Simultaneous immobilization of arsenic and cadmium in paddy soil by Fe-Mn binary oxide: A field-scale study

Paddy soil in south China has long been haunted by the co-contamination of arsenic (As) and cadmium (Cd), resulting in the relatively high accumulation of As and Cd in rice, which puts humanity into a food safety dilemma. Therefore, it is paramount to restrain the migration of contaminants from soil to rice grains to cushion their impact on human health. However, the opposite biogeochemical behaviors of As and Cd in paddy soils under flooding condition make it a great challenge to simultaneously immobilize both As and Cd, particularly for the large-scale remediation. In this work, lime, Fe2O3, and Fe-Mn binary oxides (FM) were performed for immobilizing As and Cd in paddy soil at a field-scale experiment, and their associated mechanisms were discussed. Results showed that 0.10 wt% of Lime reduced Cd in grain (36.68%), 0.60 wt% of Fe2O3 decreased the accumulation of As and Cd (28.32% and 26.91%, respectively), and 0.60 wt% of FM significantly decreased As and Cd (42.42% and 36.49%, respectively). Analytical results of As speciation in rhizosphere soils demonstrated that FM played a dual role in oxidation and adsorption toward As immobilization. The DGT-measured As and Cd concentrations in rhizosphere soils showed that 0.60 wt% of FM significantly reduced the bioavailability of As and Cd in the paddy soil by 65.63% and 52.98%, respectively. Moreover, 0.60 wt% of FM promoted the formation of Fe/Mn-plaque on root surface, which significantly enhanced the adsorption of As and Cd upon Fe/Mn-plaque (44.06% and 32.14%, respectively) and further inhibited the uptake of As and Cd by rice. Hence, the mechanism for As and Cd immobilization by FM can be summarized: (1) oxidation of As(III) to As(V) and transformation and immobilization of As and Cd in rhizosphere soil and (2) promotion of Fe/Mn-plaque formation on root surface to retard the uptake of As and Cd by rice. These efforts attempt to set up a theory-to-practice solution for remediating As and Cd co-contamination in paddy soil.