Sustainable phosphorus recovery from wastewater by layered double hydroxide/biochar composites for potential agricultural application

Aiming at the increasing depletion of phosphorus (P) resources and the eutrophication caused by P-containing wastewater in the phosphate fertilizer industry, recovering P from wastewater for agricultural use is significant for establishing a sustainable closed-loop P cycle. Biochar is a promising P adsorbent due to low cost, facile preparation, rich feedstocks, and environmental friendliness. This study employed three methods to modify biochar with calcium-aluminum layered double hydroxides (CaAl LDH) to enhance adsorption performance of composite materials. The P adsorption capacity of the CaAl LDH/biochar composite material was studied, and the feasibility of using P-rich adsorbents as potential slow-release P fertilizers was evaluated. The CaAl LDH/ biochar prepared by co-precipitation exhibits the best adsorption capacity. The P adsorption conforms to pseudo- second-order kinetics and the Langmuir model, with a maximum theoretical adsorption capacity of 254.62 mg/g. This adsorbent demonstrates excellent pH adaptability and P selectivity. The adsorption mechanisms include surface complexation, ion exchange, and electrostatic attraction. Although the P-rich adsorbent has less-stable P of 53.10 %, the P release is stable throughout multiple successive extractions, significantly enhancing pea growth. This research provides a strategy for utilizing biochar adsorbents as slow-release P fertilizers to recover P from wastewater, establishing a sustainable closed-loop P cycle.