Investigation of Adsorption Mechanisms in Competitive and Non-Competitive Systems: Cadmium Immobilization in Contaminated Calcareous Soil Amended with Bone-Derived Biochar

Biochar (BC) has emerged as a promising amendment to reduce the bioavailability and toxicity of heavy metals like cadmium (Cd) in contaminated soils. This study focuses on cow bone-derived biochar (CBBC), produced via pyrolysis at 550°C and further activated with ZnCl₂ (A-CBBC), to assess its effectiveness in immobilizing Cd in calcareous soils. The research includes comprehensive characterization of CBBC and raw bone powder (BP) using elemental analysis, XRF, BET, SEM, and FTIR. Cd mobility was evaluated through fractionation and adsorption isotherms under both competitive and non-competitive conditions. Results showed a 10% reduction in the accessible Cd fraction and mobility factor (MF) in soils treated with 5–7% CBBC after six months. Adsorption isotherms revealed that soluble Cd concentrations were twice as high in competitive systems compared to non-competitive ones. Cd levels in untreated soil were 2.7 times higher than in CBBC- and BP-treated soils. CBBC and BP enhanced Cd adsorption by 300 and 140 µmol kg⁻¹, respectively, compared to control. These findings highlight the potential of CBBC and BP to reduce Cd mobility in calcareous soils through cation exchange and surface complexation, offering a sustainable strategy for remediating heavy metal-contaminated alkaline soils.