Release behavior and risk assessment of heavy metals from nitric acid leach residue of phosphate ore

The nitric acid phosphate process offers resource efficiency and environmental performance, yet heavy metals (HMs) containment in its byproduct-acid leaching residue (ALR)-remains a bottleneck for safe disposal and valorization. Herein, we elucidate environmental behavior and health risks of HMs in ALR via mineralogical characterization, dynamic column leaching, and multi-criteria risk assessment. ALR contains hazardous As, Cr, Pb, Hg, and Cd, with As leaching exceeding GB 18599-2020 limits. Sequential extraction reveals Cd and As pose acute migration risks due to high labile fractions, whereas Pb, Hg, and Cr are predominantly residual. Kinetically, neutral leaching (pH 6.9 ± 0.1) follows rapid dissolution-diffusion regimes (second-order models), while acidic conditions (pH 5.0 ± 0.05) induce distinct mechanisms: Hg and Cd conform to the two-constant rate equation, whereas As, Cr, and Pb exhibit complex "drop-surge-decline" trajectories described by the Elovich equation. Ecological risk prioritization (Hg > As > Cd > Cr > Pb) identifies Hg as priority contaminant owing to extreme toxicity and bioaccumulation, followed by As (high concentrations/mobility) and Cd (significant enrichment; elevated I_geo). Health risk assessment demonstrates carcinogenic potential for As and Cr, with children exhibiting 5.7-fold higher total Hazard Index (HI = 3.87). Notably, children's dermal contact risks for Cr and Hg approach warning thresholds. These findings necessitate solidification or extraction pretreatment prior to ALR resource utilization.