Molecular weight fraction-specific transformation of natural organic matter during hydroxyl radical and sulfate radical oxidation

Aquatic natural organic matter (NOM) is one of the main scavengers of reactive species (e.g., (OH)-O-center dot and SO4 center dot-) during oxidation processes and undergoes complex transformations. Here, the organic content, optical properties, and redox state in various MW fractions and bulk Suwannee River NOM (SRNOM) were simultaneously evaluated during (OH)-O-center dot and SO4 center dot- oxidation processes for the first time. The SRNOM transformation pathways were specific to radical species and MW fractions, which was evidenced by a proportional decrease in electron-donating moieties and chromophores during (OH)-O-center dot oxidation but a higher decrease in electron-donating moieties than chromophores during SO4 center dot- oxidation, particularly in lower MW fractions (<3 kDa). The (OH)-O-center dot decomposed reactive moieties mainly via aromatic ring opening or depolymerization in all MW fractions. SO4 center dot- oxidation followed a similar pathway in higher MW fractions (>3 kDa) but mainly formed compounds with UV-absorbing properties (e.g., quinones) in lower MW fractions (<3 kDa). With competitive kinetics and depolymerization model, (OH)-O-center dot was quantified as approximately 10 times more reactive towards MW fractions than SO4 center dot-. However, the SO4 center dot- concentrations were approximately 10 times those of (OH)-O-center dot, resulting in comparable performance to (OH)-O-center dot oxidation. SRNOM depolymerization by (OH)-O-center dot and SO4 center dot- resulted in formation of more precursors of disinfection byproducts than the parent constituents, especially for (OH)-O-center dot. This work improves our understanding of the transformation of specific SRNOM fractions during radical oxidation processes.