Cooperative Spin Alignment Enhances Selective Singlet Oxygen Generation in Wastewater Pre-Oxidation System

Selective nonradical oxidation to enhance biodegradability is an attractive route for wastewater pretreatment, but molecular control of peroxymonosulfate (PMS) activation remains elusive. Here we report a Fe-N₄ catalyst with second-shell boron coordination (FeN₄/B) synthesized by supramolecular self-assembly. The second-shell B imposes long-range electronic regulation and cooperative spin alignment, shifting the Fe-PMS^* adduct to a high-spin configuration and promoting spin-allowed interfacial charge transfer. Concomitantly, covalent B─H bonding with PMS generates the key SO₅ ^* intermediate that channels PMS activation toward singlet oxygen. This structural control delivers ≥96% ¹O₂ selectivity with an observed rate constant k_obs = 1.64 min^-1, a ¹O₂ production rate of 227 µmol L^-1 min^-1, and 98% PMS-to-¹O₂ conversion. FeN₄/B rapidly degrades 15 highly biotoxic pollutants within 10 min and retains activity under pH fluctuations, common ion/DOM interference, and across diverse natural waters. Implemented as a hydrogel-immobilized ALICR-FeN₄/B module, the catalyst operated continuously for 168 h, increased effluent biodegradability (BOD/COD > 0.50), reduced acute toxicity, and lowered downstream biological treatment load at an estimated pretreatment cost of ∼$0.14 t^-1. These results establish cooperative spin alignment as a central lever of second-shell engineering, bridging atomistic design and scalable water treatment to enable practical, economically favorable pretreatment based on selective PMS-to-¹O₂ conversion.