Abatement of dichloromethane using persulfate activated by alkali: A kinetic study

Abstract

The alkaline activation of persulfate (PS) has been tested in this study as in situ oxidation technology for the abatement of dichloromethane (DCM), a chlorinated volatile organic pollutant commonly found in groundwater due to its wide use as solvent in the chemical industry during the last decades. The addition of an alkali (NaOH) as persulfate activator generates hydroxyl and superoxide radicals, achieving the total degradation of the pollutant (XDCM = 99%, 96 h, CDCM = 1.2 mmol L−1, CPS = 42 mmol L−1, CNAOH = 169 mmol L−1). From the results obtained in the presence of a hydroxyl radical-scavenger (methanol, CMeOH = 313 mmol L−1), it has been concluded that only hydroxyl radicals are responsible of DCM abatement. Trichloroethylene was identified as intermediate compound and formic acid and chloride as degradation products. The conversion of DCM increased as the initial concentration of the pollutant decreased (from 1.2 mmol L−1 to 0.1 mmol L−1), whereas an increase in the oxidant concentration (from 8 to 168 mmol L−1) led to an increase in the degradation rate of the pollutant. Finally, the molar ratio NaOH:PS (in the range 1–4) did not affect the degradation of the pollutant when a pH high enough (>12) was maintained. A kinetic model, able to predict adequately the experimental DCM concentration profiles as a function of the concentration of DCM, PS and NaOH, has been developed. The kinetic model discriminated was zero order, first order and hyperbolic order for NaOH, PS and DCM concentration, respectively.