Preparation and Properties of Ion-Imprinted Hollow Particles for the

Jan 14, 2015 - Four kinds of silver ion-imprinted particles (Ag-IIPs) with different morphologies were prepared by the surface ion-imprinting technolo...
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Preparation and Properties of Ion-Imprinted Hollow Particles for the Selective Adsorption of Silver Ions Hongbin Hou,†,‡ Demei Yu,*,†,‡ and Guohe Hu† †

Key Laboratory for Non-Equilibrium Synthesis and Modulation of Condensed Matter, Ministry of Education, School of Science, and State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Four kinds of silver ion-imprinted particles (Ag-IIPs) with different morphologies were prepared by the surface ion-imprinting technology (SIIT) and were used for the selective removal and concentration of silver ions from wastewater. The favorable adsorptivity and selectivity of Ag-IIPs for Ag+ were confirmed by a series of adsorption experiments at a suitable pH value. The adsorption mechanism was elucidated by analyzing the adsorption isotherms, adsorption thermodynamics, and adsorption kinetics systematically. The Ag+ adsorption onto the Ag-IIPs was well-described by the Langmuir isotherm model, and it was likely to be a monolayer chemical adsorption. This conclusion was also confirmed by the thermodynamic parameters. Moreover, the adsorption kinetics indicated that the adsorption rate would be controlled jointly by the intraparticle diffusion and the inner surface adsorption process, and the latter process was generally associated with the formation and breaking of chemical bonds. Finally, the effects of different morphologies of the Ag-IIPs for Ag+ adsorption were also investigated. In aqueous solution, the adsorptivity of the Ag+ ion-imprinting single-hole hollow particles (Ag-IISHPs) for Ag+ was highest (80.5 mg g−1) because of a specific morphology that features a single hole in the shell. In an oil−water mixture, Ag+ in the water phase could be adsorbed efficiently by the Ag+ ion-imprinting Janus hollow particles (AgIIJHPs), with emulsifiability originating from the Janus structure.

1. INTRODUCTION Elemental silver and silver ions (Ag+) have been widely applied in many fields, such as aerospace, chemical industry, electronic industries, and medical applications,1−3 but Ag+ in drinking water can be accumulated in organisms and do harm to human health.4 Moreover, the silver resource is depleted because of excessive applications.5 Therefore, the effective removal and recycling of Ag+ from wastewater has become an important issue for health and the environment. Currently, adsorption as a technically and economically feasible method is widely used to remove Ag+ from wastewater,6,7 and some adsorbents have been fabricated, including natural zeolite8,9 and silicotitanate.10 However, a comprehensive study on the development of new adsorbents, which can selectively and efficiently separate Ag+ from wastewater, is still desired. Ion-imprinting technology (IIT) derived from molecularly imprinted technology (MIT) has attracted more and more attention. IIT can generate recognition sites by reversible immobilization of template ions on some polymer matrixes.11 Numerous metal ion-imprinted polymers have been developed by bulk polymerization, including materials that recognize Pd2+,12 Cu2+,13 Zn2+,14 and Ni2+.15 However, these template ions embedded in the matrices are hard to elute, and the number of recognition sites is so limited that the adsorptivity of these materials are poor. Therefore, surface ion-imprinting technology (SIIT) has been developed by coating the ion© XXXX American Chemical Society

imprinted functional layer onto a solid matrix, which improves ion transfer and reduces permanent immobilization of the template ions, increasing the amount of recognition sites on the adsorbent surface.16 Microsized polystyrene (PSt) hollow particles show some promising potential as confined reaction vessels, encapsulation of molecules, and removal of pollutants.17 In particular, hollow particles with a single hole in the shell are attractive because of the higher specific surface area and the higher uptake capacity for drugs and pollutants compared to other types of hollow particles.18 Moreover, Janus hollow particles with individual polymeric and inorganic parts can work effectively in oil−water mixtures because of their amphiphilic properties.19 In our previous studies, a series of hollow particles was prepared with glycidyl methacrylate (GMA) as the functional monomer,20 and the epoxy groups in the GMA molecule were further modified by a ring-opening reaction. Chitosan (CTS) is a biopolymer with abundant functional groups, such as amino and hydroxyl, which can interact with metal ions.21 CTS has been applied in wastewater treatment because of high adsorptivity and selectivity for metal ions;22 however, these applications are limited because of the higher Received: August 16, 2014 Revised: January 10, 2015

A

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formation of chemical bonds between epoxy groups in GMA and amino groups in CTS. These Ag-IIPs were prepared in a certain concentration of CTS in 20.0 g of 2 wt % acetic acid solution at 50 °C for 12 h with mild agitation. The optimal CTS content in Ag-IIPs was determined by measuring the Qe values of the Ag-IIPs in a series of mass ratios of CTS and 0.2 g of polymer particles (0.25:1, 0.5:1, 0.75:1, 1:1, and 1.25:1), as shown in Figure S2a of the Supporting Information. Then, a certain amount of AgNO3 was added to the mixture and stirred for about 6 h in the dark, with the mass ratio of CTS/AgNO3 being 1:0.20, to determine the optimal template ion content, as shown in Figure S2b of the Supporting Information. The resulting particles were washed with 5 wt % acetic acid and deionized water to remove the unreacted CTS entirely. Subsequently, the template ions were eluted by the Na2S2O3 aqueous solution (6.0 g L−1, 20 mL) for 3 h (in the dark) and then rinsed repeatedly by deionized water. Finally, the above particles were dried in a vacuum oven at 50 °C. Four kinds of Ag-IIPs with different morphologies were prepared, including Ag+ ion-imprinting solid particles (Ag-IISPs), Ag+ ionimprinting hollow particles (Ag-IIHPs), Ag+ ion-imprinting single-hole hollow particles (Ag-IISHPs), and Ag+ ion-imprinting Janus hollow particles (Ag-IIJHPs). 2.4. Characterization. The internal structures and morphologies of the Ag-IIPs were observed by JEOL JEM-2100 transmission electron microscopy (TEM) and JEOL JSM-6700 scanning electron microscopy (SEM). The samples were coated with platinum before SEM characterization. Fourier transform infrared apectroscopy (FTIR, Nicolet AVATARIR 360) was employed to record the infrared spectra of the Ag-IIPs with a solid KBr tablet method. Spectra in the optical range of 800− 4000 cm−1 were obtained at a resolution of 4 cm−1/s. The thermostability of the Ag-IIPs was evaluated by thermogravimetry analysis (TGA) using a NETZSH TG 209. The samples (∼8 mg) with approximately equal mass were heated from room temperature to 600 °C continuously at a heating rate of 10 °C min−1. 2.5. Adsorption Capacity Testing. A total of 0.05 g of Ag-IIPs was added to a 25.0 mL AgNO3 aqueous solution with a certain concentration at a suitable pH value. The mixture was stirred on a magnetic stirrer at 150 rpm. The adsorption time was held constant. The blank experiments were carried out using non-ion-imprinting particles (NIPs) in the same conditions. After centrifugation, both the initial and final concentrations of metal ions in solution were measured by a graphite furnace atomic absorption spectrophotometer (GFAAS). The adsorption capacity (Qe, mg g−1) was calculated using the following equation (eq 1):30

cost and poor chemical stability of CTS in acidic solutions. In addition, CTS has been used in the form of membranes for Ag+ adsorption in aqueous solution,23−26 but according to the literature,35 Fan et al. believed that the specific surface area of microsized particles are more effective than those of membranes for adsorption. Therefore, SIIT has been used by chemical coating the CTS onto the surfaces of microsized particles, and the chemical stability and specific surface area of the CTS can be exploited. Peng et al.27 confirmed that the microsized particles with the GMA as a functional monomer could be surface-modified and coated further by CTS with the cross-linking effects between the epoxy group in GMA and amino in CTS. Herein, four kinds of silver ion-imprinted particles (Ag-IIPs) with different morphologies were prepared by SIIT. The adsorption capacity (Q) was evaluated to optimize Ag+ adsorption onto the Ag-IIPs. Moreover, the adsorption mechanism was elucidated by analyzing the adsorption isotherms, thermodynamics, and kinetics. Finally, the selectivity and effects of different morphologies of the Ag-IIPs for Ag+ adsorption were investigated to explore the possibility for practical applications.

2. EXPERIMENTAL SECTION 2.1. Materials. Styrene (Tianjin FuChen Chem. Co., China) was distilled under reduced pressure. GMA (Shanghai JiYuan Chem. Co., China) and toluene (Tianjin TianDa Chem. Co., China) were of analytical grade and used without further purification. Divinylbenzene (DVB, 55%) was washed with 1 mol/L NaOH aqueous solution to remove polymerization inhibitors. CTS (≥90% deacetylation) was purchased from Sinopharm Chemical Reagent Co., and AgNO3 was obtained from Shanghai ShenBo Chemical Co. Benzoyl peroxide (BPO, Tianjin HongYan Chem. Co., China) and 2,2′-azobis(isobutyronitrile) (AIBN, Shanghai ShanPu Chem. Co., China) as initiators were purified by recrystallization. Sodium dodecyl sulfate (SDS) was purchased from Chengdu KeLong Chem. Co., China. Tetraethyl orthosilicate (TEOS) was received from Tianjin Kemiou Co., Ltd. Polyvinylpyrrolidone (PVP) was purchased from BASF Co. and used as a stabilizer without further purification. Deionized water was used in all experiments. 2.2. Preparation of Microsized Hollow Particles with Different Morphologies. Microsized PSt−GMA solid particles were prepared by dispersion polymerization. A total of 9.0 g of styrene and 1.0 g of PVP were added to ethanol (90 mL)/water (10 mL) medium with 0.3 g of AIBN as an initiator at 70 °C for 8 h. Then, 0.5 g of GMA was slowly introduced into the system for another 4 h of reaction. Microsized hollow and single-hole hollow particles were prepared by seed emulsion polymerization with a one-step swelling method.28,29 A total of 0.5 g of PSt seed particles and 0.2 g of SDS were added to 100 mL of deionized water to form the seeded emulsion. Subsequently, 1.0 g of styrene, 3.0 g of DVB, 1.0 g of GMA, 0.9 g of toluene, and 0.05 g of BPO were added to the emulsion. The emulsion was stirred at 35 °C for about 4 h. Finally, the hollow particles were prepared by polymerization at 70 °C for 6 h, and the single-hole hollow particles were obtained by introduction of acrylamide (∼15 mg) to regulate the polarity of the solvent. Microsized Janus hollow particles were obtained by seed emulsion polymerization in the presence of PSt@SiO2 seed particles and toluene as a swelling agent. Specific preparation technology of PSt@SiO2 core−shell particles and Janus hollow particles has been described elsewhere.20 All of the resulting particles were separated and sequentially washed by centrifugation (8000 rpm for 10 min) with ethanol to remove the emulsifier and unreacted monomers and then dried under vacuum at 50 °C. 2.3. Preparation of the Ag-IIPs with Different Morphologies. CTS was introduced onto microsized polymer particles by the

Qe =

(C0 − Ce)V m

(1)

where C0 and Ce are the initial and equilibrium concentrations of metal ions (mg L−1), respectively, V is the volume of the solution (L), and m is the adsorbent mass (g). Adsorption capacity represents the adsorbed amount of metal ions by per unit mass of the adsorbent. 2.6. Selective Adsorption Experiments. A total of 0.05 g of AgIIPs were immersed to the aqueous solution (50 mL) containing Ag+, Cu2+, and Zn2+ (100 mg L−1 for each) to perform the selective adsorption experiments. The mixture was stirred on a magnetic stirrer at 150 rpm. The adsorption time was held constant. The adsorption capacities of the Ag-IIPs for these interfering ions were investigated by detecting the residual concentration of each ion by a GFAAS. The blank experiments were carried out by the NIPs in the same conditions

3. RESULTS AND DISCUSSION 3.1. FTIR and TGAs. The FTIR spectra of the resulting particles are presented in Figure 1. For the CTS (curve a of Figure 1), the strong absorption bands at 3453 cm−1 were due to −NH2 and −OH stretching vibrations. The absorption bands at 1653 and 1593 cm−1 were attributed to −NH2 stretching vibration and N−H deformation vibration, respectively. The absorption band at 1086 cm−1 was due to C−O B

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Figure 2. TGA curves of (a) CTS, (b) PSt−GMA, and (c) Ag-IISP. Figure 1. FTIR spectra of (a) CTS, (b) PSt−GMA, (c) Ag-IISP, (d) Ag-IISP@Ag+, (e) Ag-IIHP@Ag+, (f) Ag-IISHP@Ag+, and (g) AgIIJHP@Ag+.

a one-stage degradation mechanism and both the initial degradation stages occurred at higher temperatures (280 and 320 °C, respectively). Moreover, the temperatures with 10 and 50% weight loss of the Ag-IISP were higher than those of the CTS, which revealed that the thermal stability of the Ag-IISP was evidently better. These results also indicated that the fine coating was beneficial to improve the thermal stability of the CTS because of the cross-linking effects between the CTS and PSt−GMA particles and the application performance of the CTS was significantly enhanced in practice. 3.2. Effects of the pH Value on Ag+ Absorption onto the Ag-IIPs. The pH value of the solution is an important parameter in adsorption studies and affects the adsorptivity of Ag-IIPs strongly. The Ag+ adsorption onto the Ag-IIPs is sensitive to pH and usually is unable to finish at a low pH value.35 The effects of the pH value on adsorption capacities (Qe) of the Ag-IIPs were investigated over the range from 3.0 to 7.0, and the results are shown in Figure S1 of the Supporting Information. As shown in Figure S1 of the Supporting Information, the Qe values of the Ag-IIPs were all increased as the pH value increased from 3.0 to 5.0 and the maximum Qe values were observed when the pH value was 5.0. This is attributed to facile protonation of the −NH2 groups, which are expected to be the most effective adsorption sites on the Ag-IIP surface. The protonation of −NH2 would induce an electrostatic repulsion with Ag+ and obstruct the metal complex formation. At higher pH, protonation of the amino groups is weakened, allowing for coordination with Ag+ and adsorption onto the Ag-IIP surface. However, the declining Qe values of the Ag-IIPs at a pH value higher than 5.0 were attributed to the formation of metal hydroxide, which suppresses Ag+ coordination. Hence, pH 5.0 was chosen as the optimal pH value in the following investigations. 3.3. Adsorption Isotherms. To study the effects of the initial Ag+ concentration on the adsorptivity of the Ag-IIPs and NIPs, absorption isotherm experiments were performed with initial Ag+ concentrations from 50 to 1200 mg L−1 (Figure 3). The adsorption isotherm is fundamental to describe the interaction between the adsorbate and adsorbent and is important in designing an adsorption system. Herein, the Langmuir and Freundlich isotherm models were applied to analyze the equilibrium adsorption isotherm. The Langmuir isotherm model is established on the basis of the following assumptions: chemical adsorption occurs between the adsorbate and the adsorbent; the adsorbent surface distribution is uniform; and there is no interaction between adsorbed molecules. The linear form for the Langmuir isotherm

stretching vibration from primary hydroxyl groups.21 For the PSt−GMA particles (curve b of Figure 1), the absorption bands at 3100−2800 cm−1 originated from the C−H stretching vibration of the benzene ring and methylene groups. The absorption bands at 760 and 700 cm−1 represented the monosubstituted aromatic group. The absorption bands at 1270 cm−1 corresponded to vibration of the epoxy skeleton, and the absorption bands at 904 and 850 cm−1 were attributed to the asymmetric stretching vibration of the epoxy group. Obviously, the presence of the epoxy group suggested that the GMA molecules were introduced successfully as a functional monomer.29 For the Ag-IISPs (curve c of Figure 1), the stretching vibration peak of −NH2 and −OH in CTS was shifted to 3440 cm−1 and the absorption bands of the epoxy group in GMA nearly disappeared. These would be expected for the ring-opening reactions between epoxy groups in GMA and amino in CTS.27 The infrared spectra of different Ag-IIPs with Ag+ (Ag-IIPs@ Ag+) were measured after the completion of adsorption, as shown in curves d−g of Figure 1. The shapes of most absorption bands were similar in these samples. In comparison to curve c, the absorption bonds at 3440 cm−1 assigned to −NH2 and −OH in CTS were shifted to low frequency in the presence of imprinted ions and the −NH2 stretching vibration bands at 1650 cm−1 and the N−H deformation vibration bands at 1597 cm−1 were both shifted to high frequency. Moreover, the C−O stretching vibration bands of primary hydroxyl groups at 1086 cm−1 were also shifted to high frequency. These shifts of absorption bands indicated that −NH2 and −OH in the AgIIPs could engage in the coordination reaction with Ag+.25 The electronic configuration of Ag+ is 4d105s05p0, and the lone pair electrons in the N atom of −NH2 and the O atom of −OH in CTS can donate to the empty 5s and 5p orbitals of Ag+ to form the complex via s−p hybridization. Therefore, the ionimprinted performance of Ag-IIPs was based on the coordination reaction between Ag+ and CTS. Thermal degradation properties of the CTS, PSt−GMA particles, and Ag-IISPs were examined by TGA, and the temperatures with 10 and 50% weight loss were presented for all samples in Figure 2. For the CTS, the TGA curve displayed a rapid and two-stage degradation mechanism. The initial weight loss took place in the temperature range of 120−250 °C because of the decomposition of the oligomer, and the second degradation stage was located at 250 °C.21 Different from the CTS, the TGA curves of the PSt−GMA and Ag-IISP displayed C

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in Table 1 were mainly consistent with that experimentally obtained and higher than those in other literature;24,34 these also indicated that the adsorption process was restricted to a monolayer. Different from the multilayer physical adsorption, the monolayer adsorption was a chemical adsorption with the formation and breaking of chemical bonds. It was indicated that the ion-imprinting adsorption might be accompanied by the Ag+ chelation mechanism with the CTS. On the contrary, the fine fit with both the Langmuir and Freundlich isotherm models implied that Ag+ adsorption onto the NIPs would probably be dominated jointly by the monolayer chemical adsorption and multilayer physical absorption. 3.4. Adsorption Thermodynamics. The thermodynamic parameters for Ag+ adsorption onto the Ag-IIPs and NIPs are listed in Table 2, including the changes in Gibbs free energy

Figure 3. Langmuir and Freundlich isotherm models for Ag+ adsorption onto the Ag-IIPs and NIPs.

Table 2. Thermodynamic Parameters for Ag+ Adsorption onto the Ag-IIPs and NIPsa

model is expressed in Table 1. kL represents the Langmuir adsorption constant (L mg−1) and is related to the energy of adsorption. kL is calculated from a plot of Ce/Qe versus Ce.31 To predict the favorability of adsorption, the Langmuir equation can also be expressed in terms of a dimensionless constant separation factor RL, which indicates the favorability and adsorptivity of the system. It is considered to be a favorable adsorption process when the value is within the range of 0−1.0. The Freundlich isotherm model is an empirical equation based on the adsorption on heterogeneous surfaces.32 The linear form for the Freundlich isotherm model is expressed in Table 1. kF represents the Freundlich adsorption constant (mg g−1). kF and 1/n are calculated from a plot of ln Qe versus ln Ce, and it is considered to be a favorable adsorption when the 1/n value is within the range of 0.1−1.0.33 As shown in Figure 3, all of the Qe values of Ag-IIPs increased rapidly at first and approached saturation at a certain initial Ag+ concentration. However, in comparison to the AgIIPs, all of the Qe values of NIPs increased slowly along with the increasing initial Ag+ concentration and the Qm values at saturation were lower. Because the RL values in Table 1 were all within the range of 0−1.0, Ag+ adsorption onto the Ag-IIPs and NIPs appeared to be a favorable process. In addition, the lower RL values of Ag-IIPs implied that the interaction between Ag+ in solution and Ag-IIPs might be relatively strong. Furthermore, the better fit with the Langmuir isotherm model (Figure 3) suggested that Ag+ adsorption onto the Ag-IIPs was dominated mainly by the monolayer adsorption. The Qm values of Ag-IIPs

ΔG (kJ mol−1) ΔH (kJ mol−1)

ΔG = ΔH − TΔS sorbent

298 K

308 K

318 K

Ag-IISP Ag-IIHP Ag-IISHP Ag-IIJHP NISP NIHP NISHP NIJHP

−9.59 −10.69 −11.70 −10.50 −5.23 −6.02 −8.88 −6.56

−10.35 −11.43 −12.37 −11.21 −6.22 −6.92 −9.46 −7.32

−11.11 −12.16 −13.03 −11.92 −7.20 −7.82 −10.05 −8.08

ΔS (J mol−1 K−1)

ln(Qe/Ce)=(ΔS/R)−(ΔH/RT) 13.01 11.16 8.10 10.63 24.06 20.69 8.62 16.07

75.86 73.33 66.47 70.90 98.31 89.66 58.71 75.96

R is the gas constant (8.314 J mol−1 K−1), and T is the absolute temperature (K). ΔH and ΔS are calculated from the slope and intercept of the line plotted by ln(Qe/Ce) versus 1/T, respectively.35 a

(ΔG, kJ mol−1), enthalpy (ΔH, kJ mol−1), and entropy (ΔS, J mol−1 K−1). All of the ΔG values in Table 2 were negative and indicated that Ag+ adsorption onto the Ag-IIPs and NIPs was spontaneous. The decreased ΔG values suggested that the adsorption was more favorable with increasing temperature in the test temperature range from 25 to 45 °C. This process was different from the simple physical adsorption, which only depended upon the van der Waals force, and the high temperature was adverse to the adsorption. Moreover, the

Table 1. Adsorption Isotherm Constants for Ag+ Adsorption onto the Ag-IIPs and NIPsa Langmuir

Freundlich

(Ce/Qe) = (Ce/Qm) + (1/kLQm)

ln Qe = ln kF + (1/n)ln Ce

sorbent

Qm (mg g−1)

kL (L mg−1)

R2

RLb

kF (mg g−1)

1/n

R2

Ag-IISP Ag-IIHP Ag-IISHP Ag-IIJHP NISP NIHP NISHP NIJHP

90.2 124.9 166.0 117.2 18.0 35.5 70.8 33.1

0.966 1.144 1.380 1.141 0.756 0.434 0.915 0.699

0.971 0.969 0.969 0.971 0.979 0.972 0.974 0.975

0.0013 0.0011 0.0009 0.0011 0.0017 0.0029 0.0014 0.0018

45.153 68.717 100.887 64.272 7.792 10.783 34.311 13.692

0.703 0.680 0.657 0.678 0.734 0.829 0.709 0.753

0.952 0.946 0.945 0.949 0.984 0.983 0.976 0.989

a Qe is the equilibrium adsorption capacity (mg g−1). Qm is the maximum adsorption capacity of the adsorbent (mg g−1). Ce is the equilibrium concentration of Ag+ (mg L−1). bRL is the separation factor, with RL = 1/(1 + CmKL). Cm is the maximal Ag+ initial concentration in solutions (mg L−1).

D

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Figure 4. (a) Pseudo-first-order model, (b) pseudo-second-order model, (c) intraparticle diffusion model, and (d) linear fit plots of Qt versus t for Ag+ adsorption onto the Ag-IIPs and NIPs.

Table 3. Kinetics Parameters of Pseudo-First-Order Model, Pseudo-Second-Order Model, and Intraparticle Diffusion Model for Ag+ Adsorption onto the Ag-IIPs and NIPsa

a

pseudo-first-order

pseudo-second-order

intraparticle diffusion model

ln(Qe − Qt) = ln Qe − k1t

(t/Qt) = (1/k2Qe2) + (t/Qe)

Qt = kintt1/2 + C

sorbent

k1

R2

k2

R2

kini

C

R2

Ag-IISP Ag-IIHP Ag-IISHP Ag-IIJHP NISP NIHP NISHP NIJHP

0.0125 0.0129 0.0135 0.0117 0.0097 0.0103 0.0115 0.0111

0.952 0.955 0.967 0.973 0.992 0.996 0.989 0.997

0.0149 0.00988 0.0171 0.0101 0.0106 0.00964 0.00795 0.00773

0.997 0.992 0.991 0.996 0.994 0.997 0.986 0.996

2.217 3.284 4.294 3.028 0.540 0.629 2.245 0.958

−2.145 −4.237 −1.904 −2.756 −3.475 −2.401 −12.12 −6.068

0.950 0.974 0.977 0.964 0.983 0.999 0.994 0.984

Qe and Qt are the adsorption capacity of the adsorbent (mg g−1) at equilibrium and at time t (min).

negative values of ΔH and ΔS confirmed an endothermic process with increased entropy, but the simple physical adsorption was an exothermic process with decreased entropy. Increased entropy can originate from an increased randomness at the interface between the adsorbate and adsorbent with the formation and breaking of chemical bonds. Therefore, the Ag+ adsorption onto the Ag-IIPs and NIPs might be regulated jointly by the physical adsorption and chemical adsorption. Furthermore, the ΔG and ΔH values of Ag-IIPs were lower than NIPs; therefore, Ag+ adsorption onto the Ag-IIPs was more favorable, presumably because of the ion-imprinting process. Interestingly, ΔH and ΔS of Ag+ adsorption onto the Ag-IISHP were less than those onto the other Ag-IIPs. This might be due to the specific morphology of Ag-IISHP with a single hole in the shell, which provides greater internal and external surface areas to adsorb Ag+ from solution.

3.5. Adsorption Kinetics. To study the effects of the contact time on the adsorptivity of the Ag-IIPs and NIPs, adsorption kinetics experiments were performed with initial Ag+ concentrations and contact times from 60 to 480 min (Figure 4). Adsorption kinetics was used to reveal the ratecontrolling step of adsorption and determine a reasonable contact time. Herein, the adsorption kinetics for Ag + adsorptions onto the Ag-IIPs and NIPs were investigated by two kinetics models, namely, the Lagergren pseudo-first-order model (G. E. Boyd liquid film diffusion equation) and pseudosecond-order model. The Lagergren rate equations are the most widely used equations for the adsorption of solute from a liquid solution.36 The Lagergren pseudo-first-order and pseudosecond-order models are expressed in Table 3. k1 (L min−1) and k2 (g mg−1 min−1) are the equilibrium rate constants of the pseudo-first-order and pseudo-second-order models, respectively. k1 is calculated from a plot of ln(Qe − Qt) versus t E

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Langmuir (Figure 4a), and k2 is calculated from a plot of t/Qt versus t (Figure 4b). In general, the pseudo-first-order model is used to describe the initial stage of adsorption, i.e., the liquid film diffusion. If the line plotted by ln(Qe − Qt) versus t is straight and through the origin, the liquid film diffusion is the only rate-controlling step. Different from the pseudo-first-order model, the pseudosecond-order model aims to describe the whole adsorption process, including the liquid film diffusion, the intraparticle diffusion, and the inner surface adsorption. Remarkably, the inner surface adsorption was generally associated with the formation and breaking of chemical bonds for the chemical adsorption. According to the data presented in Table 3, the R2 values of the pseudo-second-order model for the Ag-IIPs were more close to 1.0; therefore, Ag+ adsorption onto the Ag-IIPs was dominated by the pseudo-second-order model, and the liquid film diffusion could not be the rate-controlling step. In other words, the rate-controlling step would likely be the intraparticle diffusion and the inner surface adsorption, which means that Ag+ was strongly held onto the binding sites of Ag-IIPs by chemical bonds. In addition, because all of the R2 values of the Lagergren models for the NIPs were close to 1.0, it was implied that Ag+ adsorption onto the NIPs was controlled by the liquid film diffusion, the intraparticle diffusion, and the inner surface adsorption. The Weber−Morris intraparticle diffusion model has been used to further investigate the mechanism for Ag+ adsorption onto the Ag-IIPs and NIPs. The kinetics data can estimate whether the intraparticle diffusion is the rate-controlling step of the adsorption.37 The Weber−Morris intraparticle diffusion model is expressed in Table 3. kint is the intraparticle diffusion rate constant (mg g−1 min−1/2). C is the intercept, which is an indicator for expressing the boundary layer thickness. kint and C are calculated from the slope and intercept of the line plotted by Qt versus t1/2 (Figure 4c), respectively. According to the Weber−Morris intraparticle diffusion model, if the line plotted by Qt versus t1/2 is straight and through the origin, the intraparticle diffusion is the ratecontrolling step. All of the fitting lines in Figure 3c were not through the origin, and the R2 values of the Ag-IIPs were far away from 1.0 in Table 3, suggesting that the Ag+ adsorption onto the Ag-IIPs was not dominated by intraparticle diffusion and the inner surface adsorption was also a rate-controlling step. This conclusion could indirectly prove that the ionimprinting performance was determined by the formation and breaking of chemical bonds between Ag+ and the binding sites on the surface of the Ag-IIPs. However, for the NIPs, the fine linear correlation with the pseudo-first-order model and the intraparticle diffusion model indicated that the Ag+ adsorptions onto the NIPs were controlled by the liquid film diffusion and intraparticle diffusion, and the effects of the inner surface adsorption on the whole adsorption could be negligible. The rate-controlling step for NIPs is likely to be a physical process between the adsorbent and adsorbate. In conclusion, Ag+ adsorption onto the Ag-IIPs could be divided into three stages, as shown in Figure 4d. First, the fast initial adsorption rate was due to Ag+ in solution diffusing to the Ag-IIP surface through the liquid film, but the liquid film diffusion was not the rate-controlling step with the faster rate. After 180 min, Ag+ diffused further from the Ag-IIP surface to the interior and then Ag+ was strongly held onto the binding sites of Ag-IIPs by the coordinate bonds. The intraparticle

diffusion and the inner surface adsorption were the ratecontrolling steps because of the slower rate. Finally, the Ag+ diffusion rate was decreased with the declining Ag + concentration in solution and the reduced amount of the binding sites, in which the adsorption tended toward saturation. 3.6. Adsorption Selectivity. To investigate the selective recognition capability of the Ag-IIPs for Ag+, adsorption experiments were designed with Cu2+ and Zn2+ as interfering ions (Figure 5). The relevant parameters, such as the

Figure 5. Selectivity for Ag+ absorption onto the Ag-IIPs and NIPs in the presence of interfering ions.

distribution coefficients (Kd, mL g−1), the selectivity coefficient (k), and the relative selectivity coefficient (k′), are listed in Table 4.38 These coefficients could be used to evaluate the selectivity for Ag+ absorption onto the Ag-IIPs in the presence of interfering ions. In the presence of interfering ions, all of the Qe and Kd values for Ag+ absorption onto the Ag-IIPs were higher than those of the interfering ions and the k values to Cu2+ and Zn2+ were in the range of 4.5−6.5. These results indicated that Ag-IIPs have the ability to selectively adsorb Ag+, presumably because of the ion-imprinting method used to prepare the particles. Because the ionic radii (R) of the interfering ions were similar to that of Ag+ (RAg+ = 144 pm; RCu2+ = 145 pm; and RZn2+ = 142 pm), other features of Cu2+ and Zn2+ must give rise to the observed selectivity. For the NIPs, all of the Qe and Kd values for each metal ion were similar and lower and the k values for Cu2+ and Zn2+ were close to 1.0. This indicates that the NIPs were unable to adsorb the Ag+ ions selectively. The high k′ values further support the conclusion that Ag-IIPs are more selective for Ag+ than NIPs. 3.7. Effects of Different Morphologies on Ag + Absorption onto the Ag-IIPs. In this study, four kinds of Ag-IIPs were prepared using particles with different morphologies as the matrices, especially the matrices of Ag-IISHP and Ag-IIJHP, as shown in Figure 6. According to the Qe values for Ag+ absorption onto the Ag-IIPs in Figure 7, in aqueous solution, the Qe value of Ag-IISHP was highest and was 2.1 times that of Ag-IISP. This might be due to the specific morphology of Ag-IISHP with a single hole in the shell. CTS should diffuse easily into the cavity of Ag-IISHP through the single hole, and the ion-imprinted functional layer could also be formed on the internal surface. Therefore, both the internal and F

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Langmuir Table 4. Selective Absorption Parameters for Ag+, Cu2+, and Zn2+ Adsorption onto the Ag-IIPs and NIPsa Qe (mg g−1) Qe = (C0 − Ce)V/m +

sorbent

Ag

Ag-IISP NISP Ag-IIHP NIHP Ag-IISHP NISHP Ag-IIJHP NIJHP

9.6 1.97 14.2 2.1 19.8 7.9 13.6 3.2

Cu

2+

2.6 1.8 3.8 1.9 5.8 5.4 3.6 2.7

Zn

2+

2.3 1.7 3.1 2 5.4 5.5 3.2 2.5

Ag

Kd (mL g−1)

k

Kd = ((C0 − Ce)/Ce)(V/m)

k=Kd(imprintingion)/Kd(interfering ion)

+

126.32 20.72 220.16 22.16 392.08 98.44 206.06 34.78

Cu

2+

27.81 18.85 41.99 19.95 67.84 62.43 39.56 28.95

Zn

2+

24.40 17.75 33.60 21.05 62.43 63.77 34.78 26.67

Cu

2+

4.54 1.10 5.24 1.11 5.78 1.58 5.21 1.20

k′ k = k(Ag-IIPs)/k(NIPs)

2+

Zn

Cu2+

Zn2+

5.18 1.17 6.55 1.05 6.28 1.54 5.92 1.30

4.13

4.44

4.72

6.22

3.67

4.07

4.34

4.54

C0 and Ce are the initial and equilibrium concentrations of metal ions (mg mL−1), respectively. V is the volume of the solution (mL). m is the adsorbent mass (g).

a

Figure 6. SEM and TEM microphotographs of the matrixes of (a) Ag-IISHP and (b) Ag-IIJHP. The insets on the top right corner are TEM microphotographs at 15000× magnification.

Figure 7. Effects of the morphologies on Ag+ absorption onto the Ag-IIPs.

external surfaces of Ag-IISHP could be used to adsorb Ag+ in solution, and the adsorptivity was improved significantly. The Qe values of Ag-IIHP and Ag-IIJHP were lower than that of AgIISHP, but they were higher than that of Ag-IISP because of the cavity structures. These cavities could provide the mass transfer between the inner cavities and the outer solution during the adsorption, and the increased contact frequency was in favor of the diffusion of Ag+ in the ion-imprinted function layer; therefore, the adsorptivity of Ag-IIHP and Ag-IIJHP was also improved.

In the oil−water mixture (toluene and AgNO3 aqueous solution), the Qe value of Ag-IIJHP was highest because of the specific Janus structure. In comparison to the other Ag-IIPs in the immiscible mixture in Figure 7, the Ag-IIJHP could adsorb Ag+ efficiently in the oil-in-water emulsion. The emulsification originated from the amphiphilic property of Ag-IIJHP, because Ag-IIJHP could be located in the oil−water interface with the hydrophobic polymer part in the oil phase and hydrophilic silica part in the water phase. Conversely, the other Ag-IIPs could only disperse slightly in the water phase with the Pickering effect, and most Ag-IIPs were gathered in the oil G

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Figure 8. Schematic for Ag+ adsorption onto the Ag-IIPs.

was much stronger than other Ag-IIPs because of specific morphology with a single hole in the shell, and both the internal and external surfaces of Ag-IISHP could be used to adsorb Ag+. In an oil−water mixture, the highest adsorptivity of Ag-IIJHP of all Ag-IIPs was attributed to the specific Janus structure and Ag+ could be adsorbed efficiently in the stable oilin-water emulsion. These conclusions were significant to systematically investigate the performance of the Ag-IIPs and further explore their practical applications.

phase. Hence, a stable emulsion was difficult to form in contrast to Ag-IIJHP, and Ag+ in the water phase was unable to contact the other Ag-IIPs sufficiently. The adsorptivity of the other AgIIPs, except Ag-IIJHP, were impaired seriously in the oil−water mixture. The mechanism for Ag+ adsorption onto the Ag-IIPs is illustrated in Figure 8. CTS has been coated tightly onto the surface of the particles by a ring-opening reaction between epoxy groups in GMA and amino groups in CTS. The template Ag+ was then captured via formation of metal coordination bonds originating from Ag+ with CTS amino and hydroxyl groups. The Ag-IIPs with imprinted binding sites were obtained after the template Ag+ was removed by the eluant. These binding sites should be complementary to the Ag+ structure and size; therefore, the Ag-IIPs could adsorb Ag+ in solution selectively.

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ASSOCIATED CONTENT

S Supporting Information *

(1) Effects of the pH value on Ag+ adsorption onto the Ag-IIPs, (2) effects of CTS and template Ag+ content on Ag+ absorption onto the Ag-IIPs, (3) reusability for Ag+ adsorption onto the Ag-IIPs after 7 reuse cycles, (4) linear fit of Langmuir and Freundlich isotherm models for Ag+ adsorption onto the AgIIPs and NIPs, (5) nonlinear fit of pseudo-first-order and pseudo-second-order models for Ag+ adsorption onto the AgIIPs and NIPs, and (6) selectivity parameters for Ag + adsorption in the presence of interfering ions. This material is available free of charge via the Internet at http://pubs.acs.org.

4. CONCLUSION AND OUTLOOK In summary, the CTS-modified Ag-IIPs with different morphologies were prepared by the SIIT. The favorable adsorptivity and selectivity of Ag-IIPs for Ag+ were confirmed by a series of adsorption experiments at a suitable pH value. FTIR and TGA suggested that the preparation of the Ag-IIPs was based on the chemical cross-linking process between GMA and CTS and the chelation effects of Ag+ with CTS. The adsorption isotherms suggested that Ag+ adsorption onto the Ag-IIPs was dominated mainly by the monolayer chemical adsorption. This conclusion was also evidenced by thermodynamic parameters, which indicated that the adsorption process was spontaneous and endothermic with the increased entropy. The adsorption kinetics indicated that Ag+ adsorption onto the Ag-IIPs featured a fast initial step, and then the adsorption rates were slower until reaching equilibrium. The fittings to the pseudo-second-order model and the intraparticle diffusion model implied that the intraparticle diffusion and the inner surface adsorption were the rate-controlling steps for the AgIIPs. The Ag+ adsorption mechanism of the Ag-IIPs was discussed further. During the preparation, the template Ag+ was captured in CTS by metal coordination of Ag+ by CTS amino and hydroxyl groups. After the template Ag+ was removed by the eluant, the Ag-IIPs could adsorb Ag+ selectively, presumably using the ion-imprinted coordination sites. Finally, the effects of the morphologies of the Ag-IIPs for Ag+ absorption were also investigated. In aqueous solution, the adsorptivity of Ag-IISHP

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are thankful for support from the Key Laboratory for Non-Equilibrium Synthesis and Modulation of Condensed Matter, Ministry of Education, School of Science, Xi’an Jiaotong University, and the State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University.

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