Sustainable removal of soil arsenic by naturally-formed iron oxides on plastic tubes.

Arsenic (As) pollution in paddy fields is a major threat to rice safety. Existing As remediation techniques are costly, require external chemical addition and degrade soil properties. Here, we report the use of plastic tubes as a recyclable tool to precisely extract As from contaminated soils. Following insertion into flooded paddy soils, polyethylene tube walls were covered by thin but massive Fe coatings of 76.9-367 mg Fe m-2 in 2 weeks, which adsorbed significant amounts of As. The formation of tube-wall Fe oxides was driven by local Fe-oxidizing bacteria with oxygen produced by oxygenic phototrophs (e.g., Cyanobacteria) or diffused from air through the tube wall. The tubes with As-bound Fe oxides can be easily separated from soil and then washed and reused. We tested the As removal efficiency in a pot experiment to remove As from ~ 20 cm depth/40 kg soils in a 2-year experiment and achieved an overall removal efficiency of 152 mg As m-2 soil year-1, comparable to phytoremediation with the As hyperaccumulator Pteris vittata. The cost of Fe hooks was estimated at 8325 RMB ha-1 year-1, and the profit of growing rice (around 16080 RMB ha-1 year-1 can be still maintained. The As accumulated in rice tissues was markedly decreased in the treatment (>11.1 %). This work provides a low-cost and sustainable soil remediation method for the targeted removal of As from soils and a useful tool for the study and management of the biogeochemical Fe cycle in paddy soils.

Soil organic matter amount determines the behavior of iron and arsenic in paddy soil with microbial fuel cells.

Arsenic (As) mobility in paddy soils is mainly controlled by iron (Fe) oxides and iron reducing bacteria (IBR). The Fe reducing bacteria are also considered to be enriched on the anode of soil microbial fuel cells (sMFC). Thus, the sMFC may have an impact on elements' behavior, especially Fe and As, mobilization and immobilization in paddy soils. In this study, we found dissolved organic matter (DOC) abundance was a major determinate for the sMFC impact on Fe and As. In the constructed sMFCs with and without water management, distinctive behaviors of Fe and As in paddy soil were observed, which can be explained by the low or high DOC content under different water management. When the sMFC was deployed without water management, i.e. DOC was abundant, the sMFC promoted Fe and As movement into the soil porewater. The As release into the porewater was associated with the enhanced Fe reduction by the sMFC. This was ascribed to the acidification effect of sMFC anode and the increase of Fe reducing bacteria in the sMFC anode vicinity and associated bulk soil. However, when the sMFC was coupled with alternating dry-wet cycles, i.e. DOC was limited, the Fe and As concentrations in the soil porewater dramatically decreased by up to 2.3 and 1.6 fold, respectively, compared to the controls under the same water management regime. This study implies an environmental risk for the in-situ application of sMFC in organic matter rich wetlands and also points out a new mitigation strategy for As management in paddy soils.