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

Abstract

Several approaches can be used to activate persulfate anion (S2O82−) and peroxymonosulfate anion (HSO5−) to generate sulfate free radical (SO4−∙), a powerful oxidizing agent. In this study, we investigate ferrous ion (Fe2+) as an activator to generate SO4−∙ 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.