Decrease In Redox Potential Research Articles

Site-Directed Mutagenesis of Two-Domain Laccase ScaSL for Obtaining a Biocatalyst with Improved Characteristics

Analysis of the structure of two-domain laccase ScaSL from Streptomyces carpinensis VKM Ac-1300 (with a middle-redox potential) revealed determinants that could affect the increased potential of ScaSL. Site-directed mutagenesis of the ScaSL laccase was carried out, and mutants H286A, H286T, H286W, and F232Y/F233Y were obtained. Replacement of His 286 with Ala led to a decrease in redox potential (0.45 V) and an increase in stability at pH 9 and 11; replacement with Thr led to an increase in redox potential (0.51 V) but to a decrease in the thermal stability of the protein; replacement with Trp did not affect the enzyme properties. Replacement of Phe residues 232 and 233 with Tyr led to a shift in enzyme activity to the acidic pH range without changing the redox potential and a decrease in the thermostability and pH stability of the enzyme. All mutants more efficiently oxidized phenolic substrate 2,6-DMP and were able to participate in direct electron transfer (DET) with MWCNT-modified electrodes. The F232Y/F233/Y mutant was unable to degrade triphenylmethane dyes without a mediator but showed a greater degree of decolorization of azo dyes in the presence of the mediator. The crystal structure of laccase with the highest potential was determined with high resolution.

Functional Group-Driven Competing Mechanism in Electrochemical Reaction and Adsorption/Desorption Processes toward High-Capacity Aluminum-Porphyrin Batteries.

Nonaqueous organic aluminum batteries are considered as promising high-safety energy storage devices due to stable ionic liquid electrolytes and Al metals. However, the stability and capacity of organic positive electrodes are limited by their inherent high solubility and low active organic molecules. To address such issues, here porphyrin compounds with rigid molecular structures present stable and reversible capability in electrochemically storing AlCl2 +. Comparison between the porphyrin molecules with electron-donating groups (TPP-EDG) and with electron-withdrawing groups (TPP-EWG) suggests that EDG is responsible for increasing both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, resulting in decreased redox potentials. On the other hand, EWG is associated with decreasing both HOMO and LUMO energy levels, leading to promoted redox potentials. EDG and EWG play critical roles in regulating electron density of porphyrin π bond and electrochemical energy storage kinetics behavior. The competitive mechanism between electrochemical redox reaction and de/adsorption processes suggests that TPP-OCH3 delivers the highest specific capacity ~171.8 mAh g-1, approaching a record in the organic Al batteries.

Functional Group‐Driven Competing Mechanism in Electrochemical Reaction and Adsorption/Desorption Processes toward High‐Capacity Aluminum‐Porphyrin Batteries

AbstractNonaqueous organic aluminum batteries are considered as promising high‐safety energy storage devices due to stable ionic liquid electrolytes and Al metals. However, the stability and capacity of organic positive electrodes are limited by their inherent high solubility and low active organic molecules. To address such issues, here porphyrin compounds with rigid molecular structures present stable and reversible capability in electrochemically storing AlCl2+. Comparison between the porphyrin molecules with electron‐donating groups (TPP‐EDG) and with electron‐withdrawing groups (TPP‐EWG) suggests that EDG is responsible for increasing both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, resulting in decreased redox potentials. On the other hand, EWG is associated with decreasing both HOMO and LUMO energy levels, leading to promoted redox potentials. EDG and EWG play critical roles in regulating electron density of porphyrin π bond and electrochemical energy storage kinetics behavior. The competitive mechanism between electrochemical redox reaction and de/adsorption processes suggests that TPP‐OCH3 delivers the highest specific capacity ~171.8 mAh g−1, approaching a record in the organic Al batteries.

Atmospheric oxygenation at the onset of Earth’s Great Oxidation forced enhanced marine anoxia

Abstract Capturing the loss of mass-independent sulphur isotope fractionation (MIF-S), the correlative South African Duitschland and Rooihoogte formations are widely held to bear the isotopic fingerprint of the first atmospheric oxygenation at the onset of the so-called Great Oxidation Event (GOE). Surprisingly, however, while the multiple sulphur isotope systematics of these formations remain central to our understanding of the GOE, until now, comparatively little work has been done to elucidate the repercussions within the marine realm. Here we present chemostratigraphic records from four drill cores covering a large area of the Transvaal Basin, transcending these crucial units and continuing into the overlying Timeball Hill Formation (TBH), that document the immediate, yet counterintuitive, marine response to atmospheric oxygenation. Specifically, irrespective of the interpretative framework employed, our basin-wide redox-sensitive trace element data document an environmental change from oxic/suboxic conditions within the lower and middle parts of the Duitschland and Rooihoogte formations to suboxic/anoxic conditions within their upper reaches. Interestingly, in concert with a ~35‰ negative δ34S excursion that implicates increased sulphate availability and bacterial sulphate reduction, δ98/95Mo3134+0.25 values increase by ~1.0 to 1.5‰. Combining these observations with increased Fe/Mn ratios, elevated total sulphur and carbon contents and a trend towards lower δ13Corg values imply a shift toward less oxygenated conditions across the Transvaal Basin. The combined observations in the mentioned parameters expose a geobiological feedback-driven causality between the earliest oxygenation of the atmosphere and decreased redox potentials of medium to deep marine environments, at least within the Transvaal Basin.

Processes and mechanisms in remediation of aqueous chromium contamination by sulfidated nano-scale zerovalent iron (S-nZVI): Experimental and computational investigations

Sulfidated nano-scale zerovalent iron (S-nZVI) has emerged as an advanced functional nanomaterial for efficiently remediating Cr(VI) contamination in aqueous environments. However, there is an insufficient understanding of its coherent process, removal pathway, and hydrochemical reactive mechanisms, presenting potential challenges for its future environmental applications. To address this gap, this study successfully synthesized S-nZVI through a chemical precipitation method and effectively applied it for the removal of Cr(VI). Additional characterization revealed that the removal of Cr(VI) followed a sequence of rapid chemisorption and intraparticle diffusion processes, concomitant with an increase in pH and a decrease in oxidation-reduction potential. The remediation mechanism encompassed a synergistic reduction of Cr(VI) to Cr(III) and simultaneous immobilization via Cr2FeO4 coprecipitation. The highest Cr(VI) removal capacity of 75 mg/g was attained during dynamic removal experiments in the sand column packed with S-nZVI. Further computational analysis, employing density functional theory calculations based on the experimental data, revealed the involvement of multiple molecular orbitals of Cr(VI) in the removal process. It also elucidated a step-by-step reduction pathway for Cr(VI) characterized by decreasing free energy. These findings provide evidence-based insights into Cr(VI) remediation using S-nZVI and can serve as valuable technical support for future environmental management of heavy metals.

Arsenic in the groundwater aquifers of the Venetian Plain: geochemical modelling and occurrence of As-sulfides minerals, a review of data from the medio Brenta domain (Italy)

The Venetian Plain is known for areas with high concentrations of arsenic (As) in groundwater (up to more than 400 μg/L; exceptionally 647 μg/L, in selected areas). A study area was chosen, north of Padua, which exhibits typical residential, industrial, and agricultural characteristics similar to most Western countries and lacks hydrothermal, volcanic, or anthropogenic sources of arsenic. The pilot area was the focus of several studies which are reviewed in this note. The objectives of the studies were to verify the distribution of As concentrations in groundwater and sediments (mineralogical and geochemical analysis of groundwater sediments and of filtered and unfiltered groundwater) and to model the mobility of arsenic arising from water-rock interaction. The grain size of aquifer reservoirs includes gravel, sand, silt, and clay. The amount of organic matter in the aquifer sediments of the study area seems peculiar (higher) compared to other plains in the world; it influences the redox potential and the relative concentration of As in groundwater. Arsenic contamination in groundwater and redox conditions varied greatly in the area. Groundwater under oxidizing and highly reducing conditions had much lower arsenic concentrations compared to groundwater under intermediate reducing conditions. Arsenic minerals (such as realgar-pararealgar) occur in aquifer sediments and they were documented in the studied materials by different analytical techniques for the first time in the context of the Italian plains. Since these minerals are rare throughout the world in plain sediments not affected by volcanic or hydrothermal activity, their occurrence is a distinctive feature of the Venetian Plain aquifer. These arsenic minerals were found in peat sediments of the study area, consistent with geochemical modeling results, which require highly reducing conditions for their precipitation from groundwater. Modeling suggests that under oxidizing and up to slightly reducing conditions (from 200 mV to -50 mV), arsenic is adsorbed on solid phases, but a further decrease in redox potential leads arsenic desorption from solids and consequent groundwater contamination (from -50 mV to -250 mV). If the redox potential becomes even more negative (below -250 mV), geochemical conditions are favorable to the formation of arsenic sulfides. The precipitation of the realgar-pararealgar phases, predicted by the geochemical model, proceeds by extracting arsenic from the groundwater and quantitatively accounts for the lower arsenic concentration measured in the highly reducing groundwater of the study area.

Fe-Co-Ni trimetallic Prussian blue analogs with tunable electrochromic energy storage properties

Prussian blue (PB) and its analogs (PBAs) have become a research hotspot in the fields of electrochemical energy storage and electrochromism due to their good reversible redox activity, and reversible color change response in different redox states. Herein, a series of trimetallic PBAs (Co/NiHCF with Co to Ni molar ratios of 1:0, 3:1, 1:1, 1:3, and 0:1) are synthesized by a facile co-precipitation method. In particular, the impact of Co to Ni molar ratio on the lithium storage and electrochromic properties of Co/NiHCF materials are systemically investigated. All five samples possess good crystallinity but the particle size decreases with the reduction of Co/Ni molar ratio. The color change, redox reaction potential, and lithium storage activity of the samples can be readily controlled by adjusting the molar ratio of Co to Ni. The lithium storage activity of the samples strengthens with the increase of Ni content in Co/NiHCF, accompanied by a decrease in redox potentials. With the increase of Ni content, the color of the Co/NiHCF film in the oxidation state changes from purple-red to orange, and finally to yellow; in the reduction state, the color of the film changes from green to light green and then to colorless. All the samples exhibit superior rate capability, excellent cycling stability, and short switching time (in the range of 1.6 to 4.3 s). The present work demonstrates a facile strategy for readily tunning the lithium storage activity, redox reaction potentials, and realizing rich color changes of PBA-based bifunctional materials.

Biochar application can enhance phosphorus solubilization by strengthening redox properties of humic reducing microorganisms during composting

Phosphorus (P) in nature mostly exists in an insoluble state, and humic reducing microorganisms (HRMs) can dissolve insoluble substances through redox properties. This study aimed to investigate the correlations between insoluble P and dominant HRMs amenable to individual culture during biochar composting. These analyses revealed that, in comparison to the control, biochar addition increased the relative abundance of dominant HRMs by 20.3% and decreased redox potential (Eh) levels by 15.4% hence, enhancing the moderately-labile-P and non-labile-P dissolution. The pathways underlying the observed effects were additionally assessed through structural equation modeling, revealing that biochar addition promoted insoluble P dissolution through both the direct effects of bacterial community structure as well as the direct effects of HRMs community structure and indirect effects based on Eh of HRMs community structure. This research offers a better understanding of the effect of HRMs on insoluble P during the composting process.

Pectin supplementation accelerates post-antibiotic gut microbiome reconstitution orchestrated with reduced gut redox potential.

Antibiotic-induced gut dysbiosis (AID) presents a big challenge to host health, and the recovery from this dysbiosis is often slow and incomplete. AID is typically characterized by elevation in redox potential, Enterobacteriaceae load, and aerobic metabolism. In our previous study, a pectin-enriched diet was demonstrated to decrease fecal redox potential and modulate the gut microbiome. Therefore, we propose that pectin supplementation may modulate gut redox potential and favor post-antibiotic gut microbiome reconstitution from dysbiosis. In the present study, rats with AIDwere used to investigate the effects of pectin supplementation on post-antibiotic gut microbiome reconstitution from dysbiosis. The results showed that pectin supplementation accelerated post-antibiotic reconstitution of gut microbiome composition and function and led to enhancement of anabolic reductive metabolism and weakening of catabolic oxidative pathways. These results were corroborated by the measurement of redox potential, findings suggesting that pectin favors post-antibiotic recovery from dysbiosis. Pectin-modulated fecal microbiota transplantation accelerated the decrease in antibiotics-elevated redox potential and Enterobacteriaceae load similarly to pectin supplementation. Moreover, both pectin supplementation and Pectin-modulated fecal microbiota transplantation enriched anaerobic members, primarily from Lachnospiraceae orchestration with enhancement of microbial reductive metabolism in post-antibiotic rats. These findings suggested that pectin supplementation accelerated post-antibiotic gut microbiome reconstitution orchestrated with reduced gut redox potential and that the effect of pectin on redox potential was mediated by remodeling of the intestinal microbiota.

Gold nanoparticles activate hydrogenase synthesis and improve heterotrophic growth of Ralstonia eutropha H16.

Ralstonia eutropha is a facultative chemolithoautotrophic aerobic bacterium that grows using organic substrates or H2 and CO2. Hydrogenases (Hyds) are synthesized under lithoautotrophic, or energy-limited heterotrophic conditions and are used in enzyme fuel cells (EFC) as anodic catalysts. The effects of chemically synthesized gold nanoparticles (Au-NPs) on R. eutropha H16 growth, oxidation-reduction potential (ORP) kinetics, and H2-oxidizing Hyd activity were investigated in this study. Atomic force microscopy showed that thin, plate-shaped Au-NPs were in the nanoscale range with an average size of 5.68 nm. Compared with growth in medium without Au-NPs (control), the presence of Au-NPs stimulated growth, and resulted in a decrease in ORP to negative values. H2-oxidizing activity was not detected in the absence of Au-NPs, but activity was significantly induced (12 U/g CDW) after 24 h of growth with 18 ng/ml, increasing a further 4-fold after 72 h of growth. The results demonstrate that Au-NPs primarily influence the membrane-bound Hyd. In contrast to R. eutropha, Au-NPs had a negligible or negative effect on the growth, Hyd activity, and H2 production of Escherichia coli. The findings of this study offer new perspectives for the production of oxygen-tolerant Hyds and the development of EFCs.

Destructive selective logging in tropical forests causes soil carbon loss through forest degradation and soil redox change

Soil organic C (SOC) stock in tropical forests can be reduced significantly after logging, not only due to lowered above ground biomass (AGB) and consequentially reduced organic matter (OM) input, but also consumption on sequestrated soil C protected by OM-mineral association. In this study, we studied the loss of mineral-associated OM after logging in the main rooting zone (e.g., 0–30 cm), and examined a hypothetical twofold underlying mechanism of SOC reduction: first, decrease in AGB limiting OM input; second, decrease in soil redox potential (Eh) leading to the reductive dissolution of short-range-ordered iron (SRO Fe) minerals that destabilize some mineral-associated OM. Thirty-five plots at different degradation levels were investigated for both above- and belowground properties. AGB and tree community composition were employed as indicators of forest degradation gradient; multiple physicochemical properties in top-30 cm soils were measured every 10 cm depth. We analysed the relationship between AGB and soil C with the mediator SRO Fe, and investigated the role of soil Eh in linking the changes in AGB and SRO Fe. Our results revealed simultaneous decreases in AGB, SRO Fe and soil C in top-30 cm soils, and SRO Fe mediated 43 % of the hypothetical causal effect of AGB on soil C in deeper (20–30 cm) soil. Soil Eh decreased with decreasing AGB in a logistic pattern, dropping to a reductive regime (Eh7 ≤ 0.05 V) when more than half of the AGB was removed from the pristine level. Mean SRO Fe stock in the reductive soils was significantly lower than that in the oxic soils (Eh7 > 0.05 V), and the loss of SRO Fe correspond to the decomposition of OM in 10–30 cm soils. We suggest that the reductive shift in soil is caused by the weakened evapotranspiration as destructive selective logging inflicts canopy openness and succession failure. This study presents the possibility that forest degradation may reduce the size of protected soil C pool resulting from the loss of soil SRO Fe minerals.

Effect of Fermentation Scale on Microbiota Dynamics and Metabolic Functions for Indigo Reduction.

During indigo dyeing fermentation, indigo reduction for the solubilization of indigo particles occurs through the action of microbiota under anaerobic alkaline conditions. The original microbiota in the raw material (sukumo: composted indigo plant) should be appropriately converged toward the extracellular electron transfer (EET)-occurring microbiota by adjusting environmental factors for indigo reduction. The convergence mechanisms of microbiota, microbial physiological basis for indigo reduction, and microbiota led by different velocities in the decrease in redox potential (ORP) at different fermentation scales were analyzed. A rapid ORP decrease was realized in the big batch, excluding Actinomycetota effectively and dominating Alkalibacterium, which largely contributed to the effective indigo reduction. Functional analyses of the microbiota related to strong indigo reduction on approximately day 30 indicated that the carbohydrate metabolism, prokaryotic defense system, and gene regulatory functions are important. Because the major constituent in the big batch was Alkalibacterium pelagium, we attempted to identify genes related to EET in its genome. Each set of genes for flavin adenine dinucleotide (FAD) transportation to modify the flavin mononucleotide (FMN)-associated family, electron transfer from NADH to the FMN-associated family, and demethylmenaquinone (DMK) synthesis were identified in the genome sequence. The correlation between indigo intensity reduction and metabolic functions suggests that V/A-type H+/Na+-transporting ATPase and NAD(P)H-producing enzymes drive membrane transportations and energization in the EET system, respectively.

Enhanced Oxidation of Cr(III)-Fe(III) Hydroxides by Oxygen in Dark and Alkaline Environments: Roles of Fe/Cr Ratio and Siderophore.

The current understanding of Cr(III)-Fe(III) hydroxide (Cr1-xFex(OH)3) oxidation in the dark is primarily focused on strong oxidants, yet the role of oxygen has generally been overlooked. Meanwhile, the effects of organic ligands on the Cr(III) oxidation are poorly known. Herein, we determined the kinetics of Cr1-xFex(OH)3 oxidation by oxygen in the dark as a function of pH and Fe/Cr ratio with/without the presence of a representative organic ligand-siderophore. Results showed that the Cr(III) oxidation rate increased linearly with increasing pH and Fe/Cr ratio. Thermodynamic calculations suggested that the enhanced Cr1-xFex(OH)3 oxidation with increasing pH was primarily due to the decreased ΔG value (i.e., the Gibbs free energy change) at higher pH. The decreased redox potentials (Eh) of Cr1-xFex(OH)3 suspensions with increasing Fe/Cr ratio accounted for the enhanced Cr(III) oxidation of iron-rich Cr1-xFex(OH)3. The siderophore greatly accelerated the Cr1-xFex(OH)3 oxidation at alkaline pH by promoting the formation of soluble organically complexed Cr(III), which can be oxidized readily by oxygen via mineral-surface catalyzed oxidation. Overall, this study highlights the specific role of oxygen and its synergistic role with the siderophore in the oxidation of solid Cr1-xFex(OH)3, which should be taken into consideration in assessing the long-term stability of Cr(III)-Fe(III) hydroxides.

Scholastic modeling of pH and redox potential changes in olive tree leaf alcohol and acids containing incubation media designed for the steady growth of Acetobacter aceti and Saccharomyces cerevisiae

Modeling of pH and redox potential changes was investigated instructionally in incubation media designed for a stable growth of Acetobacter aceti and Saccharomyces cerevisiae. Olive tree leaf, phosphoric acid, vinegar, acetic acid and ethyl alcohol were used in incubation for extraction and symbiotic purposes. Structure imaging of olive tree leaf powder was performed using the Field Emission Gun – Scanning Electron Microscope (FEG-SEM). The incubation experiments were carried out at initially lowest pH and high temperatures of 30oC and 35oC for eight days in liquid state fermentation process. A steady A. aceti and S. cerevisiae growth was observed during the incubation. Increase in pH value displayed increase in redox potential in water+ phosphoric acid, vinegar+A. aceti+phosphoric acid, S. cerevisiae+A. aceti+acetic acid+phosphoric acid and S. cerevisiae+A. aceti+phosphoric acid solution processes at 30oC, and acetic acid+phosphoric acid and vinegar+phosphoric acid solution processes at 35oC. Decrease in pH value displayed decrease in redox potential in A. aceti+alcohol+phosphoric acid, vinegar+phosphoric acid, S. cerevisiae+A. aceti+acetic acid+phosphoric acid and S. cerevisiae+A. aceti+phosphoric acid solution processes at 30oC, and vinegar+A. aceti+phosphoric acid, S. cerevisiae+A. aceti+acetic acid+phosphoric acid and S. cerevisiae+A. aceti+phosphoric acid solution processes at 35 oC.

Microbial Degradation of Plant Residues Rapidly Causes Long-Lasting Hypoxia in Soil upon Irrigation and Affects Leaching of Nitrogen and Metals

With this study, we aim to relate the substrate quality of different organic materials derived from plant residues to the respiratory activity of soil microorganisms after amendment, the formation of oxygen gradients upon irrigation, and the leaching of macronutrients and metals in soil. Elemental analyses were performed to determine the chemical composition of wheat straw, green compost, and a biochar product, showing that carbon availability, C/N ratio, and metal contents varied markedly. Consequently, after application to well-aerated sandy loam soil at 1% w/w, only straw increased microbial activity substantially, and nitrate was depleted within one week. Upon intense irrigation of soil columns packed with differently amended soils, strong hypoxia formed only in straw–soil, where microbial oxygen demand for straw degradation was high. This was enhanced after the application of mineral fertilizers, and nitrate leaching was mitigated. With the decreasing redox potential in straw–soil, the leaching of Fe, Mn, Al, Ni, Co, and As was increased. However, nitrate from mineral fertilizer mitigated the reduction of redox potential and, thus, the leaching of these metals. Measuring oxygen at different depths revealed near anoxic conditions at −15 cm of straw–soil with NP-fertilizer applied within 12 h after the start of irrigation and remained for at least 60 h, while oxygen showed extensive fluctuations in the upper few centimeters. This study showed that organic soil amendments with high carbon availability induce microbial respiration to the extent that causes strong and long-lasting hypoxia upon irrigation, even in sandy soil, which leads to substantial effects on the mobility of nutrients and toxic metals. In contrast, organic soil amendments with low carbon availability did not cause such effects.

Formate-producing capacity provided by reducing ability of Streptococcus thermophilus nicotinamide adenine dinucleotide oxidase determines yogurt acidification rate

Yogurt is made by fermenting milk with 2 lactic acid bacteria, Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus. To comprehensively understand the protocooperation mechanism between S. thermophilus and L. bulgaricus in yogurt fermentation, we examined 24 combinations of cocultures comprising 7 fast- or slow-acidifying S. thermophilus strains with 6 fast- or slow-acidifying L. bulgaricus strains. Furthermore, 3 NADH oxidase (Nox)-deficient mutants (Δnox) and one pyruvate formate-lyase deficient mutant (ΔpflB) of S. thermophilus were used to evaluate the factor that determines the acidification rate of S. thermophilus. The results revealed that the acidification rate of S. thermophilus monoculture determined the yogurt fermentation rates, despite the coexistence of L. bulgaricus, whose acidification rate was either fast or slow. Significant correlation was found between the acidification rate of S. thermophilus monoculture and the amount of formate production. Result using ΔpflB showed that the formate was indispensable for the acidification of S. thermophilus. Moreover, results of the Δnox experiments revealed that formate production required Nox activity, which not only regulated dissolved oxygen, but also the redox potential. The Nox provided the large decrease in redox potential required by pyruvate formate-lyase to produce formate. A highly significant correlation was found between formate accumulation and Nox activity in S. thermophilus. In conclusion, the formate production ability provided by the action of Nox activity determines the acidification rate of S. thermophilus, and consequently, regulates yogurt coculture fermentation.

Corrosion of austenitic stainless steel at 90 °C under highly saline and anoxic conditions: A microspectroscopic study

The stainless steel AISI 309 S used for containers for vitrified high-level radioactive waste was exposed to 90 °C under highly saline and anoxic conditions for different exposure times up to 294 days. The surface damage was limited and no pitting could be detected. The corrosion layer is made of an inner-layer mostly of chromium (hydr)oxides and an outer-layer made of Fe- and Ni-based spinel compounds with admixed nickel (hydr)oxides. Minor contributions of magnetite and layered double hydroxide could be identified. Dissolved amounts of metal ions were very low, the pH increased only slightly and the redox potential decrease was limited.

New insights into sulfur input induced methylmercury production and accumulation in paddy soil and rice

Sulfur has a high affinity for mercury (Hg) and can serve as effective treating agent for Hg pollution. However, conflict effects between reducing Hg mobility and promoting Hg methylation by sulfur were found in recent studies, and there is a gap in understanding the potential mechanism of MeHg production under different sulfur-treated species and doses. Here, we investigated and compared the MeHg production in Hg-contaminated paddy soil and its accumulation in rice under elemental sulfur or sulfate treatment at a relatively low (500 mg·kg−1) or high (1000 mg·kg−1) level. The associated potential molecular mechanisms are also discussed with the help of density functional theory (DFT) calculation. Pot experiments demonstrate that both elemental sulfur and sulfate at high exposure levels increased MeHg production in soil (244.63–571.72 %) and its accumulation in raw rice (268.73–443.50 %). Coupling the reduction of sulfate or elemental sulfur and decrease of soil redox potential leads to the detachment of Hg-polysulfide complexes from the surface of HgS which can be explained by DFT calculations. Enhancement of free Hg and Fe release through reducing Fe(III) oxyhydroxides further promotes soil MeHg production. The results provide clues for understanding the mechanism by which exogenous sulfur promotes MeHg production in paddies and paddy-like environments and give new insights for decreasing Hg mobility by regulating soil conditions.

Solubility of soil phosphorus in extended waterlogged conditions: An incubation study

Understanding how extended excess soil moisture exacerbated by extreme weather events affects changes in iron (Fe) chemistry is crucial for assessing environmental risk associated with soil phosphorus (P) in high P soils. The objective of our study was to assess the effects of three soil moisture regimes (field capacity, water saturation, and waterlogging), two Fe3+ nitrate level (Fe3+ nitrate addition and no Fe3+ nitrate addition), and the duration of incubation (0, 3, 7, 14, 21, 28, 35, 49, 63, 90, and 120 days) on the (i) reduction of ferric (Fe3+) to ferrous (Fe2+) iron, (ii) solubility of soil P, and (iii) soil microbial biomass and greenhouse gas emissions. Surface soils (0–20 cm) were collected from a maize silage field located in the Fraser Valley (British Columbia, Canada). Decreased redox potential (Eh) of 155 mV in waterlogged soils coincided with the reduction of Fe3+ to Fe2+ of about 1190 mg kg−1 and an increase in soil pH of 0.8 unit compared to field capacity regime at 120 days after pre-incubation (P < 0.001). The increase of pH is due to the microbially-mediated reduction of metal cations which consumes H+ cations. Water-extractable P (Pw) concentrations increased with increasing soil moisture regimes from 1.47 to 2.27, and 2.58 mg kg−1 under field capacity, water saturation, and waterlogged regime respectively. Mehlich-3 extractable P concentrations significantly decreased from 196 to 184 and 172 mg kg−1 under water saturation, field capacity, and waterlogged regime respectively. Concomitant to Pw concentrations, microbial biomass carbon and nitrogen as well as DOC, CO2 and N2O emissions increased with increasing soil moisture regimes. The Fe3+ nitrate addition had an inhibitory effect on Fe reduction, Pw concentration at the first 35 days, and DOC but a stimulating effect on N2O emission. A high N2O emission at the first 63 days, CO2 emission after 35 days, and a non-remarkable concentration of Fe2+ at the first 63 days with Fe3+ nitrate addition under waterlogged soil suggests that NO3− is more preferable than Fe3+ as an electron acceptor. Our results showed that soils maintained under extended anoxic conditions could increase the soluble and available P and subsequent risk of P transport to surface and drainage waters, whereas Fe3+ nitrate addition could minimize or delay this effect.

Enhanced dissolution of arsenic in anaerobic soils upon organic amendment application: acid detergent-soluble organic matter as a potential indicator

Application of organic amendments (OAMs) often enhances arsenic (As) dissolution in paddy soils. Therefore, understanding the properties of OAMs that determine the extent of As dissolution is essential for appropriate soil management. Since As dissolution increases with decrease in soil redox potential caused by microbial respiration, the decomposability of OAMs might be a critical factor controlling As dissolution in amended soils. We hypothesized that contents of acid detergent-soluble organic matter (ADSOM, mainly composed of non-fiber organic matter and hemicellulose) in OAMs can help estimate the potential of OAMs in accelerating As dissolution in soils with added OAMs. Therefore, two contrasting soil types, Andosol and Fluvisol, were mixed with 24 different OAMs and subjected to anaerobic incubation for 14 weeks. Changes in soil Eh and dissolved As contents were monitored throughout the incubation period, and As species in solid phases and ferrous iron (Fe(II)) contents in soils were measured after 2 and 6 weeks of incubation. The higher the ADSOM content in soils with OAMs, the higher the dissolved As contents in soils and the lower the Eh values. Dissolved As also positively correlated with the proportion of As(III) in solid phases and Fe(II) content after 2 and 6 weeks of incubation, indicating that decomposition of ADSOM led to reducing soil conditions, thereby promoting the reduction of As(V) and As-bearing Fe oxides and subsequent As dissolution. The results were consistent between the two types of soils, despite dissolved As content in the Andosol being two orders lower than that in Fluvisol. This is the first study to demonstrate that ADSOM can be a prominent indicator of the potential of OAMs, for promoting As dissolution, when applied to paddy soils.