Cu(II)-catalyzed THM formation during water chlorination and monochloramination: a comparison study

The catalytic effect of Cu(II) on trihalomethane (THM) formation during chlorination and monochloramination of humic acid (HA) containing water was comparatively investigated under various pH conditions. Results indicate that in the presence of Cu(II), the formation of THMs was significantly promoted as pH decreased in both chlorination and monochloramination. More THMs were formed during Cu(II)-catalyzed monochloramination which was partially due to enhanced hydroxyl radical (OH) generation as demonstrated by electron spin resonance (ESR) analysis. To discriminate the reactive moieties of HA, nine model compounds, which approximately represented the chemical structure of HA, were individually oxidized by chlorine or monochloramine. Results show that Cu(II) could promote THM formation through reacting with citric acid and similar structures in HA. During chlorination and monochloramination of citric acid in the absence of Cu(II), major intermediates including chlorocarboxylic acid, chloroacetone and chloroacetic anhydride were identified. However, the catalysis of Cu(II) did not produce any new intermediate. The complexation of Cu(II) with model compounds was characterized via FTIR analysis. The reaction mechanism for Cu(II)-catalyzed THM formation was proposed to comprise two pathways: (1) indirect catalysis in which OH oxidizes the large molecules of HA into small ones to enhance THM formation; and (2) direct catalysis in which Cu(II) complexes with HA to accelerate the decarboxylation steps for THM formation.