pH dependence of persulfate activation by EDTA/Fe(III) for degradation of trichloroethylene

The ability of free ferrous ion activated persulfate (S2O82−) to generate sulfate radicals (SO4−) for the oxidation of trichloroethylene (TCE) is limited by the scavenging of SO4− with excess Fe2+ and a quick conversion of Fe2+ to Fe3+. This study investigated the applicability of ethylene-diamine-tetra-acetic acid (EDTA) chelated Fe3+ in activating persulfate for the destruction of TCE in aqueous phase under pH 3, 7 and 10. Fe3+ and EDTA alone did not appreciably degrade persulfate. The presence of TCE in the EDTA/Fe3+ activated persulfate system can induce faster persulfate and EDTA degradation due to iron recycling to activate persulfate under a higher pH condition. Increasing the pH leads to increases in pseudo-first-order-rate constants for TCE, S2O82− and EDTA degradations, and Cl generation. Accordingly, the experiments at pH 10 with different EDTA/Fe3+ molar ratios indicated that a 1/1 ratio resulted in a remarkably higher degradation rate at the early stage of reaction as compared to results by other ratios. Higher persulfate dosage under the EDTA/Fe3+ molar ratio of 1/1 resulted in greater TCE degradation rates. However, increases in persulfate concentration may also lead to an increase in the rate of persulfate consumption.