Improving ozone-based processes for the degradation of paraquat and diquat as persistent pesticides

The widespread use of bipyridylium herbicides like paraquat (PQ) and diquat (DQ) poses a significant environmental threat due to their high toxicity and recalcitrance. This study evaluates advanced oxidation processes based on ozonation as a robust alternative for their degradation. Simple ozonation was tested at various dosages (0.9 to 1.8 gh⁻¹) to determine the oxidant's efficiency in breaking down the parent molecules, while catalytic (Fe₃O₄) and photocatalytic (TiO₂/solar) systems were evaluated to enhance mineralization and overcome ozone's selectivity toward oxidation intermediates. Experiments progressed from 1 L lab-scale reactors to a solar compound parabolic collector (CPC) pilot system to assess scalability under complex conditions. The results showed that while simple ozonation effectively removed the parent herbicides (up to 83% for DQ), it was insufficient for complete mineralization. The addition of Fe₃O₄ did not significantly improve the process; however, photocatalytic ozonation at a pilot scale achieved the highest mineralization rates and total contaminant removal. Ecotoxicity assays using Aliivibrio fischeri revealed toxicity in generated transformation products, particularly at lower ozone dosages, necessitating optimized exposure times to ensure safe effluent discharge. Furthermore, by leveraging renewable solar radiation and optimizing the ozone dosage, the overall energy demand is significantly reduced, offering a highly sustainable and cost-effective approach. This work demonstrates the effectiveness of integrated solar ozonation processes for the treatment of point-source herbicide contamination.