Microwave-Assisted Synthesis of Chalcopyrite/Silver Phosphate Composites with Enhanced Degradation of Rhodamine B under Photo-Fenton Process

Abstract

A new composite by coupling chalcopyrite (CuFeS₂) with silver phosphate (Ag₃PO₄) (CuFeS₂/Ag₃PO₄) was proposed by using a cyclic microwave heating method. The prepared composites were characterized by scanning and transmission electron microscopy and X-ray diffraction, Fourier-transform infrared, UV–Vis diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy. Under optimum conditions and 2.5 W irradiation (wavelength length > 420 nm, power density = 0.38 Wcm⁻²), 96% of rhodamine B (RhB) was degraded by CuFeS₂/Ag₃PO₄ within a 1 min photo-Fenton reaction, better than the performance of Ag₃PO₄ (25% degradation within 10 min), CuFeS₂ (87.7% degradation within 1 min), and mechanically mixed CuFeS₂/Ag₃PO₄ catalyst. RhB degradation mainly depended on the amount of hydroxyl radicals generated from the Fenton reaction. The degradation mechanism of CuFeS₂/Ag₃PO₄ from the photo-Fenton reaction was deduced using a free radical trapping experiment, the chemical reaction of coumarin, and photocurrent and luminescence response. The incorporation of CuFeS₂ in Ag₃PO₄ enhanced the charge separation of Ag₃PO₄ and reduced Ag₃PO₄ photocorrosion as the photogenerated electrons on Ag₃PO₄ were transferred to regenerate Cu²⁺/Fe³⁺ ions produced from the Fenton reaction to Cu⁺/Fe²⁺ ions, thus simultaneously maintaining the CuFeS₂ intact. This demonstrates the synergistic effect on material stability. However, hydroxyl radicals were produced by both the photogenerated holes of Ag₃PO₄ and the Fenton reaction of CuFeS₂ as another synergistic effect in catalysis. Notably, the degradation performance and the reusability of CuFeS₂/Ag₃PO₄ were promoted. The practical applications of this new material were demonstrated from the effective performance of CuFeS₂/Ag₃PO₄ composites in degrading various dyestuffs (90–98.9% degradation within 10 min) and dyes in environmental water samples (tap water, river water, pond water, seawater, treated wastewater) through enhanced the Fenton reaction under sunlight irradiation.

1. Introduction

Recently, concerns have been raised worldwide toward the harm caused by residual organic pollutants in surface water and groundwater, threatening ecosystems and aquatic species [1,2,3]. Among the many sources of water pollution, wastewater from the printing and dyeing industry is a major concern. The decolorization and degradation of most chromophores in dyes are difficult because of their stable aromatic structures, leading to prolonged toxic effects and environmental hazards [4,5,6]. Furthermore, dyes can absorb sunlight and reduce water clarity, preventing photosynthesis in aquatic plants, decreasing dissolved oxygen in water, affecting microbial diversity, and disrupting the self-purification capacity of water [7]. The removal of these deleterious and hazardous pollutants from industrial wastewater has become an urgent environmental need in the world [8].

Many studies have evaluated the removal of organic dyes by using photocatalytic degradation, ideally using sunlight, with the vision of sustainable water treatment. TiO₂ is the most widely used photocatalytic semiconductor because of its nontoxic and stable nature, with a relatively low cost and abundant production resulting from a mature manufacturing process [9,10]. However, its absorption and photocatalytic activity depends entirely on ultraviolet light, which restricts its application in large-scale wastewater treatment [11,12,13]. Studies have been attempting to identify efficient sunlight-responsive photocatalysts, and Ag₃PO_4, with a quantum efficiency of >90%, is considered an excellent candidate that is sunlight responsive [14,15,16]. However, it experiences considerable photocorrosion during photolysis. In addition to the photocatalytic method, Cu/Fe-bearing solids such as chalcopyrite (CuFeS₂) have been widely used as catalysts in advanced oxidation processes (AOPs) for wastewater treatment [17,18,19,20,21]. In general, AOPs are based on Fenton’s chemistry, which utilizes hydroxyl radicals produced from the Fenton reaction between H₂O₂ and Cu⁺/Fe²⁺ ions to degrade organic dyes (Equations (1)–(6)) [22,23]. However, the reusability of CuFeS₂ is a challenge due to its dissolution during water treatment and the slow kinetics of Fe²⁺ regeneration:

CuFeS₂(s) + 8H₂O₂ → Fe³⁺ + Cu²⁺ + 2SO₄²⁻ + 8H₂O + 2H⁺

(1)

CuFeS₂(s) + 16Fe³⁺ + 8H₂O → 17Fe²⁺ +Cu²⁺ + 2SO₄²⁻ + 16H⁺

(2)

CuFeS₂(s) + 4O₂ → Fe²⁺ + Cu²⁺ + 2SO₄²⁻

(3)

Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + OH·

(4)

Cu²⁺ + H₂O₂ → Cu⁺ + O₂⁻· + 2H⁺

(5)

Cu⁺ + H₂O₂ → Cu²⁺ + OH⁻ + OH·

(6)

Fe³⁺ + H₂O → Fe(OH)²⁺ + H⁺

(7)

Fe(OH)²⁺ + hν → Fe²⁺ + OH·

(8)

To address CuFeS₂ reusability, attempts have been made to regenerate the Fenton catalysts with UV and/or visible-light irradiation, known as the photo-Fenton process [19,24,25]. Under UV and/or visible-light irradiation, Fe³⁺ complexes are formed from the Fenton reaction (Equation (7)) to produce both Fe²⁺ and hydroxyl radicals (Equation (8)). The photogenerated Fe²⁺ ions can catalyze the Fenton reaction to form Fe³⁺, thus demonstrating the recyclability of Fenton catalysts (Equation (4)). For instance, Dotto et al. demonstrated that the prepared citrate–CuFeS₂ materials possessed 90% catalytic efficiency for bisphenol A (BPA) degradation with a 15 min photo-Fenton process for its rapid generation of hydroxyl radicals and efficient H₂O₂ consumption [24]. Dotto et al. also reported that the catalytic efficiency and stability was sustained after four consecutive photoregeneration cycles. However, the preparation of the novel CuFeS₂ samples requires a high power and expensive microwave reactor (1400 W, 200 °C, 7 min). In another simpler attempt in material preparation, Pastrana-Martinez et al. used the mineral of CuFeS₂ mined from Jendouba, Tunisia, to catalyze tyrosol (TY) degradation by using a UV light-emitting diode (LED)-assisted photo-Fenton reaction [19]. Higher total organic carbon (TOC) conversions (85.0%) and lower iron leaching (0.89 mg·L⁻¹) were attained when the purified CuFeS₂ samples were used in the photo-Fenton-like process within 60 min (0.50 mM TY at stoichiometric H₂O₂ concentrations). However, UV light irradiation is not considered sustainable because it requires a high energy input. The limitations of the methods used in all these studies underline the need to improve the visible-light absorption ability and efficiently boost the degradation performance and stability of CuFeS₂. To our knowledge, CuFeS₂ coupled with other semiconductors has not been examined in the Fenton process under visible-light irradiation.

Here, we synthesized CuFeS₂ coupled with Ag₃PO₄ (CuFeS₂/Ag₃PO₄) through cyclic microwave heating to address the challenges in material preparation, the stability of materials, and degradation performance. The breakthrough in our synthesis is that CuFeS₂/Ag₃PO₄ could be prepared using a domestic 336 W microwave oven within 12 min. Our previous report indicated that Ag₃PO₄ is an efficient photocatalyst responsive to visible light and sunlight, so incorporating Ag₃PO₄ into the CuFeS₂ Fenton reaction system might greatly increase the visible-light absorption while improving the regeneration of Fe²⁺/Cu⁺ ions. Here, we used CuFeS₂/Ag₃PO₄ composites for the degradation of organic dyes (rhodamine B (RhB), methyl red (MR), rhodamine 6G (R6G), fluorescein, and propidium iodide (PI)). We also proposed that the degradation mechanism of CuFeS₂/Ag₃PO₄ and the reactive species and successfully demonstrated the regeneration of the CuFeS₂ catalyst and the application of CuFeS₂/Ag₃PO₄ in the treatment of environmental samples.

2. Materials and Methods

2.1. Preparation of Ag₃PO₄, CuFeS_2, and CuFeS₂/Ag₃PO₄

All chemicals were purchased from Sigma Aldrich (St. Louis, MO, USA) and were of analytical grade and thus used without further purification. First, 0.212 g of AgNO₃ was added to 20 mL of deionized water (18.2 MΩ·cm) with stirring. Then, a colorless Ag(NH₃)₂⁺ ion solution was produced by adding 6.2 mL of 14 M NH₃ solution dropwise into the AgNO₃ solution. After 30 min stirring, 3.5 mL of H₃PO₄ (14.6 M) was used to neutralize the mixed solution to pH 7.0, and the solution was stirred again for 30 min in the dark. The yellow precipitate was filtered and subsequently rinsed with copies of deionized water and anhydrous ethanol. Finally, the as-synthesized Ag₃PO₄ was dried at 55 °C for 12 h [26].

CuFeS₂ was prepared using the cyclic microwave heating method [27]. First, 19.7 mg of CuCl, 25.35 mg of FeCl_3, and 48.46 mg of L-cysteine were added to 20 mL of deionized water with stirring for 15 min. Then, the mixed solution underwent 10 cycles of 36 s heating and a 36 s pause in a domestic microwave oven (power: 336 W). After the black precipitate was cooled to room temperature, it was rinsed with deionized water and anhydrous ethanol. Finally, the as-synthesized CuFeS₂ was dried at 55 °C for 12 h. Cu₂S and Fe₂S₃ were prepared following similar method without adding FeCl₃ and CuCl precursor, respectively. The CuFeS₂/Ag₃PO₄ with a molar ratio of 2.5:1 was prepared as followed: 20 mg of Ag₃PO₄, 19.7 mg of CuCl, 25.35 mg of FeCl_3, and 48.46 mg of L-cysteine were added to 20 mL of deionized water with stirring for 15 min. Then, the mixed solution underwent 10 cycles of 36 s heating and a 36 s pause in a domestic microwave oven (power: 336 W). After the black precipitate was cooled to room temperature, it was rinsed with deionized water and anhydrous ethanol. Finally, the prepared CuFeS₂/Ag₃PO₄ was dried at 55 °C for 12 h and the weight of CuFeS₂/Ag₃PO₄ was 41.7 mg. Thus, the weight percentage of CuFeS₂ in CuFeS₂/Ag₃PO₄ was 52%. In this condition, we estimated the weight of CuFeS₂ and Ag₃PO₄ in the CuFeS₂/Ag₃PO₄ was 21.7 mg and 20 mg, respectively. Therefore, the molar ratio of CuFeS₂/Ag₃PO₄ was calculated to be 2.5:1 [27]. In addition, the preparation of CuFeS₂/Ag₃PO₄ with different molar ratios (0.4:1 and 1:1) followed the same method and was prepared by decreasing the adding amounts of CuCl and FeCl₃ precursors at 20 mg Ag₃PO₄.

2.2. Characterization of Ag₃PO₄, CuFeS_2, and CuFeS₂/Ag₃PO₄

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 through powder X-ray diffraction (XRD) on SMART APEX II (Bruker AXS, Billerica, MA, USA) using Cu Kα radiation (λ = 1.5406 Å). Fourier-transform infrared (FT-IR) spectra were obtained using an Agilent Cary 600 FT-IR spectrometer (Agilent Technologies, Santa Clara, CA, USA). An Evolution 2000UV–Vis spectrophotometer (Thermo Fisher Scientific Inc., Madison, WI, USA) with integrating spheres and reflectance standard material BaSO₄ was applied to obtain the UV–Vis diffuse reflectance spectroscopy (DRS). The binding energy of elements was determined through X-ray photoelectron spectroscopy (XPS) on a VG ESCA210 (VG Scientific, West Sussex, UK).

2.3. Degradation Procedure by Using Ag₃PO₄, CuFeS_2, and CuFeS₂/Ag₃PO₄

RhB degradation was used to assess the degradation activity of the prepared samples. The photoreactor PCX-50C (Beijing Perfectlight Technology Co. Ltd., Beijing, China) was equipped with a low-power white LED irradiation (2.5 W, power density = 0.38 W·cm⁻², wavelength > 420 nm). For the photo-Fenton 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. Before 10 min, to turn the light on and add H₂O₂, we measured the absorbance to check the adsorption ability of the prepared samples. Subsequently, the LED lamp was turned on and 200 μL of H₂O₂ (35%) was added. After given time intervals, 1 mL of suspension was taken, quenched immediately by adding 0.1 mM NaN₃ and filtered by a 0.22 μm syringe filter organic membrane to remove the catalyst sample. 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 dyestuffs (MR, R6G, fluorescein, and PI). The RhB degradation (20 ppm, 50 mL) for the photocatalytic and Fenton reactions was performed using the same protocols (20 mg catalysts) as mentioned above, but without the addition of H₂O₂ and LED light irradiation, respectively. After the experiment, TOC concentration was determined on an Elementar Acquray TOC analyzer (Elementar Analysensysteme GmbH, Langenselbold, Germany) to evaluate the extent of mineralization.

2.4. Evaluation of Charge Separation and Recombination Rate of Ag₃PO₄, CuFeS_2, and CuFeS₂/Ag₃PO₄

The charge separation efficiency and recombination rate of electron–hole pairs for the prepared composites were evaluated according to our earlier reports [28,29,30]. Photocurrent was measured to evaluate the charge separation efficiency of Ag₃PO₄, CuFeS₂, and CuFeS₂/Ag₃PO₄ composites under constant white LED irradiation at 60 s intervals. To determine the recombination rate of electron–hole pairs, the photoluminescence (PL) spectra of samples were obtained using λ_ex = 250 nm with a Varian Cary Eclipse fluorescence spectrophotometer (Agilent Technologies, Santa Clara, CA, USA).

2.5. Free Radical Trapping Experiment of Ag₃PO₄, CuFeS_2, and CuFeS₂/Ag₃PO₄

To investigate the active species generated during RhB degradation over Ag₃PO₄, CuFeS₂, and CuFeS₂/Ag₃PO₄, the trapping experiment was conducted using ethylenediaminetetraacetate (EDTA), tert-butanol (t-BuOH), and p-benzoquinone (BQ) (each 1 mM) as the capturing agent for holes, hydroxyl radicals, and oxygen radicals, respectively. The implemented trapping experimental procedure was identical to the steps mentioned in the degradation section except for the capturing agent being added at each run.

3. Results and Discussion

3.1. Morphology and Crystal Phase of Ag₃PO₄, CuFeS_2, and CuFeS₂/Ag₃PO₄

The morphology of the prepared Ag₃PO₄, CuFeS_2, and CuFeS₂/Ag₃PO₄ composites was analyzed through SEM and TEM (Figure 1). As shown in Figure 1, Ag₃PO₄ appears as tetrapod-like structures, with its four cylindrical arms being approximately 7–10 μm in length and with its average diameter being 2–3 μm. The CuFeS₂ crystals appear irregular and sheet-like with sizes of 0.2–2 μm. As for the prepared Ag₃PO₄/CuFeS₂ composites, CuFeS₂ sheets were randomly deposited on the Ag₃PO₄ surface, and this deposition has no effect on the morphology and chemical composition of the Ag₃PO₄ particles. The corresponding magnified TEM image revealed clear lattice fringes with a d spacing of 0.249 and 0.312 nm, which are in good agreement with the (210) and (112) lattice planes of Ag₃PO₄ and CuFeS₂, respectively. The energy dispersive spectrometer (EDS) spectra of the prepared Ag₃PO₄, CuFeS_2, and CuFeS₂/Ag₃PO₄ composites (Figure 2) confirm the presence of Ag, P, O, Cu, Fe, and S elements in their crystals accordingly. In addition, we also found C element in the presence of CuFeS₂ and CuFeS₂/Ag₃PO₄ composites. This is because we used L-cysteine precursor in the preparation of CuFeS₂ and CuFeS₂/Ag₃PO₄ composites.

The XRD spectra of the prepared Ag₃PO₄, CuFeS_2, and CuFeS₂/Ag₃PO₄ composites are displayed in Figure 3. From the black curve of Ag₃PO₄, the diffraction peaks at 21.5°, 30.1°, 33.4°, 36.7°, 42.5°, 48.7°, 53.6°, 55.3°, 57.5°, 62.4°, 65.1°, 70.7°, 72.2°, 74.6°, and 78.2° were identified and assigned to the (110), (200), (210), (211), (220), (310), (222), (320), (321), (400), (330), (420), (421), (332), and (442) faces of the cubic Ag₃PO₄, respectively (JCPDS No. 06-0505) [26]. The XRD pattern of CuFeS₂ is presented in the red curve of Figure 3, and the diffraction peaks at 28.9°, 46.4°, and 56.2° corresponded well to the (112), (220), and (312) faces of the tetragonal chalcopyrite CuFeS₂, respectively (JCPDS No. 01-0842) [27]. As a confirmation of good composite quality, the diffraction patterns of the prepared CuFeS₂/Ag₃PO₄ composites match with the patterns of Ag₃PO₄ and CuFeS₂. According to Scherer’s equation, the average crystallite size of Ag₃PO₄ and CuFeS₂ was 55.8 and 43.5 nm, respectively. In addition, they did not change in the CuFeS₂/Ag₃PO₄ composites (Ag₃PO₄: 57.0 nm, CuFeS₂: 46.5 nm).

3.2. Optical Property of Ag₃PO₄, CuFeS_2, and CuFeS₂/Ag₃PO₄

The FT-IR spectra of the prepared Ag₃PO₄, CuFeS_2, and CuFeS₂/Ag₃PO₄ composites are shown in Figure 4. The IR spectrum for Ag₃PO₄ shows four major peaks located at 560, 1012, 1670, and 3200 cm⁻¹, which corresponded to the PO₄³⁻ stretching and O–H bending vibration, respectively. The O–H bending in the prepared Ag₃PO₄ may be attributed from Ag₃PO₄ adsorbed water from the air. For CuFeS₂, it shows three peaks located at 1045, 1640, and 3400 cm⁻¹, which corresponded to the C=S stretching and O–H bending vibration, respectively. The O–H bending in the prepared CuFeS₂ may be attributed from CuFeS₂ adsorbed water from the air. The C=S stretching in the prepared CuFeS₂ may be attributed from the L-cysteine precursor. In addition, the EDS spectra for CuFeS₂ and CuFeS₂/Ag₃PO₄ composites also confirmed the presence of C element. For CuFeS₂/Ag₃PO₄ composites, the spectrum exhibited PO₄³⁻, C=S, and O–H vibration peaks in the present on their crystals.

To study the optical absorption characteristics of the prepared Ag₃PO₄, CuFeS_2, and CuFeS₂/Ag₃PO₄ composites, UV–Vis DRS was performed, as shown in Figure 5. The absorption band-edge for Ag₃PO₄, CuFeS₂ and CuFeS₂/Ag₃PO₄ is at approximately 525, 427, and 487 nm, respectively. The bandgap energy (Eg) of Ag₃PO₄ is theoretically estimated to be 2.38 eV using the equation αhν = A(hν − Eg)^n/2 (n = 1 for Ag₃PO₄). Furthermore, the band-edge potentials for the conduction band (CB) and valence band (VB) can be calculated as E_VB = χ − E_H + 0.5 Eg and E_CB = E_VB − Eg, respectively; here, χ is the electronegativity of the constituent atom (5.96 eV for Ag₃PO₄) and E_H is the energy of the free electrons relative to the standard hydrogen reduction potential (approximately 4.5 eV). Thus, E_VB and E_CB of Ag₃PO₄ can be estimated to be 2.65 and 0.27 eV, respectively.

As another quality assurance method, the XPS analysis of the prepared Ag₃PO₄, CuFeS₂, CuFeS₂/Ag₃PO₄ composites (Figure 6, Figure 7 and Figure 8, full scan) revealed that the prepared composites contained their own elements: Ag, P, O, Cu, Fe, and S. High-resolution XPS revealed Ag_3d, P_2p, O_1s, Cu_2p, Fe_2p, and S_2p in CuFeS₂/Ag₃PO₄ composites (Figure 8B–G, respectively). In Figure 8B, the binding energies are located at 367.8 and 373.3 eV, corresponding to Ag⁺ 3d_5/2 and Ag⁺ 3d_3/2, respectively. Furthermore, the peak at 132.9 eV correspond to P⁵⁺ 2p (Figure 8C). O 1s spectra can be deconvoluted into two component peaks of 531.8 and 530.7 eV (Figure 8D). The peak centered at 530.7 eV associated with the O₂ in Ag₃PO_4. The other peak centered at 531.8 eV the presence of –OH group or a water molecule absorbed on the surface of the prepared composites. In Figure 8E, the peaks at 932.3, 934.2, 952.2, and 953.8 eV corresponded to Cu⁺ 2p_3/2, Cu²⁺ 2p_3/2, Cu⁺ 2p_1/2, and Cu²⁺ 2p_1/2, respectively. The peaks at 712.5, 718.2, 722.5, and 731.8 eV corresponded to Fe²⁺ 2p_3/2, Fe³⁺ 2p_3/2, Fe²⁺ 2p_1/2 and Fe³⁺ 2p_1/2, respectively (Figure 8F), whereas that at 162.5 and 167.8 eV corresponded to S²⁻ 2p and S⁶⁺ 2p, respectively (Figure 8G).

3.3. Degradation Performance of Ag₃PO₄, CuFeS_2, and CuFeS₂/Ag₃PO₄

To evaluate the degradation activity of the prepared Ag₃PO₄, CuFeS_2, and CuFeS₂/Ag₃PO₄ composites, RhB was selected as the target pollutant. Figure 9 shows the concentration ratio (C/C₀, where C₀ and C represented the RhB concentration at the initial condition and at time t, respectively) of RhB by using Ag₃PO₄, CuFeS_2, and CuFeS₂/Ag₃PO₄ composites under three different degradation conditions: photocatalytic, Fenton, and photo-Fenton. First, under 30 min LED light irradiation (Figure 9A), the degradation efficiencies of RhB in CuFeS₂ and CuFeS₂/Ag₃PO₄ are only 8.9% and 16.9%, respectively, whereas the efficiency reaches 39.5% in the presence of Ag₃PO₄. This suggests that the photocatalytic performance of Ag₃PO₄ is better than that of CuFeS₂ and CuFeS₂/Ag₃PO₄, attributable to the reactive radical formation from the effective photoinduced charge separation for RhB degradation. Nevertheless, these reactive radicals must be determined in a later experiment. CuFeS₂/Ag₃PO₄ is responsive to visible light caused by its diluted amount of Ag₃PO₄, resulting in a weaker degradation performance than that of pure Ag₃PO₄. Second, the degradation efficiency of the prepared Ag₃PO₄, CuFeS_2, and CuFeS₂/Ag₃PO₄ composites through the Fenton reaction in the dark was studied (Figure 9B). The RhB concentration barely changes in the presence of Ag₃PO₄, but the RhB degradation efficiency reaches 76.1% and 93.7% within 1 min in the presence of CuFeS₂ and CuFeS₂/Ag₃PO_4, respectively. This is due to the production of hydroxyl radicals that degrade RhB from the oxidation of the Cu⁺/Fe²⁺ ions on the CuFeS₂ to the formation of Cu²⁺/Fe³⁺ ions in the presence of H₂O₂. Finally, the degradation performance of the prepared Ag₃PO₄, CuFeS_2, and CuFeS₂/Ag₃PO₄ composites was evaluated with the photo-Fenton reaction in the presence of H₂O₂, and the results are shown in Figure 9C. The degradation efficiency of Ag₃PO₄ reaches 25% with a 10 min photo-Fenton reaction, whereas that of CuFeS₂ reaches 87.7% within 1 min. Notably, the degradation efficiency of CuFeS₂/Ag₃PO₄ reaches 96% within 1 min. Moreover, the relative standard deviation of the degradation performance for three different batches of the prepared composites was less than 9%, indicating the high reproducibility of the preparation methods for the proposed composites. In addition, we also analyzed the degradation performances of Cu₂S, Fe₂S₃, Cu₂S/Ag₃PO₄ and Fe₂S₃/Ag₃PO₄ composites. As shown in Figure 10, the RhB degradation efficiency reaches 75.2%, 86.5%, 81.4% and 90.4% within 1 min in the presence of Cu₂S, Fe₂S₃, Cu₂S/Ag₃PO₄ and Fe₂S₃/Ag₃PO_4, respectively. These results suggested that the addition of CuFeS₂/Ag₃PO₄ promotes the production of hydroxyl radicals through the photo-Fenton reaction to degrade RhB at a higher efficiency than CuFeS₂, Cu₂S, Fe₂S₃ alone, indicating a synergistic effect between CuFeS₂ and Ag₃PO₄. Figure 9D–F showed the corresponded pseudo-first order linear transform of RhB degradation under photocatalytic reaction, Fenton reaction and photo-Fenton reaction by Ag₃PO₄, CuFeS₂, and CuFeS₂/Ag₃PO₄ composites. The apparent rate constants for RhB degradation by Ag₃PO₄, CuFeS₂, and CuFeS₂/Ag₃PO₄ composites are listed in

Table 1. Pseudo-first-order rate constants for rhodamine B (RhB) degradation by Ag₃PO₄, CuFeS₂, and CuFeS₂/Ag₃PO₄ composites at different conditions.

Series	Degradation Mode	Pseudo-First-Order Kinetic Equation	k(min−1)	R2
Ag3PO4	Photocatalytic reaction	y = 0.016x + 0.048	0.016	0.96
	Fenton reaction	y = 0.0026x + 0.0088	0.0026	0.92
	Photo-Fenton reaction	y = 0.0047x + 0.055	0.0047	0.80
CuFeS2	Photocatalytic reaction	y = 0.0007x − 0.039	0.0007	0.72
	Fenton reaction	y = 1.5x − 0.16	1.5	0.93
	Photo-Fenton reaction	y = 2.8x − 0.29	2.8	0.94
CuFeS2/Ag3PO4	Photocatalytic reaction	y = 0.018x − 0.0015	0.018	0.94
	Fenton reaction	y = 2.2x − 0.156	2.2	0.99
	Photo-Fenton reaction	y = 3.3x − 0.0322	3.3	0.97

.

To maximize the degradation performance of CuFeS₂/Ag₃PO₄, the following factors were systematically studied: the molar ratio of CuFeS₂ to Ag₃PO₄, the mechanical mixing of CuFeS₂ and Ag₃PO₄ particles, and the added amounts of H₂O₂ and CuFeS₂/Ag₃PO₄. Figure 11A presents the plots of the RhB concentration ratio (C/C₀) against the reaction time for CuFeS₂/Ag₃PO₄ prepared at different precursor molar ratios from CuFeS₂ to Ag₃PO₄. The degradation efficiency can be visually related to the highest drop in the RhB concentration ratio in the plots during the reaction. The higher molar ratio of the precursors used to make CuFeS₂/Ag₃PO₄ resulted in higher degradation efficiency as the hydroxyl radicals generated from the photo-Fenton reaction dominated the RhB degradation. Above the 2.5:1 ratio, the suspension of excessive black CuFeS₂ particles in the CuFeS₂/Ag₃PO₄ solution was observed. This raised a concern as the excessive CuFeS₂ particles were not coupled with Ag₃PO₄ during the synthesis, and they cannot be regenerated during the photolysis. CuFeS₂/Ag₃PO₄ with the molar ratio of 2.5:1 was therefore selected as the optimal condition for this study. The second experiment was performed by mechanically mixing both CuFeS₂ and Ag₃PO₄ particles with the molar ratio of 2.5:1 to prove the synergistic effect within CuFeS₂/Ag₃PO₄ in the enhanced degradation activity (Figure 11B). The degradation efficiency for the mechanically mixed sample with 10 min photocatalytic, Fenton, and photo-Fenton reactions is 4.2%, 83.5%, and 87.7%, respectively—all evidently lower than that for composites formed with cyclic microwave heating (Figure 9). These results strongly suggest that the coupling interaction between CuFeS₂ and Ag₃PO₄ is critical for the significantly enhanced degradation activity of CuFeS₂/Ag₃PO₄ composites. Then, because the amount of H₂O₂ is key for the photo-Fenton reaction, its content in the degradation process may influence the performance of CuFeS₂/Ag₃PO₄ composites. As shown in Figure 11C, the degradation efficiency increased with an increase in the H₂O₂ amount up to 200 μL, above which the degradation efficiency decreased, probably because excess reactive radicals react with one another. Thus, we selected 200 μL of H₂O₂ as the optimum required amount of H₂O₂. Then, the degradation efficiency at different amounts of CuFeS₂/Ag₃PO₄ composites was evaluated. As shown in Figure 11D, the degradation efficiency increased with an increase in the CuFeS₂/Ag₃PO₄ amount of up to 20 mg, above which the degradation efficiency plateaued. On the basis of this result, 20 mg of CuFeS₂/Ag₃PO₄ was used for further investigations. Finally, the degradation efficiency for a higher RhB concentration at 20 mg CuFeS₂/Ag₃PO₄ was evaluated. As shown in Figure 11E, the degradation efficiency at a higher RhB concentration decreased within 10 min of photo-Fenton reaction. However, the degraded RhB amount was higher at a higher RhB concentration. The results also indicated that the maximum degraded amount of RhB at 20 mg CuFeS₂/Ag₃PO₄ was 54.9 ppm.

According to the literature, deethylation and chromophore cleavage are analogous competitive photodegradation reactions during the photocatalytic decomposition of organic pollutants. Based on the literature, the hypsochromic shifts (blue shift of the maximum absorption band) are attributed to the formation of a series of N-deethylated intermediates of RhB [31,32]. In this study, the absorption at 550 nm for RhB decreased with the increase in the reaction time and exhibited a slight hypochromic shift (dash line in Figure 12A). Therefore, a possible method to degrade RhB is through chromophore cleavage, which can be observed with an insignificant hypochromic shift in the UV–Vis spectra. Another approach to study organic compound degradation is to measure the TOC of the solution before and after the reaction. Figure 12B plots the temporal evolution of TOC content with RhB degradation by CuFeS₂/Ag₃PO₄. Initially, the color vanished in the RhB solution with the photo-Fenton reaction for 1 min; the TOC content decreased to 13.5 ppm (decomposing only 2.17% of the original concentration (13.8 ppm)). The decomposition of RhB with CuFeS₂/Ag₃PO₄ increased to as high as 78.3%, but only 3.0 ppm TOC remained in the solution with a 30 min photo-Fenton reaction. Evidence from both experimental approaches strongly supports RhB chromophore cleavage and CO₂ production as the predominant pathways of degradation by CuFeS₂/Ag₃PO₄.

3.4. Degradation Mechanism of Ag₃PO₄, CuFeS_2, and CuFeS₂/Ag₃PO₄

In photodegradation, the rate and the stability of electron–hole pairs generated from the excitation source are critical factors to be considered in the development of ideal photocatalysts. In this study, the separation efficiency and recombination rate of the electron–hole pairs for the prepared Ag₃PO₄, CuFeS₂, and CuFeS₂/Ag₃PO₄ composites were investigated through photocatalytic degradation without any H₂O₂. Figure 13A shows the photocurrent density of Ag₃PO₄, CuFeS₂, and CuFeS₂/Ag₃PO₄ under LED light irradiation; all samples produced a sharp photocurrent peak on turning the light on and no peak as the light turned off, with photocurrent response during the on/off cycles. Under LED light irradiation, the current density decreased in the following order: Ag₃PO₄ > CuFeS₂/Ag₃PO₄ > CuFeS₂. This result is consistent with the photocatalytic degradation results shown in Figure 9A, indicating that the differences in the photodegradation activity was due to the differences in the optoelectrical properties of selected materials. In addition, the different noise levels and shape forms of the photocurrent density for different composites may be attributed to the different size and morphology of the composites deposited on the fluorine-doped tin oxide (FTO) substrates. As a further experiment, the intensity of PL emission for three materials was measured to quantify the recombination rate of the electron–hole pairs; a lower rate implies a lower luminescence emission intensity and higher photocatalytic activity. Figure 13B presents the PL emission spectra of the prepared Ag₃PO₄, CuFeS₂, and CuFeS₂/Ag₃PO₄ composites at λ_ex = 250 nm. The emission intensity of CuFeS₂ was the weakest due to the lowest yield of electron–hole pairs but not due to a low recombination rate. The difference in the emission intensity between Ag₃PO₄ and CuFeS₂/Ag₃PO₄ is nonsignificant, indicating comparable electron–hole pair recombination rates. This demonstrates the ability of CuFeS₂/Ag₃PO₄ to generate electron–hole pairs with a low recombination rate, providing critical information to understand the mechanism discussed later.

As a key mechanistic study, the active species involved in the degradation reaction were identified systematically using the free radical trapping experiments (Figure 14). EDTA, BQ, and t-BuOH were used as holes, oxygen radicals, and hydroxyl radical scavengers, respectively. After adding EDTA to the reaction mixture, RhB degradation in Ag₃PO₄ samples was inhibited, indicating that holes are the major species involved in the photo-Fenton degradation (Figure 14A). In contrast to the Ag₃PO₄ sample, RhB degradation in CuFeS₂ and CuFeS₂/Ag₃PO₄ composites with the photo-Fenton reaction was inhibited by adding t-BuOH, indicating that hydroxyl radicals are the major active species (Figure 14B,C).

Hydroxyl radical production was further detected using the fluorescent luminescence (FL) technique to study the photo-Fenton reaction. The FL emission spectra, excited at 370 nm in the coumarin solution in the absence and presence of the prepared samples, were evaluated for 10 min of irradiation. Figure 15 shows that an FL signal was observed at 460 nm for each sample and that the maximum FL intensity was observed in CuFeS₂/Ag₃PO₄. This suggests that CuFeS₂/Ag₃PO₄ produced the highest amount of hydroxyl radicals among these three materials, thereby leading to more chemical reactions with coumarin to generate fluorescence [33]. Hence, the hydroxyl radical was considered the direct reactive oxidation species in the CuFeS₂/Ag₃PO₄ composites for RhB degradation. Moreover, CuFeS₂/Ag₃PO₄ composites with maximal degradation activity produced excess reactive hydroxyl radicals than CuFeS₂, which is consistent with the previously discussed results.

On the basis of the results described above, the degradation mechanisms of the Ag₃PO₄, CuFeS_2, and CuFeS₂/Ag₃PO₄ in the photo-Fenton reaction were proposed (Scheme 1). For Ag₃PO₄, the electron–hole pairs are generated under LED irradiation (Scheme 1A). The electrons could not form the oxygen radicals because of their higher position in the standard redox potential of oxygen/oxygen radical (0.13 V) than the CB potential (0.27 V). The photogenerated hole can produce the hydroxyl radical because the oxidation potential of the hydroxyl radical (1.99 V) is lower than the VB potential of Ag₃PO₄ (2.65 V). However, the recombination rate in Ag₃PO₄ is too fast to produce sufficient hydroxyl radicals by the holes. Thus, the holes are responsible for RhB degradation. The poor charge separation of CuFeS₂ limits its ability to generate electron–hole pairs, but it is effective in generating hydroxyl radicals through the Fenton reaction (Scheme 1B). However, the Fenton reaction causes an increase in the oxidation state of Cu and Fe within the CuFeS₂ solids, alters the crystal structure, and notably, weakens the integrity of the CuFeS₂ solids. This is the main reason CuFeS₂ is hardly reused in water treatment. When Ag₃PO₄ couples with CuFeS₂ to form the CuFeS₂/Ag₃PO₄ composites, the photogenerated electrons can be easily captured by the oxidized Fe³⁺/Cu²⁺ on the surface of CuFeS₂, leading to the regeneration of Fe²⁺/Cu⁺ at the CuFeS₂/Ag₃PO₄ interface (Scheme 1C). As a strong synergistic effect, the formation of hydroxyl radicals through the Fenton reaction on CuFeS₂ and photogenerated hole on Ag₃PO₄ is favored to enhance the degradation activity and stability of CuFeS₂/Ag₃PO₄ composites. In addition, we considered that the degradation mechanism for the prepared Fe₂S₃/Ag₃PO₄ and Cu₂S/Ag₃PO₄ composites was similar to that of CuFeS₂/Ag₃PO₄ composites. However, the degree of the charge separation efficiency, recombination rate of electron–hole pairs and the amount of the major reactive species were slight different, resulting in different degradation performance as shown in Figure 10.

3.5. Stability and Practical Applications of CuFeS₂/Ag₃PO₄

The stability of the catalyst is an essential parameter for the development of practical water treatment applications. To investigate the stability of the prepared Ag₃PO₄, CuFeS₂, and CuFeS₂/Ag₃PO₄ composites, the results of cyclic RhB degradation tests were evaluated (shown in Figure 16A); in each cycle, RhB and H₂O₂ were reintroduced into the catalyst. In this study, RhB degradation by Ag₃PO₄ and CuFeS₂/Ag₃PO₄ after ten cycles maintained a similar degradation efficiency (15.3% to 14% after ten cycles for Ag₃PO₄; 99.6% to 91.3% after ten cycles for CuFeS₂/Ag₃PO₄), whereas that by CuFeS₂ resulted in a considerable loss of efficiency (from 97.5% to 15.0% after ten cycles). Furthermore, the corresponding XRD results (Figure 16B) suggest a negligible change in the phase structure of Ag₃PO₄ and CuFeS₂/Ag₃PO₄ samples after the repeated reactions, indicating the good stability of the samples. However, the initial phase structure of CuFeS₂ disappeared after three reaction cycles. The SEM image shown in Figure 16D displays that the morphology of the CuFeS₂ sample changed from an irregular sheet to spherical shape, indicating that the Fenton reaction caused structural and chemical change for CuFeS₂ when it is used alone. Unlike CuFeS₂, the comparison of SEM images for Ag₃PO₄ and CuFeS₂/Ag₃PO₄ composites before and after reaction cycles only showed a slight change of morphology (Figure 16C,E). Evidently, the stability of CuFeS₂ in the CuFeS₂/Ag₃PO₄ composite improved considerably because of its coupling with Ag₃PO₄ nanoparticles as the photogenerated electrons in the CB of Ag₃PO₄ reduce Fe³⁺/Cu²⁺ ions and keep them intact in CuFeS₂/Ag₃PO₄. This prevented the structural disintegration of CuFeS₂ during the Fenton reaction, effectively demonstrating its reusability for catalysis.

To assess the practical applications of CuFeS₂/Ag₃PO₄, various organic dyes (MR, M6G, fluorescein, and PI) were degraded (Figure 17A). Compared with TiO₂ (P25), CuFeS₂/Ag₃PO₄ exhibited excellent degradation efficiency toward all selected dyestuffs, with nearly 95% degradation achieved within 10 min (except PI with only 78.5% degradation efficiency). In addition, the degradation performance of CuFeS₂/Ag₃PO₄ under sunlight irradiation was evaluated from November to December 2019 from 11:00 a.m. to 2:00 p.m. daily at the National Changhua University of Education, Changhua, Taiwan. As shown in Figure 17B, sunlight-induced RhB degradation in the absence and presence of CuFeS₂/Ag₃PO₄ was very poor without H₂O₂. However, using the sunlight-assisted Fenton reaction for RhB degradation achieved nearly 98.9% degradation efficiency within 1 min. This is because the combined UV and visible light in sunlight hastened the production of hydroxyl radicals in the presence of H₂O₂ and CuFeS₂/Ag₃PO₄. With the promise of the sunlight-assisted Fenton reaction, the pH effect in the water samples on the degradation performance of CuFeS₂/Ag₃PO₄ was also investigated. As shown in Figure 17C, the degradation efficiency decreased at pH 12.0 because the Fenton reaction was considerably hindered at a high pH as Fe²⁺ cations form inactive porphyrin ferryl complexes (FeO²⁺) in the alkaline solution [24]. Finally, the prepared CuFeS₂/Ag₃PO₄ composites were used to degrade RhB in the environmental water samples (Figure 17D). CuFeS₂/Ag₃PO₄ exhibited excellent degradation efficiency through the photo-Fenton reaction for RhB degradation, with nearly 90% degradation within 1 min. A notable difference in the degradation time for RhB was observed for the seawater and treated wastewater samples (90% and 80% degradation within 10 min, respectively) compared with the other environmental water samples (100% degradation within 10 min) probably because of the presence of anions or radical scavengers in the seawater and treated wastewater samples that reduced the degradation activity of CuFeS₂/Ag₃PO₄. Nevertheless, the studies on the environmental water samples strongly support the benefits of this newly developed CuFeS₂/Ag₃PO₄-based photo-Fenton water treatment option.

4. Conclusions

The currently prepared CuFeS₂/Ag₃PO₄ composites exhibited higher RhB degradation efficiency through the photo-Fenton reaction than did Ag₃PO₄ and CuFeS₂ alone. This high enhancement in the degradation efficiency was attributed to the synergistic effect in material stability and the hydroxyl radical production. The constituent Ag₃PO₄ in the newly developed composite not only provides the visible-light absorption ability in degrading organic compounds but also acts as a rich electron source to stabilize the crystal structure of CuFeS₂ under light irradiation. Consequently, Cu²⁺/Fe³⁺ ions produced by the Fenton reaction can be reduced and regenerated into Cu⁺/Fe²⁺ ions, and the reactive hydroxyl radicals partially from the photogenerated holes of Ag₃PO₄ and predominantly from the Fenton reaction of CuFeS₂ can be continuously produced to degrade organic compounds. The CuFeS₂/Ag₃PO₄ composite has several attractive features not realized in the other reported photo-Fenton reactions (

Table 2. Comparison of the degradation performance and practical applications using the photo-Fenton reaction.

Samples	Preparation	Degradation Performance	Sunlight Irradiation	Target	Ref.
Citrate–CuFeS2	Microwave heating	90% degradation (0.2 g catalyst/50 ppm BPA) within 15 min (4 W fluorescent lamp)	-	BPA	[24]
FS–TiO2 disk	Dip-coating method	95% degradation (50 ppm phenazone (PNZ)) within 180 min (36 W UV light)	95% degradation of 50 ppm PNZ within 90 min	PNZ	[34]
Mined CuFeS2	Milling process	85% TOC conversion (1.0 g catalyst/0.5 mM tyrosol (TY)) within 60 min (10 W UV LED light)	-	TY	[19]
Fe–N–Ag–TiO2 clay bead	Surface impregnation method	-	77% degradation of 50 ppm cephalexin (CEX) within 60 min	CEX	[35]
FS/FA/TiO2 clay bead	Dip-coating method	89% degradation (50 ppm CEX) within 4 h (36 W UV light)	94% degradation (50 ppm CEX) within 3.5 h	CEX	[36]
Fe2O3–TiO2 film	Sol–gel method	80% degradation (10 ppm ciprofloxacin (CIPRO), sulfamethoxazole (SMX), and trimethoprim (TMP) mixture) within 240 min by a solar simulator (Solarbox Model 1500e)	-	CIPRO, SMX, TMP	[37]
CuFeS2/Ag3PO4	Cyclic microwave heating	96% degradation (20 mg catalyst/20 ppm RhB) within 1 min (2.5 W white-light LED)	99.8% degradation (0.15 g catalyst/15 ppm MB) within 6 min	RhB, MR, R6G, Fluorescein, PI, phenol	This study

). First, the prepared CuFeS₂/Ag₃PO₄ composite had 96% RhB degradation performance under low-power white LED illumination within 1 min. In addition, various dyestuffs (MR, R6G, fluorescein, and PI) with 95% degradation efficiency could be achieved. Through sunlight-assisted Fenton reaction, the RhB degradation efficiency was further improved to 98.9%. For the recycling used ability, the CuFeS₂/Ag₂PO₄ composite is stable enough to be reused through the input of sustainable energy source. Hence, this study discovered the synergistic catalysis CuFeS₂/Ag₃PO₄ and successfully demonstrates the application of the sunlight-assisted Fenton reaction on environmental water samples. The current findings can be used for the applications of advanced oxidation technology in wastewater treatment in the future.