Single-atom catalysts (SACs) are emerging as transformative materials for advanced oxidation processes (AOPs), offering high atomic efficiency and tuneable electronic environments for sustainable water purification. However, challenges remain in scalable synthesis, precise coordination regulation, and long-term operation. Here, we present recent progress in atomic-level engineering of cobalt- and iron-based SACs for peroxymonosulfate activation. Through multidimensional coordination modulation, including planar Co–N₄ frameworks with axial Cl and second-shell S dopants, we achieve ultrafast non-radical electron transfer pathways with turnover frequencies up to 1.82 min⁻¹, enabling complete pollutant removal within minutes at ultralow cost. Complementarily, synergistic roles of Co–N₃C sites and graphitic nitrogen functionalities were elucidated, highlighting pollutant adsorption and additional PMS activation beyond metal centers. Extending this concept, we demonstrate the direct upcycling of waste plastics into Fe SACs with FeN₄Cl motifs, where axial chloride incorporation enhances conductivity, spin-state tuning, and PMS activation, delivering stable column operation in practical water treatment. Collectively, these advances showcase scalable routes to engineer SACs from abundant or waste resources, reveal fundamental structure–activity relationships, and chart a path toward cost-effective, selective, and sustainable catalytic AOP technologies for environmental remediation. Outcomes will advance the water purification technologies using green and cutting-edge solar driven oxidation technology to secure water safety in the post-COVID era.