Phosphorus-iron co-modified coconut shell biochar: A dual-functional soil amendment enabling simultaneous lead immobilization and inorganic phosphorus activation in contaminated soils

Phosphorus-iron co-modified coconut shell biochar (P-Fh@BC) was synthesized to simultaneously immobilize lead (Pb) and activate phosphorus (P) in contaminated soils. The adsorption capacity and remediation efficacy of biochars produced at 350, 500, and 650°C were evaluated using batch adsorption experiments, soil incubation studies, and pot experiments with Catharanthus roseus L. Among the studied biochars, P-Fh@BC exhibited the highest specific surface area (179.38 m2 g-1) and maximum Pb(II) adsorption capacity (255.96 mg g-1). Adsorption kinetics and isotherms were best fitted by the pseudo-second-order and Langmuir models, indicating monolayer chemisorption as the dominant mechanism. Additionally, X-ray photoelectron spectroscopy and synchrotron-based micro-X-ray fluorescence analyses revealed that Pb immobilization primarily occurred via the formation of Pb-phosphate precipitates (e.g., PbHPO4) and inner-sphere complexation with ferrihydrite, resulting in stable ternary complexes (Fe-O-P-Pb). In soil experiments, P-Fh@BC significantly reduced EDTA-extractable Pb (by up to 80% in granite soil) and decreased the acid-soluble Pb fractions, while concurrently enhancing residual Pb pools. Phosphorus activation was evidenced by increased labile inorganic P fractions (e.g., H2O-Pi, NaHCO3-Pi, NaOH-Pi) and a concomitant reduction in stable residual P, attributed to phosphate release and Fe-mediated retention. Pot experiments further confirmed that P-Fh@BC promoted plant growth (biomass increased up to 24-fold), reduced shoot Pb accumulation (by 80.6%), and improved plant P uptake. Redundancy and correlation analyses indicated that Pb immobilization was strongly associated with increased soil pH, cation exchange capacity, and inorganic P fractions. Collectively, P-Fh@BC is a highly effective dual-function amendment for Pb-contaminated soils remediation, offering integrated benefits of metal stabilization and nutrient recovery.