Stability of Lead Immobilization by Hydroxyapatite and Biochar in a Typical Brown Soil (Haplic Cambisol)

Abstract Soil contamination with lead remains an acute geoecological problem, necessitating effective in situ immobilization methods. This study aimed to evaluate the efficiency and temporal stability of biochar, hydroxyapatite (HAP), and their combination in reducing lead mobility in a typical brown soil (Haplic Cambisol, southern Primorsky Krai). The intrinsic sorption capacity of the soil for lead ions was found to be 24.3–25.0 mg/g; according to X-ray diffraction (XRD) analysis, the binding occurs predominantly via reversible mechanisms on clay minerals, without the formation of distinct Pb-containing phases. Hydroxyapatite exhibited a sorption capacity (q m = 1095 mg/g) nearly 45 times higher than that of the soil. In the incubation experiment, HAP ensured a stable, non-diminishing over time reduction of lead mobility at moderate and high contamination levels—by 18.5–59.1% in the water-soluble fraction and by 8.6–46.4% in the exchangeable lead fraction. This reduction was achieved without significant alkalization of the medium (pH shift of + 0.06 to + 0.17), which enables the direct application of HAP in acidic soils without liming. Low-ash biochar demonstrated a temporally unstable effect comparable in magnitude to the intrinsic capacity of the soil. In the model binary system, it precluded an unambiguous assessment of synergism with HAP due to the substantially more rapid lead binding by hydroxyapatite. The obtained results suggest that hydroxyapatite serves as an effective and stable long-term barrier against mobile forms of lead in acidic Haplic Cambisols, whereas the role of biochar in such a system is limited to the improvement of soil structure.