Biochar-supported nanoscale zero-valent iron can simultaneously decrease cadmium and arsenic uptake by rice grains in co-contaminated soil

Cadmium (Cd) and Arsenic (As) in rice grains are a primary exposure source for human beings. However, the simultaneous stabilization of Cd and As in soil becomes difficult due to the opposite properties of those. In this study, we investigated the simultaneous effects of biochar-supported nanoscale zero-valent iron (nZVI-BC) and water management on the decrease of Cd and As bioaccumulation in rice grain. Compared to the control, 0.25-1.00% nZVI-BC coupled with alternate wetting and drying (AWD) management simultaneously decreased the bioaccumulation of Cd and As in rice grains by 15.85-69.16% and 23.06-59.45%, respectively. The cancer risk associated with rice consumption effectively reduced by 15.60-52.41% after the application of nZVI-BC, and the lowest cancer risk was detected in 1.00% nZVI-BC under AWD management. Furthermore, rice cultivated under AWD management had a lower total cancer risk than that cultivated under continuous flooded (CF) management with the same amendment of type and dose. The reduction of soil Cd and As availability and the formation of iron plaque dominated the decrease of Cd and As uptake by rice grains. The elevated soil pH was responsible for Cd adsorption, and the dominant mechanism for As immobilization was the formation of complexes. The iron plaque was double-edged, promoting and inhibiting Cd uptake by rice, wherein the inhibition was predominant under aerobic conditions. In addition, iron plaque was a barrier to preventing the As accumulation by rice, a larger amount of As was immobilized on the iron plaque with nZVI-BC treatment. This study sheds new insights on the simultaneous remediation of Cd and As co-contaminated paddy fields.