Abstract
Presence of chlorinated organic pesticides in water is a serious health concern, whereas oxidation under high pressure and temperature is a promising treatment method for water and wastewater contaminated with organics. This paper aims to shed light on degradation and mineralization of 2,4-dichlorophenoxyacetic acid (2,4-D) in water with subcritical and supercritical conditions, using laboratory batch experiments. The experiments with 0.45 mM aqueous solution of 2,4-D were carried out under five different pressures (4.1-28.5 MPa) and five different temperatures (250-400oC). We found that 2,4-D did not degrade at 250oC and 4.1 MPa. The pesticide gradually degraded with an elapsed reaction time at 300oC and 8.8 MPa, but aromatic intermediates such as 2,4-dichlorophenol (2,4-DCP), 4-chlorophenol and others were produced. The pesticide almost disappeared at higher temperature and pressure in subcritical (360oC, 19.5 MPa) and supercritical (380-400 oC, 24.0-28.5 MPa) regions during the same reaction time. Chloride ion concentrations gradually increased with decreasing 2,4-D concentrations in a series of experiments. Furthermore, total organic chlorine (TOX) and total organic carbon (TOC) decreased remarkably at the supercritical condition (400 oC, 285 MPa). Dechlorination was thought to be a major step in 2,4-D oxidation by this method. The results exhibit that supercritical water oxidation of chlorinated aromatic pesticides like 2,4-D is a very promising purification method.
