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.