Experimental and theoretical investigations of CuO-catalyzed ozonation of humic acid

• The degradation of humic acid was evaluated upon catalytic and noncatalytic ozonation.
• The addition of catalyst positively affects the mechanism of ozonation.
• The experimental data was verified by theoretical model DFT.
• The formation of hydroxyl radicals was shown in the final stage of reactions.

In this study, the efficiency of copper oxide (CuO) as a catalyst in the ozonation process of humic acid (HA) was investigated in both experimental and theoretical respects. Ozonation and catalytic ozonation processes were conducted in a lab setting. HA concentration was determined by measurement of the surrogate organic parameters. The results show that the degradation of HA by catalytic ozonation in the presence of CuO was found to be much more effective than the ozonation process alone. The experimental data was verified by means of theoretical modeling. Density Function Theory (DFT) was used to calculate the decomposition of ozone in the catalytic processes. The reactions on the surface of metal oxides were evaluated with quantum-chemical calculations to explain the mechanisms of catalytic ozonation. Two models of adsorption were investigated: when only О3 is attached to the surface and when О3 and Н2О are simultaneously adsorbed by the active center of catalyst. Each is a barrierless reaction, as follows from the calculations mentioned. The result of the first reaction is one oxygen molecule and atomic oxygen being adsorbed on the CuO surface. The second reaction’s final products are О2 and hydroxyl-radicals, which are adsorbed on the CuO surface. These particles behave as powerful oxidizing agents in the further reactions with HA. Comparison of the two mechanisms shows that the second reaction with the water molecule participation is preferable to the first one based on energy levels.

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