Differential UV-vis absorbance can characterize the reaction of organic matter with ClO2.

UV-vis differential spectroscopy was applied to characterize and quantify the spectral changes after ClO2 oxidation of ten humic substances, seven aromatic model compounds and four surface waters. The differential spectra of ten humic substances after ClO2 oxidation all exhibited a peak near 230 nm and a broad absorbance band with a maximum at around 316 nm. The differential spectra after ClO2 oxidation were distinguished from the one after chlorination, which was indicative of their different oxidation mechanisms. The differential spectra after ClO2 treatment were well fitted by seven Gaussian bands with maxima at about 200, 225, 240, 276, 316, 385 and 457 nm. Differential absorbance at 316 nm and 400 nm (denoted as DA316 and DA400, respectively) were found to best quantify the degradation of organic matters during ClO2 oxidation with negligible interferences from water matrixes. Oxidation of substituted functional groups on aromatic structures, rather than destruction of aromatic rings, was more responsible for chlorite formation. Spectral parameters-DA316 and DA400 showed strong correlations with ClO2 consumption and chlorite formation during ClO2 oxidation of humic substances and surface water samples. The results demonstrate that DA316 and DA400 can serve as promising indicators of chlorite formation and ClO2 consumption, which provide a practical approach for online water quality monitoring during ClO2 water purification.