High-Value Utilization of Phosphate Tailings: Phosphorus Recovery and In-Situ Synthesis of Layered Double Hydroxides for Stable Immobilization of Heavy Metals

Acid leaching-precipitation-synthesis enables high-value utilization of PT. • 99% of P in the leachate was recovered as white hydroxyapatite via pH adjustment. • PLDH exhibits exceptional mineralization capacities for Cd 2+ , Cu 2+ , Zn 2+ , and Pb 2+ . • Mineralization mechanism involves isomorphic substitution and chemical precipitation. • PLDH could effectively reduce the risks of Cd, Cu, Zn, and Pb contamination in soil. High-value utilization of phosphate tailings (PT) is of great significance for addressing environmental issues while advancing circular economy goals. This study proposed an integrated acid leaching–precipitation–synthesis strategy for high-value utilization of PT. After recovering phosphorus (P) via acid leaching and precipitation, a functional layered double hydroxide (PLDH) was successfully synthesized by recycling the remaining leachate. Results showed that the leaching efficiencies of P, calcium (Ca), and magnesium (Mg) reached approximately 100% at 25 °C using 0.4 M HCl at a liquid-to-solid ratio of 50 mL/g for 60 min. The recovered P primarily occurred as hydroxyapatite, with P 2 O 5 content as high as 31.2%. The synthesized PLDH exhibited excellent mineralization performance toward cadmium, copper, zinc, and lead ions (Cd 2+ , Cu 2+ , Zn 2+ , and Pb 2+ , respectively) in aqueous solutions, with maximum capacities of 561.28, 349.65, 340.14, and 775.54 mg/g, respectively, surpassing those of most reported sorbents. Mechanistic investigations revealed that Cd 2+ , Cu 2+ , and Zn 2+ were mineralized through isomorphic substitution at Ca 2+ sites within the PLDH lattice, whereas Pb 2+ was mineralized through precipitation to form Pb 3 (CO 3 ) 2 (OH) 2 . An exploratory soil column experiment further suggested that PLDH could effectively limit heavy metal migration under soil-column conditions. After 42 d of soil remediation, 4% PLDH reduced heavy-metal bioavailability by 59.9–77.3% and TCLP-leachable concentrations by 49.5–83.6%. Wheat growth experiments further showed no apparent phytotoxicity of 4% PLDH under the tested conditions. Lastly, techno-economic analysis indicated the economic potential of the proposed strategy, providing strong support for industrial applications. Download: Download high-res image (300KB) Download: Download full-size image