Bone biochar as a bifunctional catalyst for sustainable co-removal of nitrate and phosphate in constructed wetlands

Constructed wetlands (CWs) are increasingly adopted for tertiary nutrient polishing as nature-based solutions. However, CWs treating secondary effluent typically suffer from low nutrient removal efficiency due to insufficient carbon sources and inadequate phosphate precipitation. Here, we developed a novel bone-derived biochar CW filler (BC-400) with dual catalytic functions, realizing highly efficient co-removal of nitrate and phosphate under carbon source-deficient conditions (C/N = 2). BC-400 achieved removal rates of 4.77 gN/(m³·d) and 0.70 gP/(m³·d) in CWs treating secondary effluent over 1000 days, exceeding most nutrient polishing fillers in CWs. This performance is derived from bone biochar's unique carbon-mineral composite structure, comprising amorphous carbon crosslinked with hydroxyapatite networks. The amorphous carbon layers enriched Gram-negative denitrifying bacteria and promoted the biosynthesis of flavin-based electron shuttles. Flavin-mediated extracellular indirect electron transfers increased electron flux, improving denitrification under low C/N conditions. Meanwhile, the hydroxyapatite framework provided structurally matched templates for phosphate crystallization with naturally occurring Ca²⁺ in wastewater, enabling spontaneous calcium phosphate deposition for sustainable phosphate removal. BC-400 CWs can reduce the dosage of carbon sources and phosphorus removal agents of the upstream wastewater treatment plant by 39.9% and 100.0%, respectively, reducing carbon emissions by 21.4%. This catalytic-filler strategy opens new pathways for tertiary nutrient polishing and advances wastewater treatment toward more sustainable paradigms.