Hydroxyl radical generation in peroxymonocarbonate/Co²⁺ systems: kinetic and mechanistic insights

This study investigates the generation of hydroxyl radicals (˙OH) in a peroxymonocarbonate (PMC)-based advanced oxidation process (AOP) catalyzed by Co²⁺. Steady-state ˙OH concentrations ([˙OH]_ss) were quantified using terephthalic acid (TA) as a probe. The presence of PMC alone had negligible effect on ˙OH production, whereas Co²⁺ markedly enhanced radical formation. Specifically, [˙OH]_ss increased from 4.03 × 10^-17 M in the H₂O₂-only system to 2.25 × 10^-16 M in H₂O₂ + Co²⁺, and from 2.26 × 10^-17 M in PMC-only to 3.38 × 10^-16 M in PMC/Co²⁺. This enhancement is attributed to a Fenton-like mechanism involving Co²⁺. Kinetic analysis revealed first-order dependence on TA concentration (R ² ≈ 0.99), Langmuir-type dependence on PMC concentration, and linear correlation with Co²⁺ concentration. Inorganic anions exhibited diverse roles, with Cl^- enhancing ˙OH generation by ∼26%, whereas SO₄ ^2- and HPO₄ ^2- suppressed it by ∼35 and ∼25%, respectively. A kinetic model describing radical generation agreed well with experimental data, offering valuable mechanistic insights and highlighting practical applicability of PMC-based AOPs for controlled ˙OH generation.