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 and SO 4 •− ) 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 and SO 4 •− 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 oxidation but a higher decrease in electron-donating moieties than chromophores during SO 4 •− oxidation, particularly in lower MW fractions (<3 kDa). The • OH decomposed reactive moieties mainly via aromatic ring opening or depolymerization in all MW fractions. SO 4 •− 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 was quantified as approximately 10 times more reactive towards MW fractions than SO 4 •− . However, the SO 4 •− concentrations were approximately 10 times those of • OH, resulting in comparable performance to • OH oxidation. SRNOM depolymerization by • OH and SO 4 •− resulted in formation of more precursors of disinfection byproducts than the parent constituents, especially for • OH. This work improves our understanding of the transformation of specific SRNOM fractions during radical oxidation processes.