**4. Conclusions**

The prepared H-CuFeS<sup>2</sup> samples showed higher RhB degradation efficiency through the Fenton-like reaction than the prepared C-CuFeS2, FeS2, Cu2S 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<sup>2</sup> samples possessed smaller size and higher surface area. Based on the results of scavenger test and radicals' quantitation experiments, H-CuFeS<sup>2</sup> catalyzed Na2S2O<sup>8</sup> to produce •SO<sup>4</sup> − 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<sup>2</sup> samples possessed several attractive features. First, the prepared H-CuFeS<sup>2</sup> samples in the presence of Na2S2O<sup>8</sup> 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<sup>2</sup> showed performance deterioration due to the change in the crystal phase of used H-CuFeS2. Further research on the high recycling-used ability of other heterojunction CuFeS<sup>2</sup> composites, such as those doped by Ag@Ag3PO<sup>4</sup> nanoparticles, is now underway in our laboratory. Finally, the prepared H-CuFeS<sup>2</sup> samples were used to degrade RhB with 10.1% mineralization improvement comparing to traditional Fenton reaction (Fe2+/H2O2). It is also easy to recover H-CuFeS<sup>2</sup> catalyst comparing to Fe2+ ions. In addition, H-CuFeS<sup>2</sup> 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<sup>2</sup> 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.


**Table 1.** Comparison of degradation performance using the (photo-) Fenton-like reaction.

In summary, this study discovered the hydrothermal synthesis of CuFeS<sup>2</sup> 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.

**Author Contributions:** Conceptualization, P.-Y.W. and Y.-W.L.; methodology, Y.-W.L.; software, Y.-W.L.; validation, P.-Y.W., T.-Y.L. and T.W.; formal analysis, P.-Y.W., T.-Y.L. and Y.-W.L.; investigation, P.-Y.W.; resources, Y.-W.L.; data curation, P.-Y.W. and T.-Y.L.; writing—original draft preparation, Y.-W.L.; writing—review and editing, T.W. and Y.-W.L.; visualization, Y.-W.L.; supervision, Y.-W.L.; project administration, Y.-W.L.; funding acquisition, Y.-W.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by the Ministry of Science and Technology of Taiwan under contract (MOST 110-2113-M-018-001).

**Conflicts of Interest:** The authors declare no conflict of interest.
