Degradation and mechanism of 2,4-dichlorophenoxyacetic acid (2,4-D) by thermally activated persulfate oxidation.

The chlorinated phenoxy herbicide of 2,4-dichlorophenoxyacetic acid (2,4-D) was oxidized by thermally activated persulfate (TAP). This herbicide was studied for different persulfate dosages (0.97-7.29 g L-1), for varying initial pH levels (3-12) and temperatures (25-70 °C). Compared with Fe2+/PS, TAP could achieve a higher total organic carbon (TOC) removal under wider pH ranges of 3-12. Increasing the mole ratio of PS to 2,4-D favored for the decay of 2,4-D and the best performance was achieved at the ratio of 50. The 2,4-D degradation rate constant highly depended on the initial pH and temperature, in accordance with the Arrhenius model, with an apparent activation energy of 135.24 kJ mol-1. The study of scavenging radicals and the EPR confirmed the presence of both SO4- and OH. However, SO4- was the predominant oxidation radical for 2,4-D decay. The presence of both Cl- and CO32- inhibited the degradation of 2,4-D, whereas the effect of NO3- could be negligible. Verified by GC/MS, HPLC and ion chromatography, a possible degradation mechanism was proposed.