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

Sulfate radical (SO4 center dot-) 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 center dot- 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 center dot- was investigated. Four well characterized hydrophobic DOM fractions extracted from different surface water sources were selected as model CDOM. SO4 center dot- 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 (UVA(254)) 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 center dot- was calculated by plotting UVA(254) decrease versus PMS exposure; where a R-ct 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 UVA(254), 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 center dot--based water treatment processes. (C) 2019 Elsevier Ltd. All rights reserved.