The increasing incidence of heavy metal contamination in soils has prompted significant interest in sustainable and cost-effective remediation approaches. Among these, biochar—a carbon-rich material derived from the pyrolysis of biomass—has demonstrated promising capabilities for immobilizing toxic metals in soils. However, the remediation potential of agricultural waste such as Delonix regia pod powder has not been extensively explored, particularly in tropical environments. This study investigates and compares the efficacy of Delonix regia pod powder and hardwood biochar in immobilizing lead (Pb), copper (Cu), and cadmium (Cd) in tropical soils subjected to variable conditions including soil pH, contaminant concentrations, organic matter content, and aging periods. Employing a 0.01 M CaCl₂ extraction method, the immobilization efficiency of the amendments was assessed across different scenarios.

Results indicated that both materials were effective in reducing the bioavailability of heavy metals, with biochar consistently outperforming Delonix regia powder under most conditions. Biochar was particularly efficient in soils with low pH and limited organic matter, while Delonix regia performed better in organic matter-rich soils. These findings provide essential insights for tailoring amendment-based remediation strategies in both agricultural and industrially impacted landscapes.