Proxymonosulfate activation by KBN-doped TiO₂@rGO: Multi-site radical and non-radical degradation of antibiotics

The effective synthesis of advanced composite photocatalysts for wastewater treatment remains a major challenge in attaining sustainability goals. In this study, a KBN-doped TiO₂@rGO composite was developed to activate proxymonosulfate (PMS) for enhanced photocatalytic degradation of emerging pollutants. The KBN@TiO₂@rGO/PMS system achieved complete ciprofloxacin (CIP) degradation (100 %) and 74.23 % total organic carbon (TOC) removal under optimal conditions (CIP: 10 mg L⁻¹, catalyst: 0.1 g L⁻¹, PMS: 0.5 mM, 90 min, and 950 W m⁻²), outperforming TiO₂/PMS (45.53 %). The superior performance was attributed to enhanced electron transfer and generation of both radical (•OH, O₂•⁻, and SO₄•⁻) and non-radical (¹O₂) reactive oxygen species, facilitated by structural defects, O–C=O groups, and Ti-oxide sites. CIP degradation efficiency increased 6.6- and 2.1-fold compared with pristine TiO₂ and KBN@TiO₂@rGO, respectively, indicating strong synergistic effects of ternary (K, B, N) doping, rGO integration, and PMS activation. Co-existing substances such as HPO₄²⁻ and HCO₃⁻ enhanced CIP degradation, while Cl⁻ and SO₄²⁻ had negligible influence; conversely, SO₄²⁻, PO₄³⁻, and high humic acid levels inhibited performance. The photocatalyst maintained high performance across various water matrices, with only a slight reduction due to natural quenchers. ESR and scavenging experiments confirmed reactive species formation, while UPLC-MS/MS revealed degradation pathways. Response Surface Methodology (RSM) optimisation identified optimal conditions (pH 8.17, time 101.2 min, photocatalyst 0.13 g L⁻¹, and [CIP] 9.99 mg L⁻¹), achieving predicted 100 % CIP degradation and 75.91 % TOC removal with 100 % desirability. This system shows strong potential for sustainable removal of pharmaceuticals from water.