Single-atom cobalt enables efficient catalytic ozonation for advanced wastewater treatment: Mechanisms and application

• Single-atom Co catalysts were successfully fabricated via the spital node strategy. • Single-atom Co based catalytic ozonation exhibited superior removal for OA and pHBA. • The inherent differences between OA and pHBA led to distinct oxidation pathways. • Single-atom Co based HCO presented excellent treatment for landfill leachate. Heterogeneous catalytic ozonation (HCO) triggered by single-atom catalysts (SACs) is an emerging and promising advanced wastewater treatment technology. The catalytic ozonation mechanisms involving single-atom sites and the application of SAC-based HCO require further exploration. Herein, a single-atom Co catalyst with Co-N 4 active sites was synthesized for HCO. Oxalic acid (OA) and p-hydroxybenzoic acid (pHBA) with different structural and chemical properties were utilized as the model contaminants. Compared with ozonation and Co nanoparticle-based HCO, single-atom Co-based HCO exhibited superior removal for the two model pollutants, with the removal rate constants of 0.098 and 0.076 min −1 for OA and pHBA, respectively. In the single-atom Co-based HCO, ozone was decomposed and converted to nonradical ROS of surface-adsorbed atomic oxygen (*O ad ) and singlet oxygen ( 1 O 2 ). OA was mainly removed on the catalyst surface, relying on the adsorption by the catalyst and the oxidation of *O ad , while electron-rich pHBA was oxidized by O 3 and 1 O 2 in solution. Furthermore, using 3–5 mm Al 2 O 3 pellets as the carrier, single-atom Co-doped carbon-Al 2 O 3 pellets were fabricated and applied for the advanced treatment of landfill leachate, and the COD decreased from 110 mg/L to 38 mg/L after treatment. This work provided new insights into the oxidation mechanisms and applications of SAC-based HCO.