(4) [44]: C  e q e = C  e  X  m + 1 q m K  L  (4)Where, q e istheamountofPb(II)adsorbedatequilibrium(mg/g), C  e  istheequilibriumconcentration(mg/l),  X  m  and K  L   areLangmuirconstants related to the adsorption capacity and energy of adsorp-tion, respectively. The linear plot of   C  e / q e  versus  C  e  (Fig. 8) shows that the Pb(II) removal by SPSP obeys the Langmuir model too.  X  m and  K  L   were calculated from the slope and intercept of the plotand are presented in Table 2. On the basis of correlation coeffi- Fig. 7.  Linearized Freundlich isotherm plot for adsorption of Pb(II) by SPSP.  252  K. Jayaram et al. / Colloids and Surfaces B: Biointerfaces 71 (2009) 248–254  Table 2 Langmuir and Freundlich isotherm parameters for Pb(II) adsorption by SPSP.Biosorbent Experimental value Langmuir isotherm parameters Freundlich isotherm parameters q e  (mg/g)  X  m  (mg/g)  K  L   ( 1/mg)  R 2 K  f   (mg/g)  n R 2 SPSP 12.651 16.420 0.065 0.990 2.969 1.259 0.984 cients,itcanbeobservedinTable2thattheexperimentaldatawas better fitted to the Langmuir equation than that of the Freundlichequation.  3.7. Separation factor (R L ) The shape of the Langmuir isotherm can be used to predictwhether a sorption system is favourable or unfavourable in abatch adsorption process [39,45]. Accordingly, the essential char- acteristics of Langmuir isotherm can be expressed in terms of adimensionless constant called the separation factor or equilibriumparameter,  R L  that is expressed as Eq. (5): R L   = 1(1 + K  L  C  i ) (5)Where  R L   is a dimensionless equilibrium parameter or separa-tion factor,  K  L   constant from Langmuir equation and  C  i  is initialmetal ion concentration.Theparameter, R L, indicatestheshapeoftheisothermandnatureof the sorption process as given below: R L   value Type of isotherm R L   >1 Unfavorable isotherm R L   =1 Linear isotherm R L   =0 Irreversible isotherm0< R L   <1 Favourable isotherm Thevaluesof  R L  forPb(II)werecalculatedandplottedagainstini-tialmetalconcentration.Thedata(Fig.9)showedthat,thesorption of Pb(II) on the SPSP increased as the initial metal concentra-tion increased from 20 to 60mg/l, indicating that adsorption iseven favourable at the higher initial metal ion concentrations. Thesorption process was favourable for Pb(II) at all the tested concen-trations investigated. Fig. 8.  Linearized Langmuir isotherm plot for adsorption of Pb(II) by SPSP.  3.8. Surface coverage values (   ) ToaccountfortheadsorptionbehaviourofthemetalionsontheSPSP, the Langmuir type equation related to surface coverage wasused. The Eq. (6) is expressed as follows: KC  i  =   1 −    (6)Where  K   is the adsorption coefficient,  C  i  is initial concentrationand     is surface coverage. The fraction of biomass surface coveredby metal ions was studied by plotting the surface coverage val-ues (   ) against metal ions concentration. The data is presented inFig. 9. The figure shows that increase in initial metal ion concen-tration for Pb(II) increased the surface coverage on the biomassuntil the surface was nearly fully covered with a monomolecularlayer. Further examination of  Fig. 10 reveals that the surface cover- ageceasedtovarysignificantlywithhigherconcentrationsofPb(II)and the reaction rate became independent of the Pb(II) concen-tration. The overall adsorption process indicates that the biomasswill be highly effective in removing Pb(II) ions in aqueous efflu-ent.  3.9. Thermodynamic equilibrium constant   ( K  0 c   )Thethermodynamicequilibriumconstant( K  0c )atdifferentPb(II)concentrations using SPSP as adsorbent was obtained at 28 ◦ C. TheEq. (7) is expressed as follows: K  0c  = C  q C  e (7)Where  K  0c  is the equilibrium constant,  C  q  is concentration of Pb(II)ontheadsorbentatequilibriuminmg/land C  e  istheequilib-riumconcentrationofPb(II)insolutioninmg/l.Thevaluesobtainedare presented in Table 3. Fig. 9.  The calculated separation factor ( R L  ) for Pb(II) as a function of metal ionconcentration (mg/l).
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