Effects of in situ Fe oxide precipitation on As stabilization and soil ecological resilience under salt stress

Iron (Fe) oxide precipitation is a promising method for stabilizing arsenic (As) in contaminated soils; however, the addition of salts during the process can negatively affect soil functions. This study investigated the effects of in situ Fe oxide precipitation on As stabilization and the impact of salt stress on soil functions and microbial communities. Fe oxide precipitation reduced the concentration of bioaccessible As by 84% in the stabilized soil, resulting in the formation of ferrihydrite and lepidocrocite, as confirmed by XANES. Nevertheless, an increase in salt stress reduced barley development, microbial enzyme activities, and microbial diversity compared to those in the original soil. Despite this, the stabilized soil exhibited natural resilience and potential for enhanced microbial adaptations, with increased retention of salt-tolerant bacteria. Washing the stabilized soil with water restored EC1:5 to the level of the original soil, resulting in increased barley growth rates and enzyme activities after 5-d and 20-week incubation periods, suggesting soil function recovery. 16S rRNA sequencing revealed the retention of salt-tolerant bacteria in the stabilized soil, while salt-removed soil exhibited an increase in Proteobacteria, which could facilitate ecological functions. Overall, Fe oxide precipitation effectively stabilized soil As and exhibited potential for restoring the natural resilience and ecological functions of soils through microbial adaptations and salt removal. Reducing the bioaccessible As concentration rather than the absolute As concentration in soil is critical for in situ applications. In situ precipitation of Fe oxides using ferric nitrate and NaOH effectively lowered the chemical extractability of As and its bioaccessibility in soil by generating more stable forms of Fe oxides that encapsulate As. However, the impacts of added salts on the soil function and ecological resilience must be considered. Removing salts from stabilized soil enhances the ecological functions, as demonstrated by recovered barley growth and restored microbial communities. This highlights the importance of considering long-term effects when stabilizing As-contaminated soils. [Display omitted] •In situ Fe oxide precipitation is effective for stabilizing As in soil•Stabilized soil exhibited natural resilience•Resilience potential could be naturally enhanced via microbial adaptations•Further enhancement is possible through natural circumstances, such as rai