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 propose