Sunlight-driven peroxydisulfate activation by floating Ca/Fe-TiO2@polystyrene spheres for efficient moxifloxacin degradation in circulated water: Critical roles of Ca/Fe bimetallic ions crosslinking on the stability of catalyst structure and electron transfer of system

• Ca/Fe bimetallic ions bonded with COO – of G and M units in SA reinforced TiO 2 immobilization. • Efficient electron transfer driven by Fe ion redox cycling for active species generation. • 95% MOX degradation was achieved by Ca/Fe-TiO 2 @PS coupling with reactor in circulated water. Owing to superior gas–solid mass transfer, light utilization, and recyclability, floating catalysts driven peroxydisulfate (PDS) activation systems exhibited better application potential compared to powder catalyst. However, structural instability, limited PDS activation ability and insufficient reactive species generation were still need to be solved. This study developed a Ca/Fe bimetallic cross-linked sodium alginate (SA) strategy to achieve robust immobilization of TiO 2 nanoparticles on polystyrene (PS) floating carriers. Results indicated that the structure stability and TiO 2 immobilization were enhanced by Ca/Fe bimetallic ions bonded with COO – of α-L-guluronic acid (G) and β-D-mannuronic acid (M) in SA . Additionally, the tight combining with Fe-crosslinker (Iron alginate) to form TiO 2 -Iron alginate complexes promoted the light absorbance range of gel complex. Besides, the photogenerated electrons participated in the valence conversion of Fe-crosslinker (Fe(Ⅱ)/Fe(Ⅲ)), thereby improving the separation of electrons and holes. Significantly, PDS activation driven by Fe ion redox cycling and efficient electron transfer jointly achieved generation of SO 4 − , OH, O 2 – and 1 O 2 oxidative active species. Besides, novel floating Ca/Fe-TiO 2 @PS-driven PDS system cooperated with homemade reactor achieved 95 % degradation and 81 % mineralization of 1 L moxifloxacin (MOX) with concentration of 10 mg·L −1 in circulated water, accompanied by significant reduction of MOX toxicity via piperazine ring opening, defluorination, hydroxylation and decarboxylation. The work offered a new insight for optimizing stability and PDS activation ability of floating catalysts.