Enhanced Photocatalytic Reduction of Cr(VI) by Combined Magnetic TiO₂-Based NFs and Ammonium Oxalate Hole Scavengers

Abstract

Heavy metal pollution of wastewater with coexisting organic contaminants has become a serious threat to human survival and development. In particular, hexavalent chromium, which is released into industrial wastewater, is both toxic and carcinogenic. TiO₂ photocatalysts have attracted much attention due to their potential photodegradation and photoreduction abilities. Though TiO₂ demonstrates high photocatalytic performance, it is a difficult material to recycle after the photocatalytic reaction. Considering the secondary pollution caused by the photocatalysts, in this study we prepared Ag/Fe₃O₄/TiO₂ nanofibers (NFs) that could be magnetically separated using hydrothermal synthesis, which was considered a benign and effective resolution. For the photocatalytic test, the removal of Cr(VI) was carried out by Ag/Fe₃O₄/TiO₂ nanofibers combined with ammonium oxalate (AO). AO acted as a hole scavenger to enhance the electron-hole separation ability, thereby dramatically enhancing the photoreduction efficiency of Cr(VI). The reaction rate constant for Ag/Fe₃O₄/TiO₂ NFs in the binary system reached 0.260 min⁻¹, 6.95 times of that of Ag/Fe₃O₄/TiO₂ NFs in a single system (0.038 min⁻¹). The optimized Ag/Fe₃O₄/TiO₂ NFs exhibited high efficiency and maintained their photoreduction efficiency at 90% with a recyclability of 87% after five cycles. Hence, taking into account the high magnetic separation behavior, Ag/Fe₃O₄/TiO₂ NFs with a high recycling capability are a potential photocatalyst for wastewater treatment.

1. Introduction

With the advancement of various industries comes serious industrial water pollution. Such wastewaters usually contain a complicated mixture of constituents, often involving the co-existence of multiple contaminants such as heavy metals and organic pollutants. With the development of electroplating, metallurgy, leathermaking and more, heavy metal pollution has become a serious threat to human survival and development. One such heavy metals released into industrial wastewater is Cr(VI), which is both toxic and carcinogenic. It has been the first type of carcinogen listed by the World Health Organization’s International Cancer Research Institute since 2012. Cr(VI) is easily accumulated in living organisms and can result in vomiting, liver damage, and severe diarrhea. Compared to Cr(VI), trivalent chromium (Cr(III)) is less toxic and more vital for animals and humans [1,2]. The conventional approach for the reduction or removal of Cr(VI) includes electrochemical precipitation [3,4], adsorption [5,6], bacterial reduction [7,8], ion exchange [9,10], photoreduction [11,12,13,14,15,16], etc. Compared to the above methods, photocatalytic reactions are considered a clean and promising technology owing to its highly efficient photoreduction of Cr(VI) to the less harmful Cr(III).

TiO₂ is a well-known photocatalyst widely applied for environmental purification due to its advantages, such as its highly active photocatalytic properties, chemical inertness, environmental-friendliness, non-toxicity, and cost-effectiveness [17,18,19,20,21,22,23,24]. It shows great potential in solving the difficult problem of reducing Cr(VI) to Cr(III) in industrial wastewaters. Though TiO₂ demonstrates a high photocatalytic performance, it is difficult to recycle following the photocatalytic reaction. Traditional separation approaches such as filtration and centrifugation have been widely adopted. However, the recycling efficiency is hindered by the loss of photocatalysts. Considering the secondary pollution caused by the photocatalysts, combining TiO₂ with Fe₃O₄ to form magnetic composite materials for the magnetic separation under modest magnetic fields has been seen as a benign and effective resolution [25,26,27,28]. To date, there have been many facile methods used to synthesize magnetic iron oxides/TiO₂ hybrid nanomaterial such as sol–gel, metal–organic chemical vapor deposition, the seed-mediated method, and hydrothermal treatment. In spite of introducing magnetic separation by doping Fe₃O₄, the photocatalytic performance could be further enhanced by modifying the shape of Fe₃O₄ to increase the active surface area [29]. In addition, modifying the structure of TiO₂ is also a common method used to enhance photocatalytic performance. Furthermore, combing the ultrafine Fe₃O₄ with one-dimension TiO₂ nanofibers can provide a superior charge transport in a one-dimensional direction, and show high activity.

A great deal of literature has indicated that incorporating Fe₃O₄ into TiO₂ does not improve the photocatalytic properties of TiO₂ as expected [30,31,32]. The crystallinity of TiO₂ depends on the calcination process, which plays a crucial role in the photocatalytic performance. At the same time, calcination also decreases the saturation magnetization of Fe₃O₄. With the increasing calcination temperature, Fe₃O_4, which has a superparamagnetic phase, would undergo a phase transition to γ-Fe₂O₃ and finally become α-Fe₂O₃, which has a soft ferromagnetic phase [33]. Another problem is the small bandgap of Fe₃O₄, which leads to the fast electron-hole pair recombination in Fe₃O₄/TiO₂ composite material [31]. Therefore, in order to enhance the photocatalytic activity and to maintain the magnetic properties, a lot of research has focused on doping metals to obtain the desired effect [34,35,36,37,38]. In particular, doping Ag into TiO₂ not only enhances the separation of electron-hole pairs, but also maintains the magnetic performance of the Fe₃O₄/TiO₂ composite material. For the Ag-doped TiO₂, the Ag dopants act as the photo-generated electron trapper that enhances the separation of the electron-hole pair and even creates a local electrical field to facilitate electron excitation [39,40,41,42,43].

In this study, in order to achieve both a high photocatalytic activity and a high magnetic property, we prepared Ag and Fe₃O₄ co-doped TiO₂ nanofibers (Ag/Fe₃O₄/TiO₂ NFs) via hydrothermal synthesis followed by a calcination treatment. The Ag/Fe₃O₄/TiO₂ NFs were studied systematically through synchrotron X-ray diffractometer, UV-Vis spectroscopy, field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). For the photocatalytic test, the removal of Cr(VI) was carried out by Ag/Fe₃O₄/TiO₂ NFs combined with ammonium oxalate (AO). Hence, taking into account the high magnetic separation behavior, Ag/Fe₃O₄/TiO₂ NFs with a high recycling capability are a potential photocatalyst for wastewater treatment.

2. Results

Prior to combining magnetic NPs into TiO₂, a basic characterization of Fe₃O₄ was investigated and summarized in Figure 1. The synchrotron X-ray diffractometer was applied to characterize the crystal structure of the Fe₃O₄ NPs as shown in Figure 1a. The characteristic peaks could be indexed to standard Fe₃O₄ (JCPDS No. 019-0629). The method used to determine the bandgap of Fe₃O₄ NPs from the diffusion reflectance is shown schematically in Figure 1b. It was calculated according to ${\left[F\left(R\right)hv\right]}^{1/2}$ versus the energy of incident light based on Kubelka–Munk function spectra, F(R). According to Figure 1b, the band gap of Fe₃O₄ NPs was ~0.8 eV. The magnetic property of the Fe₃O₄ NPs were investigated using a (Superconducting quantum interference device magnetometer) SQUID at 10 K. The magnetic hysteresis loop shown in Figure 1c indicates the ferromagnetic property that exists in Fe₃O₄ NPs. The inset of Figure 1d shows the magnetic separation of Fe₃O₄ NPs from the aqueous dispersion attracted by the Nd-Fe-B magnets. The collected Fe₃O₄ NPs indicated that it could be controlled by an applied magnetic field.

The calcination temperature for the magnetic material is seen as an important factor. For example, as the calcination temperature exceeds 600 °C, it results in a phase transformation from magnetite (Fe₃O₄) to maghemite (γ-Fe₂O₃) to hematite (α-Fe₂O₃) conversions. This phase transformation behavior would cause magnetic material to lose its magnetic properties. In order to maintain the ability to magnetically separate the synthesized TiO₂, the calcination temperature was fixed at 550 °C for the study. Figure 2a shows the synchrotron X-ray patterns Fe₃O₄/TiO₂ with various doping concentrations that depend on the Fe₃O₄/TiO₂ ratio (wt %). The characteristic peaks, which centered at 2θ around 16.71°, 24.27°, 24.81°, 25.31°, 31.33°, 34.99° and 35.71°, could be indexed to anatase phase TiO₂. Pristine TiO₂ exhibited characteristic peaks at 2θ around 18.94°, 19.75° and 22.01° that could be identified as TiO₂ low-temperature phase, β-TiO₂ monoclinic. With the incorporation of Fe₃O₄, the anatase phase TiO₂ became the only phase in the crystal structure, and no characteristic peaks from other phases could be detected. In addition, the radius of an Fe ion (Fe²⁺ ~ 0.76 Å, Fe³⁺ ~ 0.64 Å) is slightly smaller than that of a Ti ion (Ti⁴⁺ ~ 0.68 Å), indicating that some of doped Fe ion might enter to interstitial voids of TiO₂ lattice [30,44]. The Fe ions in the TiO₂ lattice would act as carrier traps, leading to the electron-hole recombination. Taking into account the recombination phenomenon, the photocatalytic performance could be affected when Fe ions were incorporated into the catalyst. When the doping concentration reached 25.0 wt %, both anatase TiO₂ and Fe₃O₄ peaks were detected. The excessive Fe₃O₄ NPs in the Fe₃O₄-TiO₂ lead to non-uniform doping and to the decrease of the crystallinity of TiO₂. From the magnetic hysteresis loop shown in Figure 2b, as the amount of Fe₃O₄ increased, the magnetization increased as well. In addition, the magnetization was proportional to the doping amount. To optimize the doping concentration, the photocatalytic activity was measured by photoreduction of Cr(VI) in K₂Cr₂O₇ aqueous solution under UV-B irradiation. The photoreduction of Cr(VI) using TiO₂-based catalyst usually follows Langmuir–Hinshelwood kinetics. It can be mathematically simplified to first-order kinetics in the early stage described as ln(C₀/C) = kt, where C₀ is the initial concentration of Cr(VI) in K₂Cr₂O₇, C is the remaining Cr(VI) concentration at various times, k is the apparent reaction rate constant, and t is the photodegradation time. The blank experiment was performed under the same conditions but without the existence of the photocatalyst. For the dark experiment, 10.0 wt %-Fe₃O₄/TiO₂ was also tested in dark conditions to observe the adsorption–desorption behavior. From Figure 2c, 10.0 wt %-Fe₃O₄/TiO₂ calcined at 550 °C showed the highest photoreduction performance among other Fe₃O₄/TiO₂ photocatalysts due to the highest crystallinity among the Fe₃O₄/TiO₂ series. High crystallinity can hinder the recombination of photoexcited electrons and holes and thus result in high photocatalytic activity. With further increasing the Fe₃O₄ doping concentration to 15.0 wt % and 25.0 wt %, the excessive dopant might destroy the lattice of TiO₂, thus decreasing the crystallinity of TiO₂ dramatically and form the impurity phases composed of Fe₃O₄, γ-Fe₂O₃ and α-Fe₂O₃. In addition, all of the Fe₃O₄/TiO₂ showed poorer performance compared to the pristine TiO₂, which is in accordance with the XRD spectra. The Fe³⁺ as carrier traps leading to recombination phenomenon and decreased the photocatalytic performance compared with pristine TiO_2.

Ag was co-doped with 10.0 wt % of Fe₃O₄ into TiO₂ to improve the electron-hole separation further. Figure 3a shows the synchrotron X-ray patterns of the Ag/Fe₃O₄/TiO₂ series with various Ag doping concentrations that depended on the amount of Ag (mol %) co-doped with 10 wt % Fe₃O₄/TiO₂. The characteristic peaks of Ag/Fe₃O₄/TiO₂ could all be assigned to anatase phase TiO₂ without any Ag signal. The results indicated that the incorporation of Fe₃O₄ and Ag did not destroy the crystal structure of TiO₂. The magnetic hysteresis loop (Figure 3b) illustrates that as the amount of Ag increased, the magnetization decreased. When the excessive Ag dopant was 10.0 mol %, it resulted in a decay of saturation magnetization compared to Fe₃O₄/TiO₂, due to the contribution of the volume of non-magnetic material to the total sample volume. Therefore, the magnetism of the 10.0 mol % Ag/Fe₃O₄/TiO₂ was too low for magnetic separation by adding a magnetic field. Figure 3c demonstrates the C/C_o curves for photoreduction of Cr(VI) under UV-B irradiation over the Ag/Fe₃O₄/TiO₂ series with different Ag doping concentrations. The blank experiment was also performed under the same conditions but without the presence of the photocatalyst. For the dark experiment, 5.0 mol % Ag/Fe₃O₄/TiO₂ was also tested in dark conditions to eliminate the adsorption–desorption behavior. The 10.0 mol % Ag/Fe₃O₄/TiO₂ showed the highest photoreduction performance, even higher than that of pristine TiO₂. Although 10.0 mol % Ag/Fe₃O₄/TiO₂ possessed the highest reduction performance, after considering the ability to be magnetically separated, we selected the 5.0 mol % doping level as the optimal photocatalyst. We could also observe that the photoreduction for Ag co-doped with 10 wt % Fe₃O₄/TiO₂ showed the higher performance after the incorporation of Ag compared to 10 wt % Fe₃O₄/TiO₂. This enhancement could be interpreted by the energy level theory, namely that the conduction band of Fe₃O₄ is lower than the conduction band of TiO₂, so the conduction band of TiO₂ becomes an electron capture position. With the further introduction of Ag into Fe₃O₄/TiO₂, Ag could act as another electron trap to enhance the electron-hole separation ability [45].

After the optimization process, pristine TiO₂, 10.0 wt % Fe₃O₄/TiO₂ (Fe₃O₄/TiO₂) and 5.0 mol % Ag/Fe₃O₄/TiO₂ (Ag/Fe₃O₄/TiO₂) were compared. The FESEM images of TiO₂-based NFs before and after combining magnetic NPs and Ag are shown in Figure 4. The image shows that the surface of the pristine TiO₂ was very clean and smooth (Figure 4a). When incorporated with Fe₃O₄ NPs, there was no significant morphological change for the Fe₃O₄/TiO₂ (Figure 4b). For Ag/Fe₃O₄/TiO₂, the surface became relatively rough and some particles aggregated on it (Figure 4c). On increasing the silver content, the surface charge of TiO₂-based material would gradually decrease. With small amounts of Ag dopant, Ag₂O and AgO might disperse on the surface of TiO₂-based material. When increasing Ag doping concentration, the decrease in surface charge was attributed to an agglomeration of the silver species and a reduction to Ag⁰ on the TiO₂ surface [46]. The EDS-characterized elemental compositions and the corresponding results are listed in

Table 1. The corresponding ratios of Fe/Ti and Ag/Ti for pristine TiO₂, Fe₃O₄/TiO₂ and Ag/Fe₃O₄/ TiO₂.

Sample	Fe/Ti (%)	Ag/Ti (%)
Pristine TIO2	0.0	0.0
Fe3O4/TiO2	2.9	0.0
Ag/Fe3O4/ TiO2	3.1	0.4

. For Fe₃O₄/TiO₂, the ratio of Fe/Ti and Ag/Ti were ~2.9% and ~0.0%, respectively. After incorporating Ag, the ratio of Fe/Ti was ~3.1%, which was approximately the same as Fe₃O₄/TiO₂, and the ratio of Ag/Ti increased to 0.4%. The corresponding ratios of Fe/Ti and Ag/Ti illustrated the existence of Ag in the Ag/Fe₃O₄/TiO₂, together with the leading component Ti and Fe. The distinct signals of these elements present in the spectrum confirmed the successful inclusion of Ag ions into the host TiO₂ lattice.

The Kubelka–Munk function spectra of TiO₂-based materials are shown in Figure 5a. Pristine TiO₂ only showed absorption behavior in the UV range. However, compared to pristine TiO₂, the F(R) spectra of Fe₃O₄/TiO₂ and Ag/Fe₃O₄/TiO₂ showed an obvious extension to the visible light region, and the band gap energy also decreased from 3.1 eV to 2.1 eV and 2.0 eV, respectively (Figure 5b). This could be ascribed to the introduction of Fe₃O₄. During the calcination process, the introduced Fe³⁺ could exchange with the lattice position of Ti⁴⁺ and therefore form an impurity band. Fe₃O₄/TiO₂ and Ag/Fe₃O₄/TiO₂ with a decreased forbidden bandwidth could successfully narrow the band gap for the higher absorption behavior in the visible region. This enhanced absorption behavior could generate a lot of photo-excited electrons and holes for photocatalytic reactions.

The photocatalytic activity test was examined by photoreduction of Cr(VI) to Cr(III). The photoreduction pathways of Cr(VI) on the surface of TiO₂ through UV irradiation can be described by the following reaction sequence (Equations (1)–(6)). After UV light irradiation, photo-excited electron-hole pairs are generated. During the photoreduction reaction of Cr(VI), electrons dominate the entire reaction. Meanwhile, the hole will oxidize ${\mathrm{H}}_2\mathrm{O}$ to form the reactive oxygen species $OH$, which will further react with Cr(III) to generate Cr(VI).

$$Ti{O}_2+hv \to h^{+}+e^{-}$$

(1)

$$C{r}_2{O}_7{}^{2-}+14H^{+}+6e^{-}\to 2C{r}^{3+}+7H_{2}O$$

(2)

$$e^{-}+h^{+}\to recombination$$

(3)

$$h^{+}+H_{2}O\to ·OH+H^{+}$$

(4)

$$h^{+}+O{H}^{-}\to ·OH$$

(5)

$$3·OH+C{r}^{3+}\to 3O{H}^{-}+C{r}^{6+}$$

(6)

It is unfavorable to reduce Cr(VI) to Cr(III) while Cr(VI) participates in the reaction alone, due to the electron-hole recombination and the oxidation of Cr(III). Figure 6a shows the photoreduction of Cr(VI) over pristine TiO₂, Fe₃O₄/TiO₂ and Ag/Fe₃O₄/TiO₂ in a single system for which only Cr(VI) existed in the initial condition. The reduction of Cr(VI) was greatly promoted by the coexistence of ammonium oxalate (AO), and the corresponding results for single systems are also plotted for comparison (Figure 6b). AO is a type of hole scavenger that is widely used for detecting reactive oxygen species during the photocatalytic reaction in order to better understand the reaction mechanism. Therefore, AO would capture the photogenerated holes during the photocatalysis reaction, leaving the photogenerated electrons on the surface of the TiO₂-based NFs. With the help of AO, the separation of the electron-hole was greatly facilitated and thus the reduction performance of Cr(VI) was enhanced. The poor enhancement of pristine TiO₂ compared with Fe₃O₄/TiO₂ and Ag/Fe₃O₄/TiO₂ could be due to the bandgap of each sample. A decrease in the bandgap for Fe₃O₄/TiO₂ and Ag/Fe₃O₄/TiO₂ resulted in a greater absorption of photons, which was beneficial for the production of electrons and holes required for the photocatalytic reactions. However, the photoexcited electron-hole pair in the Fe₃O₄/TiO₂ and Ag/Fe₃O₄/TiO₂ favored a transfer to Fe₃O₄. Holes can provide a faster reaction route with AO, rather than recombining with the electron. Further, the residual electron on the surface of Fe₃O₄/TiO₂ and Ag/Fe₃O₄/TiO₂ can reduce Cr(VI) to Cr(III). Therefore, the photoreduction performance for Fe₃O₄/TiO₂ and Ag/Fe₃O₄/TiO₂ showed a dramatic enhancement. The reaction rate constant for Ag/Fe₃O₄/TiO₂ in binary system achieved 0.260 min⁻¹, which was 6.95 times that of Ag/Fe₃O₄/TiO₂ in a single system at 0.038 min⁻¹. These results confirmed the synergetic promotion effect of ammonium oxalate.

The stability and recyclability of the photocatalyst is an important index for practical application. In order to examine the stability and recyclability of Ag/Fe₃O₄/TiO₂, the photoreduction of Cr(VI) was repeated five times. Each time, the photocatalysts were recycled by adding a magnetic field. This exhibited a slight decay of reduction efficiency after each cycle, which accounted for the weight loss during every recycle process. After five cycles, the photoreduction efficiency was maintained at 90% (Figure 7a), and the amount of the remaining photocatalyst was 87% (Figure 7b). The stability and recyclability tests proved that the Cr(VI) photoreduction efficiency over Ag/Fe₃O₄/TiO₂ has consistently high stability and recyclability. Therefore, Ag/Fe₃O₄/TiO₂ is a potential photocatalyst for wastewater treatment.

3. Materials and Methods

3.1. Synthesis of Fe₃O₄ Magnetic NPs

The synthesis of Fe₃O₄ NPs was carried out by the co-precipitation method, in which the iron(II) chloride (FeCl₂·4H₂O, Acros, 99+%) and iron(III) chloride ((FeCl₃·6H₂O, Acros, 99+%) were used as the raw materials with a molar proportion of 1:2. First, they were dissolved in deionized water and preheated to 60 °C. After that, a 10 M sodium hydroxide aqueous solution (NaOH) acting as a precipitation reagent was added into the mixture solution under continuous stirring for 1 h. The Fe₃O₄ suspension was magnetically separated and washed with deionized water repeatedly until the pH was 7. Finally, the product was air dried at 60 °C.

3.2. Synthesis of Ag/Fe₃O₄/TiO₂ NFs

The TiO₂-based NFs were synthesized by hydrothermal method and crystallized by heat treatment. First, 2.5 g anatase phase TiO₂ powder (98%, Sigma-Aldrich, St. Louis, MO, USA), as-synthesized Fe₃O₄ NPs, and silver nitrate (AgNO₃, extra pure, Choneye, Taipei, Taiwan) with various stoichiometric ratios were suspended into separate 62.5 mL of 10 M NaOH. The suspension was dispersed uniformly into an ultrasonic bath. After that, the reactants were transferred into a polytetrafluoroethylene-lined autoclave for thermal treatment at 150 °C for 24 h to obtain sodium titanate (Na₂Ti₃O₇). Then, various forms of Na₂Ti₃O₇ were washed with 0.10 M hydrochloric acid (HCl, 37%, Sigma-Aldrich, St. Louis, MO, USA) to exchange the sodium ion for protons. Finally, the sodium hydrogen titanate (Na_xH_2−xTi₃O₇) was filtered and air dried at 80 °C. The dried Na_xH_2−xTi₃O₇ was calcined at 550 °C for 12 h at a 5 °C/min heating rate to obtain magnetic TiO₂-based NFs.

3.3. Characterization

To observe the crystal structure, the synchrotron X-ray spectra were collected from 5° to 45° of 2θ with a scan rate of 0.02°/s and a wavelength of ~ 1.025 Å. The Kubelka–Munk function, F(R), spectra were measured and recorded by UV/Vis spectrophotometer (Jacso, V-650, Tokyo, Japan) from 200 to 900 nm wavelength. The magnetic properties of Fe₃O₄ NPs and magnetic TiO₂-based NFs were measured at 10 K temperature using a SQUID magnetometer (MPMS3, Quantum Design, San Diego, CA, USA). The microstructure was characterized by transmission electron microscopy (TEM, spherical-aberration corrected ULTRA-HRTEM, JEM-ARM200FTH, JEOL Ltd., Tokyo, Japan). The morphology and atomic ratio of TiO₂-based NFs were measured by FE-SEM (SU8010, Hitachi, Tokyo, Japan) equipped with EDS (XFlash Detector 5030, Bruker AXS, Karlsruhe, Germany).

3.4. Photocatalytic Measurement

For the measurement of the photoreduction of Cr(VI), 20.0 mg of magnetic TiO₂-based photocatalyst was dispersed into 150.0 mL of potassium dichromate (K₂Cr₂O₇, 0.0167 M, Fisher Scientific, CA, USA) with an initial concentration of 1.0 ppm at ambient conditions. As the control group, 20.0 mg of pristine TiO₂ was also dispersed into 150.0 mL K₂Cr₂O₇ with an initial concentration of 1.0 ppm at ambient conditions. The two UV-B light lamps (G15T8E, λ_max ~312 nm, 8.0 W, Sankyo Denki, Osaka, Japan) were placed ~10.0 cm above the reaction system. Before exposure to light irradiation, the suspensions were put in the dark for 30 min in order to achieve the adsorption equilibrium and thus minimize the surface adsorption behavior. The concentration of retained Cr(VI) was measured by the diphenylcarbazide method. By comparing the intensity of the Cr(VI) characteristic peak located at λ = 540 nm with the calibration curve examined previously, we can obtain its corresponding concentration. In order to examine the mechanism of Cr(VI) photoreduction, 142.2 µL tert-butanol ((CH₃)₃COH, $\ge$99.0%, J.T.Baker, Phillipsburg, NJ, USA) and 24.0 mg ammonium oxalate (C₂H₈N₂O₄, 98%, Vetec, trademark of Sigma-Aldrich, St. Louis, MO, USA) were added into K₂Cr₂O₇ in the beginning, respectively. The stability and recyclability of the photocatalysts were measured by cycling experiments. After the Cr(VI) photoreduction for each cycle, the magnetic TiO₂ was collected by Nd-Fe-B magnet wrapped with PVC film. After removing the magnetic field, the magnetic TiO₂ was washed three times with ethanol to remove residual ions and molecules and then dried at 80 °C. The fresh 1.0 ppm K₂Cr₂O₇ aqueous solution was mixed with the used photocatalyst to perform the second run of photoactivity testing. Similarly, the photocatalyst was recycled to perform the third, fourth, and fifth tests.

4. Conclusions

In this study, we successfully synthesized Ag and Fe₃O₄ co-doped TiO₂ NFs using hydrothermal synthesis followed by thermal treatment in order to achieve high photocatalytic performance and a feasible recycle process. The synthesized Ag/Fe₃O₄/TiO₂ exhibited a relatively narrower band gap (2.0 eV) than that of pristine TiO₂ (3.1 eV). For the photoreduction of Cr(VI), electrons dominated the photoreduction efficiency. The photocatalytic process paired with ammonium oxalate could greatly facilitate the separation of electron-hole pairs and thus enhance the reduction rate of Cr(VI). After five cycles of the stability and recyclability test, the photoreduction efficiency was maintained at 90%, and the amount of remaining photocatalyst was maintained at 87%. Consequently, taking into account the high magnetic separation behavior and the high stability, Ag/Fe₃O₄/TiO₂ showed great potential to be used for practical wastewater treatment.