Efficient and stable photocatalytic reduction of aqueous hexavalent chromium ions by polyaniline surface-hybridized ZnO nanosheets

Journal of Molecular Liquids 279 (2019) 133–145

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Efficient and stable photocatalytic reduction of aqueous hexavalent chromium ions by polyaniline surface-hybridized ZnO nanosheets Chongzhuo Bao a, Mingxin Chen a, Xin Jin a, Dongwen Hu a, Qiang Huang a,b,⁎ a b

School of Materials Science and Engineering, Yunnan University, Kunming, Yunnan 650091, PR China Yunnan Province University Science and Technology Innovation Team of Volatile Organic Compound Sensors and Functional Materials, Yunnan University, 650091, PR China

a r t i c l e

i n f o

Article history: Received 16 November 2018 Received in revised form 22 January 2019 Accepted 22 January 2019 Available online 24 January 2019 Keywords: Photocatalytic reduction Hexavalent chromium ZnO/PANI nanocomposites Synergistic effect

a b s t r a c t ZnO/PNAI nanocomposites were obtained via a simple two-step method, in which a hierarchically flower-like ZnO, assembled from nanosheets, was first synthesized by low temperature (80 °C) solution method without surfactant/template and then, an in-situ polymerization of aniline onto the ZnO nanosheets was carried out. The characterization was actualized on the synthesized nanocomposite using XRD, Raman, TG-DSC, UV–vis-DRS, PL, XPS, SEM, TEM and N2 adsorption-desorption analysis. An efficient and stable photocatalytic activity was exhibited by the synergistic ZnO/PNAI nanocomposites on the reduction of toxic Cr(VI) to benign Cr(III) in comparison with the pristine ZnO and other semiconductor/PANI materials reported. The influence of parameters such as solution pH, initial Cr(VI) concentration and the addition of organic sacrifices on the Cr(VI) removal efficiency was investigated. It was found that the Cr(VI) removal was strongly pH-dependent and the optimum pH range was 4–7 for ZnO/PANI composite catalyst. The Langmuir–Hinshelwood kinetic examination showed that the surface photoreaction was the rate-determining step to compare the adsorption equilibrium. The organic additives such as EDTA and citric acid can promote the photocatalytic efficiency and the solution of EDTA/HCl can well restore the photoreduction ability of ZnO/PANI catalyst. Moreover, the reusability of photocatalyst was also tested to estimate the practicability. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Chromium (Cr) is recognized as a heavy metal element that is harmful to organisms. The extensive use of chromium in industrial activities such as electroplating, leather tanning, metal corrosion inhibition and alloy manufacturing increases the emission of chromium into nature. The two significant oxidized forms of chromium are hexavalence (Cr (VI)) and trivalence (Cr(III)) whereas the Cr(VI) is more pernicious than the Cr(III). The oxyanion (CrO42− or HCrO42−) and polyoxyanion (Cr2O72−) are notorious for their carcinogenicity and high mobility in environment [1,2]. The Chinese Environmental Protection Board (EPB) has regulated Cr(VI) as one of the top-priority toxic pollutants. The World Health Organization (WHO) recommends a maximum concentration limit of 0.05 and 0.10 mg/L by the Cr(VI) ions for drinking water and industrial wastewater, respectively [3]. For that reason, it is very necessary to remove Cr(VI) from wastewater before its release into nature. Comparatively, Cr(III) is much less toxic than Cr(VI) and can be readily precipitated or adsorbed as Cr(OH)3 by organic and inorganic substrates at ambient condition [1,4]. Moreover, limited ⁎ Corresponding author at: School of Materials Science and Engineering, Yunnan University, Kunming, Yunnan 650091, PR China. E-mail address: [email protected] (Q. Huang).

https://doi.org/10.1016/j.molliq.2019.01.122 0167-7322/© 2019 Elsevier B.V. All rights reserved.

hydroxide solubility leads to its relative immobility and less availability for biological uptake [5]. Consequently, toxicity of Cr(VI) could be decreased and become less bioavailable as reduced to Cr(III). In fact, most of the methods to remove or lessen Cr(VI) contaminants are based on the reduction of Cr(VI) to Cr(III) followed by precipitation of Cr(III). Treatment technologies applied for eliminating Cr(VI) from wastewater include adsorption, chemical precipitation, membrane filtration, extraction, ion exchange, bioremediation etc. [6–11] However, these traditional methods generally required a large amount of reductants or other auxiliary materials that resulted in high cost and energy consumption, low efficiency and easy to cause secondary waste products. These obvious shortcomings have prompted researchers to develop more rational approaches for Cr(VI) removal. Photocatalytic reduction is being explored for the decontamination of high valent toxic metal ions from industrial wastewaters (Hg(II), Pb (II), Cd(II), U(II), and Cr(VI)), which is considered an efficient, green and promising approach [12–14]. It is identified when photocatalysts are irradiated by light with photon energies higher than their bandgaps, electrons in the valence bands (VBs) are excited into the conduction bands (CBs) and then, the photogenerated electrons (e−) or the resulting active species reduce the high valence to its lower valence in aquatic environments. The light-driving process ordinarily draws support from semiconductor photocatalysts such as TiO2, SnO2, WO3,

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Fe2O3, CdS and ZnO [15,16]. Among them, TiO2 received the most attention due to its highly photocatalytic activity, low toxicity, and high stability to light illumination [17]. ZnO is another attractive semiconductor photocatalyst and has similar band gap energy with TiO2. There are evidences that ZnO exhibited better photo-electric conversion efficiency than TiO2 [18,19]. It is demonstrated that ZnO particles can availably photo-reduce high hazardous Cr(VI) ions to less toxic Cr(III) [20–22]. However, the present use of ZnO powders as photocatalyst is still faced with several problems to be solved, just like poor stability and dispersity, fast combination of charge carriers, and narrow range of light response, etc. It is well known that the photochemical activity of ZnO is significantly dominated by its crystal size, surface defects, and microstructure. Therefore, it is an effective strategy to modify the particle surface through physical or chemical methods to change the state of surface atoms [15,23]. In our previous works, it was demonstrated that the surface-modified ZnO nanoparticles by polymers showed better stability and dispersion, as well as better photocatalytic ability than pristine one [24,25]. S. Jana et al. demonstrated that polyaniline (PANI) hybridized surface defective ZnO nanorods exhibited long-term stable photoelectrochemical activity [26]. Y. Cheng et al. prepared a pompon-like ZnO-PANI heterostructure and investigated its applications for the treatment of typical water pollutants [27]. They inferred that the p-n type heterojunction between ZnO and PANI enhanced the production of photo-induced •OH and O2−• active species, and also promoted the separation efficiency of photogenerated electron-hole pairs, so its photocatalytic ability was greatly improved. On the other hand, PANI is a p-type organic semiconductor containing conjugated system and can easily be hybridized with various metal oxide semiconductors (ZnO, TiO2, SnO2, WO3 etc.) [26,28–34]. The resulting PANI-metal oxide composites are often provided with several advantages in photocatalytic process: (i) improved charge separation efficiency; (ii) broadened light absorption; and (iii) suppressed photocorrosion. Although the treatment of water pollutants by ZnO/ PANI photocatalysts has been the subject of many fundamental investigations, most of them focused on the degradation of organic pollutants. The possible utilization of ZnO/PANI photocatalyst on treatment of inorganic pollutants has not received similar attention. In addition, the photodegradation of organic compounds and the photoreduction of metal ions are considered to be a synergistic and mutually reinforcing process [35–37], but when they exist simultaneously in solution, the effect of mutual inhibition are often exhibited [38–40]. Such result requires researchers to explore their reaction mechanisms in greater depth. In present work, ZnO/PNAI nanocomposites were prepared by a simple low temperature (80 °C) solution method for the synthesis of ZnO nanosheets from surfactant/template-free precursor and then, PANI was coated onto the ZnO nanosheets to prepare ZnO/PANI nanocomposites through in-situ polymerization of aniline monomer. The reduction of Cr(VI) to Cr(III) by ZnO/PANI photocatalyst in aqueous suspensions was investigated to evaluate the influence of parameters such as solution pH, initial Cr(VI) concentration and the addition of organic sacrifices on the reduction efficiency, as well as to discuss the reaction mechanism. Moreover, the reusability of photocatalyst was also tested to estimate the practicability.

2. Experimental 2.1. Chemicals The monomer aniline in analytical grade was distilled into colorless under reduced pressure prior to use. Other chemicals were of analytical grade, purchased locally and used as received without further purification. The experimental water was selfmade deionized water.

2.2. Preparation of ZnO/PANI nanocomposites ZnO/PANI nanocomposites were prepared by a tow-step process. A typical preparation is described below: 4.38 g (20 mmol) zinc acetate dehydrate, Zn(CH3COO)2·2H2O, was dissolved in 100 mL deionized water and then, aqueous NaOH (1 M, 100 mL) was poured into at room temperature. After the formative white slurry was irradiated with ultrasound for 15 min, the mixture was moved into an oven at 80 °C for 24 h. The precipitates obtained by centrifugation were washed with water and dried to get white ZnO powders. Then, after the obtained ZnO was added into 50 mL 0.01 M HCl solution in which 0.2 mL aniline (C6H7N) was previously dissolved. After the suspension was stirred for 30 min, an aqueous solution (0.1 M, 25 mL) of ammonium persulfate ((NH4)2S2O8, APS) was added drop wise to the suspension under ice bath (0–5 °C) and left for 4 h stirring. A dark green precipitate was filtered and thoroughly washed with deionized water followed by ethanol and acetone. The final product was dried in a vacuum oven at 60 °C for 12 h. For comparison, PANI without ZnO was prepared by using the similar method. 2.3. Characterization The X-ray diffraction (XRD) pattern was recorded on a RIGAKU D/ max-TTR III diffractometer (Japan) with Cu Kα1 radiation (λ = 0.15406 nm). Raman spectrum was recorded by a Renishaw inVia microscopic confocal Raman spectrometer (UK) using 514.5 nm line of an Ar+ laser at room temperature in air operating at 1 mW laser power. Scanning electron microscopy (SEM) images were obtained on FEI Quanta 200 (Holland). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were taken on JEM-2100 (Japan) with an acceleration voltage of 200 KV. The X-ray photoelectron spectroscopy (XPS) was performed on Thermo ESCALAB 250 (America), operating at 15 kV and 15 mA with an alumina target (Al–Kα, hv = 1486.6 eV). Thermogravimetric/differential scanning calorimetry analysis (TG-DSC) was performed using SDT Q600 (America). Photoluminescence spectra (PL) were measured on a Hitachi F-4500 fluorospectrophotometer (Japan) using a Xe lamp of 325 nm as the excitation wavelength at room temperature. The specific surface area (SBET) was determined by Brunauer-Emmet-Teller (BET) N2 adsorption-desorption isotherm measurements at liquid nitrogen temperature (77 K) using Micrometrics TriStar II 3020 instrument (America). 2.4. Aqueous Cr(VI) ions removal experiments Cr(VI) ions removal experiments were carried out in a batch model. The reaction flask was surrounded by thermostatic water circulation to maintain a temperature of 30 ± 1 °C. The wavelength of UV source irradiation was 320–400 nm (Spectroline EA-180, USA). A stock solution (1000 mg·L−1) of Cr(VI) was prepared by dissolving K2Cr2O7 in deionized water, which was further diluted to the required concentration before use. The solution pH was adjusted using 0.1 mol·L−1 NaOH or 0.1 mol·L−1 HCl. In a typical Cr(VI) removal experiment, 100 mL of Cr (VI) solution (10–50 mg·L−1) was poured into a quartz flask with a fixed amount of photocatalyst (50 mg) and the flask was oscillated for 30 min in dark. For the photocatalytic reduction, the suspension was irradiated with UV source. At a certain interval, 5 mL of the suspension was withdrawn and centrifuged quickly for solid–liquid separation. The total chromium concentration [Cr] in the supernatant liquid was measured by ICP analysis (ICP-AES1000, Shimadzu Japan). Residual Cr (VI) concentration [Cr(VI)] in liquid was analyzed by spectrophotometer (U-4100, Hitachi Japan) using 1,5‑diphenylcarbazide (DPC) as the complexing agent at a wavelength of 540 nm. The Cr(III) concentration [Cr(III)] is calculated by the total chromium concentration minus the Cr (VI) concentration. The ratio of [Cr(VI)] to initial concentration [Cr(VI)]0 ([Cr(VI)]/[Cr(VI)]0) is used to indicate the removal rate of Cr(VI) ions

C. Bao et al. / Journal of Molecular Liquids 279 (2019) 133–145

and the ratio of [Cr(III)] to the total chromium concentration ([Cr(III)]/ [Cr]) denotes the percent reduction of Cr(VI) ions in the solution. 3. Results and discussion 3.1. Characterization of ZnO nanosheets hybridized with PANI 3.1.1. XRD and Raman analyses Fig. 1a shows the XRD patterns of as-prepared ZnO and ZnO/PANI. All the obviously observed peaks can be well indexed to hexagonal ZnO wurtzite structure (JCPDS card No.36-1451, Space group F63mc (186)). After coated with PANI, there is almost no significant change in the 2θ position of each corresponding diffraction peak, and the calculated grain sizes from Scherrer equation are also provided with similar values, suggesting that the incorporation of PANI did not destroyed the crystalline structure of ZnO. The existence of PANI in the composite can be confirmed by Raman spectroscopic analysis (Fig. 1b). It can be seen that the Raman spectrum of pristine ZnO presents a typical wurtzite hexagonal phase. The Raman active zone-center optical phonon predicted by the group theory is A1 + E1 + 2E2. The phonons of A1 and E1 symmetry are polar phonons and, hence, exhibit different frequencies for the transverse-optical (TO) and longitudinal-optical (LO) phonons. Nonpolar phonon modes with symmetry E2 have two frequencies. E2H is associated with oxygen atoms and E2L is associated with Zn sublattice [41]. The bands at 330, 435, 579, 658 and 1147 cm−1 can be attributed to E2H-E2L, E2H, E1(LO), TO + LO and their multilevel optical phonon modes, respectively [42]. In the Raman spectrum of ZnO/PANI, peaks at 1588, 1537 and 1341 cm−1 come from benzenoid rings, quinonoid and/or semiquinonoid rings, and C\\N stretching vibrations, respectively, a typical emeraldine salt form of PANI [43]. In ZnO/PANI, the Raman modes of wurtzite ZnO can also be observed, but the corresponding bands slightly shifted to lower wavenumbers, which suggests defective ZnO crystal faces and an intensive chemical bond on the interface [44]. 3.1.2. TG/DSC analysis TG/DSC analysis was used to determine the relative content of substances with different thermal stability in the composites. The TG curve shows three main stages of weight loss and the corresponding DSC curve provides three major peaks (Fig. 1c). The first broaden endothermic peak below 200 °C is related to the gradual release of solvent molecules and unpolymerized monomers adsorbed into the composites, corresponding to about 12.5% weightlessness. A loss in TG measurement associated with the wide exothermic peak at ~350 °C may be involved in the decomposition of aniline oligomers, corresponding to around 2.5% weight loss. The strong exothermic peak at ~450 °C is attributed to thermolysis of the main body of PANI. The total residual 79.2% was obtained after completing decomposition. The TG/DSC analysis suggests that the thoroughly dried ZnO/PANI composites contain about 8.3% polyaniline and oligoaniline, and also indicates that heat treatment below 200 °C can remove impurities in the samples.

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3.1.3. XPS studies XPS analysis was performed to further investigate the chemical bonding state and the surface composition of element. Fig. 2 shows the XPS core-level spectra of as-prepared ZnO and ZnO/PANI, and their Gaussian-divided results are listed in Table 1. From Fig. 2a, the doublet spectral lines of Zn 2p3/2 and 2p1/2 due to spin-orbit coupling are observed at the binding energy of 1021.6 (1022.2) and 1044.9 (1045.4) eV for pure ZnO (ZnO/PANI) respectively, which are close to the standard ZnO binding energy values [45], and the chemical shifts between Zn 2p1/2 and 2p3/2 peaks are almost equal (23.3 eV for ZnO and 23.2 eV for ZnO/PANI). These results correspond to Zn 2p1/2 and Zn 2p3/2 core levels of the zinc(II) ion (Zn2+). Both Zn 2p peaks in ZnO/ PANI line shifted to a slightly lower energy compared to the pure ones, implying the formation of chemical bonds between Zn2+ and PANI molecular chain [29], consistent with the above Raman analysis. The deconvolutions of XPS spectra for O 1 s and N 1 s core levels are shown in Fig. 2b, c and d. In the deconvoluted XPS spectra, the black solid line represents the experimental data, the red dashed denotes the fitting curves, and the individual peak is described by other colored lines. In Fig. 2b and c, the deconvolutions illustrate the presence of three different O 1 s bands in the samples. The peaks centered at 530.02 and 530.54 eV (O1) are attributed to the O2– ions in wurtzite ZnO, while the peaks at 531.74 and 531.39 eV (O2) are associated with O2– ions in the zinc oxyhydroxide species (ZnO-OH) that are in oxygendeficient regions of the ZnO matrix [46]. The bands with higher binding energies (533.16 and 532.66 eV, O3) are usually ascribed to the chemisorbed oxygen, such as O2 and H2O. It is also found that the FWHM value and the atomic concentration of each deconvoluted peak were changed after hybridized with PANI, whereas the binding energy of each fitted peak remained essentially unchanged (see Table 1). These phenomena indicate that the relative content of each O species on the surface was changed by PANI hybridizing, but the type of O species kept basically uniform. It is worth pointing out that the relative content of O2 in both ZnO and ZnO/PANI samples is larger than that of O1, indicating abundant zinc oxyhydroxide species and oxygen defect on the surface. The N 1 s core-level signal (Fig. 2d) in ZnO/PANI can be detected and resolved into three bands, centered at ∼399.30 (N1), ∼400.15 (N2) and ∼401.25 eV (N3) assigned to _N\\, \\NH\\ and \\NH+/_N+\\, respectively [26], revealing the construing units of PANI chain. The appearance of protonated nitrogen species (\\NH+/_N+\\) reinforced the interaction between PANI and OH groups on ZnO surface in ZnO/ PANI. The proportional intensity of the N1, N2 and N3 peaks was 24.6, 54.0 and 21.4%, respectively (a ratio of N1:N2:N3 = 1:2.20:0.87), reflecting their relative content in PANI. 3.1.4. Optical properties Fig. 3a depicts the UV–vis diffused reflectance spectra (DRS) of ZnO and ZnO/PANI samples. It is clear that ZnO/PANI composite exhibits a wide visible-light absorption in the range of 400–800 nm as well as the bandgap absorption of ZnO. The improvement in visible absorption

Fig. 1. (a) XRD patterns, (b) Raman spectra of ZnO and ZnO/PANI, and (c) TG-DSC curve of ZnO/PANI.

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Fig. 2. XPS core-level spectra: (a) Zn 2p of ZnO and ZnO/PANI, (b) O 1 s of ZnO, (c) O 1 s and (d) N 1 s of ZnO/PANI nanocomposites.

can be considered due to the interactions of PANI with ZnO and π − π* transition from PANI molecules [26]. In this respect, it is well known that PANI is able to absorb visible as well as near infrared light [47]. The incorporation of PANI into ZnO in ZnO/PANI composites can therefore be the reason for the enhancement of visible-light absorption [26,48]. Furthermore, the bandgap was calculated by the optical absorbance data with Stern equation [49]. The obtained values of optical direct bandgap energy are 3.18 and 3.13 eV for ZnO and ZnO/PANI, respectively. The photoluminescence property of ZnO/PANI composites has also been altered in addition to the change of optical absorption property. The photoluminescence (PL) spectroscopy in Fig. 3b shows a near-UV

emission (~395 nm) and a broad superimposed emission in visible region (~461 nm) for bare ZnO, while the position of luminescence center shifted to lower region (~387 and 443 nm) especially visible emissions and the intensity was significantly decreased for ZnO/PANI. The UV emission is due to the band edge transition or the free exciton recombination which is usually considered a characteristic emission of ZnO, while the visible emission is related to the intrinsic defects in ZnO lattices [42]. The changes in luminescence position reflected the ZnO surface states before and after PANI hybridization. The decrease in intensity may be owing to the suppression of electron-hole recombination occurred by loading PANI on ZnO. Lower intensity of PL bands indicates the lower recombination of electron-hole pairs which should

Table 1 The peak integration results of XPS. Sample

Peak

Position (eV)

FWHM (eV)

Atomic concentration (%)

ZnO

Zn 2p3/2 Zn 2p1/2 O1 O2 O3 Zn 2p3/2 Zn 2p1/2 O1 O2 O3 N1 N2 N3

1021.6 1044.9 530.02 531.74 533.16 1222.2 1045.4 530.54 531.39 532.66 399.30 400.15 401.25

2.32 1.83 1.14 2.19 1.36 2.73 2.24 1.95 1.21 1.69 2.21 1.68 3.13

66.5 33.5 21.1 49.6 29.3 67.1 32.9 15.1 29.3 55.6 24.6 54.0 21.4

ZnO/PANI

Attribution Spin-orbit coupling Spin-orbit coupling Zn-O bonds (O2−) ZnO-OH (defective O) Chemisorbed O Spin-orbit coupling Spin-orbit coupling Zn-O bonds (O2−) ZnO-OH (defective O) Chemisorbed O _N\ \ \ \NH\ \ \ \NH+/_N+\ \

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Fig. 3. (a) UV–Vis absorption spectra measured by diffused reflectance method, (b) Determination of direct band gap energy and, (c) Photoluminescence measurements with 325 nm excitation source for ZnO and ZnO/PANI.

enhance the photochemical activity of catalysts as described in many literatures [26,29,34]. 3.1.5. Textural characterization The textural properties of ZnO and ZnO/PANI were determined by BET N2 adsorption-desorption measurement and the obtained parameters is described in Table 2. It is found that the calculated SBET of ZnO/ PANI was decreased in some extent due to surface coating of PANI whereas the existence of PANI caused a larger pore volume and average pore size, suggesting large proportion of macropores in ZnO/PANI. The textural characteristic of ZnO/PANI is conductive to ion diffusion and electrolyte permeation [50]. SEM and TEM techniques were used to observe the morphology of samples. From Fig. 4a it is distinct that the detailed morphology of asprepared ZnO is hierarchically flower-like microstructures with size of several microns, assembled by many arranged nanosheets as petals. After PANI coating, a slight change was occurred in geometry. Some of the assemblies were disassembled to form more dispersed nanosheets (Fig. 4b). The similar morphology was also found in TEM microgram (Fig. 4c). Fig. 4d shows the HRTEM image of a side face. An amorphous PANI thin layer and the ZnO phase with clear lattice fringes can be detected. The d-spacing of about 0.28 nm is ascribed to (100) plane of hexagonal ZnO. SEM and TEM micrograms revealed that well dispersed ZnO nanosheets hybridized with PANI thin layer were successfully obtained. 3.2. Photoreduction of aqueous Cr(VI) ions 3.2.1. Effect of initial pH The effect of initial pH on Cr(VI) removal efficiency was investigated over a pH range of 3–9 with initial Cr(VI) concentration of 20 mg·L−1. The pH range was chosen because lower pH values were able to cause dissolution of the ZnO phase and higher pH values resulted in precipitation of chromium hydroxide. Fig. 5a and b represent the timedependent Cr(VI) concentration variation plots at different initial pH. The apparent first-order kinetic model was chosen to estimate photocatalytic removal of Cr(VI) qualitatively. The expression is given by Eq. (1):

ln

½CrðVIÞ0 ¼ kobs t ½CrðVIÞ

ð1Þ

Table 2 Textural properties of ZnO and ZnO/PANI samples. Sample ZnO ZnO/PANI

SBET (m2·g−1)

Pore volume (cm3·g−1)

Average pore size (nm)

45.8 39.6

0.34 0.48

23.8 41.7

where kobs is the apparent rate constant (min−1), [Cr(VI)]0 is the initial Cr(VI) concentration at time t = 0 and [Cr(VI)] is the concentration at time t. The value of kobs is equal to the corresponding slope of the fitting line as provided in Table 3. For pristine ZnO photocatalyst, the highest kobs value (0.0238 min−1) occurred at neutral pH over the chosen range. The Cr(VI) removal rate was ~86% at this pH over 120 min UV irradiation. At pH b7, the removal rate decreased with decrease in solution pH from 5 to 3. It is clear that at pH 3 ZnO exhibited little photoreductive ability, which was mainly due to the dissolution of ZnO at the lower pH. Researchers have reported that the hydroxide-stabilized ZnO surface was damaged at pH below 3.8 [51]. On the other hand, the optimum kobs value for ZnO/PANI catalyst was found to be 0.0476 min−1 at pH 4, which was increased about 16-fold compared to that of uncovered ZnO at the same pH. The Cr(VI) removal rate reached to ~97% at this pH. With the increase of initial pH to 5 and 7, the removal rates decreased gradually. When pH was 3, the removal rate was significantly lower than did the neighbor, but it still reached about 76%, which was much higher than the removal rate of pure ZnO photocatalyst for the same pH solution. Under alkalescence, the efficiency of ZnO/PANI was also greater than that of the pure ZnO. By comparing kobs values, it is clear that the PANI-hybridized ZnO sample exhibited higher photocatalytic activity than did the pristine ZnO over the tested pH range. The incorporation of PNAI significantly improved the photo-reductive ability of ZnO. Considering that ZnO was provided with rather good photoreduction of high valent metal ions under UV irradiation, and that ZnO/PANI still exhibited high photocatalytic efficiency even in the pH condition where ZnO was likely dissolved, it is reasonable that there existed a synergic effect between ZnO intrinsic surface and PANI to enhance the photocatalytic activity. In order to further verify this view, a controlled experiment was carried out using mechanically blended ZnO and PANI (ZnO-B-PANI) as a photocatalytic reference over the same pH range. The results can be seen from Fig. S1 and Table S1 in the Supporting information. It is found that the ZnO-B-PANI exhibited the similar kobs values with pristine ZnO at the corresponding pH, showing that the blend of ZnO and PANI, which is not chemically bonded, had no notable promotion on the photoreduction of Cr(VI). It also showed that the improved photocatalysis of the ZnO/PANI composites was due to the synergistic effect between the interfaces of the PANI-hybridized ZnO. From the effect of initial pH on Cr(VI) removal and the changes of rate constant (see Fig. 5 and Table 3), it can be detected that the photocatalytic process is markedly pH-dependent. For pure ZnO photocatalyst, the optimal pH was nominated to be neutral, and the acidification or alkalization resulted in a sharp decrease in efficiency. For ZnO/PANI, the recommended pH range was 4–7, in which a better efficiency can be achieved. These results suggested that the PANI hybridization broadened the pH range of ZnO used in acidic solution. Moreover, a pH decrease from 7 to 4 caused nearly double the reaction rate, i.e. from 0.0253 to 0.0476 min−1. As photocatalysts assisted with

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Fig. 4. SEM images of (a) ZnO and (b) ZnO/PANI, (c) TEM micrograph of ZnO/PANI and (d) TRTEM of a single ZnO/PANI sheet.

ZnO for Cr(VI) reduction, it is of great practical significance that they can be efficiently utilized in acidic solution. This is mainly because (i) the chromium-containing wastewater is acidic in general, and (ii) the alkaline environment results in the precipitation of trivalent chromium ions on the catalyst surface, which can usually abate the activity of semiconductor photocatalysts [38,52]. The observed pH effect can be ascribed to several reasons, as follows: (i) Depending on the pH and concentration of the solution, Cr(VI)

species participate in the following equilibriums Eqs. (2)–(4) [2]: 2HCrO4 − ↔Cr2 O7 2− þ H2 O

pKa1 ¼ −1:52

ð2Þ

H2 CrO4 ↔HCrO4 − þ Hþ

pKa2 ¼ 0:8

ð3Þ

HCrO4 − ↔CrO4 2− þ Hþ

pKa3 ¼ 6:5

ð4Þ

Fig. 5. Time-dependent Cr(VI) concentration variation plots at different initial pH using (a) ZnO and (b) ZnO/PANI as catalysts: [Cr(VI)]0 = 20 mg·L−1, [Cat] = 0.5 g·L−1, T = 30 ± 1 °C; (c) Percent reduction of Cr(VI) to Cr(III) at different time.

C. Bao et al. / Journal of Molecular Liquids 279 (2019) 133–145 Table 3 Apparent rate constant (kobs) for Cr(VI) removal at different pH. Sample

ZnO ZnO/PANI

Parameter

kobs (min−1) R2 kobs (min−1) R2

pH 3

4

5

7

9

– – 0.0184 0.966

0.00282 0.818 0.0476 0.970

0.0134 0.912 0.0297 0.983

0.0238 0.974 0.0253 0.994

0.0121 0.994 0.0148 0.975

Under the present cases, HCrO4− ions are the dominant species in solution. Furthermore, the acidic solution is in favor of\\NH2 and\\N_ in PANI to form their protonated counterpoints, facilitating the attraction of negative HCrO4− ions and the subsequent photocatalytic reduction. This was evidenced by Fig. 5b where the extent of dark adsorption was up to 25% in the more acidic solution but insignificant at higher pH values. The control butter using ZnO-B-PANI catalyst also showed the same trend (Fig. S1). These results proved that acidic solution was favorable to the adsorption of chromium anions by PANI. (ii) The Nernst reduction potential of HCrO4−/Cr3+ increases with degressive pH, which help to enhance its oxidative ability [30,53]. On the other hand, it is well-known that the photocatalytic activity of ZnO is attributed to the photogenerated charges and their separation efficiency [15,19]. According to the XRD and XPS results, the surface state of ZnO phase has not been obviously changed by PANI modification. Furthermore, although PANI can be theoretically excited by both UV and visible light, it had no evidence of photocatalytic activity according to the photocatalytic test of ZnO-B-PANI catalyst and the reports else [30,32]. Therefore, the enhanced photo-reductive efficiency of ZnO/ PANI is expected due to a promotion of interfacial charge separation by PANI hybridization. The efficient charge transfer between ZnO and PANI can be confirmed from the observation of decreasing intensity of PL band edge emission of ZnO. It is clear that the PL spectrum of ZnO/ PANI (Fig. 3c) presents a lower intensity of emission peak compared to the pristine ZnO, suggesting that the electron was transferred from excited ZnO via interfacial charge transfer mechanism [15]. As discussed, PANI is an excellent p-type semiconductor material for transporting holes through specific π-conjugated structure [32]. The formed heterostructure with the n-type semiconductor ZnO can create an internal electric field at the interface to easy to the separation of photogenerated carriers [26,27,29,44]. Furthermore, visible emissions are also found in the PL spectra. Various intrinsic/extrinsic surface defects can be responsible for their appearance. These surface deficiencies especially oxygen defects in ZnO nanosheets have already approved by Raman (Fig. 1b) and XPS (Fig. 2) analyses. It was identified that the surface oxygen defects can conduct charge carrier traps and restrain the recombination of electrons and holes [15,19,26]. Consequently, the synergic effect of surface defects and PNAI layers facilitated efficient charge separation to promote the catalytic activity of ZnO/PANI photocatalyst. Fig. 5c shows the percent reduction of Cr(III) to total chromium at different time. It can be seen that the distribution of percent reduction is in agreement with the trend of Cr(VI) removal ratio for both ZnO and ZnO/PANI catalysts, demonstrating that Cr(VI) was reduced to Cr (III) during photocatalysis. It is worth noting that, during the dark adsorption, N40% of hexavalent chromium was reduced to trivalent form for ZnO/PANI system while ignorable trivalent chromium was found in the solution for pure ZnO catalysis at this period indicating the PANI layer in the composite provided a chemical reduction ability to reduce hexavalent chromium. Further evidence will be given in the discussion below. 3.2.2. Effect of initial Cr(VI) concentration To evaluate the effect of initial concentration on Cr(VI) photoreduction, a series of reactions were performed by using different initial Cr (VI) concentrations ranging from 10 to 50 mg·L−1. The results are

139

shown in Fig. 6. The linear fitted values of kobs are given in Table 4. It can be found that a lower concentration of Cr(VI) resulted in a higher apparent rate constant, indicating that the catalysts were suitable for the removal of low concentration Cr(VI) solution. Moreover, ZnO/PANI revealed a higher kobs value than did the ZnO catalyst over all concentrations. A heterogeneous photocatalysis usually involves adsorption and surface reaction processes. The removal rate (r) and initial concentration of Cr(VI) for a heterogeneous catalytic process can be depicted by Langmuir–Hinshelwood kinetic model shown as Eqs. (5) and (6) [54]. r ¼ kC

K LH ½CrðVIÞ ¼ kobs ½CrðVIÞ 1 þ K LH ½CrðVIÞ

1 1 ½CrðVIÞ0 ¼ þ kobs kC K LH kc

ð5Þ

ð6Þ

where kc (mg·L−1·min−1) and KLH (L·mg−1) are the kinetic rate constant of surface reaction and Langmuir–Hinshelwood adsorption equilibrium constant, respectively. KLH and kc can be gotten by linear fitting 1/kobs as a function of [Cr(VI)]0. The obtained results are showed in Fig. 6b and d. The kinetic parameters calculated were listed in Table 4. The experimental data are in agreement with the Langmuir– Hinshelwood model, suggesting that the photoreduction of Cr(VI) catalyzed by ZnO and ZnO/PANI was geared to a concurrent action of adsorption and surface reaction. For ZnO/PANI, the reaction rate constant and the adsorption equilibrium constant were calculated to be kc = 4.36 mg·L−1·min−1 and KLH = 0.0185 L·mg−1, respectively. The kc value is much larger than KLH, indicating that the reaction rate was dominated by the surface reaction. This result can also be seen from Fig. 6a that the concentration of Cr (VI) decreases rapidly after the photocatalytic reaction. For pristine ZnO, a similar tendency was exhibited, but both values of kc and KLH are smaller than that of the composite photocatalyst. These results indicated that the ZnO/PANI catalyst was provided with a higher equilibrium adsorption capacity and faster reaction rate than did the pristine ZnO over the concentration range. 3.2.3. Effect of sacrificial electron donor In the present work, some organic compounds such as ethylenediaminetetraacetic acid (Na2EDTA), citric acid, oxalic acid and formic acid were added to evaluate the effect of organic sacrifice on the photocatalytic reduction of Cr(VI). For this purpose, 10 mg·L−1 of each compound was added to 20 mg·L−1 Cr(VI) solution without pH adjusting. The intrinsic pH measured was 6.6, 6.2, 5.7 and 5.2 after EDTA, citric acid, oxalic acid and formic acid were added respectively. The comparative results are shown in Fig. 7. It is illustrated that EDTA was provided with the most positive impact on enhancing the reduction, whereas formic and oxalic acid had negative effect on catalytic efficiency. Many researchers have studied the effect of organic additives on the photocatalytic reduction of Cr(VI) by semiconductors [35–37,39]. The improvement was often attributed to the fact that organic additives as an effective electron donor captured positively charged hole (h+) in valence band and assisted more negative electrons (e−) to participate in the reduction of Cr(VI) and thus, the efficiency of reduction was enhanced. On the contrary, the inhibition was mainly due to the evidence that holes are only filled by the formation of radicals [37,39]. However, different views have been put forward on the mechanism of EDTA and citric acid promoting Cr(VI) photoreduction. M. I. Litter et al. found that the formation of Cr(III)-EDTA complex inhibited the deposition of Cr(III) onto catalytic surface and consequently, the deactivation of catalyst was restrained [55,56]. R. O. Cristóvão et al. [57] and R. Djellabi et al. [58] also pointed out that citric acid was provided with the similar efficacy for the extraction of deactivated Cr(III) ions. In the current case, in order to understand the mechanism, HPLC analysis was carried out on the supernatants before and after UV exposure, and no evidence of EDTA degradation was detect. Therefore, it is considered that the promotion of photoreduction by EDTA was not due to its contribution as

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Fig. 6. (a) Time-dependent Cr(VI) concentration variation plots at different initial concentrations and (b) relationship between 1/kobs and initial Cr(VI) concentration by ZnO/PANI catalyst: [Cat] = 0.5 g·L−1, pH = 4.0, T = 30 ± 1 °C; (c) Time-dependent Cr(VI) concentration variation plots at different initial concentrations and (d) relationship between 1/kobs and initial Cr(VI) concentration by ZnO catalyst: [Cat] = 0.5 g·L−1, pH = 7.0, T = 30 ± 1 °C.

a sacrificial agent, but rather to the complexation of Cr (III) by EDTA. In fact, many studies have demonstrated that the photocatalyst was deactivated by the reductive deposition of Cr(III) species on the surface [38,56]. The cleaning of Cr(III) on the catalytic surface should help to enhance photocatalytic efficiency. 3.3. Cr(VI) ions removal mechanism It is essential to determine the chemical valence state of chromium on the catalyst surface, which is conductive to comprehend the mechanism of Cr(VI) removal. XPS was utilized to identify the possible chromium species on the surface. Fig. 8 shows the Cr 2p high-resolution spectra of ZnO/PANI photocatalyst after adsorption in dark and

irradiation by UV light. Before photoreduction, the detected Cr 2p bands can be resolved into four Gauss peaks, located at 577.79, 583.25, 587.06 and 591.72 eV (Fig. 8a). The resolved peaks at binding energy 577.79 and 587.06 eV correspond Cr 2p3/2 and 2p1/2 bands of Cr(III), respectively. The higher binding energy peaks at 583.25 and 591.72 eV can be ascribed to Cr 2p3/2 and 2p1/2 bands of Cr(VI), respectively. The simultaneous appearance of Cr(III) and Cr(VI) species indicated that the adsorbed Cr(VI) on the surface was partially reduced to Cr(III) during the dark adsorption. The area of fitted peaks and their integrated ratio reflected the relative content of each chromium species. The calculated percentage of Cr(III) to the total chromium is ~67%, recommending that about 2/3 Cr(VI) was reduced to Cr(III) during the adsorption. It was widely reported that the conjugated amine and imine groups in PANI

Table 4 Kinetic parameters for the photocatalytic reduction of Cr(VI) at different initial Cr(VI) concentrations: [Cat] = 0.5 g·L−1, T = 30 ± 1 °C. [Cr(VI)]0 (mg·L

kobs

−1

)

ZnO/PANI 10 20 30 50 ZnO 10 20 30 50

(min

r0 −1

)

Removal ratio

(mg·L

−1

·min

−1

)

R2

(%)

KLH (L·mg

kc −1

)

(mg·L−1·min−1)

0.0510 0.0476 0.0392 0.0322

0.510 0.952 1.176 1.610

97.2 96.8 93.5 91.1

0.972 0.970 0.983 0.981

0.0185

4.36

0.0276 0.0238 0.0202 0.0144

0.276 0.476 0.606 0.720

89.4 86.1 83.2 74.3

0.973 0.974 0.970 0.985

0.0322

1.19

C. Bao et al. / Journal of Molecular Liquids 279 (2019) 133–145

141

Fig. 7. Time-dependent Cr(VI) concentration variation plots at different initial concentrations using (a) ZnO and (b) ZnO/PANI as catalysts: [Cr(VI)]0 = 20 mg·L−1, [organic acid] = 10 mg·L−1, [Cat] = 0.5 g·L−1, pH = 7.0, T = 30 ± 1 °C.

can reduce high potential Cr(VI) to Cr(III) [5,59]. After the photoreduction, negligible Cr(VI) was found on the surface of catalyst, suggesting that the remaining Cr(VI) was reduced to Cr(III). This result was consistent with the distribution of chromium species in the liquid phase (Fig. 5c). Moreover, the binding energy of Cr(III) shifted to a lower value after photoreduction, indicating that the chemical bonding between Cr(III) and the catalyst surface changed. XPS analysis illustrated that both chemical reduction and photoreduction contributed to the Cr(VI) decontamination. Herein, the photochemical reduction mechanism of ZnO/PANI is mainly discussed. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of PANI are −3.39 eV and −5.74 eV (vs vacuum), respectively [60]. The conduction band (CB) and valence band (VB) of ZnO are located at −4.40 eV and −7.70 eV (vs vacuum), respectively [61]. Under UV irradiation, the electrons photoexcited in the LUMO of PANI transfer to the CB of ZnO, and the holes excited in the VB of ZnO transfer to the HOMO due to the potential differences. Moreover, the stable p-n heterostructure drove the photoinduced electron and hole transport in reverse directions, and such an anti-recombination effect can be confirmed by the PL quenching of ZnO/PANI composites under UV light (λ = 325 nm) [62–64]. Based on the above theoretical analysis and experimental results, as well as previous reports on the photo-reductive mechanism of Cr(VI), a tentative Cr(VI) removal mechanism over the ZnO/PANI photocatalyst can be schematically proposed in Fig. 9. In an acidic solution, the negatively charged HCrO− ions were attracted onto the

catalyst surface by protonated amine and imine groups in PANI. Due to their oxidizability, partial Cr(VI) ions were reduced to Cr(III) by PANI. Meanwhile, the reduced Cr(III) was released into the solution until the ion transfer equilibrium reached. During photocatalysis, the equilibrium was broken. When UV light fell on the catalytic surface, a conduction band (e−) and a valence band (h+) were generated. A series of photochemical reactions ensued. Under a normal non-intense laboratory UV illumination, most photocatalytic reactions occur through successive mono-electronic reactions due to the low frequency of photon absorption [65]. For Cr(VI), the highest oxidized state of chromium, the thermodynamically allowed one-electronic transfer steps are considered two types: (i) direct reduction by photoinduced electrons (e−) and (ii) indirect reduction by reducing radicals generated by hole (h+) oxidation of electron donors [66,67]. At the present case, since no evidence of EDTA degradation was detected, EDTA should participate in the photochemical reactions with a non-sacrificial electron donor [55–58]. Thus, the mechanism of Cr(VI) photocatalytic reduction is inferred as the type I, that is direct reduction by photogenerated electrons. The reaction involving in Cr(VI) photoreduction are shown as Eqs. (7)– (11). þ

ZnO=PANI þ hv→ðZnO=PANIÞ h −e−




HCrO− þ 7Hþ þ 3e− →Cr3þ þ 4H2 O

Fig. 8. XPS core-level Cr 2p spectra of ZnO/PANI after (a) dark adsorption and (b) UV irradiation: [Cr(VI)]0 = 20 mg·L−1, [Cat] = 0.5 g·L−1, pH = 4.0, T = 30 ± 1 °C.

ð7Þ ð8Þ

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Fig. 9. Proposed schematic mechanism for Cr(VI) photoreduction over ZnO/PANI composites under UV irradiation.

þ

H2 O þ h →˙OH þ Hþ

ð9Þ

OH− þ h →˙OH

þ

ð10Þ

PANI þ ˙OH→PANI

ð11Þ

The ZnO/PANI catalyst is excited by UV irradiation, causing the creation of electron-hole pairs (h+–e−) (Eq. (7)). Because the redox potential of HCrO4−/Cr3+ is more than that of the conduction band, Cr(VI) is directly reduced to Cr(III) by e− experiencing Cr(V) and Cr(IV), which are produced by successive one-electron reactions (Eq. (8)). Cr(III) was proved to be the final species either as adsorbate on the catalyst surface by the XPS determination or as soluble complexes when citric acid or EDTA exist. Cr(V) formation has been detected in photocatalytic process by electronic paramagnetic resonance (EPR) [68,69]. Simultaneously, the holes may be oxidized H2O molecules into hydroxyl radical (•OH) (Eqs. (9) and (10)). These •OH species were probably consumed through participating in PANI redox (Eq. (11)). This may also prevent the reoxidation of the low valence chromium ion to the high valence state by holes and reactive oxidation species. Moreover, the p-n heterojunction should provide a boosted separation efficiency of electron-hole pairs, as reflected in the PL spectrum (Fig. 3c). As results,

acidic conditions facilitated reduction of Cr(VI) according to Eq. (8), whereas alkaline solution promoted formation of •OH according to Eqs. (9) and (10). Therefore, the removal efficiency of Cr(VI) was higher at lower pH (Fig. 5). These plausible reactions have also been presented elsewhere by other researchers [35,70]. The photocatalytic mechanism over PANI hybridized semiconductor catalysts has been extensively discussed. The π–π⁎ transition from PANI, which can be excited by visible light, and subsequent electron transfer to the conduction band of semiconductor, are often considered as the beginning of enhanced photocatalytic activity [29–33]. However, when the photon energy of the light source is equal or greater than the bandgap of the semiconductor, the origin of the excited electron remains controversial. In this sense, the current mechanism is still worth discussing.

3.4. Reusability Regeneration of the used catalyst contributes to the practicability and examining the mechanism. HCl (1.0 mM), NaOH(1.0 mM), EDTA (1.0 mM) and EDTA/HCl (1:1 volume ratio) solutions were selected as a scavenger to clean the used ZnO/PANI for the next cycle. Water washing was also tested as a control experiment. The result is shown in Fig. 10. It is clear that HCl, EDTA and EDTA/HCl solutions can well

Fig. 10. (a) Removal ratio variation using different scavengers and (b) cycling runs for Cr(VI) removal by ZnO/PANI with EDTA/HCl scavenger at pH 4.0 and 7.0.

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Table 5 Comparison of catalytic performances of semiconductor/PANI catalysts for Cr(VI) reduction. Catalyst

ZnO/PANI SnO2/PANI PANI/SnS2/NRG a MoS2/PANI b PANI/MnO2/TiO2 ZnO/PANI a b

Light source (wavelength) Simulated solar light Visible light (N420 nm) Visible light (N420 nm) UV – UV(320–400 nm)

Lighting time (min)

Cycles

300 210 140 1440 5 90

– 5 4 5 5 5

Efficiency (%)

Reference

Fresh catalyst

Used catalyst

92.7 ~99 ~98 – 99.9 98

– 78 ~80 – 99.9 ~80

[27] [30] [71] [72] [73] This work

Data of catalytic efficiency was not reported in the Reference [72]. Chemical reduction was performed using HCOOH as an oxidant.

scavenge the impurities adsorbed, whereas NaOH solution was the worst one. After EDTA/HCl cleaning, the Cr(VI) removal ratio was still up to 95% by the recovered catalyst. Since Cr (III) is considered the most important deactivator, their removal should facilitate the photoactive restoration of the catalyst. In an acidic solution, H+ is competitive with Cr(III) and the protonation of PANI chain to form nitrogenium segments is helpful to the desorption of Cr(III) [6]. The complexation of EDTA to Cr (III) reduces the concentration of free Cr (III) ions in liquid, which is favorable for the release of Cr (III) from the surface of catalyst. The durability of the ZnO/PANI catalyst was also tested, as shown in Fig. 10b. Before each run, the recovered catalyst was immersed in EDTA/ HCl solution for 30 min, thoroughly washed with water and then dried overnight under vacuum. During five cycles, the Cr(VI) removal ratio decreased from 96.5% to 83.7% when the initial pH was 4.0, whereas when the pH was 7.0 the removal ratio lessened from 88.9% to 79.8%. It is revealed that a slight decrease in removal ratio occurred in comparison with the previous cycle and a satisfactory efficiency of close to 80% was still achieved after five cycles. Therefore, it is possible that ZnO/ PANI photocatalyst can be easily recovered and reused several times. For pure ZnO and ZnO-B-PANI samples, on the contrary, it is found that the Cr(VI) removal ratio was sharply decreased to b20% during three cycles. The drastic descent in photoactivity of ZnO can be ascribed to the photocorrosion effect [29,32]. It is apparent that the photocorrosion of ZnO was availably inhibited by PANI hybridization. Many investigations have indicated that the photogenerated holes were the dominant species for photocorrosion on ZnO nanocrystals [32,38]. After ZnO was hybridized by PANI, a p-n heterojunction formed. When ZnO absorbed UV light to generate electron-hole pairs, the holes in VB directly transferred to the HOMO of PANI. Electrons moved to the opposite direction from holes. The p-n heterojunction reduced the recombination of photogenerated electrons and holes, and caused charge separation more efficient. As a result, photoinduced holes rapidly transferring to the solution facilitated the inhibition of photocorrosion. Moreover, the shielding effect of H+ attack can also be explained as the reason of resistance to chemical corrosion at a wide pH range after PANI hybridization [25,51]. 3.5. Comparison with other semiconductor/PANI catalysts Table 5 shows the comparative results of semiconductor/PANI photocatalysts for Cr(VI) removal reported in recent years. Zou et al. reported that the pompon-like ZnO/PANI heterostructure pohocatalyst could reduce aqueous Cr(VI) to Cr(III) under simulated solar light with an acceptable efficiency (92.7%) [27]. Polyaniline modified SnO2 nanoparticles and PANI/SnS2/N-doped reduced graphene oxide (NRG) ternary composites demonstrated a higher efficiency, shorter time and satisfactory reusability under visible light [30,71]. It is worth noting that the PANI/MnO2/TiO2 ternary catalyst reveals an excellent conversion (99.9%) through chemical reduction method assisted with HCOOH oxidant [73]. After repeated use of this nanocomposite for five times, no loss of catalytic conversion was achieved. In comparison,

simple preparation, low cost, high and stable efficiency are the advantages of the catalyst in this work. 4. Conclusion In summary, ZnO/PANI nanocomposites were successfully prepared via a two-step method. The obtained ZnO/PANI composites exhibited higher photoreduction efficiencies over a wider range of pH in comparison with the pristine ZnO under UV irradiation. The reaction rate constant (kc) and the adsorption equilibrium constant (KLH) according to Langmuir–Hinshelwood kinetic model were calculated to be kc = 4.36 mg·L−1·min−1 and KLH = 0.0185 L·mg−1 respectively, indicating that the photochemical reaction was the control step for reaction rate. The PANI hybridization effectively promoted charge transfer and suppressed recombination of photogenerated electrons and holes, resulting in the improved photoreduction of aqueous Cr(VI). In addition, the ZnO/ PANI composites also showed good stability and recycling performance. This study demonstrates that ZnO/PANI composite is an effective photocatalyst for the treatment of Cr (VI) polluted water, which is expected to be applied in environmental remediation in the future. Acknowledgement The authors are grateful for the financial support from the National Natural Science Foundation of China (No. 51364040). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2019.01.122. References [1] A. Bakshi, A.K. Panigrahi, A comprehensive review on chromium induced alterations in fresh water fishes, Toxicol. Rep. 5 (2018) 440–447. [2] Y. Zheng, W. Wang, D. Huang, A. Wang, Kapok fiber oriented-polyaniline nanofibers for efficient Cr(VI) removal, Chem. Eng. J. 191 (2012) 154–161. [3] D. Mamais, C. Noutsopoulos, I. Kavallari, E. Nyktari, A. Kaldis, E. Panousi, G. Nikitopoulos, K. Antoniou, M. Nasioka, Biological groundwater treatment for chromium removal at low hexavalent chromium concentrations, Chemosphere 152 (2016) 238–244. [4] M.A. Omole, O.A. Sadik, I.O. K'Owino, Palladium nanoparticles for catalytic reduction of Cr(VI) using formic acid, Appl. Catal., B 76 (2007) 158–167. [5] K.K. Krishnani, S. Srinives, B.C. Mohapatra, V.M. Boddu, Jumin Hao, X. Meng, Ashok Mulchandani, Hexavalent chromium removal mechanism using conducting polymers, J. Hazard. Mater. 252–253 (2013) 99–106. [6] C. Bao, M. Chen, G. Liu, X. Xin, Efficient adsorption/reduction of aqueous hexavalent chromium using oligoaniline hollow microspheres fabricated by a template-free method, J. Chem. Technol. Biotechnol. 93 (2018) 1147–1158. [7] D. Mohan, C.U. Pittman, Activated carbons and low cost adsorbents for remediation of tri- and hexavalent chromium from water, J. Hazard. Mater. 137 (2006) 762–811. [8] S.S. Hosseinia, E. Bringas, Ni.R. Tan, I. Ortiz, M. Ghahramani, M.A.A. Shahmirzadi, Recent progress in development of high performance polymeric membranes and materials for metal plating wastewater treatment: a review, J. Water Proc. Eng. 9 (2016) 78–110. [9] Q. Zhao, C. Liu, B. Li, R. Zevenhoven, H. Saxén, M. Jiang, Recovery of chromium from residue of sulfuric acid leaching of chromite, Process. Saf. Environ. Prot. 113 (2018) 78–87.

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