A new low-cost hydroxyapatite for efficient immobilization of lead

Distiller waste (DW), a common by-product of soda ash plants, was used as the unique calcium source to produce low-cost hydroxyapatites (HAPs) for the first time. The DW-derived HAPs were characterized by SEM, TEM, XRD and BET methods and investigated as amendments for soil Pb immobilization. The DW-derived HAPs displayed relatively smaller particle size (30-80 nm) and larger BET specific surface areas (60.00-64.06 m²/g) compared with two selected commercial HAPs (technical grade HAP and biological reagent HAP designated as HAP-TG and HAP-BR, respectively). The maximum sorption capacity of Pb on the DW-derived HAPs predicted from Langmuir sorption isotherm model was 726-734 mg/g higher than the commercial HAPs and even other well-designed sorbents. TCLP leaching experiments and BCR sequential extraction experiments were conducted to determine the effectiveness of HAP additions on Pb immobilization in soil. A lower addition ratio is required for the DW-derived HAPs than commercial HAPs to reduce Pb leachability to below the harmless level. Meanwhile, the DW-derived HAPs were found to be superior to commercial HAPs in transformation of readily bioavailable forms of Pb to stable speciation in soil, with residual fraction of Pb increased from 6.7% in non-amended soil to 60.7-61.4% in DW-derived HAPs amended soils, 58.3% in HAP-TG amended soil and 42.6% in HAP-BR amended soil, respectively. This study strongly demonstrated the feasibility and low-cost of HAPs derived from the distiller waste for reducing the environmental risks and bioavailability of Pb in soil.