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Reactivity of chromophoric dissolved organic matter (CDOM) to sulfate radicals: Reaction kinetics and structural transformation

Abstract:

Sulfate radical (SO4•−) has been extensively studied as a promising alternative in advanced oxidation processes (AOPs) for water treatment. However, little is known about its reactivity to the ubiquitous dissolved organic matter (DOM) in water bodies. SO4•− would selectively react with electron rich moieties in DOM, known as chromophoric DOM (CDOM), due to its light absorbing property. In this study, the reactivity and typical structural transformation of CDOM with SO4•− was investigated. Four well characterized hydrophobic DOM fractions extracted from different surface water sources were selected as model CDOM. SO4•− was produced through the activation of peroxymonosulfate (PMS) by Co(II) ions at pH 8 in borate buffer. The reactivity of CDOM was studied based on the decrease in its ultraviolet absorbance at 254 nm (UVA254) as a function of time. The reactivity of CDOM changed with time where fast and slow reacting CDOMs (i.e., CDOMfast and CDOMslow) were clearly distinguished. A second-order rate constant of CDOMfast with SO4•− was calculated by plotting UVA254 decrease versus PMS exposure; where a Rct value (i.e., ratio of sulfate radical exposure to PMS exposure) was calculated using pCBA as a probe compound. The transformation of CDOM was studied through the analysis of the changes in UVA254, electron donating capacity, fluorescence intensity, and total organic carbon. A transformation pathway leading to a significant carbon removal was proposed. This new knowledge on the kinetics and transformation of CDOM would ultimately assist in the development and operation of SO4•−-based water treatment processes.