Comparison of advanced oxidation processes in the decomposition of diuron and monuron – efficiency, intermediates, electrical energy per order and the effect of various matrices

Abstract

The decomposition of diuron and monuron, widely used phenylurea pesticides, via UV-induced photolysis (UV_254nm), ozonation (O₃), their combination (UV_254nm/O₃) and heterogeneous photocatalysis (TiO₂/UV) were investigated and compared. The UV_254nm/O₃ and TiO₂/UV methods proved to be effective from the aspects of both the transformation and the mineralization. The transformation rates changed in the order: O₃ ≪ TiO₂/UV < UV_254nm < UV_254nm/O₃. Comparing the electric energy per order (E^c_EO) values calculated for the transformation, the most efficient method was UV_254nm/O₃. However, the lowest E^TOC_EO value calculated for the mineralization was obtained using TiO₂/UV. Identification of the aromatic intermediates revealed that the first step in the decomposition involves transformation of the aliphatic chain, in parallel with dechlorination and hydroxylation of the aromatic ring. The amount and quality of the intermediates formed depends strongly on the method applied. Matrices such as natural waters (from the River Tisza and thermal water from Kistelek) and inorganic salts exert significant effect on the transformation rates only in the case of TiO₂/UV. Humic acids behave as “light filter” and consequently decrease the rate of photoinitiated transformation using UV_254nm and UV_254nm/O₃ methods, whereas they slightly enhance the effect of ozonation, most likely because humic acids and/or their intermediates promote the decomposition of ozone and increase the radical concentration. Regarding this parameter, the most sensitive method was heterogeneous photocatalysis, most likely because the well adsorbed humic acid strongly inhibits the formation of hydroxyl radical.