*2.3. Degradation Mechanism of H-CuFeS<sup>2</sup>*

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<sup>3</sup> were used as •OH and •SO<sup>4</sup> − scavengers, respectively. Comparing to methanol, NaN<sup>3</sup> inhibit RhB degradation more, indicating that •SO<sup>4</sup> − 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, Fe2+/Cu<sup>+</sup> ions on the CuFeS<sup>2</sup> surface catalyzed S2O<sup>8</sup> <sup>2</sup><sup>−</sup> to produce •SO<sup>4</sup> <sup>−</sup> radicals (Equations (1) and (2)). Due to high oxidation activity of •SO<sup>4</sup> − radicals (E<sup>0</sup> = 2.5–3.1 V), they were utilized to degrade dyes and to oxidize Fe2+/Cu<sup>+</sup> ions (Equations (3)–(5)). Then, •OH radicals also produced from the oxidation reaction between •SO<sup>4</sup> <sup>−</sup> radicals and H2O/OH<sup>−</sup> to degrade the dyes (Equations (6)–(8)). Thus, after adding methanol to the reaction mixture, RhB degradation in CuFeS<sup>2</sup> samples was slightly decreased, indicating that production of •OH radicals are considered as the indirect active species in the CuFeS<sup>2</sup> catalyzed RhB degradation (red curve in Figure 11A).

$$\text{Fe}^{2+} + \text{S}\_2\text{O}\_8\text{}^{2-} \rightarrow \text{Fe}^{3+} + \text{SO}\_4\text{-}^- + \text{SO}\_4\text{}^{2-} \tag{1}$$

$$\text{Cu}^+ + \text{S}\_2\text{O}\_8^{2-} \rightarrow \text{Cu}^{2+} + \text{SO}\_4\text{-}^- + \text{SO}\_4^{2-} \tag{2}$$

$$\text{CO}\_4\text{-}^- + \text{RhB} \rightarrow \text{CO}\_2 + \text{H}\_2\text{O} \tag{3}$$

$$\text{+SO}\_4\text{-}^- + \text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + \text{SO}\_4{}^{2-} \tag{4}$$

$$\text{SO}\_4\text{-}^- + \text{Cu}^+ \rightarrow \text{Cu}^{2+} + \text{SO}\_4\text{2}^- \tag{5}$$

$$\text{SO}\_4\text{-}^- + \text{H}\_2\text{O} \rightarrow \text{SO}\_4^{2-} + \cdot\text{OH} + \text{H}^+ \tag{6}$$

$$\cdot\text{SO}\_4\cdot^- + \text{OH}^- \rightarrow \text{SO}\_4^{2-} + \cdot\text{OH} \tag{7}$$

$$\cdot\text{OH} + \text{RhB} \rightarrow \text{CO}\_2 + \text{H}\_2\text{O} \tag{8}$$

**Figure 11.** (**A**) Free radical trapping experiment and (**B**) absorbance spectra at different condition. **Figure 11.** (**A**) Free radical trapping experiment and (**B**) absorbance spectra at different condition.

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 S2O82− and Fe2+/Cu+ ions on the H-CuFeS2 surface to degrade RhB (Equations (1)–(8)). Then, Fe2+/Cu+ ions were regenerated through a series reduction of S2− anions (Equations (11)–(13)). Moreover, it is also possible to produce Fe2+ ions by reduction reaction between Cu+ and Fe3+ ions (Equation (14)). •SO<sup>4</sup> − radical production in the Fenton-like reaction was further studied using the spectrophotometric method [50]. According to Equations (9) and (10), I<sup>3</sup> − solution (light yellow) was found from chemical reaction between S2O<sup>8</sup> <sup>2</sup><sup>−</sup> and KI. The absorbance spectra of the S2O<sup>8</sup> <sup>2</sup>−/KI solution in the absence and presence of the prepared CuFeS<sup>2</sup> 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<sup>2</sup> samples (blue curve in Figure 11B). This suggests that S2O<sup>8</sup> <sup>2</sup><sup>−</sup> produced the highest amount of I<sup>2</sup> compared to others, thereby leading to more chemical reactions with KI to generate I<sup>3</sup> <sup>−</sup>. Due to a high specific surface area and high content of Fe2+ ions, H-CuFeS<sup>2</sup> effectively catalyzed S2O<sup>8</sup> <sup>2</sup><sup>−</sup> to produce •SO<sup>4</sup> − radicals, as a result of a few I<sup>2</sup> production. Thus, the absorbance intensity at 358 nm of H-CuFeS2/S2O<sup>8</sup> <sup>2</sup>−/KI mixing solution (black

curve in Figure 11B) was lower than that of C-CuFeS2/S2O<sup>8</sup> <sup>2</sup>−/KI mixing solution (red curve in Figure 11B).

$$\mathrm{S\_2O\_8}^{2-} + 2\mathrm{I}^- \to 2\mathrm{SO\_4}^{2-} + \mathrm{I\_2} \tag{9}$$

$$\text{I}\_2 + \text{KI} \rightarrow \text{I}\_3^- + \text{K}^+ \tag{10}$$

On the basis of the results described above, the degradation scheme of the H-CuFeS<sup>2</sup> samples in the Fenton-like reaction was proposed (Scheme 1). •SO<sup>4</sup> <sup>−</sup> radicals and •OH radicals were produced from the Fenton-like reaction between S2O<sup>8</sup> <sup>2</sup><sup>−</sup> and Fe2+/Cu<sup>+</sup> ions on the H-CuFeS<sup>2</sup> surface to degrade RhB (Equations (1)–(8)). Then, Fe2+/Cu<sup>+</sup> ions were regenerated through a series reduction of S2<sup>−</sup> anions (Equations (11)–(13)). Moreover, it is also possible to produce Fe2+ ions by reduction reaction between Cu<sup>+</sup> and Fe3+ ions (Equation (14)). *Catalysts* **2022**, *11*, x FOR PEER REVIEW 13 of 21

$$\rm{S^{2-}} + \rm{Fe^{3+}/Cu^{2+}} \rightarrow \rm{Fe^{2+}/Cu^{+}} + \rm{S\_2}^{2-} \tag{11}$$

$$\rm{^2S\_2^{2-}} + \rm{Fe^{3+}/Cu^{2+}} \rightarrow \rm{Fe^{2+}/Cu^{+}} + \rm{S\_n}^{2-} \tag{12}$$

$$\rm{S\_n}{^{2-}} + \rm{Fe}^{3+} / \rm{Cu}^{2+} \rightarrow \rm{Fe}^{2+} / \rm{Cu}^{+} + \rm{SO\_4}{^{2-}} \tag{13}$$

$$\rm Cu^{+} + Fe^{3+} \rightarrow Fe^{2+} + Cu^{2+} \tag{14}$$

**Scheme 1.** Possible scheme of Fenton-like reaction for the H-CuFeS2 samples. **Scheme 1.** Possible scheme of Fenton-like reaction for the H-CuFeS<sup>2</sup> samples.

### *2.4. Stability and Practical Applications of H-CuFeS2 2.4. Stability and Practical Applications of H-CuFeS<sup>2</sup>*

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 (FeO2+) are formed as Fe2+ ions in the alkaline solution. As a result, a weakened 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-CuFeS<sup>2</sup> 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<sup>4</sup> <sup>−</sup> 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 (FeO2+) are formed as Fe2+ ions in the alkaline solution. As a result, a weakened degradation result at pH 10.0 was found (Figure 12).

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 Cu2+ 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, Fe2+/Cu+ ions were regenerated through reduction between S2− anions and Fe3+/Cu2+ ions (Equations (11)– In the study of copper ion effect as shown in Figure 13, RhB degradation efficiencies by H-CuFeS<sup>2</sup> in the presence of Cu<sup>+</sup> 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 Cu2+ 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<sup>4</sup> − radicals are produced in the presence of Cu<sup>+</sup> ions (Equation (2)). In addition, Fe2+/Cu<sup>+</sup> ions were regenerated through reduction between S2<sup>−</sup> anions and Fe3+/Cu2+ ions (Equations (11)–(13)). As a result, an improve degradation at pH 10.0 was found.

(13)). As a result, an improve degradation at pH 10.0 was found.

system.

*Catalysts* **2022**, *11*, x FOR PEER REVIEW 14 of 21

**Figure 12.** Fenton-like reaction for RhB degradation by the H-CuFeS2 samples at different pH value **Figure 12.** Fenton-like reaction for RhB degradation by the H-CuFeS<sup>2</sup> samples at different pH value system. system.

**Figure 12.** Fenton-like reaction for RhB degradation by the H-CuFeS2 samples at different pH value

**Figure 13.** RhB degradation efficiency by the H-CuFeS2 samples at different pH value system in the presence of copper ions. **Figure 13.** RhB degradation efficiency by the H-CuFeS<sup>2</sup> samples at different pH value system in the presence of copper ions.

**Figure 13.** RhB degradation efficiency by the H-CuFeS2 samples at different pH value system in the

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,

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

presence of copper ions.

underway in our laboratory.

destruction of the H-CufeS2 sample crystalization. In addition, EDS spectrum found that the atomic ratio (Cu:Fe:S) for the third used H-CuFeS2 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@Ag3PO4 nanoparticles, is now

**Figure 14.** (**A**) Fenton-like reaction for RhB degradation by the H-CuFeS2 samples for the recycling -used test, (**B**) XRD, (**C**) Raman spectra, and (**D**) SEM image of the 3rd used samples. **Figure 14.** (**A**) Fenton-like reaction for RhB degradation by the H-CuFeS<sup>2</sup> samples for the recycling -used test, (**B**) XRD, (**C**) Raman spectra, and (**D**) SEM image of the 3rd used samples.

To assess the practical applications of H-CuFeS2 as a new water treatment option, various dyes (R6G, MB, and MO) and colorless organic compound (BPA) were tested (Figure 15A). H-CuFeS2 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-CuFeS2 comparing to a traditional Fenton reaction (Fe2+/H2O2) was evaluated. From the TOC analysis (Figure 15B), mineralization efficiency for the Fe2+/H2O2 and H-CuFeS2/S2O82− system was 70.0% and 80.1%, respectively, representing 10.1% improvement of RhB degradation. Finally, the prepared H-CuFeS2 samples were used to degrade RhB in the environmental water samples (pond water and seawater). H-CuFeS2 exhibited adequate mineralization effi-For recyling-used study, Figure 14A showed RhB degradation by H-CuFeS<sup>2</sup> 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<sup>2</sup> samples after the repeated reactions, indicating the destruction of the H-CufeS<sup>2</sup> sample crystalization. In addition, EDS spectrum found that the atomic ratio (Cu:Fe:S) for the third used H-CuFeS<sup>2</sup> 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@Ag3PO<sup>4</sup> nanoparticles, is now underway in our laboratory.

ciency 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-CuFeS2. Nevertheless, the studies on the environmental water samples strongly support the benefits of this newly developed H-CuFeS2-based Fenton-like water treatment option. To assess the practical applications of H-CuFeS<sup>2</sup> as a new water treatment option, various dyes (R6G, MB, and MO) and colorless organic compound (BPA) were tested (Figure 15A). H-CuFeS<sup>2</sup> 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<sup>2</sup> comparing to a traditional Fenton reaction (Fe2+/H2O2) was evaluated. From the TOC analysis (Figure 15B), mineralization efficiency for the Fe2+/H2O<sup>2</sup> and H-CuFeS2/S2O<sup>8</sup> <sup>2</sup><sup>−</sup> system was 70.0% and 80.1%, respectively, representing 10.1% improvement of RhB degradation. Finally, the prepared H-CuFeS<sup>2</sup> samples were used to degrade RhB in the environmental water samples (pond water and seawater). H-CuFeS<sup>2</sup> 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-CuFeS2. Nevertheless, the studies on the environmental water samples strongly support the benefits of this newly developed H-CuFeS2-based Fenton-like water treatment option.

**Figure 15.** Fenton-like reaction of (**A**) different dyestuff by the H-CuFeS2 samples, (**B**) TOC analysis of different degradation systems by using Fe(II) and the H-CuFeS2 samples in the different environmental water samples. **Figure 15.** Fenton-like reaction of (**A**) different dyestuff by the H-CuFeS<sup>2</sup> samples, (**B**) TOC analysis of different degradation systems by using Fe(II) and the H-CuFeS<sup>2</sup> samples in the different environmental water samples.
