Effect of preozonation on the characteristic transformation of fulvic acid and its subsequent trichloromethane formation potential: presence or absence of bicarbonate

The effects of bicarbonate on the characteristic transformation of fulvic acid (FA) and its subsequent trichloromethane formation potential (TCMFP) were investigated in the process of preozonation. Dissolved organic carbon (DOC) removal rate and the residual aqueous ozone concentration during preozonation were measured with different bicarbonate concentration. The presence of bicarbonate inhibited DOC removal and decreased TCMFP yields in the initial oxidation period. In order to explain these phenomena, the molecular weight (MW) distribution (<5, 5-10, 10-30, and >30 kDa) and corresponding TCMFP were investigated for FA and its subsequent oxidation products. Furthermore, transformation of molecular structure, based on MW distribution, was also characterized with Fourier transform infrared (FTIR) spectrum. Bicarbonate showed different inhibiting effects on TCMFP of organic species with different MW, and more significant TCMFP decrement was observed for the high MW fraction (>30 kDa) than for the low MW fractions. Preozonation led to obvious reduction on DOC and UV254 in most of MW fractions wherever bicarbonate was present or not, demonstrating that ozone contributed to both organics mineralization and structure variation, synchronously. As being indicated from the results of FTIR and gas chromatography-mass spectrometry, the functional groups such as alcohols, epoxides and phenols, the formation of which was promoted with hydroxyl radicals (.OH) and would be remarkably inhibited by bicarbonate, were responsible for the increment of TCM precursor's concentration during ozonation. Results of these studies confirmed low dosage bicarbonate affecting the ozonation pathways, influencing the intermediate species formation and impacting its subsequent TCMFP yields through inhibiting the .OH radicals reactions mainly occurred in high MW fractions.