Micro-nano-bubble ozonation enhanced thiamethoxam mineralization and toxicity alleviation in wastewater

Neonicotinoid insecticides (NNIs), including thiamethoxam (TMX), clothianidin, and imidacloprid, are widely used in agriculture to control pests. Consequently, they have been frequently detected in wastewater, posing significant ecotoxicological risks. Conventional ozonation is widely applied for NNI removal but is limited by low mineralization efficiency and high effluent toxicity. However, the mechanisms of the performance limitations and increased toxicity remain unclear, hindering the effective application of ozonation in wastewater treatment. This study constructed a novel micro-nano-bubble ozonation (MNB-O₃) system, which enhanced the degradation rate of TMX, a representative NNI, by 34.7% and the mineralization efficiency by 176.5%, compared to conventional bubble ozonation (CB-O₃). MNB-O₃ also significantly reduced both acute toxicity and neurotoxicity in the ozonated effluents, addressing the issue of high toxicity associated with CB-O₃. Mechanistically, the formation of hydroxyl radicals (·OH) and singlet oxygen (¹O₂) increased substantially and was identified as the primary contributor to TMX degradation. Transformation product (TP) analysis revealed that formaldehyde and acetaldehyde were the key contributors to effluent toxicity, both accumulating in the CB-O₃ effluent. In contrast, MNB-O₃ achieved significant reductions in formaldehyde (87.4%) and acetaldehyde (34.6%) concentrations, substantially lowering effluent toxicity. Furthermore, a large-scale MNB-O₃ application demonstrated excellent performance in removing NNIs and reducing toxicity. This study provides valuable insights into the mechanisms underlying toxicity reduction in MNB-O₃ and highlights its potential for low-carbon wastewater treatment. By addressing the limitations of CB-O₃ and reducing the NNIs-related environmental risks, MNB-O₃ represents a promising advancement in the field of wastewater treatment.