Supercritical water oxidation for the destruction of recalcitrant chlorinated organic solvents: Kinetics and chlorine balances

Abstract

The reactivity and mineralization of six chlorinated organics (dichloromethane, trichloromethane, chlorobenzene, 2-chlorophenol, 2-chlorobenzoic acid and 1-chloro-2-nitrobenzene) serving as model recalcitrant halogenated pollutants was investigated in a lab-scale supercritical water oxidation (SCWO) reactor. Different reaction conditions (temperature, reaction time, pH, nature of oxidant) were investigated to understand their effects on the mineralization of the organic chlorine to inorganic chloride. Complete or near-complete (>95 %) mineralization was achieved for all compounds, though at different conditions, reflecting their different reactivities. Reactivity generally decreased with increasing chlorine content, and aromatic chlorinated compounds were less reactive to SCWO than aliphatic ones. Electron-donating functional groups enhanced the reactivity of chlorinated aromatics, consistent with proposed reaction mechanisms. The effects of the presence of a co-pollutant and of alkali addition were examined. Isopropanol (IPA) as co-pollutant increased the mineralization rate of 2-chlorophenol but not that of dichloromethane, likely by increasing radical concentrations. NaOH slightly improved mineralization but introduced complications due to salt precipitation on reactor surfaces, potentially affecting both NaOH availability and reactor long-term stability. H₂O₂ was found to be a more reactive oxidant than oxygen from air, especially for aromatic compounds. Overall, this study demonstrated that SCWO is an effective method for the complete mineralization of organochlorine compounds. It also provided new insights into the reaction kinetics and mechanisms of chlorinated compounds during SCWO.