**1. 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 [2–8].

Among many treatment processes, the in-site chemical oxidation method is to inject and mix oxidants into the underground environment aiming to degrade pollutants

**Citation:** Wen, P.-Y.; Lai, T.-Y.; Wu, T.; Lin, Y.-W. Hydrothermal and Co-Precipitated Synthesis of Chalcopyrite for Fenton-like Degradation toward Rhodamine B. *Catalysts* **2022**, *12*, 152. https:// doi.org/10.3390/catal12020152

Academic Editor: Roberto Fiorenza

Received: 31 December 2021 Accepted: 25 January 2022 Published: 26 January 2022

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in groundwater and soil [9–11]. Under ideal conditions, this chemical treatment can convert organic pollutants into less toxic molecules, such as carbon dioxide, water, and inorganic salts. The commonly used oxidants are permanganate (MnO<sup>4</sup> −), Fenton reagent (Fe2+/H2O2), and ozone (O3) [12–15]. The mechanism of Fenton reagent (Fe2+/H2O2) in reactions has been known to produce •OH radicals, which can cleave C–H bonds of organic compounds, turning them into environmentally benign final products [16,17]. However, this Fenton method has some drawbacks including the specific pH working range and Fe sludge precipitation at the end of the Fenton reaction [18,19]. Another alternative way to generate radicals with a wider pH working range is by using persulfate salts [20–26]. Persulfate salts can come from two types: peroxymonosulfate (HSO<sup>5</sup> −) and peroxydisulfate (S2O<sup>8</sup> <sup>2</sup>−), both of which contain an O–O bond (peroxide group) capable of generating •SO<sup>4</sup> <sup>−</sup> radicals and •OH radicals in Fenton-like reaction for degradation of organic compounds [27,28]. Persulfate salts are strong oxidant (E<sup>0</sup> = 2.1 V), yet they are very stable for transportation and prolong storage making them very attractive oxidants for underground water treatment [29].

There are a few ways of activating persulfate to generate radicals, such as thermal decomposition, alkaline activation, transition metal ions activation, and heterogeneous catalysis [30]. Among them all, transition metal ions activation is considered the simplest and most benign method with no external energy requirement and recyclability of transition metal ions [31–34]. Cobalt ions are commonly used in activating persulfate in research, but their hazardous nature makes them unsuitable for water treatment [35,36]. Thus, it is necessary to find an alternative transition metal catalyst that can be used in water treatment.

Recently, Cu/Fe-bearing solids such as chalcopyrite (CuFeS2) have been widely used as catalysts in advanced oxidation processes (AOPs) for wastewater treatment [37–41]. For instance, Dotto et al. demonstrated the ability of their prepared citrate-CuFeS<sup>2</sup> materials to degrade 90% of bisphenol A (BPA) in a 15-min Fenton process [42]. Their novel CuFeS<sup>2</sup> samples were prepared with a microwave reactor (1400 W, 200 ◦C, 7 min). Pastrana-Martinez et al. used the mineral of CuFeS<sup>2</sup> mined from Jendouba, Tunisia, to catalyze tyrosol degradation (85.0% degradation within 60 min) by using a UV light-assisted Fenton reaction [43]. However, for ground water treatment, this method requires UV light as external energy requirement. Chang et al. proposed that the microwave-assisted synthesis of CuFeS2/Ag3PO<sup>4</sup> with enhanced rhodamine B (RhB) degradation (96% degradation within 1 min) under visible light-Fenton process [44]. However, these methods also need light irradiation to improve the degradation performance of CuFeS2.

Herein, we synthesized CuFeS<sup>2</sup> samples through hydrothermal and co-precipitated method to realize the advantages in material preparation, stability of materials, and degradation performance in new water treatment option. We expected higher temperature and pressure treatment (hydrothermal process) to make the prepared particles with small size and high special surface area compared to the co-precipitated process, resulting in higher catalytic activity [45–47]. In order to prove this, the prepared CuFeS<sup>2</sup> samples in the presence of Na2S2O<sup>8</sup> were used to evaluate the degradation efficiency of various dyestuff (RhB, rhodamine 6G [R6G], methylene blue [MB], methyl orange [MO], and BPA). The degradation mechanism of CuFeS<sup>2</sup> was elucidated and the reactive species were identified. Finally, the practical applications of CuFeS<sup>2</sup> samples in the treatment of environmental samples were demonstrated.
