Taguchi optimization approach for Pb(II) and Hg(II) removal from aqueous

Journal of Hazardous Materials 192 (2011) 1046–1055Contents lists available at ScienceDirectJournal of HazardousMaterialsj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j h a z m atTaguchi optimization approach for Pb(II)and Hg(II)removal from aqueous solutions using modified mesoporous carbonGhasem Zolfaghari a ,Abbas Esmaili-Sari a ,Mansoor Anbia b ,∗,Habibollah Younesi a ,Shahram Amirmahmoodi b ,Ali Ghafari-Nazari caDepartment of Environment,Faculty of Natural Resources and Marine Sciences,Tarbiat Modares University,Noor,Mazandaran,P.O.Box:46414-356,Iran bResearch Laboratory of Nanoporous Materials,Faculty of Chemistry,Iran University of Science and Technology,Farjam Street,Narmak,Tehran 16846,Iran cLoabiran Company,Research and Development Group,Shiraz,Irana r t i c l ei n f oArticle history:Received 22January 2011Received in revised form 2June 2011Accepted 6June 2011Available online 12 June 2011Keywords:Lead Mercury Zn-OCMK-3Taguchi method Adsorptiona b s t r a c tUsing the Taguchi method,this study presents a systematic optimization approach for removal of lead (Pb)and mercury (Hg)by a nanostructure,zinc oxide-modified mesoporous carbon CMK-3denoted as Zn-OCMK-3.CMK-3was synthesized by using SBA-15and then oxidized by nitric acid.The zinc oxide was loaded to the modified CMK-3by the equilibrium adsorption of Zn(II)ions from aqueous solution followed by calcination to convert zinc nitrate to zinc oxide.The CMK-3had porous structure and high specific surface area which can accommodate zinc oxide in a spreading manner,the zinc oxide connects to the carbon surface via oxygen atoms.The controllable factors such as agitation time,initial concentration,temperature,dose and pH of solution have been optimized.Under optimum conditions,the pollutant removal efficiency (PRE)was 97.25%for Pb(II)and 99%for Hg(II).The percentage contribution of each controllable factor was also determined.The initial concentration of pollutant is the most influential factor,and its value of percentage contribution is up to 31%and 43%for Pb and Hg,respectively.Our results show that the Zn-OCMK-3is an effective nanoadsorbent for lead and mercury pollution ngmuir and Freundlich adsorption isotherms were used to model the equilibrium adsorption data for Pb(II)and Hg(II).© 2011 Elsevier B.V. All rights reserved.1.IntroductionHeavy metal contamination such as lead (Pb)and mercury (Hg)has been identified as a concern in environment,due to discharges from industrial wastes,agricultural and urban sewage [1].Pb has been well recognized for its negative effect on the environment where it accumulates readily in living systems [2].There is a great concern about Hg pollution,which is due to its toxicity,persistent character in the environment and biota as well as biomagnifica-tion along the food chain [3].Increasing pollution by Pb and Hg has been reported in recent years in Iran [4,5].The results from the data collected in the rivers around Caspian Sea and Anzali Wetland,North Iran,show the various degrees of Pb and Hg con-centrations [6].Thus,the increased awareness of toxicity of heavy metal and the stringent environmental safety regulations demand the removal of them from the various discharges to avoid con-tamination of the biological ecosystem.Several methods such as coagulation [7],chemical oxidation [8],photocatalytic degradation [9],adsorption [10],etc.are being used for the removal of heavy∗Corresponding author.Tel.:+982177240516;fax:+982177491204.E-mail address:anbia@iust.ac.ir (M.Anbia).metal from water.Among these methods,adsorption is still the most versatile and widely used.Alternative adsorbents studied include biosorbents [11],clays [12],activated carbon [13],resins [14]and zeolites [15].Increasingly stringent standard on the quality of drinking water has stimulated a growing effort on the develop-ment of new high efficient adsorbents.The development of porous materials with large surface areas is currently an area of exten-sive research,particularly with regard to potential applications as environmental remediation.An upsurge began in 1992with the development by the Mobil Oil Company of the class of silicate meso-porous materials known as the M41S phase [16].It was reported that ordered mesoporous silica (OMS)can be used as an adsorbent for removal of numerous compounds [17–19].Recently,the syn-thesis of ordered mesoporous carbons (OMC)using OMS as hard templates has attracted a great deal of attention.Ryoo et al.[20]first used MCM-48silica as the template to synthesize CMK-1carbon.Subsequently,this research group reported the synthesis of CMK-2[21],CMK-3[22],CMK-4[23],and CMK-5[24].Very recently,Guo et al.[25]reported the synthesis of OMC by using SBA-16.Some researchers modified and functionalized the surface of the OMC for more applications [26,27].In the present study,CMK-3was synthesized using SBA-15,modified by HNO 3treatment and then functionalized by Zn(NO 3)2·4H 2O.This modified nanostructure,0304-3894/$–see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.jhazmat.2011.06.006G.Zolfaghari et al./Journal of Hazardous Materials 192 (2011) 1046–10551047zinc oxide-modified mesoporous carbon CMK-3called Zn-OCMK-3.The novelty of the present study consists of Taguchi optimization method for the removal of lead and mercury from aqueous solution and experimental data are presented in order to demonstrate the practical usage of zinc oxide-modified mesoporous carbon CMK-3.The optimum conditions with the higher pollutant removal effi-ciency (PRE)for batch adsorption studies were determined using the Taguchi method.In this study,agitation time,initial concentra-tion,temperature,dose and pH were studied as the controllable factors.Accordingly,the percentage contribution of each afore-mentioned experimental parameter to the process is determined using the Taguchi method.Furthermore,Langmuir and Freundlich adsorption isotherms were studied to explain the sorption mecha-nism.2.Design of experiments (DOE)Design of experiments (DOE)is developing a scheme of experiment different conditions.The Taguchi method which was established by Genichi Taguchi has been generally adopted to optimize the design variables because this approach can signifi-cantly minimize the overall testing time and the experimental costs [28–31].The Taguchi crossed array layout consists of an inner array and an outer array [32].The inner array is made up of the orthogonal array (OA)selected from all possible combinations of the control-lable ing the orthogonal array specially designed for the Taguchi method,the optimum experimental conditions can be eas-ily determined [28].Accordingly,an analysis of the signal-to-noise (S/N)ratio is needed to evaluate the experimental u-ally,three types of S/N ratio analysis are applicable:(1)lower is better (LB),(2)nominal is best (NB),and (3)higher is better (HB)[33].Because the target of this study is to maximize the pollutant removal efficiency,the S/N ratio with HB characteristics is required,which is given by Eq.(1):S /N =−10log 101n1PRE i2 (1)where n is the number of repetitions under the same experimentalconditions,and PRE represents the results of measurements.3.Experimental3.1.MaterialsThe reactants used in this study were tetraethyl orthosilicate (TEOS,98%)as a silica source,distilled water (DW),Pluronic ®P123(EO 20PO 70EO 20)as a surfactant and phosphoric acid (H 3PO 4,85%)from Aldrich (U.K.),sucrose as a carbon source,sodium hydroxide (NaOH),ethanol,sulfuric acid (H 2SO 4,98%)and zinc nitrate tetrahy-drate (Zn(NO 3)2·4H 2O)from Merck,Germany.Hydrochloric acid (HCl,37%)and sodium hydroxide were used to control the pH of water samples.3.2.Ordered mesoporous silica:SBA-15SBA-15was synthesized as reported by Colilla et al.[34].The synthesis procedure was based on the use of P123as direct-ing agent of the silica and phosphoric acid as catalyst.The molar compositions of the initial solutions here presented are as follows:SiO 2/P123/H 3PO 4/H 2O =1/0.017/1.5/208.These solutions were stirred at 35◦C for 24h in sealed Teflon beakers and then fur-ther heated at 100◦C for 24h.The obtained products were filtered,washed with DW and then dried at 90◦C for 12h in air.The surfac-tant removal was done by heating first at 250◦C (3h)and then at 550◦C (4h)in air.3.3.Ordered mesoporous carbon:CMK-3CMK-3was prepared according to the synthesis procedure described by Jun et al.[22].SBA-15was used as a hard template.1g SBA-15was added to a solution obtained by dissolving 1.25g of sucrose and 0.14g of H 2SO 4in 5g H 2O.The resultant mixture was dried in an oven at 100◦C (6h),and subsequently,the oven temperature was increased to 160◦C (6h).After that,the SBA-15containing the partially carbonizing organic masses was added in aqueous solution consisting sucrose (0.75g),H 2SO 4(0.08g)and water (5g).The resultant mixture was dried again at 100◦C (6h),and subsequently,the oven temperature was increased to 160◦C (6h).This powder sample was heated to 900◦C using a fused quartz reactor.The carbon–silica composite obtained was washed with 1M aqueous ethanolic NaOH solution (50vol.%NaOH solution and 50vol.%ethanol)twice at 90◦C,in order to remove the silica tem-plate.The carbon samples obtained after the silica removal were filtered,washed with ethanol and dried at 120◦C.3.4.Modification of CMK-3:OCMK-3The texture and surface chemistry of synthesized CMK-3was modified by HNO 3to optimize their ability of dispersing active metal particles [35].0.1g of dried CMK-3powder was treated with 15ml of HNO 3solution (2M)at 80◦C under refluxing.Samples were recovered and washed thoroughly with DW until the pH was close to 7.Finally,carbon supports were filtered,washed with distilled water and dried at 108◦C.It was denoted as OCMK-3.3.5.Functionalization of OCMK-3:Zn-OCMK-3The zinc oxide was loaded to the OCMK-3by the equilibrium adsorption of Zn(II)ions from aqueous solution followed by calci-nation in air at 350◦C (2h)to convert zinc nitrate to zinc oxide.The Zn(II)solution was prepared by dissolving Zn(NO 3)2.4H 2O in THF (1.5mol/l).80ml of Zn(II)solution including 2g of the OCMK-3was agitated at 150rpm for 3days to reach the equilibrium state.The solid was separated by filtration and then dried in oven at 110◦C before calcinations at 350◦C.The ZnO functionalized OCMK-3was referred as Zn-OCMK-3.The filtrate was analyzed by Inductively Coupled Plasma to ensure complete adsorption.It was observed that the Zn was taken up by OCMK-3.3.6.CharacterizationThe X-ray powder diffraction (XRD)patterns were recorded on a Philips 1830diffractometer using Cu K ␣radiation.The diffractograms were recorded in the 2Ârange of 0.5–10◦with a 2Âstep size of 0.01◦.The surface area was measured by nitro-gen adsorption/desorption at 77K using Brunaure–Emmet–Teller (BET)method.Scanning electron microscopy (SEM)images were obtained with JEOL 6300F SEM.Transmission electron microscopy (TEM)images for determination of ZnO dispersion and morphology of samples were obtained using a 300kV Philips CM-30TEM.Table 1Controllable factors and their levels.Factor DescriptionLevel 1Level 2Level 3Level 4A Agitation time (min)1030120240B Concentration (mg/l)10100200400C Dose (g/l)0.10.30.50.7D Temperature (◦C)20253035EpH24671048G.Zolfaghari et al./Journal of Hazardous Materials 192 (2011) 1046–10553.7.Adsorption procedure3.7.1.Optimization studiesThis study considers five controllable factors,and each factor has four levels (Table 1).Therefore,an L 16(45)orthogonal array is chosen,and the experimental conditions (Table 2)can be obtained by combining Table 1and the L 16(45)orthogonal array.Stock solu-tions of 1000mg/l Pb(II)and Hg(II)were prepared from Pb(NO 3)2as the Pb source and Hg(NO 3)2as the Hg source in deionized water.A series of aqueous solutions of Pb and Hg with the agita-tion time of 10–240min,initial concentration of 10–400mg/l,dose of 0.1–0.7g/l,temperature of 20–35◦C,and pH of 2–7were pre-pared (Table 2).Batch adsorption tests were performed by shaking 500ml Amber Winchester bottles containing 100ml of the Pb(II)and Hg(II)in a Gallenkamp incubator shaker,stirring at 150rpm.The concentration of heavy metal in the supernatant was analyzed with an Inductively Coupled Plasma Atomic Emission Spectrome-ter (ICP-AES –Perkin-Elmer 4300DV Model).The PRE is calculated using Eq.(2):PRE =C 0−C e C 0×100(2)where C 0and C e are the initial and equilibrium concentrations of pollutant (mg/l),respectively.The analysis of mean (ANOM)statistical approach is adopted herein to develop the optimal conditions [31].Initially,the mean of the S/N ratio of each factor at a certain level should be calculated.For example,(M )Level =iFactor =I,the mean of the S/N ratio of factor I in level i ,is given by Eq.(3):(M )Level =iFactor =I =1n Iin Iij =1S NLevel =iFactor =Ij(3)where n Ii represents the number of appearances of factor I in thelevel i ,and [(S /N)Level =iFactor =I ]j is the S/N ratio of factor I in level i ,andits appearance sequence in Table 2is the j th.By the same measure,the mean of the S/N ratios of the other factors in a certain level can be determined.Thereby,the S/N response table is obtained,and the optimal conditions are established.Finally,the confirmation experiments on solidification under these optimal conditions are carried out.In addition to ANOM,the analysis of variance (ANOVA)[31]statistical method is also used to analyze the influence of eachcontrollable factor on removal of lead and mercury.The percentage contribution of each factor,R F ,is given by Eq.(4):R F =SS R −(DOF R V ER )SS T×100(4)In Eq.(4),DOF R represents the degree of freedom for each factor,which is obtained by subtracting one from the number of the level of each factor (L ).The total sum of squares,SS T ,is given by Eq.(5):SS T =mj =1⎛⎝nj =1PRE 2i⎞⎠−mn (PRE T )2(5)where PRE T =˙m j =1(˙n i =1PRE i )j /(mn ),m represents the number of experiments carried out in this study,and n represents the numberof repetitions.The factorial sum of squares,SS F ,is given by Eq.(6):SS F =mn LLk =l PRE Fk −PRE T2(6)where PRE Fk is the average value of the measurement results of a certain factor in the k th level.Additionally,the variance of error,V Er ,is given by Eq.(7):V Er =SS T −EF =ASS Fm (n −1)(7)3.7.2.Sorption isotherm studiesFor sorption isotherm studies,a series of aqueous solutions of lead and mercury with the initial concentration of 10,100,200and 400mg/l,were prepared.The experiments were carried out under stirring at 150rpm at temperature of 25◦C for 120min contact time,dose 0.5g/l Zn-OCMK-3and pH 6.The amount of lead and mercury adsorption at equilibrium,q e (mg/g)was calculated by Eq.(8):q e =C 0−C eWV (8)where C 0and C e (mg/g)are the liquid phase initial and equilibrium concentrations of the heavy metals respectively.V is the volume of the solution (l),and W is the mass of dry adsorbent used (g)[36].The sorption equilibrium data of Pb(II)and Hg(II)onto theTable 2The S/N ratio of each test.Tests FactorPRE (%)S/NPb(II)Hg(II)ABCDEPRE 1PRE 2PRE 1PRE 2Hg(II)Tests 110100.120228.0028.3030.5030.3828.9829.66Tests 2101000.325431.0028.7435.7535.5029.4831.03Tests 3102000.530663.0063.0147.5447.9535.9833.57Tests 4104000.735748.4046.5047.2047.1033.5133.46Tests 530100.330765.5065.3574.2574.1336.3137.40Tests 6301000.135655.5055.4555.5055.0034.8834.84Tests 7302000.720432.5032.4945.5045.4030.2333.15Tests 8304000.525226.6026.0035.0035.3028.3930.91Tests 9120100.535478.5078.3597.2097.0037.8839.74Tests 101201000.730268.5066.3051.5051.4036.5634.22Tests 111202000.125751.0051.1045.5045.7434.1533.18Tests 121204000.320641.0041.1043.0043.5032.2632.71Tests 13240100.725695.7595.7597.4097.6739.6239.78Tests 142401000.520768.0068.9864.0064.0036.7136.12Tests 152402000.335248.5048.0034.1034.1433.6630.66Tests 162404000.130432.7032.6045.0043.4030.2732.90PRE 1and PRE 2for both pollutants represent the pollutant removal efficiency at first and second test pieces,respectively.The boldfaces correspond to the maximum value of S/N ratio among the 16tests.G.Zolfaghari et al./Journal of Hazardous Materials192 (2011) 1046–10551049Fig.1.XRD patterns of CMK-3,OCMK-3and Zn-OCMK-3.Zn-OCMK-3were analyzed in terms of Langmuir and Freundlich isotherm models[37].The Langmuir model is given by Eq.(9):C eq e=1q m b+1q mC e(9)where q e and C e are the equilibrium concentrations of metal ions in the adsorbed and liquid phases in mg/g and mg/l,respectively.q m (mg/g)and b(l/mg)are the Langmuir constants.In other word,q m is the maximum monolayer capacity and b is the adsorption affinity onto the adsorption sites and it is related to energy of adsorption. The Langmuir constants can be calculated from the slope and inter-cept of the linear plot,C e/q e versus C e.The essential characteristics of the Langmuir isotherm can also be expressed in terms of a dimen-sionless constant of separation factor or equilibrium parameter,R L, which is defined asR L=11+C0b(10) where b is the Langmuir constant and C0is the initial concentra-tion of metal ions.The R L value indicates the shape ofisothermFig.2.Images of samples.(A)SBA-15:SEM,(B)CMK-3:SEM,(C)ZnO supported on OCMK-3(Zn-OCMK-3):SEM,(D)hexagonal structure of CMK-3:TEM,(E)oxidized mesoporous carbon(OCMK-3):TEM(F)zinc oxide-modified mesoporous carbon(Zn-OCMK-3):TEM.1050G.Zolfaghari et al./Journal of Hazardous Materials192 (2011) 1046–1055 Table3S/N ratio response table for Pb(II).Factor/level[(S/N)LevelFactor ]j(M)LevelFactorj=1j=2j=3j=4A/128.9829.4835.9833.5132.00 A/236.3134.8830.2328.3932.46 A/337.8836.5634.1532.2635.22 A/439.6236.7133.6630.2735.07 B/128.9836.3137.8839.6235.70 B/229.4834.8836.5636.7134.41 B/335.9830.2334.1533.6633.51 B/433.5128.3932.2630.2731.12 C/128.9834.8834.1530.2732.08 C/229.4836.3132.2633.6632.93 C/335.9828.3937.8836.7134.75 C/433.5130.2336.5639.6234.99 D/128.9830.2332.2636.7132.05 D/229.4828.3934.1539.6232.92 D/335.9836.3136.5630.2734.79 D/433.5134.8837.8833.6634.99 E/128.9828.3936.5633.6631.91 E/229.4830.2337.8830.2731.97 E/335.9834.8832.2639.6235.69 E/433.5136.3134.1536.7135.18 The boldface corresponds to the maximum value of the mean of the S/N ratios of a certain factor among the four levels.[38].R L values between0and1indicate favorable adsorption,while R L>1,R L=1,and R L=0indicate unfavorable,linear,and irreversible adsorption isotherms.Also,the Freundlich model is given by Eq.(11):ln q e=ln k f+1nln C e(11)where q e and C e are the equilibrium concentrations of metal ions in the adsorbed and liquid phases in mg/g and mg/l,respectively.k f (mg/g)and n(l/mg)are the Freundlich constants which are related to the sorption capacity and intensity,respectively.The Freundlich constants k f and n can be calculated from the intercept and slope of the linear plot with ln q e versus ln C e.4.Results and discussion4.1.Characterization4.1.1.XRD patterns and BET surface areaThe XRD of synthesized samples are shown in Fig.1.The XRD patterns of CMK-3showed three diffraction peaks that can be indexed to(100),(110),and(200)in the2Ârange from0.5◦to10◦,representing well-ordered hexagonal mesopores[22].The XRD patterns show well-resolved reflections indicating that CMK-3 nicely maintains its original structure even after the modification with HNO3and functionalization with ZnO.CMK-3had porous structure and high surface area(998m2/g)which can accommodate ZnO in a spreading manner,the ZnO connects to the carbon surface via oxygen atoms.The lower surface area of OCMK-3(985m2/g) compared with pure CMK-3is mainly attributed to the presence of dense carboxylic surface groups.It is considered that calcina-tion at350◦C makes the Zn(II)ions adsorbed on the carbon bind to the mesoporous carbon via oxygen atoms upon the formation of zinc oxide,in which the surface area of Zn-OCMK-3also may progressively increase(1030m2/g).4.1.2.SEM and TEM imagesSEM images of the synthesized samples are shown in Fig.2A–C. Fig.2A reveals that the SBA-15sample consists of many rope-like domains with relatively uniform sizes of≈1␮m,which are aggregated into wheat-like macrostructures.The preservation of Table4S/N ratio response table for Hg(II).Factor/level[(S/N)LevelFactor]j(M)LevelFactorj=1j=2j=3j=4A/129.6631.0333.5733.4631.94A/237.4034.8433.1530.9134.08A/339.7434.2233.1832.7134.97A/439.7836.1230.6632.9034.87B/129.6637.4039.7439.7836.65B/231.0334.8434.2236.1234.06B/333.5733.1533.1830.6632.64B/433.4630.9132.7132.9032.50C/129.6634.8433.1832.9032.65C/231.0337.4032.7130.6632.96C/333.5730.9139.7436.1235.09C/433.4633.1534.2239.7835.16D/129.6633.1532.7136.1232.92D/231.0330.9133.1839.7833.73D/333.5737.4034.2232.9034.53D/433.4634.8439.7430.6634.68E/129.6630.9134.2230.6631.37E/231.0333.1539.7432.9034.21E/333.5734.8432.7139.7835.23E/433.4637.4033.1836.1235.05 The boldface corresponds to the maximum value of the mean of the S/N ratios of a certain factor among the four levels.rope-like morphology during the SBA-15templating synthesis of CMK-3from sucrose provides additional confirmation that carbon is faithful replica of the SBA-15(Fig.2B).Furthermore,the carbon replica possesses the same wheat-like shape similar to the SBA-15.Fig.2C is the SEM picture of the ZnO particle on CMK-3.TEM image showed in Fig.2D confirm that the structure of CMK3is well-ordered hexagonal arrays of mesopores.Fig.2E shows that after oxidation treatment,ordered structure is maintained.Fig.2F clearly demonstrates the ZnO particulates deposited in the pores of the carbon.As it can be seen,ZnO is well dispersed on the carbon support.4.2.Optimization4.2.1.Optimum conditionsThe pollutant removal efficiency in Tests1–16measured for Pb(II)and Hg(II)according to the method and Eq.(2)(Table2).Sub-stituting the number of experimental repetitions and results of the measurement(PRE)into Eq.(1),the S/N ratio of each test condition are determined(Table2).The boldfaces in Table2refer to the maxi-mum value of S/N ratio among the16tests.Subsequently,the values of the S/N ratio were substituted into Eq.(3)and the mean of theS/N ratios of a certain factor in the i th level,(M)LevelFactor,was obtained(Table3(Pb)and Table4(Hg)).The(M)LevelFactorshows the effect of each level of each factor on the response,independently.It is calculated by averaging the S/N ratio values of all the experiments where the level of that factor has been used.In Tables3and4the boldfaces refer to the maximum value of the mean of the S/N ratios of a cer-tain factor among four levels,and thus it indicates the optimum conditions for adsorption process.From these Tables,the optimum conditions for removal of Pb(II)and Hg(II)are as follows:(1)The agitation time is120min;(2)the initial concentration is10mg/l;(3)the temperature is35◦C;(4)the dose is0.7g/l;and(5)the pH is6.The confirmation experiment was carried out according to the aforementioned optimum conditions,the PRE of Pb and Hg regis-tered,and the S/N ratio was calculated(Table5).The value of the S/N ratio under optimum conditions(Pb:39.75and Hg:39.91)slightly exceeds that in Test13(Pb:39.62and Hg:39.78),and the average PRE under optimum conditions(Pb:97.25%and Hg:99%)indeed exceeds that in Test13(Pb:95.75%and Hg:97.54%).Although the difference of the S/N ratio between the optimum conditions andG.Zolfaghari et al./Journal of Hazardous Materials192 (2011) 1046–10551051 Table5The optimum conditions for Pb(II)and Hg(II)adsorption.A B C D E PRE1PRE2S/NTest13for Pb(II)240100.725695.7595.7539.62 Optimization condition for Pb(II)120100.735697.0097.5039.75 Test13for Hg(II)240100.725697.4097.6739.78 Optimization condition for Hg(II)120100.735698.5099.5039.91PRE1and PRE2for both pollutants represent the pollutant removal efficiency atfirst and second test pieces,respectively.Test13is very little,the agitation time substantially decreases from 4h(Test13)to2h(optimum conditions).Furthermore,the PRE increases from95.75to97.25%and from97.54to99%for Pb(II)and Hg(II),respectively.These results are pretty exciting due to the fact the lower agitation time corresponds to a better adsorption system.4.2.2.Agitation timeThe(M)LevelFactorare shown in Figs.3–7for the experimental con-ditions proposed by Taguchi method.Fig.3shows the effect of agitation time on the S/N ratio in the removal of Pb(II)and Hg(II). The distribution of adsorbate between adsorbent and solution is influenced by agitation time.It is seen that the time required for equilibrium is120min.In the other words,the optimum time is 2h.This is obvious from the fact that a large number of vacant sur-face sites are available for the adsorption during the initial stage and with the passage of time,the remaining vacant surface sites are difficult to be occupied due to repulsive forces between the solute molecules on the solid phase and in the bulk liquid phase. Heavy metal adsorption slightly decreases after120min.It may be related to desorption process at upper time.4.2.3.Initial concentrationFig.4shows that the S/N for Pb concentration decreases from 35.70to31.12by increasing concentration from10to400mg/l and the S/N for Hg concentration decreases from36.65to32.50 by increasing concentration from10to400mg/l.The efficiency of metal removal is affected by the initial metal concentration,with the removal percentage decreases as the concentration increases. This effect has been attributed to the surface binding of low-affinity surface sites as high-affinity ones begin to reach saturation,lead-ing to a reduction in the removal efficiency.Seco et al.[39]have reported that the amount of adsorption was decreased with the increase in initial concentration.4.2.4.Adsorbent amountThe present study shows that the Zn-OCMK-3is an effective adsorbent for the removal of Pb(II)and Hg(II)from aqueous solu-tion(97.25%for Pb and99%for Hg).It can be seen from Fig.5that the dose for Pb(II)and Hg(II)was increased with the increase in adsorbent.Amount of Zn-OCMK-3was varied from0.1to0.7g/l. It is apparent that the equilibrium concentration in solution phase decreases with increasing carbonous adsorbents amount for a given initial Pb(II)and Hg(II)concentration.Since the amount of metals removed from the aqueous phase increases as the sorbent amount is increased in the batch vessel with afixed initial heavy metal concentration.This result was anticipated because for afixed ini-tial solute concentration,increasing amount of adsorbent provides greater surface area.4.2.5.TemperatureThe plot of adsorption capacity as a function of temperature is shown in Fig.6.The uptake of Pb and Hg onto the Zn-OCMK-3 increased with increase in temperature from20to35◦C,indicat-ing more chemical interaction between the sorbate and the surface functionalities of the Zn-OCMK-3.Adsorption efficiency is observed to increase on raising the temperature,as previously reported [40,41].Seco et al.[39]have investigated the adsorption of heavy metals onto activated carbon.They concluded that increase in tem-perature results in greater removal efficiency.The adsorption of Pb(II)onto another carbon material has been reported by Wang et al.[42].In their study,the Pb(II)adsorption capacity increased with the rise of temperature.4.2.6.Solution pHOne of the critical parameter in the treatment of heavy metals by the sorption medium is pH,because the surface charge of an adsorbent could be modified by changing pH of the solution.Fig.7 shows that the S/N ratio increased with the pH value from2to6.The removal of Pb(II)and Hg(II)from water by Zn-OCMK-3was found to be dependent on the solution pH value.This can be explained on the basis of Surface Complex Formation(SCF)theory[39].In the SCF theory,adsorption of inorganic species onto hydrous solids is described as a chemical coordination and electrostatic process involving specific interactions at the solid/solution interface.The increase in the metal removal as the pH increases is on the basis of a decrease in competition between proton and metal species for the surface sites,and by the decrease in positive surface charge, which results in a lower coulombic repulsion of the sorbingmetal. Fig.3.The effect of agitation time on the S/N ratio in the removal of Pb(II)(left)and Hg(II)(right).Circles onfigures indicate optimum time for adsorption process.。