Mechanistic Insight into the Abiotic Interactions of Selenate and Selenite with Natural Organic Matter

Impact of land use changes on selenium distribution and bioavailability in Mollisol of Sanjiang Plain

Recent changes in land use in the Mollisol region of the Sanjiang Plain have significantly impacted speciation and bioavailability of selenium (Se) in soils. This study examines Se species and distribution across various land use types, focusing on the interactions between iron oxides and organic matter in Se migration and transformation. The total Se content in woodland, dryland, and paddy soils is 0.79 mg/kg, 0.80 mg/kg, and 0.82 mg/kg, respectively, showing a uniform distribution. Soil organic carbon (SOC) is positively correlated with total Se, with complex effects on Se species. In paddy soils, SOC is positively associated with organic-bound Se and residual Se; Se content decreases with increasing soil depth in both profiles, following a typical surface accumulation pattern. Additionally, water management in paddy fields plays a critical role: prolonged water retention at the surface promotes Se enrichment in the topsoil and enhances dissolved Se leaching from deeper layers. This research indicated that coupling iron oxides and organic matter is crucial in Se fixation, particularly in paddy soils. Bioaccumulation factor (BCF) analysis shows a significant correlation between Se content in crops and soil Eh and pH, with higher Eh enhancing Se uptake. This study highlights the influence of land use and soil properties on Se bioavailability.

Effects of glomalin-related soil protein on soil selenium availability in farmland: a non-negligible component of organic matter

The selenium (Se) soil environmental behaviour directly regulates the Se enrichment effect of crops and the risk of Se ecotoxicity and is critically influenced by soil organic matter fractions. Recent studies have shown that glomalin-related soil protein (GRSP) plays an important role in regulating heavy metal transport transformation. However, there is still a lack of systematic knowledge of the effects of GRSP on the Se environmental behaviour in soil, which is a knowledge gap that might hinder the efficient and safe utilization of Se resources in farmland. This study combined field sampling across Se-rich and Se-toxicity farmlands with laboratory adsorption experiments to investigate the effects of GRSP on the soil Se availability. Field sampling revealed the impact of GRSP on soil Se availability in farmland cannot be ignored. Adsorption of Se by GRSP significant reduced soil Se availability in Se-rich farmland, but had a significant promoting effect in Se-toxicity farmland. This was related to the differential Se adsorption modes due to the adsorption saturation. Hydrogen bonding and ion exchange were the primary and secondary modes of Se adsorption by GRSP, respectively, demonstrated by laboratory results. The adsorption process of Se by GRSP started with Se reduction caused by carbonylation of hydroxyl groups on the surface. The Se adsorption was finally completed by abundant carbonyl and hydroxyl groups on GRSP surface through ion exchange generation and hydrogen bonding, respectively. This enhances our understanding of how organic matter, particularly GRSP, affects the environmental risk associated with Se in soil.

Effects of straw amendment on the bioavailability of selenite in soil and its mechanisms

Dissolved organic matter (DOM) released by straw returning for decomposition interacts with selenium (Se) in soil, which affects the speciation distribution of Se and its bioavailability. However, the relative mechanisms involved are slightly understood. This study investigated the effects of straw-derived DOM on two levels of exogenous selenite (low-Se and high-Se treatments) in two types of soil with distinct pH. Interactions between DOM and Se were revealed through three-dimensional excitation emission matrix (3D-EEM) fluorescence spectroscopy and two-dimensional correlation spectroscopy (2D-COS). Results showed that straw amendment significantly enhanced selenite bioavailability in alkaline Lou soil regardless of Se application rates (p < 0.05). However, only the high-Se treatment generated remarkable Se content in wheat grains in acidic krasnozems (p < 0.05). Selenite predominantly incorporated with phenolic and etheric C-O functional groups of DOM in soil, which mainly existed in aromatic DOM such as humic acid (HA). Consequently, HA-Se was more likely to form in krasnozems enriched with HA. 2D-COS evidenced that HA mineralization promoted Se bioavailability in krasnozems with high-Se treatment. After selenite complexed with saturated and unsaturated aliphatic carboxyl groups (CO) of DOM, it formed Hy-Se and FA-Se in Lou soil, which could be directly absorbed by wheat roots. Therefore, the composition and functional group reaction sequences of DOM in different soils manipulated selenite bioavailability in soil. These findings could provide a basis for regulating Se bioavailability during biofortification in soils.

Thioarsenate sorbs to natural organic matter through ferric iron-bridged ternary complexation to a lower extent than arsenite

Understanding processes regulating thioarsenate (HxAsSnO4-n3-x; n = 1 - 3; x = 1 - 3) mobility is essential to predicting the fate of arsenic (As) in aquatic environments under anoxic conditions. Under such conditions, natural organic matter (NOM) is known to effectively sorb arsenite and arsenate due to metal cation-bridged ternary complexation with the NOM. However, the extent and mechanism of thioarsenate sorption onto NOM via similar complexation has not been investigated. By equilibrating monothioarsenate (representative of thioarsenate) with a peat (model NOM) with different Fe(III) loadings, this study shows that NOM can sorb monothioarsenate considerably via Fe(III)-bridging. Iron and As K-edge XAS analysis of the monothioarsenate-treated Fe-loaded peats revealed that monothioarsenate forms bidentate mononuclear edge-shared (1E) (RAs···Fe: 2.89 ± 0.02 Å) and bidentate binuclear corner-shared (2C) (RAs···Fe: 3.32 Å) complexes with organically bound Fe(O,OH)6 octahedra, in addition to direct covalent bonds with oxygen-containing functional groups (e.g., -COOH and -OH) (RAs···C: 2.74 ± 0.02 Å), upon equilibration with the Fe(III)-loaded peat. However, the extent of monothioarsenate sorption was considerably less than that of its precursor As species, arsenite, due to higher electrostatic repulsion between the negatively charged monothioarsenate and peat. This study implies that thioarsenate formation under anoxic conditions would increase As mobility by decreasing its sorption onto the NOM.

Prediction and pathway models for assessing soil properties influencing soil selenium enrichment and bioavailability in Aksu Prefecture, northwest China

Selenium (Se) in soil is the primary source of human Se intake, and its content and bioavailability are influenced by soil physicochemical properties. However, the influence of soil physicochemical properties on Se enrichment and bioavailability in soil remains uncertain. Therefore, this study investigated 536 soil samples and their corresponding wheat grain samples collected from the oasis zone of Aksu Prefecture, located in northwest China. The Se content, spatial distribution, and bioaccumulation factor (BCF) in soil and wheat grains as well as soil Se fractions were examined, and the effects of soil physicochemical properties on Se enrichment and bioavailability were assessed. The results indicated that the mean Se content of soil (0.32 mg/kg) surpassed the national Se background level for Chinese soil by a factor of 1.10. The average Se content (0.13 mg/kg) in wheat grains met the national standard for Se-rich cereal products. The prediction model established using multiple linear regression showed that soil calcium carbonate (CaCO3), organic matter (OM), cation exchange capacity (CEC), and electrical conductivity (EC) were the main physicochemical factors influencing Se enrichment in soil, accounting for 29 % of the variation. The main physicochemical factors affecting Se bioavailability were CaCO3, OM, and iron, and the main Se fractions were the exchangeable and humic acid-bound Se fractions, which together explain 12 % of the variance. Additionally, a structural equation model was employed to analyze the pathways and interactions of soil factors on soil Se enrichment and bioavailability. The results indicated that soil CaCO3 and OM were the most critical factors affecting Se enrichment and bioavailability in soil. These findings can provide technical guidance for the cultivation and layout of Se-rich wheat in local and other similar regions around the world.

DGT and kinetic analyses differentiate Se and Cd bioavailability in naturally enriched paddy soils

Naturally selenium (Se)-rich soils often contain elevated cadmium (Cd) levels, complicating safe production of Se-enriched rice. This study employed diffusive gradients in thin-films (DGT) and DGT-induced fluxes in soils (DIFS) model to determine Se and Cd bioavailability in paddy soils. We investigated desorption kinetics and accumulation patterns in rice using paired rhizosphere and grain samples from 65 field sites in Guangxi, China, encompassing Se-enriched karst and non-karst soils. Despite greater total Se and Cd contents in karst soils, their elevated pH, along with greater soil organic matter and total Fe, Mn, and Ca contents, constrained Se and Cd bioavailability, resulting in similar accumulation levels in rice grains from both soil categories. DIFS-derived kinetic data revealed that Se was replenished 75.4 times faster than Cd, but Cd had an 83.2 times larger labile pool, leading to a stronger overall Cd resupply capacity. DGT-based labile Se:Cd molar ratios showed that rice Cd content declined sharply as the ratio increased from 0.7 to 4.0, stabilizing at its lowest level when exceeding 20. Moreover, DGT measurements demonstrated stronger correlations with grain Se and Cd concentrations compared to traditional methods. Our findings highlight the effectiveness of DGT and kinetic analyses in determining Se and Cd bioavailability in high-background paddy soils, offering insights for balancing Se fortification and Cd risk mitigation in rice production.

Zero-valent iron (ZVI) facilitated in-situ selenium (Se) immobilization and its recovery by magnetic separation: Mechanisms and implications for microbial ecology

Selenium (Se(VI)) is environmentally toxic. One of the most popular reducing agents for Se(VI) remediation is zero-valent iron (ZVI). However, most ZVI studies were carried out in water matrices, and the recovery of reduced Se has not been investigated. A water-sediment system constructed using natural sediment was employed here to study in-situ Se remediation and recovery. A combined effect of ZVI and unacclimated microorganisms from natural sediment was found in Se(VI) removal in the water phase with a removal efficiency of 92.7 ± 1.1% within 7 d when 10 mg L-1 Se(VI) was present. Soluble Se(VI) was removed from the water and precipitated to the sediment phase (74.8 ± 0.1%), which was enhanced by the addition of ZVI (83.3 ± 0.3%). The recovery proportion of the immobilized Se was 34.2 ± 0.1% and 92.5 ± 0.2% through wet and dry magnetic separation with 1 g L-1 ZVI added, respectively. The 16 s rRNA sequencing revealed the variations in the microbial communities in response to ZVI and Se, which the magnetic separation could potentially mitigate in the long term. This study provides a novel technique to achieve in-situ Se remediation and recovery by combining ZVI reduction and magnetic separation.

Insight into the availability and desorption kinetics of Se and Cd in naturally-rich soils using diffusive gradients in thin-films technique

Cadmium (Cd) contamination of selenium (Se)-rich soils may jeopardize the nutritional benefits of Se-biofortified crops. This study employed diffusive gradients in thin-films (DGT) technique and DIFS (DGT-induced fluxes in soils) model to understand the interdependency and driving factors of Se and Cd distribution and desorption kinetics across 50 soils from south China with naturally elevated levels. DGT-labile Se was the highest (up to 2.66 μg L-1) in non-carbonate/shale-derived soils, while Cd was maximal (5.53 μg L-1) in carbonate-based soils, reflecting soil background concentrations and soil characteristics. Over one-third of the soils showed labile Se:Cd molar ratio below 0.7, suggesting Cd phytotoxicity risks. The DIFS-derived response times (Tc) and desorption rate constants (k-1) suggested that Se was resupplied to the soil solution faster than Cd in soils with higher pH and SOM level, but Se resupply was still restricted due to the rapid depletion of its labile pool. As the first study of Se and Cd release kinetics in soils, our results reveal dependence on soil parent materials, with low labile Se:Cd soils presenting greater Cd hazards. By elucidating Se and Cd lability and interactions in soils, our findings help to inform management strategies to balance reduced Cd risk with adequate Se availability.