Ya Fu ,Jiaan Wu ,Hongan Zhou ,Guanping Jin ,2,*

1 Department of Application Chemistry of School of Chemical Engineering,Hefei University of Technology,Hefei 230009,PR China

2 Anhui Key Laboratory of Controllable Chemistry Reaction&Material Chemical Engineering,Hefei 230009,PR China

Keywords:Iminodiacetic acid Glycidyl methacrylate Carbon fiber Adsorption Nickel(II)

ABSTRACT Iminodiacetic acid functionalized polyglycidyl methacrylate grafted-carbon fibers(PGMA-IDA/CFs)were prepared for Ni(II)removal from aqueous solutions.The effects of solution pH value,temperature and adsorption time were investigated.The maximum adsorption capacity of Ni(II)on PGMA-IDA/CFs is 0.923 mmol·L-1 ·g-1 at pH 5.2 and 50°C.Kinetic data indicate that the adsorption process matches the pseudo-second-order model and Elovich kinetic model.Thermodynamic data suggest that the adsorption process is endothermic spontaneous reaction.

1.Introduction

Nickel is a common heavy metal pollutant in wastewater from electroplating and metal cleaning industries.Accumulation of nickel in aqueous environment may cause a serious hazard to human health,such as skin allergies,lung fibrosis,different poisoning degrees to kidney and cardiovascular system and stimulation of neoplastic transformation[1,2].Thus the removal of Ni(II)from wastewater is necessary.Many treatment methods have been investigated,such as chemical precipitation[3],ion exchange[4],electrochemical treatment[5]and adsorption[6].Adsorption is becoming a popular method due to its convenience and cost-effectiveness.

Carbon fibers have outstanding chemical stability,electric conduction,surface area and adsorption,especially easy operation compared to powderadsorbents.They have been widely used assupportmaterials[7,8]and adsorbents in the removal of heavy metals.For example,Jin et al.successfully grafted tetraoxalyl ethylenediamine melamine resin on carbon fibers for separation of Ni(II)from the spent nickel plating baths[7].Chergui et al.grafted cyclam functionalized polyglycidyl methacrylate on carbon fibers,showing excellent uptake towards Cu(II)[8].The chelating resin,iminodiacetic acid functionalized polyglycidyl methacrylate(PGMA-IDA)was successfully employed in the removal of Ni(II)[9,10].PGMA-IDA has an excellent adsorption capacity for Ni(II)[7,10].In this work,PGMA-IDA is grafted on carbon fibers by atom transfer radical polymerization(ATRP)to prepare PGMA-IDA grafted carbon fibers(PGMA-IDA/CFs)for the removal of Ni(II)from aqueous solutions.

2.Experimental

2.1.Apparatus and chemicals

All electrochemical experiments were performed with a CH660 B electrochemicalworkstation(Chenhua,Shanghai,China).Infrared spectra were measured with IR 200(Nicolet,America).pH was measured with HI 255 model pH meter(Leici,Shanghai,China).Polyacrylonitrile carbon fibers were obtained from Dingfeng Carbon Fibers Fabrication Company of Yixing(Wuxi,China).One truss carbon fibers have about 3000 branches with a diameter of(7±1)μm and they were snipped to 8 cm long in the experiments.1-(4-Aminophenyl)-ethanol was purchased from Alfa Aesar.Glycidyl methacrylete(GMA),iminodiacetic acid(IDA),hydrobromic acid and fluoroboric acid were purchased from Aladdin(Shanghai,China).Ethylenediaminetetraacetic acid(EDTA)disodium salt was obtained from Xilong Chemical Co.,Ltd.All other chemicals were of analytical grade and deionized distilled water was used throughout.

2.2.Preparation of PGMA-IDA/CFs

The synthesis processes ofPGMA-IDA/CFs adsorbentare divided into four steps and illustrated in Fig.1.

(i)Synthesis of diazonium salt,+N2–C6H4–CH(CH3)-Br(D1)[11].1-(4-Aminophenyl)-ethanol(0.4 g),tetramethylammonium bromide(0.0135 g),and hydrobromic acid(48%,5 ml)were added into 50 ml a round bottom flask,refluxed at 150°C for 16 h,and then cooled to 0 °C.Fluoroboric acid(49.5%–50.5%,3.2 ml)was added into the flask.A solution of sodium nitrite(1.7 mol·L-1,2 ml)was slowly dripped into the flask then stirred for 30 min in 0°C ice bath.The precipitated diazonium tetra fluoroborate(,+N2–C6H4–CH(CH3)-Br,D1)was filtered and washed with 5%sodium fluoroborate and methanol,then dried under vacuum.

(ii)Electrochemical treatment of carbon fibers[7].0.015 g D1 was added into 30 ml of0.1 mol·L-1tetrabutylammonium tetra fluoroborate/acetonitrile and reduced on carbon fibers by chronoamperometry for 300 s undernitrogen atmosphere,ata potential of-0.5 V(versus saturated calomel electrode).The aryl initiator modified carbon fibers(labeled CF-Br)were washed with acetone and deionized water.

(iii)Surface-initiated ATRP of GMA monomers[7].ATRP of GMA monomers at the CF-Br surface was carried out in DMF/water mixture(26.6/13.3 in ml)using GMA(3.7 ml),cuprous bromide(0.0065 g),copper(II)bromide(0.039 g),and 2,2′-bipyridine(0.068 g).ATRP was conducted at room temperature under argon atmosphere for 12 h.After polymerization,the carbon fibers(labeled PGMA/CFs)were washed with acetone and deionized water,then dried under vacuum at 40°C.

(iv)Modification of PGMA/CFs[12].PGMA/CFs were immersed in 15 ml mixed solution(3.18 g Na2CO3+0.5 g IDA,pH 12)at 75°C for 48 h.After modification,carbon fibers(labeled PGMAIDA/CFs)were washed with acetone and deionized water,then dried under vacuum.

Fig.1.The preparation process of PGMA-IDA/CF adsorbent.

2.3.Adsorption experiments

Adsorption experiments were carried out by adding 4 trusses PGMA-IDA/CFs into 15 ml Ni(II)solution with initial concentration of 17.09 mmol·L-1in 180 min.Adsorption was studied by varying the variables such as initial pH value of Ni(II)solution,temperature and adsorption time.The initial pH value of Ni(II)solution was varied from 3 to 6 at temperatures from 30 °C to 55 °C.The kinetics was studied in different time intervals and the temperature in thermodynamic study varied from 35 °C to 50 °C.The concentration of Ni(II)was determined via titration against 1.5 mmol·L-1EDTA using murexide as an indicator[13].The adsorption capacity(q,mmol·L-1·g-1)of Ni(II)is calculated by following equation:

where c0and ceare the initial and equilibrium concentrations of Ni(II)(mmol·L-1),m is the mass of PGMA-IDA/CFs(g),and V(L)is the volume of Ni(II)solution.

3.Results and Discussion

3.1.IR analysis

IR spectra of materials D1,CF-Br,PGMA/CFs,IDA and PGMA-IDA/CFs are showed in Fig.2.For CF-Br,the vibration band at 2262 cm-1characteristic of N≡N disappears,which is clearly observed in curve a.The vibration band of the aromatic ring stretching at 1510 cm-1(curve a)shifts to 1515 cm-1(curve b).These confirm the electrochemical reduction of D1 and grafting of aryl layer[11].For PGMA/CFs,the peaks at 1728 cm-1and 906 cm-1correspond to the stretching vibration of ester carbonyl group and the symmetrical stretching of epoxy group,respectively[14].For IDA and PGMA-IDA/CFs,the peak at 1715 cm-1of stretching vibration of carbonylgroup in IDA shifts to lower frequency(1650 cm-1and 1397 cm-1).The peaks at 1650 cm-1and 1397 cm-1match the asymmetric stretching vibration and symmetric stretching vibration of carboxylic acid anion,respectively[15,16].Thus IDA is successfully introduced onto the PGMA/CFs.

Fig.2.IRspectra ofmaterials D1(a),CF-Br(b),PGMA/CFs(c),IDA(d)and PGMA-IDA/CFs(e).

3.2.Effect of pH on adsorption of Ni(II)

The effect of initial pH on adsorption of Ni(II)is showed in Fig.3.The adsorption capacity of Ni(II)on PGMA-IDA/CFs increases obviously with pH value until reaching a maximum value(0.852 mmol·L-1·g-1)at pH 5.2.The synergy between electrostatic interaction and ligand chelation improves the adsorption capacity of Ni(II)with the increase of pH.The adsorption capacity decreases due to hydrolysis as pH＞5.5[17].

Fig.3.The effect of initial pH on adsorption of Ni(II).

3.3.Effect of temperature and adsorption time on adsorption of Ni(II)

Effects oftemperature and adsorption time on the adsorption ofNi(II)are shown in Fig.4.The adsorption capacity increases with temperature until reaching a maximum value at 50 °C(0.923 mmol·L-1·g-1),then changes little up to 55°C.In inset a,the adsorption capacity increases very slowly after 40 min(0.911 mmol·L-1·g-1),reaching about 98%of the maximum amount of adsorption.

Fig.4.The effect of initial pH and adsorption time(inset)on adsorption of Ni(II).

3.4.Adsorption kinetics

The kinetics of Ni(II)adsorption on PGMA-IDA/CFs is investigated at initialconcentration of[Ni(II)]=17.09 mmol·L-1,pH 5.2 and 50 °C.For a solid–liquid adsorption,experimental data can be fitted with two widely used models:pseudo- first-order[19].The pseudo- first-order model is expressed as

Integrating the equation with initialcondition qt=0 at t=0,the following expression is obtained

where k1(min-1)is the pseudo- first-order rate constant of adsorption,and qe(mmol·L-1·g-1)and qt(mmol·L-1·g-1)are the amounts of Ni(II)adsorbed at equilibrium and time t(min)[18].

The pseudo-second-order model is expressed as

With qt=0 at t=0,this equation becomes

where k2[g·(mmol·L-1)-1·min-1]is the pseudo-second-order rate constant of adsorption[19].

The kinetic parameters for the pseudo- first and pseudo-second models are determined from the linear plots of lg(qe–qt)vs.t[Fig.5(a)]and(t/qt)vs.t[Fig.5(b)],respectively.The relevant parameters obtained by linear regression are displayed in Table 1.According to the results,the pseudo-second-order model is better for fitting the adsorption of Ni(II),with higher R2value and closer qevalue.The adsorption kinetics follows the pseudo-second-order model,suggesting a chemisorption process.

Fig.5.The uptake of Ni(II)on PGMA-IDA/CFs with time at pH 5.2 and 50°C.(a)Pseudoif rst-order kinetics；(b)pseudo-second-order kinetics.

A widely used equation to describe the kinetics of chemisorption of gases on solids is the Elovich equation[20].The equation is as follows.

To simplify the Elovich equation,it is assumed that αβt? 1.With the initial condition qt=0 at t=0,this equation becomes

where α is the initial adsorption rate(mmol·L-1·g-1·min-1)and β is the desorption constant[g·(mmol·L-1)-1][20].The constants could be obtained from the slope and intercept of a straight line plot of qtvs.ln t(Fig.6).The values of α and β for the adsorption of Ni(II)on PGMA-IDA/CFs are 0.141(mmol·L-1·g-1·min-1)and 2.786[g·(mmol·L-1)-1],respectively.The value of R2is 0.999.These data suggest the applicability of Elovich kinetic model for the adsorption.

3.5.Adsorption isotherm

Fig.7 shows the isotherm of adsorption of Ni(II)on PGMA-IDA/CFs at different temperatures.The maximum adsorption capacity is 0.923 mmol·L-1·g-1at 50 °C.The adsorption data are plotted according to the Langmuir equation

Table 1 Parameters of the pseudo- first-order and pseudo-second-order kinetics for adsorption of Ni(II)on PGMA-IDA/CFs

Fig.6.The Elovich kinetic model for adsorption of Ni(II)on PGMA-IDA/CFs.

Fig.7.Adsorption isotherm ofNi(II)on PGMA-IDA/CFs atpH 5.2.■ 35 °C；○ 45 °C；▲ 50 °C；inset:c e/q e against c e.

where ce(mmol·L-1)is the equilibrium concentration of Ni(II)in the solution,qeis the equilibrium concentration of Ni(II)on PGMA-IDA/CFs(mmol·L-1·g-1),Qmax(mmol·L-1·g-1)is the maximum adsorption capacity,and KL[L·(mmol·L-1)-1]is the Langmuir isotherm constant.Plotting ce/qeagainst cegives a straight line with a slope of 1/Qmaxand an intercept of 1/KLQmax.The values of KLand Qmaxat different temperatures for adsorption of Ni(II)are displayed in Table 2.

Table 2 Langmuir constants for adsorption of Ni(II)on PGMA-IDA/CFs

The essential feature of the Langmuir isotherm model can be expressed in terms of constant separation factor(RL).Parameter RLindicates the shape of isotherm(RL＞1 is unfavorable and 0＜RL＜1 is favorable[21]).The value of RLcan be calculated from the relation

where c0(mmol·L-1)is the initial concentration of Ni(II).The values of RLbetween 0.042 and 0.132 indicate thatitis suitable to use PGMA-IDA/CFs as a adsorbent for Ni(II)removal.

3.6.Thermodynamics of adsorption

The thermodynamic parameters of enthalpy(ΔH0),entropy(ΔS0)and Gibbs free energy(ΔG0)are calculated from the van't Hoff equation[22,23].

where R(8.314 J·mol-1·K-1)is the universal gas constant and T(K)is temperature.Plotting ln KLagainst 1/T gives a straight line with a slope –ΔH0/R and an intercept of ΔS0/R.The values of ΔH0,ΔS0,and ΔG0are displayed in Table 3.The positive value of ΔH0confirms the endothermic nature of adsorption process,while the positive value of ΔS0suggests high degree of freedom of adsorption system at equilibrium due to the interaction between active sites and Ni(II)ions.The negative value ofΔG0indicates thatthe adsorption reaction is spontaneous.The increase in negative values of ΔG0with temperature implies that the adsorption is more favorable at higher temperature.

Table 3 Gibbs free energy change and entropy change foradsorption ofNi(II)on PGMA-IDA/CFs at different temperatures

3.7.Selectivity

Selective separation of Ni(II)from binary mixtures with Cu(II),Pb(II),Co(II)was studied at pH 5.2 and 50°C due to their co-existence in wastewater.The separation factors for Ni(II)over Pb(II)and Co(II)are calculated from the adsorption data using the following equation

where cA1and cA2stand for the concentration of Ni(II)before and after adsorption,respectively,cB1and cB2are the concentration of Pb(II)or Co(II)before and after adsorption.The values of separation factor of Ni(II)over Cu(II),Pb(II)and Co(II)are 1.62,3.71 and 10.8,respectively.

3.8.Regeneration

Regeneration of PGMA-IDA/CFs was investigated by repeated cycles of adsorption/desorption of Ni(II)in 0.1 mol·L-1HNO3for 36 h,with the desorption ratio calculated by

The percentage ofNi(II)adsorption from five consecutive adsorption/desorption cycles indicates a desorption efficiency as high as 91.8%.Thus the present materialpresents good durability and efficiency for repeated use.

4.Conclusions

Iminodiacetic acid functionalized polyglycidylmethacrylate graftedcarbon fibers were obtained by a combination method with electrochemical and organic synthesis in this study.Batch experiments were conducted to study the effects of adsorption time,pH,temperature,and initial concentration.Kinetic data give good correlation with the pseudo-second-order model with a chemisorption process.Thermodynamic data show that the adsorption is more favorable at higher temperature.This work provides a convenient and effective adsorbent for Ni(II)removal from aqueous solutions.