Efficient removal of low-concentration phosphate by a novel modified hydrotalcite-biochar composite

To address the limited adsorption capacity, poor pH adaptability, and lack of resource utilization pathways after saturation of conventional adsorbents in the treatment of low-concentration phosphorus-containing wastewater, an LDH@BC composite was synthesized by loading Mg Al layered double hydroxide (LDH) onto sawdust biochar via a ‘physical mixing-carbonization composite’ strategy. Batch experiments showed that at an adsorbent dosage of 0.32 g/L, LDH@BC achieved a maximum phosphate adsorption capacity of 160.59 mg/g, which was 13.3 times that of pristine biochar, and maintained removal efficiencies of 84.7%–95.5% over pH 3–11. In fixed-bed tests, 1 g of LDH@BC maintained >91% phosphate removal from a 1.0 mg/L influent, effectively treating 4440 mL, with the 0.5 mg/L breakthrough at ~7300 mL. Approximately 137 g of LDH@BC would treat 1 ton of such wastewater, demonstrating promising potential for advanced low-concentration phosphate treatment. The adsorption kinetics followed the pseudo-second-order model (R 2 > 0.97), and the isotherm fitted the Freundlich model (R 2 = 0.963), indicating heterogeneous multilayer adsorption dominated by chemisorption. Mechanistic analysis indicated that the synergistic effect of ion exchange and ligand exchange was the dominant mechanism for the efficient adsorption, in which the metal sites on the LDH layers played a key role. In addition, phosphate-saturated LDH@BC was applied in soil pot experiments and promoted the growth of marigold, preliminarily verifying its resource recovery potential as a slow-release phosphorus source. This study provides a material design strategy and an engineering-viable solution for the advanced treatment of low-concentration phosphorus-containing wastewater and phosphorus resource recycling.