conjugated polyvinyl chloride derivative nanocomposite with higher visible-light photocatalytic activity for treating Cr(VI)-polluted water

Journal Pre-proofs Magnetically recoverable MgFe2O4/conjugated polyvinyl chloride derivative nanocomposite with higher visible-light photocatalytic activity for treating Cr(VI)-polluted water Zhikang Jiang, Kexu Chen, Yongcai Zhang, Yuanyou Wang, Fang Wang, Geshan Zhang, Dionysios D. Dionysiou PII: DOI: Reference:

S1383-5866(19)33821-3 https://doi.org/10.1016/j.seppur.2019.116272 SEPPUR 116272

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

26 August 2019 29 October 2019 30 October 2019

Please cite this article as: Z. Jiang, K. Chen, Y. Zhang, Y. Wang, F. Wang, G. Zhang, D.D. Dionysiou, Magnetically recoverable MgFe2O4/conjugated polyvinyl chloride derivative nanocomposite with higher visiblelight photocatalytic activity for treating Cr(VI)-polluted water, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.116272

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Magnetically recoverable MgFe2O4/conjugated polyvinyl chloride derivative nanocomposite with higher visible-light photocatalytic activity for treating Cr(VI)-polluted water

Zhikang Jiang a, Kexu Chen a, Yongcai Zhang a,*, Yuanyou Wang b, Fang Wang c, Geshan Zhang d, Dionysios D. Dionysiou e a

School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China b

c

Yangzhou Polytechnic Institute, Yangzhou 225127, China

School of Chemistry and Materials Science of Shanxi Normal University & Key Laboratory of

Magnetic Molecules and Magnetic Information Materials of Ministry of Education, Linfen 041004, China d

College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China e

Environmental Engineering and Science Program, Department of Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221-0012, USA *E-mail address: [email protected] (Y. Zhang)

1

ABSTRACT A facile three-step method was adopted to synthesize MgFe2O4/conjugated polyvinyl chloride (CPVC) nanocomposite. XRD, Raman, XPS, SEM, EDX, element mapping, and HRTEM analyses confirmed the formation of MgFe2O4/CPVC nanocomposite. The optical and magnetic properties of the obtained MgFe2O4/CPVC nanocomposite were measured using UV-Vis spectrophotometer and vibrating sample magnetometer, respectively. Through photocatalytic reduction of aqueous Cr(VI) under visible-light (λ > 420 nm) irradiation, it can be noticed that MgFe2O4/CPVC nanocomposite had both improved visible-light photocatalytic activity (about 2.1 times that of MgFe2O4 nanoparticles) and good photocatalytic stability. Moreover, MgFe2O4/CPVC nanocomposite can also be easily recovered by use of a magnet. Based on electrochemical impedance, transient photocurrent response and Mott-Schottky measurement, the mechanism responsible for the enhanced photocatalytic activity of MgFe2O4/CPVC nanocomposite was proposed. Finally, MgFe2O4/CPVC nanocomposite was applied to photocatalytic treatment of the diluted black chromium electroplating solution at different pH. Results showed that MgFe2O4/CPVC nanocomposite had significant visible-light photocatalytic activity in treating the diluted black chromium electroplating solution over a wide pH range (1.4–10.3), and its treatment efficiency increased with the decrease of solution pH. Thus, MgFe2O4/CPVC nanocomposite has the potential to be a new magnetically recoverable, high-performance visible-light photocatalyst for treating Cr(VI)-polluted water.

Keywords: MgFe2O4/CPVC nanocomposite; Photocatalytic enhancement; Magnetic recovery; Cr(VI) treatment

2

1. Introduction The production and applications of Cr(VI)-containing compounds have brought about a massive amount of Cr(VI)-polluted water. Cr(VI) is highly toxic and mobile, and can cause serious damage to aquatic ecological environment and human health. For example, excessive exposure to Cr(VI) can cause liver diseases, kidney failure, lung cancer, diarrhea, nausea, respiratory troubles and ulcer formation to human beings. As a result, Cr(VI) is recognized worldwide as a priority pollutant in water [1–4]. In contrast, Cr(III) has much lower toxicity and is an essential trace element for creature [1–4]. Furthermore, Cr(III) is easy to precipitate out from the aqueous solution (Ksp(Cr(OH)3) = 6.3 × 10–31 [3]). Therefore, a common treatment method for Cr(VI)-polluted water is to reduce its Cr(VI) to Cr(III). Compared with routine chemical reduction, photocatalytic reduction of Cr(VI) has distinctive advantages of convenient utilization of sunlight as the reaction driving force, no or less secondary pollution, and lower cost [1–4]. So, photocatalytic reduction method is more sustainable for the treatment of Cr(VI) pollutant in water. However, at present, the lack of satisfactory photocatalysts is still a big stumbling block to the industrial application of photocatalytic reduction method. Most of the synthesized photocatalysts suffer one or more shortcomings, such as low visible-light photocatalytic activity, difficult recovery from the aqueous suspension, poor reusability or high cost, etc. Hence, great effort has been continuously devoted to the modification of existing photocatalysts or development of new visible-light photocatalysts [5–17]. Semiconductor magnetic materials, which at least have semiconducting and magnetic properties, are multi-functional materials that can be applied in numerous fields [18–28]. Especially, the research on the application of semiconductor magnetic materials as photocatalysts has recently received increasing interest, because semiconductor magnetic materials (such as spinel ferrites) not 3

only have visible-light photocatalytic activity and good photochemical stability, but can also be simply and rapidly separated from the solution by using a magnet [25–28]. Easy recovery of photocatalysts facilitates their reuse, leading to reduced treatment cost and easier realization of industrial applications. Spinel MgFe2O4 is made of abundant, cheap and nontoxic Mg, Fe and O elements. It has the typical properties of both semiconductor and magnetic material [26–30]. As a semiconducting material, MgFe2O4 has a narrow band gap of 1.7–2.0 eV [26–28], which enables it to absorb a large portion of visible-light. In addition, it has good resistance to photocorrosion [26–28]. As a magnetic material, MgFe2O4 has good magnetism, which enables it to be easily recovered from the aqueous suspension by use of a magnet [26–30]. Therefore, MgFe2O4 holds the potential for application as a magnetically recoverable visible-light photocatalyst. However, the photocatalytic activity of MgFe2O4 is low, because its photoexcited carriers have a high recombination rate. As a result, MgFe2O4 has been used mostly as photocatalyst support or modifier [30–32], rather than photocatalyst so far. Considering MgFe2O4 has the outstanding advantages of low cost, nontoxicity, excellent visible-light absorption, good photocorrosion resistance, and easy magnetic separation, it should be very meaningful to explore effective and practical ways to enhance the photoexcited carrier separation of MgFe2O4 for the development of efficient MgFe2O4-based photocatalysts [33–35]. It has been shown that conjugated polymers are effective modifiers for enhancing the photocatalytic activity of many semiconductor materials [36–40]. It is considered that conjugated polymers have the following synergistic enhancement effects on photocatalysis [36–40]. Firstly, conjugated polymers have noteworthy visible-light-absorbing ability, so they can produce additional photoexcited carriers for photocatalysis under visible-light irradiation. Secondly, conjugated 4

polymers are efficient in transporting charges, and have matched band structures with the modified semiconductor materials, which would expedite the interfacial charge transfer and photoexcited carrier separation. Nonetheless, to date, the conjugated polymers exploited for modifying semiconductor photocatalysts are mostly high cost polythiophene, polypyrrole, cyclized polyacrylonitrile and polyaniline [36–40]. As is a cheap and widely used polymer, polyvinyl chloride (PVC) has no conjugated structure. Nevertheless, it can undergo dehydrochlorination upon proper heating, producing conjugated polyene structure derivatives (CPVC) [41–45]. CPVC can absorb visible-light and has good ability in transferring photoexcited charges [41–45]. Therefore, CPVC has been recently studied to modify semiconductor photocatalysts (including TiO2 [41], CdS [42], SnO2 [43], g-C3N4 [44] and SnS2 [45]) to achieve more efficient visible-light photocatalysis. However, all the modified semiconductor photocatalysts have no magnetism, which would meet great difficulty in recovery and reuse. Moreover, most of the pollutants selected for photocatalytic evaluation were organic dyes [41–44]. This study investigated the feasibility of using CPVC to modify magnetic MgFe2O4 to enhance its photoexcited charge separation, thus improving its photocatalytic activity. Our aim was to develop MgFe2O4/CPVC composite as a new magnetically recoverable, high-performance visible-light photocatalyst for the treatment of Cr(VI)-polluted water. 2. Experimental 2.1. Synthesis MgFe2O4/CPVC nanocomposite was produced under the optimized conditions (the synthesis optimization is provided in Table S1 and Fig. S1–S2 in the Supplementary material), according to the three steps described below: 5

(1) Nanocrystalline MgFe2O4 was synthesized via solvothermal treatment of 1 mmol magnesium acetate and 2 mmol iron(III) acetylacetonate in 30 mL benzyl alcohol at 200 °C for 24 h; (2) After the dissolution of 20 mg PVC powder (which has a polymerization degree of 1350–1250) in 20 mL tetrahydrofuran, 1000 mg of the solvothermally synthesized MgFe2O4 nanoparticles were introduced into them. The resultant mixture was supersonically treated for 2 h, then dried in air at 65 °C for 4 h to remove tetrahydrofuran, leading to MgFe2O4/PVC nanocomposite; and (3) The MgFe2O4/PVC nanocomposite was heated in air at 150 °C for 2 h for transforming its PVC into CPVC. CPVC was obtained by thermal dehydrochlorination of 500 mg PVC at 150 °C for 2 h in air. 2.2. Characterization and photocatalytic evaluation The characterization of structure and composition used FEI Tecnai G2 F30 S-TWIN field-emission TEM, Hitachi S-4800 Field Emission SEM having an accessory EDX Analyzer, Renishaw Invia Raman spectrometer, Thermo Scientific ESCALAB 250Xi XPS system, and Bruker AXS D8 ADVANCE X-ray diffractometer. The measurement of optical properties used Varian Cary 5000 UV-Vis-NIR spectrophotometer. Magnetic measurements used LakeShore 7407 vibrating sample magnetometer. Electrochemical impedance measurements used Bio-Logic VMP3 electrochemical workstation. Mott-Schottky plots as well as transient photocurrent measurements used CHI660E Electrochemical Workstation with a three-electrode cell. Platinum rod, saturated calomel electrode and sample-coated glassy carbon electrode served as the counter electrode, reference electrode and working electrode, respectively. A Philips 23 W fluorescent (incandescent light) lamp was adopted as the irradiation source. Electrochemical impedance measurements were carried out in an aqueous solution containing 5 mmol/L Fe(CN)63−/Fe(CN)64− and 0.1 mol/L KCl, whereas the measurements 6

of Mott-Schottky plots and transient photocurrent responses were performed in an aqueous solution containing 0.1 mol/L Na2SO4. Photocatalytic performance of the obtained products (1 g/L) was assessed through visible-light (λ > 420 nm)-induced catalytic reduction of Cr(VI) in a mixture of 1 mL 85 g/L citric acid (CA) aqueous solution and 300 mL 20 mg/L K2Cr2O7 aqueous solution. In addition, MgFe2O4/CPVC nanocomposite was also applied to treat the black chromium electroplating solution (containing 250 g/L CrO3, 20 g/L H3BO3, 7 g/L NaNO3 and 0.1 mL/L H2SiF6) after 10000-fold dilution, with the addition of 157.6 g/L CA. The role of CA was to scavenge the photoinduced holes. The Cr(VI) concentrations in the solution were determined using the diphenylcarbazide colorimetric method. The detailed photocatalytic procedures and instrument are given in the Supplementary materials. 3. Results and discussion C=C MgFe2O4/CPVC MgFe2O4

(440)

Intensity (a. u.)

(511)

(422)

(400)

(220) MgFe2O4

Intensity (a. u.)

(b)

(311)

(a)

MgFe2O4/CPVC

C-C

A1g Eg

CPVC

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Fig. 1 The XRD patterns of the obtained samples are provided in Fig. 1(a). CPVC displays no distinct diffraction peaks, revealing its amorphous nature. The solvothermally synthesized MgFe2O4 shows the typical diffraction peaks of spinel MgFe2O4 (JCPDS File no. 17-0464). After introduction of CPVC, MgFe2O4/CPVC composite also demonstrates similar diffraction peaks as MgFe2O4. Due to 7

its low content and amorphous structure, the diffraction peaks of CPVC are not detected in the obtained MgFe2O4/CPVC composite. But, the following Raman, XPS, EDX, element mapping and HRTEM analyses verify that there exists CPVC in the obtained MgFe2O4/CPVC composite. The Raman spectra of the obtained MgFe2O4/CPVC and MgFe2O4 are provided in Fig. 1(b). MgFe2O4 displays discernable Raman peaks at about 318, 476 and 689 cm−1, which in sequence correspond to the Eg, F2g and A1g modes of spinel MgFe2O4 [46–49]. In contrast, MgFe2O4/CPVC displays not only the typical Raman peaks of MgFe2O4, but also the C=C Raman peak at approximately 1530 cm−1 and the C–C Raman peak at approximately 1132 cm−1 originating from CPVC [50,51]. Fig. 2 shows the XPS analysis of the composition and elemental valence of the obtained MgFe2O4/CPVC. As indicated by the survey XPS spectrum, the obtained MgFe2O4/CPVC contains Mg, Fe, O, Cl and C elements. The high resolution XPS spectrum of Mg 1s displays a peak at about 1302.7 eV, suggesting Mg element exists in the form of Mg2+ [52]. In the Fe 2p core level XPS spectrum, three peaks are identified at approximately 724.5, 719.2 and 710.8 eV, which can be in turn assigned to Fe 2p1/2, shake-up satellite and Fe 2p3/2 of Fe3+ [52]. The XPS spectrum of O 1s core level has been fitted into three peaks at about 532.7, 531.4 and 529.9 eV, which in sequence correspond to C–O/C=O, surface –OH, lattice O2– [52]. The XPS spectrum of C 1s core level has been divided into three peaks at about 288.3, 286.1 and 284.6, which may correspond to the C species in C=O, C–O and C=C/C–H/C–C [52], respectively. Besides, the XPS spectrum of Cl 2p has been deconvoluted into two peaks at 201.2 and 199.5 eV, which respectively belong to Cl 2p1/2 and Cl 2p3/2 of CPVC [52]. Because MgFe2O4 and its precursors contain no Cl, the Cl in the obtained MgFe2O4/CPVC should originate from CPVC. Hence, the above XPS analysis manifests that the 8

obtained MgFe2O4/CPVC is composed of MgFe2O4 and CPVC. Survey

C 1s Cl 2p

Intensity (a. u.)

O 1s

Fe 2p

Intensity (a. u.)

Mg 1s

Mg 1s

1200

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800 600 400 Binding energy (eV)

Fe 2p

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1306

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2p3/2

M-O

Intensity (a. u.)

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2p1/2

satellite

735

730

725 720 715 Binding energy (eV)

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705

-OH

C=O/C-O

535

534

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532 531 530 529 Binding energy (eV)

Cl 2p

C 1s

2p3/2

292

C=O

290

Intensity (a. u.)

Intensity (a. u.)

C=C/C-C/C-H

C-O

288 286 284 Binding energy (eV)

282

206

280

Fig. 2

9

2p1/2

204

202 200 198 Binding energy (eV)

196

194

Fig. 3 The SEM image, EDX spectrum and element mapping images of the obtained MgFe2O4/CPVC are provided in Fig. 3(a), (b) and (c–h), respectively. The SEM image in Fig. 3(a) shows that the obtained MgFe2O4/CPVC consists of nanoparticle aggregates. From the EDX spectrum in Fig. 3(b), it can be seen that the obtained MgFe2O4/CPVC contains Mg, Fe, O, C and Cl elements. Furthermore, 10

the element mapping images demonstrate that Mg, Fe, O, C and Cl elements are uniformly distributed in the obtained MgFe2O4/CPVC composite (Fig. 3(c–h)).

Fig. 4 The TEM image of MgFe2O4 is shown Fig. 4(a), whereas the TEM and HRTEM images of MgFe2O4/CPVC composite are respectively presented in Fig. 4(b1) and (b2). From Fig. 4(a) and (b1), it is seen that both MgFe2O4 and MgFe2O4/CPVC comprise nanoparticles with sizes of about 3–9 nm. The morphology and size of both samples show no noticeable difference. The HRTEM image of 11

MgFe2O4/CPVC composite in Fig. 4(b2) displays clearly the lattice fringes corresponding to (311) crystal planes (d = 0.26 nm) of spinel MgFe2O4, as well as amorphous CPVC on the surface of MgFe2O4 nanocrystallites. Therefore, the foregoing Raman, XPS, EDX, element mapping and HRTEM analyses confirmed the synthesis of MgFe2O4/CPVC nanocomposite. (a)

MgFe2O4 MgFe2O4/CPVC CPVC

1.0

Absorbance

0.8 0.6 0.4 0.2 0.0 200

300

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MgFe2O4/CPVC

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Fig. 5 Fig. 5(a) shows the UV-vis absorbance spectra of CPVC, MgFe2O4/CPVC and MgFe2O4, which were converted from their diffuse reflection spectra according to the Kubelka-Munk formula [53–55]. It is observed from Fig. 5(a) that all of the three samples have the ability to absorb visible-light. Nevertheless, their visible-light absorption capacities are apparently in the order of CPVC < 12

MgFe2O4/CPVC < MgFe2O4. Furthermore, the Eg values of CPVC, MgFe2O4/CPVC and MgFe2O4 are obtained to be 1.88, 1.80 and 1.72 eV, respectively, based on the Tauc plots drawn in Fig. 5(b) and (c). Hence, the higher photocatalytic activity of MgFe2O4/CPVC is unlikely due to its weaker visible-light absorption capacity, but should be ascribed to its more efficient separation and transfer of photoexcited carriers (Fig. 6(a) and (b)). 0.10

(a)

MgFe2O4/CPVC MgFe2O4 CPVC

(b)

CPVC MgFe2O4 MgFe2O4/CPVC

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Fig. 6 The separation and transfer efficiency of photoexcited carriers of CPVC, MgFe2O4 and MgFe2O4/CPVC was investigated through transient photocurrent response and electrochemical impedance spectra. Fig. 6(a) shows the transient photocurrent responses of CPVC, MgFe2O4 and MgFe2O4/CPVC when exposed to periodic irradiation from a fluorescent lamp. CPVC shows almost zero photocurrent, suggesting its photoexcited electron-hole pairs end up in complete recombination. MgFe2O4 exhibits very small photocurrent, suggesting its photoexcited electron-hole pairs recombine very rapidly. In contrast, MgFe2O4/CPVC exhibits much larger photocurrent. The photocurrent generated by MgFe2O4/CPVC is almost 11 times as that generated by MgFe2O4 under the same fluorescent irradiation. So, compared with MgFe2O4 and CPVC, MgFe2O4/CPVC is much more efficient in the separation of photoexcited carriers [56–59]. Besides, as can be observed from the 13

electrochemical impedance spectra of CPVC, MgFe2O4 and MgFe2O4/CPVC in Fig. 6(b), the Nyquist arc radius of MgFe2O4/CPVC is shorter than those of MgFe2O4 and CPVC. The Nyquist arc radius is inversely proportional to the transfer efficiency of charge carriers [60–62]. Thus, the photoexcited carriers of MgFe2O4/CPVC have higher transfer efficiency, which can in turn expedite their separation [60–62]. The enhancement in the separation and transfer of photoexcited carriers should make contribution to the better photocatalytic activity of MgFe2O4/CPVC nanocomposite. 3.0

(a)

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(b) -1

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MgFe2O4/CPVC: k=0.017 min; R =0.983

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MgFe2O4: k=0.008 min ; R =0.997

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CPVC MFO MFO/CPVC

ln(Ci0/Cit)

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Fig. 7 The performance of MgFe2O4, MgFe2O4/CPVC and CPVC in dark adsorption and visible-light-induced catalytic reduction of aqueous Cr(VI) is shown in Fig. 7(a). It is noticed from Fig. 7(a) that under the dark condition, the Cr(VI) concentration showed almost no decrease as the adsorption time was prolonged from 100 to 120 min, indicating that the adsorption of Cr(VI) by the three samples all reached equilibrium after 100 min. The equilibrium Cr(VI) adsorption amounts by CPVC, MgFe2O4 and MgFe2O4/CPVC were in turn 2.4%, 28.6% and 32.5%. When the visible-light irradiation was started, the concentration of Cr(VI) in the aqueous suspensions of MgFe2O4 and MgFe2O4/CPVC further decreased. Moreover, the Cr(VI) concentration in the aqueous suspensions of MgFe2O4 and MgFe2O4/CPVC showed continual decrease with the extension of irradiation time. 14

After 160 min of visible-light irradiation, the Cr(VI) concentration in the aqueous suspensions of MgFe2O4/CPVC and MgFe2O4 has decreased to zero and 15.8%, respectively. Thus, it can be inferred that both MgFe2O4/CPVC and MgFe2O4 have visible-light photocatalytic activity for the reduction of Cr(VI). By contrast, the Cr(VI) concentration in the aqueous suspension of CPVC showed little decrease with the prolongation of irradiation time, suggesting the photocatalytic activity of CPVC was negligible. This is because the photoexcited electron-hole pairs of CPVC undergo complete recombination. For quantitative comparison of photocatalytic efficiency, the Cr(VI) reduction reaction rate constants (k) in the aqueous suspensions of MgFe2O4 and MgFe2O4/CPVC were acquired by adopting the pseudo-first-order reaction kinetic equation (Eq. 1) [63–65]: ln(Ci0/Cit) = kti

(1)

where Cit and Ci0 represent in turn the Cr(VI) concentration at the irradiation time (ti) of t and 0 min. From the plots of ln(Ci0/Cit) vs. ti in Fig. 7(b), the k values in the aqueous suspensions of MgFe2O4 and MgFe2O4/CPVC were obtained to be 0.008 and 0.017 min–1, respectively. Evidently, the photocatalytic efficiency of MgFe2O4/CPVC is about 2.1 times that of MgFe2O4. Hence, coupling with CPVC is effective for increasing the photocatalytic activity of MgFe2O4. 1.0 1st

2nd

3rd

4th

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Ct/C0

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560 840 Time (min)

Fig. 8 15

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Intensity (a. u.)

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Fig. 9 The photocatalytic reusability and stability are also the major concerns for a new photocatalyst. Fig. 8 shows the reusability of MgFe2O4/CPVC during the first five cycles of photocatalytic reduction of aqueous Cr(VI). Each cycle consisted of dark adsorption for 120 min and visible-light irradiation for 160 min. Whenever each photocatalytic cycle was finished, MgFe2O4/CPVC was separated by use of a magnet, washed with H2O, and finally dried in air at 85 °C for 10 h. Considering the loss of MgFe2O4/CPVC in regular spectrophotometric determination of Cr(VI) concentrations, the former photocatalytic cycle was repeated twice for accumulation of sufficient sample for the next cycle. As seen from Fig. 8, the photocatalytic efficiency of MgFe2O4/CPVC showed no significant decrease during the first five cycles. At the end of the fifth cycle, the reduced percentage of Cr(VI) could still reach 96%. Furthermore, the XRD pattern (Fig. 9(a)), Raman spectrum (Fig. 9(b)) and component element (including Mg, Fe, O, C and Cl) XPS spectra (Fig. 10) of the MgFe2O4/CPVC recovered after photocatalytic application (MgFe2O4/CPVC-R) remained almost unchanged compared with those of fresh MgFe2O4/CPVC, suggesting MgFe2O4/CPVC kept the same composition after experiencing the photocatalytic reactions and played only the role of catalyst. Therefore, it can be deduced that MgFe2O4/CPVC has favorable reusability and stability for 16

application in photocatalytic treatment of Cr(VI)-polluted water. Survey

1200

1000

C 1s

800 600 400 Binding energy (eV)

Fe 2p

Cl 2p

Intensity (a. u.)

O 1s Cr 2p

Intensity (a. u.)

Fe 2p

Mg 1s

Mg 1s

200

0

1306

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Intensity (a. u.)

Intensity (a. u.)

2p1/2

satellite

735

730

725 720 715 Binding energy (eV)

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-OH

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535

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528

527

Cl 2p C=C/C-C/C-H

292

C=O

290

Intensity (a. u.)

Intensity (a. u.)

2p3/2

C-O

288 286 284 Binding energy (eV)

282

280

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17

2p1/2

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202 200 198 Binding energy (eV)

196

194

Cr 2p 2p3/2

Intensity (a. u.)

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600

595

590

585 580 575 Binding energy (eV)

570

565

Fig. 10 The reduced product of Cr(VI) was also identified by XPS analysis of MgFe2O4/CPVC-R. The survey XPS spectrum of MgFe2O4/CPVC-R in Fig. 10 showed the existence of O, Fe, Mg, C, Cl and Cr. The peak shapes and positions of O 1s, Mg 1s, Fe 2p, C 1s and Cl 2p XPS spectra of MgFe2O4/CPVC-R (Fig. 10) displayed no appreciable difference from those of fresh MgFe2O4/CPVC (Fig. 2), suggesting the chemical states of the five component elements of MgFe2O4/CPVC have not changed. The XPS spectrum of Cr 2p core level of MgFe2O4/CPVC-R showed two peaks with the binding energies at 587.6 and 576.9 eV, which can be in turn assigned to 2p1/2 and 2p3/2 of Cr(III) [52]. Therefore, through the photochemical processes catalyzed by MgFe2O4/CPVC, the high-toxicity Cr(VI) is reduced to low-toxicity Cr(III). For obtaining the band structures of CPVC and MgFe2O4 to illuminate the photocatalytic enhancement mechanism of MgFe2O4/CPVC nanocomposite, the Mott-Schottky plots of CPVC and MgFe2O4 were determined. According to Fig. 11, the flat band potentials of CPVC and MgFe2O4 are about –0.66 and –0.36 V vs. SCE (correspondingly, –0.42 and –0.12 V vs. NHE), respectively. Moreover, the straight-line portions of their Mott-Schottky plots have the positive slopes, manifesting that CPVC and MgFe2O4 can be considered as n-type semiconductors. There is a 18

consensus that the conduction band (CB) potentials of n-type semiconductors are usually 0.1 V more negative relative to their flat band potentials [66–68]. Thus, the CB potential of MgFe2O4 and the lowest unoccupied molecular orbital (LUMO) potential of CPVC are in turn –0.22 and –0.52 V vs. NHE. Accordingly, the valence band (VB) potential of MgFe2O4 and the highest occupied molecular orbital (HOMO) potential of CPVC are +1.50 and +1.36 V vs. NHE, respectively, obtained based on the following computational formula (Eq. 2): ECB = EVB − Eg

(2)

where ECB and EVB denote respectively CB and VB potential. Evidently, CPVC and MgFe2O4 have matched energy band structures for forming type-II heterojunction (Scheme 1). When MgFe2O4/CPVC nanocomposite is under visible-light irradiation, both CPVC and MgFe2O4 can be activated to generate photoexcited electrons (e–) and holes (h+). Under the drive of potential difference, h+ in the VB of MgFe2O4 can transfer to the HOMO of CPVC, whereas e– in the LUMO of CPVC can transfer to the CB of MgFe2O4. Because e– and h+ of MgFe2O4/CPVC nanocomposite move in opposite directions, their recombination probability is effectively reduced. The enhanced charge separation of MgFe2O4/CPVC can supply more e– and h+ for photocatalysis, resulting in higher photocatalytic efficiency. Because the reduction potentials of Cr2O72−/Cr3+ and CrO42−/Cr(OH)3 redox couples (E(Cr2O72−/Cr3+) = +1.36 V, and E(CrO42−/Cr(OH)3) = –0.13 V vs. NHE) are more positive than the CB potential (–0.22 V vs. NHE) of MgFe2O4, it is thermodynamically feasible for e– in the CB of MgFe2O4 to reduce Cr(VI) to Cr(III). Thus, e– in the CB of MgFe2O4 took the responsibility to reduce Cr(VI) to Cr(III). Meantime, h+ in the HOMO of CPVC were depleted in the oxidation of CA (Scheme 1).

19

1.6

1.0 MgFe2O4

CPVC

4

C (10 F cm )

1.2

9

-2

0.6

0.8

-2

-2

9

-2

4

C (10 F cm )

0.8

0.4

0.4

0.2

0.0 -1.0

-0.8

-0.6

-0.4 -0.2 0.0 0.2 Potential (V vs. SCE)

0.4

0.0 -1.0

0.6

-0.8

-0.6 -0.4 -0.2 Potential (V vs. SCE)

0.0

0.2

Fig. 11

Scheme 1. Photocatalytic reduction of Cr(VI) by MgFe2O4/CPVC nanocomposite under visible-light irradiation in the presence of CA.

Since the recovery of a photocatalyst is also an important factor affecting its practical applications, the magnetic property of MgFe2O4/CPVC nanocomposite and its magnetic recovery after photocatalytic use were further examined. Fig. 12(a) shows the room temperature magnetization curves of MgFe2O4/CPVC nanocomposite and pure MgFe2O4. The saturation magnetization of 20

MgFe2O4/CPVC nanocomposite was determined to be 24.2 emu/g, which is slightly smaller than that (25.5 emu/g) of pure MgFe2O4. This suggests that MgFe2O4/CPVC nanocomposite also has relatively strong magnetism, which makes it feasible for magnetic recovery. Indeed, MgFe 2O4/CPVC nanocomposite could be easily separated from the aqueous suspension by using a piece of magnet, as illustrated in Fig. 12(b). 30

(a)

20

MgFe2O4 MgFe2O4/CPVC

M (emu/g)

10 0 -10 -20 -30 -20

-15

-10

-5

0

5

10

15

20

H (kOe)

Fig. 12 3.5

(a)

1.0

pH = 10.3 pH = 7.3 pH = 5.6 pH = 3.5 pH = 1.4

0.8

Ct/C0

0.4

-1

2

pH=1.4: k=0.018 min; R =0.994 -1 2 pH=3.5: k=0.014 min; R =0.990 -1 2 pH=5.6: k=0.011 min; R =0.999 -1 2 pH=7.3: k=0.010 min; R =0.999 -1 2 pH=10.3: k=0.006 min; R =0.990

3.0 2.5 ln(Ci0/Cit)

0.6

(b)

2.0 1.5 1.0

0.2 0.5 Dark

0.0

Irradiated

0.0

0

40

80

120 160 t (min)

200

240

0

20

40

60

80 ti (min)

100

120

140

Fig. 13 MgFe2O4/CPVC nanocomposite was applied to photocatalytic treatment of the black chromium electroplating solution (which contained 250 g/L CrO3, 20 g/L H3BO3, 7 g/L NaNO3 and 0.1 mL/L 21

H2SiF6) after 10000-fold dilution. The pH of the diluted black chromium electroplating solution was adjusted from 1.4 to 10.3, using 1 mol/L HCl or NaOH aqueous solution. As seen in Fig. 13, MgFe2O4/CPVC nanocomposite showed significant visible-light photocatalytic activity in treating the diluted black chromium electroplating solution with pH values ranging from 1.4 to 10.3. This indicated that MgFe2O4/CPVC nanocomposite can be applied to photocatalytic treatment of Cr(VI)-polluted water over a wide pH range. Nevertheless, the Cr(VI) removal efficiency by MgFe2O4/CPVC nanocomposite was increased with lowering the pH value of the diluted black chromium electroplating solution. When the initial pH values of the diluted black chromium electroplating solution were 10.3, 7.3, 5.6, 3.5 and 1.4, the dark equilibrium adsorption amounts of Cr(VI) by MgFe2O4/CPVC nanocomposite were in sequence 12.8%, 17.7%, 25.1%, 32.3% and 51%, whereas the k values obtained from the pseudo-first-order reaction kinetic plots in Fig. 13(b) were in turn 0.006, 0.010, 0.011, 0.014 and 0.018 min–1. The current pH effects are in consistence with the previous reports [69–75], and can be justified by the following aspects: (i) the surface of photocatalyst would be more positive at lower pH, so increasing the electrostatic attraction of CrO42− and Cr2O72− anions (Cr(VI) exists mainly as CrO42− in the aqueous solution with pH value above 7, whereas mostly as Cr2O72− in the aqueous solution with pH value below 7 [76]). Moreover, the proportion of Cr2O72− increases at lower solution pH; (ii) the reduction of Cr(VI) by e– of photoexcited MgFe2O4/CPVC nanocomposite to Cr(III) may be via the overall reactions of Eq. (3) (for acidic solution) and Eq. (4) (for alkaline solution): Cr2O72− + 14H+ + 6e− → 2Cr3+ + 7H2O CrO42− + 4H2O + 3e− → Cr(OH)3 + 5OH−

(3) (4)

According to Le Chatelier’s principle, lower pH (that is, more H+ or less OH−) would facilitate the 22

above (3) and (4) reactions; (iii) the Nernst reduction potential of CrO42−/Cr(OH)3 is much lower than that of Cr2O72−/Cr3+. Therefore, from the thermodynamic viewpoint, Cr2O72− is more prone to be reduced in comparison with CrO42−. Moreover, with lowering one pH unit, the Nernst reduction potentials of HCrO4−/Cr3+ and Cr2O72−/Cr3+ redox couples can go up 138 mV, whereas the CB potentials of semiconductors would increase 59 mV [70–72]. As a result, the thermodynamic driving force for the reduction of Cr(VI) by e– of photoexcited semiconductors would increase 79 mV when the solution pH is lowered by one unit; and (iv) with the lowering of solution pH, the sedimentation of Cr(III) species on the surface of photocatalyst decreased, leaving more surface active sites available for adsorption and photocatalytic reduction of Cr(VI). 4. Conclusions MgFe2O4/CPVC nanocomposite photocatalyst was prepared via a simple and practical three-step method. The obtained MgFe2O4/CPVC nanocomposite photocatalyst had the following three outstanding advantages: (i) much improved visible-light photocatalytic activity (about 2.1 times that of pristine MgFe2O4 nanoparticles) for the reduction of aqueous Cr(VI); (ii) good photocatalytic reusability and stability; and (iii) fast and easy recovery from the aqueous suspension by using a magnet. The photoelectrochemical analyses indicated that the effective separation of photoexcited carriers played a major role in enhancing the photocatalytic efficiency of MgFe2O4/CPVC nanocomposite. Besides, MgFe2O4/CPVC nanocomposite demonstrated noteworthy visible-light photocatalytic activity in treating the diluted black chromium electroplating solution over a wide pH range (1.4–10.3), and lower solution pH led to higher treatment efficiency. This work not only produced

a

new

magnetically

recoverable,

high-performance

visible-light

photocatalyst

(MgFe2O4/CPVC nanocomposite) for the treatment of Cr(VI)-polluted water, but also provided a 23

simple and effective modification method for enhancing the photocatalytic efficiency of semiconductor materials using low cost PVC.

Acknowledgments This study is supported by The Natural Science Foundation of Jiangsu Province (BK20171282), Science and Technology Cooperation Funds of Yangzhou City and Yangzhou Polytechnic Institute (No. YZ2018148), and Powerchina Huadong Engineering Corporation Limited (contract 2018330101000285 and project KY2017-02-52), China.

24

References [1] I. Kretschmer, A.M. Senn, J.M. Meichtry, G. Custo, M.I. Litter, Photocatalytic reduction of Cr(VI) on hematite nanoparticles in the presence of oxalate and citrate, Appl. Catal. B-Environ. 242 (2019) 218–226. [2] R. Yin, L. Ling, Y. Xiang, Y. Yang, Enhanced photocatalytic reduction of chromium (VI) by Cu-doped TiO2 under UV-A irradiation, Sep. Purif. Technol. 190 (2018) 53–59. [3] Y.C. Zhang, L. Yao, G. Zhang, D.D. Dionysiou, One-step hydrothermal synthesis of high-performance visible-light-driven SnS2/SnO2 nanoheterojunction photocatalyst for the reduction of aqueous Cr(VI), Appl. Catal. B-Environ. 144 (2014) 730–738. [4] Z. Wu, X. Yuan, G. Zeng, L. Jiang, H. Wang, Highly efficient photocatalytic activity and mechanism of Yb3+/Tm3+ codoped In2S3 from ultraviolet to near infrared light towards chromium (VI) reduction and rhodamine B oxydative degradation, Appl. Catal. B-Environ. 225 (2018) 8–21. [5] C. Zhou, P. Xu, C. Lai, C. Zhang, G. Zeng, Rational design of graphic carbon nitride copolymers by molecular doping for visible-light-driven degradation of aqueous sulfamethazine and hydrogen evolution, Chem. Eng. J. 359 (2019) 186–196. [6] Y. Yang, C. Zhang, D. Huang, G. Zeng, Boron nitride quantum dots decorated ultrathin porous g-C3N4: Intensified exciton dissociation and charge transfer for promoting visible-light-driven molecular oxygen activation, Appl. Catal. B-Environ. 245 (2019) 87–99. [7] C. Zhou, C. Lai, D. Huang, G. Zeng, C. Zhang, Highly porous carbon nitride by supramolecular preassembly of monomers for photocatalytic removal of sulfamethazine under visible light driven, Appl. Catal. B-Environ. 220 (2018) 202–210. [8] W. Wang, Z. Zeng, G. Zeng, C. Zhang, Sulfur doped carbon quantum dots loaded hollow tubular 25

g-C3N4 as novel photocatalyst for destruction of Escherichia coli and tetracycline degradation under visible light, Chem. Eng. J. 378 (2019) 122132. [9] D. He, C. Zhang, G. Zeng, Y. Yang, A multifunctional platform by controlling of carbon nitride in the core-shell structure: From design to construction, and catalysis applications, Appl. Catal. B-Environ. 258 (2019) 117957. [10] W. Wang, P. Xu, M. Chen, G. Zeng, C. Zhang, Alkali metal-assisted synthesis of graphite carbon nitride with tunable band-gap for enhanced visible-light-driven photocatalytic performance, ACS Sustainable Chem. Eng. 6 (2018) 15503–15516. [11] C. Zhou, C. Lai, C. Zhang, G. Zeng, Semiconductor/boron nitride composites: Synthesis, properties, and photocatalysis applications, Appl. Catal. B-Environ. 238 (2018) 6–18. [12] C. Zhang, W. Wang, A. Duan, G. Zeng, Adsorption behavior of engineered carbons and carbon nanomaterials for metal endocrine disruptors: Experiments and theoretical calculation, Chemosphere 222 (2019) 184–194. [13] D. Maruthamani, S. Vadivel, M. Kumaravel, B. Saravanakumar, Fine cutting edge shaped Bi2O3 rods/reduced graphene oxide (RGO) composite for supercapacitor and visible-light photocatalytic applications, J. Colloid Interf. Sci. 498 (2017) 449–459. [14] R. Bomila, S. Srinivasan, S. Gunasekaran, A. Manikandan, Enhanced photocatalytic degradation of methylene blue dye, opto-magnetic and antibacterial behaviour of pure and La-doped ZnO nanoparticles, J. Supercond. Nov. Magn. 31 (2018) 855–864. [15] P. Thilagavathi, A. Manikandan, S. Sujatha, S.K. Jaganathan, S.A. Antony, Sol–gel synthesis and characterization studies of NiMoO4 nanostructures for photocatalytic degradation of methylene blue dye, Nanosci. Nanotechnol. Lett. 8 (2016) 438–443. 26

[16] G. Mathubala, A. Manikandan, S.A. Antony, P. Ramar, Enhanced photocatalytic activity of spinel CuxMn1–xFe2O4 nanocatalysts for the degradation of methylene blue dye and opto-magnetic properties, Nanosci. Nanotechnol. Lett. 8 (2016) 375–381. [17] A. Manikandan, E. Manikandan, S. Vadivel, M. Kumaravel, S. Hariganesh, Photocatalysis: Present, past and future, In: Inamuddin, A.M. Asiri, A. Mohammad (Eds.), Organic Pollutants in Wastewater I, Methods of Analysis, Removal and Treatment, Materials Research Forum LLC, Millersville, 2018. [18] Y. Slimani, A. Baykal, A. Manikandan, Effect of Cr3+ substitution on AC susceptibility of Ba hexaferrite nanoparticles, J. Magn. Magn. Mater. 458 (2018) 204–212. [19] M.F. Valan, A. Manikandan, S.A. Antony, A novel synthesis and characterization studies of magnetic Co3O4 nanoparticles, J. Nanosci. Nanotechnol. 15 (2015) 4580–4586. [20] A. Manikandan, M. Durka, M.A. Selvi, S.A. Antony, Sesamum indicum plant extracted microwave combustion synthesis and opto-magnetic properties of spinel MnxCo1−xAl2O4 nano-catalysts, J. Nanosci. Nanotechnol. 16 (2016) 448–456. [21] S. Suguna, S. Shankar, S.K. Jaganathan, A. Manikandan, Novel synthesis of spinel MnxCo1−xAl2O4 (x = 0.0 to 1.0) nanocatalysts: Effect of Mn2+ doping on structural, morphological, and opto-magnetic properties, J. Supercond. Nov. Magn. 30 (2017) 691–699. [22] B.A. Josephine, A. Manikandan, V.M. Teresita, S.A. Antony, Fundamental study of LaMgxCr1−xO3−δ perovskites nano-photocatalysts: Sol-gel synthesis, characterization and humidity sensing, Korean J. Chem. Eng. 33 (2016) 1590–1598. [23] A. Silambarasu, A. Manikandan, K. Balakrishnan, Room-temperature superparamagnetism and enhanced photocatalytic activity of magnetically reusable spinel ZnFe2O4 nanocatalysts, J. 27

Supercond. Nov. Magn. 30 (2017) 2631–2640. [24] V.M. Teresita, A. Manikandan, B.A. Josephine, S. Sujatha, S.A. Antony, Electromagnetic properties and humidity-sensing studies of magnetically recoverable LaMgxFe1−xO3−δ perovskites nano-photocatalysts by sol-gel route, J. Supercond. Nov. Magn. 29 (2016) 1691–1701. [25] A. Silambarasu, A. Manikandan, K. Balakrishnan, S.K. Jaganathan, E. Manikandan, J.S. Aanand, Comparative study of structural, morphological, magneto-optical and photo-catalytic properties of magnetically reusable spinel MnFe2O4 nano-catalysts, J. Nanosci. Nanotechnol. 18 (2018) 3523–3531. [26] L. Sharma, R. Kakkar, Magnetically retrievable one-pot fabrication of mesoporous magnesium ferrite (MgFe2O4) for the remediation of chlorpyrifos and real pesticide wastewater, J. Environ. Chem. Eng. 6 (2018) 6891–6903. [27] J. Jiang, W. Fan, X. Zhang, S. Yuan, W. Shi, Rod-in-tube nanostructure of MgFe2O4: Electrospinning synthesis and photocatalytic activities of tetracycline, New J. Chem. 40 (2016) 538–544. [28] M. Shahid, J. Liu, Z. Ali, I. Shakir, M. Nadeem, Photocatalytic degradation of methylene blue on magnetically separable MgFe2O4 under visible light irradiation, Mater. Chem. Phys. 139 (2013) 566–571. [29] I. Shakir, M. Sarfraz, Z. Ali, M.F.A. Aboud, P.O. Agboola, Magnetically separable and recyclable graphene-MgFe2O4 nanocomposites for enhanced photocatalytic applications, J. Alloy. Compd. 660 (2016) 450–455. [30] T. Zhou, G. Zhang, P. Ma, X. Qiu, H. Zhang, Efficient degradation of rhodamine B with magnetically separable Ag3PO4@MgFe2O4 composites under visible irradiation, J. Alloy. Compd. 28

735 (2018) 1277–1290. [31] J. Chen, D. Zhao, Z. Diao, M. Wang, S. Shen, Bifunctional modification of graphitic carbon nitride with MgFe2O4 for enhanced photocatalytic hydrogen generation, ACS Appl. Mater. Interfaces 7 (2015) 18843–18848. [32]

B.T.

Huy,

D.

Jung,

N.T.K.

Phuong,

Y.

Lee,

Enhanced

photodegradation

of

2,4-dichlorophenoxyacetic acid using a novel TiO2@MgFe2O4 core@shell structure, Chemosphere 184 (2017) 849–856. [33] A.G. Abraham, A. Manikandan, E. Manikandan, S. Vadivel, S.K. Jaganathan, A. Baykal, Enhanced magneto-optical and photo-catalytic properties of transition metal cobalt (Co2+ ions) doped spinel MgFe2O4 ferrite nanocomposites, J. Magn. Magn. Mater. 452 (2018) 380–388. [34] A. Manikandan, M. Durka, K. Seevakan, S.A. Antony, A novel one-pot combustion synthesis and opto-magnetic properties of magnetically separable spinel MnxMg1−xFe2O4 (0.0 ≤ x ≤ 0.5) nanophotocatalysts, J. Supercond. Nov. Magn. 28 (2015) 1405–1416. [35] I.J.C. Lynda, M. Durka, A. Dinesh, A. Manikandan, S.K. Jaganathan, A. Baykal, Enhanced magneto-optical and photocatalytic properties of ferromagnetic Mg1−yNiyFe2O4 (0.0 ≤ y ≤ 1.0) spinel nano-ferrites, J. Supercond. Nov. Magn. 31 (2018) 3637–3647. [36] M. Faisal, F.A. Harraz, A.A. Ismail, A.M. El-Toni, S.A.Al-Sayari, Polythiophene/mesoporous SrTiO3 nanocomposites with enhanced photocatalytic activity under visible light, Sep. Purif. Technol. 190 (2018) 33–44. [37] B. Yan, Y. Wang, X. Jiang, K. Liu, L. Guo, Flexible photocatalytic composite film of ZnO-microrods/polypyrrole, ACS Appl. Mater. Interfaces 9 (2017) 29113–29119. [38] Q. Luo, X. Yang, X. Zhao, D. Wang, Facile preparation of well-dispersed ZnO/cyclized 29

polyacrylonitrile nanocomposites with highly enhanced visible-light photocatalytic activity, Appl. Catal. B-Environ. 204 (2017) 304–315. [39] L. Liu, L. Ding, Y. Liu, W. An, W. Cui, A stable Ag3PO4@PANI core@shell hybrid: Enrichment photocatalytic degradation with π-π conjugation, Appl. Catal. B-Environ. 201 (2017) 92–104. [40] B. Niu, Z. Xu, A stable Ta3N5@PANI core-shell photocatalyst: Shell thickness effect, high-efficient photocatalytic performance and enhanced mechanism, J. Catal. 371 (2019) 175–184. [41] D. Wang, L. Shi, Q. Luo, An efficient visible light photocatalyst prepared from TiO2 and polyvinyl chloride, J. Mater. Sci. 47 (2012) 2136–2145. [42] D. Wang, C. Bao, Q. Luo, Improved visible-light photocatalytic activity and anti-photocorrosion of CdS nanoparticles surface-modified by conjugated derivatives from polyvinyl chloride, J. Environ. Chem. Eng. 3 (2015) 1578–1585. [43] Q. Luo, L. Wang, D. Wang, Preparation, characterization and visible-light photocatalytic performances of composite films prepared from polyvinyl chloride and SnO2 nanoparticles, J. Environ. Chem. Eng. 3 (2015) 622–629. [44] D. Wang, H. Sun, Q. Luo, An efficient visible-light photocatalyst prepared from g-C3N4 and polyvinyl chloride, Appl. Catal. B-Environ. 156–157 (2014) 323–330. [45] Y. Zhang, F. Zhang, Z. Yang, H. Xue, D.D. Dionysiou, Development of a new efficient visible-light-driven photocatalyst from SnS2 and polyvinyl chloride, J. Catal. 344 (2016) 692–700. [46] A. Manohar, C. Krishnamoorthi, Photocatalytic study and superparamagnetic nature of Zn-doped MgFe2O4 colloidal size nanocrystals prepared by solvothermal reflux method, J. Photochem. Photobiol. B 173 (2017) 456–465. 30

[47] L. Sharma, R. Kakkar, Magnetically retrievable one-pot fabrication of mesoporous magnesium ferrite (MgFe2O4) for the remediation of chlorpyrifos and real pesticide wastewater, J. Environ. Chem. Eng. 6 (2018) 6891–6903. [48] W. Wang, B. Liu, K. Xu, Y. Zu, J. Song, In-situ preparation of MgFe2O4-GO nanocomposite and its enhanced catalytic reactivity on decomposition of AP and RDX, Ceram. Int. 44 (2018) 19016–19020. [49] S. Wang, D. Li, C. Yang, G. Sun, J. Zhang, A novel method for the synthesize of nanostructured MgFe2O4 photocatalysts, J. Sol-Gel Sci. Technol. 84 (2017) 169–179. [50] Y. Su, C. Wang, Y. Zhang, Z. Yang, D.D. Dionysiou, Design and preparation of SnO2/SnS2/conjugated polyvinyl chloride derivative ternary composite with enhanced visible-light photocatalytic activity, Mater. Res. Bull. 118 (2019) 110524. [51] X. Yang, Y. Zhang, Y. Zhang, T. Peng, A. Zhu, Modification of SnO2 nanoparticles by conjugated derivative of polyvinyl chloride for efficient photocatalytic reduction of Cr(VI) under visible-light, Mater. Lett. 218 (2018) 173–176. [52] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray photoelectron spectroscopy, J. Chastain (Eds.), Perkin-Elmer Corporation, Eden Prairie, 1992. [53] F. Zhang, Y. Zhang, G. Zhang, Z. Yang, D.D. Dionysiou, A. Zhu, Exceptional synergistic enhancement of the photocatalytic activity of SnS2 by coupling with polyaniline and N-doped reduced graphene oxide, Appl. Catal. B-Environ. 236 (2018) 53–63. [54] Y.C. Zhang, J. Li, H.Y. Xu, One-step in situ solvothermal synthesis of SnS2/TiO2 nanocomposites with high performance in visible light-driven photocatalytic reduction of aqueous Cr(VI), Appl. Catal. B-Environ. 123–124 (2012) 18–26. 31

[55] Y.C. Zhang, Z.N. Du, K.W. Li, M. Zhang, Size-controlled hydrothermal synthesis of SnS2 nanoparticles with high performance in visible light-driven photocatalytic degradation of aqueous methyl orange, Sep. Purif. Technol. 81 (2011) 101–107. [56] F. Zhang, Y. Zhang, C. Zhou, Z. Yang, D.D. Dionysiou, A new high efficiency visible-light photocatalyst made of SnS2 and conjugated derivative of polyvinyl alcohol and its application to Cr(VI) reduction, Chem. Eng. J. 324 (2017) 140–153. [57] L. Zhang, C. Niu, C. Liang, X. Wen, G. Zeng, One-step in situ synthesis of CdS/SnO2 heterostructure with excellent photocatalytic performance for Cr(VI) reduction and tetracycline degradation, Chem. Eng. J. 352 (2018) 863–875. [58] C. Zhou, Z. Zeng, G. Zeng, D. Huang, R. Xiao, M. Cheng, C. Zhang, Visible-light-driven photocatalytic degradation of sulfamethazine by surface engineering of carbon nitride: Properties, degradation pathway and mechanisms, J. Hazard. Mater. 380 (2019) 120815. [59] Y. Yang, Z. Zeng, G. Zeng, D. Huang, R. Xiao, C. Zhang, Ti3C2 Mxene/porous g-C3N4 interfacial Schottky junction for boosting spatial charge separation in photocatalytic H2O2 production, Appl. Catal. B-Environ. 258 (2019) 117956. [60] Y. Yang, Z. Zeng, C. Zhang, D. Huang, G. Zeng, Construction of iodine vacancy-rich BiOI/Ag@AgI Z-scheme heterojunction photocatalysts for visible-light-driven tetracycline degradation: Transformation pathways and mechanism insight, Chem. Eng. J. 349 (2018) 808–821. [61] G. Zhang, D. Chen, N. Li, Q. Xu, J. Lu, Fabrication of Bi2MoO6/ZnO hierarchical heterostructures with enhanced visible-light photocatalytic activity, Appl. Catal. B-Environ. 250 (2019) 313–324. [62] S. Ghosh, H. Remita, R.N. Basu, Visible-light-induced reduction of Cr(VI) by PDPB-ZnO 32

nanohybrids and its photo-electrochemical response, Appl. Catal. B-Environ. 239 (2018) 362–372. [63] H. Wei, Q. Zhang, Y. Zhang, Z. Yang, D.D. Dionysiou, Enhancement of the Cr(VI) adsorption and photocatalytic reduction activity of g-C3N4 by hydrothermal treatment in HNO3 aqueous solution, Appl. Catal. A-Gen. 521 (2016) 9–18. [64] F. Chen, Q. Yang, Y. Wang, F. Yao, G. Zeng, Efficient construction of bismuth vanadate-based Z-scheme photocatalyst for simultaneous Cr(VI) reduction and ciprofloxacin oxidation under visible light: Kinetics, degradation pathways and mechanism, Chem. Eng. J. 348 (2018) 157–170. [65] B. Li, C. Lai, P. Xu, G. Zeng, Facile synthesis of bismuth oxyhalogen-based Z-scheme photocatalyst for visible-light-driven pollutant removal: Kinetics, degradation pathways and mechanism, J. Clean. Prod. 225 (2019) 898–912. [66] P. Wang, X. Li, J. Fang, D. Li, J. Chen, A facile synthesis of CdSe quantum dots-decorated anatase TiO2 with exposed {001} facets and its superior photocatalytic activity, Appl. Catal. B-Environ. 181 (2016) 838–847. [67] S.S. Kalanur, H. Seo, Facile growth of compositionally tuned copper vanadate nanostructured thin films for efficient photoelectrochemical water splitting, Appl. Catal. B-Environ. 249 (2019) 235–245. [68] K. Maeda, K. Domen, Preparation of BaZrO3–BaTaO2N solid solutions and the photocatalytic activities for water reduction and oxidation under visible light, J. Catal. 310 (2014) 67–74. [69] Y. Li, Z. Liu, Y. Wu, J. Chen, P. Na, Carbon dots-TiO2 nanosheets composites for photoreduction of Cr(VI) under sunlight illumination: Favorable role of carbon dots, Appl. Catal. B-Environ. 224 (2018) 508–517. [70] H. Wei, C. Hou, Y. Zhang, Z. Nan, Scalable low temperature in air solid phase synthesis of 33

porous flower-like hierarchical nanostructure SnS2 with superior performance in the adsorption and photocatalytic reduction of aqueous Cr(VI), Sep. Purif. Technol. 189 (2017) 153–161. [71] J. Li, T. Peng, Y. Zhang, Polyaniline modified SnO2 nanoparticles for efficient photocatalytic reduction of aqueous Cr(VI) under visible light, Sep. Purif. Technol. 201 (2018) 120–129. [72] Q. Wang, Q. Gao, H. Wu, Y Cong, In situ construction of semimetal Bi modified BiOI-Bi2O3 film with highly enhanced photoelectrocatalytic performance, Sep. Purif. Technol. 226 (2019) 232–240. [73] B.A. Marinho, R.O. Cristóvão, R. Djellabi, J.M. Loureiro, V.J.P. Vilar, Photocatalytic reduction of Cr(VI) over TiO2-coated cellulose acetate monolithic structures using solar light, Appl. Catal. B-Environ. 203 (2017) 18–30. [74] R. Djellabi, L. Zhang, X. Zhao, Sustainable self-floating lignocellulosic biomass-TiO2@Aerogel for outdoor solar photocatalytic Cr(VI) reduction, Sep. Purif. Technol. 229 (2019) 115830. [75] Y. Deng, L. Tang, G. Zeng, Z. Zhu, Insight into highly efficient simultaneous photocatalytic removal of Cr(VI) and 2,4-diclorophenol under visible light irradiation by phosphorus doped porous ultrathin g-C3N4 nanosheets from aqueous media: Performance and reaction mechanism, Appl. Catal. B-Environ. 203 (2017) 343–354. [76] V.G. Poulopoulou, E. Vrachnou, S. Koinis, D. Katakis, The Cr2O72−-CrO42−-HCrO4− system revisited, Polyhedron 16 (1997) 521–524.

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Figure captions Fig. 1. (a) XRD patterns of MgFe2O4, MgFe2O4/CPVC and CPVC; (b) Raman spectra of MgFe2O4/CPVC and MgFe2O4. Fig. 2. Survey and element XPS spectra of MgFe2O4/CPVC. Fig. 3. (a) SEM image, (b) EDX spectrum, and (c) selected area SEM image, (d) Mg, (e) Fe, (f) O, (g) C and (h) Cl element mapping images of MgFe2O4/CPVC. Fig. 4. (a) TEM image of MgFe2O4; (b1) TEM image and (b2) HRTEM image of MgFe2O4/CPVC. Fig. 5. (a) UV-vis absorbance spectra of MgFe2O4, MgFe2O4/CPVC and CPVC; (b) Tauc plots of MgFe2O4 and MgFe2O4/CPVC; (c) Tauc plot of CPVC. Fig. 6. (a) Fluorescent light-induced transient photocurrent responses and (b) Electrochemical impedance spectra of MgFe2O4, MgFe2O4/CPVC and CPVC. Fig. 7. (a) Performance of MgFe2O4, MgFe2O4/CPVC and CPVC in dark adsorption and visible-light-induced catalytic reduction of aqueous Cr(VI); (b) Plots of ln(Ci0/Cit) vs. ti for obtaining the k values in the presence of MgFe2O4 and MgFe2O4/CPVC. Fig. 8. Cycle performance of MgFe2O4/CPVC in photocatalytic treatment of aqueous Cr(VI). Fig. 9. (a) XRD pattern and (b) Raman spectrum of MgFe2O4/CPVC-R. Fig. 10. Survey and element XPS spectra of MgFe2O4/CPVC-R. Fig. 11. Mott-Schottky plots of MgFe2O4 and CPVC. Fig. 12. (a) Room temperature magnetization curves of MgFe2O4 and MgFe2O4/CPVC; (b) Photograph of the recovery of MgFe2O4/CPVC from aqueous suspension by using a magnet. Fig. 13. (a) Photocatalytic treatment of the diluted black chromium electroplating solution by MgFe2O4/CPVC at different pH; (b) Plots of ln(Ci0/Cit) vs. ti to obtain the k values at different pH. 35

Highlights

● First preparation and study of MgFe2O4/CPVC nanocomposite as photocatalyst. ● MgFe2O4/CPVC has higher visible-light photocatalytic activity than MgFe2O4. ● MgFe2O4/CPVC can be recovered from the aqueous suspension by using a magnet. ● The photocatalytic enhancement mechanism of MgFe2O4/CPVC was proposed. ● MgFe2O4/CPVC can treat black chromium electroplating solution over a wide pH range.

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Conflict of interests

We declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Zhikang Jiang, Kexu Chen, Yongcai Zhang *, Yuanyou Wang, Fang Wang, Geshan Zhang, Dionysios D. Dionysiou

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