Ligand-regulated titanium coagulation coupled with algal biochar adsorption enables sequential separation and resource recovery of algal cells and antimony

Harmful algal blooms (HABs) and antimony (Sb) pollution increasingly co-occur in eutrophic waters, posing combined ecological and health risks that demand integrated mitigation strategies. However, no sequential, resource-efficient strategy has yet linked their management. Here, we establish a two-stage sequential strategy that first removes algae using ligand-regulated titanium coagulants, then repurposes the resulting titanium-rich flocs as a functional biochar for Sb(III) adsorption. By systematically comparing five organic ligands with distinct coordination configurations, we identify acetylacetone, which forms a stable six-membered chelate ring, most effective for achieving high algal removal and Sb(III) adsorption capacity, preserving the highest Ti-OH active site density (32.28%) and achieving >99% algal removal with a floc growth rate of 110 μm/min. Crucially, the physicochemical properties of the precursor coagulant, including porosity and titanium-related surface chemistry, were found to persist through pyrolysis in a ligand-dependent manner: the acetylacetone-derived biochar exhibits a high specific surface area (201.22 m 2 /g) and attains an Sb(III) adsorption capacity of 33.27 mg/g, 3.5-fold higher than the weakest ligand system, to which Ti-O-Sb ligand exchange is considered a likely major contributor. Life cycle assessment indicates that this waste-to-resource strategy has the potential to reduce environmental footprint compared with conventional coagulation-plus-disposal and commercial adsorbent routes. This study was conducted under controlled batch conditions; biochar regeneration and real-water validation remain to be addressed. It offers a sustainable, waste-to-resource solution for managing HABs and heavy metal pollution via a two-stage sequential strategy, advancing both water treatment and sludge valorization in eutrophic aquatic environments.