Immobilization of lead and zinc in contaminated soil using taro stem-derived biochar and apatite amendments: a comparative study of application ratios and pyrolysis temperatures†

Soil contamination by heavy metals presents a substantial environmental challenge. Remediation strategies employing biochar and apatite offer promise for restoring compromised sites. However, the efficacy of apatite and biochar derived from various biomass sources remains an under-investigated area. Taro stem-derived biochar produced at 300 and 500 °C (TSB300 and TSB500) and apatite amendments were incubated in contaminated soil for one month at various ratios (biochar 3%, 6%, 10%, mixture of biochar/apatite 3 : 3%, and 6 : 6% w/w) to investigate their potential to immobilize Pb and Zn. The initial concentrations of Pb and Zn in the contaminated soil were 4165.1 ± 19.6 mg kg⁻¹ and 3424.9 ± 20.4 mg kg⁻¹, respectively. Soil samples were subjected to Tessier's sequential extraction for analysis of Pb and Zn in five chemical fractions (F1: exchangeable fraction; F2: carbonate fraction; F3: Fe/Mn oxide fraction; F4: organic carbon fraction and F5: residual fraction). The results indicated that one-month biochar and/or apatite amendment significantly increased soil pH, organic carbon (OC), and electrical conductivity (EC) compared to the control (p < 0.05). Amendments also notably reduced exchangeable fractions of Pb and Zn (F1) up to 71.8% and 61.5%, respectively, while enhancing their presence in more stable fractions (F4 and F5). This immobilization effect peaked at the 10% biochar application and 6 : 6% biochar–apatite combination. These findings suggest that TSB300, TSB500, and their blends with apatite hold promise for immobilizing Pb and Zn in heavily contaminated soil, potentially mitigating environmental risks.