Cascading purification of algal-laden surface water by Far-UVC/Fe(VI)-UF: Overcoming the organic matter transformation-water quality risk trade-off.

Algal-laden surface water poses a persistent challenge to drinking water safety. Preoxidation coupled with ultrafiltration (UF) is a widely used treatment strategy, but preoxidation intensity entails an inherent trade-off between organic matter transformation and downstream water quality risk control. To overcome this trade-off, this study developed a Far-UVC/Fe(VI)-UF system that achieved cascading purification through the successive functions of enhanced preoxidation, in-situ adsorption/coagulation, and membrane surface prefiltration. During enhanced preoxidation, Fe(VI), Fe(V)/Fe(IV), and ROS jointly contributed to algal cell inactivation and selective organic matter transformation. Aromatic and unsaturated components were preferentially transformed through putative pathways involving oxygen addition, dehydrogenation, dealkylation, and decarboxylation, accompanied by the relative enrichment of more saturated and oxygenated products. Nascent Fe(III) generated from Fe(VI) reduction subsequently mediated in-situ adsorption/coagulation through adsorption bridging and sweep flocculation, promoting the aggregation and partial removal of inactivated algal cells, residual organic matter, and oxidation products, particularly higher m/z components. During UF, suspended nascent Fe(III) deposited on the membrane surface under permeation-driven drag, forming a porous layer that served as a third purification and antifouling unit through pore sieving, surface adsorption, and interfacial regulation. Consequently, total DOC removal reached 48.3%, the third cycle terminal normalized flux was 0.41, and total fouling resistance decreased by 52.6%. Importantly, Far-UVC/Fe(VI)-UF reduced the formation potentials of THMs and HANs and had a preliminary partial operating cost estimate of 0.419-0.502 CNY m-3. Therefore, Far-UVC/Fe(VI)-UF provides a promising cascading purification strategy to overcome the trade-off between organic matter transformation and downstream water quality risks.