Direct and hydrogen peroxide-assisted UV degradation of halonitromethanes and haloacetonitriles

Abstract

Halonitromethanes (HNMs) and haloacetonitriles (HANs) are emerging contaminants posing potential threats to human health even at low concentrations because they have been found a few orders of magnitudes more toxic than regulated carbonaceous DBPs. The controls of HNMs, HANs, and their major precursors cannot be achieved via conventional treatment processes for drinking water supply. Thus, their removals by direct and hydrogen peroxide (H₂O₂)-assisted UV degradation were investigated. Preliminary studies revealed that the direct UV photolysis was effective in removing HNMs, but the H₂O₂-assisted UV degradation was needed in removing HANs. The removal rates of HNMs followed first-order degradation kinetics and those of HANs followed pseudo first-order degradation kinetics. The degradation rates of trichloronitromethane (TCNM) were low at all pHs tested, while those of bromonitromethane (BNM), dichloronitromethane (DCNM) and dibromonitromethane (DBNM) increased with increasing pH. The increasing degradation rates of BNM, DCNM, and DBNM were correlated with their pH-dependent molar absorptivities and determined by the concentrations of their deprotonated fractions. Half removals of deprotonated HNMs could be achieved at neutral to alkaline pH with UV dose similar to that of UV disinfection processes. A homolytic cleavage pathway was proposed for the protonated BNM, DCNM, and DBNM and TCNM. A heterolytic cleavage pathway was proposed for deprotonated mono- and di-HNMs. The H₂O₂-assisted UV degradation rates of monochloroacetonitrile (MCAN), dichloroacetonitrile (DCAN), and trichloroacetonitrile (TCAN) are determined by the H₂O₂-assisted hydrolysis rates and the hydroxyl radical oxidation rates of them. The H₂O₂-assisted hydrolysis rates of chlorinated acetonitriles (CANs) increase with increasing degrees of halogenation of CANs and hydroperoxide ion concentrations in the solution. The hydroxyl radical oxidation rates of CANs generally decrease with increasing degrees of halogenation of CANs. The H₂O₂-assisted hydrolysis was considered to dominate the degradation of TCAN at tested pH and DCAN at pH 7.5 and H₂O₂ concentrations of 5 mM and 10 mM over the hydroxyl radical oxidation. Complete hydrolysis of TCAN at pH 7.5 was achieved within 20 minutes at 1 mM H₂O₂, but produced another toxic DBPs TCAcAm. The H₂O₂-assisted hydrolysis and the hydroxyl radical oxidation pathways of CANs were proposed.

Date of Award	2014
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology