Jackfruit seed biochar–apatite amendments: investigating changes in lead and zinc’s fractionation in the multi-metal-contaminated soil

Multi-metal contamination in soil presents major environmental and agricultural challenges globally, impacting the feasibility of phytoremediation. This study investigated the efficacy of jackfruit seed-derived biochar (JSB) produced at 300 °C (JSB300) and 600 °C (JSB600), combined with apatite, to mitigate potentially toxic elements (PTEs), thus influencing bioavailability, in soils heavily contaminated with lead (Pb) and zinc (Zn). The primary objective was to determine how these amendments altered the chemical fractions of Pb and Zn using Tessier’s sequential extraction procedure. Soil samples with initial concentrations of 3052.5 ± 15.6 mg kg⁻¹ Pb and 1531.0 ± 20.2 mg kg⁻¹ Zn were treated with biochar and apatite at 5%, 10%, 2.5:2.5%, and 5:5% (w/w). Results revealed that JSB600 and JSB300 at a 10% ratio, achieved the most significant reduction in exchangeable Pb and Zn fractions, decreasing them by up to 49.3 and 48.6%, respectively, within one month. This substantial decrease in readily available metal fractions, alongside concurrent increases in soil pH (+22.6%), organic carbon (+290.3%), and electrical conductivity (+249.0%), suggests that jackfruit seed biochar and apatite can significantly improve soil conditions for phytostabilization, by reducing metal bioavailability, or potentially for phytoextraction by influencing specific metal chemical fractions, in multi-metal-contaminated environments, enhancing soil conditions for remediation. This study presents a pioneering investigation into the remediation of heavy metal-contaminated agricultural soil in Vietnam country using a combination of apatite and biochar derived from jackfruit seeds. By examining the changes in heavy metal chemical speciation, the research offers a novel approach to addressing the environmental challenges posed by Pb/Zn mining in the region.