Pathway-Resolved Kinetics of Carbonate Radical Reactions with Aromatic Contaminants

Carbonate and bicarbonate are pervasive in natural and engineered waters and can suppress advanced oxidation by shifting the oxidizing species toward the highly selective carbonate radical (CO 3 •– ). Although single-electron transfer (SET) is often assumed to dominate, direct experimental discrimination from hydrogen atom transfer (HAT) has remained limited. Here, this mechanistic ambiguity is resolved by combining laser flash photolysis with quantum chemical calculations for a suite of aromatic contaminants. In a UV/peroxodisulfate/bicarbonate system, clean and quantitative conversion of the sulfate radical to CO 3 •– was directly observed, enabling reliable kinetic measurements under environmentally relevant conditions. Electron-donating aromatics react rapidly with CO 3 •– (up to 4.3 × 10 8 M –1 s –1 ), and their reactivity exhibits strong linear correlations with Hammett constants, electrode potentials, and Gibbs energy changes, consistent with SET. In contrast, electron-withdrawing aromatics react much more slowly (10 4 –10 5 M –1 s –1 ) and exhibit no such dependence on these electronic parameters. These results clarify how carbonate-rich conditions reshape oxidant chemistry and reaction pathways for aromatic oxidation.