Fe3O4 Nanomaterials for the Immobilization of Potentially Toxic Metals in Contaminated Mediterranean Agricultural Soils

Heavy metal contamination poses a major threat to soil quality and agricultural sustainability, particularly in Mediterranean regions affected by both anthropogenic activities and naturally metal-rich parent materials. This study evaluated the potential of iron oxide nanomaterials (Fe3O4) as a soil amendment for reducing heavy metal mobility in three agricultural soils from Western Macedonia, Greece, representing contamination from (i) lignite-fired power plant emissions, (ii) mining activities, and (iii) geogenic enrichment associated with ultramafic (ophiolitic) parent material. Fe3O4 nanomaterials were applied at rates of 0.5 and 1.0% (w/w), followed by a 45-day laboratory incubation alongside untreated controls. The application of Fe3O4 nanomaterials modified selected soil physicochemical properties, with the strongest response observed in the acidic mining soil, where soil pH increased from 5.80 ± 0.08 to 6.02 ± 0.07 and cation exchange capacity increased by approximately 10%, indicating more favorable conditions for metal retention. Changes in metal mobility and fractionation, assessed using calcium chloride (CaCl2) and diethylenetriaminepentaacetic acid (DTPA) extractions and the Community Bureau of Reference (BCR) sequential extraction procedure, showed a marked reduction in the most readily extractable metal fractions, although the magnitude of the response varied according to soil properties, initial metal mobility, and geochemical distribution. The strongest immobilization response was observed in the mining soil, where the 1.0% rate reduced CaCl2-extractable Cu, Zn, Cd and Pb by approximately 50, 40, 47 and 68% relative to the untreated control, whereas the changes in the geogenic soil were small. Metals lost from the exchangeable and acid-soluble fraction were recovered mainly in the reducible fraction, indicating redistribution among soil fractions rather than removal from the soil. Overall, Fe3O4 nanomaterials effectively immobilized potentially toxic metals under the conditions examined, although their efficiency depended on soil properties, contamination source, initial metal mobility, and geochemical distribution. These findings indicate that remediation performance cannot be predicted solely from pseudo-total metal concentrations and highlight the need to adapt Fe3O4 application strategies to site-specific soil characteristics and contaminant fractionation. As these results derive from a 45-day laboratory incubation, longer-term and field-scale evaluation is required before wider agricultural application can be recommended.