A study of the reaction mechanisms of the degradation of 2,4-dichlorophenoxyacetic acid by oxalate-mediated photooxidation

The oxidation of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) by FeII/H2O2/UV (FHU) and ferrous-oxalate/H2O2/UV (FOHU) processes was investigated and compared. The initial decay rate and the overall removal percentage were used as the performance indexes. To extensively explore the associated processes, the intermediates and the end products were also examined and compared. The results showed that the degradation of 2,4-D by FHU is slower than that of FOHU. The involvement of ferrous-oxalate in the reaction can greatly improve the initial decay rate of 2,4-D because of the higher light sensitivity of the organometallic complexes. The total removal of 2,4-D and its corresponding intermediates was found to be strongly dependent on the initial hydrogen peroxide concentration; an excessively low initial [H2O2] will terminate the process due to the deficiency of hydroxyl radicals in the solution. All of the major primary intermediates (2,4-dichlorophenol, 2-chloro-4-hydroxyphenoxyacetic acid, 4-chloro-2-hydroxyphenoxyacetic acid, 5-hydroxy-2,4-dichlorophenoxyacetic acid and 6-hydroxy-2,4-dichlorophenoxyacetic acid) have been identified and confirmed by LC-MS. A reaction mechanism was proposed for FHU and FOHU and verified by the evidence presented in this study.