**3. Discussion**

An innovative solution was applied that has not yet been used as a cosmetic wastewater treatment technique, nor has it appeared in other industries. After separate analysis of the effectiveness of each catalyst, metallic iron with hematite and metallic iron with magnetite [33], the compounds were combined to create a unique mixture in order to check its properties and effectiveness during the treatment of cosmetic wastewater. The highest efficiency of the process was achieved when using the catalyst proportion in which there was a significant advantage of metallic iron and a comparable lower dose of magnetite and hematite (quantitative ratio of the compounds was 250/250/1500 mg/L). The lowest efficiency of the process was obtained when the catalyst was used, in which a significantly lower dose of metallic iron of 250 mg/L, and slightly higher concentrations of hematite and magnetite were used, 375 mg/L in all cases. When using a total concentration of 2000 mg/L of the mixed catalyst, the process was the most effective. The lowest concentrations were obtained with the use of a lower dose of 1000 mg/L reagent. The lower dose was insufficient to efficiently perform oxidation. When a higher dose of hydrogen peroxide was used, the process was also not as effective as when a higher dose of the mixed catalysts was used.

Analyzing the obtained results, they complied with those of earlier research [33], indicating that the excess of hydrogen peroxide adversely affects the performed process, decreasing its effectiveness.

The use of a higher dose of the oxidant caused a lower efficiency in TOC removal from CW. Excess hydrogen peroxide is an inhibitor of the reaction. Then, a process may take place (reaction Equation (5)) where a hydroperoxide radical is formed (oxidationreduction potential 1.7 V) which is much less reactive than the hydroxyl radical (2.8 V). Hydroperoxide radicals react with hydroxyl radicals (reaction Equation (6)) to form a water molecule and an oxygen molecule. Reactive molecules merge with each other to form substrates that are not strong oxidants.

$$\text{HO}^{\bullet} + \text{H}\_2\text{O}\_2 \rightarrow \text{HO}\_2^{\bullet} + \text{H}\_2\text{O} \tag{5}$$

$$\rm{HO}\_2\rm{}^{\bullet} + \rm{HO}^{\bullet} \rightarrow \rm{H}\_2\rm{O} + \rm{O}\_2\tag{6}$$

$$\text{Fe}^{2+} + \text{H}\_2\text{O}\_2 \rightarrow \text{FeO}^{2+} + \text{H}\_2\text{O} \tag{7}$$

$$\text{FeO}^{2+} + \text{H}\_2\text{O}\_2 \rightarrow \text{Fe}^{2+} + \text{O}\_2 + \text{H}\_2\text{O} \tag{8}$$

$$\text{FeO}^{2+} + \text{RH} \rightarrow \text{Fe}^{2+} + \text{ROH} \tag{9}$$

$$\text{[Fe(C}\_2\text{O}\_4)] + \text{H}\_2\text{O}\_2 \rightarrow \text{[Fe(C}\_2\text{O}\_4)]^+ + \text{OH}^- + \text{OH}^\bullet \tag{10}$$

$$\text{[Fe(C}\_2\text{O}\_4)\_3]^{3-} + \text{hv} \rightarrow [\text{Fe(C}\_2\text{O}\_4)\_2]^{2-} + \text{C}\_2\text{O}\_4 \text{°{}^{\bullet}\$} \tag{11}$$

$$\text{Fe}^{3+} + \text{C}\_2\text{O}\_4^{\bullet-} \rightarrow \text{Fe}^{3+} + 2\text{CO}\_2 \tag{12}$$

$$\text{Fe(RCOO)}^{2+} + \text{hv} \rightarrow \text{Fe}^{2+} + \text{R}^{\bullet} + \text{CO}\_2 \tag{13}$$

Intermediate compounds are formed in the catalytic cycle. One is oxoiron FeIVO [34], which is formed as a result of the reaction initiating the Fenton process, described by reaction Equation (7).

With an excess of hydrogen peroxide, oxoion reacts with it and forms Fe2+, oxygen, and water (reaction (8)). This is a reaction that stops the process. Oxoiron is also involved in the oxidation of organics (reaction (9)), which is much slower than the one with the hydroxyl radical.

Factors accelerating the formation of radicals may be ligands present in CW, which form complexes and chelates with iron. The oxalate ligand reacts with hydrogen peroxide to form a hydroxyl radical according to reaction Equation (10). Oxalates (diethyl, dimethyl, diisopropyl, diisobutyl, sodium) are present in cosmetics and thereby also in CW. In acidic conditions, at a low pH of 3.0, acid hydrolysis of oxalates esters could take place.

The process was supported by UV radiation (full emission spectrum is shown in Figure S3), so its influence on reaction kinetics in the presence of oxalates should be considered. Absorbed radiation causes the decarboxylation of the ligand with the release of CO<sup>2</sup> and reduction in Fe3+ to Fe2+ (reactions (11) and (12)). The oxidation of organic compounds occurs at high speed in the presence of oxalates. Complex compounds with carboxylic acids under UV radiation reduce the iron, and alkyl radical and carbon dioxide are produced (reaction (13)). These reactions lead to the fast mineralization of organic compounds in wastewater.

Improperly selected doses of catalysts (too small an amount of iron) have a negative effect on the high-efficiency treatment process. Regardless of the amount of catalyst and hydrogen peroxide, the effectiveness of the treatment processes increases with time.

The longest process time, 120 min, was the most effective. The 15 min process time was the least effective, as this was not enough time to carry out the treatment process. However, the duration and higher costs of the process should be considered, and the optimal time should be selected so as to maintain high efficiency with an appropriate cost of treatment. For lightless processes and 120-min process time, the process is no longer profitable, and an equally high efficiency was achieved after 60 min of treatment. Such an observation was not made in the case of light-assisted processes. For them, extending the process time to 120 min is still profitable. The process showed the greatest efficiency in relation to its duration during the first 15 min, and TOC was mostly decreased in a short period of time. Then, sequential TOC measurements showed lower speed of the treatment process.

Most of the studies that used hematite or magnetite were carried out on pollutants present in components. For example, hematite was used as a catalyst by Araujo et al. [35]. In total, 20 g of hematite and a dose of 800 mg/L of hydrogen peroxide allowed for achieving 99% treatment efficiency of the process after 120 min. Our research was carried out for much lower catalyst doses and a different type of contaminant with a much more complex matrix (wastewater from the cosmetic industry).

In samples taken after 15- and 30-min process time, a large amount of evolving gas bubbles was visible because of the decomposition of unreacted hydrogen peroxide. The color of the precipitate depended on the ratio of iron to magnetite and hematite. Dependence was visible; the greater the amount of metallic iron in the sample was, the more orange the sediment was. No greenish sediment was observed, which would indicate the presence of Fe2+ ions. On this basis, it was concluded that the oxidant doses were correctly selected.

There was a separation in the sediment phases into an upper rusty one (oxidized iron) and lower black one (magnetite, metallic iron). The exact mechanism of the separation process is unknown, but this could be due to the difference between densities, as iron hydroxide has a lower density than that of magnetite and metallic iron.
