Oxidation of 2,4-Dichlorophenoxyacetic Acid by Persulfate or Peroxymonosulfate with Iron(II) as an Activator

Several approaches can be used to activate persulfate anion $({\mathrm{S}}_2{\mathrm{O}}_8^{2-})$ and peroxymonosulfate anion $({\mathrm{HSO}}_5^{-})$ to generate sulfate free radical $({\mathrm{SO}}_4^{-}\bullet )$, a powerful oxidizing agent. In this study, we investigate ferrous ion (Fe²⁺) as an activator to generate ${\mathrm{SO}}_4^{-}\bullet$ for the oxidation of 2,4-dichlorophenoxyacetic acid (2,4-D), a common herbicide. Experiments were conducted using different molar ratios of 2,4-D, oxidants, and Fe²⁺. The 2,4-D concentrations used in the experiments were fixed at 0.045 mM. Results showed that 2,4-D was rapidly oxidized within minutes but plateaued to an asymptotic concentration. A possible reason for the slowdown in oxidation was the rapid conversion of Fe²⁺ to Fe³⁺, which in turn slowed the activation process. Experiments with sequential addition of Fe²⁺ at different times resulted in further 2,4-D oxidation, which indicated that Fe²⁺ was limiting in the activation process. The optimal molar ratio of persulfate:Fe²⁺ or peroxymonosulfate:Fe²⁺ for the oxidation of 2,4-D was found to be 1:1. Of the two kinetic models used, the bi-exponential first-order model gave the best fit of the experimental data with R² values greater than 0.9 as compared with the availability-adjusted first-order model. Results of the experiments showed that a low cost material such as Fe²⁺ can be used to activate persulfate or peroxymonosulfate for the removal of 2,4-D and other organic compounds in contaminated groundwater or drinking water.