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Recovery of Silver from Wastewater Using a New Magnetic Photocatalytic Ion-Imprinted Polymer Xiaocui Yin, Jian Long, Yu Xi, and Xubiao Luo* Key Laboratory of Jiangxi Province for Persistent Pollutant Control and Resource Recycling, Nanchang Hangkong University, 696 Fenghenan Avenue, Nanchang, Jiangxi Province 330063, People’s Republic of China

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S Supporting Information *

ABSTRACT: A novel magnetic, photocatalytic, and Ag(I)imprinted thiol-functionalized polymer (Fe3O4@SiO2@TiO2IIP) was prepared as functionalized IIP for selective removal and recycling of Ag+ ions from actual wastewater. The material used in this study exhibited a promising silver saturation adsorption capacity of 35.475 mg/g under the optimum pH of 6 within 80 min. The specific Ag+ ion adsorption property of the material was excellently offered by the Ag(I)-imprinted thiolfunctionalized polymer. The selectivity separation factors for Ag+ with respect to Li+, Co2+, Cu2+, and Ni2+ are 10.626, 27.829, 13.276, and 68.109, respectively. In the presence of TiO2 and methanol used as the sacrificial agent (methanol/water 15:40), the adsorbed Ag(I) can be reduced to Ag(0) and then separated from the imprinted polymers after the ultrasound. The reduction rate is 0.00566 min−1 based on a pseudo-first-order kinetic model. The retained adsorption capacity of the Ag-IIP was 68.51% after one round of photocatalysis and ultrasound, which was closed to three rounds of acid elution. We also conducted an experiment with real wastewater and validated the great potential of Fe3O4@SiO2@TiO2-IIP in advanced wastewater treatment. The results showed that 1.3 mg of silver was recovered from 100 mL of 50 mg/L AgNO3 solution with 0.1 g of the IIP. Accordingly, the functionalized IIP constructed and applied in this study demonstrated (a) the promising selective adsorption capacity of Ag, (b) the efficient photoreduction potential of Ag, (c) gentle and ecofriendly regeneration conditions, and (d) excellent magnetic separation ability, and it has great potential in future practical wastewater treatment. KEYWORDS: Ag+ ion-imprinted polymer, Magnetic, Photocatalytic reduction, Recovery

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INTRODUCTION

attention has been paid to the development of techniques for such a goal. Surface imprinting technology, which could produce the reversible fixed template adsorption sites, has widely attracted the attention of global researchers. Ion-imprinted polymers (IIPs) is a branch of molecular imprinting that has shown great potential in selective removal ions by the capacity of ion recognition.13,14 So far, different kinds of IIPs have been synthesized, including Zn(II),15 Pd(II),16 Ni(II),17 Ca(II),18 Cr(III),19 and Ag(I).20 Some IIPs can regenerate by using temperature, pH, and light stimuli. The most common methods of IIP regeneration are to rinse the absorbent with solution (e.g., alcohol, acid, alkaline, water, etc.). However, this process is not ecofriendly and usually causes solvent pollution. Other methods for IIP regeneration mainly include thermosensitive polymers or photoresponsive polymers. The thermosensitive polymers remove or combine the template by phase transformation at different temperatures. Photoresponsive polymers

Silver (Ag), as the most common precious metal, has been widely applied in many fields, such as medical applications, electronic industries, and chemical applications.1,2 However, excess Ag+ in drinking water can be accumulated in organisms and is harmful to human health.3 In most cases, the silver ions are present at low concentrations in industrial discharges; thus, it is necessary to enrich and separate silver ions from solution. In addition, enrichment of silver is prior to recovery.4,5 Currently, methods such as electrochemical deposition, ultrafiltration, reverse osmosis, ion exchange adsorption, biological treatment, and so forth6−10 are used for removal of silver ions from wastewater. However, most of those techniques still have disadvantages, such as poor selectivity, high cost, and being time-consuming, and may cause secondary pollution.11 Additionally, most of the adsorption processes are nonspecific, showing low selectivity toward a particular heavy metal.12 Generally, different kinds of metal ions, especially precious metal ions, are co-present at low concentrations in the wastewater. Therefore, it is of great significance to selectively remove or enrich the precious metal ions. More and more © 2017 American Chemical Society

Received: August 7, 2016 Revised: January 24, 2017 Published: February 6, 2017 2090

DOI: 10.1021/acssuschemeng.6b01871 ACS Sustainable Chem. Eng. 2017, 5, 2090−2097

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process. Under the protection of nitrogen, 0.782 g of AgNO3 was dissolved in 60 mL of methanol and DMF solutions (v/v = 1:1). With the addition of 4 mL of MPTES, the mixture was subjected to reflux for 3 h at 60 °C with continuous stirring to obtain “Solution A”. Then, 5 g of Fe3O4@SiO2@TiO2 was suspended in methanol, and the mixture was quickly stirred to achieve a uniform distribution of Fe3O4@SiO2@TiO2 particles. Then, 50 mL of the Fe3O4@SiO2@ TiO2−methanol suspension and 2 mL of ultrapure water was added to Solution A, and the mixture was subject to reflux overnight with continuous stirring. The resulting solid substance was separated using a magnet and washed three times with methanol. Then, the solid substance was washed by using 1 mol/L nitric acid until Ag(I) ion was completely eluted. Last, they were washed thoroughly to neutral by distilled water. The magnetic nonimprinted polymer (NIP) (Fe3O4@SiO2@TiO2NIP) was also synthesized according to the same procedures in the absence of AgNO3. Photocatalytic Performance. Photocatalytic activity testings on the reduction of Ag+ were performed by irradiating with a UV lamp (ZF-1, wavelength 245−365 nm, power 20 W) and Xe lamp (PLSSXE300, wavelength 320−780 nm, power 300 W) performed in a 100 mL beaker containing 55 mL of aqueous solution, 25 mg of AgNO3, and 0.1 g of catalyst under stirring at 300 rpm at room temperature. The photocatalystic activities can be valuated by the following equation

have been constructed by appropriate proportion of lightsensitive azobenzene functional monomers [e.g., azobenzene (azo) monomers, and N-isopropylacrylamide (NIPAAm)] and cross-linkers.21−24 The reaction mechanism of photoresponsive polymers is as follows: UV/vis light changes the conformational −NN− bond via cis−trans isomerization, thus binding or releasing the template. However, these reaction conditions are very strict. Therefore, it is necessary to develop a facile and ecofriendly way to regenerate the IIP. In recent years, TiO2 materials have been widely applied to various fields ranging from photoelectric and light catalysis to sensors and photoelectrochemical cells.25,26 The anatase phase TiO2 has been broadly applied in photocatalytic degradation due to their low cost, strong oxidation and mineralization ability, and excellent stability. Electrons in the valence band in anatase phase TiO2 adsorbed the photon of UV light and jumped to the conduction band, which could form a highly active electronic e−, while the valence band could produce positively charged holes. The conduction band electrons can reduce metal ions, while the holes can mineralize organic species to CO2 and H2O in solution.27 The ability of UVirradiated TiO2 to reduce Ag+ has been reported previously by Hermann et al.28 After photocatalysis, separation of the photocatalytic materials and the ions for recycling is needed. Magnetic nanoparticles have attracted considerable attention due to their low toxicity, high biocompatibility, convenient catalyst recycling, and easy separation from a liquid system by an external magnetic field.29,30 When IIP particles are incorporated with Fe3O4, they can be easily separated by application of an external magnetic field. In the recognition and adsorption of copper ions, Ren and Zhang used such an approach to collect the spent composite.31 Recently, the use of magnetic IIPs for selective adsorption of heavy metal ions is rather common.32−34 Zhang et al.35 prepared a core−shell structure of magnetic surface Pb IIPs for selective extraction of Pb2+, and Cui et al.36 used ion-imprinted magnetic microspheres to monitor trace levels of lead ions in water. Combining TiO2 with Fe3O4 also has numerous research reports.37,38 In order to decrease the adverse effect of Fe3O4 on the photocatalytic activity of TiO2 and effectively protect Fe3O4 from chemical corrosion, introduction of a SiO2 layer on the surface of Fe3O4 is very necessary. Such magnetically photocatalytic material has not only excellent photocatalytic ability but also efficient magnetic recycled ability. Thus, far, there are only a limited number of reports of photocatalysis combined with IIPs. In this work, we aim to prepare new magnetic Ag+-imprinted polymers that can reduce the number of adsorbed Ag+ ions using TiO2 photocatalysis in place of acid elution. In light of the strong interaction between Ag+ and −SH, we designed a new type of Ag+ IIP (Fe3O4@ SiO2@TiO2-IIP) that is functionalized with −SH groups. We adopted 3-mercaptopropyl trimethoxysilane (MPTS) as a functional monomer, Ag+ as a template, and Fe3O4@SiO2@ TiO2 as a support. The adsorption capacity and selectivity of the Fe3O4@SiO2@TiO2-IIP for Ag+ removal were investigated in detail. The illumination time, the concentration of the sacrificial agent, and the ultrasound times were optimized in the process of Ag+ reduction, and the reaction rates were calculated.

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⎛ C − C1 ⎞ R=⎜ 0 ⎟ × 100 (%) ⎝ C0 ⎠

(1) +

C1 is the concentration of Ag at real time t, and C0 stands for the initial concentration of Ag+ ions. The concentration of Ag+ ions was analyzed by atomic absorption spectrophotometry (AAS). Adsorption Capacity Study. The adsorption of Ag+ from aqueous solutions on the imprinted polymer was determined in a batch system. The effect of pH was investigated in the range of 2.0− 7.0. In addition, the effects of the initial Ag+ concentration and contact time on the adsorption capacity were also examined. For this purpose, the adsorption amount was determined in a thermostatic concussion incubator. The amount of adsorbed Ag+ ions (Q mg/g) can be calculated according to the following equation

Qe =

(C0 − Ce)· V m

(2) −1

where Qe is the adsorption capacity (mg·g ), C0 and Ce represent the initial and equilibrium concentrations of Ag+ ions (mg·L−1), respectively, V denotes the volume of the metal ion solution (L), and m is the polymer mass (g). Adsorption Selectivity. Competitive adsorption of Li+, Co2+, Cu2+, and Ni2+ with respect to Ag+ was investigated for both IIPs and NIPs. The metal ion concentrations in the solution were analyzed by AAS. The selectivities of the IIPs and NIPs were evaluated from the distribution ratio (D). The selectivity factor of Ag+ relative to other heavy metal ions (βAg+/Mn+) and the relative selectivity coefficient (βr) were calculated using the following equations39

D=

C i − Cf V × Cf m

β Ag+ /Mn+ =

βr =

(3)

D Ag+ D Mn +

(4)

β Ag+‐IIP βNIP

(5)

where Ci, Cf, m, and V denote the initial and final metal ion concentrations, the polymer mass, and the solution volume, respectively. DAg+ and DMn+ are the distribution ratios of Ag+ and other metal ions. βAg‑IIP and βNIP represent the selectivity coefficients of IIPs and NIPs, respectively.

EXPERIMENTAL METHODS

Synthesis of the Ag+-Imprinted Polymer. The Fe3O4@SiO2@ TiO2 preparation is described in detail in Supporting Information section S1. Fe3O4@SiO2@TiO2-IIP was synthesized via a sol−gel 2091

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Figure 1. SEM images of (A) Fe3O4; (B) Fe3O4@SiO2; (C) Fe3O4@SiO2@TiO2; and (D) Fe3O4@SiO2@TiO2-IIP (inset, TEM images) and the synthesis route for Fe3O4@SiO2@TiO2-IIP. Desorption and Reuse. A desorption test was conducted by comparing traditional acid elution with photocatalytic reduction. The Ag+ IIPs were placed into the 100 mg/L Ag+ solution and vibrated for 6 h. Then, the final concentration of the Ag+ ion in the solution was determined, as described above. Fe3O4@SiO2@TiO2-IIP was separated magnetically and washed with distilled water. The particles were transferred to a 100 mL quartz beaker with a 55 mL aqueous solution and irradiated under ultraviolet light with mechanical agitation. Meanwhile, a 1 mol/L HNO3 solution was used to release Ag+. The polymers were separated magnetically and washed with water. Synthesis Real Wastewater. To verify the potential applicability of Fe3O4@SiO2@TiO2@IIP for the removal of Ag+ ions in real wastewater, a wastewater experiment was investigated. Because there are few silver ions in the wastewater of most factories, we needed to synthesize a real wastewater system that contained a lot of silver ions. The wastewater (Dingxin Co., Ltd., Shangrao China) was selected and analyzed. The wastewater is described in Table S1. The wastewater contained large amounts of heavy metals (Li, K, Zn, Co, Cu, Ni, and Cd) and had a pH value of 6. Thus, we added AgNO3 into real wastewater and ensured sufficient Ag+ to perform the adsorption experiment (100 mg/L).

Characteristic of XRD. In order to further verify the presence of a crystalline structure, the as-prepared samples were examined by XRD. The resulting peaks were analyzed with the database in MDI Jade 5.0X software. The spectrum diagram of Fe3O4 is shown in Figure 2a, compared to standard

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RESULTS AND DISCUSSION Characterization. SEM/TEM Characterization. The morphologies of Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2@TiO2, and Fe3O4@SiO2@TiO2-IIP were characterized via scanning electron microscopy and transition electron microscopy. As shown in Figure 1a, Fe3O4 was monodispersed and had a size of approximately 50 nm. After a layer of SiO2 was immobilized on the Fe3O4 surface, the surface of Fe3O4@SiO2 was smooth, with a size of approximately 200 nm (Figure 1b). The particles of Fe3O4 were aggregated seriously due to introduction of the SiO2 shells. The average particle size of Fe3O4@SiO2@TiO2 was 320 nm, which was larger than that of Fe3O4@SiO2 (Figure 1c), and numerous TiO2 nanoparticles were spread on the surface. The imprinted shell of Fe3O4@SiO2@TiO2-IIP was only 30 nm, which was likely to be cross-linked on the surface of the core. The results corresponded to the TEM characterization (Figure S1). There were numerous cavities in the IIP, which were available for Ag+ adsorption. The N2 adsorption− desorption characterization results confirmed that there is a thick polymer layer on the surface of Fe3O4@SiO2@TiO2, which also corresponded to the characterization of BET (section S2.1). In addition, the results of thermogravimetric analysis suggested that the concentration of the thiol functional groups on the surface of the IIP was higher than that of NIP (section S2.2).

Figure 2. XRD patterns of (a) Fe3O4, (b) Fe3O4@SiO2, (c) Fe3O4@ SiO2@TiO2, (d) Fe3O4@SiO2@TiO2-IIP, and (e) the separated Ag particles reduced by photocatalysis.

JCPDS card No. 19-0629; a face-centered cubic structure of magnetite was obtained. The coating of the SiO2 layer did not disrupt the structure of Fe3O4 (Figure 2b). In addition, Fe3O4@ SiO2 wrapped up an anatase TiO2 layer (JCPDS card No. 211272). The peaks of the Fe3O4@SiO2@TiO2 in Figure 2c could be seen in the synthesized Fe3O4@SiO2@TiO2-IIP with slightly decreased intensity. Moreover, the peaks’ positions were unchanged upon coating polymerization, which suggested that the crystalline structure of anatase TiO2 and the magnetite were not essentially changed. As the functional monomer on the surface of the TiO2 parcel formed amorphous polymers, polymerization only weakened the peaks of the kernel without changing its peaks’ positions. This was consistent with the results described in the literature.40,41 The IIPs before and after adsorption were analyzed by XPS. Figure 3A shows the full scan spectra of IIPs before (a) and after adsorption (b) in the range of 0−1200 eV. The data show that there are O, Si, Ti, C, and S elements peaks in the IIP before and after adsorption. The difference is that the peaks of Ag 3d and Ag 3p were observed after adsorption, which verified that Ag+ ions were adsorbed into the cavities of the IIP. However, the Fe element peaks are 2092

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at least in part to the ease of abstraction of α-hydrogen from these molecules.42 In the subsequent studies, 15 mL of methyl alcohol was selected as a sacrificial agent according to the promising reduction rate (Figure 4).

Figure 3. (A) XPS fully scanned spectra for (a) IIP and (b) IIP after adsorption; (B) XPS spectra of S 2p for (a) IIP and (b) IIP after adsorption; and (C) XPS spectrum of Ag 3d for IIP after adsorption. Figure 4. Reduction of Ag+ after 1 h of irradiation with different sacrificial agents.

not observed in the IIP before and after adsorption because it is hard to detect these elements inside of the material with XPS. In order to identify the photocatalytic reduction of Ag(I) to Ag(0), the separated Ag particles were examined by XRD. The characteristic diffraction peaks of the face-centered cubic structure of Ag were observed at 2θ = 38.1, 44.2, and 64.4° (JCPDS card No. 04-0783), which indicates that the Ag+ ions have been reduced to crystallized Ag nanoparticles by photocatalysis. Photocatalytic Reduction. The reduction rate constants of the Ag+ ion under the Xe lamp and the UV lamp were fitted by the pseudo-first-order kinetic model, which is presented as below ⎛ C − C1 ⎞ ln⎜ 0 ⎟ = −kt ⎝ C0 ⎠

Effect of Adsorption Conditions. Effect of pH on Adsorption. pH is an important parameter that could directly affect the adsorption of Ag+ ions. The protonation of −SH groups from IIPs and the solution chemistry of Ag+ ions will vary with pH changes. These factors subsequently induced changes in the equilibrium characteristics of adsorption. The heavy metal ions will precipitate gradually when the pH value of the aqueous solution is higher than 7; thus, the pH sensitivity experiments were performed in the pH range of 2−7. Figure 5

(6)

The results of the model fitting are illustrated in Table 1. In comparing the rate constants (k) and the correlation coefficient Table 1. Reduction Rates of Different Lamps UV lamp xenon lamp

reaction rate constant k

R2

0.00218 0.00106

0.9911 0.9890

(R2), the catalyst showed better performance under UV lamp irradiation. Moreover, TiO2 can only absorb at wavelengths of ultraviolet light lower than 400 nm. It is known that the output wavelengths of the UV lamp are below 365 nm. However, 20% output wavelengths of the Xe lamp are below 400 nm. In addition, the power of the UV lamp is much less than that of the Xe lamp. Therefore, the UV lamp was applied as the light source. To determine the photocatalytic activity of as-prepared Fe3O4@SiO2@TiO2-IIP photocatalyst for Ag+ reduction in wastewater, batch photodissolution experiments were conducted under different conditions. By comparing with Ri, the best conditions could be determined. Different types of sacrificial agents were added to improve the photocatalytic reduction. Methyl alcohol performed better than the other reagents, including ethanol and methyl orange. This was related

Figure 5. Effect of pH on the adsorption capacity of Ag+ by Fe3O4@ SiO2@TiO2-IIP. Conditions: sorbent, 50 mg; volume, 50 mL; C0(Ag+) = 50 mg/L; temperature, 25 °C.

demonstrates the effect of pH on the IIP adsorption capacity for Ag+ ions. The adsorption capacity increased first with increasing pH and finally attained a maximum value at pH 6. These results might be explained as follows. At lower pH, adsorption sites for Ag+ ions will be occupied by more H+ ions,43−45 which decreases the amount of activated sites available for Ag+. Adsorption Kinetics. The adsorption kinetics of Ag+ adsorption on Fe3O4@SiO2@TiO2-IIP and Fe3O4@SiO2@ TiO2-NIP were investigated, and the results are described in 2093

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adsorption capacities were 30.55 and 17.21 mg/g for IIPs and NIPs, respectively. Because the Ag+-IIPs are perfectly designed and synthesized containing silver ion recognition sites, it is not surprising that the adsorption capacity of the IIP is nearly two times that of the NIP. Langmuir and Freundlich isotherms are generally used to fit adsorption data. The Langmuir and Freundlich results are listed in section S3.2. According to the slope and intercept in Figure S5, the values of the calculated Qmax were 36.44 and 21.80 mg/g for IIPs and NIPs, respectively. They were very close to their experimental Qmax values (30.55 mg/g for IIP and 17.21 mg/g for NIP). This suggested that the adsorption sites were essentially homogeneous. The synthetic wastewater adsorption experiment with 50 mg of IIP obtained a silver adsorption capacity of 28 mg/g, which was also very close to the experimental Qmax values driven from individual adsorption experiments. The Dubinin−Radushkevich (D−R) model was used to determine the adsorption energy, and the results are analyzed in section S3.3, which indicate that the adsorption process was chemical adsorption. Comparison with other effective adsorbents is given in Table S4. Although the adsorption of Fe3O4@SiO2@TiO2-IIP is lower than that of chitosan adsorbents due to a high weight core without adsorption ability for Ag+ ions, it has excellent selectivity for Ag+ ions and provides a new ecofriendly regeneration method. Adsorption Selectivity. Competitive adsorption of Ag+/Li+, Ag+/Cu2+, Ag+/Co2+, and Ag+/Ni2+ pairs were investigated from their binary mixed solutions to determine the selectivity of the imprinted polymers. The initial concentration of each metal ion in a binary mixture was 50 mg/L. The selectivity coefficients relative to other heavy metal ions using IIPs and NIPs are presented in Table 2. It can be seen that the relative

Figure 6. It was apparently noticed that Fe3O4@SiO2@TiO2IIP showed much higher adsorption capacity than Fe3O4@

Figure 6. Effect of equilibrium time on the adsorption of Ag+ by Fe3O4@SiO2@TiO2-IIP and Fe3O4@SiO2@TiO2−NIP. Conditions: sorbent, 100 mg; volume, 100 mL; C0(Ag+) = 50 mg/L; temperature, 25 °C; pH, 6.0 ± 0.2.

SiO2@TiO2-NIP. In addition, Fe3O4@SiO2@TiO2-IIP and Fe3O4@SiO2@TiO2-NIP demonstrated good performance in the first 50 min and then reached equilibrium after 100 min. To discern the rate-controlling step in the adsorption process of Ag+ ions, rate constants of adsorption were analyzed by the pseudo-first-order and pseudo-second-order kinetic models, respectively. The results are shown in section S3.1, and from the correlation coefficients (R2) data, we think that the adsorption process of Ag+ ions is more suitable for the pseudo-second-order kinetic model. The theoretical Qe values for pseudo-second-order models were 26.19 mg/g, which is close to the experimental value. Adsorption Isotherm. Adsorption equilibrium experiments were carried out to examine the adsorption capacity of IIPs and NIPs for Ag+ ions with an initial concentration range of 10− 300 mg/L at pH 6.0. According to Figure 7, the adsorption capacities for Ag + ions increased with increasing Ag + concentration, and the maximum adsorption amount or monolayer coverage was achieved at an initial concentration of 100 mg/L. It was also observed that the maximum

Table 2. Selective Adsorption Property of Imprinted and Nonimprinted Polymers selectivity coefficient βAg/M ions

IIP

NIP

relative selectivity coefficient βY

Li Co Cu Ni

10.626 27.829 13.276 68.109

1.806 4.065 2.587 28.291

5.884 6.847 5.132 2.407

selectivity coefficient of IIPs for each heavy metal ion was bigger than 1. Because IIPs provide perfect match sites with −SH ligand groups and a geometry-suitable cavity for Ag(I) coordination in the imprinted process, Ag+ ions will enter easily into the imprinted cavities and have stronger affinity with the thiol ligands (Figure S6); see comparisons with Li(I), Co(II), Cu(II), and Ni(II). Hence, the IIP is able to selectively adsorb Ag(I) in the complex mixed solution. Photocatalytic Removal of Ag(I). To explore the feasibility of photocatalytic removal, repetitive adsorption tests were designed. Upon comparison of the first adsorption capacity (Q1) and the second adsorption capacity (Q2), it was found that the reduced Ag was hardly separated from the imprinted polymers. Thus, the ultrasonic separation was studied (Table 3). The second adsorption experiments were conducted under the same conditions as the first experiments. The adsorption capacity was obviously improved after ultrasound. Compared with the different methods of ultrasound, the efficiency of the first method was better, and as it was much easier to determine the best conditions of ultrasound and

Figure 7. Effects of Ag+ initial concentration on the adsorption of Ag+ by Fe3O4@SiO2@TiO2-IIP and Fe3O4@SiO2@TiO2−NIP. Conditions: sorbent, 50 mg; volume, 50 mL; temperature, 25 °C; pH, 6.0 ± 0.2. 2094

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ACS Sustainable Chemistry & Engineering Table 3. Different Methods for Separationa ultrasound after illuminate ultrasound with illuminate without ultrasound

Q1 (mg/g)

Q2 (mg/g)

Q2/Q1 (%)

22.85 21.7 20.8

12.9 12.1 1.35

56.46 55.76 6.490

d[Ag +] = −k rxn[Ag +] dt

(7)

where krxn is the pseudo-first-order rate constant and [Ag+] is the concentration of reduced Ag dissolved in HNO3. The reaction rates predicted by the pseudo-first-order rate constant are plotted in Figure S7. To investigate the regeneration and reusability of IIPs as a photocatalyst and adsorbent, desorption experiments were carried out using three different methods, namely, photocatalysis with ultrasound, with 1 M HNO3 after photocatalysis with ultrasound, and with 1 M HNO3. The imprinted cavities could regenerate 68.51% by one round of photocatalytic reduction. However, imprinted cavities could only regenerate 77.85% by 1 M HNO3 after three rounds of elution. Compared with using one round of photocatalytic reduction, the imprinted cavities could regenerate to 74.40% while the materials were washed by 1 M HNO3 once after the photocatalysis and ultrasonic treatment. It was noticed that the regeneration rate of IIPs that had been first treated by photocatalytic reduction and ultrasound could be further improved by rinsed the adsorbent with 1 M HNO 3 . However, the dissolved concentration of Ag should be more than 2 M HNO3. These indicated that some adsorbed Ag+ ions were left unreduced. The repetition rates of IIPs treated by photocatalysis and ultrasonic are shown in Figure S8 As the results in Figure S8 show, the adsorption capacity of IIPs decreased greatly after the first run. The possible reasons are listed below. First, the Ag+ adsorbed on the surface of the IIP were not removed fully. Second, a few of the reduced Ag deep in the IIP layer could not be separated from the material. Finally, a small amount of polymer maybe lost in each recovery cycle process. Nevertheless, compared with the first run, the recovery did not exhibit obvious loss after four cycles, indicating its promising stability and reusability. From the perspective of practical application, the development of the recyclable IIP photocatalysts would be a convenient, practical, and ecofriendly method to treat the recovery of Ag+. Discussion of the Mechanism. In order to investigate the mechanism further, XPS eperiments were conducted. As shown in Figure 3A,c, typical peaks of Ag+ appearing at binding energies of 374.5 and 368.5 eV were founded in XPS spectra after adsorption, which indicated that IIPs can effectively adsorb Ag+ ions from wasterwater. In order to reveal special Ag+ adsorption sites, the high-resolution XPS spectra of the S 2p are analyzed in Figure 3B. As shown in Figure 3B, two peaks centered at 164.6 and 163.4 eV corresponded well to S 2p1/2 and S 2p3/2 binding energies with H, respectively, which both existed in the spectrum of the IIP before and after adsorption. After adsorption, there were two other peaks observed at 163.4 and 162.2 eV, which corresponded well to S−Ag 2p1/2 and S− Ag 2p3/2. Recovery of Silver. As the particles were prepared through the ion-imprinting method, the IIP showed favorable adsorptivity and selectivity. The performance of Fe3O4@ SiO2@TiO2-IIP with real wastewater was tested. After adsorption equilibrium with 0.1 g of Fe3O4@SiO2@TiO2-IIP in 100 mL of AgNO3 and other interfering ions in the synthesized real wastewater system, the adsorption capacity of Ag+ was 22.38 mg/g. With the presence of TiO2 and the organic reagent, the adsorbed Ag+ could be reduced to Ag by photocatalysis. Besides that, the IIP could be separated by magnetism from the core of Fe3O4. The process continued with

a Conditions: 50 mg of IIP with 50 mL of 50 mg/L Ag+ after adsorption equilibrium; illuminate for 3 h; ultrasound power, 100%.

illumination, ultrasound after illumination was applied as the optimal photocatalytic condition. To determine the best conditions for the recovery of Ag+ ions, batch recycle adsorption experiments were carried out. The effects of illumination time, ultrasonic time, and ultrasonic power on Fe3O4@SiO2@TiO2-IIP were investigated, and the results are shown in Figure 8. The regeneration rate of the

Figure 8. Effects of (a) illumination time (30 min at 70% power); (b) ultrasound power (ultrasound for 30 min after 3 h of illumination); and (c) ultrasound time (after 3h of illumination and at 70% power).

imprinted cavities increased with increasing illumination time and nearly achieved equilibrium after 3 h. With increasing power and ultrasound time, the reduced Ag could be separated from the IIP easier. However, the surface-imprinted functional groups were also removed simultaneously. Therefore, the ultrasound power and the time at which the repetition rate was the highest were 60% and 45 min, respectively. Photocatalytic Reduction Kinetics Experiments. The photocatalytic reduction experiments were performed after the adsorption equilibrium. As the adsorption model better matched with the Langmuir equation, theoretical binding sites on the adsorbent material were uniformly distributed, and each adsorption sites could only hold one ion at the same time. Hence, it is assumed that the adsorbed ion concentrations remain constant before the reduction experiment. The IIP was separated by magnetite after adsorption equilibrium and then irradiated by the UV lamp (1−4h) with 15 mL of methanol and 40 mL of water. Then, the reduced Ag particles were separated from the IIP materials by ultrasound for 30 min at 70% power. The photocatalytic reduction ratio of the adsorbed Ag+ was only related to the time, neglecting possible errors in the separation of Ag. The amounts of reduced Ag were calculated by dissolving in HNO3 and measuring by AAS. Accordingly, Ag+ reduction kinetics can be approximately described using a pseudo-first-order kinetic model 2095

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3 h of illumination and 45 min of ultrasound, in which the materials were separated from the solution containing reduced silver. The mixed solution containing silver was dried for 4 h at 60 °C and then dissolved in concentrated nitric acid until a constant volume of 100 mL was reached. The reduced Ag+ concentration was 12.435 mg/L. The reduction ratio was calculated to be 55.56%, neglecting possible errors from the repetition rate. In total, 1.3 mg of silver was recovered from 100 mL of 50 mg/L Ag+ solution with 0.1 g of Fe3O4@SiO2@TiO2IIP. The route of silver recovery in real wastewater by photocatalysis is presented in Figure 9. This process was used

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01871. Details about chemical materials, characterization methods, synthesis of Fe3O4@SiO2@TiO2 core−shell structured microspheres, nitrogen adsorption−desorption characterization, thermogravimetric analysis, analyses of adsorption kinetics and the isotherm, calculation of the adsorption energy and basic properties of the real wastewater sample, and full-scale application (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86 791 3953372. Fax: +86 791 395373. ORCID

Xubiao Luo: 0000-0002-3935-1268 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the Natural Science Foundation of China (51238002, 51272099), the National Science Fund for Excellent Young Scholars (51422807), the Key Project of Science and Technology Department of Jiangxi Province (20143ACG70006), and the Cultivating Project for Academic and Technical Leader of Key Discipline of Jiangxi Province(20153BCB22005).

Figure 9. Route of silver recovery by photocatalysis.

full-scale for batch production with mechanical stirring, ultrasound generator, and electromagnetic separation equipment (section S4). Additional field and experimental studies are required to further elucidate the mechanism of Ag(0) transformation.

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REFERENCES

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CONCLUSIONS

In this present work, a novel Ag(I)-imprinted photocatalyst was synthesized using a surface-imprinting technique. With the inclusion of Fe3O4 nanoparticles, the adsorbent was magnetic. With TiO2 and methanol used as sacrificial agents, the absorbed Ag(I) could be reduced to Ag(0) and then separated from the imprinted polymers upon exposure to ultrasound. The batch experiments showed that Ag+ adsorption on the imprinted polymers better matched the Langmuir equation, which assumed that the ions were adsorbed at a fixed number of well-defined sites. The rate-controlling step was chemical sorption. The Ag(I) reduction by photocatalysis suggested that methanol performed better than the other sacrificial agents. The desorption rate of photocatalytic reduction was similar to that of acid elution. Considering the operational error and other influence factors, the reduction rate likely matched the pseudo-first-order kinetic model. Additional field and experimental studies were required to further elucidate the mechanism of Ag(0) transformation. Compared with other methods of adsorbing Ag+, this work demonstrated selective adsorption of Ag+ ions and ion reduction to elemental silver, indicating great potential in terms of feasibility and economic value. 2096

DOI: 10.1021/acssuschemeng.6b01871 ACS Sustainable Chem. Eng. 2017, 5, 2090−2097

Research Article

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DOI: 10.1021/acssuschemeng.6b01871 ACS Sustainable Chem. Eng. 2017, 5, 2090−2097