<p>Diatomic catalysts have emerged as highly efficient materials for peroxymonosulfate (PMS) activation; however, the mechanistic role of phosphate species in such systems remains poorly understood. In this study, a synergistic copper–cobalt (Cu–Co) diatomic catalyst was constructed, and it was demonstrated that different phosphate species could selectively regulate metal- and carbon/nitrogen-related active sites, thereby altering the PMS activation pathways. Under alkaline conditions, singlet oxygen (¹O₂) and sulfate radicals (SO₄^•−) dominated, whereas SO₄^•− and hydroxyl radicals (^•OH) were the primary reactive species under neutral conditions. Phosphate species primarily modulate C–C/C=C, C=O, graphitic N and pyridinic N, Cu²⁺, Cu⁰, Co²⁺, and Co⁰ site activity, thereby influencing the reaction kinetics and reactive oxygen species distribution. The presence of HPO₄²⁻ promoted PMS activation by biochar to generate SO₄^•− and ¹O₂, whereas H₂PO₄⁻ had a negligible effect on PMS activation. The system exhibited concentration-dependent behavior when HPO₄²⁻ and H₂PO₄⁻ coexisted. Low phosphate concentrations promoted the PMS activation, whereas high concentrations inhibited it. Density functional theory (DFT) calculations further confirmed that the phosphate species regulated the PMS activation mechanism by influencing the adsorption energies, electron transfer, and O–O bond lengths. This study elucidated the long-standing controversial phosphate regulation mechanism and highlighted that controlling phosphate speciation is crucial for improving the performance of diatomic catalyst/PMS systems in realistic aqueous environments.</p>