Comparison of Photodegradation Performance of 1,1,1-Trichloroethane in Aqueous Solution with the Addition of H2O2 or S2O82- Oxidants

The chemical oxidation of 1,1,1-trichloroethane (TCA), a widely detected groundwater pollutant, by UV/H2O2 and UV/S2O82- processes was investigated. The effects of various factors were evaluated, including peroxide/TCA molar ratio, solution pH, Cl- and HCO3- anions, and humic acid (HA). The results showed that TCA oxidation fit to a pseudo-first-order kinetic model. The optimum H2O2/TCA molar ratio was 5:1, with TCA removal of 54.2% in 60 min. In the UV/S2O82- process, higher molar ratios (from 1/1 to 10/1) resulted in higher TCA oxidation rates, and TCA could be completely removed after 60 min with a S2O82-/TCA molar ratio of 3/1. In addition, acidic conditions were favorable for TCA removal in the UV/S2O82- process, while maximum TCA removal was observed at pH 6 in the UV/H2O2 process. Both Cl- and HCO3- anions adversely affected TCA oxidation performance, and higher concentration of HA resulted in a lag phase for TCA oxidation in both processes. Several reaction intermediates, including 1,1,1,2-tetrachloroethane, carbon tetrachloride, chloroform, tetrachloroethylene, 1,1-dichloroethylene, and tri- and dichloroacetic acids, were first identified during TCA oxidation by S2O82- chemistry, while only monochloroacetic acid was detected in the UV/H2O2 process. The results indicated that the UV/S2O82- process was much more effective than the UV/H2O2 process, but the latter was more environmentally friendly because fewer toxic intermediates were produced.