Magnetic Hierarchical Photocatalytic Nanoreactors: Toward Highly

Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Magnetic Hierarchical Photocatalytic Nanoreactors: Toward Highly Selective Cd2+ Removal with Secondary Pollution Free Tetracycline Degradation Ziyang Lu,†,‡ Fan He,† Cheng Yu Hsieh,§ Xiangyang Wu,*,† Minshan Song,∥ Xinlin Liu,⊥ Yang Liu,‡ Shouqi Yuan,# Hongjun Dong,∇ Song Han,† Peng Du,○ and Guozhong Xing*,§

ACS Appl. Nano Mater. Downloaded from pubs.acs.org by 46.161.63.16 on 03/12/19. For personal use only.

†

School of the Environment and Safety Engineering, Institute of Environmental Health and Ecological Security, Jiangsu University, Zhenjiang, Jiangsu 212013, China ‡ Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Changchun, Jilin 130103, China § United Microelect Corporation Ltd., 3 Pasir Ris Drive 12, Singapore 519528, Singapore ∥ School of Science, Jiangsu University of Science and Technology, Zhenjiang, Jiangsu 212003, China ⊥ School of Energy and Power Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China # Research Center of Fluid Machinery Engineering and Technology, Jiangsu University, Zhenjiang, Jiangsu 212013, China ∇ School of Chemistry & Chemical Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu, China ○ Department of Electronic Engineering, Institute for Wearable Convergence Electronics, Kyung Hee University, Yongin-si, Gyeonggi-do 17104, Republic of Korea S Supporting Information *

ABSTRACT: In the present work, a new type of ecofriendly and recyclable magnetic hierarchical porous Cd2+ imprinted photocatalytic nanoreactors (MHP-Cd) are developed by utilizing the ion imprinting technique. Owing to numerous Cd2+ cavities generation and corresponding high adsorption capacity of Cd2+ of 154.99 mg/g in the imprinted layer, the as-prepared nanoreactors exhibit excellent selectivity of Cd2+ adsorption under corroboration of enhanced kions of Cd2+ to other ions [kions(Cd2+/Fe3+) = 2.836, kions(Cd2+/Cu2+) = 2.303, and kions(Cd2+/Zn2+) = 3.064]. Importantly, with coexistence of mesoporous and sodium pyrrolidone carboxylate, i.e., light transmittance materials, toxic tetracycline can easily contact with CdS and most of the light is amenable to being adsorbed by CdS, consequently promoting higher photocatalytic activity for degradation of tetracycline (i.e., degradation rate reaches 75.32%). Such developed MHP-Cd photocatalysts demonstrate a highly selective adsorption of Cd2+ and simultaneous tetracycline degradation with effective inhibition of the secondary pollution. With a promoted stability for recycling, our work provides a new promising technique for environmentfriendly selective adsorption of targeted heavy metal ions and synchronous degradation of antibiotic containment in mixed water environments. KEYWORDS: magnetic photocatalytic nanoreactors, ion imprinting technique, secondary pollution free, Cd2+ selective removal, synchronous tetracycline degradation

1. INTRODUCTION

hinders the procedure of various water treatments. In addition, vast cadmium ion amounts in water are a common heavy metal pollution,8 mainly from surface runoff and industrial wastewater.9,10 When the environment is subject to cadmium pollution, Cd2+ is more difficult to remove than other heavy metal ions; hence, it can cause chronic poisoning. On account of extreme toxicity of Cd2+ and tetracycline pollution to the

1,2

Nowadays, the problem of water environment pollution is becoming increasingly serious. Finding an appropriate solution to deal with the pollutants in a water environment efficiently and economically is one of the topics that researchers constantly explore. It comes to light that the most troublesome contaminants existing in a water body are residual antibiotics3 and heavy metal ions.4 Tetracycline5 is used frequently in people’s daily life6 especially in aquaculture.7 However, there is no doubt that excess tetracycline present in the environment leads to adverse influence on the water environment and © XXXX American Chemical Society

Received: January 20, 2019 Accepted: March 5, 2019 Published: March 5, 2019 A

DOI: 10.1021/acsanm.9b00113 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

presence of Cd2+ imprinted cavities; most of Cd2+ can be quickly adsorbed by the Cd2+ imprinted cavities around the mesoporous channels and on the surface of the imprinted layer; thereby the imprinted layer can effectively reduce the possibility of secondary pollution. Therefore, the introduction of sodium pyrrolidone carboxylate and P123 figure out the puzzle of weakening the photocatalytic ability of as-prepared material and significantly decrease the secondary pollution of CdS for the coverage of the hierarchical porous Cd2+ imprinted layer. Besides, common materials are difficult to collect and isolate, have low reuse, and have other issues that also limited their application; the introduction of magnetic materials can be a good solution to solve this problem, and Fe3O4 is an excellent magnetic material36 which possesses good magnetic separation property.37 As a result, Fe3O4 could be regarded as a supporter which can easily separate and then collect materials. In addition, Fe3O4 can also transfer electrons photogenerated by CdS because its conductivity is as high as 1.9 × 106 S m−138,39 and further improve photocatalytic activity;40,41 however, the effective combination of photocatalysis and ion imprinting techniques to achieve the treatment of multiple contaminants in complex water environments is rarely reported.42 Moreover, to our knowledge, using photocatalytic technology and ion imprinted technique to selectively adsorb heavy metal ion and efficiently degrade antibiotic residue synchronously is unprecedented. Therefore, as-prepared materials are more innovative and practical in complex water pollution treatment. Nanocrystals have been focused on many fields for their potential applications.43−50 In this work, MHP-Cd was compounded by ion imprinting technique. The structural properties of materials were further characterized by XRD; XPS and FT-IR were both used to further determine chemical composition, TEM was conducted to gain the corresponding morphology. UV−vis DRS and VSM also were used to investigate corresponding property. What’s more, several influencial factors were tested. In the end, photocatalytic degradation experiment, selectivity, stability, secondary pollution test, and mechanism of MHP-Cd were also researched.

ecology and the human body, there is a very meaningful activity to selectively adsorb Cd2+11,12 and availably remove tetracycline. Consequently, it is meaningful and significant to create a material which could synchronously selectively remove Cd2+ and remove tetracycline in water. For removing tetracycline, photocatalytic technology,13 which has served as a valid and environment-protected solution, relies on its superiorities such as resource savings, ecological protection, and low expense.14 Photodegradation methods15,16 decompose many antibiotics to specific less harmful organics and transform them into harmless compounds.17,18 A group of semiconductors, including TiO2,19 ZnO,20 CdS,21 and others, have been extensively studied for the photodecomposition of contaminant with irradiation. CdS shows fair photocatalytic activity with visible light illumination, in comparison with TiO2 and ZnO; the band gap of CdS is relatively narrow,22,23 which makes it more practical. Therefore, in order to maximize the degradation of tetracycline with visible light illumination, CdS was chosen as the first choice for photocatalysts. However, CdS is easy to decompose by light, resulting in secondary pollution, which greatly limits its development. Therefore, the secondary pollution of CdS should be effectively inhibited, so that its value will be greatly enhanced.24 On the other hand, for the selective adsorption of Cd2+ and restraint of secondary pollution of CdS by photocorrosion, ion imprinting technique is used. Ion imprinting technique is an extension of the molecular imprinting technique.25,26 Ion imprinting technique27,28 makes materials possess the ability to selectively recognize a specific ion. As for imprinted polymers, vast imprinted cavities with the same shape and units are formed by template ion.29,30 Consequently, ion imprinted polymers have ion recognition capacity and high energy to bind a specific template ion.31,32 Covering an imprinted layer on CdS not only ensures the material exhibiting excellent adsorbing selectivity of Cd2+ but also greatly suppresses the secondary pollution of CdS. Nevertheless, an ion imprinted layer33 suppresses the light absorption by CdS and restrains the tetracycline residue touching with CdS, leading to a great decline in the photocatalytic ability of the composite. In order to solve the above problems, sodium pyrrolidone carboxylate34 and P12335 are used as functional monomer and porogen, respectively. On the one hand, sodium pyrrolidone carboxylate as the functional monomer can achieve the purpose of coating the imprinted layer on the surface of CdS/Fe3O4; meanwhile, it has good light transmittance which can effectively reduce the light blocking of the ion imprinted layer, making most of light being absorbed by CdS. Compared to some conducting polymers as the functional monomer to form the imprinted layer, electrons and holes can be effectively separated, but tetracycline cannot enter the imprinted cavities to contact the conductive polymers and be further degraded. Even if tetracycline can enter the imprinted cavities, it will have a competitive relationship with Cd2+; this is not conducive to the degradation of tetracycline and adsorption of Cd2+. On the other hand, P123 could serve as a porogen to produce mesopores which possess better ability to make mesoporous channels due to their long chainlike molecular structure, and the approach to remove P123 is more facile, which prevents material loss. These mesopores ensure tetracycline contacting with CdS can then be degraded. Although, during light irradiation, CdS will be corroded and leaks into solution through mesoporous channels, due to the

2. EXPERIMENTAL SECTION 2.1. Materials. Ferric trichloride (FeCl3, AR), sodium acetate (NaAc, AR), cadmium monosulfate (CdSO4, AR), ethylene glycol (AR), thiourea (AR), ammonia (NH4OH, GR), poly(ethylene glycol) 4000 (PEG4000, PR), cadmium nitrate tetrahydrate (Cd(NO3)2· 4H2O), sodium pyrrolidone carboxylate, ethylene glycol dimethacrylate (EGDMA), P123 (PEO−PPO−PEO, 5800), acetone (AR), methylbenzene (AR), DMPO (AR), tetracyclne, triethanolamine (TEOA, AR), tertiary butanol (t-BuOH, CP), coumarin (CP), sodium sulfate (Na2SO4, AR), potassium chloride (KCl, AR), potassium ferricyanide (K3Fe(CN)6, AR), and potassium ferrocyanide (K4Fe(CN)6, AR) were bought from Shanghai Chemical Reagents. Irgacure784 was obtained from Alibaba. 2.2. Experimental Methods. 2.2.1. Synthesis of Fe3O4. The synthesis of Fe3O4 was done according to literature51 with a slight change. A 1.35 g amount of FeCl3·6H2O and 3.6 g of CH3COONa were simultaneously distributed in 50 mL of ethylene glycol; after stirring, this solution was transferred and kept at 200 °C for 8 h. When the reaction finished, the black product was collected and rinsed many times. At last, the sample was obtained after drying for 12 h. 2.2.2. Synthesis of CdS/Fe3O4. The synthesis of CdS/Fe3O4 was done according to the literature52 with several changes. At first, Fe3O4 (0.4 g) was dispersed in DI (50 mL) with mechanical agitation for a while at ambient temperature. Then, 5.5 mL of ammonia, 0.513 g of B


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Cd2+ or tetracycline was measured with ICP or UV−vis spectroscopy. The adsorption capacity of Cd2+ or tetracycline was computed by the corresponding formula.60,61 2.5. Photodegradation Performance. Photocatalytic degradation of tetracycline was examined with xenon lamp irradiation (300 W, 1.8 × 105 lux) and light wavelength was from 380 to 780 nm (other wavelength was kept away), which reacted in a vessel containing the same tetracycline aqueous solution as adsorption experiment, 50 mg of the sample with stirring at ambient temperature. After reaching the desired adsorption time, turning on the light. The degradation rate could be calculated based on the following equation:62

cadmium monosulfate, and 0.3 g of thiourea were dispersed in the above-mentioned solution with mechanical agitation for 3 h in 60 °C. The as-obtained products were gathered and rinsed for some time and dried in 40 °C. 2.2.3. Synthesis of Magnetic Hierarchical Porous Cd2+ Imprinted Photocatalytic Nanoreactor (MHP-Cd). Nowadays, multifunctional nanocomposites have become potential candidates in application.53−59 At first, CdS/Fe3O4 (0.5 g) and poly(ethylene glycol) 4000 (4 g) were dispersed into 20 mL of DI, with 0.5 h of ultrasonic treatment, then denoted as solution A. At the same time, a certain amount of Cd (NO3)2·4H2O and 0.3 g of sodium pyrrolidone carboxylate were weighed into 20 mL of toluene, followed by adding 0.8 mL of ethylene glycol dimethacrylate (EGDMA) and 0.015 g of visible light initiator (Irgacure784) with ultrasound for 10 min to complete dissolution, denoted as solution B. Second, solution A and solution B were moved to a flask and a certain mass of P123 was added. Third, the mixture was placed in a visible light photocatalytic reactor, and magnetic stirring was started under N2 atmosphere for 20 min. The reaction product was washed for many times, at last, solid product was dried at 40 °C. The obtained sample proceeded to P123 removal with acetone at 60 °C for 1 day. Afterward, these sample was eluted with 100 mL of 0.5 g/L EDTA at 30 °C for a half-day for Cd2+ elution.60 The solid samples were separated and then rinsed until neutral (pH = 7). Finally, the solid sample was dried at 40 °C. In Figure 1, the schematic illustration of synthesis processes of MHP-Cd was presented.

degradation rate =

C0 − Ct × 100% C0

(1)

where C0 and Ct were the concentration of Cd2+ or tetracycline after adsorbing and at different times. 2.6. Selectivity. Adsorption capacities of Cu2+, Zn2+, Fe3+, and Cd2+ were researched for different samples. The sample (50 mg) was dispersed in a photocatalytic reactor which contained 100 mg/L above-mentioned ions mixed solution (100 mL). The metal ion concentrations were determined by ICP. The selectivity was evaluated from adsorption capacity (Q); the kions and kmaterials were computed according to literature sources.60,63. 2.7. Secondary Pollution Test. For testing the secondary pollution of MHP-Cd, the experiment was shown as below. The sample (50 mg) was diffused in 100 mL of DI water; then a given mass of solution was extracted every 10 min with stirring for 1 h without illumination; after being separated and collected, the concentrations of Cd2+ were measured by ICP. 2.8. Cycling Experiment. A 0.5 g/L EDTA solution and irradiation for 3 h were selected to elute saturated Cd2+ and tetracycline, respectively. After each photocatalytic reaction, MHP-Cd is magnetically separated by a magnet, then washed and dried. When Cd2+ and tetracycline was no longer monitored, the recycled MHPCd was further reused, and its mass after drying was measured to calculate the recovery rate.64 The regenerated MHP-Cd was reused for 10 cycles to show the good reusability of MHP-Cd. 2.9. Photoelectrochemical Measurements. The parameter of EIS was followed by literature.6 Testing material was distributed in a clean conducting glass. Electrolyte was 0.5 M Na2SO4 solution. Photocurrent measurements were examined with irradiation followed by a similar approach as EIS, except using the different solution of 0.1 M KCl, 5 mM K3Fe(CN)6, and 5 mM K4Fe(CN)6 as electrolyte. Mott−Schottky plots were analyzed at a frequency of 1000 and 2000 Hz without light irradiation. 2.10. Electron Spin Resonance (ESR) Spectroscopy. •OH and • O2− were captured by 5,5-diamethyl-1-pyrroline N-oxide (DMPO),65 measurement of which was carried out on a Bruker A300 ESR spectrometer. 2.11. Radical Capture Experiment. The radical capture experiments were conducted for the tetracycline degradation test over MHP-Cd. To begin with photocatalytic degradation, 1 mmol of TEOA or 1 mmol of tertiary butanol (t-BuOH) were dispersed in the tetracycline solution being usedg as h+ or •OH capturer, respectively. Besides, N2 was continuously aerated into the tetracycline solution to detect •·O2− via inhibiting its generation. Another experimental process was the same as the degradation experiment. 2.12. Mineralization Test. The mineralization test was conducted followed by the procedure in section 2.5, the difference being that, after photocatalytic degradation, the solution was determined with the TOC analyzer, and the corresponding result was computed based on the following equation:

Figure 1. Schematic illustration of preparation processes of MHP-Cd. 2.3. Characterization. The morphology of samples was researched by transmission electron microscopy (TEM, JEM-2100, 100 kV) and scanning electron microscopy (SEM, JSM6700F, 5.0 kV) with energy dispersive spectrometer detector (EDS). The crystal phase was conducted by powder X-ray diffraction (XRD) obtained by X-ray diffractometer (MAC Science, Japan). Surface electronic states were examined on X-ray photoelectron spectroscopy (XPS, PerkinElmer PHI 5300). FT-IR (America Thermo-electricity Co.) in the range 400−4000 cm−1. N2 adsorption−desorption isotherms were studied by Tristar II 3020 M. The magnetic properties were researched by using vibrating sample magnetometer (VSM). The optical characteristics of the materials were examined by UV−vis diffuse reflectance spectroscopy (DRS) with a Specord 2450 spectrometer (Shimazu, Japan). Determination of the products of tetracycline after degradation was conducted by Thermo LXQ mass spectrometry (MS). 2.4. Adsorption Experiment. The corresponding experiments were conducted as follows: 50 mg of the different materials was dispersed in the vessel that contained 100 mL of 100 mg/L Cd(NO3)2 solution or 100 mL of 20 mg/L tetracycline solution; after the reaction, a given mass of solution was extracted every 10 min with stirring for 60 min without light and filtered and the concentration of

mineralization rate ij organic carbon content after reaction yzz zz × 100% = jjj1 − j z total organic carbon content k { C

(2)


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Figure 2. (A) XRD patterns and (B) FT-IR spectra of (a) Fe3O4, (b) CdS/Fe3O4, and (c) MHP-Cd.

Figure 3. XPS fine scans of MHP-Cd: (a) C 1s, (b) O 1s, (c) N 1s, (d) Cd 3d, and (e) S 2p.

3. RESULTS AND DISCUSSION 3.1. Factor Investigation of Different Adding Amounts of Cd(NO3) 2·4H2O on Adsorption of Cd2+ and Different Adding Doses of P123 on Degradation of Tetracycline during the Synthesis of MHP-Cd. For researching the mesoporous and Cd2+ imprinted cavities on the material properties, the additions of template ions (Cd(NO3)2·4H2O) and porogen (P123) were changed respectively for investigating the adsorption ability and photocatalytic activity; the corresponding results were shown in Supporting Information Figure S1. Clearly, upon adding an amount less than 0.5 g, more Cd(NO3)2·4H2O was added than more Cd2+ imprinted cavities were produced on the surface of the material; such Cd2+ imprinted cavities possess a unique recognition capacity for Cd2+. However, when the adding amount was more than 0.5 g, the imprinted cavities could be saturated, in turn leading to the stacking of the imprinted cavities and further destroying the specific recognition sites of the imprinted cavities, eventually leading to a decline in the adsorption activity. Accordingly, when the adding dose of (Cd(NO3)2·4H2O) was 0.5 g, MHP-Cd showed the best Cd2+ adsorption capacity in comparison with the other doses (such as 0.05 and 0.1−0.7 g; interval was 0.1 g). Beyond that, more P123 was added, more mesoporous channels appeared on the imprinted layer, and tetracycline could readily touch with CdS through these mesoporous channels and further be degraded, consequently; when the adding dose of P123 was 3.0 mL,

MHP-Cd showed the best degradation ability compared with the other doses (ranging from 1.0 to 4.0 mL; interval was 0.5 mL). Therefore, 0.5 g of Cd(NO3)2·4H2O and 3.0 mL of P123 were used to synthesize the MHP-Cd. 3.2. Characteristics. So as to verify the structure composition of the different materials, XRD patterns were employed. Based on the standard JCPDS Card No. 19-629,66 the diffraction peaks of Fe3O4 at 2θ values of 30.06°, 35.41°, 43.04°, 53.39°, 56.91°, and 62.50° were assigned to the reflections (220), (311), (400), (422), (511), and (440), respectively (Figure 2A), which suggested that Fe 3 O 4 possessed the face-centered cubic structure. Different from Fe3O4, three additional diffraction peaks appeared in CdS/ Fe3O4; the peaks displayed in 28.22°, 44.05°, and 54.03° marked by the indexes (111), (220), and (311), respectively, were in line with JCPDS Card No, 21-0829;67 this consequence confirmed that CdS had been loaded on Fe3O4. Moreover, the XRD pattern of MHP-Cd was almost consistent with CdS/Fe3O4, suggesting that the crystal structure of CdS and Fe3O4 remained consistent after the imprinted layer formed. Besides, in Figure S2, the diffraction peaks of other samples in each case corresponded to MHP-Cd; the major diffraction peaks and their intensities kept consistent The FT-IR spectra of Fe3O4, CdS/Fe3O4, and MHP-Cd were presented in Figure 2B, and other materials were displayed in Figure S3. It could be observed that the peaks from 500 to 550 cm−1 were ascribed to Fe−O, and peaks from D


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ACS Applied Nano Materials 600 to 650 cm−1 were assigned to Cd−S. Compared with Fe3O4 and CdS/Fe3O4, several peaks emerged; the adsorption peaks at 1170 and 1700 cm−1 were ascribed to C−O and C O in EGDMA.68 Similarly, the adsorption peak at 1456 cm−1 was ascribed to C−N in sodium pyrrolidone carboxylate. The above results further demonstrated that MHP-Cd was compounded as expected, which was consistent with the consequences in XRD and XPS. The FT-IR spectra of other samples in Figure S3 were in accordance with that of the MHP-Cd, because EGDMA and sodium pyrrolidone carboxylate both exsited in these materials. For further verifying the chemical composition of the MHPCd, XPS analysis was carried out and corresponding results were exhibited in Figure 3, Figure S4, and Figure S5. Compared with XPS spectra of Fe3O4 and CdS/Fe3O4, the binding energy of O was shifted due to interaction between Fe3O4 and CdS. And the XPS spectra of MHP-Cd exhibited that many distinct peaks were ascribed to C 1s, O 1s, N 1s, Cd 3d, and S 2p peaks. The Fe element peaks were not clearly found in the MHP-Cd because it was hard to detect these elements inside of the material with XPS, while Fe element was detected in Fe3O4 and CdS/Fe3O4. The peaks of 283.24 and 282.88 eV (C 1s) can be attributed to the CO and C−O, which also demonstrated EGDMA existed (Figure 3). Besides, the peaks of 403.73 eV (N 1s) can be assigned to -NH from sodium pyrrolidone carboxylate.69 Moreover, the binding energies of Cd, O, and S were slightly shifted after the imprinted layer modification. The above results indirectly demonstrated that MHP-Cd was successfully synthesized. The morphologies of the samples were characterized and displayed in Figures 4, 5, and S6. It can be explicitly found that

Figure 5. Selected area for elemental mapping images of Fe, O, Cd, S, N, and C elements of MHP-Cd.

Moreover, the SAED pattern proved the presence of Fe3O4 and CdS as well. The elemental mapping images of Fe, O, Cd, and S also confirmed that CdS/Fe3O4 was synthesized as expected (Figure. 5); furthermore, the elemental mapping images of C, N, and O also demonstrated that the imprinted layer was compounded as expected. The results in SEM and corresponding elemental mapping images (Figure S6) of different samples were also in accordance with those in Figure 4 and Figure 5. In addition, low-angle XRD was conducted to certify the mesoporous structure; the result was shown in Figure S7. As displayed in Figure S7, the obvious diffraction peak could be detected, indicating that the mesoporous structure existed in MHP-Cd.80 So as to confirm the existence of mesoporous and Cd2+ imprinted cavities, corresponding tests were examined. As shown in Figure 6 and Figure S8, due to plenty of CdS being stacked on Fe3O4, the BET specific surface area of CdS/Fe3O4 (18.47 m2/g) was larger than that of Fe3O4 (11.76 m2/g), in addition, the average pore diameters of Fe3O4 and CdS/Fe3O4 were 6.01 and 5.98 nm. Furthermore, Figure 6b showed that the MHP-Cd possessed type IV isotherms and H4 type hysteresis loops,81 demonstrating that MHP-Cd had mesoporous structure. It was obvious that MHP-Cd possessed the biggest surface area (72.96 m2/g), which was much superior to other materials, and the smallest average pore diameter (2.73 nm), which was smaller than others, resulted from the coexistence of mesoporous and Cd2+ imprinted cavities in MHP-Cd. Thus, these consequences further demonstrate that abundant cavities were formed on MHP-Cd. The optical absorption property of photocatalysts made significant effect on photocatalytic performance, which were studied by the UV−vis DRS. In Figure S9, CdS had excellent light response in a wide wavelength range and Fe3O4 possessed the full spectrum absorption properties; accordingly, CdS/ Fe3O4 presented a wide light absorption range. And even coating the imprinted layer, MHP-Cd still showed good adsorption ability in the whole light range, because the functional monomer (sodium pyrrolidone carboxylate) in the imprinted layer had good light transmittance. Moreover, for investigating the photocatalytic mechanism, UV−vis DRS spectra and the following formula82 were conducted to confirm the band gap:

Figure 4. TEM images of Fe3O4 (a), CdS/Fe3O4 (b), MHP-Cd (c), HRTEM image (d), and SAED pattern (e) of MHP-Cd.

Fe3O4 was homodispersed and its average diameter was 400 nm. After CdS was loaded, the surface of CdS/Fe3O4 was tough, and the average diameter of CdS/Fe3O4 was larger than that of Fe3O4. Similarly, after forming the imprinted layer on the CdS/Fe3O4, the average diameter of MHP-Cd was also larger than that of CdS/Fe3O4. TEM technique is not only effective but also significant for structure measurement in the materials engineering fields.70−78 The HRTEM images further confirmed the connection between layer and CdS/Fe3O4.79 The marked lattice spaces of 0.2529 and 0.3360 nm stood for the (311) and (111) planes of Fe3O4 and CdS, respectively.

Ahν 2/ n = k(hv − Eg )

(3)

where A was absorption coefficient, h was planck constant, ν was light frequency, k was proportionality constant, and Eg was band gap. n = 1 or 4 was determined by type of semiconductor. E


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Figure 6. N2 adsorption−desorption isotherms and pore size distribution curves of CdS/Fe3O4 (a and d), MHP-Cd (b and e), and nonmesoporous nonimprinted photocatalyst (c and f).

Figure 7. (A) Degradation rate of tetracycline and (B) selectivity for adsorption of Cd2+ with CdS (a in A), CdS/Fe3O4 (b in A; e in B), mesoporous nonimprinted photocatalyst (c in A; b in B), nonmesoporous imprinted photocatalyst (d in A; c in B), MHP-Cd (e in A; a in B), and nonmesoporous nonimprinted photocatalyst (f in A; d in B).

Because CdS was direct transition absorption, n = 1 and the Eg of CdS was calculated by a plot of (Ahν)2 vs (hν) was approximately 2.4 eV. The magnetic property of MHP-Cd was measured by VSM. Figure S10 displayed the magnetic hysteresis loops of Fe3O4, CdS/Fe3O4, and MHP-Cd, and the magnetic property was measured by putting a magnet close to the sample (inset). The Ms value of Fe3O4 was highest (31.40 emu/g) among the above three samples. After loading CdS and coating the imprinted layer, the Ms values of CdS/Fe3O4 and MHP-Cd dropped; nevertheless, it could be explicitly seen from the photograph (inset) that the MHP-Cd still could be effectively separated with a magnet. The above result indicated that MHP-Cd had great magnetic separation property. Especially, the photograph of dispersion of the magnetic material in the solution under magnetic stirring was shown in Figure S11; it can be seen that magnetic stirring does not affect the dispersion of materials. From the results of adsorption and degradation, it can also reflect that the material is in full contact with the contaminants, and therefore, the effect is not affected by the magnetic properties. Photocurrent, EIS, and PL (the excitation wavelength of 345 nm) were conducted to further trace the transfers of e− and h+, which were shown in Figure S12. Compared with CdS/Fe3O4

and MHP-Cd, CdS represented the worst photocurrent response, highest impedance, and largest PL intensity, demonstrating that Fe3O4 could availably separate the photogenerated e− produced by CdS.38 Moreover, as shown in Figure S12b,c, the results of photocurrent, impedance, and PL of MHP-Cd resembled that of CdS/Fe3O4, demonstrating that the separation of the photogenerated e− was not influenced though the existence of the imprinted layer. 3.3. Adsorption Ability. The adsorption properties of various materials were studied. In Figure S13, it could be easily found that, for Cd2+ adsorption, the adsorption capacity of MHP-Cd without light in 60 min was the greatest (154.99 mg/ g), due to abundant Cd2+ imprinted cavities which had a certain capacity of specific recognition ability for Cd2+ formed in the layer of MHP-Cd. The kinetics were analyzed based on kinetics equations83 and displayed in Figure S14 and Table S1. It could be explicitly found that Cd2+ adsorption complied with pseudo-second-order kinetics, manifesting that the procedure was primarily chemical adsorption, providing evidence that Cd2+ was adsorbed by specific chemical groups of functional monomer. By calculation, it could be found that the maximum adsorption amount of the ideal Cd2+ imprinted cavities in MHP-Cd to Cd2+ was higher than the sum of the maximum drop amount of Cd2+ by CdS photocorrosion and the F


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ACS Applied Nano Materials Table 1. Selectivity (kions and kmaterials) kions sample

kions(Cd2+/Fe3+)

kions(Cd2+/Cu2+)

kions(Cd2+/Zn2+)

kmaterials

MHP-Cd mesoporous nonimprinted photocatalyst nonmesoporous imprinted photocatalyst nonmesoporous nonimprinted photocatalyst CdS/Fe3O4

2.836 0.778 0.847 0.773 0.709

2.303 0.636 0.622 0.523 0.475

3.064 0.974 1.082 0.992 0.928

2.47 2.69 3.49 4.74

maximum amount of Cd2+ pollutant in solution. Therefore, there were enough Cd2+ imprinted cavities in the imprinted layer to adsorb Cd2+. Furthermore, in Figure S13 (left), because many mesopores existed in the layer, the tetracycline adsorption capacity of MHP-Cd and mesoporous nonimprinted photocatalyst without light in 60 min were remarkably superior to those of other samples. In addition, after a half-hour, the desired adsorption stage was achieved. As a result, a half-hour was set as the desired adsorption time in the degradation experiments. The above results manifested that Cd2+ imprinted cavities and mesopores significantly worked in Cd2+ and tetracycline adsorption, respectively. Consequently, MHP-Cd showed decent adsorption ability for the above pollution adsorption. 3.4. Photocatalytic Degradation. For studying the degradation property of different materials, the experiment was carried out and the degradation rate was shown in Figure 7A. Prior to photodegradation, an adsorption procedure was conducted for a half-hour without irradiation for reaching the desired stage. So as to underline the excellent photocatalytic property of MHP-Cd, a series of experiments were carried out. It can be explicitly discovered that due to Fe3O4 being able to transfer a photoinduced electron from CdS, the degradation rate of CdS/ Fe3O4 was higher than CdS. More importantly, due to the existence of mesoporous and sodium pyrrolidone carboxylate (light transmittance material) in the imprinted layer, tetracycline could readily touch with CdS and most of the light can be adsorbed by CdS; the degradation rate of MHPCd (75.32%) was similar to that of CdS/Fe3O4 (80.12%). It was easy to find that the degradation rate of tetracycline by MHP-Cd was not much lower than that of CdS/Fe3O4. Because the imprinted layer could not transfer the electron, tetracycline can only be degraded by contacting the surface of CdS through the mesoporous channel. By comparing the degradation rate of tetracycline by MHP-Cd and CdS/Fe3O4, it was known that the presence of the imprinted layer did not inhibit the degradation of tetracycline by the MHP-Cd, which further demonstrated that the mesopores have a good mass transfer effect and a small mass transfer resistance. Moreover, the degradation rate of MHP-Cd was remarkably superior to other contrastive photocatalysts. It was worth mentioning that the concentration of tetracycline in the solution after light irradiation for 1 h was significantly lower than the concentration in the dark (Figure S15), indicating that the majority of tetracycline was degraded by reaching the surface of CdS and a small amount of tetracycline was adsorbed on the mesoporous channels. The results indicated that MHP-Cd had decent photocatalytic capacity for tetracycline degradation. 3.5. Selectivity. Competitive adsorption of various heavy metal ions was conducted from their mixed solutions, which was used to determine the selectivity of the MHP-Cd, which was shown in Figure 7B. Because a great deal of Cd2+

imprinted cavities were formed in the MHP-Cd, which could recognize Cd2+, in contract to others, MHP-Cd exhibited the best adsorption capacity (154.99 mg/g) for adsorption of Cd2+. In addition, on account of the difference between various ions, the Cd2+ imprinted cavities presented a relatively inferior adsorption capacity. Meanwhile, the anions selective adsorption properties of MHP-Cd were also researched. And the competitive adsorption of NO3−, Cl−, and SO42− with respect to Cd2+ was investigated for MHP-Cd shown in Figure S16, which also exhibited the higher adsorption capacity than anions in each solution. What’s more, the kions and kmaterials were exhibited in Table 1. It could be easily be found that the kions of Cd2+ to Fe3+, Cu2+, and Zn2+ over MHP-Cd (kions(Cd2+/Fe3+) = 2.836, kions(Cd2+/ Cu2+) = 2.303, and kions(Cd2+/Zn2+) = 3.064) were all remarkably superior to other materials. More importantly, kmaterials of MHP-Cd vs others were all greater than 1. The above consequences demonstrated that MHP-Cd owned the special recognition ability for Cd2+ selective adsorption. 3.6. Secondary Pollution Test. The ability of the imprinted layer to inhibit secondary pollution of CdS was investigated and presented in Figure S17. It could be explicitly found that after the light irradiation for 1 h, the concentration of Cd2+ was not increased too much for MHP-Cd, while, for CdS/Fe3O4, the concentration of Cd2+ was obviously increased. The cause was that during the light irradiation, although CdS was corroded and leaked into solution through mesoporous channels, on account of the existence of Cd2+ imprinted cavities around the mesoporous channels and on the surface of the imprinted layer, most of Cd2+ can be quickly adsorbed by abundant Cd2+ imprinted cavities; accordingly, the layer of MHP-Cd inhibited the secondary pollution of CdS. 3.7. Cycling Experiment. In order to prove the reusability of MHP-Cd, the recycled MHP-Cd was used for 10 cycles. As shown in Figure S18, after 10 cycles MHP-Cd had a high recovery rate; for degradation of tetracycline, the MHP-Cd still possessed a superior photocatalytic property, and similarly, after 10 cycles for adsorption of Cd2+, MHP-Cd still had a high adsorption capacity. It can be obviously found that MHP-Cd was able to be reused for many cycles with a negligible loss of photocatalytic property and adsorption ability, confirming that due to the magnetic property of the MHP-Cd, it can ensure sufficient separation and recovery of MHP-Cd at the end of each reaction, further demonstrating MHP-Cd was a sustainable material which owned superior stability and recyclability. 3.8. Photocatalytic and Selective Mechanism. For exploring the photocatalytic mechanism of tetracycline degradation with MHP-Cd, the corresponding experiments were carried out. Different quenchers were added during the photodegradation process. The degradation rates in existence of disparate quenchers were shown in Figure 8. It could be explicitly found that the additions of TEOA and N2 both had G


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Figure 8. Photodegradation rates for tetracycline degradation over the MHP-Cd by adding different quenchers (a), ESR signals DMPO−· OH (b) and ESR signals DMPO−·O2− (c) with irradiation of MHPCd. Figure 9. Proposed photocatalytic and selective mechanism of the MHP-Cd.

crucial impact on the photocatalytic activity; in contract, the existence of t-BuOH made a slight effect on the photocatalytic activity. As a result, h+ and •O2− were the dominant oxidative species in the photodegradation reaction; •OH played a smaller role. Panels b and c of Figure 8 also proved •OH and • O2− were both generated during the photodegradation reaction. In order to calculate the band positions of CdS, a Mott− Schottky experiment was conducted. As displayed in Figure S19, the flat band potential of CdS was −1.16 V vs Ag/AgCl. The flat band potential of CdS could be calculated to −0.54 V vs NHE according to the Nernst equation (eq 7).6 As known, the CB of n-type semiconductors bears 0−0.1 eV energy that exceeded the flat band potentials.6 In our experiment, the voltage difference between CB and the flat band potential was chosen as 0 eV; consequently, the calculated position of CB of CdS was −0.54 eV vs NHE. Combined with Eg of CdS (2.4 eV), the VB of CdS was 1.86 eV. Consequently, according the CB position and VB position of CdS, •OH was only generated from •O2− and cannot be generated form h+. θ E NHE = EAg/AgCl + 0.059pH + EAg/AgCl

CdS (e−) + O2 → •O2− •

transfer CdS (e−) ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Fe3O4 (e−)

(6)

(7)

•

O2 + H /H 2O2 → OH

(8)

→ CO2 + H 2O + other molecules •

(9)

O2− + tetracycline → CO2 + H 2O + other molecules (10)

•

OH + tetracycline → CO2 + H 2O + other molecules (11)

3.9. Photodegradation Intermediates Analysis. The mineralization rate of tetracycline after photodegradation with MHP-Cd was determined by TOC measurement; the result was presented in Figure S20. It could be easily found that about 44% tetracycline had been decomposed into carbon dioxide and water over MHP-Cd. The mineralization rate was obviously inferior to the degradation rate, demonstrating that a large number of products were formed during the photocatalytic procedure. MS was used to confirm the photodegradation intermediates to further research the photodegradation process. As shown in Figure S21a−c, the supposed photodegradation intermediates were speculated. Moreover, in Figure S22, m/z of 445 was tetracycline molecular ion, the tetracycline was framented into small molecules during photodegradation, the detailed process was as follows: m/z = 445 → 402 → 317 → 276 and m/z = 445 → 461 → 374 → 287. Eventually, tetracycline could be gradually decomposed by time into CO2, H2O, and other molecules. In addition, in order to prove that the product after degradation is less toxic than tetracycline, toxicological experiments were conducted on the intermediates. The results of acute toxicity tests in Figure S23 and Table S2, which indicated that photodegradation for 60 min can significantly decrease the toxicity of tetracycline.

where ENHE and EAg/AgCl were potential vs NHE and Ag/AgCl, respectively. The EθAg/AgCl was 0.197 eV.6,15 pH = 7.02. Combined with the above consequences, the supposed mechanism was presented in Figure 9. On the one hand, when MHP-Cd was dispersed in 100 mL of 100 mg/L Cd2+ solution, the majority of Cd2+ were selectively adsorbed by Cd2+ imprinted cavities and Cu2+, Fe3+, and Zn2+ were kept outside. On the other hand, when MHP-Cd was added into 20 mg/L tetracycline solution and irradiated by the light, the light can be absorbed by CdS through the transparent imprinted layer and the e− and h+ were formed in CdS; meanwhile, a vast amount of tetracycline could arrive in the surface of CdS by mesoporous channels. Afterward, one part of e− in CdS transferred into Fe3O4,38 which was beneficial to restrain the combination of h+ and e−; meanwhile, another part of e− was trapped by O2 to form •O2− and •OH. Eventually, tetracycline was transferred to CO2, H2O, and smaller molecules by h+, • O2−, and •OH. The corresponding reaction procedure were presented as follows: (5)

+

CdS (h+) + tetracycline

(4)

MHP −Cd + hν → CdS (h+) + CdS (e−)

−

4. CONCLUSION In summary, MHP-Cd had been compounded through the ion imprinting technique; 0.5 g and 3.0 mL were the optimal adding doses of Cd(NO3) 2·4H2O and P123, respectively. The as-prepared nanoreactor possessed good light response property, magnetic separation ability, a superior stability and recyclability. In addition, the existence of Cd2+ imprinted H



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cavities improved the selective adsorption ability of Cd2+. Meanwhile, the existence of mesoporous and sodium pyrrolidone carboxylate (light transmittance material) in the imprinted layer realized the contact of tetracycline to CdS and allowed most of the light to be absorbed by CdS, which resulted in great photocatalytic activity for tetracycline degradation. Therefore, the as-prepared nanoreactor realized the aim for selectively adsorbing Cd2+ and synchronously degrading tetracycline. The photodegradation rate of the asprepared nanoreactor for tetracycline degradation with irradiation of 60 min was 75.32%. The kions of Cd2+ to Fe3+, Cu2+, and Zn2+ over MHP-Cd (kions(Cd2+/Fe3+) = 2.836, kions(Cd2+/Cu2+) = 2.303, and kion (Cd2+/Zn2+) = 3.064) were all superior to others, and the kmaterials of MHP-Cd relative to other materials was all greater than 1. Above all, the coexistence of imprinted layer and Cd2+ imprinted cavities availably restrained the secondary pollution of CdS. Herein, our work provides a new technical approach for selective adsorption of targeted heavy metal ions and synchronous degradation of antibiotic contaminants in mixed water.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00113. Results of factor investigation, additional XRD, FT-IR, SEM, N2 adsorption−desorption isotherms results, dynamic curves of adsorption and adsorption kinetics, Mott−Schottky curve of CdS, mineralization rate and degradation product analysis, method and analysis of cation selectivity, and acute toxicity tests (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.W.). *E-mail: [email protected]; [email protected] (G.X.). ORCID

Ziyang Lu: 0000-0001-8873-3695 Xiangyang Wu: 0000-0003-3366-6012 Peng Du: 0000-0002-8668-038X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 21607062), the Natural Science Foundation of Jiangsu Province (Grant Nos. BK20160494 and BK20150536), the China Postdoctoral Science Foundation (Grant Nos. 2016M600378 and 2017T100333), Opening Project of Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University (Grant No. 2017001), the Social Development Project of Key Research Program of Zhenjiang (Grant Nos. SH2018021 and SH2016018), the Youth Talent Development Program of Jiangsu University, and the Jiangsu Collaborative Innovation Center of Technology and Material of Water Treatment. I


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