Selective Removal of Arsenic(V) from Aqueous Solution Using A

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Selective Removal of Arsenic(V) from Aqueous Solution Using A Surface-Ion-Imprinted Amine-Functionalized Silica Gel Sorbent Hong-Tao Fan,†,‡ Xuelei Fan,† Jing Li,† Mengmeng Guo,† Dongsheng Zhang,† Feng Yan,† and Ting Sun*,‡ †

College of Applied Chemistry Shenyang, University of Chemical Technology, Shenyang, 110142, China College of Sciences, Northeastern University, Shenyang 110004, China

‡

ABSTRACT: A new surface-ion-imprinted amino-functionalized silica gel sorbent was prepared by the surface imprinting technique with As(V) as the template, 3-(2-aminoethylamino)propyltrimethoxysilane as the functional monomer, silica gel as the support, and epichlorohydrin as the cross-linking agent and was characterized by FTIR, SEM, nitrogen adsorption, and the static adsorption−desorption experiment method. The results showed that the maximum static adsorption capacity of the imprinted silica gel sorbent was 16.1 mg·g−1, the adsorption equilibrium could be reached in 20 min, there was no influence of pH values on adsorption capacity of the imprinted silica gel sorbent in the range of 3.7−9.2, and the imprinted silica gel sorbent could be used repeatedly and indicated high selectivity even in the presence of the other metal ions. The Langmuir adsorption model was more favorable than the Freundlich adsorption model. Kinetic studies indicated that the adsorption followed a pseudosecond-order model. Various thermodynamic parameters such as ΔGo, ΔHo, and ΔSo were evaluated with results indicating that this system was a spontaneous and endothermic process.

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INTRODUCTION It is well-known that arsenic is a toxic element for humans and has been identified as a public health problem because it has serious toxic effects even at low exposure levels.1 Arsenic is one of the most toxic pollutants introduced into natural waters by geochemical reactions, industrial waste discharges, agricultural use of arsenic pesticides, discharges from coal fired thermal power plants, herbicides, fertilizers, petroleum refining, ceramic industries, etc.2 Various techniques such as precipitation method, ion exchange method, membrane method, adsorption method, and biological method have been applied for arsenic removal from the contaminated water.3,4 In these methods, the adsorption techniques are simple and convenient, and the research of new adsorption materials has attracted more interest. Molecular imprinting is a state-of-the-art technique for imparting molecular recognition properties to a synthetic polymeric matrix and has become a powerful method for the preparation of robust materials that have the ability to recognize a specific chemical species.5,6 Molecular imprinting is a process where functional and cross-linking monomers are copolymerized in the presence of the target analyte (the imprint molecule), which acts as a molecular template. The functional monomers initially form a complex with the imprint molecule, and following polymerization, their functional groups are held in position by the highly cross-linked polymeric structure. Subsequent removal of the imprint molecule reveals binding sites that are complementary in size and shape to the analyte. In that way, a molecular memory is introduced into the polymer, which is now capable of rebinding the analyte with a very high specificity.7,8 For metal ions, molecular imprinting can be interpreted as ionic imprinting exactly. Ionic imprinting is a process, in which a functional monomer is allowed to selfassemble around templated ions and subsequently is cross© 2012 American Chemical Society

linked as required. In the ion imprinted process, the selectivity of a specific target ion is obtained by providing the polymers with cavities, in which complexing ligands are arranged so as to match the charge, coordination number, coordination geometry, and size of the target ion.9 This process creates the ionic recognition site, which is a specific location for the target ion by chemical functionality and spatial orientation. Conventionally, the imprinting technique is easily carried out using bulk imprinting, where imprinted polymers are prepared in large chunks and post-treatment processes like grinding and sieving are then required. However, the creation of binding sites within the polymeric bulk and the issue of the hindrance of adsorbate diffusion during template rebinding limit the imprinted polymers prepared for practical applications.10 Thus over the years, many efforts to address the limitations of conventional imprinting techniques have resulted in new imprinting methodologies. Surface imprinting is one of the important imprinting methods. Surface imprinted polymer not only possesses high selectivity but also avoids problems with mass transfer, and shows significant promise for industrial applications.10−17 So far there are some metal ions imprinted polymers, including U(VI),11 Zr(IV),12 Ni(II),13 Fe(III),14 and Cd(II)15−17 imprinted polymers, have been prepared by the surface imprinting technique making the functional group immobilized on the surface of the silica gel for selective separation of heavy metals from aqueous solution. However, few studies have been reported on As(V) imprinted polymers. Recently, Chen et al. have reported that 3-(2aminoethylamino)propyltrimethoxysilane (AAPTS) modified Received: Revised: Accepted: Published: 5216

November 18, 2011 March 8, 2012 March 18, 2012 March 19, 2012 dx.doi.org/10.1021/ie202655x | Ind. Eng. Chem. Res. 2012, 51, 5216−5223

Industrial & Engineering Chemistry Research

Article

imprinted silica gel sorbent. All reagents used were of at least analytical grade. 3-(2-Aminoethylamino)propyltrimethoxysilane (AAPTS, Sigma) was used in this study. All metal stock solutions (1 g·L−1) were purchased from the National Research Center for Standard Materials (NRCSM, Beijing, China). The working solutions were prepared by series dilution of the stock solutions immediately prior to their use. All the other chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Synthesis and Characterization of As(V)-Imprinted Amino-Functionalized Silica Gel Sorbent. Silica gel (80− 120 mesh) was activated by refluxing in 33% methanesulfonic acid for 8 h and then washed thoroughly with the deionized water and dried before undergoing chemical surface modification. To prepare the As(V)-imprinted sorbent, 3.56 g of Na3AsO4·12H2O was dissolved in 80 mL of methanol under stirring and heating and then mixed with 4 mL of AAPTS. The solution was stirred and refluxed for 1 h, to which 6 g of activated silica gel was added. After 20 h of stirring and refluxing the mixture, 8 mL of epichlorohydrin was added at 70 °C for 4 h. The product was recovered by filtration, washed with ethanol, and stirred in 50 mL of 6 mol L−1 HCl for 2 h for the removal of As(V). The final product was recovered by filtration, washed with doubly deionized water up to the eluent pH 4−5 and dried under vacuum at 80 °C for 12 h. For comparison, the nonimprinted functionalized silica gel sorbent was also prepared using an identical procedure, but without the addition of Na3AsO4·12H2O. The As(V)-imprinted amino-functionalized silica gel sorbent was characterized by SEM and IR spectra. The surface area was determined by adsorption−desorption isotherms of nitrogen at 77 K. As(V) Adsorption Tests. To determine the static sorption capacity of the imprinted silica gel sorbent for As(V) as well as the effect of the contact time (5−60 min), aqueous metals concentration (10−200 mg L−1), and solution pH (pH 2.7− 9.2), static sorption experiments were performed by batch equilibration at 25 ± 2 °C keeping the concentration of the imprinted silica gel sorbent constant at 50 mg/25 mL. The suspensions were brought to the desired pH by adding sodium hydroxide and nitric acid. Adsorption capacity (mg g−1) was calculated as the difference between As(V) ions concentration of the pre- and postadsorption solutions divided by the weight of dry imprinted and nonimprinted silica gel sorbent. The adsorption capacity of sorbent can be obtained from equilibrium binding data according to eq 1.

ordered mesoporous silica has been used for the selective separation and preconcentration of As(V).18 The study shows that As(V) can be adsorbed by AAPTS modified ordered mesoporous silica sorbent through electrostatic effect because the As(III) compounds are neutral in charge while the As(V) compounds are negatively charged in the pH range 3−9,18 whereas previous research show that AAPTS can complex with other heavy metal ions (such as Cd(II), Cu(II), Ni(II), Pb(II), and Zn(II)) by coordination bond.19−21 In the presence of other heavy metal ions, the selective adsorption of AAPTSmodified ordered mesoporous silica for As(V) will be unremarkable because it is without the stereochemical interactions between AAPTS and As(V). In this study, a simple procedure was developed to synthesize a new As(V)-imprinted amino-functionalized silica gel sorbent by surface imprinting technique. The characterization and adsorption property of As(V)-imprinted amino-functionalized silica gel sorbent were described and discussed in detail.

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EXPERIMENTAL SECTION Apparatus and Reagents. Infrared spectroscopy (4000− 400 cm−1) was performed on a Perkin-Elmer Spectrum One FT-IR spectrometer with a resolution of 1 cm−1. Samples were dispersed in potassium bromide pellets for analysis. The surface morphology of the sorbent was examined by a Shimadzu SSX550 scanning electron microscopy at the desired magnification operating at 30 kV. The specific surface area was measured by N2 adsorption−desorption method using a Micromertics ASAP 2010C apparatus following the Brunauer−Emmett−Teller (BET) procedure. An atomic absorption spectrometer coupled to hydride generation (HG-AAS, PE, USA) was used to measure the concentration of As in aqueous solution after appropriate dilutions for the study of the uptake of As and the selectivity of the prepared new sorbent. For hydride generation, 3% NaBH4 (prepared in 1% NaOH) and 1.5% HCl solutions were reacted with the samples for As determinations. A calibration curve, with a concentration series of 0, 2, 4, 6, 8, and 10 μg L−1, was achieved using dilution prepared from a commercially available 1 g L−1 standard As solution and had a correlation coefficient (R2) always higher than 99.9%. The method had a detection limit of approximately 50 ng L−1. Highpurity nitrogen was used as a purge gas for the hydride generation unit, and the glass reaction vessel and quartz cell were continuously purged with nitrogen to eliminate any interference from air at 194.3 nm. The RSD% of 5 μg L−1 sample (n = 6) were 2.5%. These conditions were found to give the best results for As determinations. The concentrations of metals (Cu, Cd, Zn, Co, Pb, and Ni) in solutions were determined by flame atomic absorption spectrometry (FAAS) (AA-700, PE, USA) after appropriate dilutions and acidification to pH ∼2 using HNO3. The solution blanks were below the instrument detection limits of 10 μg L−1 for Cd(II), 10 μg L−1 for Cu(II), 15 μg L−1 for Co(II), 20 μg L−1 for Ni(II), 20 μg L−1 for Zn(II), and 300 μg L−1 for Pb(II). The measurement of P(V) was performed by UV−vis spectrometer (Shimadzu Corporation, Japan) utilizing the molybdovanadophosphoric yellow method at λ = 315 nm.22 Analytical standards were prepared in the concentration range 0−250 μg L−1 (prepared in 2% v/v HCl) from a 1 mg L−1 P working standard. A pHS-10C digital pH meter (Shanghai Precision & Scientific Instrument Co., LTD, Shanghai, China) was used for the pH adjustments. Silica gel (80−120 mesh, Qingdao Ocean Chemical Co., Qingdao, China) was used as the support to prepare the

Q = (C i − C f )V /(1000W )

(1) −1

where Q represents the adsorption capacity (mg g ); Ci and Cf are the initial and final concentrations of As(V) (mg L−1), respectively. V is the volume of the solution (mL); W is the mass used of the imprinted and nonimprinted silica gel sorbent (g). The selective adsorption experiments of Cd(II), Cu(II), Pb(II), Ni(II), and P (V) ions with respect to As(V) ions were conducted using the imprinted and nonimprinted silica gel sorbent. The imprinted silica gel sorbent (50 mg) were added to 25 mL of containing 25 mg L−1 As(V)/Cd(II), As(V)/ Cu(II), As(V)/Pb(II), As(V)/P(V), and As(V)/Ni(II) in the flasks with stirring. The pH value of solutions except the mixed solution of As(V)/P(V) was 6. The pH value of the mixed solution of As(V)/P(V) was 9. After adsorption equilibrium, the concentrations of As(V), Cd(II), Cu(II), Pb(II), P(V), and 5217

dx.doi.org/10.1021/ie202655x | Ind. Eng. Chem. Res. 2012, 51, 5216−5223


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Figure 1. IR spectra of As(V)-imprinted amino-functionalized silica gel sorbent and silica gel.

silica gel sorbent. The As(V) ions were removed from the sorbent by washing with 5 mL of 3 mol L−1 HCl for 4 h. The sorbent was rinsed several times with deionized water and then neutralized to pH 6−7 using 0.1 mol L−1 NaHCO3 solution for up to 24 h to ensure complete H+ neutralization. The sorbent was washed again with deionized water and dried under vacuum at 60 °C overnight before another extraction cycle. The sorption ratio of each cycle was calculated as a percentage of the uptake at the first sorption. Removal of As(V) Ion from Aqueous Solution. The selective removal of As(V) was performed at a pH of 6 at room temperature, in the flasks stirred magnetically, by adding 50 mg of imprinted silica gel sorbent from a multimetal mixture solution (25 mL) containing 50 mg L−1 heavy metal ions (Cd(II), Cu(II), Pb(II), Ni(II), Co(II), and Zn(II) individually), spiked with 25, 50, and 100 mg L−1 As(V) ions. After adsorption equilibrium, the concentration of the As(V) in the remaining solution was measured. In all above batch experiments, the mixtures were magnetically stirred for the desired treatment periods at room temperature, and the concentrations of As(V), Cd(II), Cu(II), Co(II), Pb(II), and Zn(II) ion in the aqueous phases which were centrifuged at 3000 rpm for 5 min were measured by HGFAAS or FAAS. Statistical Analysis. The experiments were performed in replicates of three, and the samples were analyzed in replicates of three as well. For each set of data present, standard statistical methods were used to determine the mean values and standard deviations. Confidence intervals of 95% were calculated for each set of samples to determine the margin of error.

Ni(II) ion in the remaining solution were measured. The distribution and selectivity coefficients of Cd(II), Cu(II), Pb(II), P(V), and Ni(II) with respect to As(V) can be obtained from equilibrium binding data according to eqs 2 and 3. Kd = [(C i − C f )/C f ](V /m)

(2)

In eq 2, Kd represents the distribution coefficient; Ci and Cf are the initial and final concentrations of metal ions (mg L−1), respectively. V is the volume of the solution (mL); m is the mass of sorbent used (g). k = Kd(As(V))/Kd(X)

(3)

In eq 3, k is the selectivity coefficient, and X represents Cd(II), Cu(II), Pb(II), P(V), and Ni(II) ions. A comparison of the k values of the imprinted and nonimprinted silica gel sorbent with those metal ions allows an estimation of the effect of imprinting on selectivity. A relative selectivity coefficient k′ (eq 4) can be defined as k′ = k imprinted/k non − imprinted

(4)

Results from the comparison of the k′ values of the imprinted silica gel sorbent with nonimprinted silica gel sorbent allow an estimation of the effect of imprinting on selectivity. Desorption and Reusability Studies. Several stripping agents such as EDTA, HNO3, and HCl, were used to desorb the As(V) from the imprinted sorbent. One-tenth of a gram of the imprinted silica gel sorbent was placed in 5 mL of desorption medium and stirred continuously at a stirring rate of 600 rpm from 1 to 4 h at room temperature. The final concentrations of metal ions in the aqueous phase were determined by FAAS. The desorption ratio is calculated as follows:

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RESULTS AND DISCUSSION Synthesis of As(V)-Imprinted Amino-Functionalized Silica Gel Sorbent. As known, silica gel is an amorphous inorganic polymer. The silanol groups (Si−OH) are responsible for chemical modifications and facilitate the introduction of the organic groups which covalently bind to the silica surface. In this work, 33% methanesulfonic acid solution was used for the activation of the silica. AAPTS is a kind of organotriethoxysilane with amino groups and covalently attached hydrophobic groups its molecular structure. The complex was formed between As(V) and AAPTS, then cohydrolyzed and co-

desorption ratio (%) =

amount of ions desorbed to the elution medium (mg) amount of ions adsorbed onto the sorbent (mg) × 100

(5)

The sorption/desorption cycle was performed up to five times to evaluate the possibility of repeated reuse of the imprinted 5218



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competing ions and partly that the pH of the solution influences the chemical speciation of the metal ions as well as the ionization of the functional groups onto the sorbent surfaces. The pH of the solution affects the charge on the surface of the sorbent, so the change in pH also affects the adsorption process and the H+ may react with the functional groups on the active sites of the adsorption surface. The effects of initial pH of solutions on the removal efficiency of As(V) and As(III) are shown in Figure 3. In the selective pH range of

condensed with the activated silica gel. The remnant AAPTS was removed by ethanol and could be recycled. After polymerization, the As(V) of the product was removed by 6 mol L−1 HCl solution. The As(V)-imprinted functionalized sorbent which contained a tailor-made cavity for As(V) was formed on the surface of silica. Characterization of As(V)-Imprinted Amino-Functionalized Silica Gel Sorbent. FT-IR spectra of the As(V)imprinted functionalized sorbent could be able to give some information about its chemical structure. It was observed that the peaks at 3506 and 1525 cm−1 are due to the vibration band of NH2 groups, those at 3340 and 1634 cm−1 are due to the vibration band of surface silanol groups with hydrogen bond and the remaining adsorbed water molecules,23 that at 1465 cm−1 corresponds to δ(N−CH2) in the phase twist,20 that at 1095 cm−1 is due to the siloxane vibrations of (SiO)n, those at 1054 and 1180 cm−1 are due to the vibration of δs(Si-CH2),20 that at 974 cm−1 is due to the stretching vibration band of Si− OH groups, that at 787 cm−1 is due to the stretching vibration band of Si−O−Si groups, and the peak at 468 cm−1 is due to the bending vibration band of Si−O−Si groups (shown in Figure 1). These results suggested that AAPTS had been successfully grafted onto the surface of silica gel after modification. The rough surface of the imprinted silica gel sorbent was exemplified by the electron micrographs. Figure 2 presents the

Figure 3. Effect of pH on the adsorption capacity of As(V)-imprinted amino-functionalized silica gel sorbent for As(V) and As(III): Concentration of As(V) = 100 mg L−1, concentration of As(III) = 100 mg L−1, time = 20 min, temperature = 25 °C.

2.7−9.2, the removal efficiency of As(V) increased with increasing pH, was nearly constant above pH 3.7. The reduction of As(V) adsorption at pH below 3.7 might be caused by the strong competition between proton concentration and As(V) concentration. The As(V) adsorption could be responsible for a complexation between As(V) and the nitrogen atoms of the amino groups through electrostatic effect. The arsenic(V) removal acidity for subsequent experiments was selected in the pH range 3.7−9.2. As can be seen, As(III) removal was not favorable, and the removal rate of As(III) was about 10% and remained relatively constant in the pH range tested. These results showed that in highly acidic medium, where the surfaces of imprinted silica gel sorbent were protoned and As(III) mostly existed in the form of neutral H3AsO3 species,24,25 only physical adsorption driving forces between H3AsO3 and the imprinted silica gel sorbent were present, resulting in less adsorption. Kinetic Curve of Binding Rate. The adsorption rate is an important parameter used to image the adsorption process. Figure 4 shows the time dependence of the binding amounts of the imprinted silica gel sorbent for As(V) ions. As seen here, the As(V) ions uptake was rapid up to 20 min and then, the curve levels off as equilibrium was reached at the pH 6. In the adsorption process, at low coverage, removal of As(V) ions was very rapid, but as the coverage increase, the number of available surface sites came down, and the adsorption rate decreased until equilibrium was approached. This fast adsorption equilibrium of the imprinted silica gel sorbent was most probably due to its smaller diffusion barrier for As(V) on the surface of the imprinted silica gel sorbent. The smaller diffusion resistances lead to As(V) ions easily entering into the cavities or easily binding with the recognition sites.16,17

Figure 2. SEM of (A) As(V)-imprinted amino-functionalized silica gel and (B) nonimprinted silica gel sorbent.

morphological difference of imprinted silica gel sorbent (Figure 2A) and nonimprinted silica gel sorbent (Figure 2B). However, there were obviously some holes existing within the imprinted silica gel sorbent. These strongly indicated that a template imprint was formed within the imprinted silica gel sorbent. Regarding the difference of two sorbents, it was important to mention the general role As(V) ion played in the preparation of the imprinted polymer. Assembling with metal as the pivot, as already explained, monomers were regularly positioned around the templates via a coordinating bridge. Because the coordination bond was stronger than the hydrogen bond used in the imprinted silica gel sorbent, the relative motion of monomer template was largely restricted. After polymerization and removed template, the imprint with a relative higher fidelity was thus left behind. As a result, the prepared imprinted silica gel sorbent, in logic, could be expected to show a better recognition toward the imprint As(V) ion. The specific surface area of the imprinted silica gel sorbent was found to be 96 m2 g−1. Effects of pH. pH is one of the most important parameters controlling the adsorption process of metal ions. This is due to the fact partly that hydrogen ions themselves are strong 5219



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Adsorption Capacity. The adsorption capacity is an important factor to evaluate the imprinted polymer. Figure 5

Figure 4. Adsorption rates of As(V) ions on the As(V)-imprinted amino-functionalized silica gel sorbent: Concentration of As(V) = 100 mg L−1, pH 6, temperature = 25 °C. Figure 5. Adsorption capacity of As(V)-imprinted amino-functionalized silica gel sorbent for As(V): pH 6, time = 20 min, temperature = 25 °C.

Two different kinetic models (the pseudofirst-order and pseudosecond-order kinetic models) were used to fit the experimental data. The linear expression of pseudofirst-order kinetic model for the adsorption of solid/liquid systems is given as:26 lg(qeq − qt) = lgqeq − k1t /2.303

shows the initial concentration of As(V) ions dependence of the adsorbed amount of As(V) onto the imprinted silica gel sorbent. The adsorption values increased with the increase of initial concentrations of As(V), then the adsorption capacity reached a plateau. The static adsorption capacity of the imprinted silica gel sorbent was 16.1 mg g−1. Langmuir and Freundlich equations are two common isotherm models to describe the behavior of sorbents and the correlation between adsorption parameters. In this work, the two models were used to test the adsorption process of the imprinted silica gel sorbent. The Langmuir model assumes monolayer coverage of adsorbate over a homogeneous adsorbent surface. The binding sites are also assumed to be energetically equivalent and distant from each other, so there are no interactions between molecules adsorbed on adjacent sites. The Langmuir model can be described in a linear expression as eq 8.28

(6)

where qt is the adsorption capacity at time t (mg g−1) and k1 (min−1) is the rate constant of the pseudofirst adsorption. The rate constant k1 and correlation coefficients are calculated from the linear plots of log(qeq − qt) versus t and listed in Table 1. However, linearity of the plots did not necessarily ensure the pseudofirst-order mechanism. There was a large deviation between the calculated values and the experimental values of adsorption capacity. The pseudofirst-order kinetics was therefore less likely to explain the rate processes. The kinetic data were further analyzed using Ho’s pseudosecond-order kinetics model. This model is based on the assumption the sorption follows second order chemisorptions.27 It can be represented in a linear expression as eq 7

Ceq /qeq = 1/(qmax b) + Ceq /qmax

(8)

2

t /qt = 1/k2qeq + t /qeq

(7)

where qe is the amount of adsorbed As(V) in the adsorbent (mg g−1), Ce is the equilibrium ion concentration in solution (mg L−1), b (L mg−1) is the equilibrium constant related to the adsorption energy, and qmax is the maximum adsorption capacity (mg g−1). The values of qmax and b can be determined experimentally by plotting Ceq versus Ceq/qeq. The Freundlich expression is an exponential equation that describes reversible adsorption and is not restricted to the formation of the monolayer. The linear form of the empirical equation is expressed as eq 9.29

where k 2 (g·mg−1 ·min−1) is the rate constant of the pseudosecond-order adsorption. The rate constant k2, the qeq value and the corresponding linear regression correlation coefficient r2 are calculated from the linear plots of t/qt against t and given in Table 1. The values of qeq (cal), and k2 for the imprinted silica gel sorbent were 17.4 mg g−1 and 8.84 × 10−3 g mg−1 min−1, respectively. The straight lines with extremely high correlation coefficients (r2 > 0.99) were obtained. In addition, the calculated qeq values also agreed with the experimental data in the case of pseudosecond-order kinetics. These results suggested that the adsorption data of the imprinted silica gel sorbent were well represented by pseudosecond-order kinetics model.

lg qeq = lg kF + (1/n)lg Ceq

(9)

where KF and n are the Freundlich constants; Ceq is the equilibrium ion concentration in solution (mg L−1).

Table 1. Comparison of the Pseudofirst- and Pseudosecond-Order Constants pseudofirst-order model −1

−1

−1

pseudosecond-order model 2

qeq (exp) (mg g )

k1 (min )

qeq (cal) (mg g )

r

16.1

0.05

6.9

0.7619 5220

k2 ( × 10

−3

−1

g mg

8.84

min−1)

qeq (cal) (mg g−1)

r2

17.4

0.9916



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The linear Langmuir and Freundlich plots for the adsorption of As(V) onto the imprinted silica gel sorbent were obtained by plotting Ce/qe vs Ce and lg qe vs log Ce, respectively. Table 2 Table 2. Constants of Langmuir and Freundlich Isotherms Langmuir adsorption isotherm

Freundlich adsorption isotherm

qmax = 17.1 mg g−1 b = 0.186 L mg−1 R2 = 0.9996

KF = 7.32 n = 2.52 R2 = 0.9690

shows the kinetic constants of the Langmuir and Freundlich isotherms. Compare the correlation coefficients (r2) values of isotherms, the data were demonstrated in more accordance with the Langmuir isotherm model than the Freundlich isotherm model, while a low difference of qmax values between the experiment (16.1 mg g−1) and calculation (17.1 mg g−1) was observed, indicating a good Langmuir isotherms fit to the experimental data. Thus, the adsorption behavior of the imprinted silica gel sorbent for As(V) mostly belonged to monolayer adsorption. Thermodynamic Parameters. Thermodynamic parameters for As(V) adsorption were quantified to increase the utility of measured data and fit parameters to a broader range of temperatures. The thermodynamic parameters can be determined from the thermodynamic equilibrium constant, K0 (or the thermodynamic distribution coefficient). The standard Gibbs free energy ΔGo (kJ mol−1), standard enthalpy change ΔHo (kJ mol−1), and standard entropy change ΔSo (J mol−1 K−1) are calculated using the following equations ΔGo = −RT ln K 0

Table 3. Values of Various Thermodynamic Parameters for Adsorption of As(V) on As(V)-Imprinted AminoFunctionalized Silica Gel Sorbent T (K)

(11)

K0 can be defined as:30,31 γ C a K0 = s = s s ae γe Ce

(12)

where as is the activity of adsorbed As(V), ae is the activity of As(V) in solution at equilibrium, γs is the activity coefficient of adsorbed As(V), γe is the activity coefficient of As(V) in equilibrium solution, Cs is the As(V) adsorbed on the imprinted silica gel sorbent (mmol g −1), and Ce is the As(V) concentration in equilibrium solution (mmol mL−1). The expression of K0 can be simplified by assuming that the concentration in the solution approaches zero resulting in Cs→ 0 and Ce→0 and the activity coefficients approach unity at the every low concentration; that is, the difference between the activity and the concentration of hydrogen ion diminishes as the solution becomes more dilute.30,31 The behavior of solutions approach the behavior of the ideal solution. Equation 13 can be written as lim C Cs ⎯⎯⎯→ 0 s = Ce

as = K0 ae

thermodynamic constants

298.15

308.15

318.15

K0 ΔGo (kJ mol−1) ΔHo (kJ mol−1) ΔSo (J mol−1 K−1)

31.994 −8.59 9.65 61.14

35.995 −9.18 9.65 61.14

40.882 −9.82 9.65 61.14

indicated spontaneous adsorption and the degree of spontaneity of the reaction increased with increasing temperature. ln K0 was plotted against 1/T to calculate ΔHo and ΔSo from the slope and intercept, respectively (Figure 7).32 The positive

(10)

ΔS o ΔH o − R RT

ln K 0 =

Figure 6. Plots of ln(Cs/Ce) vs Cs at various temperatures.

Figure 7. Variation in equilibrium constant (K0) as a function of temperature (1/T).

standard enthalpy change of 9.65 kJ·mol−1 for this study suggested that the adsorption of As(V) by the imprinted silica gel sorbent was endothermic, which was supported by the increasing adsorption of As(V) with increase in temperature. The positive standard entropy change (61.14 J mol−1 K−1) reflected the affinity of the imprinted silica gel sorbent toward As(V). Selective Adsorption. Competitive adsorptions of As(V)/ Cd(II), As(V)/Cu(II), As(V)/Pb(II), As(V)/Ni(II), and As(V)/P(V) were investigated in their double mixture system. A relative selectivity coefficients (k′) of the imprinted silica gel

(13)

K0 at different temperatures was determined by plotting ln(Cs/ Ce) versus Cs (Figure 6) and extrapolating Cs to zero.30,31 The straight line obtained was fitted to the points by least-squares analysis. K0 increased with temperature indicating that the adsorption was endothermic (Table 3). Negative values of ΔGo 5221



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used repeatedly without losing their adsorption capacities significantly. Adsorption capacity of the imprinted silica gel sorbent was decreased only 8.5% after five repeated adsorption−desorption cycles. Application. The removal rates of the imprinted silica gel sorbent for As(V) from the mutli-competitive synthetic wastewater (72.5, 75.6, and 76.8%, respectively) spiked with 25, 50, and 100 mg L−1 As(V) were obtained. The average removal rate of the imprinted silica gel sorbent for As(V) was about 75%. The results indicated that the imprinted silica gel sorbent could effectively remove As(V) ion from the wastewater.

sorbent for As(V)/P(V), As(V)/Cd(II), As(V)/Cu(II), As(V)/ Pb(II), and As(V)/Ni(II) were 5.67, 4.18, 4.13, 4.33 and 4.02, respectively (Table 4). The results indicated that the imprinted Table 4. Selectivity Parameters of As(V)-Imprinted AminoFunctionalized Silica Gel Sorbent for As(V) Kd metals

sorbent

Kd (As)

Kd (X)

k

k′

As(V)/Cu(II)

IIP NIP IIP NIP IIP NIP IIP NIP IIP NIP

4804 3628 3887 2656 4959 3059 3401 3303 4256 3234

968 3023 940 2775 1430 3680 730 2845 857 3687

4.96 1.20 4.14 0.957 3.47 0.831 4.66 1.16 4.97 0.877

4.13

As(V)/Pb(II) As(V)/Cd(II) As(V)/Ni(II) As(V)/P(V)

4.33

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CONCLUSIONS In this work, a new As(V)-imprinted amino-functionalized silica gel sorbent was used by surface imprinting technique. It was found that the synthesized product exhibited good adsorption characteristics, such as fast adsorption kinetics, and relatively high selectivity for As(V). The experimental data followed the Langmuir model of monolayer adsorption. The kinetic data fitted pseudosecond-order kinetic model in compared to pseudofirst-order kinetic model, due to the better correlation with the experimental data. The negative values of ΔGo at the experimental temperature for 298.15, 308.15, and 318.15 K indicated that adsorption of As(V) on the imprinted silica gel sorbent was spontaneous. The positive values of ΔHo and ΔSo exhibited that adsorption of As(V) on imprinted silica gel sorbent was endothermic and the randomness decreased with the adsorption of As(V) on imprinted silica gel sorbent. The average removal rate of the imprinted sorbent for As(V) from the mutli-competitive synthetic wastewater was about 75%. From the results obtained in this study, it was concluded that the imprinted amino-functionalized silica gel sorbent could be promising for the removal of As(V) from aqueous media.

4.18 4.02 5.67

silica gel sorbent had higher selectivity for As(V) even in the presence of P(V), Cd(II), Cu(II), Pb(II), and Ni(II) interferences in the same medium. It was because the coordination-geometry selectivity of the imprinted silica gel sorbent could provide the ligand groups arranged in a suitable way required for the coordination of As(V) ion. Although some ions had similar size with As(V) ion and some of them had high affinity with the amino ligands, the imprinted silica gel sorbent still exhibited high selectivity for extraction of As(V) in the presence of these other metal ions. Desorption and Repeated Use. Desorption of the adsorbed As(V) ions from the imprinted silica gel sorbent was also studied in a batch experimental setup. Various factors are probably involved in determining rates of As(V) desorption, such as the extent of hydration of the heavy metal ions and sorbent microstructure. However, an important factor appears to be binding strength (Table 5). In this study, the desorption

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Corresponding Author

*E-mail: [email protected]. Tel.: +86-24-83684786. Fax: +8624-83676698.

Table 5. Effects of Stripping Agents on As(V) desorption stripping agents −1

0.1 mol L 1 mol L−1 3 mol L−1 3 mol L−1 1 mol L−1 3 mol L−1 3 mol L−1 3 mol L−1

EDTA HNO3 HNO3 HNO3 HCl HCl HCl HCl

time (h)

recovery (%)

2 2 2 4 2 2 3 4

35.2 62.6 88.9 94.8 57.2 80.3 93.5 95.2

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Project was sponsored by the National Science Foundation for Young Scientists of China (Grant 21107076) and the doctoral scientific research foundation of Liaoning Province of China (Grant 20111048).

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time was found to be 4 h. Desorption ratios were very high (up to 95.2%). Compared with HNO3, HCl was cheap. Thus, 3 mol L−1 HCl solution was used as a desorption agent. It was most possibly due to complete protonation of donating nitrogen heteroatoms of binding sites in the cavities of the imprinted silica gel sorbent, when 3 mol L−1 HCl penetrated into polymeric network. The regeneration of the commercial imprinted silica gel sorbent is likely to be a key factor in improving wastewater process economics. To show the reusability of the imprinted silica gel sorbent, we repeated adsorption−desorption cycle five times by using the same imprinted silica gel sorbent. The results showed that the imprinted silica gel sorbent could be

REFERENCES

(1) Muñoz, E.; Palmero, S. Analysis and speciation of arsenic by stripping potentiometry: a review. Talanta 2005, 65, 613. (2) Budinova, T.; Petrov, N.; Razvigorova, M.; Parra, J.; Galiatsatou, P. Removal of arsenic(III) from aqueous solution by activated carbons prepared from solvent extracted olive pulp and olive stones. Ind. Eng. Chem. Res. 2006, 45, 1896. (3) Rivas, B. L.; Muñoz, C. Functional water-insoluble polymers with ability to remove arsenic(V). Polym. Bull. 2010, 65, 1. (4) Choonga, T. S. Y.; Chuaha, T. G.; Robiaha, Y.; Koaya, F. L. G.; Aznib, I. Arsenic toxicity, health hazards and removal techniques from water: an overview. Desalination 2007, 217, 139. (5) Wulff, G. Molecular imprinting in cross-linked materials with the aid of molecular templatesa way towards artificial antibodies. Angew. Chem., Int. Ed. 1995, 34, 1812.

5222



Article

(6) Masqueè, N.; Marceè, R. M.; Borrull, F. Molecularly imprinted polymers: new tailor-made materials for selective solid-phase extraction. TrAC, Trends Anal. Chem. 2001, 20, 477. (7) Haupt, K. Peer reviewed: molecularly imprinted polymers: the next generation. Anal. Chem. 2003, 75, 376A. (8) Haupt, K.; Mosbach, K. Molecularly imprinted polymers and their use in biomimetic sensors. Chem. Rev. 2000, 100, 2495. (9) Rao, T. P.; Kala, R.; Daniel, S. Metal ion-imprinted polymers novel materials for selective recognition of inorganics. Anal. Chim. Acta 2006, 578, 105. (10) Tan, C. J.; Tong, Y. W. Molecularly imprinted beads by surface imprinting. Anal. Bioanal. Chem. 2007, 389, 369. (11) Shamsipur, M.; Fasihi, J.; Ashtari, K. Grafting of ion-imprinted polymers on the surface of silica gel particles through covalently surface-bound initiators: a selective sorbent for uranyl ion. Anal. Chem. 2007, 79, 7116. (12) Chang, X.; Wang, X.; Jiang, N.; He, Q.; Zhai, Y.; Zhu, X.; Hu, Z. Silica gel surface-imprinted solid-phase extraction of Zr(IV) from aqueous solutions. Microchim. Acta 2008, 162, 113. (13) Jiang, N.; Chang, X.; Zheng, H.; He, Q.; Hu, Z. Selective solidphase extraction of nickel(II) using a surface-imprinted silica gel sorbent. Anal. Chim. Acta 2006, 577, 225. (14) Chang, X.; Jiang, N.; Zheng, H.; He, Q.; Hu, Z.; Zhai, Y.; Cui, Y. Solid-phase extraction of iron(III) with an ion-imprinted functionalized silica gel sorbent prepared by a surface imprinting technique. Talanta 2007, 71, 38. (15) Buhani; Narsito; Nuryono; Kunarti, E. S. Production of metal ion imprinted polymer from mercapto−silica through sol−gel process as selective adsorbent of cadmium. Desalination 2010, 251, 83. (16) Fang, G.-Z.; Tan, J.; Yan, X.-P. An ion-imprinted functionalized silica gel sorbent prepared by a surface imprinting technique combined with a sol−gel process for selective solid-phase extraction of cadmium(II). Anal. Chem. 2005, 77, 1734. (17) Lu, Y.-K.; Yan, X.-P. An imprinted organic-inorganic hybrid sorbent for selective separation of cadmium from aqueous solution. Anal. Chem. 2004, 76, 453. (18) Chen, D.; Huang, C.; He, M.; Hu, B. Separation and preconcentration of inorganic arsenic species in natural water samples with 3-(2-aminoethylamino) propyltrimethoxysilane modified ordered mesoporous silica micro-column and their determination by inductively coupled plasma optical emission spectrometry. J. Hazard. Mater. 2009, 164, 1146. (19) Alié, C.; Lambert, S.; Heinrichs, B.; Pirard, J.-P. Nucleation phenomenon in silica xerogels and Pd/SiO2, Ag/SiO2, Cu/SiO2 cogelled catalysts. J. Sol−Gel Sci. Technol. 2003, 26, 827. (20) Bois, L.; Bonhommé, A.; Ribes, A.; Pais, B.; Raffin, G.; Tessier, F. Functionalized silica for heavy metal ions adsorption. Colloids Surf., A 2003, 221, 221. (21) Aguado, J.; Arsuaga, J. M.; Arencibia, A.; Lindo, M.; Gascón, V. Aqueous heavy metals removal by adsorption on amine-functionalized mesoporous silica. J. Hazard. Mater. 2009, 163, 213. (22) Michelsen, O. B. Photometric determination of phosphorus as molybdovanadophosphoric acid. Anal. Chem. 1957, 29, 60. (23) Silva, A. L. P.; Sousab, K. S.; Germano, A. F. S.; Oliveira, V. V.; Espinola, J. G. P.; Fonseca, M. G.; Airoldi, C.; Arakakic, T.; Arakakia, L. N. H. A new organofunctionalized silica containing thioglycolic acid incorporated for divalent cations removala thermodyamic cation/ basic center interaction. Colloids Surf. A 2009, 332, 144. (24) Geucke, T.; Deowan, S. A.; Hoinkis, J.; Pätzold, Ch. Performance of a small-scale RO desalinator for arsenic removal. Desalination 2009, 239, 198. (25) Doušová, B.; Machovič, V.; Koloušek, D.; Kovanda, F.; Dorničaḱ , V. Sorption of As(V) species from aqueous systems. Water, Air, Soil Pollut. 2003, 149, 251. (26) Lagergren, S. About the theory of so-called adsorption of soluble substances. Kungliga Svenska Vetensk. Handl. 1898, 24, 1. (27) Ho, Y. S.; McKay, G. Pseudo-second-order model for sorption processes. Process Biochem. 1999, 34, 451.

(28) Langmuir, I. The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc. 1918, 40, 1361. (29) Freundlich, H. M. F. Ü ber die adsorption in lösungen. Z. Phys. Chem. 1906, 57, 385. (30) Calvet, R. Adsorption of organic-chemicals in soils. Environ. Health Persp. 1989, 83, 145. (31) Biggar, J. W.; Cheung, M. W. Adsorption of picloram (4-amino3,5,6-trichloropicolinic acid) on Panoche, Ephrata, and Palouse soils thermodynamic approach to adsorption mechanism. Soil Sci. Soc. Am. J. 1973, 37, 863. (32) Zheng, H.; Wang, Y.; Zheng, Y.; Zhang, H.; Liang, S.; Long, M. Equilibrium, kinetic and thermodynamic studies on the sorption of 4hydroxyphenol on Cr-bentonite. Chem. Eng. J. 2008, 143, 117.

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Selective Removal of Arsenic(V) from Aqueous Solution Using A

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