Calcium-mediated catalytic ozonation for water purification: Synergistic interplay among surface acid-base sites,...

Robust and eco-friendly catalysts are essential for heterogeneous catalytic ozonation (HCO) in water purification. This work developed a green calcium-based catalyst (Ca-C@SiO₂) via pectin-mediated coordination. It achieved 99.2% bisphenol A removal (0.2640 min^-1), 77.6% TOC removal and 92.7% oxalic acid mineralization as a benchmark for refractory intermediates. It retained 92% activity over five cycles with only 4.41% Ca²⁺ leaching, and performed efficiently across pH 3-11 and in real waters (tap water, drinking source water, lake water and secondary effluent). Combining experimental quantification and theoretical calculations revealed three findings: (i) synergistic acid-base sites, including Ca-OH basic sites and oxygen vacancies (Ovs)/C-OH acidic sites, cooperatively generated ·O₂^-, ·OH, ¹O₂ and *O, (ii) CaO surface protonation constructed an alkaline interfacial microenvironment (pH > 8.0) to promote O₃ chain decomposition, with a synergy ratio of 1.21 and (iii) Ovs captured *O to replenish lattice oxygen and sustained reactive Ca-OH sites for reaction. Quantitative contribution analysis ranked reactive sites as Ovs (35.1%) > Ca-OH (23.6%) > C-OH (21.1%). This work provided the first demonstration of dynamic oxygen supplementation and site-specific contribution in non-redox calcium catalysts, establishing a fundamental basis to design sustainable HCO systems.