Photooxidation of 2,4-dichlorophenoxyacetic acid by ferrous oxalate-mediated system.

The photodegradation of a herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) by ferrous oxalate/UV/H2O2 was studied. Ferrous oxalate, the more photoactive and reactive species, was found to react faster with hydrogen peroxide for hydroxyl radical production than ferrous ions under UV irradiation. About 2.9 times greater rate enhancement was found with the addition of 0.3 mM oxalate than that of a solution containing 0.1 mM Fe2+ and 1 mM H2O2 in the presence of UV light at 253.7 nm. The kinetics dependence of hydrogen peroxide concentration and initial solution pH were also investigated. A threefold increase in peroxide concentration could accelerate the removal of 2,4-D and nearly complete the reaction in 30 min of illumination. The initial decay rate of 2,4-D treated by ferrous oxalate/UV/H2O2 accelerated from 0.0029 to 0.0034 s(-1) and the overall removal of the starting material increased from 78% to 99.9%. The 2,4-D transformation at lower initial pH had higher reaction rates than that at higher pH and different reaction mechanisms were identified. The efficiency of the herbicide decomposition was retarded 2.6 times and 9.6 times as initial pH increased from 2.8 to 5.1 and 9.0, respectively.