TiO2 composites

Journal Pre-proofs Adsorption and photocatalytic reduction of aqueous Cr(VI) by Fe3O4-ZnAllayered double hydroxide/TiO2 composites Yanting Yang, Jing Li, Tao Yan, Rixin Zhu, Liangguo Yan, Zhiguo Pei PII: DOI: Reference:

S0021-9797(19)31416-X https://doi.org/10.1016/j.jcis.2019.11.088 YJCIS 25705

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

17 September 2019 18 November 2019 19 November 2019

Please cite this article as: Y. Yang, J. Li, T. Yan, R. Zhu, L. Yan, Z. Pei, Adsorption and photocatalytic reduction of aqueous Cr(VI) by Fe3O4-ZnAl-layered double hydroxide/TiO2 composites, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.11.088

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Adsorption and photocatalytic reduction of aqueous Cr(VI) by Fe3O4-ZnAl-layered double hydroxide/TiO2 composites

Yanting Yanga,b, Jing Lia, Tao Yana, Rixin Zhua, Liangguo Yana,*, Zhiguo Peib,*

aSchool

of Water Conservancy and Environment, University of Jinan, Key Laboratory

of Water Resources and Environmental Engineering in Universities of Shandong (University of Jinan), Jinan 250022, PR China bState

Key Laboratory of Environmental Chemistry and Ecotoxicology, Research

Center for Eco–Environmental Sciences, Chinese Academy of Sciences, PO Box 2871, Beijing 100085, PR China

* To whom all correspondence should be addressed L. Yan: Tel: + 86-531-82767617, E-mail: [email protected], [email protected]. Z. Pei: Tel: + 86-010-62849329, E-mail: [email protected].

Abstract To evaluate the synergetic adsorption capacity of layered double hydroxide (LDH) and photocatalytic reduction property of TiO2 for aqueous Cr(VI), the Fe3O4-ZnAlLDH and TiO2 composites (FLT) were prepared via sol-gel method. The removal efficiency of FLT composites for Cr(VI) changed with the mass ratio of Fe3O4LDH/TiO2, and the optimal ratio was 20% (FLT-2). The adsorption efficiencies of 100 mg FLT-2 were 74% and 64% for 20 and 50 mg/L Cr(VI) solution, and the total removal ratios of Cr(VI) were all over 97% after UV irradiation. The experimental data of adsorption kinetic, adsorption isothermal, and photocatalytic reduction were consistent with the pseudo-second-order, Langmuir, and first-order reaction equations, respectively. From the X–ray photoelectron spectra analysis, FLT composites adsorbed Cr(VI) directly via surface complexation, also reduced aqueous Cr(VI) to Cr(III). The photocurrent and electrochemical impedance spectroscopy of FLT composites indicated the photo-induced electron-hole pairs were separated effectively. The regeneration results show that the FLT composites had good stability and reusability after five consecutive cycles. Therefore, FLT composites are promising materials in the disposal of wastewater containing Cr(VI). Keywords: Hexavalent chromium; Adsorption; Photocatalytic reduction; Heavy metal removal; Synergetic effect

1. Introduction Heavy metal pollution in water environment has drawn widespread concerns for their non-biodegradability and easy bioaccumulation [1-3]. Hexavalent chromium, knows as Cr(VI), is a kind of mutagenic and toxic heavy metal, and is applied in many industries, such as electroplating, pigments, metallurgy, pharmacy, and ferrochrome industry [4, 5]. This results in a large quantity of industrial wastewater containing Cr(VI) being discharged into the aquatic system [6, 7]. The released Cr(VI) had caused acute health problems for organisms, such as gastrointestinal irritation, kidney and liver damage [8, 9]. Therefore, it is necessary to remove Cr(VI) from water prior to discharge. Various methods, for example, adsorption, photocatalytic and electrochemical reduction, reverse osmosis, ion exchange, had been used to separate or reduce Cr(VI) from the environment [10-13]. In these approaches, adsorption method is gained some advantages over others for its low-cost, simple equipment, and easy operation [14]. A variety of materials, such as fertilizer industry waste [15], cellulose [16], modified chitosan [17], iron oxides [18], activated alumina [19], layered double hydroxides (LDHs) [20, 21], and activated charcoal [22] had been applied to clean the Cr(VI)containing wastewater. However, it should be noted that the adsorption method was usually effective for wastewater with low Cr(VI) concentrations, and cannot completely remove all chromium ions at high concentrations due to the limited active sites of adsorbents. Hence, the development of new approaches to remove high concentrations of aqueous Cr(VI) is urgent. Photocatalytic reduction of Cr(VI) to less toxic Cr(III) is another choice. In

photocatalysis method, TiO2 is the most commonly used photocatalysts and has been well studied for environmental protection for its chemical stability, efficiency, and nontoxicity [23-27]. Furthermore, various TiO2-based materials were synthesized in the past 30 years for the decontamination of pollutants via synergetic adsorption and photocatalysis method [28-31]. For example, the graphitized mesoporous carbon-TiO2 nano-composite promoted the degradation of ciprofloxacin [30], Ananpattarachai and Kajitvichyanukul [32] designed the nanoTiO2 impregnated chitosan/xylan film to remove aqueous Cr(VI), Zhao et al [12] synthesized a nanocomposite of Ag/AgCl/Bi6O4(OH)4(NO3)6·H2O for the methyl orange removal at high-concentration, and Liu et al [33] fabricated the composite of g-C3N4 and TiO2 to remove methylene blue. The above work all based on the synergetic adsorption and photocatalysis. To evaluate the synergetic adsorption properties of LDHs and the reduction efficiency of TiO2 for aqueous Cr(VI), we synthesized the composites of Fe3O4-LDH and TiO2 (FLT) via the sol-gel method. The chemical composition and structure of FLT composites were analyzed using various characterization methods. The adsorption kinetics and isotherms were evaluated by the batch equilibrium methods at dark conditions. Then the Cr(VI) reduction by FLT composites was conducted under UV irradiation. Besides, the synergetic removal mechanisms of Cr(VI) by FLT composites via adsorption and photocatalysis were also discussed.

2. Experimental 2.1. Materials TiO2 (P25 Degussa) was obtained from Shanghai Maclin Biochemical Technology

Co. Ltd. K2Cr2O7, Zn(NO3)2·6H2O, Na2CO3, NaOH, H3PO4, H2SO4, Al(NO3)3·9H2O, and polyethylene glycol (PEG 200) were obtained from Tianjin Guangcheng Reagent Factory. Fe3O4 was gained from Tianjin Damao Reagent Factory.

2.2. Synthesis of FLT composites 2.2.1 Preparation of Fe3O4-LDH Solution A was obtained by dissolving of Zn(NO3)2·6H2O (11.88 g) and Al(NO3)3·9H2O (7.50 g) in 60 mL distilled water. Solution B was obtained by dissolving of NaOH (7.2 g) and of Na2CO3 (5.6 g) in 60 mL distilled water. The Fe3O4 powder (2.30 g) was put into distilled water (60 mL). After sonochemical dispersion to a uniform suspension, the solution was set in a water bath (60°C). Then the solutions A and B were added to the dispersion system containing Fe3O4. During this process, the pH of the mixture was maintained at 9-10. When the reaction finished, the product was rinsed by distilled water until the supernatant pH was about 7. The final solid products were centrifuged, dried at 60°C in the oven, and then passed through a 100 mesh sieve. 2.2.2 Synthesis of FLT composites TiO2 (0.25 g) was thoroughly mixed with 30 mL PEG200, and then Fe3O4-LDH (2.25 g) was added and dispersed ultrasonically for 0.5 h. The mixture of Fe3O4-LDH/TiO2 was then put into a reaction kettle, heated to 95°C, and held for 1 h. Finally, the obtained solid was separated by centrifugation, rinsed with distilled water to clean the residual PEG200, dried at 60°C in the oven, and then passed through a 100 mesh sieve. The sample was named as FLT-1 (10% content of TiO2). The FLT-2 and FLT-5 composites containing 20% and 50% (wt%) content of TiO2 were also prepared according the above

procedure.

2.3. Characterization methods The crystallographic structure of FLT composites was determined by a D/MAX 2200 X-ray diffraction (XRD, Rigaku, Japan) with CuKa radiation generated at 40 kV, 300 mA, and λ of 0.154 nm. The scattering angle was ranged from 10° to 80° (2θ) with a scanning speed of 0.03°/s. The Vertex70 Fourier transform infrared (FTIR) spectrometer (Perkin-Elmer, United States) was used for FTIR spectrum analysis. The morphology of FLT-2 was characterized using the S570 scanning electron microscope (SEM, Hitachi, Japan) combined with JEM-2100 EDS (JEOL, Japan), and the Tecnai G2 transmission electron microscopy (TEM, FEI Corporation, USA). The surface functional groups of FLT-2 composite were determined by an ESCALAB 250 Xi Xray photoelectron spectroscopy (XPS, Thermo Fisher, United States). The electrochemical impedance spectroscopy (EIS) experiments were performed using a PP211 electrochemical workstation (ZAHNER ZENNIUM, Germany). The photocurrent was determined using an electrochemical station (CHI 760E, Chenhua Instruments, China). The UV–vis diffuse reflectance spectra (DRS) were determined using a Cary 500 UV–vis spectrophotometer (Varian, United States).

2.4. Adsorption and photocatalytic reduction experiment The adsorption test of FLT composites for Cr(VI) was performed by batch equilibrium method. The solution pH of Cr(VI) (100 mL) was set as 3.0 using HCl or NaOH solution according to the preliminary result of the adsorption of Cr(VI) onto FLT-2 as a function of initial solution pH (Fig. S1), and then placed in a 250 mL quartz

tube with FLT composites (100 mg). The mixture was stirred to reach adsorptiondesorption equilibrium state. Then four 5 W Hg lamps (8.85 mW/cm and maximum wavelength of 254 nm) were turned on to perform the photocatalytic process of FLT composites for Cr(VI). A mixed solution (3 mL) was taken out from the quartz tube at certain time intervals and achieved solid/liquid separation by centrifugation (8000 rpm, 10 min). Then the UV2450 spectrophotometer (Shimadzu, Japan) was used to measure the Cr(VI) concentration in the supernatant according to the diphenylcarbazide colorimetric method.

3. Results and discussion 3.1. Characterization of FLT composites The FTIR spectra of FLT composites, TiO2, and Fe3O4-LDH are depicted in Fig. 1a. The broad bands around 3396 cm–1 arose from the stretching vibrations of the O-H or water molecules [30, 34]. The peak located at 1366 cm–1 could be induced from the interactions of CO32– and OH– groups in LDH interlayers. The bands at 400 - 900 cm–1 were attributed to the M-O and M-OH vibrations [35]. The band of 581 cm–1 was related to the benching vibrations of Fe-O from the synthesis process of Fe3O4 [36]. For the FLT composites with different mass ratios of Fe3O4-LDH/TiO2, all the peaks described above appeared in their FTIR spectra. This indicated that the FLT composites contained function groups of TiO2, LDH, and Fe3O4. Fig. 1b is the XRD patterns of FLT composites, TiO2, and Fe3O4-LDH. The peaks of FLT composites at 2θ of 11.7° (003), 23.6° (006), 30.3° (220), 34.7° (012), 35.6° (311), 39.2° (015), 43.2° (400), 46.9° (018), 54.0° (442), 56.9° (511), 60.3° (110), 61.7° (113),

and 62.8° (440), showed the characteristics of Fe3O4-LDH [37]. They also had the diffraction peaks of TiO2 at 2θ of 25.5°, 37.9°, 48.1°, 55.1°, 62.8°, 68.8°, 70.4°, and 75.1° which resulted from the (101), (004), (105), (211), (204), (116), (220), and (215) planes [38]. It was worth noting that no impure XRD peaks were emerged, which indicated that the FLT materials were highly purified. Generally, the angle of (003) plane indicated the large distance between interlayers of LDHs [39]. Although the FLT composites included all the peaks, the intensity of some peaks was different, such as the (003) and (006) planes of LDH, and the (101) planes of TiO2, which may be related to the different proportions of TiO2 and Fe3O4-LDH. This also demonstrated the crystalline structure was not destroyed during the preparation process. The electron microscopy images of FLT-2 in Fig. 2 revealed its surface morphology, crystal phase and composition. The spherical Fe3O4, stratified ZnAl-LDH and columnar TiO2 interlaced with each other (Fig. 2a and 2d). The lattice fringes of FLT-2 had interlayer spaces of 0.294, 0.351, and 0.266 nm (Fig. 2e), rather closed to the XRD spacings (2θ = 30.28°, d = 0.295 nm; 2θ = 25.51°, d = 0.349 nm; and 2θ = 34.77°, d = 0.264 nm). To study the element distribution of FLT-2, the elemental mapping of Al, Fe, O, Ti, and Zn were elaborated (Fig. 2c). Combined with the EDS image in Fig. 2b, all the elements appeared and distributed evenly. To provide further investigation of surface composition and electronic structure, the FLT-2 was analyzed by XPS technique (Fig. S2). The signals of Fe, Zn, Al, Ti, and O were all existed, which were in keeping with the chemical composition of the material.

3.2. Adsorption property of FLT composites

Fig. 3a depicts the adsorption of 50 mg/L Cr(VI) on FLT composites at different contact times. The Cr(VI) adsorption by three FLT composites increased quickly at the first 10 min, then maintained a slow gradual increase, and reached equilibrium state at 120 min. At equilibrium, the adsorption efficiencies of FLT-1, FLT-2, and FLT-5 for Cr(VI) were 57.7%, 66.5%, and 47.3%, suggesting that the doping content of TiO2 on FLT composites influenced the adsorption capacities. The pseudo-first-order (Eq. S1) and pseudo-second-order kinetic equations (Eq. S2) are frequently used to interpret the adsorption kinetic data [40] and also used in this work. The resulted kinetic rate constants and correlation coefficients (R2) are demonstrated in Table 1. It was obvious that the experimental data were described well by the pseudo-second-order model, implying that FLT composites adsorbed Cr(VI) ions via a chemisorption process. The occurrence of chemisorption generally meant that the chemical bond reaction between the active sites and Cr(VI) might be the key factor of the adsorption process [37]. The adsorption properties of FLT composites for Cr(VI) were further estimated using their adsorption isotherms (Fig. 3b). The Langmuir (Eq. S3) and Freundlich (Eq. S4) models were used to explain the experimental data. The corresponding R2 and isothermal parameters are shown in Table 1. Owing to the higher R2 (> 0.99), the Langmuir equation better described the Cr(VI) adsorption data. The maximum adsorption capacities followed the order of FLT-2 (47.73 mg/g) > FLT-1 (44.76 mg/g) > FLT-5 (36.14 mg/g).

3.3. Photocatalytic performances of FLT composites Generally, some materials can remove Cr(VI) from water via adsorption during dark

period. The semiconductors with photocatalytic ability, such as TiO2, a common UV photocatalyst, can photocatalytic reduce aqueous Cr(VI) to Cr(III). Then the TiO2 contents in the FLT composites may influence the photocatalytic efficiency for Cr(VI). The photocatalytic efficiencies of pure Fe3O4-LDH and TiO2 for 50 mg/L Cr(VI) were only 40.68% and 9.07% after 300 min of irradiation (Fig. 4a). However, the FLT-1, FLT-2, and FLT-5 exhibited higher activities, the removal ratios reached 71.48%, 91.52%, and 67.03%, respectively. The first-order reaction equation (Eq. S5) gave good fit to the reduction data (Fig. 4b). The k values of FLT composites were all higher than that of Fe3O4-LDH and TiO2 (Fig. 4c). Moreover, the k values of FLT-2 were 3.7 and 28 times higher, which indicated the FLT-2 had the best catalytic performance for Cr(VI) reduction. This suggested that the addition of semiconductor TiO2 in the composites created more free electrons and holes, which facilitated the reduction efficiency of FLT composite for Cr(VI).

3.4. Adsorption and photocatalytic reduction of Cr(VI) by FLT composites For 20 mg/L Cr(VI) solution, the adsorption performance and photocatalytic activity of TiO2, Fe3O4-LDH, and FLT composites were conducted under dark environment and UV irradiation, respectively (Fig. 5a). The adsorption efficiencies of TiO2, Fe3O4-LDH, FLT-1, FLT-2, and FLT-5 at 120 min were 3.74%, 88.11%, 64.67%, 73.94% and 58.53%, respectively. After another 90 min of UV irradiation, the total removal percentages of Cr(VI) by these materials increased to 34.87%, 96.07%, 84.57%, 97.88% and 96.61%. If the UV irradiation continued to 180 min, the total removal ratios were

above 93% except TiO2. The results suggested that the removal efficiencies of Fe3O4LDH and FLT composites for Cr(VI) by the adsorption-photocatalysis process were similar. Then the FLT composites can be chosen to remove aqueous Cr(VI) ions. To investigate the potential applications of FLT composites for high concentration level of Cr(VI), 50 mg/L was selected. As seen in Fig. 5b, FLT composites also exhibited the highest removal efficiencies for Cr(VI), and followed the order of FLT-2 (97.16%) > FLT-1 (87.93%) > FLT-5 (82.61%). In contrast, the removal ratio of Fe3O4-LDH under similar conditions decreased to 75.8%. This variation might result from the extra TiO2 phase in the composites and the difference of surface areas [33, 41, 42]. Then the synchronous process of adsorption and photocatalysis of Cr(VI) by FLT-2 was investigated. FLT-2 could remove 95% of Cr(VI) after 420 min under UV irradiation (Fig. S3). This suggested that the FLT composites were effective in the treatment of Cr (VI)-containing solutions. Moreover, the FLT-2 had higher efficiency for both low and high concentration of Cr-containing solutions, compared with other adsorbents and photocatalysts for Cr(VI) removal under different operating conditions (Table 2). To evaluate the stability and reusability of FLT composites, the regeneration experiments of FLT-2 were performed. The adsorption and photocatalysis sites of FLT2 after adsorption and photocatalysis of Cr(VI) were restored using 0.5 mol/L NaOH solution [43]. After drying at 60oC for 12 h, the solid was utilized to react with another Cr(VI) solution (50 mg/L). Total five successive regeneration cycles were performed (Fig. 5c). In the first adsorption/desorption cycle, the amount of desorbed Cr(VI) maintained over 63.5%. It tended to be stable after the fourth cycle, and the adsorption

removal ratio was about 43.10%. In the first adsorption-photocatalysis/desorption cycle, the removal ratio of FLT-2 for Cr(VI) was about 90.0%, and slightly decreased to 64.4% after five cycles. During the removal process, the surface active sites of FLT-2 may be covered by the generated Cr(III), which decreased the removal efficiency of FLT-2 for Cr(VI) [44]. To evaluate the potential application of FLT composites in real water environment, the removal tests of FLT for Cr(VI) were taken using the synthesized surface water. The removal efficiency of FLT-2 was higher and decreased with the increase of Cr(VI) concentration (Fig. S4). However，the removal ratio of FLT-2 was lower in the real surface water samples from Yellow River and the reclaimed water from Everbright Water Limited of Jinan (Fig. S4). The presence of anions, such as CO32–, HCO3–, SO42–, H2PO4–, Cl– and NO3–, may compete with chromate ions [51-53]. Wang et al. reported an negative impact of coexisting anions on Cr(VI) by LDHs [53]. Moreover, the existing humic acid also blocked the reactive sites and led to the inhibitory effect on Cr(VI) removal [54, 55].

3.5. Mechanisms of Cr(VI) removal by FLT composites The different form of chromium in solution depends on the pH values [56]. In acidic solutions, Cr2O72– and HCrO4− were the main existing forms, the FLT-2 was protonated and had a positive charged surface (Fig. S5). Therefore, the main adsorption mechanisms of Cr(VI) were electrostatic attractions between the positive charged surface and chromate anions, and anion exchange of CO32− ions in the surface or interlayer of LDHs with Cr2O72– and/or HCrO4− [57-60]. FLT-2 after adsorption was

also characterized by FTIR (Fig. S6). The slight red shift from 3375 to 3384 cm–1 indicated that there were molecular interactions between the OH– groups on the surface and aqueous Cr(VI) anions. Based on previous analysis [61], they were likely to be the electrostatic interactions (Fe-OH2+…HCrO4−). To further investigate the possible interaction mechanisms, the FLT-2 before and after adsorption or photocatalysis of Cr(VI) were characterized using XPS technique. According to Fig. 6a, the peaks of Cr(VI) and Cr(III) were existed in FLT-2 after adsorption and after photocatalysis. The peaks at binding energy (BE) around 578±1 eV were attributed to Cr 2p3/2 and that of 587±1 eV were resulted from Cr 2p1/2 [20, 61]. This suggested that Cr(VI) and Cr(III) were adsorbed by FLT-2 and the partially adsorbed Cr(VI) was converted into Cr(III) [62]. The Fe 2p XPS spectra in FLT-2 displayed typical spectra of Fe 2p (Fig. 6b), which were the characteristic peaks of Fe3O4 structure. The spectra of Fe 2p3/2 and Fe 2p1/2 located at 708.6 and 722.7 eV were resulted from Fe2+ from FeO and Fe3+ from Fe2O3. The BE values at 710.4 and 716.0 eV were attributed to Fe(III)-oxides and the over-lap of oxidized iron (Fe 2p3/2) [63, 64]. After adsorption and photocatalysis of Cr(VI), these changes of Fe 2p from 708.6 to 710.9, from 710.4 to 713.6, from 716.0 to 718.6, and from 722.7 to 724.8 eV could be ascribed the participating of Fe element in the Cr(VI) reaction and complexation with Cr(VI). The O 1s spectra are shown in Fig. 6c. The spectra at BE of 528.3 and 530.5 eV on FLT-2 were attributed to Ti-O and M-O (M is Zn, Al or Fe) [65]. After adsorption and photocatalysis, the peaks of M-O shifted to 531.7 and 531.6 eV. This indicated the Cr was bonded with the lattice O [66-68]. Moreover, the shift of

Ti-O peaks from 528.3 eV to 529.9 and 530.1 eV showed that the electrons of TiO2 were diverted and reduced Cr(VI) to Cr(III) [69]. According to Fig. 6d, the two outstanding peaks of TiO2 were observed at 456.9 and 462.6 eV, which were appointed to the Ti 2p 3/2 and Ti 2p 1/2 of TiO2, respectively [24, 70]. The peak shift of Ti 2p 3/2 from 456.9 to 458.3 eV and Ti 2p 1/2 from 462.6 to 464.1 eV illustrated that there were obviously distinctions of Ti chemical environments before and after Cr(VI) removal. The adjacent differences of Ti 2p 1/2 and Ti 2p 3/2 after photocatalysis and after adsorption were all 5.8 eV, illustrating that the TiO2 on the surface of FLT still had octahedrally coordinated anatase structure and remained photocatalysis activity [71]. In heterostructured photocatalyst, the interfacial charge transport can improve the separation efficiency of electron-hole pairs and photocatalytic performance for contaminant. To further analysis the reduction process of Cr(VI) by FLT composites, the UV-vis diffuse reflection spectra (Fig. S7), transient photocurrent response and EIS spectra (Fig. 7) were used to evaluate the optical and electrical properties of FLT composites. From Fig. S7, the TiO2 had a broad absorption band in UV region. Once TiO2 was added into FLT composite, the absorption intensity of FLT-2 increased in the 200-400 nm than Fe3O4-LDH [72]. This small change was caused by the formation of TiO2/Fe3O4-LDH heterostructure. To further explicate the activity promotion of FLT-2 composite, the charge transfer behavior and electrical resistivity were better elucidated by transient photocurrent response and EIS spectra (Fig. 7) [73]. It was noted that the transient photocurrent of FLT-2 was higher than that of TiO2 and Fe3O4-LDH, and the arc radius in Nyquist plots was smaller than TiO2 and Fe3O4-LDH. Therefore, the

introduction of TiO2 into the FLT composites improved the electron conductivity by depressing the charge recombination. Above all, the main reactions were as follows：

Zn2+ + HCrO4–

ZnCrO4 + H+

Fe2+ + HCrO4–

FeCrO4 + H+

Under UV light irradiation:

TiO2

e– (TiO2) + h+ (TiO2)

e– (TiO2) + FLT Fe2+ _ e–

e– (FLT)

Fe3+

Zn2+ + 2HCrO4– + 6H+ + 6e– 14H+ + Cr2O72– + 6e– Fe2+ + 2HCrO4– + 6H+ + 6e– HCrO42– + 7H+ + 3e–

ZnCr2O4 + 4H2O 2Cr3+ + 7H2O FeCr2O4 + 4H2O Cr3+ + 4H2O

4. Conclusion LDHs are huge variety of anionic clays with excellent chemical and physical properties due to their tunability of metal cations and interlayer anions. Therefore, LDHs can be used to efficiently remove Cr(VI) from aqueous solutions via adsorption. However, this process only transfers Cr(VI) from the aqueous phase to the solid adsorbent and do not decrease the toxicity of Cr(VI). Photocatalytic reduction of Cr(VI) to less toxic Cr(III) is a better choice. In this work, the FLT hybrid materials of TiO2 and Fe3O4-LDH were prepared using the sol-gel method to evaluate the synergetic adsorption property of LDHs and reduction efficiency of TiO2 for aqueous Cr(VI). The

FLT-2, optimized of 20% mass ratio of Fe3O4-LDH/TiO2, emerged preferable synergistic adsorption/photocatalysis properties for Cr(VI) removal as well as excellent recycle performance. The combination of Fe3O4-ZnAl-LDH and TiO2 improved the electron conductivity and photocatalytic performance by depressing the charge recombination from the photocurrent response and EIS spectra. This demonstrated that FLT

composites

were

one

of

the

selected

materials

via

synergistic

adsorption/photocatalysis process for the disposal of Cr(VI)-containing wastewater. Furthermore, the combination of adsorption and photocatalysis is a feasible method to remove toxic Cr(VI) from water environment.

Acknowledgements This work was funded by the Natural Science Foundation of China [grant numbers 21577048, 41771507, and 21477145].

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Figure captions: Fig. 1. FTIR spectra (a) and XRD patterns (b) of FLT composites, Fe3O4-LDH, and TiO2. Fig. 2. SEM (a), EDS images (b), elemental mapping (c), TEM (d), and HRTEM (e) of FLT-2. Fig. 3. Effect of contact time on Cr(VI) adsorption (a) (dosage: 0.1 g, concentration: 50 mg/L, time: 5-180 min) and adsorption isotherms of FLT composites (b) (dosage: 0.1 g, time: 180 min, concentration: 5-300 mg/L). Fig. 4. Photocatalytic reduction of Cr(VI) as function of time (a), linear fit of first-order kinetic equation (b), and the corresponding reduction rate constants (c) by FLT composites, TiO2, and Fe3O4-LDH (dosage: 0.1 g, concentration: 50 mg/L, time: 5-300 min). Fig. 5. Synergistic adsorption-photocatalysis of Cr (VI) by Fe3O4-LDH, TiO2 and FLT composites (dosage: 0.1 g, concentration: 20 mg/L (a), 50 mg/L (b), time: 420 min), and recycling runs of FLT-2 (c) (dosage: 0.1 g, concentration: 50 mg/L, adsorption time: 120 min, synergy time: 420 min, eluent: 0.5 mol/L NaOH). Fig. 6. XPS spectra of Cr 2p (a), Fe 2p (b), O1s (c), and Ti 2p (d) of FLT-2 before adsorption, after adsorption, and after photocatalysis. Fig. 7. Photocurrent responses (a) and EIS plots (b) of Fe3O4-LDH, TiO2, and FLT-2.

Author Contribution Statement

Yanting Yang: Conceptualization, Investigation, Methodology, Validation, Formal analysis, Writing-Original Draft Jing Li: Investigation, Validation, Formal analysis Tao Yan: Conceptualization, Methodology, Resources, Writing- Reviewing and Editing, Rixin Zhu: Formal analysis, Validation Liangguo Yan: Supervision, Conceptualization, Resources, Writing-Reviewing and Editing, Project administration, Funding acquisition Zhiguo Pei: Supervision, Conceptualization, Resources, Writing-Reviewing and Editing, Project administration, Funding acquisition

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

(a)

(b) FLT-5

1366

3000 2000 Wavenumber (cm-1)

1000

0

10

20

50

(113)

60

(215)

(116) (220) (440)

(511) (110)

(204)

(211)

(105)

40

(018)

(004)

30

(442)

Fe3O4 -LDH

(015)

581

(400)

TiO2

Fe3O4-LDH 4000

(101)

422

(012) (311)

TiO2

FLT-1

(006)

776

(220)

FLT-1

FLT-2

(003)

FLT-2

Relative intensity (a.u.)

Transmittance (a.u.)

3396

FLT-5

70

80

2-Theta (degree)

Fig. 1. FTIR spectra (a) and XRD patterns (b) of FLT composites, Fe3O4-LDH, and TiO2.

(a)

1800

(b)

1500

Zn Element C N O Al Ti Fe Zn Matrix

1200 O 900 Al

600

Ti

Fe

300 0

N

At% 00.93 03.63 54.11 10.02 09.28 06.03 15.68 ZAF

Zn

C

0

400

800

1200

1600

Energy (eV)

(c) Al

(d)

Wt% 00.37 01.69 28.72 08.97 14.74 11.18 34.01 Correction

Fe

O

Ti

Zn

(e)

Fig. 2. SEM (a), EDS images (b), elemental mapping (c), TEM (d), and HRTEM (e) of FLT-2.

(a) 70

(b)50 40

50 40 FLT-1 FLT-2 FLT-5

30 20

30 20 FLT-1 FLT-2 FLT-5

10

10 0

qe(mg/L)

% Cr (VI) adsorbed

60

0

40

80

120

160

0

t (min)

0

40

80

120

160

200

240

280

ce(mg/L)

Fig. 3. Effect of contact time on Cr(VI) adsorption (a) (dosage: 0.1 g, concentration: 50 mg/L, time: 5-180 min) and adsorption isotherms of FLT composites (b) (dosage: 0.1 g, time: 180 min, concentration: 5-300 mg/L).

1.0

(a)

0.8

c/c0

0.6 0.4

TiO2 Fe3O4-LDH

0.2 0.0

FLT-1 FLT-2 FLT-5

0

50

100

150

200

250

200

250

300

t (min)

TiO2

(b) 2.5

Fe3O4-LDH FLT-1 FLT-2 FLT-5

-ln (c/c0)

2.0 1.5 1.0 0.5 0.0 0

50

100

150

300

t (min)

(c)

1. Fe3O4-LDH 2. FLT-1 3. FLT-2 4. FLT-5 5. TiO2

k (min-1)

0.00788

0.00415

0.00359

0.00214 0.000282 1

2

3

4

5

Fig. 4. Photocatalytic reduction of Cr(VI) as function of time (a), linear fit of first-order kinetic equation (b), and the corresponding reduction rate constants (c) by FLT composites, TiO2, and Fe3O4-LDH (dosage: 0.1 g, concentration: 50 mg/L, time: 5-300 min).

(a)

TiO2

1.0

Fe3O4-LDH

FLT-1 FLT-2 FLT-5

0.8 adsorption

c/c0

0.6

photocatalysis 0.4 0.2 0.0

0

50

100

150

200

250

300

t (min)

(b)

1.0 0.8 TiO2 Fe3O4-LDH FLT-1 FLT-2 FLT-5

photocatalysis

c/c0

0.6 0.4 adsorption

0.2 0.0

0

60

120

180

240

300

360

420

t (min)

Removal percentage of Cr (VI)

(c)

100

adsorption synergy

80 60 40 20 0

1

2

3

4

5

Recycled number

Fig. 5. Synergistic adsorption-photocatalysis of Cr (VI) by Fe3O4-LDH, TiO2 and FLT composites (dosage: 0.1 g, concentration: 20 mg/L (a), 50 mg/L (b), time: 420 min), and recycling runs of FLT-2 (c) (dosage: 0.1 g, concentration: 50 mg/L, adsorption time: 120 min, synergy time: 420 min, eluent: 0.5 mol/L NaOH).

Cr(VI) 588.3 eV

Cr(III) 586.5 eV

Cr(III) 577.1 eV

Cr(VI) 587.9 eV

Cr(III) 586.0 eV

Cr(VI) 579.3 eV

724.8 eV

Fe 2p

(b)

718.6 eV

710.9 eV 713.6 eV

Cr(VI) 579.4 eV

After photocatalysis Intensity (a.u.)

Cr 2p

Intensity (a.u.)

(a)

After photocatalysis 724.3 eV 718.2 eV

710.3 eV 712.3 eV

After adsorption 722.7 eV

Cr(III) 577.0 eV

716.0 eV

708.6 eV 710.4 eV

After adsorption

595

FLT-2

590

585

580

575

740

570

730

Binding Energy (eV)

(c)

O 1s

531.6 eV

(d)

Ti 2p

700

458.4 eV

464.2 eV

530.1 eV

After photocatalysis

720 710 Binding Energy (eV)

531.7 eV

Intensity (a.u.)

Intensity (a.u.)

After photocatalysis

529.9 eV

After adsorption

464.1 eV

458.3 eV

After adsorption

530.5 eV

456.9 eV

462.6 eV

528.3 eV

FLT-2

FLT-2 536

534

532

530

528

526

465

Binding Energy (eV)

460

455

450

Binding Energy (eV)

Fig. 6. XPS spectra of Cr 2p (a), Fe 2p (b), O1s (c), and Ti 2p (d) of FLT-2 before adsorption, after adsorption, and after photocatalysis.

(a)

(b)

TiO2

- Z'' (Ohm)

Photocurrent (μA)

FLT-2

FLT-2 TiO2 Fe3O4-LDH

Fe3O4-LDH

20

40

60

80

100

0

Times (s)

10

20

30

Z' (Ohm)

Fig. 7. Photocurrent responses (a) and EIS plots (b) of Fe3O4-LDH, TiO2, and FLT-2. Table1 Calculated adsorption kinetic and isothermal parameters of Cr (VI) adsorption by FLT composites. parameter

FLT-1

FLT-2

FLT-5

Pseudo-first-order

qe (mg/g)

9.563

9.814

7.791

equation

k1 (1/min)

0.01446

0.01538

0.01444

R2

0.9827

0.9847

0.8718

Pseudo-second-order

qe (mg/g)

29.63

33.88

24.38

equation

k2 (g/(mg·min))

0.009707

0.009931

0.01633

R2

0.9987

0.9979

0.9985

qm (mg/g)

44.76

47.73

36.14

KL (L/min)

0.1301

0.05124

0.04818

R2

0.9929

0.9992

0.9944

k

5.763

3.684

3.700

n

2.183

1.904

2.251

R2

0.7362

0.9001

0.9471

Langmuir model

Freundlich model

Table 2 Comparison of removal efficiency of Cr(VI) by FLT-2 with other materials at different conditions. Catalyst

Cr(VI) concentration (mg/L)

Light source

Time (h)

Efficiency

pH

Reference

FLT-2

20

UV

8h

98.4%

3.0

This work

FLT-2

50

UV

8h

97.2%

3.0

This work

LDH-TiO2-40

20

UV

7h

96.84%

2.0

[45]

3D TiO2-graphene hydrogel

5

UV

2.5 h

≈ 100%

5.5

[46]

g-C3N4/rGH hydrogel

30

visible light

2h

≈ 100%

5.5

[47]

Calcined CoFe-LDH/g-C3N4

50

visible light

2h

≈ 100%

2.0

[48]

PAN-CDs

100

visible light

2h

≈ 100%

2.0

[49]

TiO2-TNT

10

UV

4.5 h

96%

5.0

[50]

38

Graphical Abstract

Fe3O4-LDH

TiO2

PEG(200)

Fe3O4

60℃

95℃ 1h

39

40