Abstract
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.
Highlights
Due to large-scale mining and industrial activities, emission of industrial wastes, wastewater irrigation, and pollution of toxic metal(loid)s in farmland around the mining area are becoming the most urgent environmental concerns, the potential toxic elements (PTEs), for example, arsenic (As) and cadmium (Cd; Ye et al, 2018; Liu et al, 2019; Xiao et al, 2020; Zhou et al, 2020)
It is very urgent to impede the migration of As and Cd from soil to rice, which is of great significance to alleviate the risks of As and Cd in the paddy soil in the main rice producing areas, thereby securing the safe rice (Chen et al, 2018; Carrijo et al, 2019; Zhai et al, 2020)
The results showed that As in flooding soil existed in two inorganic speciation, As(V) and As(III), accounting for 72.17% and 27.83%, respectively
Summary
Due to large-scale mining and industrial activities, emission of industrial wastes, wastewater irrigation, and pollution of toxic metal(loid)s in farmland around the mining area are becoming the most urgent environmental concerns, the potential toxic elements (PTEs), for example, arsenic (As) and cadmium (Cd; Ye et al, 2018; Liu et al, 2019; Xiao et al, 2020; Zhou et al, 2020). Lin et al: Simultaneous immobilization of As and Cd in paddy soil by Fe-Mn oxide especially in Asia, for example, contributing about 60% of the dietary As intake and 56% of the dietary Cd intake for the general population in China (Chen et al, 2018; Xu et al, 2018). As and Cd in paddy soil usually exhibit opposite geochemical behavior (Yang et al, 2018). Arsenic adsorbed on Fe(III) would be dissolved and released into the soil solution due to the reduction reaction, thereby improving the bioavailability of As. a reducing condition enables the soluble Cd(II) to form precipitates, resulting in low bioavailability (Honma et al, 2016; Yu et al, 2017)
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