From phosphate adsorption to soil remediation: utilizing spent Fe/Mg-LDH-biochar for concurrent heavy metal immobilization and phosphorus supply

This study presents a novel FeMg-layered double hydroxide (LDH) modified biochar composite (LB) designed to synergistically address phosphate recovery from wastewater and heavy metal immobilization in contaminated soil. The composite exhibits an exceptional phosphate adsorption capacity of 25.02 mg/g, representing a 169.00 % increase compared to pristine biochar. The improved performance results from precipitation and anion exchange within the LDH layers. And the competitive adsorption tests further confirm that LB better retains phosphate in simulated wastewater. When repurposed as a soil amendment, phosphate-laden LB (P-LB) demonstrates remarkable heavy metal immobilization efficiencies, achieving 41.23 % Pb, 30.52 % Cu and 25.63 % As immobilization at a 1.0 % dosage, representing an 18.70–26.42 % improvement over LB. Beyond its metal stabilization capabilities, P-LB effectively enhances soil phosphorus cycling, increasing total phosphorus by 14.41 % and bioavailable phosphorus by 9.02 %. Furthermore, the remediated soil exhibits significantly enhanced enzymatic activity, with urease and sucrase activities increasing by 21.38 % and 380.10 %, respectively. Pot experiments confirm the agronomic benefits of P-LB, demonstrating a 68.16 % increase in plant height and a 39.27 % increase in biomass accumulation in the remediated soil. This integrated approach, leveraging LDH-phosphate synergy, not only achieves phosphate recovery in sewage but also realizes metal immobilization and soil fertility remediation in contaminated soil.