Sustainable soil remediation using nano-biochar for improved food safety and resource recovery

The contamination of agricultural soils with potentially toxic elements (PTEs) poses serious environmental and health risks due to their persistence and adverse effects on crop productivity. The main objective of this study was to evaluate the potential of nano-biochar (n-BC) to immobilize PTEs in contaminated soil and its effect on PTEs bioaccumulation in lettuce (Lactuca sativa L.), with the hypothesis that n-BC-due to their unique and improved physicochemical properties-are more effective than bulk forms in reducing PTEs mobility and bioavailability. Biochars (BCs) were obtained from palm bunch (PB), rice husk (RH) and sewage sludge (SSL) at 550°C and subsequently processed into nanoscale forms. A six-week pot experiment demonstrated that n-BC amendments significantly reduced the bioavailable (extracted with H₂O and CaCl₂) fractions of Cr, Cu, Fe, Mn, Ni, Zn, and Pb in soil, with higher immobilization efficiencies by 4.2 % to even 305 % than corresponding bulk biochars (b-BC). According to NICA-Donnan modelling, the main immobilization mechanisms were precipitation and ion exchange. Application of n-BC also resulted in a notable decrease in PTEs concentrations in lettuce leaves (ranging from 29.7 % to 100 %), thereby reducing both the bioaccumulation factor and health risk index. Among the different BCs, SSL-derived n-BC demonstrated the highest immobilization capacity and the most substantial reduction in PTEs uptake by plants. These findings highlight the potential of n-BC as a highly effective and low-cost amendment for rapid mitigation PTEs contamination in agricultural soils, enhancing food safety, and supporting circular economy principles by utilizing organic waste materials.