Pristine and biochar-supported nano zero-valent iron to immobilize As, Zn and Pb in soil contaminated by smelting activities

Nano zero-valent iron (nZVI) is one of the most studied nanomaterials for environmental remediation during the past 20 years. However, few studies have focused on nZVI combination with other materials (e.g., biochar) for enhancement of soil remediation. In this study, pristine nZVI and a composite of wood sawdust biochar (BC) and nZVI (nZVI-BC) were added to a highly contaminated soil to compare their efficacy in immobilizing available arsenic (As = 28.6 mg kg^-1), zinc (Zn = 1707 mg kg^-1), and lead (Pb = 6759 mg kg^-1). Sediment quality guidelines were used to evaluate the extent of soil contamination and ascertain its source. The mineralogy of soil and slags were assessed by X-ray Diffractometry Spectroscopy (XRD), and the geochemical fractions of Pb, Zn, and As were obtained by chemical sequential extractions. The average Pollution Load Index (PLI) was 10.66, indicating elevated multi-elemental contamination. Contamination Factor (CF) values for As, Zn, Pb, cadmium (Cd), and copper (Cu) were all higher than 6 which implies extreme contamination. Secondary minerals frequently found in Pb/Zn smelter sites, such as cerussite and anglesite, were detected in the slags through XRD. Pb and Zn were mainly bound to carbonates and residual fractions in soil and presented a high risk considering the sediment quality guidelines, sequential extraction results, and XRD analysis. The treatment with nZVI-BC was more effective than pristine nZVI on concurrently decreasing 97% of available As, 84% of Pb and 81% of Zn compared to control. The application of nZVI-BC is a promising green and sustainable remediation technique for soils contaminated with potentially toxic elements of distinct chemical behavior.