Hydrothermal and Co-Precipitated Synthesis of Chalcopyrite for Fenton-like Degradation toward Rhodamine B

In this study, Chalcopyrite (CuFeS2) was prepared by a hydrothermal and co-precipitation method, being represented as H-CuFeS2 and C-CuFeS2, respectively. The prepared CuFeS2 samples were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive X-ray spectroscopy mapping (EDS-mapping), powder X-ray diffractometer (XRD), X-ray photoelectron spectrometry (XPS), and Raman microscope. Rhodamine B (RhB, 20 ppm) was used as the target pollutant to evaluate the degradation performance by the prepared CuFeS2 samples. The H-CuFeS2 samples (20 mg) in the presence of Na2S2O8 (4 mM) exhibited excellent degradation efficiency (98.8% within 10 min). Through free radical trapping experiment, the major active species were •SO4 radicals and •OH radicals involved the RhB degradation. Furthermore, •SO4 radicals produced from the prepared samples were evaluated by iodometric titration. In addition, one possible degradation mechanism was proposed. Finally, the prepared H-CuFeS2 samples were used to degrade different dyestuff (rhodamine 6G, methylene blue, and methyl orange) and organic pollutant (bisphenol A) in the different environmental water samples (pond water and seawater) with 10.1% mineral efficiency improvement comparing to traditional Fenton reaction.


Introduction
Since the industrial revolution, the development of various industries has made life in human society more convenient, but has also caused many environmental problems. Wastewater, such as cooling water and clean water for equipment, is discharged from various industrial processes. The constituents in any wastewater are diverse and complex consisting of raw materials, intermediate products, by-products, and end products. Charging these compounds directly to the environment can have detrimental consequences. For example, oxygen-containing organic compounds such as aldehydes, ketones, and ethers are reductive, meaning that they are capable of consuming dissolved oxygen in the water to a low-level endangering aquatic organisms. Wastewater can also contain a large amount of nitrogen, phosphorus, and potassium which can promote the growth of algae and triggering eutrophication pollution in water bodies [1]. Released toxic substances from wastewater can bio-accumulate in fish and eventually pass to people who consumed it. Thus, wastewater treatment is an important process to avoid these consequences. Within the treatment options, physical, biological, and chemical methods are mainly used to treat wastewater by removing pollutants in the water and reducing organic pollutants and eutrophic substances in the water [2][3][4][5][6][7][8].
Among many treatment processes, the in-site chemical oxidation method is to inject and mix oxidants into the underground environment aiming to degrade pollutants  The diameters of both CuFeS 2 samples from TEM images were consistent with the SEM results. We also found both CuFeS 2 samples possessed 0.31 nm and 0.23 nm of lattice lines, corresponding to the crystal planes of (112) and (204). The EDS spectra of the prepared CuFeS 2 samples confirm the presence of Cu, Fe, and S elements in their crystals, accordingly. The atomic ratios (Cu:Fe:S) for the H-CuFeS 2 and C-CuFeS 2 samples were determined to be 1.1:1:1.8 and 1.4:1:2.0, respectively. High content of Cu elements in the C-CuFeS 2 in the sample is consistent with lower solubility predicted from smaller Ksp value of Cu 2 S when comparing with Fe 2 S 3 (Ksp of Fe 2 S 3 : 3.7 × 10 −19 , Ksp of Cu 2 S: 2.0 × 10 −47 ).

Degradation Performance of the CuFeS 2 Samples
The degradation activity of the prepared CuFeS 2 samples was evaluated with RhB (20 ppm) first. According to our previous experience, the degradation efficiency decreased with an increasing dye concentration. This is because the excessive coverage of dye on the active surface of catalysts leads to a decrease in the catalytic activity. Thus, 20 ppm RhB was selected for the experiment. The variations in the RhB concentration (C/C 0 ), where C 0 is the initial RhB concentration, and C is the RhB concentration at time t, with the reaction time for the prepared CuFeS 2 samples in the presence of H 2 O 2 (Fenton reaction) and Na 2 S 2 O 8 (Fenton-like reaction), were found in Figure 7. Prior to the addition of the oxidant, each catalyst (0.20 g) was introduced to the 20 ppm RhB solution for 30 min in the dark (indicated as "−30 min" in Figure 7) to reach equilibrium. The RhB concentration for H-CuFeS 2 samples after this equilibration time is lower than that of C-CuFeS 2 samples, reflecting RhB adsorption on H-CuFeS 2 samples. This is because smaller size of the H-CuFeS 2 samples had higher specific surface area than C-CuFeS 2 samples. Through the Fenton reaction, the degradation efficiency within 30 min was 32.3% and 26.4% for the H-CuFeS 2 and C-CuFeS 2 samples, respectively (black and blue curve). This suggests that the degradation performance of H-CuFeS 2 is better than that of C-CuFeS 2 , attributable to adsorption ability of high specific surface area for the H-CuFeS 2 samples. The results of RhB degradation through a Fenton-like reaction by the H-CuFeS 2 and C-CuFeS 2 samples were shown in the red and pink curve. The degradation efficiency within 30 min reaction time follows this order: H-CuFeS 2 (93.7%) > C-CuFeS 2 (66.3%), indicating H-CuFeS 2 having higher catalytic activity to produce •SO 4 − radicals. Furthermore, we found that degradation performance of •SO 4 − radicals is higher than that of •OH radicals for both CuFeS 2 samples. This is because of the different lifetimes of radicals (•SO 4 − radicals: 4 s, •OH radicals: 1 µs). Thus, the degradation system of H-CuFeS 2 through a Fenton-like reaction was selected for the further study.

Degradation Performance of the CuFeS2 Samples
The degradation activity of the prepared CuFeS2 samples was evaluated with RhB (20 ppm) first. According to our previous experience, the degradation efficiency decreased with an increasing dye concentration. This is because the excessive coverage of dye on the active surface of catalysts leads to a decrease in the catalytic activity. Thus, 20 ppm RhB was selected for the experiment. The variations in the RhB concentration (C/C0), where C0 is the initial RhB concentration, and C is the RhB concentration at time t, with the reaction time for the prepared CuFeS2 samples in the presence of H2O2 (Fenton reaction) and Na2S2O8 (Fenton-like reaction), were found in Figure 7. Prior to the addition of the oxidant, each catalyst (0.20 g) was introduced to the 20 ppm RhB solution for 30 min in the dark (indicated as "−30 min" in Figure 7) to reach equilibrium. The RhB concentration for H-CuFeS2 samples after this equilibration time is lower than that of C-CuFeS2 samples, reflecting RhB adsorption on H-CuFeS2 samples. This is because smaller size of the H-CuFeS2 samples had higher specific surface area than C-CuFeS2 samples. Through the Fenton reaction, the degradation efficiency within 30 min was 32.3% and 26.4% for the H-CuFeS2 and C-CuFeS2 samples, respectively (black and blue curve). This suggests that the degradation performance of H-CuFeS2 is better than that of C-CuFeS2, attributable to adsorption ability of high specific surface area for the H-CuFeS2 samples. The results of RhB degradation through a Fenton-like reaction by the H-CuFeS2 and C-CuFeS2 samples were shown in the red and pink curve. The degradation efficiency within 30 min reaction time follows this order: H-CuFeS2 (93.7%) > C-CuFeS2 (66.3%), indicating H-CuFeS2 having higher catalytic activity to produce •SO4 − radicals. Furthermore, we found that degradation performance of •SO4 − radicals is higher than that of •OH radicals for both CuFeS2 samples. This is because of the different lifetimes of radicals (•SO4 − radicals: 4 s, •OH radicals: 1 μs). Thus, the degradation system of H-CuFeS2 through a Fenton-like reaction was selected for the further study.  To maximize the degradation performance of H-CuFeS 2 , the effect from various concentrations of Na 2 S 2 O 8 was studied. As shown in Figure 8, the degradation efficiency increased with increasing Na 2 S 2 O 8 concentration. Due to low solubility of Na 2 S 2 O 8 , we selected 4.0 mM of Na 2 S 2 O 8 as the optimum required concentration of Na 2 S 2 O 8 . Dye adsorption on H-CuFeS 2 was observed in the absence of Na 2 S 2 O 8 (black cure in Figure 9). Although direct degradation of RhB by Na 2 S 2 O 8 without H-CuFeS 2 was noticed from the experiment due to the high oxidizing strength of Na 2 S 2 O 8 (red curve in Figure 9), its rate of degradation cannot compete with H-CuFeS 2 samples in the presence of Na 2 S 2 O 8 , which achieved an impressive 98.8% within 10 min (blue cure in Figure 9). In addition, we also analyzed the degradation performances of Cu 2 S and FeS 2 nanoparticles to investigate which element is important for a Fenton-like reaction. As shown in Figure 10, the RhB degradation efficiency within 15 min reaches 64.1% and 89.0% for Cu 2 S and FeS nanoparticles , respectively. These results suggested that the FeS 2 nanoparticles catalyze Na 2 S 2 O 8 to produce •SO 4 − radicals better than Cu 2 S nanoparticles, indicating Fe component is important than Cu component for the Fenton-like reaction.
centrations of Na2S2O8 was studied. As shown in Figure 8, the degradation efficiency increased with increasing Na2S2O8 concentration. Due to low solubility of Na2S2O8, we selected 4.0 mM of Na2S2O8 as the optimum required concentration of Na2S2O8. Dye adsorption on H-CuFeS2 was observed in the absence of Na2S2O8 (black cure in Figure 9). Although direct degradation of RhB by Na2S2O8 without H-CuFeS2 was noticed from the experiment due to the high oxidizing strength of Na2S2O8 (red curve in Figure 9), its rate of degradation cannot compete with H-CuFeS2 samples in the presence of Na2S2O8, which achieved an impressive 98.8% within 10 min (blue cure in Figure 9). In addition, we also analyzed the degradation performances of Cu2S and FeS2 nanoparticles to investigate which element is important for a Fenton-like reaction. As shown in Figure 10, the RhB degradation efficiency within 15 min reaches 64.1% and 89.0% for Cu2S and FeS nanoparticles, respectively. These results suggested that the FeS2 nanoparticles catalyze Na2S2O8 to produce •SO4 − radicals better than Cu2S nanoparticles, indicating Fe component is important than Cu component for the Fenton-like reaction.

Degradation Mechanism of H-CuFeS 2
As a key mechanistic study, the active species involved in the degradation reaction were identified systematically using the free radical trapping experiments ( Figure 11A). Methanol and NaN 3 were used as •OH and •SO 4 − scavengers, respectively. Comparing to methanol, NaN 3 inhibit RhB degradation more, indicating that •SO 4 − radicals are the major species involved in the Fenton-like degradation (blue curve in Figure 11A). According to the results of the scavenger test and XPS experiment, we propose a possible degradation mechanism. First, Fe 2+ /Cu + ions on the CuFeS 2 surface catalyzed S 2 O 8 2− to produce •SO 4 − radicals (Equations (1) and (2)). Due to high oxidation activity of •SO 4 − radicals (E 0 = 2.5-3.1 V), they were utilized to degrade dyes and to oxidize Fe 2+ /Cu + ions (Equations (3)- (5)). Then, •OH radicals also produced from the oxidation reaction between •SO 4 − radicals and H 2 O/OH − to degrade the dyes (Equations (6)- (8)). Thus, after adding methanol to the reaction mixture, RhB degradation in CuFeS 2 samples was slightly decreased, indicating that production of •OH radicals are considered as the indirect active species in the CuFeS 2 catalyzed RhB degradation (red curve in Figure 11A).
•SO 4 − + OH − → SO 4 2− + •OH (7) •OH + RhB → CO 2 + H 2 O  On the basis of the results described above, the degradation scheme of the H-CuFeS2 samples in the Fenton-like reaction was proposed (Scheme 1). •SO4 − radicals and •OH radicals were produced from the Fenton-like reaction between S2O8 2− and Fe 2+ /Cu + ions on the H-CuFeS2 surface to degrade RhB (Equations (1)-(8)). Then, Fe 2+ /Cu + ions were regenerated through a series reduction of S 2− anions (Equations (11)-(13)). Moreover, it is also possible to produce Fe 2+ ions by reduction reaction between Cu + and Fe 3+ ions (Equation (14)). •SO 4 − radical production in the Fenton-like reaction was further studied using the spectrophotometric method [50]. According to Equations (9) and (10), I 3 − solution (light yellow) was found from chemical reaction between S 2 O 8 2− and KI. The absorbance spectra of the S 2 O 8 2− /KI solution in the absence and presence of the prepared CuFeS 2 samples were evaluated. Figure 11B shows that an absorbance peak was observed at 358 nm for each sample and that the maximum absorbance was observed in the absence of the prepared CuFeS 2 samples (blue curve in Figure 11B). This suggests that S 2 O 8 2− produced the highest amount of I 2 compared to others, thereby leading to more chemical reactions with KI to generate I 3 − . Due to a high specific surface area and high content of Fe 2+ ions, H-CuFeS 2 effectively catalyzed S 2 O 8 2− to produce •SO 4 − radicals, as a result of a few I 2 production. Thus, the absorbance intensity at 358 nm of H-CuFeS 2 /S 2 O 8 2− /KI mixing solution (black curve in Figure 11B) was lower than that of C-CuFeS 2 /S 2 O 8 2− /KI mixing solution (red curve in Figure 11B). S 2 O 8 2− + 2I − → 2SO 4 2− + I 2 (9) On the basis of the results described above, the degradation scheme of the H-CuFeS 2 samples in the Fenton-like reaction was proposed (Scheme 1). •SO 4 − radicals and •OH radicals were produced from the Fenton-like reaction between S 2 O 8 2− and Fe 2+ /Cu + ions on the H-CuFeS 2 surface to degrade RhB (Equations (1)-(8)). Then, Fe 2+ /Cu + ions were regenerated through a series reduction of S 2− anions (Equations (11)-(13)). Moreover, it is also possible to produce Fe 2+ ions by reduction reaction between Cu + and Fe 3+ ions (Equation (14)

Stability and Practical Applications of H-CuFeS2
The stability of the catalyst is an essential parameter for the development of practical water treatment applications. To investigate the stability of H-CuFeS2, results of pH effect, copper ions effect, and cyclic RhB degradation tests were evaluated as shown in Figures  12-14. Figure 12 showed the study of pH effect. RhB degradation by H-CuFeS2 at pH 4.0 maintained a similar degradation efficiency at pH 7.0 (98.48% at pH 4.0 and 98.49% at pH 7.0, respectively), whereas that at pH 10.0 resulted in a considerable loss of efficiency (72.13% at pH 10.0). This is because most •SO4 − radicals were converted to •OH radicals at alkaline condition (Equations (6)-(8)). Thus, •OH radicals are the major active radicals involved in dye degradation at alkaline condition. In addition, inactive porphyrin ferryl complexes (FeO 2+ ) are formed as Fe 2+ ions in the alkaline solution. As a result, a weakened degradation result at pH 10.0 was found ( Figure 12).
In the study of copper ion effect as shown in Figure 13, RhB degradation efficiencies by H-CuFeS2 in the presence of Cu + ioins were 88.64% at pH 4.0, 91.21% at pH 7.0, and 87.42% at pH 10.0, whereas those in the presence of Cu 2+ ions were 93.96% at pH 4.0, 94.21% at pH 7.0, and 91.19% at pH 10.0. Comparing to that at pH 10.0 in the absence of copper ions, an obvious improvement was found. This is because •SO4 − radicals are produced in the presence of Cu + ions (Equation (2)). In addition, Fe 2+ /Cu + ions were regenerated through reduction between S 2− anions and Fe 3+ /Cu 2+ ions (Equations (11)-(13)). As a result, an improve degradation at pH 10.0 was found.

Stability and Practical Applications of H-CuFeS 2
The stability of the catalyst is an essential parameter for the development of practical water treatment applications. To investigate the stability of H-CuFeS 2 , results of pH effect, copper ions effect, and cyclic RhB degradation tests were evaluated as shown in Figures 12-14. Figure 12 showed the study of pH effect. RhB degradation by H-CuFeS 2 at pH 4.0 maintained a similar degradation efficiency at pH 7.0 (98.48% at pH 4.0 and 98.49% at pH 7.0, respectively), whereas that at pH 10.0 resulted in a considerable loss of efficiency (72.13% at pH 10.0). This is because most •SO 4 − radicals were converted to •OH radicals at alkaline condition (Equations (6)-(8)). Thus, •OH radicals are the major active radicals involved in dye degradation at alkaline condition. In addition, inactive porphyrin ferryl complexes (FeO 2+ ) are formed as Fe 2+ ions in the alkaline solution. As a result, a weakened degradation result at pH 10.0 was found ( Figure 12).
In the study of copper ion effect as shown in Figure 13, RhB degradation efficiencies by H-CuFeS 2 in the presence of Cu + ioins were 88.64% at pH 4.0, 91.21% at pH 7.0, and 87.42% at pH 10.0, whereas those in the presence of Cu 2+ ions were 93.96% at pH 4.0, 94.21% at pH 7.0, and 91.19% at pH 10.0. Comparing to that at pH 10.0 in the absence of copper ions, an obvious improvement was found. This is because •SO 4 − radicals are produced in the presence of Cu + ions (Equation (2)). In addition, Fe 2+ /Cu + ions were regenerated through reduction between S 2− anions and Fe 3+ /Cu 2+ ions (Equations (11)-(13)). As a result, an improve degradation at pH 10.0 was found.  For recyling-used study, Figure 14A showed RhB degradation by H-CuFeS2 exhibited a considerable loss of efficiency (from 98.48% to 72.46% after three cycles). Furthermore, the corresponding XRD, Raman, and SEM results ( Figure 14B-D) suggest a decrease in the phase structure of the H-CuFeS2 samples after the repeated reactions, indicating the  For recyling-used study, Figure 14A showed RhB degradation by H-CuFeS2 exhibited a considerable loss of efficiency (from 98.48% to 72.46% after three cycles). Furthermore, the corresponding XRD, Raman, and SEM results ( Figure 14B-D) suggest a decrease in the phase structure of the H-CuFeS2 samples after the repeated reactions, indicating the destruction of the H-CufeS2 sample crystalization. In addition, EDS spectrum found the atomic ratio (Cu:Fe:S) for the third used H-CuFeS2 samples was determined t 1:1:1.9. The morphology of the third used samples still retained sphere-like struct with the average diameter ranging 20-35 nm. Further research to improve recyclingability by other heterojunction, such as those doped by Ag@Ag3PO4 nanoparticles, is underway in our laboratory. To assess the practical applications of H-CuFeS2 as a new water treatment op various dyes (R6G, MB, and MO) and colorless organic compound (BPA) were tested ure 15A). H-CuFeS2 exhibited excellent degradation efficiency toward R6G, MB, MO BPA, with 96.84%, 93.86%, 81.89%, and 75.24% degradation achieved within 10 min spectively. In addition, the mineralization performance of H-CuFeS2 comparing traditional Fenton reaction (Fe 2+ /H2O2) was evaluated. From the TOC analysis (Fi 15B), mineralization efficiency for the Fe 2+ /H2O2 and H-CuFeS2/S2O8 2− system was 7 and 80.1%, respectively, representing 10.1% improvement of RhB degradation. Fin the prepared H-CuFeS2 samples were used to degrade RhB in the environmental w samples (pond water and seawater). H-CuFeS2 exhibited adequate mineralization ciency through the Fenton-like reaction for RhB degradation. A notable difference in mineralization efficiency for RhB was observed for the seawater samples (47.9% effici within 10 min) compared with pond water samples (63.8% efficiency within 10 min), p ably because of the effect of higher concentration of anions or radical scavengers in seawater sample that reduced the degradation activity of H-CuFeS2. Nevertheless studies on the environmental water samples strongly support the benefits of this n developed H-CuFeS2-based Fenton-like water treatment option. For recyling-used study, Figure 14A showed RhB degradation by H-CuFeS 2 exhibited a considerable loss of efficiency (from 98.48% to 72.46% after three cycles). Furthermore, the corresponding XRD, Raman, and SEM results ( Figure 14B-D) suggest a decrease in the phase structure of the H-CuFeS 2 samples after the repeated reactions, indicating the destruction of the H-CufeS 2 sample crystalization. In addition, EDS spectrum found that the atomic ratio (Cu:Fe:S) for the third used H-CuFeS 2 samples was determined to be 1:1:1.9. The morphology of the third used samples still retained sphere-like structures, with the average diameter ranging 20-35 nm. Further research to improve recycling-used ability by other heterojunction, such as those doped by Ag@Ag 3 PO 4 nanoparticles, is now underway in our laboratory.
To assess the practical applications of H-CuFeS 2 as a new water treatment option, various dyes (R6G, MB, and MO) and colorless organic compound (BPA) were tested ( Figure 15A). H-CuFeS 2 exhibited excellent degradation efficiency toward R6G, MB, MO, and BPA, with 96.84%, 93.86%, 81.89%, and 75.24% degradation achieved within 10 min, respectively. In addition, the mineralization performance of H-CuFeS 2 comparing to a traditional Fenton reaction (Fe 2+ /H 2 O 2 ) was evaluated. From the TOC analysis ( Figure 15B), mineralization efficiency for the Fe 2+ /H 2 O 2 and H-CuFeS 2 /S 2 O 8 2− system was 70.0% and 80.1%, respectively, representing 10.1% improvement of RhB degradation. Finally, the prepared H-CuFeS 2 samples were used to degrade RhB in the environmental water samples (pond water and seawater). H-CuFeS 2 exhibited adequate mineralization efficiency through the Fenton-like reaction for RhB degradation. A notable difference in the mineralization efficiency for RhB was observed for the seawater samples (47.9% efficiency within 10 min) compared with pond water samples (63.8% efficiency within 10 min), probably because of the effect of higher concentration of anions or radical scavengers in the seawater sample that reduced the degradation activity of H-CuFeS 2 . Nevertheless, the studies on the environmental water samples strongly support the benefits of this newly developed H-CuFeS 2 -based Fenton-like water treatment option.

Preparation of CuFeS2
All chemicals were purchased from Sigma Aldrich (St. Louis, MO, USA) and were of analytical grade and used without further purification. In this study, hydrothermal (H) and co-precipitated method (C) were used to prepare CuFeS2 samples, representing as H-CuFeS2 and C-CuFeS2, respectively. For hydrothermal procedure, 0.989 g of CuCl, and 2.703 g of FeCl3·6H2O were added to 57 mL of deionized water, with stirring for 10 min. Then, 8 mL of Na2S ·9H2O (0.02 mol) was added dropwisely into the above green mixture.

Preparation of CuFeS 2
All chemicals were purchased from Sigma Aldrich (St. Louis, MO, USA) and were of analytical grade and used without further purification. In this study, hydrothermal (H) and co-precipitated method (C) were used to prepare CuFeS 2 samples, representing as H-CuFeS 2 and C-CuFeS 2 , respectively. For hydrothermal procedure, 0.989 g of CuCl, and 2.703 g of FeCl 3 ·6H 2 O were added to 57 mL of deionized water, with stirring for 10 min. Then, 8 mL of Na 2 S·9H 2 O (0.02 mol) was added dropwisely into the above green mixture. After stirring for 30 min, the black mixture was transferred into a Teflon-lined stainless-steel autoclave. The autoclave was sealed and heated in an electric oven at 200 • C for 10 h. After the autoclave naturally cooled to room temperature, the precipitates were centrifuged (5000 rpm, 15 min) and washed three times with ethanol and deionized water, and then dried in vacuum at 60 • C overnight. In addition, Cu 2 S and FeS 2 nanoparticles were prepared following similar method without adding FeCl 3 ·6H 2 O and CuCl precursor, respectively.
For the co-precipitated method, 4.95 mg of CuCl, and 0.0135 g of FeCl 3 ·6H 2 O were added to 20 mL of deionized water, with stirring at 70 • C for 10 min. Then, 1 mL of NH 4 OH (30%) and 1 mL of N 2 H 4 ·H 2 O (64-65%) were added dropwise into the above mixture with stirring at 70 • C for 3 h. After that, 0.024 g of Na 2 S·9H 2 O was added into the above brown mixture with stirring at 70 • C for 3 h. Finally, the black precipitates were centrifuged (5000 rpm, 15 min) and washed three times with ethanol and deionized water, and then dried in vacuum at 60 • C overnight.

Characterization of CuFeS 2
The morphological and compositional characteristics of all as-prepared samples were observed with scanning electron microscopy (SEM) on a HITACHI S-4300 (Hitachi, Tokyo, Japan) and transmission electron microscopy (TEM) on a 1200EX II (JEOL, Tokyo, Japan) equipped with a QUANTAX Annular XFlash QUAD FQ5060 (Bruker Nano, Berlin, Germany). The crystallographic texture of the samples was measured by powder X-ray diffraction (XRD) on SMART APEX II (Bruker AXS, Billerica, MA, USA) using Cu Kα radiation (λ = 1.5406 Å). Raman spectra were collected at room temperature using a confocal micro-Raman system (Thermo Scientific Inc., New York, NY, USA). A 532 nm laser line was used as the photoexcitation source with a laser power of 2 mW focused on the sample for 10 s. The binding energy of elements was determined through X-ray photoelectron spectroscopy (XPS) on a VG ESCA210 (VG Scientific, West Sussex, UK).

Degradation Procedure
RhB degradation was used to assess the degradation activity of the prepared samples. For the Fenton-like reaction, 20 mg of the prepared catalyst samples was added into the RhB solution (20 ppm, 50 mL), and the solution was stirred in the dark for 30 min. At 10 min before adding Na 2 S 2 O 8 , the absorbance at its characteristic absorption peak of 550 nm was measured to check the adsorption ability of the prepared samples. Subsequently, 100 µL of Na 2 S 2 O 8 (2 M) was added to dye solution. After a given time interval, 1 mL of suspension was sampled with a plastic pipette and this aliquot was quenched immediately by adding 10 µL NaN 3 (1 M) and filtered by a 0.22-µm syringe filter organic membrane to remove catalyst particles. The concentration of RhB was measured using a Synergy H1 hybrid multimode microplate reader (BioTek Instruments, Winooski, VT, USA) at its characteristic absorption peak of 550 nm. Similar processes were performed for other catalysts (Cu 2 S and FeS 2 ), dyestuffs (R6G, MB, and MO), and organic pollutant (BPA). After the experiment, TOC concentration was determined on an Elementar Acquray TOC analyzer (Elementar Analysensysteme GmbH, Langenselbold, Germany) to evaluate the extent of mineralization.

Free Radical Trapping Experiment
To investigate the active species generated during RhB degradation over H-CuFeS 2 , the trapping experiment was conducted using NaN 3 and methanol (each 0.1 M) as the capturing agent for •SO 4 − radicals and •OH radicals, respectively. The implemented trapping experimental procedure was identical to the steps mentioned in the degradation section with an additional step of adding the capturing agent at each run.

Conclusions
The prepared H-CuFeS 2 samples showed higher RhB degradation efficiency through the Fenton-like reaction than the prepared C-CuFeS 2 , FeS 2 , Cu 2 S nanoparticles, and previously reported samples (Table 1). This high enhancement in the degradation efficiency (98.8% RhB degradation within 10 min) was attributed to the prepared H-CuFeS 2 samples possessed smaller size and higher surface area. Based on the results of scavenger test and radicals' quantitation experiments, H-CuFeS 2 catalyzed Na 2 S 2 O 8 to produce •SO 4 − radicals and •OH radicals for the organics degradation. As we know, the three limiting factors to address prior to industrial application were viable methods of catalyst preparation, the catalyst durability and universality under operating conditions. The prepared H-CuFeS 2 samples possessed several attractive features. First, the prepared H-CuFeS 2 samples in the presence of Na 2 S 2 O 8 had 98.8% RhB degradation performance within 10 min. In addition, various organics (R6G, MB, MO, and BPA) with 75.24-96.84% degradation efficiency could be achieved. However, the repeated use of H-CuFeS 2 showed performance deterioration due to the change in the crystal phase of used H-CuFeS 2 . Further research on the high recycling-used ability of other heterojunction CuFeS 2 composites, such as those doped by Ag@Ag 3 PO 4 nanoparticles, is now underway in our laboratory. Finally, the prepared H-CuFeS 2 samples were used to degrade RhB with 10.1% mineralization improvement comparing to traditional Fenton reaction (Fe 2+ /H 2 O 2 ). It is also easy to recover H-CuFeS 2 catalyst comparing to Fe 2+ ions. In addition, H-CuFeS 2 catalyst deposited on a cellulosebased substrate is ongoing in our lab. The difficult separation and recycle of powder catalyst may result in high cost and secondary pollution, therefore, the powder form of catalyst greatly limited the commercial industrial application. More importantly, H-CuFeS 2 deposited on cellulose is very suitable for the dynamic-flow water treatment system. We will propose a new adsorption-degradation strategy for the pollutant removal in industrial level application in the future. In summary, this study discovered the hydrothermal synthesis of CuFeS 2 samples and successfully demonstrated the application of the Fenton-like reaction in the environmental water samples. The current findings can be used to the application of AOPs in wastewater treatment in the future.