Adsorptive removal of Pb2+, Co2+ and Ni2+ by hydroxyapatite chitosan

Adsorptive removal of Pb2+,Co2+and Ni2+by hydroxyapatite/chitosan composite from aqueous solutionNeha Gupta,Atul K.Kushwaha,M.C.Chattopadhyaya*Environmental Chemistry Research Laboratory,Department of Chemistry,University of Allahabad,Allahabad,(U.P.)211002,India1.IntroductionWater pollution by heavy metals through the discharge ofindustrial effluents,is a worldwide environmental problem.Studies have proved that the metals such as copper,lead,zinc,cobalt,nickel,chromium and mercury are considered to be toxic.Cobalt containing compound are widely used in many industrialapplications such as mining,metallurgical,paints,pigments andelectronics[1].Small amount of cobalt is essential for humanhealth because it is a part of vitamin B12.However,higherconcentration of cobalt may cause paralysis,diarrhoea,low bloodpressure,lung irritation and bone defects.Nickel is being widelyused in electroplating industries.It is also used in fertilizers.Exposure to nickel can cause dermatitis and allergic sensitization.Inhalation of Ni can cause respiratory cancer[2].Lead compound iswidely used in painting,plastics and batteries.At low concentra-tion lead can adversely affect the brain,the central nervous system,blood cells,and kidneys[3].Therefore,removal methods of heavy metals from wastewaterhave been investigated considerably for past several years.Anumber of physical and chemical processes exists which includeprecipitation,solvent extraction,membrane processes,filtration,ion-exchange and adsorption.Among them,adsorption is anefficient and economical process.The efficiency of techniquedepends on the nature of adsorbent.The adsorbent such asactivated carbon[4],clay[5],activated alumina[6],chitosan[7],silica[8],zeolite[9]and hydroxyapatite[10]were used for heavymetal removal.Hydroxyapatite Ca10(PO4)6(OH)2(HAp)is a main mineralconstituent of teeth,bones and phosphate mineral rocks.Itbelongs to apatite mineral family.HAp was shown to be efficientfor the removal of many toxic metal ions such as lead,cobalt,nickel,copper,zinc,cadmium and uranium in aqueous solutions[11–16].However,characteristic of HAp existing in the formof white powder therefore isolating the suspendedfine solidsfrom aqueous solutions after adsorption of metal ions is noteasy[17].Therefore,it is needed to bind HAp with polymer to solve thisproblem.There are several polymers available in nature which canbe use as binding material for HAp.In the present study we havesynthesized hydroxyapatite–chitosan(HApC)composite by precip-itation method and its application was explored as an adsorbent forthe removal of heavy metals such as lead,cobalt and nickel fromaqueous solution.The reason behind choosing chitosan as a bindingmaterial for HAp is its high abundance in nature and specialcharacteristics such as hydrophilicity,biocompatibility,biodegrad-ability,non-toxicity,adsorption properties,etc.The presence ofamino and hydroxyl groups in chitosan can serve as the active sitesfor adsorption.Journal of the Taiwan Institute of Chemical Engineers43(2012)125–131A R T I C L E I N F OArticle history:Received6March2011Received in revised form12July2011Accepted31July2011Available online17October2011Keywords:AdsorptionCobaltNickelLeadIsothermKineticsHydroxyapatite/chitosan compositeA B S T R A C TIn the present work hydroxyapatite/chitosan(HApC)composite was synthesized by precipitationmethod and was characterized by Fourier transform infrared(FT-IR)spectroscopy,X-ray diffraction(XRD)crystallography and scanning electron microscopy(SEM).The HApC composite was tested for theadsorption of lead,cobalt and nickel from aqueous solution and compared its removal capacity withhydroxyapatite(HAp).Effect of various physico-chemical parameters such as pH,adsorbent dose,contacttime,initial metal ion concentration and temperature on adsorption of metal ions onto HApC compositewas investigated.The adsorption process followed pseudo-second order kinetics and intra-particlediffusion model.Equilibrium data werefitted well in the Langmuir and Freundlich isotherm models.Thenegative values of Gibb’s free energy change(D G8)showed that the adsorption process was feasible andspontaneous in nature.The positive values of enthalpy change(D H8)indicated that the process wasendothermic.The positive values of entropy change(D S8)showed increased randomness at the solid/solution interface.ß2011Taiwan Institute of Chemical Engineers.Published by Elsevier B.V.All rights reserved.*Corresponding author.Tel.:+915322462393;fax:+915322541786.E-mail addresses:neha.evs07@(N.Gupta),atulkk2008@(A.K.Kushwaha),mcc46@(M.C.Chattopadhyaya).Contents lists available at ScienceDirectJournal of the Taiwan Institute of Chemical Engineersj ou r n a l h o m e p a g e:w w w.e l s e v i e r.co m/l o c a t e/j t i c e1876-1070/$–see front matterß2011Taiwan Institute of Chemical Engineers.Published by Elsevier B.V.All rights reserved.doi:10.1016/j.jtice.2011.07.0092.Materials and methods2.1.Synthesis of hydroxyapatite–chitosan compositeChitosan(75–85%deacetylated)was supplied by Sigma Aldrich,India.(NH4)2HPO4and Ca(NO3)2Á4H2O were obtained from Merck and was used without further purification.The HAp was synthesized by mixing the solution of Ca(NO3)2Á4H2O and (NH4)2HPO4with stoichiometric ratio of Ca/P(1.67).The pH value during mixing was maintained10–11with ammonia solution.The precipitate formed was rinsed with double distilled water to bring the pH level to7and was dried at808C for24h [18].HApC composite was also synthesized by precipitation method.Chitosan(5wt%)was dissolved in1%acetic acid and added to the mixture of diammonium hydrogen phosphate and calcium nitrate.The precipitate formed was rinsed with double distilled water to bring the pH level to7and was dried at808C for24h.2.2.Characterization of materialsFTIR study was carried out on FTLA2000spectrophotometer to analyze functional groups,using potassium bromide(KBr) disk method and the morphology of adsorbents was investigated by FEI Quanta200scanning electron microscope operated at 20kV accelerated voltage.XRD was performed on X’Pert-Pro, PANalytical diffractometer operated at30kV/30mA,to deter-mine crystallinity of the material using CuK a1radiation with wavelength of1.54A˚in the wide angle region from108to508on 2u scale.2.3.Adsorption experimentsStock solution(1000mg/L)of each metal ion was prepared by dissolving required amount of nitrate salt of metal ion(Pb(NO3)2, Co(NO3)2Á6H2O and Ni(NO3)2Á6H2O obtained from Merck)into double distilled water.The experimental solution of desired concentration was prepared by successive dilution of stock solution.In order to determine the effect of physicochemical parameters such as pH,adsorbent dose,contact time,initial metal ion concentration of solution and temperature.The adsorption experiments were performed by batch equilibrium method.The experiments were carried out in150ml of conical flasks by mixing a pre-weighed amount of adsorbent with50ml of metal ion solution.Initial pH of solutions was adjusted by 0.1M NaOH or0.1M HNO3.All experiments were performed at room temperature and kept for stirring for given period of time. Thereafter the mixture was centrifuged(Remi Research centri-fuge)and the initial andfinal metal ion concentrations were determined by Atomic Absorption Spectrophotometer(ECIL-4141).The%removal of metal ion and amount of metal ion adsorbed on HApC composite(q e)was calculated by Eqs.(1)and (2),respectively:removalð%Þ¼100C0ÀC eC0(1)q e¼ðC0ÀC eÞVM(2)where q e is the amount of metal ion adsorbed on adsorbent at equilibrium(mg/g),C0and C e are the initial and equilibrium concentration(mg/L)of metal ion in solution respectively,V is the volume of solution(L)and M is the mass of adsorbent(g).All adsorption experiments were performed in triplicate and the mean values were used in data analysis.3.Results and discussion3.1.Characterization of materialsThe IR spectra of HAp(a),Chitosan(b)and HApC(c)were recorded in the region of4000–500cmÀ1and are shown in Fig.1. The spectra of HAp showed a broad band at1113cmÀ1due to asymmetric stretching of PO43Àand corresponding symmetric stretching was observed at825cmÀ1.The absorption band at 1398cmÀ1suggested the presence of CO32À[19],which was supposed to have come from atmosphere during the precipitation process.The broad band at3500to3200cmÀ1and1636cmÀ1was due to stretching and bending of O–H groups,respectively,present in HAp[20].The FT-IR spectra of HApC showed a band at 1032cmÀ1which was due to stretching vibration of C–O–C group of chitosan confirms formation of composite.The peak of PO43Àgroup shifted from1113to1092cmÀ1due to interaction with chitosan.The stretching and bending vibration mode of–NH group of chitosan occurred at3452and1627cmÀ1,respectively which was overlapped with the bands of O–H groups in HApC composite.XRD spectra of HApC composite(a),HAp(b)and Chitosan(c)are presented in Fig.2.The crystalline peaks of HAp at2u=268,31.98, 33.18,34.28and39.98are also found in HApC composite with very small shifting in2u.This may be due to binding of HAp with chitosan.This indicated that there was no marked change in the peak structure after the composite formation and confirmed that the crystal structure of HAp is retained in HApC composite[21]. Fig.3shows the morphology of HAp(a)and HApC composite(b). The HAp powder exhibited as particles but in case of HApC composite aggregates appeared andfilm of chitosan over HAp, confirmed the formation of HApC composite.3.2.Effect of pHTo study the effect of pH on adsorption,experiments were carried out in the pH range2–5.5for Pb2+and2–6for Co2+and Ni2+.Fig.4 shows that the removal of metal ions was increased with increasing initial pH of metal ion solution and maximum value was reached at pH5.5for Pb2+and6.0for Co2+and Ni2+.The pH above5.5for Pb2+and 6for Co2+and Ni2+was not used in order to avoid the precipitation of metal ions in the form of their hydroxides[22].At lower pH, adsorption of metal ions decreased due to increased competition with H+ions for active adsorption sites.However,at higher pH values,TransmittanceWavenu mber (cm-1)Fig.1.Infrared spectra of HAp(a),chitosan(b)and HApC composite(c).N.Gupta et al./Journal of the Taiwan Institute of Chemical Engineers43(2012)125–131 126presence of H+ion in solution decreased and adsorbent surface also deprotonated,make increased adsorption of metal ions.3.3.Effect of adsorbent doseFig.5shows that the adsorption of metal ions increased from78 to93.5%,71to86%and64to77%for Pb2+,Co2+and Ni2+,respectively, as HApC dose increased from1to3g/L.This is because at higher dose of adsorbent,due to increased surface area,more adsorption sites are available causing higher removal of Pb2+,Co2+and Ni2+.Further increase in adsorbent dose,did not cause any significant increase in% removal of metal ion.This was due to the concentration of metal ions reached at equilibrium status between solid and solution phase.3.4.Effect of contact timeInfluence of contact time on adsorption of Pb2+,Co2+and Ni2+on HApC was investigated in the range of5–60min for the initial concentration of10mg/L for each metal ion(Fig.6).Maximum rate of removal occurred within5min of contact time thereafter removal rate became slow and after30min of contact time no change was observed for Pb2+(93.5%)which established that the system has reached the equilibrium point.For Co2+(88.72%)and Ni2+(80%),the equilibrium point was found to be40min.3.5.Effect of equilibrium metal ion concentrationThe effect of different concentration of Pb2+,Co2+and Ni2+on the adsorption has been investigated at303K,Pb2+,Co2+and Ni2+ adsorption capacities of HApC were given as a function of equilibrium concentration in Fig.7.It was clear that Pb2+,Co2+ and Ni2+adsorption capacities of HApC increased with increase of equilibrium concentration.The Increase in adsorption capacity with increase in equilibrium metal ion concentration for different metal ions was in the order Pb2+>Co2+>Ni2+.3.6.Kinetics of adsorptionSeveral kinetic models have been proposed to clarify the mechanism of a solute adsorption from aqueous solution onto an adsorbent.The rate constant of adsorption was determined from the pseudo-first order rate expression Eq.(3)[23]:dq t¼k1ðq eÀq tÞ(3)where q t and q e are the amount of metal ions adsorbed at time t (mg/g)and at equilibrium(mg/g),respectively and k1is the pseudo-first order rate constant(minÀ1).Integrating theaboveFig.3.SEM micrograph of HAp(a)and HApC composite(b).Fig.4.Effect of pH on adsorption of Pb2+,Co2+and Ni2+on HApC composite.1020304050cbaIntensity(a.u.)2 Th eta (degr ee)Fig.2.XRD spectra of HApC composite(a),HAp(b)and chitosan(c).N.Gupta et al./Journal of the Taiwan Institute of Chemical Engineers43(2012)125–131127equation using the boundary condition,q t =0at t =0leads to:ln ðq e Àq t Þ¼ln q e Àk 1t(4)The values of k 1and q e were calculated from the slopes and intercepts of the linear plots of ln (q e Àq t )vs.t (Fig.8)and are presented in Table 1.As shown in Table 1the values of regression correlation coefficients (R 2)were not close to unity and a large difference in the values of q e,cal and q e,exp for the adsorption of Pb 2+,Co 2+and Ni 2+onto HApC composite revealed that the pseudo-first order kinetic model was not suitable to describe the kinetic profile of the adsorption.The pseudo-second order kinetic model is presented as [24]:dq tdt¼k 2ðq e Àq t Þ2(5)where q t and q e are the amount of metal ions adsorbed at time t (mg/g)and at equilibrium (mg/g)respectively and k 2is the pseudo-second order rate constant (g/mg/min).Integrating the above equation using the boundary condition,q t =0at t =0leads to:t t ¼12eþt e (6)The values of k 2and q e were calculated from the intercepts and slopes of the linear plots of t /q t vs.t (Fig.9),respectively and are presented in Table 1.Results showed that the calculated q e values were very close to that of experimentally obtained q e and the values of correlation coefficients (R 2)were closer to unity confirmed that the adsorption of Pb 2+,Co 2+and Ni 2+on HApC composite followed pseudo-second order kinetics.Adsorption is multistep process involving the transfer of solute from bulk of solution to surface of adsorbent.Intra-particle diffusion plays an important role in the extent of adsorption and can be expressed as [25]:q t ¼k i t 0:5þI(7)where q t is the amount of metal ion adsorbed at time t (mg/g)and k iis intra-particle diffusion rate constant (mg/g/min 0.5).The values of I give an idea about the thickness of boundary layer.The values of k i and I were obtained from the slopes (k i )and intercepts (I )of the plots of q t vs.t 0.5(Fig.10),respectively and are presented in Table 1.Results showed that the diffusion rate of12345678q e (m g /g )C e (mg/L)Fig.7.Effect of equilibrium metal ion concentration on adsorption of Pb 2+,Co 2+and Ni 2+on HApC composite.-5-4-3-2-11l n (q e -q t )t (min )Fig.8.Pseudo-first order kinetic model for adsorption of Pb 2+,Co 2+and Ni 2+onHApC composite.404550556065707580859095100% R e m o v a lAdsorbent dose (g/L )Fig.5.Effect of adsorbent dose on adsorption of Pb 2+,Co 2+and Ni 2+on HApCcomposite.% R e m o v a lConta ct ti me (min )Fig. 6.Effect of contact time on adsorption of Pb 2+,Co 2+and Ni 2+on HApCcomposite.N.Gupta et al./Journal of the Taiwan Institute of Chemical Engineers 43(2012)125–131128different metal ions decreased in the order Pb 2+>Co 2+>Ni 2+which can be explained on the basis of their hydrated ionic radii,ionic mobility and diffusion rate constant [26].The hydrated ionic radius of different metal ions increases in the order Pb 2+<Co 2+<Ni 2+.Smaller the hydrated ionic radius of metal ion,higher is the ionic mobility and the rate of diffusion.Fig.10showed that all the lines did not pass through the rger the value of I ,greater was the contribution of the surface adsorption in the rate-limiting step [27].Results confirmed the presence of both surface adsorption and intraparticle diffusion mechanism.3.7.Adsorption isothermThe analysis of equilibrium data to construct adsorption isotherms is usually important for design of adsorption systems.Adsorption isotherms express the mathematical relationship between the quantity of adsorbate and equilibrium concentration of adsorbate remaining in the solution at a constant temperature.The most common types of isotherms are Langmuir and Freundlich models.The Langmuir isotherm model is applicable to monolayer adsorptions and can be expressed as:C e q e¼1bq max þ1q max C e(8)where C e is concentration of metal ions in solution at equilibrium (mg/L),q e is the amount of metal ions at equilibrium in unit mass of adsorbent (mg/g),q max and b are the Langmuir coefficient related to adsorption capacity (mg/g)and adsorption energy (L/mg),respectively.The Langmuir isotherm parameters q max and b were determined from slope (1/q max )and intercept (1/bq max )of the plot of C e /q e vs.C e (Fig.11),respectively.The empirical Freundlich isotherm is based on adsorption on a heterogeneous surface and express by the following equation:ln q e ¼ln K F þ1nln C e(9)K F and n are the Freundlich coefficient related to adsorption capacity [mg/g (mg/L)À1/n ]and adsorption intensity of adsorbent,respectively.The values of Freundlich constants n and K F were obtained from the slope (1/n )and intercept (ln K F )of the plot of ln q e vs.ln C e (Fig.12),respectively.The plots give straight lines,indicating that the adsorption of Pb 2+,Co 2+and Ni 2+onto HApC composite followed both theTable 1Kinetic parameters for adsorption of Pb 2+,Co 2+and Ni 2+on HApC composite.Metal ionsq e,exp(mg/g)Pseudo-first orderPseudo-second orderIntraparticle diffusionq e,cal (mg/g)k 1(min À1)R 2q e,cal (mg/g)k 2(g/mg/min)R 2k i (mg/g/min 0.5)IR 2Pb 2+ 3.11 2.080.1360.932 3.170.2730.9950.198 2.030.992Co 2+ 2.95 1.410.0980.895 2.980.3090.9970.117 2.210.995Ni 2+2.661.630.1190.9092.700.3000.9970.1171.930.991t /q t (m i n /m g /g )t (min )Fig.9.Pseudo-second order kinetic model for adsorptionof Pb 2+,Co 2+and Ni 2+onHApC composite.2.02.22.42.62.83.03.23.4q t (m g /g )t 0.5(mi n 0.5)Fig.10.Intraparticle diffusion model for adsorption of Pb 2+,Co 2+and Ni 2+on HApCcomposite.0.20.40.60.81.01.21.41.6C e /q e (g /L )C e (mg/L )ngmuir isotherm model for adsorption of Pb 2+,Co 2+and Ni 2+on HApCcomposite.N.Gupta et al./Journal of the Taiwan Institute of Chemical Engineers 43(2012)125–131129Langmuir and Freundlich isotherm models.The Langmuir and Freundlich isotherms constants and regression coefficients are listed in Table 2.The experimental results indicated that the order of adsorption capacity of HApC for metal ions was as follows:Pb 2+>Co 2+>Ni 2+.The values of n >1also indicated favourable adsorption conditions [28].3.8.Effect of temperature and thermodynamic parametersThe effect of temperature on the adsorption of Pb 2+,Co 2+and Ni 2+on HApC composite was investigated by conducting experi-ments for 10mg/L of initial metal ion concentrations at 303,313,and 323K.It was observed that on increasing the temperature percentage removal of metal ions increased.This showed that the adsorption process was endothermic in nature.The thermodynamic parameters Gibb’s free energy (D G 8),enthalpy (D H 8)and entropy (D S 8)were calculated using the following equations:lnq e m e ¼D S o þÀD H o(10)D G o ¼D H o ÀT D S o(11)where m is the adsorbent dose (g/L),C e is concentration of metal ions in solution at equilibrium (mg/L),q e is the amount of metal ions at equilibrium in unit mass of adsorbent (mg/g),q e /C e is called the adsorption affinity.D H 8,D S 8and D G 8are change in enthalpy (kJ/mol),entropy (J/(mol K))and free energy (kJ/mol),respectively.R is the gas constant (8.314J/mol/K)and T is the temperature (K).The values of D H 8and D S 8were obtained from the slopes and intercepts of the Van’t Hoff plots of ln (q e m /C e )vs.1/T (Fig.13),respectively,thereafter D G 8values were determined from Eq.(11).The values of thermodynamic parameters are presented in Table 3.The results showed that the D G 8values are negative and increased in their absolute values with temperature.This result suggested that a high temperature is favoured for the adsorption of Pb 2+,Co 2+and Ni 2+on HApC,indicated a spontaneous adsorption process.The values of D G 8for Pb 2+is the highest,indicated that the adsorption of Pb 2+ion was more spontaneous than Co 2+and Ni 2+ions.The values of heat of adsorption,D H 8is positive for all metal ions,indicated that the adsorption process of Pb 2+,Co 2+and Ni 2+on HApC was endothermic.A positive D S 8suggested that heavy metal ions were not stable on the adsorption sites of HApC probably due to the increase in translational energy of heavy metal ions.parative studyIn addition to the HApC composite,HAp was also used as anadsorbent for the removal of Pb 2+,Co 2+and Ni 2+and a comparative data are presented in Table 4.Study shows that the %removal of metal ion by HApC composite was almost similar to HAp which confirmed that the removal capacity of HAp was retained in HApC composite.Chitosan did not affect the adsorption capacity of HApI n q elnC eFig.12.Freundlich isotherm model for adsorption of Pb 2+,Co 2+and Ni 2+on HApC composite.Table 2Isotherm parameters for adsorption of Pb 2+,Co 2+and Ni 2+on HApC composite.Metal ions Langmuir constantsFreundlich constantsq max (mg/g)b (L/mg)R2K F [mg/g (mg/L)À1/n ]nR2Pb 2+12.040.5720.997 4.039 1.530.994Co 2+10.630.3490.994 2.619 1.590.997Ni 2+8.540.2110.993 1.5771.640.997l n (q e m /C e )1/T (K -1)Fig.13.The plots of ln (q e m /C e )vs.1/T for the adsorption of Pb 2+,Co 2+and Ni 2+on HApC composite.Table 4Comparative study of %removal of metal ions on HAp and HApC composite.Metal ions Removal of metal ions (%)Experimental condition HApHApC pHDose (g/L)Temperature (K)Contact time (min)C 0(mg/L)Pb 2+92.093.5 5.633033010Co 2+81.588.72633034010Ni 2+7680633034010Table 3Thermodynamic parameters for adsorption of Pb 2+,Co 2+and Ni 2+on HApC composite.Metal ionsD H 8(kJ/mol)D S 8(Jmol/K)ÀD G 8(kJ/mol)303K313K323KPb 2+104.861366.626 6.2269.89213.558Co 2+35.404134.041 5.209 6.5507.890Ni 2+32.537118.8873.4854.6745.863N.Gupta et al./Journal of the Taiwan Institute of Chemical Engineers 43(2012)125–131130for metal ions,it bind the HAp powder,form aggregates which prevent its dispersion in aqueous solution makes it convenient to use because dispersed HApfine particle in solution is difficult to recover.4.ConclusionsThis study showed that the binding of HAp powder with chitosan makes it convenient to use practically and its removal capability was also retained in HApC which makes HApC composite as an effective adsorbent for the removal of Pb2+,Co2+and Ni2+from aqueous solution.The order of removal efficiency(Pb>Co>Ni)for HApC composite was same as in case of pure HAp.The adsorption kinetics followed pseudo-second order kinetics and intraparticle diffusion model.The adsorption of Pb2+,Co2+and Ni2+onto the HApC composite followed both,Langmuir and Freundlich isotherm models.The negative values of D G8suggested that the adsorption was spontaneous in nature.The positive value of D H8and D S8 indicated endothermic adsorption process and increased random-ness at surface–solution interface,respectively. AcknowledgementAuthors are thankful to the NCEMP,Allahabad University for recording XRD and Prof. A.C.Pandey and Mr.P.K.Sharma of Nanotechnology Application Centre,University of Allahabad,India, for recording SEM.References[1]Ghassabzadeh H,Mostaedi MT,Mohaddespour A,Maragheh Md G,Ahmadi SJ,Zaheri P.Characterizations of Co(II)and Pb(II)removal process from aqueous solutions using expanded perlite.Desalination2010;261:73–9.[2]Turtureanu A,Georgescu C,Oprean L.Nickel Removal from Aqueous Solutionsby Flotation with Cationic Collector Determination of the Optimum Separation Conditions.Chem Bull‘‘POLITEHNICA’’Univ(Timisoara)2008;53:1–2.[3]Inglezakis VJ,Stylianou MA,Gkantzou D,Loizidou MD.Removal of Pb(II)fromaqueous solutions by using clinoptilolite and bentonite as adsorbents.Desali-nation2007;210:248–56.[4]Zaini MAA,Amano Y,Machida M.Adsorption of heavy metals onto activatedcarbons derived from polyacrylonitrilefiber.J Hazard Mater2010;180: 552–60.[5]Abu-Eishah SI.Removal of Zn,Cd,and Pb Ions from water by Sarooj clay.ApplClay Sci2008;42:201–5.[6]Bishnoi NR,Bajaj M,Sharma N,Gupta A.Adsorption of Cr(VI)on activated ricehusk carbon and activated alumina.Bioresour Technol2004;91:305–7. [7]Paulino AT,Guilherme MR,Reis AV,Tambourgi EB,Nozaki J,Muniz EC.Capacity of adsorption of Pb2+and Ni2+from aqueous solutions by chitosanproduced from silkworm chrysalides in different degrees of deacetylation.J Hazard Mater2007;147:139–47.[8]Aguado J,Arsuaga JM,Arencibia A,Lindo M,Gascon V.Aqueous heavy metalsremoval by adsorption on amine-functionalized mesoporous silica.J Hazard Mater2009;163:213–21.[9]Motsi T,Rowson NA,Simmons MJH.Adsorption of heavy metals from acidmine drainage by natural zeolite.Int J Miner Process2009;92:42–8.[10]Xu HY,Yang L,Wang P,Liu Y,Peng MS.Kinetic research on the sorption ofaqueous lead by synthetic carbonate hydroxyapatite.J Environ Manage 2008;86:319–28.[11]Bailliez S,Nzihou A,Beche E,Flamant G.Removal of Lead(Pb)by hydroxyapa-tite sorbent.Process Saf Environ Protect2004;82:175–80.[12]Smiciklas I,Dimovic S,Plecas I,Mitric M.Removal of Co2+from aqueoussolutions by hydroxyapatite.Water Res2006;40:2267–74.[13]Mobasherpour I,Salahi E,Pazouki M.Removal of nickel(II)from aqueoussolutions by nano-crystalline calcium hydroxyapatite.J Saudi Chem Soc 2011;15:105–12.[14]Corami A,Mignardi S,Ferrini V.Copper and zinc decontamination from single-and binary-metal solutions using hydroxyapatite.J Hazard Mater2007;146: 164–70.[15]Zhu R,Yu R,Yao J,Mao D,Xing C,Wang D.Removal of Cd2+from aqueoussolutions by hydroxyapatite.Catal Today2008;139:94–9.References and further reading may be available for this article.To view references and further reading you must purchase this article.[16]Simon FG,Birmann V,Peplinski B.Uranium removal from groundwater usinghydroxyapatite.Appl Geochem2008;23:2137–45.[17]Choi S,Jeong Y.The removal of heavy metals in aqueous solution by hydroxy-apatite/cellulose composite.Fiber Polym2008;9:267–70.[18]Mobasherpour I,Heshajin MS,Kazemzadeh A,Zakeri M.Synthesis of nano-crystalline hydroxyapatite by using precipitation method.J Alloy Compd 2007;430:330–3.[19]Leyva AQG,Marrero J,Smichowski P,Cicerone D.Sorption of antimony ontohydroxyapatite.Environ Sci Technol1994;28:1472–80.[20]Rayanaud S,Champion E,Assollant DB,Thomas P.Calcium phosphate apatiteswith variable Ca/P atomic ratio I Synthesis,characterisation and thermal stability of powders.Biomaterials2002;23:1065–72.[21]Sundaram CS,Viswanathan N,Meenakshi S.Uptake offluoride by nano-hydroxyapatite/chitosan composite,a bioinorganic composite.Bioresour Technol2008;99:8226–30.[22]Afkhamia A,Tehranib Md S,Bagherib H.Simultaneous removal of heavy-metalions in wastewater samples using nano-alumina modified with2,4-dinitro-phenylhydrazine.J Hazard Mater2010;181:836–44.[23]Lagergren S.About the theory of so called adsorption of soluble substances.Ksver Veterskapsakad Handl1898;24:1–6.[24]Ho YS,McKay G.Pseudo-second order model for sorption processes.ProcessBiochem1999;34:451–65.[25]Weber Jr WJ,Morris JC.Kinetics of adsorption on carbon from solution.J SanitEng Div ASCE1963;89(SA2)::31–59.[26]Ricordel S,Taha S,Cisse I,Dorange G.Heavy metals removal by adsorptiononto peanut husks carbon:characterization,kinetic study and modelling.Sep Purif Technol2001;24:389–401.[27]Kannan K,Sundaram MM.Kinetics and mechanism of removal of methyleneblue on adsorption of various carbons-a comparative study.Dyes Pigment 2001;51:25–40.References and further reading may be available for this article.To view references and further reading you must purchase this article..[28]Hameed BH,Mahmoud DK,Ahmad AL.Equilibrium modeling and kineticstudies on the adsorption of basic dye by a low-cost adsorbent:Coconut(Cocos nucifera)bunch waste.J Hazard Mater2008;158:65–72.N.Gupta et al./Journal of the Taiwan Institute of Chemical Engineers43(2012)125–131131。