Field-scale remediation of moderately and mildly contaminated farmland: from bioavailability reduction to safe wheat cultivation

Heavy metal pollution in farmland poses a threat to crop safety and human health, with cadmium (Cd), arsenic (As), and mercury (Hg) being the main pollutants due to their high toxicity and bioaccumulation. Chemical amendment and agronomic regulation are effective measures for the remediation and safe utilization of mildly and moderately polluted farmland, but the effects of different amendments and their regulation mechanisms remain to be clarified. This study investigated the effects of rhamnolipids, ethylenediaminetetraacetic acid (EDTA-Na2), and (NH4)2SO4 (applied individually/jointly) for soil remediation and wheat safety through field experiments. Physicochemical properties and heavy metal forms were determined, and heavy metal contents in different wheat parts were determined to assess migration, enrichment, and safety. Results have shown the treatment containing (NH4)2SO4 significantly increased the soil organic matter (OM, +93.3%-105.3%) content. The combination of (NH4)2SO4 and EDTA-Na2 significantly increased the cation exchange capacity (CEC, +32.7%-54.0%). The application of amendments can promote the transformation of heavy metals from available to residual forms, and soil pH, OM, and CEC are key factors affecting the transformation of heavy metal forms. The bioavailability of heavy metals can be reduced through molecular interactions and interfacial processes. The S2E2 combination significantly enhances soil OM and CEC, achieving effective passivation of soil heavy metals through chelation and adsorption. In wheat, As and Cd are mainly retained in the roots (translocation factor [TF] < 1), while Hg is transported to the stems (TF > 1). All grains meet food safety limits, confirming that the synergistic effect of chemistry and agronomy can achieve safe cultivation of wheat during the remediation process.