Beyond TOC decline: Compartment-resolved carbon mass balance for mineralization assessment in heterogeneous persulfate oxidation of doxycycline

Advanced oxidation processes (AOPs) frequently report high pharmaceutical removal without rigorously verifying mineralization. Parent-compound disappearance and TOC decline alone overestimate treatment effectiveness when transformation intermediates or catalyst-associated carbon remain unaccounted for. To address this critical gap, we developed a compartment-resolved carbon mass-balance framework that tracks aqueous organic carbon, inorganic carbon, catalyst-associated carbon, and inferred gas-phase carbon to assess mineralization in heterogeneous persulfate systems. The framework was demonstrated using doxycycline oxidation in a reduced cobalt ferrite (rCoFe2O4)-activated persulfate system. Under optimized conditions, the system achieved 96.4% doxycycline removal within 120 min and 99.89% aqueous TOC removal within 360 min, while near-complete carbon closure supported extensive mineralization rather than adsorptive removal. XPS, quenching, EPR, and LC-MS analyses linked carbon-fate behaviour to catalyst redox activity, with ·OH as the dominant reactive species. Notably, reusability tests revealed that doxycycline removal remained high (87.03% at cycle 10) while TOC removal declined sharply (36.44%), demonstrating that pollutant disappearance does not equal mineralization. This framework provides a practical, transferable approach for rigorous mineralization assessment in heterogeneous advanced oxidation processes.