Enhanced stabilization of mercury-contaminated soil using waste tyre-modified biochar: a green remediation approach

Mercury (Hg) contamination in soil is a serious environmental and health concern due to its high toxicity and bioaccumulation. This study investigated the potential of rice husk biochar (RHB), bamboo biochar (BB) and tyre-modified biochar (T-RHB and T-BB) for mitigating Hg contamination. Biochar was produced through pyrolysis of agricultural residues at 500-550 °C and characterized for surface area, elemental composition, functional groups using BET, CHNS, FTIR and SEM-EDS analysis. The results showed that Hg removal followed a chemical adsorption process. Tyre-modified biochar was more effective in removing Hg, with T-BB and T-RHB adsorbing 82.64 mg/g and 47.62 mg/g of Hg, which was much higher than the unmodified BB (38.46 mg/g) and RHB (32.36 mg/g). The Langmuir model indicated monolayer adsorption with higher Qmax and KL values, showing stronger Hg binding on modified biochar. TCLP tests revealed that tyre-modified biochar reduced Hg leaching by 85% (T-BB) and 72% (T-RHB) compared to untreated soil. Even under different pH conditions, the modified biochar maintained high Hg-binding efficiency, especially near neutral to slightly alkaline conditions (pH 7.3-9.6). Overall, tyre-modified biochar offers a cost-effective, eco-friendly and sustainable way to stabilize Hg in contaminated soil while turning waste tyre powder and agricultural residues into useful materials.