Enhanced oxidation and stabilization of arsenic in a soil-rice system by phytosynthesized iron oxide nanomaterials: Mechanistic differences under flooding and draining conditions

Rare earth elements redistribution in mine tailings soil: A comparative study of sunlit and shady slopes after in-situ leaching

The in-situ leaching of rare earth minerals results in ecological differences between sunlit and shady slopes, which may be related to differences in the distribution REEs in the associated soil matrices. Studies of REEs mine tailings in Southern China indicated higher total concentrations of REEs on sunlit slopes compared to shady ones. Specifically, the exchangeable REEs fraction (F1-REEs) was higher on the shady slopes, whereas the Fe/Mn oxides bound REEs fraction (F3-REEs) was higher on the sunlit slopes. In addition, light REE (LREE) concentrations were lower at lower elevations. With the exception of the Ce fraction which remained stable, this indicated a change in all REEs distributions, moving from F1-REEs towards the residual fraction. Hierarchical cluster and principal component analysis revealed a strong correlation between F3-REEs, organic matter bound REEs (F4-REEs), and LREEs, and a positive association of F3-REEs with sunlight exposure. Partial Least Squares Path Modeling analysis suggested that OM promoted the conversion of LREEs to F3 and F4-REEs in soil driven by sunlight exposure. Additionally, as the Feo/Fed ratio decreased, more LREEs were converted to F3. This study suggests that sunlight and elevation both play a critical role in the geochemical dynamics of REEs in in-situ tailings, advocating for environmental evaluations to be undertaken in order to accurately understand the ecological impacts of rare earth mining.

Intensive ammonium fertilizer addition activates iron and carbon conversion coupled cadmium redistribution in a paddy soil under gradient redox conditions

While over-fertilization and nitrogen deposition can lead to the enrichment of nitrogen in soil, its effects on heavy metal fractions under gradient moisture conditions remains unclear. Here, the effect of intensive ammonium (NH4+) addition on the conversion and interaction of cadmium (Cd), iron (Fe) and carbon (C) was studied. At relatively low (30-80 %) water hold capacity (WHC) NH4+ application increased the carbonate bound Cd fraction (F2Cd), while at relatively high (80-100 %) WHC NH4+ application increased the organic matter bound Cd fraction (F4Cd). Iron‑manganese oxide bound Cd fractions (F3Cd) and oxalate-Fe decreased, but DCB-Fe increased in NH4+ treatments, indicating that amorphous Fe was the main carrier of F3Cd. The variations in F1Cd and F4Cd observed under the 100-30-100 % WHC treatment were similar to those observed under low moisture conditions (30-60 % WHC). The C=O/C-H ratio of organic matter in soil decreased under the 30-60 % WHC treatment, but increased under the 80-100 % WHC treatment, which was the dominant factor influencing F4Cd changes. The conversion of NH4+ declined with increasing soil moisture content, and the impact on oxalate-Fe was greater at 30-60 % WHC than at 80-100 % WHC. Correspondingly, genetic analysis showed the effect of NH4+ on Fe and C metabolism at 30-60 % WHC was greater than at 80-100 % WHC. Specifically, NH4+ treatment enhanced the expression of genes encoding extracellular Fe complexation (siderophore) at 30-80 % WHC, while inhibiting genes encoding Fe transmembrane transport at 30-60 % WHC, indicating that siderophores simultaneously facilitated Cd detoxification and Fe complexation. Furthermore, biosynthesis of sesquiterpenoid, steroid, butirosin and neomycin was significantly correlated with F4Cd, while glycosaminoglycan degradation metabolism and assimilatory nitrate reduction was significantly correlated with F2Cd. Overall, this study gives a more comprehensive insight into the effect of NH4+ on activated Fe and C conversion on soil Cd redistribution under gradient moisture conditions.

Impact of coexisting components in acid mine drainage on Sb(Ⅲ) oxidation by biosynthesized iron nanoparticles

Despite the oxidation mechanism of antimonite (Sb(Ⅲ)) by biosynthesized iron nanoparticles (Fe NPs) has been reported, the impact of coexisting components in acid mine drainage (AMD) on the Sb(III) oxidation by Fe NPs is unknown. Herein, how the coexisting components in AMD affect Sb(Ⅲ) oxidation by Fe NPs was investigated. Firstly, Fe NPs achieved complete oxidation of Sb(Ⅲ) (100%), while only 65.0% of Sb(Ⅲ) was oxidized when As(Ⅲ) was added, due to competitive oxidation between As(Ⅲ) and Sb(Ⅲ), which was verified by characterization analysis. Secondly, the decline in solution pH improved Sb(Ⅲ) oxidation from 69.5% (pH 4) to 100% (pH 2), which could be attributed to the rise of Fe³⁺ in solution promoting the electron transfer between Sb(Ⅲ) and Fe NPs. Thirdly, the oxidation efficiencies of Sb(Ⅲ) fell by 14.9 and 44.2% following the addition of oxalic and citric acid, respectively, resulting from the fact that these two acids reduced the redox potential of Fe NPs, thereby inhibiting Sb(Ⅲ) oxidation by Fe NPs. Finally, the interference effect of coexisting ions was studied, where PO₄^3- significantly reduced Sb(Ⅲ) oxidation efficiency due to the occupation of the surface-active sites on Fe NPs. Overall, this study has significant implications for the prevention of Sb contamination in AMD.