Ionic Imprinted Silica-Supported Hybrid Sorbent with an Anchored

Article pubs.acs.org/IECR

Ionic Imprinted Silica-Supported Hybrid Sorbent with an Anchored Chelating Schiff Base for Selective Removal of Cadmium(II) Ions from Aqueous Media Hong-Tao Fan,* Jin-Xiu Liu, Hui Yao, Zhi-Gang Zhang, Feng Yan, and Wen-Xiu Li* Liaoning Provincial Key Laboratory of Chemical Seperation Technology, Shenyang University of Chemical Technology, Shenyang 110142, China ABSTRACT: A novel Cd(II)-ion-imprinted silica-supported hybrid sorbent functionalized with tetradentate Schiff bases ligands derived from 2-thiophenecarboxaldehyde and 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane for the selective removal of Cd(II) ions from aqueous media had been synthesized by a surface-imprinting technique in conjunction with a sol− gel process and characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, N2 adsorption− desorption isotherms, and thermogravimetric analysis. The results revealed that the static adsorption capacity of Cd(II) was 29.1 mg·g−1. The imprinted hybrid sorbent exhibited a fast equilibrium time with 30 min, had a stable binding capacity in the range of pH 3.5−9.5, and showed the selective adsorption for Cd(II) in binary ions systems of Cd(II)/Zn(II), Cd(II)/Ni(II), Cd(II)/ Cu(II), and Cd(II)/Pb(II). The adsorption/desorption cycles of the imprinted hybrid sorbent could be up to nine times. Langmuir, Freundlich, and Dubinin−Radushkevich isotherm models were applied to analyze the experimental data, and the best interpretation for the experimental data was given by the Langmuir isotherm equation. The adsorption kinetics could be fitted by pseudo-second-order rate equation wonderfully compared with pseudo-first-order, Elovich, and intraparticle diffusion models. Negative values of ΔG° indicated the spontaneous adsorption, and the degree of the spontaneity increased with increasing temperature. The thermodynamic parameters of ΔH° and ΔS° were 11.77 kJ·mol−1 and 74.98 J·K−1·mol−1, respectively. Thus, this novel imprinted hybrid sorbent was a favorable, useful, and promising good candidate material for the selective removal of Cd(II) ions from aqueous media. capacities and strong affinities for the target metal ions.20−25 However, the low selectivity of these materials, due to no consideration of the stereochemical interactions of the ligands with metal ion, and the low efficiency because of the negative influence of coexisting interferences in real cases strongly restrict their application. Thus, research on the development of more effective and selective sorbents to remove Cd(II) is increasingly focused. Ion imprinting is a versatile technique for preparing polymeric materials that are capable of high ionic recognition. With this technique, the selectivity of a certain metal ion can be achieved during polymer synthesis which is carried out in the presence of a complex of the given metal ion as a template with the constituent monomer containing functional groups. The subsequent removal of the template metal ion leads to the formation of cavities in the polymeric structure which are used as the specific recognition sites.26−28 Ion-imprinting technology based on the use of highly selective ion-imprinted polymer networks has widely been exploited as the new sorbents for the selective preconcentration, separation, or removal of Cd(II) ions.29−35 More recently, the Cd(II)-imprinted silica-supported hybrid sorbents using the functionality of organosilanes (e.g., 3mercaptopropyltrimethoxysilane,34 3-thiocyanatopropyltriethoxysilane,35 3-(2-aminoethylamino)propyltrimethoxysilane,36

1. INTRODUCTION Cadmium (Cd) is a member of group IIb in the periodic table of elements and one of the highly toxic elements among the priority pollutants regulated by the U.S. Environmental Protection Agency (U.S. EPA) because of nonbiodegradability and toxicity to human health.1 Cd(II) is highly toxic at low concentrations and can accumulate in living organisms by smoking cigarettes, eating contaminated food, and drinking contaminated water.2,3 It has been classified as a human carcinogen by the International Agency for Research on Cancer.4 A good number of technologies have been employed for the removal of Cd(II) from aqueous solution including solvent extraction,5 chemical precipitation,6 cementation,7 ion exchange,8 membrane separation,9 coagulation,10 flotation,11 and adsorption.12,13 Among them, adsorption is one of the most effective approaches for the removal of Cd(II) at low concentrations from aqueous solutions.14 Thus, great efforts have been made to develop various materials for the remediation of Cd(II) pollution in aquatic environments. Many kinds of sorbents are widely used with the adsorption technique for the removal of Cd(II), such as carbon materials, minerals, industrial byproducts or waste materials, agricultural waste biomass, and the synthetic sorbents.15−19 Among these materials, increasing interest in recent decades has been focused on silica-supported organic−inorganic hybrid materials with specific chelating groups containing O, N, S, and P donor atoms derived from Schiff bases reaction between an amine group and an aldehyde group due to relatively high loading © 2013 American Chemical Society

Received: Revised: Accepted: Published: 369

August 23, 2013 December 13, 2013 December 16, 2013 December 16, 2013 dx.doi.org/10.1021/ie4027814 | Ind. Eng. Chem. Res. 2014, 53, 369−378

Industrial & Engineering Chemistry Research

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and 3-aminopropyltrimethoxysilane37) by the surface-imprinted technique combined with a sol−gel processing, seem to be of great interest with outstanding advantages such as high selectivity, high adsorption capacity, fast mass transport rates, and good mechanical and thermal stabilities.34−37 However, in our knowledge, few functional precursors containing Schiff bases have been employed to prepare Cd(II)-imprinted hybrid sorbent. This work attempted to prepare new ion-imprinted silicasupported hybrid sorbents functionalized with tetradentate Schiff base ligands based on the surface-imprinted technique and sol−gel process. Its main characteristic features for the selective removal of Cd(II) from aqueous solution were described and discussed in detail. The kinetic and thermodynamic behaviors of the adsorption of Cd(II) ion onto the imprinted hybrid sorbent were also studied.

operating at 30 kV. The specific surface area was obtained from nitrogen adsorption isotherms at 77 K by a surface area analyzer (ASAP 2010C, Micromeritics, Norcross, GA, USA). The surface area was determined according to Brunauer− Emmett−Teller (BET) method. The pore size distribution was calculated using the Barrett−Joyner−Halenda (BJH) model. All of the samples were degassed at 423 K under vacuum before analysis. Thermogravimetric analysis (TGA-DTA) was performed on HCT-2 thermoanalyzer equipment (Beijing Scientific Instrument Factory, China), which was operated in a nitrogen atmosphere at a heating rate of 10 °C min−1 from 50 to 800 °C with an initial mass of approximately 5.00 mg of solid. A pHS-3C digital pH meter (Shanghai Precision & Scientific Instrument Co., Ltd., Shanghai, China) was used for the pH measurements. 2.4. Static Adsorption Experiments. Batch adsorption technique was used for the study of Cd(II) adsorption from solutions. The optimal time required for Cd(II) adsorption was determined by adding 0.1 g of the imprinted hybrid sorbent to 25 mL of Cd(II) solution (300 mg·L−1) at different contact times (5−60 min) at pH 5. The effect of Cd(II) initial concentrations on adsorption of Cd(II) by the imprinted hybrid sorbent was conducted as follows: 0.1 g of the imprinted hybrid sorbent was suspended in 25 mL of Cd(II) solutions containing different concentrations (50−500 mg·L−1) at pH 5 for 45 min. The effect of solution pH was investigated as follows: 0.1 g of the imprinted hybrid sorbent was suspended in 25 mL of Cd(II) solution (300 mg·L−1) in the range of pH 1.5−10.5 for 45 min. The pH values of the bulk solutions were adjusted using 2% HCl or NaOH solution. In order to test the effect of temperature variation on Cd(II) uptake, 0.1 g of the imprinted hybrid sorbent was added to 25 mL of Cd(II) solution (300 mg·L−1) from 25 to 45 °C at pH 5 for 45 min. To evaluate the competitive adsorption, 0.1 g of the imprinted sorbent or nonimprinted sorbent was equilibrated with 25 mL of binary metal mixture solutions of Cd(II)/Ni(II), Cd(II)/ Cu(II), Cd(II)/Pb(II), and Cd(II)/Zn(II) containing 50 mg· L−1 of above each individual heavy metal ions at pH 5 for 45 min. The selectivity parameters of the imprinted sorbent and nonimprinted sorbents for Cd(II) in binary metal mixture solutions were calculated with the following equations:38

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. The ethanol, CdCl2· 2.5H 2 O, NiCl 2 ·6H 2 O, ZnCl 2 , CuCl 2 ·2H 2 O, Pb(NO 3 ) 2 , NaOH, HCl, NH4OH, and tetraethoxysilicate were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. 3-[2-(2-Aminoethylamino)ethylamino]propyltrimethoxysilane and 2-thiophenecarboxaldehyde were purchased from SigmaAldrich. Unless stated otherwise, all reagents used were of the highest available purity and of at least analytical grade. The stock solutions of metal ions (1000 mg·L−1) were purchased from the National Research Center for Standard Materials (Beijing, China). The working solutions were prepared by the series dilution of the stock solutions immediately prior to their use. The solutions were prepared weekly and stored in a refrigerator. Cellulose filter membrane (0.45 μm pore size) was obtained by Dikma (Tianjin, China). Deionized water was used throughout this work. 2.2. Preparation of the Cadmium(II)-Imprinted Hybrid Sorbent. Solution a was prepared by mixing 0.05 mol of 2thiophenecarboxaldehyde and 0.05 mol of 3-[2-(2aminoethylamino)ethylamino]propyltrimethoxysilane in 20 mL of ethanol solution with stirring for 2 h at 60 °C and being followed by addition of 1.92 g of CdCl2·2.5 H2O which was dissolved in 20 mL of heated ethanol solution. The mixtures were stirred for 2 h at 60 °C. Solution b was prepared by mixing tetraethoxysilicate and water with the volume ratio of 2:1, and the solution was adjusted to pH 2 with the addition of 6 mol·L−1 HCl solution. The mixtures were stirred for 30 min. Solutions a and b were mixed, and 0.1 mol·L−1 of NH4OH solution was added up to pH ≈ 5. The obtained gel was left for a night and rinsed with 60/40 (v/v) of ethanol/water for 12 h. The product was obtained by filtration, washed with 1 mol·L−1 HCl to remove Cd(II) from the sorbent, and then neutralized by water (pH ≈ 7) as well as dried under a vacuum at a temperature of 80 °C for 12 h. The final product was ground and sieved by 200 mesh size. For comparison, the nonimprinted hybrid sorbent was also prepared using an identical procedure, but without the addition of CdCl2·2.5 H2O. 2.3. Characterization of the Cadmium(II)-Imprinted Hybrid Sorbent. The imprinted hybrid sorbent was characterized using Fourier transmission infrared (FT-IR) spectroscopy (Spectrum One, Perkin-Elmer) with a resolution of 1 cm−1 in the range from 400 to 4000 cm−1. The surface morphologies of the imprinted and nonimprinted hybrid sorbents were observed using a Shimadzu SSX-550 scanning electron microscope (SEM) at the desired magnification

q = (C0 − Ce)V /1000W

(1)

D = q/Ce

(2)

α = DCd /DM

(3)

αr = αimprinted /αnonimprinted

(4)

where q represents the experimental value of adsorption capacity (mg·g−1), C0 and Ce represent the initial and final concentrations of metal ions (mg·L−1), W is the mass of sorbent (g), V is the volume of metal ion solution (mL), D is the distribution ratio (L·g−1), α is the selectivity coefficient, and DCd and DM represent the distribution ratios of Cd(II) to Ni(II), Pb(II), Cu(II), or Zn(II). αr is the relative selectivity coefficient; αimprinted and αnonimprinted represent the selectivity factors of the imprinted and nonimprinted hybrid sorbents. 2.5. Desorption and Regeneration. To investigate the possibility of recycling of the imprinted hybrid sorbents, the desorption and regeneration processes were also studied. The desorption experiments were performed using 1 mol·L−1 HCl solution in batch method. A 0.5 g amount of Cd(II)-loaded 370

dx.doi.org/10.1021/ie4027814 | Ind. Eng. Chem. Res. 2014, 53, 369−378


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Scheme 1. Preparation Procedure of Cadmium(II)-Imprinted Hybrid Sorbents

imprinted hybrid sorbent were formed by the self-condensation and co-condensation of the complexes and tetraethoxysilicate. Finally, after the removal of Cd(II) ions by acid leaching, the Cd(II)-imprinted hybrid sorbent with a predetermined arrangement of ligands and tailored binding sites for Cd(II) was obtained. 3.2. Characteristics. The FT-IR spectra of the imprinted hybrid sorbents are shown in Figure 1. The strong absorption

imprinted hybrid sorbents was suspended in 25 mL of 1 mol· L−1 HCl solution for 2 h. The solution mixture was filtered and the sorbent washed several times with distilled water in order to remove excess acid. It was then treated with 25 mL of Cd(II) solution (300 mg·L−1), and the above procedure was repeated for nine cycles using the same sorbent. 2.6. Selective Removal of Cadmium(II) Ion from Mine Wastewater. The selective removal of Cd(II) was performed at room temperature, in the flasks stirred magnetically, by adding 200 mg of the imprinted hybrid sorbents in 50 mL of untreated mine wastewater which was collected from the surface of a wastewater canal (Nanpiao, China) on Oct. 28, 2013. After adsorption equilibrium, the concentrations of the metals in the remaining solution were measured. Major cation concentrations, chemical oxygen demand (COD), and pH of the untreated mine wastewater were as follow: [K(I)] = 7.1 mg· L−1, [Na(I)] = 48.7 mg·L−1, [Ca(II)] = 75.2 mg·L−1, [Mg(II)] = 9.9 mg·L−1, [Cu(II)] = 0.27 mg·L−1, [Cd(II)] = 0.54 mg·L−1, [Pb(II)] = 2.24 mg·L−1, [Ni(II)] = 0.21 mg·L−1, [Zn(II)] = 0.45 mg·L−1, [COD] = 208.2 mg·L−1, and pH = 5.1. Unless stated otherwise, all laboratory measurements were performed in triplicate. In all of the above batch experiments, the mixtures were magnetically stirred with a constant rate of 200 rpm at 25 °C. The samples were centrifuged at 3000 rpm for 5 min and then filtered with a 0.45 μm pore size cellulose filter membrane to remove any undissolved particles before analysis. The concentrations of metal ions in the aqueous solutions were measured by an AA-6300c flame atomic absorption spectrometer (FAAS, Shimadzu Corp., Kyoto, Japan) after appropriate dilutions and acidification to pH ∼ 2 adjusted with HNO3.

Figure 1. IR spectra of Cd(II)-imprinted hybrid and nonimprinted hybrid sorbents.

bands at 3418 cm−1 represent stretching vibrations of the -OH groups due to the remaining silanol groups (Si−OH) in the imprinted hybrid sorbents.39 The weak intense band at 2887 cm−1 is assigned to methylene groups. The CN stretching vibration of azomethine groups is found at 1635 cm−1.40 The stretching vibration of azomethine hydrogen (NCH) gives three bands at 2979 cm−1.41 The bending vibrations of N−H groups of amino groups appear at 1558 cm−1.42 The absorption bands at 1068 and 791 cm−1 are attributed to Si−O−Si and Si− O stretching vibrations, respectively. The band at 959 cm−1 is due to Si−OH stretching vibrations. The absorption peak at 457 cm−1 corresponds to the bending vibration of Si−O−Si groups. These results suggested that the ligands containing

3. RESULTS AND DISCUSSION 3.1. Preparation. The schematic representation of the synthesis route and ion-recognition ability of Cd(II)-imprinted hybrid sorbent are shown in Scheme 1. First, the tetradentate ligands containing N and S donor atoms were formed by Schiff base reaction between the NH 2 groups of 3-[2-(2aminoethylamino)ethylamino]propyltrimethoxysilane and the aldehyde groups of 2-thiophenecarboxaldehyde. Second, Cd(II) ions strongly interacted with the Schiff base ligands and formed stable complexes in the synthesis process. Third, the Cd(II)371



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Figure 2. SEM of Cd(II)-imprinted (A) and nonimprinted (B) hybrid sorbents.

steadily due to Cd(II) ions used as the templates in the synthesis process. The results implied that the imprinted cavities and specific binding sites of Cd(II) ion on the surface of silica were formed. The thermogravimetric curve of the imprinted hybrid sorbent is shown in Figure 3. An abrupt weight loss occurred

Schiff base had been grafted successfully onto the surface of silica. The surface morphologies of the imprinted and nonimprinted hybrid sorbents are evaluated with SEM and the results are presented in Figure 2. As it could be seen, a flat and smooth surface was observed for the nonimprinted sorbent. Compared with the nonimprinted sorbent, the imprinted hybrid sorbent synthesized presented an irregular, rough surface, which was more beneficial to the fast binding of Cd(II) ions in the cavities on the surface of the imprinted hybrid sorbent. These results were in agreement with specific surface area data, where the imprinted hybrid sorbent presented a value of 254.2 m2·g−1, much higher than nonimprinted sorbent with 156.6 m2·g−1. The effect of Cd(II) ions, as the templates in the synthesis process, was fundamental and significant for the morphological features of the imprinted hybrid sorbent. The physical surface characteristics, such as BET specific surface area, BJH adsorption, and desorption cumulative volumes, BJH adsorption and desorption pore diameters, and average pore diameters are given in Table 1. The imprinted and Table 1. Structural Properties of the Imprinted and Nonimprinted Hybrid Sorbents physical characteristics BET surface area BJH adsorption cumulative volume BJH adsorption pore diameter BJH desorption cumulative volume BJH desorption pore diameter average pore diameter

imprinted hybrid sorbent

nonimprinted hybrid sorbent

254.2 m2·g−1 0.531 cm3·g−1

156.6 m2·g−1 1.231 cm3·g−1

5.6 nm

4.3 nm

0.569 cm3·g−1

1.234 cm3·g−1

4.9 nm

5.6 nm

8.4 nm

32.1 nm

Figure 3. DTA-TGA of Cd(II)-imprinted hybrid sorbents.

from 200 to 580 °C because of the exothermic decomposition of the organic groups on the imprinted hybrid sorbent as shown by the DTA pattern. The TGA curve showed that imprinted hybrid sorbent until 200 °C presented a loss of 3 wt % due to the adsorbed water or residual solvent corresponding with two small endothermic peaks at 135 and 153 °C in the DTA curve. From 200 to 580 °C, a weight loss of 22 wt % occurred because of the elimination of organic groups corresponding to a broad exodothermic peak at 255 °C in the DTA curve. The weight loss stayed constant above 600 °C. The total weight loss was 25 wt %. 3.3. Effect of pH. The initial solution pH is an important parameter in the adsorption process of metal ions from aqueous solutions and affects both the speciation and solubility of metal ions and functional groups of sorbents. To investigate the relationship between pH and adsorption capacity, the adsorption capacity of Cd(II) with pH value was observed from pH 1.5 to 10.5 as shown in Figure 4. It could be seen that a weak adsorption in strong acidic medium (pH 1.5−3.5) was obtained due to the protonation of active sites, which adversely

nonimprinted hybrid sorbent exhibited the average pores diameter of 8.4 and 32.1 nm, respectively. The BJH adsorption pore volumes of the imprinted and nonimprinted hybrid sorbent were 0.531 and 1.231 cm3·g−1, respectively. The BJH desorption pore volumes of the imprinted and nonimprinted hybrid sorbents were 0.569 and 1.234 cm3·g−1, respectively. Compared with the nonimprinted hybrid sorbent, the pore volumes and pore sizes of imprinted hybrid sorbents decreased 372



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3.5. Adsorption Kinetics. Figure 6 shows the effect of contact time on the adsorption of Cd(II) ions by the imprinted

Figure 4. Effect of pH on the adsorption capacity of Cd(II)-imprinted hybrid sorbents for Cd(II): Concentration of Cd(II) = 300 mg·L−1; pH = 5; time = 45 min; temperature = 25 °C. Figure 6. Effect of contact time on the adsorption equilibrium for Cd(II): Concentration of Cd(II) = 300 mg·L−1; pH = 5; temperature = 25 °C.

affected the adsorption capacity. The adsorption capacity of Cd(II) increased simultaneously with increasing pH from pH 1.5 to 3.5, remained constant in the range of pH 3.5−9.5, and decreased over pH 9.5. At pH above 9.5, hydroxides of Cd(II) were formed and led to the drop of adsorption capacity of Cd(II). It was clear that stable adsorption of Cd(II) occurred in the range of pH 3.5−9.5, and decreased at lower and higher pH. The pH of solution was controlled at pH 5 in all further experiments. 3.4. Adsorption Capacity. The adsorption capacity is one of the basic parameters required for the design of any batch adsorption system. The plot of the Cd(II) absorbed by the imprinted hybrid sorbent against the initial concentrations of Cd(II) is indicated in Figure 5. It could be seen that the

hybrid sorbent. As it could be seen, the adsorption capacity of Cd(II) ions increased with the contact time during the 60 min. The adsorption capacities of Cd(II) ions on the imprinted hybrid sorbent showed a rapid increase with increasing contact time in the first 30 min. After this fast initial step, the rate of the adsorption process became slower and an equilibrium value was attained. The results indicated that adsorption capacities of Cd(II) ions almost reached equilibrium with 30 min. In the adsorption process, at low coverage, removal of Cd(II) ions was very rapid, whereas with an increase of coverage, the number of available surface binding sites came down, and the adsorption rate decreased until the adsorption equilibrium was reached. This fast adsorption equilibrium of the imprinted hybrid sorbents was most probably due to the smaller diffusion barrier for Cd(II) on its surface. The contact time of 45 min was enough for further experiments. To investigate the mechanism of adsorption kinetics, several models, such as pseudo-first-order,43 pseudo-second-order,44 Elovich,45 and intraparticle diffusion models,46 were used to test the experimental data of adsorption processes. The linear form of the pseudo-first-order rate expression is log(qe − qt ) = log qe − k1t /2.303

(5)

−1

where k1 (min ) is the rate constant of the pseudo-first-order adsorption. qe and qt (mg·g−1) are the adsorption capacities at equilibrium and at time t (min), respectively. The pseudo-second-order rate expression is linearly expressed as

Figure 5. Adsorption capacity of Cd(II)-imprinted hybrid sorbents for Cd(II): pH = 5; time = 45 min; temperature = 25 °C.

t /qt = 1/k 2qe 2 + t /qe −1

(6)

−1

where k2 (g·mg ·min ) is the rate constant of the pseudosecond-order adsorption. The linear form of the Elovich equation can be expressed as

adsorption capacity of Cd(II) onto the imprinted hybrid sorbent increased with increasing initial concentrations of Cd(II) from 50 to 300 mg·L−1; then the adsorption capacity reached a plateau after 300 mg·L−1 Cd(II) solution. At high concentrations of Cd(II), a unit mass of the sorbents was exposed to a large number of Cd(II) ions, and a progressively higher number of Cd(II) ions was taken up with the gradual filling up of the appropriate binding sites. The adsorption capacity of the imprinted hybrid sorbents for Cd(II) using equilibrium experiments was 29.1 mg·g−1.

qt = 1/β ln(αβ) + 1/β ln t

(7) −1

−1

where α is the initial adsorption rate (mg·g ·min ) and β is the desorption constant (g·mg−1) during any one experiment. The intraparticle diffusion model is linearly expressed as

qt = kit 0.5 373

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Table 2. Calculated Kinetic Parameters for the Adsorption of Cd(II) onto the Imprinted Hybrid Sorbents pseudo-first-order model

pseudo-second-order model

Elovich model

intraparticle diffusion model

k1 = 0.0465 min−1 qe(calcd) = 21.4 mg·g−1 r2 = 0.9611

k2 = 5.62 × 10−3 g·mg−1·min−1 qe(calcd) = 34.1 mg·g−1 r2 = 0.9948

α = 5.09 mg·g−1·min−1 β = 0.127 g−1·mg r2 = 0.9771

ki = 3.347 mg·g−1·min−1

where ki is the intraparticle diffusion rate constant (mg·g−1· min−0.5). The rate constants k1 and qe and correlation coefficients r2 were calculated using the slope and intercept of plots of log(qe − qt) versus t. The rate constants k2 and qe and correlation coefficients r2 were calculated from the linear plots of t/qt versus t. A plot of qt vs ln t gave a linear trace with a slope of (1/β) and an intercept of 1/β ln(αβ). And ki was calculated by the slope of the straight-line portion of plotting qt vs t0.5. The results are given in Table 2. Among these models, according to the values of correlation coefficients (r2), the correlation coefficients for the pseudo-firstorder (r2 = 0.9611), Elovich (r2 = 0.9771), and intraparticle diffusion (r2 = 0.9061) models were lower compared with the pseudo-second-order kinetic model (r2 = 0.9948). The correlation coefficients for the second-order kinetic model were very close to unity, while the calculated value (34.1 mg· g−1) of static adsorption capacity almost agreed with the experimental data (29.1 mg·g−1). On the basis of the regression coefficient and calculated values of static adsorption capacity, the adsorption process of Cd(II) on the surfaces of the imprinted hybrid sorbent was found to obey the pseudosecond-order kinetic model. From these results, it supports the assumption that the rate limiting step may be a chemical sorption involving valence forces through sharing of electrons between the imprinted hybrid sorbent adsorbent and Cd(II) until the active sites on the surface of the imprinted hybrid sorbent are fully occupied by Cd(II) ions through chemical interaction.47 Similar results were reported by some researchers for a variety of adsorbate−adsorbent systems.48,49 It suggested that the imprinted cavities and the specific binding sites of Cd(II) ions on the surface of the imprinted hybrid sorbent were readily available and easily accessible probably in the sorption processes. 3.6. Adsorption Isotherms. Different models of adsorption, such as Langmuir, Freundlich, and Dubinin−Radushkevich isotherm, were used to describe the adsorption data, to calculate the corresponding constants, and to predict the theoretical capacities of Cd(II) on the surface of the imprinted hybrid sorbent. The Langmuir adsorption isotherm is the best known of all isotherms describing adsorption, is often used to describe adsorption of a solute from a liquid solution, and can be described in a linear expression as50 Ce/qe = 1/(qmax b) + Ce/qmax

r2 = 0.9061

log qe = log kF + (1/n) log Ce

(10)

where KF and n are the Freundlich constants. The Dubinin−Radushkevich isotherm is more general because it does not assume a homogeneous surface or constant adsorption potential.52,53 The linear form can be represented as ln qe = ln qs − kadε 2

(11)

where kad is the Dubinin−Radushkevich isotherm constant (mol2·kJ−2), qs is the theoretical isotherm saturation capacity (mg·g−1), ε is the Polanyi potential and calculated as follows: ε = RT ln(1 + 1/Ceq)

(12) −1

−1

where R is universal gas constant (8.314 J·mol ·K ); T is temperature (K). Ce is the concentration of Cd(II) in equilibrium solution (mol·L−1). The experimental isotherm data of Langmuir, Freundlich, and Dubinin−Radushkevich isotherms equations had been analyzed separately by linear methods from the plots of Ce/qe vs Ce, log qe vs log Ce, and ln qe vs ε2, respectively. The values of the Langmuir, Freundlich, and Dubinin−Radushkevich constants are presented in Table 3. Table 3. Isotherms Parameters for the Adsorption of Cadmium(II) by the Imprinted Hybrid Sorbents Langmuir adsorption isotherm

Freundlich adsorption isotherm

Dubinin−Radushkevich isotherm

qmax = 31.4 mg·g−1 b = 0.108 L·mg−1 r2 = 0.9987

KF = 6.67 n = 3.323 r2 = 0.9219

kad = 3 × 10−9 mol2·kJ−2 qs = 65.8 mg·g−1 r2 = 0.9547

Among the available models for adsorption data analysis, it was concluded that the Langmuir isotherm model was slightly better than Fruendlich and Dubinin−Radushkevich isotherms to fit Cd(II) adsorption data well according to the values of correlation coefficients (r2). There was no significant difference between the calculated (31.4 mg·g−1) and experimental values (29.1 mg·g−1) of saturation adsorption capacity. The results indicated that Fruendlich and Dubinin−Radushkevich isotherms could not be used for explaining the equilibrium relationship of Cd(II) with the imprinted hybrid sorbent and the experimental equilibrium data were found to be fitted well with the Langmuir isotherm model which showed the monomolecular adsorption of Cd(II) onto the imprinted hybrid sorbent. 3.7. Selectivity. In order to prove the selectivity of the imprinted hybrid sorbent, different potential interfering ions such as Pb(II), Zn(II), Cu(II), and Ni(II) were added to the mixed solutions with equal quantities of metals because these metal ions had similar chemical properties; likewise, they were divalent and could form complexes with N, S donor atoms, and the coordination number of the above metals complex usually was 4.54 Competitive adsorption of Cd(II)/Pb(II), Cd(II)/ Zn(II), Cd(II)/Cu(II), and Cd(II)/Ni(II) from their mixtures were investigated by using the imprinted and nonimprinted hybrid sorbents, respectively. As shown in Table 4, the D values

(9)

where qe is the equilibrium amount of adsorbed metals in the sorbent (mg·g−1), Ce is the equilibrium ion concentration in solution (mg·L−1), b is the Langmuir isotherm constant related to the free energy of adsorption (L·mg−1), and qmax is the calculated values of maximum adsorption capacity for a monolayer coverage (mg·g−1). The Freundlich expression is an exponential equation that describes reversible adsorption and is not restricted to the formation of the monolayer.51 The linear form equation of the Freundlich adsorption isotherm can be represented by 374



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Table 4. Selectivity Parameters of the Imprinted Hybrid Sorbents for Cadmium(II) metals

sorbents

D(Cd)

D(X)

α

αr

Cd(II)/Ni(II)

IIP NIP IIP NIP IIP NIP IIP NIP

5689 824 5982 1042 6285 578 5714 756

2731 5193 2447 5494 2093 6405 2672 5605

2.083 0.1587 2.445 0.1897 3.003 0.09024 2.138 0.1349

13.1

Cd(II)/Cu(II) Cd(II)/Pb(II) Cd(II)/Zn(II)

12.9 33.3 15.9

of the imprinted hybrid sorbent for Cd(II) were greater than that of nonimprinted sorbent due to the random distribution of ligand functionalities in the polymeric network of nonimprinted sorbent, while D values of the imprinted hybrid sorbent for Pb(II), Zn(II), Cu(II), and Ni(II) decreased significantly. The results implied that the imprinted cavities and specific binding sites in a predetermined orientation on the surface of the imprinted hybrid sorbent were formed. The relative selectivity coefficient (αr) values, for Cd(II)/Pb(II), Cd(II)/Cu(II), Cd(II)/Zn(II), and Cd(II)/Ni(II) systems were 33.3, 12.9, 15.9, and 13.1, respectively. The radius of Cd(II) (97 pm) is smaller than that of Pb(II) (120 pm) and is larger than those of Zn(II) (74 pm), Cu(II) (72 pm), and Ni(II) (72 pm) ions. These metal ions did not match with the imprinted cavity of Cd(II). These results indicated that the imprinted hybrid sorbent synthesized had a good selectivity for the Cd(II) ion. 3.8. Thermodynamic Study. The thermodynamic parameters can be determined from the thermodynamic equilibrium constant, K0. The standard Gibbs free energy ΔG° (kJ·mol−1), standard enthalpy change ΔH° (kJ·mol−1), and standard entropy change ΔS° (J·mol−1·K−1) are calculated using the following equations: ΔG° = −RT ln K 0

Figure 7. Plots of ln(Cs/Ce) vs Cs at various temperatures.

calculated from the slope and intercept of a linear plot of ln K0 versus 1/T, respectively (as shown in Figure 8).

(13)

Figure 8. Variation of equilibrium constant K0 as a function of temperature (1/T).

ΔS° ΔH ° − (14) R RT −1 −1 R is the universal gas constant, 8.314 J·mol ·K , and T is the solute temperature (K). K0 can be defined as55 γ C a K0 = s = s s ae γe Ce (15) ln K 0 =

Table 5 presents the thermodynamic parameters. ΔH° and ΔS° were found to be 11.77 kJ·mol−1 and 74.98 J·mol−1·K−1, Table 5. Various Thermodynamic Constants for Adsorption of Cadmium(II) on the Imprinted Hybrid Sorbents for given temperature

where as is the activity of adsorbed metals, ae is the activity of metals in solution at equilibrium, γs is the activity coefficient of adsorbed metals, γe is the activity coefficient of metals in equilibrium solution, Cs is the metal adsorbed on the imprinted hybrid sorbent (mmol·g−1), and Ce is the metal concentration in equilibrium solution (mol·L−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 very low concentration.55 Equation 15 can be written as lim

Cs → 0

Cs a = s = K0 Ce ae

thermodynamic constants

298.15 K

308.15 K

318.15 K

K0 ΔG° (kJ·mol−1) ΔH° (kJ·mol−1) ΔS° (J·mol−1·K−1)

71.05 −10.57 11.77 74.98

85.24 −11.39 11.77 74.98

95.71 −12.07 11.77 74.98

respectively. The Gibbs free energies were calculated from eq 13, setting the temperature at 298.15, 308.15, and 318.15 K, and corresponded to the values of −10.57, −11.39, and −12.07 kJ·mol−1, respectively. Considering the negative ΔG° value for the adsorption process, it could be concluded that the adsorption of Cd(II) ions onto the imprinted hybrid sorbent was thermodynamically feasible and spontaneous. The positive ΔH° value confirmed that the Cd(II) ions adsorption was endothermic in nature which was supported by increasing adsorption capacity of Cd(II) ions with an increase of temperature. The positive ΔS° reflected that the randomness

(16)

K0 at different temperatures was determined by plotting ln(Cs/ Ce) versus Cs (Figure 7) and extrapolating Cs to zero.55 The straight line obtained was fitted to the points by least-squares analysis. Based on eq 14, the values of ΔH° and ΔS° were 375



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of the different imprinted polymers in the literature for adsorption of Cd(II) ions were compared with the value for the imprinted hybrid silica obtained in this study. Although the direct comparison of the imprinted hybrid silica obtained with the other imprinted materials was difficult, owing to the different experimental conditions, it has been seen that the adsorption properties of Cd(II) on the imprinted hybrid silica obtained is either better or comparable with many other imprinted sorbents listed in Table 6.30−32,34,36,56−60

increased in the adsorption process due to the release of water molecules in the absorption reaction between the metal ions and the functional groups on the surfaces of the sorbent. 3.9. Regeneration. Regeneration of any exhausted sorbents is an important factor in the adsorption process for improving the process economics. The adsorption−desorption cycles can regenerate the sorbents close to its initial properties for effective reuse. The same sample of the imprinted hybrid sorbent was used for the adsorption of metals from solutions. After nine adsorption/desorption cycles, the adsorption capacity of Cd(II) was not significantly decreased and was found to be about 82% of the fresh sorbent in Figure 9. These results indicated that the imprinted hybrid sorbent owned a good regeneration ability for Cd(II).

4. CONCLUSION A simple procedure for the synthesis of a Cd(II)-imprinted Schiff-functionalized silica-supported hybrid sorbent was developed by a surface-imprinting technique in conjunction with the sol−gel process. The prepared imprinted hybrid sorbent had higher capacity and selectivity than the nonimprinted sorbent and could be used several times as a sorbent of Cd(II) ion. Cd(II) ions adsorption followed the pseudosecond-order kinetics model, and adsorption equilibrium was achieved within 30 min. Equilibrium data of adsorption were in good agreement with the Langmuir isotherm, with the adsorption capacity of 29.1 mg·g−1. Thermodynamic studies confirmed that the process was spontaneous and endothermic in nature. Results showed that the Cd(II)-imprinted Schifffunctionalized silica-supported hybrid sorbent was one of the potential alternatives for the selective removal of toxic Cd(II) ions from aqueous solution.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: 86-24-89383297. *E-mail: [email protected]. Tel.: 86-24-89388215.

Figure 9. Regeneration of Cd(II)-imprinted hybrid sorbent.

Notes

The authors declare no competing financial interest.

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3.10. Application. The removal rates of the imprinted hybrid sorbent for Cd(II), Ni(II), Cu(II), Pb(II), and Zn(II) from the untreated mine wastewater were 92.2%, 5.6%, 9.4%, 2.8%, and 4.8%, respectively. The results indicated that the imprinted hybrid sorbent could effectively and selectively remove Cd(II) ions from the untreated mine wastewater. 3.11. Comparison with Other Cadmium(II)-Ion-Imprinted Materials for the Adsorption of Cadmium(II). The adsorption capacities, adsorption time, and the selectivity

ACKNOWLEDGMENTS The project was sponsored by the National Natural Science Foundation for Young Scientists of China (Grant No. 21107076), by the Doctoral Scientific Research Foundation of Liaoning Province of China (Grant No. 20111048), by the program for Liaoning Excellent Talents in University of China (Grant No. LJQ2012032), and by the Public Welfare Scientific

Table 6. Adsorption Results of Cadmium(II) Ions from the Literature by Various Imprinted Sorbents relative selectivity coefficients ligands a

SPANDS MACb biomassc MPSd chitosan 1-vinylimidazole 4-VPe dual-ligand monomerf DAAB and 4-VP poly(ethyleneimine) Schiff base

polymerization method bulk bulk surface surface surface bulk suspension bulk bulk surface surface

Cd(II)/Ni(II)

Cd(II)/Cu(II)

Cd(II)/Pb(II)

6.6 9.9

4.44

157.5

25.5 51.2 13.1

12.9

Cd(II)/Zn(II)

adsorption capacity (mg·g−1)

7.4 8.9

0.27 3.0 32.1 31.9 128.1 4.56 0.48 32.6 10.4 18.7 29.1

220 120 1.38 35.3 45.6

88.2 33.3

15.9

equilibrium time (min) 50 30 60 60 20

20 60 30 30

ref 30 31 32 34 36 55 56 57 58 59 this study

a

2-(p-Sulphophenylazo)-1,8-dihydroxy-3,6-naphthalene disulfonic acid trisodium salt. bN-Methacryloyl-(L)-cysteine methyl ester. cNannochloropsis sp. d3-Mercaptopropyltrimethoxysilane. e4-Vinylpyridine. f(2Z)-N,N′-Bis(2-aminoethylic)but-2-enediamide. 376



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Research Project of Liaoning Province of China (Grant No. 2012003003).

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Ionic Imprinted Silica-Supported Hybrid Sorbent with an Anchored

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