## 3 Results
### The Use of Ozone for Dye Removal  
In the next stage, ozone was applied for the removal of Indigo Carmine, Tartrazine, and Allura Red AC (Figure 6). The removal of Indigo Carmine after 30 min reached 91%. In contrast, Tartrazine and Allura Red AC did not undergo significant degradation; after 30 min, the removal efficiency for Tartrazine was only 8% and for Allura Red AC was merely 4%. No statistically significant difference was observed in the Indigo Carmine values at different time points. The \(p\)-values for all three dyes indicate no significant differences between the time points, suggesting that the changes in dye concentration over time are not statistically significant. Similar results were obtained by [PERSON] et al. (2016) [4] in the removal of Indigo Carmine during ozonation. After 32 min, an efficiency of 94.4% was reported. In order to optimize this process, a heterogeneous catalyst was used, which was steel shavings (Figure 7). The addition of the catalyst accelerated the reaction of dye decomposition. After 1 min for pH = 5.5, a 3-fold increase in the efficiency of Indigo Carmine removal was noted compared to the process conducted without the catalyst. However, the process conducted in the reaction environment, where pH = 3, did not result in an increase in the efficiency of Indigo Carmine removal. In the case of Tartrazine, no changes in the removal efficiency were noted after introducing the catalyst to the system for pH = 5.5, while at pH = 3, the process did not occur. In the next step, an additional amount of oxidant in the form of hydrogen peroxide was introduced into the catalyst system (Figure 8). A significant increase in the removal of Tartrazine and Allura Red was noted; after 30 min, these values were 47.14% and 64% for pH 5.5 and 67.36% and 81.14% for pH = 3, respectively. Complete removal of Indigo Carmine was obtained after 5 min for pH = 3, while the higher pH resulted in the process being slower in the first minutes compared to the process where pH = 3 and without the addition of oxidant. The chemical structure of this dye is probably more susceptible to  
Figure 6: The effect of ozone on dye removal. Figure 7: The effect of the ozonation process with the addition of a heterogeneous catalyst on the removal of dyes; (**a**) pH = 5.5; (**b**) pH = 3.  
the action of hydroxyl radicals in an acidic environment. In an environment with a higher pH, the degradation process of this substance may be more difficult due to the reduced reactivity of the oxidant and/or the greater stability of the dissociated form of the dye. The addition of an oxidant significantly increases the efficiency of the dye degradation process, which is probably due to the production of more hydroxyl radicals. Highly reactive radicals accelerate the decomposition of organic pollutants by non-selective oxidation of chemical bonds in dye molecules. The removal efficiency using ozone depends on the chemical structure of the contaminant, the presence of oxidizable functional groups, and the reaction conditions. Ozone is a strong oxidizing agent that primarily reacts with double bonds, aromatic groups, and electron-donating functional groups in dye molecules. The oxidation mechanism by ozone is a complex process that occurs through two pathways: direct reaction with dissolved O\({}_{3}\) or indirect oxidation through the formation of radicals (-OH). If ozone-mediated oxidation is carried out under acidic or near-neutral pH conditions, the degradation of contaminants primarily occurs through direct reactions between O\({}_{3}\) and organic molecules. The main targets of ozone attack are the double bonds -C=C- or -N=N-[19]. Indigo Carmine contains an aromatic system and double bonds within its indigo structure, which are highly susceptible to oxidation. This dye contains one sulfone group (-SO\({}_{3}\)H), which affects its water solubility and stability.