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Identification of Oxidation Intermediates in Humic Acid Oxidation

ABSTRACT The effects of using ultraviolet irradiation (UV), ozone (O3) and the combined O3-UV advanced oxidation process (AOP) on humic acid (HA), which is regarded as the main disinfection by-products (DBPs) precursor, have been evaluated and compared. In particular, aromatic acids, short-chain aldehydes and carboxylic acids were measured. The purpose of this study was to determine the different classes and yields of oxidation intermediates, compare O3, UV and O3-UV for HA removal, as well as investigate the effects of O3, irradiation time, UV intensity on the mineralization of HA. Based on that, the knowledge gap of DBPs generation was made up. The results showed that by UV irradiation and O3 oxidation, HA broke down into smaller molecules that were more hydrophilic, namely formaldehyde, acetaldehyde, propaldehyde, butyraldehyde, glyoxal, methyl-glyoxal, formic, acetic, fumaric, benzoic, phthalic, protocatechuic and 3-hydroxybenzoic acids. Meanwhile, unsaturated conjugated double bonds in the structure of HA were destroyed, which lead to UV254 a slight decrease. Due to the synergistic effect of O3-UV, DOC and UV254 significantly decreased and remained stable after the reaction of 10 min, indicating that O3-UV system had stronger potential of mineralization and lower nonselectivity. Besides, the kinds and concentration levels of the intermediates were obviously reduced with light intensity increasing. Hydroxyl radical (.OH) could mineralize some organics that could not be mineralized by O3.