General transformation law of chlorinated acetanilide herbicides by reactive oxidative species (ROSs) during photocatalytic process

Five chlorinated acetanilide herbicides with similar structure, namely alachlor (Al), acetochlor (Ac), butachlor (Bu), propachlor (Pr) and metolachlor (Me), were selected to explore general laws of the transformation mechanism by reactive oxidative species (ROSs) during photocatalytic process. The degradation kinetics followed the order of Al > Ac > Me > Bu > Pr, and OH was the main active species for causing the degradation of chlorinated acetanilide herbicides. After calculation of frontier electron densities (FEDs), point charges (PCs) and Ea of the proposed degradation intermediates, carbon atom connected to amino group and its para-position on benzene ring were elucidated to take radical reactions with OH easily, while the chlorinated acetanilide and ether side chain were likely to be adsorbed on the catalyst surface and then reacted with photohole at the initial stage. Combination of calculated results and identified degradation intermediates by HPLC and HPLC/MS/MS, three degradation pathways were thereby proposed by ROSs for these five chlorinated acetanilide herbicides, such as the addition of hydroxyl radicals to parent molecules of chlorinated acetanilide herbicides, the left of ether side chain on nitrogen atom and the cleavage of chlorinated acetanilide.