Biosorption of nickel from protonated rice bran

Biosorption of nickel from protonated rice bran
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  Journal of Hazardous Materials 143 (2007) 478–485 Biosorption of nickel from protonated rice bran Muhammad Nadeem Zafar a , ∗ , Raziya Nadeem b , Muhammad Asif Hanif  b a  Institute of Chemistry, University of the Punjab, Lahore 54590, Pakistan b  Department of Chemistry, University of Agriculture, Faisalabad 38040, Pakistan Received 10 June 2006; received in revised form 18 September 2006; accepted 19 September 2006Available online 22 September 2006 Abstract In the present study biosorption technique, the accumulation of metals by biomass was used for the removal of nickel from aqueous medium.The rice bran in its acid treated (H 3 PO 4 ) form was used as a low cost sorbent. The adsorption characteristics of nickel on protonated rice bran wereevaluated as a function of pH, biosorbent size, biosorbent dosage, initial concentration of nickel and time. Within the tested pH range (pH 1–7), theprotonatedricebrandisplayedmoreresistancetopHvariation,retainingupto102mg/gofthenickelbindingcapacityatpH6.Meanwhile,atlowerpH values the uptake capacity decreased. The % age removal of nickel was maximum at 0.25g of biosorbent dose and 0.25mm biosorbent size.At the optimal conditions, metal ion uptake was increased as the initial metal ion concentration increased up to 100mg/L. Kinetic and isothermexperiments were carried out at the optimal pH 6.0 for nickel. The metal removal rate was rapid, with 57% of the total adsorption taking placewithin 15–30min. The Freundlich and Langmuir models were used to describe the uptake of nickel on protonated rice bran. The Langmuir andFreundlich model parameters were evaluated. The equilibrium adsorption data was better fitted to Langmuir adsorption isotherm model. Theadsorption followed pseudo second-order kinetic model. The thermodynamic assessment of the metal ion-rice bran biomass system indicated thefeasibility and spontaneous nature of the process and   G ◦ values were evaluated as ranging from  − 22.82 to  − 24.04kJ/mol for nickel sorption.The order of magnitude of the   G ◦ values indicated an ion-exchange physiochemical sorption process.© 2006 Elsevier B.V. All rights reserved. Keywords:  Biosorption; Nickel; Adsorption isotherms; Water treatment; Rice bran 1. Introduction As today’s technology progresses, the natural environmentsuffers from the detrimental effects of pollution. The naturalprocess of transportation of metal ions between soil and waterconsolidates metal contamination in high concentrations thataffect the areas of natural ecosystems [1]. The majority of toxic metal pollutants are waste products of industrial and metallurgi-cal processes. Their concentrations have to be reduced to meetever increasing legislative standards. According to the WorldHealth Organization [2], the metals of most immediate concern are cadmium, chromium, cobalt, copper, lead, nickel, mercuryand zinc. The effluents from metal finishing processes may con-tain up to 10mg/L of copper, chromium, nickel and zinc.Heavymetalreleasestotheenvironmenthavebeenincreasingcontinuously as a result of industrial activities and technologi- ∗ Corresponding author at: Tel.: +92 3454593635.  E-mail address: (M.N. Zafar). cal development, posing a significant threat to the environmentand public health because of their toxicity, accumulation in thefood chain and persistence in nature. It is therefore important todevelopnewmethodsformetalremovalandrecoveryfromdilutesolutions (1–100mg/L) and for the reduction of heavy metalions to very low concentrations. The use of conventional tech-nologies, such as ion exchange, chemical precipitation, reverseosmosis and evaporative recovery for this purpose is often inef-ficient and/or very expensive [3–6].Biosorption, which is a property of certain types of inac-tive, dead microbial biomass to bind and concentrate heavymetals from even very dilute aqueous solutions, is one of themost promising technologies involved in the removal of toxicmetals from industrial waste streams and natural waters [7,8].Biosorption can be considered a collective term for a number of passive, metabolism independent, accumulation processes andmayincludephysicaland/orchemicaladsorption,ionexchange,coordination, complexation, chelation and microprecipitation.Biomass cell walls, consisting mainly of polysaccharides, pro-teinsandlipids,offermanyfunctionalgroupsthatcanbindmetal 0304-3894/$ – see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.jhazmat.2006.09.055   M.N. Zafar et al. / Journal of Hazardous Materials 143 (2007) 478–485  479 ions such as carboxylate, hydroxyl, sulphate, phosphate andamino groups. In addition to these functional binding groups,polysaccharides often have ion-exchange properties [9,7,10].Non-living biomass appears to present specific advantages incomparison to the use of living microorganisms. Dead cellsare not subjected to metal toxicity and nutrient supply is notnecessary. Moreover, the pretreatment and killing of biomasseither by physical or chemical treatments [11,10] or cross-linking [4] are known to improve the biosorption capacity of  biomass.Branisaby-productfromthemillingofrice,consistingofthelarge kernel with a part the germ. It is used in some applicationsmainly as a fertilizer or fuel [12]. Rice bran contains different vitamins, carbohydrates, potassium, nitrogen and phosphoruscompounds,whichinducetowatertocontactwithit.Thesecom-pounds not only have no pollution effects but they are nutritiousto the plants. Therefore, the use of bran to eliminate pollutionfrom water reveals the significance of the bran or natural prod-ucts. Unfortunately up to now only a few studies have beencarried out in this field. Rice bran was used in  in vitro  studyfor determination of capacity for Hg, Cd and Pb [13], bioscav- enging of Cu(II) ion from aqueous solution [14] and removal of  mercury from waste water [15]. In this study, efficiency of rice bran in removal of heavy metal nickel was investigated in detailin its acid treated form. The influence of initial concentration of heavy metal, pH, biosorbent size, biosorbent dose and contacttime on biosorption of metal ions were studied. 2. Methodology 2.1. Chemicals and instruments All chemicals were pro-analysis grade and were purchasedfrom E. Merck Company (Darmstadt, Germany). Metal con-centrations were measured with a Perkin-Elmer (AAnalyst 300)atomicabsorptionspectrophotometer.ATOA.V.pHmeter(HM30P) was used to check the pH of the metal solutions. Otherinstruments such as shaker (PA 250/25. H), Octagon siever(OCT-DIGITAL4527-01)andShimadzu(AW220)electricbal-ance were used. 2.2. Rice bran collection The biosorbent ice bran was collected from different shellersofSialkotandHafizabad,Pakistan.Thesampleswerethandriedat 70 ◦ C for 1 week. 2.3. Protonation of rice bran with acids Rice bran was protonated with three acids HCl, H 2 SO 4  andH 3 PO 4  according to Ref. [4]. The biomass after each treat- ment was washed with deionized water until the pH of the washsolution was in near neutral range (pH 6.8–7.2). After this thebiomass was dried at 60 ◦ C for 24h in a drying oven. 2.4. Batch laboratory binding capacity experiments Binding capacity experiments were performed with the pro-tonated biomass as well as controls for metal studied. Theexperiments were conducted using solutions of nickel in theform of NiSO 4 · 6H 2 O. The solutions prepared using distilledwater had an initial metal concentration of 100mg/L. Knownamounts of biomass were contacted with each metal solution.The reaction mixture was agitated at 120rpm on orbital shaker(PA 250/25. H) for 4h, filtrate was obtained by filtering thereaction mixture through a filter and analysed for metal concen-tration. Nickel adsorption losses to the flask walls and to thefilter paper were negligible. All experiments were carried out at30 ◦ C. The effects of following parameters such as pH, biomasssize, biomass dose, initial metal ion concentration and contacttime were studied. Bioadsorption experiments were carried outin duplicate. 2.5. Metal analysis Metal concentrations were measured using a Perkin-Elmer(AAnalyst 300) atomic absorption spectrophotometer. Thewavelength used for the analysis of the metal in this study was232nm. The instrument was calibrated within the linear rangeof analysis and a correlation coefficient of 0.98 or greater wasobtained for the calibration curve. The instrument was period-ically checked throughout the analysis with known standards.Threereadingswereobtainedforeachsample,andameanvaluewas computed along with standard deviations for each sample.The amount bound on the biomass was assumed to be the differ-ence between the initial metal concentration and that found inthe supernatant. The nickel uptake was calculated by the simpleconcentration difference method [16]. 3. Results and discussion In order to protonate the biomass rice bran, it was treatedby HCl, H 2 SO 4  and H 3 PO 4 . Time given to the biosorbent foracidic treatment was 2, 4 and 12h. The adsorption capacity of acid treated rice bran and untreated rice bran was compared andit was noted that among the three acids, H 3 PO 4  was greatlyenhanced the adsorption capacity. The adsorption capacity foruntreated rice bran was 39.76mg/g. The adsorption capacity forHCl treated biosorbent for 2, 4 and 12h was 53.50, 66.80 and68.70mg/g, respectively. Similarly the adsorption capacity forH 2 SO 4  treatedbiosorbentfor2,4and12hwas62.80,72.80and79.30mg/g, respectively, and for H 3 PO 4  treated biosorbent for2,4and12hwas68.50,77.60and101.90mg/g,respectively.Sothe highest metal uptake (101.9mg/g) was obtained by H 3 PO 4 treated rice bran which was than used for further study. 3.1. pH profile studies for nickel binding Sorption of heavy metals from aqueous solutions dependson properties of adsorbent and molecules of adsorbate transferfromthesolutiontothesolidphase.IthasbeenalsoreportedthatbiosorptioncapacitiesforheavymetalsarestronglypHsensitiveand that adsorption increases as solution pH increases [17,18].  480  M.N. Zafar et al. / Journal of Hazardous Materials 143 (2007) 478–485 Fig. 1. Effect of pH on biosorption of nickel. Initial investigation of biosorption capability of rice bran fornickel at different values of pH (1–7) (Fig. 1) showed that the rice bran possessed maximum sorption capacity for the cationicmetalionatpHvalue6.AtpHvaluesabovetheisoelectricpoint,there was a net negative charge on the cell wall components andthe ionic state of ligands such as carboxyl, phosphate and aminogroups will be such as to promote a reaction with metal cations.AtlowerpHtheoverallsurfacechargeonthecellsbecameposi-tiveandthepresenceofH + ionshindertheaccessofmetalionsbyrepulsive forces to the surface functional groups, consequentlydecreasingthepercentageofmetalremoval[19].SoatpHbellow 3,anuptakeofnickelwasless,probablyduetothecationcompe-titioneffectswithoxonium(hydronium)ionH 3 O + .Furthermoreat higher pH poorly soluble hydroxyl species was formed andprecipitation of nickel would occur [20]. So at pH 7 biosorption of cationic metal decreased probably because of chemical pre-cipitation.SorptionstudiesweremeaninglessabovepH7duetotheformationofinsolubleproductsininvestigatedsolution,whatis in accordance with the solubility products of metal hydroxideasfollows: K  sp (Ni(OH) 2 )=10–14[21].Accordingtotheresults of this initial experiment, the further biosorption investigationswere performed at pH 6 as an optimal value. 3.2. Effect of biosorbent size and doze The effect of altering the sorbent particle size showed thatthere was a more removal of nickel by smaller particles (Fig. 2).The adsorption capacities ( q ) for different granular sizes of acid pretreated  Oryza sativa  bran 0.250, 0.335, 0.500 and Fig. 2. Effect of biosorbent size on biosorption of nickel.Fig. 3. Effect of biosorbent doze on biosorption of nickel. 0.710mm were 113.60, 82.20, 48.20 and 19.00mg/g and per-centageremovalsofnickelwere55.60,40.25,23.60and9.30%,respectively. This was most probably due to increase in the totalsurface area which provided more sorption sites for metal ions.The enhanced removal of sorbate by smaller particles has beennotedpreviouslyduringastudyintotheremovalofcolourbysil-ica[22].Themaximumadsorptionwasoccurredwith0.250mm biosorbent size.Biosorbent dose seemed to have a great influence in biosorp-tion process. Dose of biomass added into the solution determinethe number of binding sites available for adsorption. The resultsof the effect of protonated rice bran dose (Fig. 3) showed that nickel uptake values increased with a decrease in biomass dose,thoughthiscannotbeattributedtoagreaterbiosorptioncapacity.In this case the relevant parameter is the percentage of removednickel. An increase in biomass quantities strongly affects thequantitiesofnickelremovedfromaqueoussolutionstoacertainlimit and than decreases. Adsorption capacities ( q ) for differentdosesofbiosorbent0.05,0.01,0.15,0.20,0.25and0.30g,atpH6.0 were 106.80, 57.00, 41.20, 31.90, 26.10 and 21.00mg/g andpercentage removals of nickel were 52.20, 55.70, 60.40, 62.40,63.70 and 61.50%, respectively. The critical value of dose of protonated rice bran was 0.25g for nickel. 3.3. Kinetic study Kinetic study revealed that maximum biosorption capacitiesand metal removal efficiencies for nickel were achieved gener-ally in the first 30min of contact. Metal removal and sorptionwerealsorapidduringthisperiod.Fig.4showsthetimecourseof  Fig. 4. Effect of sorption time on biosorption of nickel.   M.N. Zafar et al. / Journal of Hazardous Materials 143 (2007) 478–485  481Fig. 5. Effect of initial metal concentration on biosorption of nickel. biosorption, when the initial pH was 6 and the initial metal con-centrations ( C  0 ) were 100mg/L. In the first 5min, sorption took place very rapidly and then it continued at a relatively slowerrateuptomaximumsorption.Equilibriumwasreachedinacon-tact time of 4h. This figure also verifies that sorption took placein two stages: a very rapid surface adsorption and a slow intra-cellular diffusion. Similar results were reported by [23,22,17],while in some other studies single-step uptake was suggestedfor different biosorbents [11]. 3.4. Effect of initial concentrations of nickel ions Theapparentcapacityofprotonatedricebranfornickelmetalwas determined at the different concentrations. Fig. 5 clarifiestherelationbetweencapacitiesandthemetalionconcentrations,which shows that as the metal ion concentration increased thecapacity increased until 44.90mg/g. At lower 25, 50, 100ppmconcentrations, the percentage removal of nickel was 52.50,55.40, 58.00%, respectively. The maximum removal of nickelwas occurred at 100ppm. In general, the data indicated thatsorption capacity increased with increase in initial metal ionconcentration for nickel on the biomass. This sorption charac-teristic indicated that surface saturation was dependent on theinitial metal ion concentrations. At low concentrations, adsorp-tion sites took up the available metal more quickly. However, athigher concentrations, metal needed to diffuse to the biomasssurface by intraparticle diffusion and greatly hydrolyzed ionswill diffuse at a slower rate. The maximum metal ion sorptionof an adsorbent may be determined from column experiments,by the use of a large excess of the adsorbate. 3.5. Kinetic modeling Inordertoinvestigatethemechanismofbiosorptionofnickelby protonated rice bran and the potential rate-controlling steps,such as mass transport and chemical reactions, kinetic modelswere used to test experimental data. Mathematical models thatcan describe the behaviour of batch biosorption process oper-ated under different experimental conditions are very useful forscale up process optimization. A number of models with vary-ing degrees of complexicity have been developed to describethe kinetics of metal biosorption in batch systems. According Fig. 6. The pseudo first-order (Lagergren) plot. to the kinetic model selection criteria, proposed by [24], sev- eral reaction-based and diffusion-based models were tested forthe stimulation of the obtained experimental data. The finallyselected kinetic models will be those, which not only fit closelythe data, but also represent reasonable sorption mechanism. Inthis study two different kinetic models were used to adjust theexperimental data of nickel biosorption on protonated rice bran.Thesekineticmodelsincludedpseudofirst-orderLagergrenandpseudo second-order [25,26]. The pseudo first-order Lagergren model is expressed aslog( q e − q ) = log q e − k 1 , ads 2 . 303(1)where q e  (mg/g)and q aretheamountsofadsorbedmetalionsonthe biosorbent at the equilibrium and at any time  t  , respectively; k  1,ads  (min − 1 ) is the Lagergren rate constant of the first-orderbiosorption.  q e  and  k  1,ads  can be calculated from the slopes andthe intercept of the plot log( q e − q ) versus  t   (Fig. 6).The Lagergren first-order rate constant  k  1  and  q e  determinedfrom the model indicates that this model had failed to estimate q e  sincetheexperimentalvalueof  q e  differsfromestimatedone.The best fit for the experimental data of this study wasachievedbytheapplicationofpseudosecond-orderkineticequa-tion.Thepseudosecond-ordermodelisbasedontheassumptionthat biosorption follows a second-order mechanism. So, the rateof occupation of adsorption sites is proportional to the square of the number of unoccupied sites: t q = 1 k 2 , ads q 2e + 1 q e t  (2)where  k  2,ads  is the rate constant of second-order biosorption(g/mgmin).  q e  and  k  2,ads  can be calculated from the slope andthe intercept of the plot  t   /  q  versus  t   (Fig. 7).It is important to notice that it is not necessary to estimatethe experimental value of   q e  for the application of such a model.The coefficient of correlation for second-order kinetic modelwas equal to 1 and the estimated value of   q e  also agreed withthe experimental one. Both factors suggest that the sorption of nickel ions followed the second-order kinetic model, indicatingthat the rate-limiting step was a chemical biosorption processbetween nickel and protonated rice bran. Similar conclusions  482  M.N. Zafar et al. / Journal of Hazardous Materials 143 (2007) 478–485 Fig. 7. The pseudo second-order plot.Table 1The pseudo first-order (Lagergren) parameters q exp  (mg/g) 25.04 k  1  (min − 1 ) 0.00921 q e  (mg/g) 3.38  R 2 0.9712Table 2The pseudo second-order parameters q exp  (mg/g) 25.04 k  2  (g/mgmin) 0.00882 q e  (mg/g) 25.20  R 2 1.000 were found by Ho and McKay as a result of an analysis of datafrom literature. They reported that most of the sorption systemsfollow a pseudo second-order kinetic model [27]. The pseudo first-order Lagergren model and pseudo second-order parame-ters are given in Tables 1 and 2, respectively. 3.6. Equilibrium modeling The biomass exhibited adsorption isotherms of Langmuirand Freundlich, which is a characteristic of the biomass sub-stratecontainingbothmicroporesandmesopores[28].Modeling the equilibrium data is fundamental for the industrial applica-tion of biosorption since it gives information for comparisonamong different biomaterials under different operational condi-tions, designing and optimizing operating procedures [29]. To examine the relationship between sorbed ( q e ) and aqueous con-centrations ( C  e ) at equilibrium, sorption isotherm models arewidelyemployedforfittingthedata,ofwhichtheLangmuirandFreundlich equations are the most widely used. To get the equi-librium data, initial nickel concentrations were varied while thebiomass weight in each sample was kept constant. Four hoursof equilibrium periods for sorption experiments were used toensure equilibrium conditions. This time was chosen consider-ing the results of kinetics of nickel removal by protonated ricebran which was presented prior to this one.If the metal ions are taken up independently on a single typeofbindingsiteinsuchawaythattheuptakeofthefirstmetalion Fig. 8. The Langmuir plot. doesnotaffectthesorptionofthenextion,thenthesorptionpro-cess would follow the Langmuir adsorption isotherm equation,which was linearised to the form: C e q e = 1 q 0 K L + 1 q 0 C e (3)where  q 0  and  K  L  are the Langmuir constants.The capacity of protonated rice bran biomass in binding withnickel was determined by plotting  C  e  /  q e  against  C  e  using theabove equation. Fig. 8 shows the data linearised to fit the Lang- muir equation. The plots of specific sorption ( C  e  /  q e ) againstequilibrium concentration ( C  e ) gave the linear isotherm param-eters of   q 0 ,  K  L  and the coefficient of determination and theseare presented in Table 3. The  R 2 values suggested that the Lang-muir isotherm provided a good model of the sorption system.The sorption capacity,  q 0  which is a measure of the maximumadsorption capacity corresponding to complete monolayer cov-erage, showed that the protonated rice bran had a mass capacityfor nickel (46.51mg/g). The adsorption coefficient,  K  L  whichis related to the apparent energy of adsorption for nickel was0.00943dm 3  /g. This indicates that not all binding sites may beavailable for nickel binding due to its relatively larger hydrationenergy.The Freundlich equation is another model which has beencommonly used to describe adsorption isotherms. Its linearisedform is represented by Eq. (4) [30]:log q e  = log K F + 1 n log C e (4)where  q e  is the amount adsorbed per unit mass of adsorbent and C  e  is the equilibrium concentration (mg/L).The plot of log q e  versus log C  e  was linear (Fig. 9) and con- stants  K  F  and  n  can be evaluated from the slopes and intercepts.The Freundlich constants are shown in Table 4. It was found that the adsorption equilibrium data was better fitted by the Table 3The Langmuir isotherm parameters q exp  (mg/g) 44.90 q 0  (mg/g) 46.51 K  L  (dm 3  /g) 0.00943  R 2 0.9414
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