**1. Introduction**

The cosmetics market is booming and it is one of the fastest growing consumer markets. It globally generated EUR 474.2 billion in 2019. The coronavirus pandemic resulted in a decrease in industry revenues in 2020 by only 1.2%, to EUR 468.3 billion [1].

The constantly increasing production of cosmetics is accompanied by the side effect of producing increasing amounts of waste and wastewater. Cosmetic wastewater (CW) is created by washing production lines with water with surfactants and disinfectants, so CW contains the same compounds as those that are present in cosmetics. A typical industrial-scale CW treatment method is coagulation coupled with dissolved air flotation (C/DAF) followed by biological treatment [2,3]. This method is highly effective, but not enough [4] to remove micropollutants considered to be particularly harmful, such as polycyclic musk, UV filters, heavy metals, and microplastics [5–11]. Fragrances and UV filters are contaminants of emerging concern (CEC) [5]. The most commonly used and thus detected in the polycyclic musk environment are galaxolide (1,3,4,6,7,8-hexahydro-4,6,6,7,8,8,-hexamethyl-cyclopenta[g]benzopyran, HHCB) and tonalide (6-acetyl-1,1,2,4,4,7 hexamethyltetraline, AHTN), while the most important UV filters are benzophenone-3 (2-hydroxy-4-methoxyphenyl)-phenylmethanone, BP-3) and 4-MBC (4-methylbenzylidene camphor). These compounds often have the potential for bioaccumulation and also show estrogenic activity [6]. Heavy metals such as Zn, Cu, and Fe are typically used in cosmetics as physical UV filters, dyes, or enzyme components. However, even metals such as silver or bismuth are used as bactericides or mask ingredients [7]. Plastics are usually chemically

**Citation:** Bogacki, J.; Marcinowski, P.; Bury, D.; Krupa, M.; Scie ´ zy ´nska, D.; ˙ Prabhu, P. Magnetite, Hematite and Zero-Valent Iron as Co-Catalysts in Advanced Oxidation Processes Application for Cosmetic Wastewater Treatment. *Catalysts* **2021**, *11*, 9. https://dx.doi.org/10.3390/catal110 10009

Received: 20 November 2020 Accepted: 22 December 2020 Published: 24 December 2020

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inert, but under environmental conditions, they are broken down into microscopic grains that penetrate the body even at the cellular level. Their content in organisms increases as they move up the food chain [8]. Due to their persistence to decomposition, they form layers or even islands floating on the water or they accumulate in the soil or bottom sediments, depending on their density [9]. There is a need to develop a policy for dealing with substances that are components of cosmetics [10]. Cosmetic micropollutants during treatment in a biological treatment plant do not decompose but pass into the sludge phase [6]. Their presence is detected in globally collected environmental samples, in concentrations usually below 100 µg/L or 100 µg/kg, depending on sample type [11].

In order to increase the effectiveness of CW treatment, the possibilities of improving classically used coagulation and DAF processes were investigated [12–15]. Advanced oxidation processes (AOPs) [15–19] and the improvement of biological treatment [20–23] were also tested. Attempts were also made to improve the entire treatment, including both chemical and biological methods [24–26].

Many alternatives to classical coagulation and DAF for CW treatment technologies are being developed. Promising ones are AOPs, consisting of the effective generation of strong oxidants, namely radicals. In the case of AOPs in which the production of radicals is catalyzed by the presence of Fe2+ ions (Fenton's process and its modifications), a major problem [27] is to ensure the appropriate quantity and availability of Fe2+ ions. The amount of Fe2+ ions in a solution is influenced by many factors, including pH, the efficiency of Fe2+ ion recovery from Fe3+, and the rate of Fe2+ ion release from the carrier. This problem is solved in two ways: by controlling the Fe2+/Fe3+ ions ratio or by the controlled continuous introduction of Fe2+ ions into the solution. Both strategies pose numerous technical difficulties when applied in practice; therefore, iron-based heterogeneous cocatalysts are gaining interest. Among them are Fe<sup>0</sup> (metallic iron, zero-valent iron, ZVI), Fe2O<sup>3</sup> (hematite), and Fe3O<sup>4</sup> (magnetite) [28–31]. Oxides act through coordinating surface sites of Fe2+ that form complexes with contaminants and reduce them [28].

The aim of this study is to determine the effectiveness of the joint use of Fe<sup>0</sup> , Fe2O3, and Fe3O<sup>4</sup> as mutually supportive catalysts using synergy effects in the AOP treatment of industrial wastewater. This is the first article where Fe2O3, Fe3O4, and Fe<sup>0</sup> were mutually used in one process as co-catalysts supporting modified Fenton processes to treat cosmetic wastewater.
