Sonolytic degradation of pesticide metazachlor in water: The role of dissolved oxygen and ferric sludge in the process intensification

The study of sonolytic degradation of metazachlor (MZC), which was activated by either oxygen or insoluble Fe(III) was performed. Herbicide MZC was selected as a target compound due to its wide use in agriculture. The results indicated the positive effect of oxygen used as undoubtedly the most harmless H-scavenger promoting the penetration of HO from the gaseous phase of the cavitation bubble into the liquid phase and the interphase region. Another approach in activation of sonolysis was the application of Fe2O3 used as a source of iron in order to utilize H2O2 generated as a result of combination of HO∙-s in the interphase region of the cavitation bubble. The present study revealed the synergistic effect of the combination of sonolysis and Fenton-like oxidation. The pseudo-first order rate constant of MZC sonolytic degradation increased from 1.11 × 10−2 min-1 for conventional sonolysis to 1.79 × 10−2 and 2.88 × 10−2 min-1 for O2-saturated and Fe2O3-added solutions, respectively. The application of ultrasound demonstrated the amplifying effect on the detachment of Fe(III) from insoluble ferric oxyhydroxide (Fe2O3·nH2O) in acidic environment into the aqueous solution, thereby improving the oxidation of organic compounds. MZC followed two degradation pathways, which are the cleavage of the aliphatic sites of MZC with the initial hydroxylation reactions and oxidation of pyrazole group.