Arsenic methylation and microbial communities in paddy soils under alternating anoxic and oxic conditions.

Arsenic (As) methylation in soils affects the environmental behavior of As, excessive accumulation of dimethylarsenate (DMA) in rice plants leads to straighthead disease and a serious drop in crop yield. Understanding the mobility and transformation of methylated arsenic in redox-changing paddy fields is crucial for food security. Here, soils including un-arsenic contaminated (N-As), low-arsenic (L-As), medium-arsenic (M-As), and high-arsenic (H-As) soils were incubated under continuous anoxic, continuous oxic, and consecutive anoxic/oxic treatments respectively, to profile arsenic methylating process and microbial species involved in the As cycle. Under anoxic-oxic (A-O) treatment, methylated arsenic was significantly increased once oxygen was introduced into the incubation system. The methylated arsenic concentrations were up to 2-24 times higher than those in anoxic (A), oxic (O), and oxic-anoxic (O-A) treatments, under which arsenic was methylated slightly and then decreased in all four As concentration soils. In fact, the most plentiful arsenite S-adenosylmethionine methyltransferase genes (arsM) contributed to the increase in As methylation. Proteobacteria (40.8%-62.4%), Firmicutes (3.5%-15.7%), and Desulfobacterota (5.3%-13.3%) were the major microorganisms related to this process. These microbial increased markedly and played more important roles after oxygen was introduced, indicating that they were potential keystone microbial groups for As methylation in the alternating anoxic (flooding) and oxic (drainage) environment. The novel findings provided new insights into the reoxidation-driven arsenic methylation processes and the model could be used for further risk estimation in periodically flooded paddy fields.

Organic matter in geothermal springs and its association with the microbial community.

Organic matter (OM) plays an important role in the biogeochemical cycles of carbon, nitrogen, and other elements, shaping the structure of the microbiome and vice versa. However, the molecular composition of OM and its impact on the microbial community in terrestrial geothermal environments remain unclear. In this study, we characterized the OM in water and sediment from a typical geothermal field using ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry. By combining high-throughput amplicon sequencing and multivariate analyses, we deciphered the association between OM components and microbial community. A surprisingly high chemodiversity of OM was observed in the waters (11,088 compounds) and sediments (7772 compounds) in geothermal springs. Sulfur-containing organic compounds, a characteristic molecular signature of geothermal springs, accounted for 21 % ± 5 % in waters and 33 % ± 4 % in sediments. Multivariate analyses revealed that both labile and recalcitrant fractions of OM (e.g., carbohydrates intensity and tannins chemodiversity) influenced the structure and function of the microbial community. Co-occurrence networks showed that Proteobacteria and Crenarchaeota accounted for most of the connections with OM in waters (33 % and 15 %, respectively) and sediments (15 % and 12 %, respectively), highlighting their key roles in carbon cycling. This study expands our understanding of the molecular compositions of OM in geothermal springs and highlights its potentially important role in global climate change through microbial carbon cycling.

Fast On-Site Speciation and High Spatial Resolution Imaging of Labile Arsenic in Freshwater and Sediment Using the DGT-SERS Sensor.

Diffusive gradients in thin films (DGT) technique is renowned for in situ passive sampling but not for rapid on-site analysis, whereas surface-enhanced Raman spectroscopy (SERS) excels in ultrasensitive on-site detection but is limited by substrate contamination from complex matrices. Here, a hierarchical nanostructure of silver (Ag) mirror-supported large Ag nanoparticles (∼120 nm) was grown in situ in polyacrylamide hydrogel with a restricted pore size (PAM/Ag mirror/AgNPs) to serve as both the DGT binding phase and the SERS substrate. The substrate exhibited a maximum electric field enhancement factor of 9.9 × 108 and a signal relative standard error of 4.8%. Using the DGT-SERS sensor, As(III) and As(V) in freshwater were simultaneously detected at limits of 0.9 and 0.8 µg L-1, respectively, applicable across a wide range of environmental conditions. The DGT-SERS effectively mitigated the interfacial reduction of As(V) caused by humic acid by excluding it from plasmonic hotspots through size exclusion of the diffusive layer. The Raman analysis of a DGT sample in the field requires only 2 s using a portable spectrometer without DGT device disassembly. More importantly, the DGT-SERS captured the first two-dimensional image of As(III) and As(V) in one DGT at the micron scale resolution, revealing their spatially supplementary distribution patterns at the sediment-water interface. This study paves the way for next-generation speciation imaging DGT and the application of SERS in complex environments.

Phosphate ligand-mediated production of reactive oxygen species during oxygenation of Fe(II)-phosphate complexes.

Reactive oxygen species (ROS) production upon the oxygenation of reduced iron minerals is of critical importance to redox cycles of Fe and the fate of refractory organic contaminants. The environmental impact factors during this process, however, have been underappreciated. In this study, prominently enhanced production of hydroxyl radicals (•OH) was observed by oxygenation of Fe(II) with 5-50 mM phosphate. The results of spin trap electron spin resonance (ESR) experiment showed that Fe(II)-phosphate complexes facilitated the generation of •OH. The degradation experiment of p-nitrophenol (PNP) confirmed that •OH formation was dominated by a consecutive one-electron O2 reduction (90.2-96.9 %), and the quantification of PNP degradation products revealed that Fe(II)/phosphate molar ratios regulated the O2 activation pathways for O2•- or •OH production. The further experimental and theoretical investigation demonstrated that the coordination of phosphate with Fe(II) plays a dual role in ROS generation that facilitated O2•- formation by lowering the energy barrier for Fe(II) oxidation and altered the reaction pathway of •OH formation due to its occupation of sites for electron transfer. The present work highlights an important role of natural oxyanions in O2 activation by Fe(II) and raises the possibility of in situ degradation of contaminants in subsurface environment.

Detecting antimony(III) on-site using novel gel-based techniques: Colorimetric diffusive equilibrium in thin films for two-dimensional imaging and surface-enhanced Raman scattering for sensitive quantification.

Antimony (Sb) pollution has raised increasing public concerns and its rapid on-site screening is central for the risk assessment. Herein, we proposed two gel-based methods based on colorimetric diffusive equilibrium in thin films (DET) and surface-enhanced Raman scattering (SERS), for two-dimensional imaging and sensitive detection of Sb(III) by revisiting the phenylfluorone (PhF) complexation reaction. PhF was well dispersed in the polyvinyl alcohol (PVA) hydrogel and reacted with Sb(III) in the DET gel to form a strong PhF-Sb(III) complex. The distribution of Sb(III) was easily visualized at a submillimeter resolution using computer imaging densitometry, with a detection limit (LOD) of ∼100 nmol L-1. Field application in the Sb mine area reveals limited dissolved Sb(III) penetrating the redox barrier below the sediment-water interface by 20 mm in rivers and tailing pond sediments. To improve the detection sensitivity and apply the principle to trace Sb quantification, a SERS platform was established by anchoring PhF on the hydrogel-stabilized Ag nanoparticles via C-O-Ag bonding to specifically detect Raman-inactive Sb(III). Benefiting from the high SERS activity of PhF and enrichment ability of hydrogel, Sb(III) was quantified with a LOD of 1.2-10.7 nmol L-1 depending on the sample volume. The coexisting ions at a 100-fold higher concentration than Sb(III) resulted in only 3.3-10.4 % variation in SERS intensity, indicating a negligible interference on the SERS platform. The platform exhibited a RSD of 6.6-13.1 % and acceptable recoveries for various environmental matrices, highlighting its promise in on-site application.

Incorporation of Shewanella oneidensis MR-1 and goethite stimulates anaerobic Sb(III) oxidation by the generation of labile Fe(III) intermediate.

Dissimilatory iron-reducing bacteria (DIRB) affect the geochemical cycling of redox-sensitive pollutants in anaerobic environments by controlling the transformation of Fe morphology. The anaerobic oxidation of antimonite (Sb(III)) driven by DIRB and Fe(III) oxyhydroxides interactions has been previously reported. However, the oxidative species and mechanisms involved remain unclear. In this study, both biotic phenomenon and abiotic verification experiments were conducted to explore the formed oxidative intermediates and related processes that lead to anaerobic Sb(III) oxidation accompanied during dissimilatory iron reduction. Sb(V) up to 2.59 µmol L-1 combined with total Fe(II) increased to 188.79 µmol L-1 when both Shewanella oneidensis MR-1 and goethite were present. In contrast, no Sb(III) oxidation or Fe(III) reduction occurred in the presence of MR-1 or goethite alone. Negative open circuit potential (OCP) shifts further demonstrated the generation of interfacial electron transfer (ET) between biogenic Fe(II) and goethite. Based on spectrophotometry, electron spin resonance (ESR) test and quenching experiments, the active ET production labile Fe(III) was confirmed to oxidize 94.12% of the Sb(III), while the contribution of other radicals was elucidated. Accordingly, we proposed that labile Fe(III) was the main oxidative species during anaerobic Sb(III) oxidation in the presence of DIRB and that the toxicity of antimony (Sb) in the environment was reduced. Considering the prevalence of DIRB and Fe(III) oxyhydroxides in natural environments, our findings provide a new perspective on the transformation of redox sensitive substances and build an eco-friendly bioremediation strategy for treating toxic metalloid pollution.

Arsenite S-Adenosylmethionine Methyltransferase Is Responsible for Antimony Biomethylation in Nostoc sp. PCC7120.

Antimony (Sb) biomethylation is an important but uninformed process in Sb biogeochemical cycling. Methylated Sb species have been widely detected in the environment, but the gene and enzyme for Sb methylation remain unknown. Here, we found that arsenite S-adenosylmethionine methyltransferase (ArsM) is able to catalyze Sb(III) methylation. The stepwise methylation by ArsM forms mono-, di-, and trimethylated Sb species. Sb(III) is readily coordinated with glutathione, forming the preferred ArsM substrate which is anchored on three conserved cysteines. Overexpressing arsM in Escherichia coli AW3110 conferred resistance to Sb(III) by converting intracellular Sb(III) into gaseous methylated species, serving as a detoxification process. Methylated Sb species were detected in paddy soil cultures, and phylogenetic analysis of ArsM showed its great diversity in ecosystems, suggesting a high metabolic potential for Sb(III) methylation in the environment. This study shows an undiscovered microbial process methylating aqueous Sb(III) into the gaseous phase, mobilizing Sb on a regional and even global scale as a re-emerging contaminant.

Iron oxide-supported gold nanoparticle electrode for simultaneous detection of arsenic and sulfide on-site.

The environmental behavior of arsenic (As) has garnered significant attention due to its hazardous nature. The fate of As often couples with sulfide, thus co-detecting arsenic and sulfide on-site is crucial for comprehending their geochemical interactions. While electrochemical methods are suitable for on-site chemical analysis, there currently exists no electrode capable of simultaneously detecting both arsenic and sulfide. To address this, we developed a dual-metal electrode consisting of iron oxide-encased carbon cloth loaded with gold nanoparticles (Au/FeOx/CC) using the electrochemical deposition method. This electrode enables square wave stripping voltammetry (SWASV) binary detection of As and sulfide. Comparison experiments reveal that the reaction sites for sulfide primarily reside on FeOx, while the interface synergy of iron oxide and gold nanoparticles enhances the response to arsenite (AsIII). Arsenate (AsV) is directly reduced to As0 on Fe0, obviating the need for an external reducing agent. The electrode achieves detection limits of 1.5 µg/L for AsV, 0.25 µg/L for AsIII, and 11.6 µg/L for sulfide at mild conditions (pH 7.8). Field validation was conducted in the Tengchong geothermal hot spring region, where the electrochemical method exhibited good correlation with the standard methods: Total As (r = 0.978 vs. ICP-MS), AsIII (r = 0.895 vs. HPLC-ICP-MS), and sulfide (r = 0.983 vs. colorimetric method). Principal component analysis and correlation analysis suggest that thioarsenic, could potentially be positive interferents for AsIII. However, this interference can be anticipated and mitigated by monitoring the abundance of sulfide. The study provides new insights and problems for the electrochemical detection of coexisted As and sulfide.

Sequential separation of critical metals from lithium-ion batteries based on deep eutectic solvent and electrodeposition.

The rise and development of electric vehicles have brought much attention to the recycling of lithium-ion batteries (LIBs). However, the recovery of critical metals from LiNixCoyMn1-x-yO2 (NCM) is a challenge, especially for the nickel and cobalt, which have similar chemical properties. Here, a novel ternary deep eutectic solvent (DES) composed of choline chloride, ethylene glycol, and tartaric acid was proposed. Our protocol of DES synthesis, nickel separation, and leaching of cobalt and manganese were integrated into one step, which significantly simplified the recovery process. The crystallization occurring during DES leaching was subjected to detailed investigation. The lithium, nickel, and cobalt were sequentially separated as Li2CO3, NiO, and Co(OH)2 by anterior formic acid leaching and posterior electrodeposition. After electrodeposition, DES was reused. This work provides new ideas for the sequential separation of critical metals from NCM and has great application prospects.

A Multienzyme Cascade Pathway Immobilized in a Hydrogen-Bonded Organic Framework for the Conversion of CO2.

The reduction of carbon dioxide to valuable chemicals through enzymatic processes is regarded as a promising approach for the reduction of carbon dioxide emissions. In this study, an in vitro multi-enzyme cascade pathway is constructed for the conversion of CO2 into dihydroxyacetone (DHA). This pathway, known as FFFP, comprises formate dehydrogenase (FDH), formaldehyde dehydrogenase (FaldDH), formolase (FLS), and phosphite dehydrogenase (PTDH), with PTDH serving as the critical catalyst for regenerating the coenzyme NADH. Subsequently, the immobilization of the FFFP pathway within the hydrogen-bonded organic framework (HOF-101) is accomplished in situ. A 1.8-fold increase in DHA yield is observed in FFFP@HOF-101 compared to the free FFFP pathway. This enhancement can be explained by the fact that within FFFP@HOF-101, enzymes are positioned sufficiently close to one another, leading to the elevation of the local concentration of intermediates and an improvement in mass transfer efficiency. Moreover, FFFP@HOF-101 displays a high degree of stability. In addition to the establishment of an effective DHA production method, innovative concepts for the tailored synthesis of fine compounds from CO2 through the utilization of various multi-enzyme cascade developments are generated by this work.

Microbial electron flow promotes naphthalene degradation in anaerobic digestion in the presence of nitrate electron acceptor: Focus on electron flow regulation and microbial interaction succession.

Microbial electron flow (MEF) is produced from microbial degradation of organic compounds. Regulating MEF to promote organic pollutants biodegradation such as naphthalene (Nap) is a potential way but remains a lack of theoretical basis. Here, we regulated MEF by adding electron acceptor NO3- to achieve 2.6 times increase of Nap biodegradation with cyclodextrin as co-metabolism carbon source. With the NO3- addition, the genes inhibited by Nap of electron generation significantly up-regulated. Especially, key genes ubiD and nahD for anaerobic Nap degradation significantly up-regulated respectively 3.7 times and 6.7 times. Moreover, the ability of electron transfer in MEF was also improved consistent with 7.2 times increase of electron transfer system (ETS) activity. Furthermore, total 60 metagenome-assembled genomes (MAGs) were reconstructed through the metagenomic sequencing data with assembly and binning strategies. Interestingly, it was also first found that the Klebsiella MAG. SDU (Shandong University) 14 had the ability of simultaneous Nap biodegradation and denitrification. Our results firstly offered an effective method of regulating MEF to promote polycyclic aromatic hydrocarbons (PAHs) degradation and simultaneous methanogenesis.

Occurrence, Fate, Human Exposure, and Toxicity of Commercial Photoinitiators.

Photoinitiators (PIs) are a family of anthropogenic chemicals used in polymerization systems that generate active substances to initiate polymerization reactions under certain radiations. Although polymerization is considered a green method, its wide application in various commercial products, such as UV-curable inks, paints, and varnishes, has led to ubiquitous environmental issues caused by PIs. In this study, we present an overview of the current knowledge on the environmental occurrence, human exposure, and toxicity of PIs and provide suggestions for future research based on numerous available studies. The residual concentrations of PIs in commercial products, such as food packaging materials, are at microgram per gram levels. The migration of PIs from food packaging materials to foodstuffs has been confirmed by more than 100 reports of food contamination caused by PIs. Furthermore, more than 20 PIs have been detected in water, sediment, sewage sludge, and indoor dust collected from Asia, the United States, and Europe. Human internal exposure was also confirmed by the detection of PIs in serum. In addition, PIs were present in human breast milk, indicating that breastfeeding is an exposure pathway for infants. Among the most available studies, benzophenone is the dominant congener detected in the environment and humans. Toxicity studies of PIs reveal multiple toxic end points, such as carcinogenicity and endocrine-disrupting effects. Future investigations should focus on synergistic/antagonistic toxicity effects caused by PIs coexposure and metabolism/transformation pathways of newly identified PIs. Furthermore, future research should aim to develop "greener" PIs with high efficiency, low migration, and low toxicity.

Production of reactive oxygen species from oxygenation of Fe(II)-carbonate complexes: The critical roles of carbonate.

Hydroxyl radicals (•OH) production upon the oxygenation of reduced iron minerals at the oxic/anoxic interface has been well recognized. However, little is known in the influencing environmental factors and the involved mechanisms. In this study, much more •OH could be efficiently produced from oxygenation of Fe(II) with 20-200 mM carbonate. Both carbonate concentration and anoxic reaction time play a critical role in •OH production. High carbonate facilitates the formation of Fe(II)high reactivity, i.e., surface-adsorbed and structural Fe(II) with low crystalline that is reactive toward O2 reaction for •OH production, while long anoxic reaction time enables the transfer from Fe(II)high reactivity to Fe(II)low reactivity, i.e., Fe(II) at interior sites with high crystalline, that is hardly oxidized by O2. Furthermore, the degradation pathway of p-nitrophenol (PNP) is highly dependent on the carbonate concentration that low carbonate facilitates •OH oxidation of PNP (80.2%) while high carbonate enhanced O2•- reduction of PNP (48.7%). Besides, carbonate also influences the structural evolution of Fe mineral during oxygenation by retarding its hydrolysis and following transformation. Our finding sheds new light on understanding the important role of oxyanions such as carbonate in iron redox cycles and directing contaminant attenuation in subsurface environment.

Phylogenetic distance affects the artificial microbial consortia's effectiveness and colonization during the bioremediation of polluted soil with Cr(VI) and atrazine.

Soils co-contaminated with heavy metals and organic pollutants are common and threaten the natural environment and human health. Although artificial microbial consortia have advantages over single strains, the mechanism affecting their effectiveness and colonization in polluted soils still requires determination. Here, we constructed two kinds of artificial microbial consortia from the same or different phylogenetic groups and inoculated them into soil co-contaminated with Cr(VI) and atrazine to study the effects of phylogenetic distance on consortia effectiveness and colonization. The residual concentrations of pollutants demonstrated that the artificial microbial consortium from different phylogenetic groups achieved the highest removal rates of Cr(VI) and atrazine. The removal rate of 400 mg/kg atrazine was 100%, while that of 40 mg/kg Cr(VI) was 57.7%. High-throughput sequence analysis showed that the soil bacterial negative correlations, core genera, and potential metabolic interactions differed among treatments. Furthermore, artificial microbial consortia from different phylogenetic groups had better colonization and a more significant effect on the abundance of native core bacteria than consortia from the same phylogenetic group. Our study highlights the importance of phylogenetic distance on consortium effectiveness and colonization and offers insight into the bioremediation of combined pollutants.

Enzyme-based electrochemical biosensor for antimonite detection in water.

Antimonite (SbIII) is a naturally occurring contaminant demanding on-site ultrasensitive detection. The enzyme-based electrochemical (EC) biosensors are promising, but the lack of specific SbIII oxidizing enzymes hindered the past efforts. Herein, we modulated the specificity of arsenite oxidase AioAB toward SbIII by regulating its spatial conformation from tight to loose using the metal-organic framework ZIF-8. The constructed EC biosensor, AioAB@ZIF-8, exhibited the substrate specificity toward SbIII at 12.8 s-1 µM-1, an order of magnitude higher than that of AsIII (1.1 s-1 µM-1). Relaxing AioAB structure in ZIF-8 was evidenced by the break of the S-S bond and the conversion of α helix to the random coil as suggested by Raman spectroscopy. Our AioAB@ZIF-8 EC sensor exhibited a dynamic linear range in 0.041-4.1 µM at a response time of 5 s, and the detection limit at 0.041 µM at a high sensitivity of 1894 nA µM-1. The insights into tuning the specificity of an enzyme shed new light on biosensing metal(loid)s without specific proteins.

A Novel Whole-Cell Biosensor for Bioavailable Antimonite in Water and Sediments.

Antimony (Sb) is an emerging contaminant, and its on-site speciation analysis is central to the accurate evaluation of its bioavailability and toxicity. The whole-cell biosensors (WCBs) for Sb(III) are promising but challenging due to the lack of Sb(III)-specific recognition components. Here, we constructed a novel Sb(III)-specific WCB using an Sb(III) transcriptional regulator (antR) and its cognate promoter (Pant). To prevent the promoter leakage of Pant, an additional regulatory gene, antR, was inserted downstream of the Sb(III)-inducible promoter, improving the sensitivity of the WCB by an order of magnitude and reaching the detection limit at 0.009 µM, which is lower than the WHO drinking water standard of Sb. Moreover, the WCB with double antR showed a high specificity toward Sb(III) compared with interfering ions at 3 orders of magnitude higher concentrations. This WCB was capable of measuring Sb(III) bioavailability in natural waters and sediments on-site, and its results were not statistically different from the chemical analysis. The insights gained from this work demonstrate that the addition of regulatory genes prevents promoter leakage and improves the sensitivity of WCBs in field applications. IMPORTANCE Antimony (Sb) is a redox-sensitive pollutant ubiquitous in the environment. Sb(III) is dominant in the subsurface and is readily oxidized to less toxic Sb(V) upon exposure to air, and therefore, on-site Sb speciation analysis is essential to evaluate its bioavailability and toxicity. Dissolved Sb concentration and speciation can be determined accurately using on-site chemical sensors, but chemical sensors have difficulty determining the bioavailable Sb(III) that is taken up by the cells. Here, we constructed an Sb(III)-specific whole-cell biosensor (WCB) using double Sb(III) transcriptional regulators (antR) downstream of its cognate promoter Pant. With an additional antR, the sensitivity of the WCB was improved by approximately 10 times, and the promoter leakage commonly found in WCBs was inhibited. Integrated with a tea-bag design, the WCB is able to measure Sb(III) bioavailability in natural water and sediments on-site. This study demonstrates the importance of inserting one more regulatory gene to improve sensitivity.

Arsenic mobilization in nZVI residue by Alkaliphilus sp. IMB: Comparison between static and flowing incubation.

Arsenate reducing bacteria (AsRB) enhance arsenic (As) release via reducing As(V) to As(III), and As mobility is usually controlled by As(III) re-uptake on in-situ formed secondary iron minerals. The re-uptake of As(III) under groundwater flow conditions significantly impacts the fate and transport of As. Herein, a novel As(V)-reducing bacterium Alkaliphilus IMB was isolated in an As-contaminated soil. Scanning transmission X-ray microscopy showed that dissolved As(V) was mainly bound to the cell walls whereas dissolved As(III) was homogeneously distributed around IMB, indicating that As(V) reduction occurs outside the cell membrane. To explore the effect of IMB on As mobility, IMB was incubated with As-loaded nanoscale zero-valent iron (nZVI) residues under static and flowing conditions. IMB reduced 100% dissolved As(V) to As(III) even in a short contact time (∼1 h) during flowing incubation. The formation of As(III) did not influence As mobility under static condition as evidenced by the comparable concentrations of released As in the presence of IMB (8.5% to total As) and the abiotic control (10% to total As). Biogenic As(III) was re-adsorbed on the solids as shown by the higher ratio of solid-bound As(III) to total As in the presence of IMB (54%) than that in the abiotic control (12%). By contrast, the degree of As(III) re-adsorption was inhibited in the flowing environment, as suggested by the lower As(III) ratio in the solid (31%). This inhibition can be ascribed to the relatively slow adsorption of As(III) compared with the quick reduction of As(V) (∼1 h). Thus, IMB significantly enhanced As release during flowing incubation as shown that 9.8% As was released in the presence of IMB while 2.1% As in the abiotic control. This study found the contrary effect of AsRB on As mobility in static and flowing environments, highlighting the importance of re-adsorption rate of As(III).

Facet-Dependent Atomic Distances Shape Vanadate Adsorption Complexes on Hematite Nanocrystals.

The environmental fate of vanadate (V(V)) is significantly influenced by iron oxide nanocrystals through adsorption. Nevertheless, the underlying driving force controlling V(V) adsorption on hematite (Fe2O3) facets is poorly understood. Herein, V(V) adsorption on the {001}, {110}, and {214} Fe2O3 facets was explored using batch adsorption experiments, spectroscopic studies, and density functional theory (DFT) calculations. Adsorption experiments suggested that the order of V(V) adsorption capacity followed {001} > {110} > {214}. However, the affinity of V(V) to the {001} facet was the weakest, as evidenced by its least resistance to phosphate and sulfate competition. Our extended X-ray absorption fine structure (EXAFS) study indicated the formation of the inner-sphere monodentate mononuclear (1V) complex on the {001} facet and bidentate corner-sharing (2C) complexes on the {110} and {214} facets. Density functional theory (DFT) calculations showed the 1V complex is preferable when the adjacent Fe-Fe atomic distance is significantly larger than the O-O atomic distance of V(V). Otherwise, the 2C complex is formed if the distance is comparable. This determining factor in surface complex formation can be safely extended to other oxyanions that the compatibility in the atomic distance of Fe-Fe on Fe2O3 facets and O-O in oxyanions shapes the surface complex. The molecular-level understanding of the facet-dependent adsorption mechanism provides the basis for the design and application of oxyanion adsorbents.

Active microbial arsenic methylation in saline-alkaline paddy soil.

Seawater rice has been cultivated to ensure food security. The salt-tolerant rice strains are resistant to saline and alkali but may be vulnerable to elevated arsenic (As) near coastal regions. Herein, the saline-alkaline paddy soil was incubated with natural irrigation river for three months to explore the mobility and transformation of As. The incubation results showed that 65 ± 1.2 % solid-bound As(V) was reduced to As(III) within two weeks with the release of As(III) to porewater. The dissolved As(III) was methylated after two weeks, resulting in dimethyl arsenate (DMA) as the dominant As species (87 %-100 %). The elevated As methylation was attributed to the most abundant arsenite methyltransferase gene (arsM) (4.1-10.4 × 107/g dry soil), over three orders of magnitude higher than As redox-related genes. The analysis of arsM operational taxonomic units (OTUs) suggested the highest sequence similarity to Proteobacteria (25.7-39.5 %), Actinobacteria (24.9-30.5 %), Gemmatimonadetes (7.5-11.9 %), Basidiomycota (5.1-12.5 %), and Chloroflexi (4.1-8.7 %). Specifically, Chloroflexi and Actinobacteria are salt-tolerant bacteria, probably responsible for As methylation. The As in grain was within a safe regulatory level, and the dominance of methylated As in porewater did not enhance its accumulation in rice grains.