Coupling Microbial Transformation and Adsorption for Organic Phosphorus Removal in Sludge Biochar-Based Biofilter

Organic phosphorus (OP) constitutes an important and chemically diverse fraction of total phosphorus (TP) in aquatic environments, yet its removal mechanisms in substrate-based treatment systems remain insufficiently understood. In particular, the relative contributions of adsorption and microbial transformation to OP removal and their coupling effects are still unclear. To address this issue, gravel-, sludge-, and sludge biochar-based biofilters were operated under controlled phosphorus inputs with varying OP/inorganic phosphate (IP) compositions. Phosphorus removal performance, effluent phosphorus speciation, phosphatase activity, and microbial community characteristics were systematically analyzed to distinguish physicochemical and biological pathways. Results indicated that phosphorus removal was dominated by adsorption at early operational stages, with comparable performance across substrates. As the operation progressed, sludge-based substrates exhibited more stable removal than gravel, attributable to stronger Fe/Al-associated adsorption. Biologically active sludge biochar systems consistently maintained higher TP removal efficiencies (87.1–93.3%) than abiotic systems. Phosphatase-mediated OP mineralization governed phosphorus speciation transformation, while effective removal depended on subsequent immobilization of transformation products. Overall, the results demonstrate that efficient OP removal relies on a coupled bio–physicochemical mechanism, in which microbial transformation and substrate adsorption act synergistically. This insight offers guidance on optimizing phosphorus control in biofilters and constructed wetlands (CWs), especially for robust biofilters and CWs designed to treat OP-rich wastewaters.