Fe Reduction Rates Research Articles

Influence of crop residue-induced Fe-DOC complexation on nitrate reduction in paddy soil

Although complexation between dissolved organic matter (DOM) and ubiquitous Fe is known to have a major influence on electron transferring ability in redoximorphic soil, it was unclear whether and how this complexation affected nitrate reduction and N2O productivity. The nitrate reduction of paddy soil in the presence of crop residues returning under flooding conditions was explored in this study. The rate of nitrate reduction in control soil was 0.0677 d−1, while it improved 1.99 times in treatment soil with Chinese milk vetch (CMV) straw returning. During a 28-day incubation period, N2O productivity decreased 0.08–0.91 ppb in CMV soil and 0.43–0.50 ppb in rice straw soil compared with control. The presence of crop residue increased DOC content and Fe (III) reduction rate, which aided in the formation of Fe (II)-DOC complexation. Meanwhile, the addition of CMV increased the content of DOC by 5.14–78.77 mg/kg and HCl extractable Fe (II) by 35.12–1221.03 mg/kg. Crop residues returning to soil increased the relative abundance of iron reductive and electroactive genera, as well as denitrifying genera with more copies of denitrification genes (Archangiaceae, Gemmatimonadaceae, and Burkholderiaceae). The synergistic effect of Fe-DOC complexation, electroactive genera, and denitrifying genera contributed to up-regulated expression of napA and narG (5.84 × 106 and 3.39 × 107 copies increased in the CMV soil compared to the control) numbers and equally accelerated reduction of nitrate to nitrite, while further nitrite reduction was primarily attributed to the abiotic reaction by Fe (II). From a bio-electrochemical point of view, this work provided new insight into the nitrate reduction of paddy soil impacted by Fe-DOC complexation.

Unraveling nitrogen loss in paddy soils: A study of anaerobic nitrogen transformation in response to various irrigation practice

Soil nitrogen (N) transformation processes, encompassing denitrification, anaerobic ammonium oxidation (anammox), and anaerobic ammonium oxidation coupled with iron reduction (Feammox), constitute the primary mechanisms of soil dinitrogen (N2) loss. Despite the significance of these processes, there is a notable gap in research regarding the assessment of managed fertilization and irrigation impacts on anaerobic N transformations in paddy soil, crucial for achieving sustainable soil fertility management. This study addressed the gap by investigating the contributions of soil denitrification, anammox, and Feammox to N2 loss in paddy soil across varying soil depths, employing different fertilization and irrigation practices by utilizing N stable isotope technique for comprehensive insights. The results showed that anaerobic N transformation processes decreased with increasing soil depth under alternate wetting and drying (AWD) irrigation, but increased with the increasing soil depth under conventional continuous flooding (CF) irrigation. The denitrification and anammox rates varied from 0.41 to 2.12 mg N kg−1 d−1 and 0.062–0.394 mg N kg−1 d−1, respectively, which accounted for 84.3–88.1% and 11.8–15.7% of the total soil N2 loss. Significant correlations were found among denitrification rate and anammox rate (r = 0.986, p < 0.01), Fe (Ⅲ) reduction rate and denitrification rate (r = 0.527, p < 0.05), and Fe(Ⅲ) reduction rate and anammox rate (r = 0.622, p < 0.05). Moreover, nitrogen loss was more pronounced in the surface layer of the paddy soil compared to the deep layer. The study revealed that denitrification predominantly contributed to N loss in the surface soil, while Feammox emerged as a significant N loss pathway at depths ranging from 20 to 40 cm, accounting for up to 26.1% of the N loss. It was concluded that fertilization, irrigation, and soil depth significantly influenced anaerobic N transformation processes. In addition, the CF irrigation practice is best option to reduce N loss under managed fertilization. Furthermore, the role of microbial communities and their response to varying soil depths, fertilization practices, and irrigation methods could enhance our understanding on nitrogen loss pathways should be explored in future study.

Redox regulation of layered Fe-Cr hydroxides during Cr(VI) reduction by sulfate green rust: The overlooked electron conductivity and reactivity of Fe(III)-Cr(III) residues

In this study, unique layered Fe(III)-Cr(III) oxides were formed during the Cr(VI) removal by sulfate green rust (GRSO4). Solid state characterization confirmed that the hexagonal plate and layered structure of GRSO4 remained after fast reduction of Cr(VI). Then, sodium dithionite was further employed to reduce the Fe(III) to Fe(II) in these layered Fe(III)-Cr(III) oxides, enabling stable Fe(II)/Fe(III) cycle in the hexagonal plates. As the Fe(II) content increased in the layered Fe(III)-Cr(III) oxides, there was no significant Fe(II) and Cr(III) release into the solution and the GR-like plate remained intact, indicating that this hexagonal layer Fe-Cr oxides has stable structure during the reduction of Fe(III). Interestingly, better crystallinity was observed at higher Fe(III) reduction rate, and the layered metal oxide structure was confirmed with a basal spacing of 0.935 nm when ∼ 100 % of Fe(III) was reduced to Fe(II). Multiple treatment cycles experiment demonstrated that Cr(III) loading on the layered Fe(III)-Cr(III) oxides could be increased in the following 6 runs of Fe(II)/Fe(III) redox cycle. This study provides new evidence to confirm the electron conductivity and redox reactivity of the Fe(III)-Cr(III) residues under ambient condition, which might help to formulate novel strategies for elimination of Cr(VI) or related heavy metals.

Coupled Fe(III) reduction and phenanthrene degradation by marine-derived Kocuria oceani FXJ8.057 under aerobic condition

Diverse aerobic actinobacteria possess the capacity to degrade polycyclic aromatic hydrocarbons (PAHs) and have recently been shown to reduce Fe(III). However, the coupling of the two processes under oxic conditions remains unclear. Here, the co-metabolism of phenanthrene (PHE) and Fe(III) by marine-derived Kocuria oceani FXJ8.057 was realized under aerobic condition. In the presence of both PHE and Fe(III), the rates of PHE degradation (83.91 %) and Fe(III) reduction (50.00 %) were synchronously enhanced, compared to those with PHE (67.34 %) or Fe(III) (38.00 %) alone. Transcriptome analysis detected upregulation of PHE biodegradation and riboflavin biosynthesis in the strain cultured with both PHE and Fe(III) compared to that with PHE alone. Metabolite analysis indicated that, with the addition of Fe(III), the strain could efficiently degrade PHE via three pathways. Moreover, the strain secreted riboflavin, which acted as a shuttle to promote electron transfer from PHE to Fe(III). It also secreted organic acids that could delay Fe(II) reoxidation. Finally, H2O2 secreted by the strain caused extracellular Fenton reaction to generate •OH, which also played a minor role in the PHE degradation. These findings provide the first example of an aerobic bacterium that couples PAH degradation to Fe(III) reduction and extend our understanding of Fe(III)-reducing microorganisms.

Geochemical processes of phosphorus‑iron on sediment-water interface during discharge of groundwater to freshwater lakes: Kinetic and mechanistic insights

Groundwater is widely recognized as a source of lake materials. When it discharges into lakes, phosphorus(P)‑iron(Fe) geochemical reactions occur due to environmental changes, affecting P discharge from groundwater. However, redox kinetics of Fe and associated P geochemical processes at the sediment-water interface are not fully understood. Taking Dongting Lake as an example, this study explored Fe and P geochemical processes at the sediment-water interface under groundwater discharge with high Fe and P concentrations. We incubated sediments from Dongting Lake under anoxic-oxic conditions with different initial aqueous P/Fe ratios and pH. Aqueous PO43−-P and Fe2+, and solid P and Fe phases in sediments were analyzed, and experimental data were further simulated using numerical reactive models. At the beginning of the experiment, aqueous P and Fe were adsorbed rapidly on sediments. Under anoxic conditions, the Fe reduction rate decreased with decreasing content of poorly crystalline ferric (oxyhydr)oxides, and the addition of aqueous P and Fe at neutral pH enhanced the reduction rate. The increased aqueous P was dominated by desorption caused by sediment Fe reduction and then fixed by gibbsite adsorption and hydroxyapatite precipitation. Under oxic conditions, Fe(II) oxidation under was pH- and (P:Fe)ini-independent, with a sharp rate decline. Furthermore, the final sediment Fe(II) content was higher than the initial content, indicating the formation of a low-oxidizability Fe(II) phase. The P dynamics were dominated by adsorption on the produced Fe-oxides. The numerical models also suggested that heterogeneity in natural sediments promotes hydroxyapatite formation at low pH, but restricts it at high pH. The findings reveal that although aqueous P concentration decreased during groundwater discharge to lakes, PO43−-P concentration remained much higher than that in natural lake water, increasing the risk of lake eutrophication. The paper provides references for further understanding of P loading from groundwater discharge into lakes.

The Contribution of Fe(III) Reduction to Soil Carbon Mineralization in Montane Meadows Depends on Soil Chemistry, Not Parent Material or Microbial Community

AbstractThe long‐term stability of soil carbon (C) is strongly influenced by organo‐mineral interactions. Iron (Fe)‐oxides can both inhibit microbial decomposition by providing physicochemical protection for organic molecules and enhance rates of C mineralization by serving as a terminal electron acceptor, depending on redox conditions. Restoration of floodplain hydrology in montane meadows has been proposed as a method of sequestering C for climate change mitigation. However, dissimilatory microbial reduction of Fe(III) could lead to C losses under increased reducing conditions. In this study, we explored variations in Fe‐C interactions over a range of redox conditions and in soils derived from two distinct parent materials to elucidate biochemical and microbial controls on soil C cycling in Sierra Nevada montane meadows. Soils derived from basalt showed greater rates of Fe(III)‐reduction at increasing soil moisture levels than granitic soils. Increases in Fe(III) reduction, however, were only associated with elevated rates of C mineralization in one basalt soil. Known Fe(III)‐reducing taxa were present in all samples but neither the relative abundance nor richness of Fe(III)‐reducers corresponded with measured rates of Fe(III) reduction. Under reducing conditions, Fe(III)‐reduction was only coupled to C mineralization in the soil with the greatest amount of Fe‐oxide bound C. However, Fe‐oxide ‐bound C was below theoretical limits for C sorption onto Fe‐oxides and not detectable in all soils. Overall, our results suggest that “what's there” in terms of soil chemistry may be a more important driver of C mineralization coupled to Fe(III) reduction than “who's there” in the microbial community.

Promoting vivianite recovery: Crucial role of tightly-bound extracellular polymeric substances

Vivianite is an important bio-induced product of dissimilated iron reduction (DIR), which exists widely in activated sludge and digested sludge of wastewater treatment plants (WWTPs). Extracellular polymeric substances (EPS) surrounding dissimilatory iron-reducing bacteria (DIRB) were reported to be transient media for microbial extracellular electron transfer (EET) and DIR. However, what happens when a layer of EPS envelops microbial cells during vivianite recovery? This study tried to assess the relationship between iron reduction and electron transfer by stripping EPS layer-by-layer, then clarified the delamination of different EPS layers on the performance of vivianite recovery. After stripping total EPS (PC-EPS batches) of DIRB, the Fe (III) reduction efficiencies (RFe), Fe (III) reduction rate (VFe), and vivianite formation efficiency (RV) decreased by 33%, 37%, and 30%, respectively. Tightly bound-EPS (TB-EPS) was confirmed to play a crucial role in vivianite formation, which contributed 28–51% to vivianite recovery and 34–55% to iron reduction. Electronic shuttle substances were contained in TB-EPS, which enhanced the EET rate and efficiency between cells and Fe (III). The polysaccharide content of TB-EPS was positively related to adhesiveness between cells and mineral, as well as the iron reduction efficiency. Furthermore, redox proteins (PpcA, OmcB) were embedded in EPS, which shortened the distance of electron hopping to across the EPS layer and further accelerated vivianite recovery from wastewater. Long-term operation of anaerobic reactors confirmed that TB-EPS played critical roles in bio-induced vivianite formation.

Iron–organic carbon associations stimulate carbon accumulation in paddy soils by decreasing soil organic carbon priming

Iron-bound organic carbon (Fe-OC) complexes are important for stabilizing soil organic carbon (SOC) against biodegradation. However, it is unclear how the stabilization of OC and its release from Fe minerals subsequently affect the priming effect on SOC mineralization. To address the knowledge gap, we incubated typical paddy soil for 60 days by adding 2-line ferrihydrite (2LFh) or 6-line Fh (6LFh)-bound glucose, each with both high and low amounts of glucose, under anaerobic conditions. Approximately 21% more CO2 was derived from 2LFh-bound glucose than from 6LFh-bound glucose. Glucose addition alone stimulated SOC mineralization and caused a positive priming effect (0.27% of SOC). In contrast, 2LFh- and 6LFh-bound glucose inhibited SOC mineralization to both CO2 and CH4 and subsequently induced a negative priming effect, ranging from −0.33% to −0.55% SOC. Compared to 2LFh-bound glucose, 6LFh-bound glucose induced a lower priming effect on CO2 emissions (2-fold lower), which was attributed to the lower Fe-reduction rate of 6LFh and OC released. In addition, the available nutrients adsorbed by 6LFh were more difficult to release than those by 2LFh, which aggravated microbial nutrient limitation, and further decreased microbial activity. The priming effect for CH4 emissions was directly proportional to the glucose level loaded. The Fe reduction rates were higher in Fh-bound high amount of glucose than that in the Fh-bound low amount of glucose, which subsequently provided more available C sources for methanogens. Thus, Fe minerals have a high capacity for SOC accumulation, as they prevent bound OC from mineralization and decrease native SOC priming. Moreover, the protection of SOC by Fe minerals depended on its crystalline structure and the amount of OC loading. Our results show that promoting the transformation from weakly crystalline Fe oxides to more crystalline forms would increase SOC accumulation and stability over the complete rice-growing period.

Biochar-derived dissolved black carbon accelerates ferrihydrite microbial transformation and subsequent imidacloprid degradation

The effects of an electron shuttle (dissolved black carbon (DBC) derived from biochar) on the microbial reduction of ferrihydrite and subsequent imidacloprid (IMI) degradation were studied. The results showed that DBC addition enhanced the microbial reduction of Fe(III) in ferrihydrite and increased the quantity of Fe(II) released into the liquid phase. The electron transfer capacity of DBC was significantly influenced by the content of redox-active oxygen-containing functional groups (e.g., quinone, hydroquinone, and polyphenol groups), which was dependent on the pyrolysis temperature. The electrochemical characteristics of DBC resulted in enhanced electron transfer, which promoted Fe(III) reduction and mediated the microbial transformation of ferrihydrite. The microbial transformation of ferrihydrite resulted in the formation of secondary minerals such as siderite and vivianite. The IMI degradation efficiency was related to the Fe(III) reduction rate and the pyrolysis temperature used in DBC production, and the degradation pathways were nitrate reduction and imino hydrolysis induced by the Fe(II) generated from the reduction of Fe(III) in ferrihydrite. The results obtained in this study provide new data for understanding the multifunctional roles of biochar-derived DBC in the redox and transformation processes of iron minerals induced by iron-reducing bacteria, the related biogeochemical cycles of iron and the fate of pollutants.

Quantifying the Influence of Defects on Selectivity of Electrodes Encapsulated by Nanoscopic Silicon Oxide Overlayers.

Encapsulation of electrocatalysts and photocatalysts with semipermeable nanoscopic oxide overlayers that exhibit selective transport properties is an attractive approach to achieve high redox selectivity. However, defects within the overlayers─such as pinholes, cracks, or particle inclusions─may facilitate local high rates of parasitic reactions by creating pathways for facile transport of undesired reactants to exposed active sites. Scanning electrochemical microscopy (SECM) is an attractive method to determine the influence of defects on macroscopic performance metrics thanks to its ability to measure the relative rates of competing electrochemical reactions with high spatial resolution over the electrode. Here, we report the use of SECM to determine the influence of overlayer defects on the selectivity of silicon oxide (SiOx) encapsulated platinum thin-film electrocatalysts operated under conditions where two competing reactions─the hydrogen evolution and Fe(III) reduction reactions─can occur. After an SECM methodology is described to determine spatially resolved selectivity, representative selectivity maps are correlated with the location of defects that are characterized by optical, electron, and atomic force microscopies. This analysis reveals that certain types of defects in the oxide overlayer are responsible for ∼60-90% of the partial current density toward the undesired Fe(III) reduction reaction. By correcting for defect contributions to Fe(III) reduction rates, true Fe(III) permeability values for the SiOx overlayers were determined to be over an order of magnitude lower than permeabilities determined from analyses that ignore the presence of defects. Finally, different types of defects were studied revealing that defect morphology can have varying influence on both redox selectivity and calculated permeability. This work highlights the need for spatially resolved measurements to evaluate the performance of oxide-encapsulated catalysts and understand their performance limits.

Iron Reduction Controls Carbon Mineralization in Aquaculture Shrimp Pond Sediments in Subtropical Estuaries

AbstractExpanding worldwide aquaculture has greatly increased greenhouse gas emissions; however, the underlying microbial mechanisms are poorly understood. In particular, the role of ferric iron [Fe(III)] (hydro)oxides in carbon mineralization in aquaculture pond sediments remains unclear. Here, we studied the rates of microbial Fe(III) reduction, sulfate reduction, methanogenesis, and carbon mineralization in aquaculture shrimp (Litopenaeus vannamei) ponds of various salinities before, during, and after shrimp farming in subtropical estuaries in southeast China. Sediment samples (0–10 cm) were collected to investigate the content of iron species, characteristics of organic matter, and abundance of Geobacter, a proxy of iron reducers. Overall, Fe(III) reduction (46.1% ± 19.1%) dominated carbon mineralization, followed by sulfate reduction (39.6% ± 16.8%) and methanogenesis (1.5% ± 1.1%). Microbial Fe(III) reduction contributed more to carbon mineralization during farming than before and after farming. This enhancement in Fe(III) reduction is attributed to a significant increase in Fe(III) content during farming. Additionally, the contributions of microbial Fe(III) reduction to carbon mineralization were lower in the high‐salinity ponds than in the low‐salinity ponds due to the suppression of sulfate reduction, abiotic Fe(III) reduction by sulfides, and lower oxidation‐reduction potential. Our findings demonstrate that microbial Fe(III) reduction plays a significant role in carbon mineralization in aquaculture pond sediments. Future carbon flux prediction models of aquaculture pond systems should fully integrate microbial Fe(III) reduction.

Thiosulfate driving bio-reduction mechanisms of scorodite in groundwater environment

Reductive dissolution of scorodite results in the release and migration of arsenic (As) in groundwater. The purpose of this study was to explore the possible abiotic and biotic reduction of scorodite in groundwater environment and the effect of microbial-mediated sulfur cycling on the bio-reduction of scorodite. Microcosm experiments consisting of scorodite with bacterium Citrobacter sp. JH012-1 or free sulfide were carried out to determine the effects of thiosulfate on the mobilization of As/Fe. The results show arsenic release is positively correlated with iron reduction. The arsenate [As(V)] released can agglomerate with Fe(II) on the surface of scorodite to form crystalline parasymplesite, while no parasymplesite was detected in the abiotic reduction of scorodite by sulfide. The reduction of scorodite and As(V) was affected by thiosulfate. When 0.5 mM thiosulfate was added, the Fe(III) reduction rate increased from 32% to 82%, and the As(V) reduction rate rose from 54% to 64%. When the addition of thiosulfate was increased from 0.5 mM to 2 mM and 5 mM, Fe(III) reduction rate added 4% and 8%, and As(V) reduction rate increased 11% and 16%, respectively. In addition, the presence of thiosulfate promoted the scorodite almost completely converting to parasymplesite. Therefore, the effect of microbial-mediated sulfur cycling should be considered in arsenic migration and reduction from scorodite.

Microbial iron reduction and greenhouse gas production in response to organic matter amendment and temperature increase of periglacial sediments, Bolterdalen, Svalbard

ABSTRACT Arctic permafrost soils store substantial reserves of organic matter (OM) from which microbial transformation contributes significantly to greenhouse gas emissions of CH4 and CO2. However, many younger sediments exposed by glacier retreat and sea level change in fjord landscapes lack significant organic carbon resources, so their capacity to promote greenhouse gas emissions is unclear. We therefore studied the effects of increased temperatures (4°C and 21°C) and OM on rates of Fe(III) reduction, CO2 production, and methanogenesis in three different Holocene sedimentary units from a single site within the former marine limit of Adventdalen, Svalbard. Higher temperature and OM addition generally stimulated CH4 production and CO2 production and an increase in Bacteria and Archaea abundance in all units, whereas an equal stimulation of Fe(II) production by OM amendment and an increase in temperature to 21°C was only observed in a diamicton. We observed an accumulation of Fe(II) in beach and delta deposits as well but saw no stimulating effect of additional OM or increased temperature. Interestingly, we observed a small but significant production of CH4 in all units despite the presence of large reservoirs of Fe(III), sulfate, and nitrate, indicating either the availability of substrates that are primarily used by methanogens or a tight physical coupling between fermentation and methanogenesis by direct electron transfer. Our study clearly illustrates a significant challenge that comes with the large heterogeneity on a narrow spatial scale that one encounters when studying soils that have complex histories.

Feammox is more important than anammox in anaerobic ammonium loss in farmland soils around Lake Taihu, China

Ammonium (NH4+) oxidation is a key step in nitrogen transformation in ecosystems. Prior to the recent discovery of Feammox (anaerobic NH4+ oxidation coupled with iron reduction), anammox (anaerobic NH4+ oxidation coupled with nitrite reduction) was thought of as the only pathway by which anaerobic NH4+ loss (NH4+ directly to N2) occurs in soils. Experimental evidence has confirmed that both anammox and Feammox contribute to anaerobic NH4+ loss; however, their relative contributions to this process in farmland soils are largely unknown. Therefore, in this study, we examined the seasonal activities of anammox and Feammox in conventional tillage (CT) and no-tillage (NT) soils around Lake Taihu, China. Isotopic tracing experiments showed higher anammox and Feammox rates in summer than in other seasons, and the contribution of Feammox to anaerobic NH4+ loss from the farmland soils (54.6%–69.3%) was higher than that of anammox. Further, the Feammox rates corresponding to the two soil tillage practices were significantly different, whereas their corresponding anammox rates showed no significant differences. Furthermore, molecular analysis showed that the abundance of Geobacteraceae differed significantly with season and tillage practice, whereas the abundance of anammox bacteria showed no significant differences between CT and NT practices. Structural equation modeling also revealed that the anammox rate was directly or indirectly driven by N availability and season, whereas the Feammox rate was driven by soil moisture content, Fe(III) concentration, Fe(III) reduction rates, tillage practice, and season. Overall, this study enhances understanding regarding anaerobic NH4+ oxidation in farmland soils and highlight the importance of Feammox in NH4+ loss in such an ecosystem.

Effect of bacterial cell addition on Fe(III) reduction and soil organic matter transformation in a farmland soil

The coupled biogeochemical processes of Fe(III) reduction and organic matter transformation profoundly impact terrestrial carbon cycling. However, little is known about the microbial role in soil organic matter (SOM) transformation during Fe(III) bio-reduction. Here we investigated the bio-reduction behavior of a black farmland soil and corresponding SOM transformation under circumneutral and anoxic conditions. A model dissimilatory Fe reducing bacterium, Geobacter sulfurreducens, was added in either live or dead form in order to enhance Fe(III) reduction in soil and to evaluate the accompanying transformation of SOM. The progress of Fe reduction was monitored and SOM transformation was characterized by various spectroscopy methods.Results showed that addition of either dead or live G. sulfurreducens cells increased the Fe(III) reduction rate and extent. Without cell addition, SOM transformation was insignificant within 13 days of incubation, only with some consumption of aliphatic/protein compounds, apparently due to their higher bio-degradability. Addition of dead or live cells resulted in more drastic SOM transformation, but through different mechanisms. With dead cell amendment, cell necromass and debris stimulated the activity of indigenous soil microbes by serving as extra carbon/energy sources. During Fe(III) reduction, the aliphatic/protein compounds were preferentially consumed by indigenous microbial communities, similar to the treatment without cell addition but with a greater extent of consumption. In comparison, addition of live G. sulfurreducens cells stimulated degradation of less bioavailable compounds, including more saturated and higher molecular weight SOM. It is possible that fast depletion of labile SOM by live G. sulfurreducens cells favored utilization of less bioavailable molecules (such as those with more aromatic structures) by the originally dormant species in native microbial community, suggesting an active role of live Fe(III)-reducing bacteria in affecting SOM transformation. In addition, fast assimilation of microbial related carbon, such as aromatic proteins and microbial byproducts, into SOM pools was also observed.Overall, our results suggest that addition of Fe(III)-reducing bacteria to a native soil can enhance Fe(III) reduction and accelerate the turnover of SOM through various mechanisms. The study provides new insights into coupled Fe(III) reduction and SOM transformation, as well as the “priming effect” of SOM using microbial cells as substrate.

Transformation Behavior of Iron Minerals in High-Iron Red Mud During High-Pressure Hydrothermal Reduction.

Nowadays, the comprehensive utilization of red mud is still a difficult problem. Using starch as the reducing agent, the "high-pressure hydrothermal reduction method" is used to separate and recover sodium, aluminum, and iron in high-iron red mud in one step in an alkaline hydrothermal system. This article focuses on the hydrothermal reduction behavior of the iron ore phase. The results showed that starch formed a strong reduction system through degradation under alkaline hydrothermal conditions, in which Fe2O3 was first partially dissolved and reduced to form Fe[OH]+, then Fe[OH]+ further reacted with Fe2O3 to form Fe3O4. At a temperature of 260°C, a C/S (calcium to silicon ratio) of 3.5, a Na2O concentration of 250g/L, Whigh-iron red mud/Wstarch = 10, the Fe reduction rate was 98.46%, and the transformation slag at a magnetic field strength of 1800 Gs resulted in a magnetic iron concentrate powder with a TFe grade of 65.75%.

Ocean Acidification Effect on the Iron-Gallic Acid Redox Interaction in Seawater

Ocean acidification impacts the iron (Fe) biogeochemistry both by its redox and its complexation reactions. This has a direct effect on the ecosystems due to Fe being an essential micronutrient. Polyphenols exudated by marine microorganisms can complex Fe(III), modifying the Fe(II) oxidation rates as well as promoting the reduction of Fe(III) to Fe(II) in seawater. The effect of the polyphenol gallic acid (GA; 3,4,5-trihydroxy benzoic acid) on the oxidation and reduction of Fe was studied. The Fe(II) oxidation rate constant decreased, increasing the permanence of Fe(II) in solutions at nM levels. At pH = 8.0 and in the absence of gallic acid, 69.3% of the initial Fe(II) was oxidized after 10 min. With 100 nM of gallic acid (ratio 4:1 GA:Fe), and after 30 min, 37.5% of the initial Fe(II) was oxidized. Fe(III) is reduced to Fe(II) by gallic acid in a process that depends on the pH and composition of solution, being faster as pH decreases. At pH &amp;gt; 7.00, the Fe(III) reduction rate constant in seawater was lower than in NaCl solutions, being the difference at pH 8.0 of 1.577 × 10–5 s–1. Moreover, the change of the Fe(III) rate constant with pH, within the studied range, was higher in seawater (slope = 0.91) than in NaCl solutions (slope = 0.46). The Fe(III) reduction rate constant increased with increasing ligand concentration, being the effect higher at pH 7.0 [k′ = 1.078 × 10–4 s–1; (GA) = 250 nM] compared with that at pH 8.0 [k′ = 3.407 × 10–5 s–1; (GA) = 250 nM]. Accordingly, gallic acid reduces Fe(III) to Fe(II) in seawater, making possible the presence of Fe(II) for longer periods and favoring its bioavailability.

A coupled function of biochar as geobattery and geoconductor leads to stimulation of microbial Fe(III) reduction and methanogenesis in a paddy soil enrichment culture

Biochar can participate in biogeochemical electron transfer processes due to its electron-accepting and donating capabilities (i.e., geobattery) and electron conductivity (i.e., geoconductor). These two functions were separately demonstrated to play a role in biogeochemical iron cycling and methane formation. Yet, little is known about the coupled effect of both electron transfer mechanisms, even though naturally occurring electron transfer through biochar is expected to simultaneously rely on both geobattery and geoconductor mechanisms. Here, we incubated an anoxic paddy soil enrichment culture with acetate as the substrate to investigate how biochar's coupled electron transfer mechanisms influence the electron transfer pathways between microbes and Fe(III) minerals and how it impacts the soil microbial community composition. We found that biochar simultaneously stimulated microbial Fe(III) reduction and methanogenesis by 2.6 and 2.3 fold, but these processes were spatially decoupled. Small biochar particles (5–20 μm) caused higher Fe(III) reduction and methanogenesis rates than large particles (50–100 μm). The addition of biochar enriched a syntrophic acetate-oxidizing co-culture with dominating Fe(III)-reducing Geobacteraceae taxa and acetoclastic methanogenic Methanosarcina taxa. After acetoclastic methanogenesis stopped, the observed continuing methanogenesis was likely due to interspecies electron transfer caused by biochar functioning as a geoconductor transferring electrons from Geobacteraceae to Methanosarcina. In summary, the simultaneous occurrence of Fe(III) reduction and methanogenesis leads to the formation of a cell-biochar-mineral battery network and a cell-biochar-cell conductive network in an enrichment culture from a paddy soil.

The role of gentisic acid on the Fe(III) redox chemistry in marine environments

Phenolic compounds excreted by marine microalgae are part of the ligand pool in natural waters. The effect of the polyphenol gentisic acid (GA; 2,5 dihydroxybenzoic acid) on the Fe(III) reduction as a function of organic ligand concentration (100 nM – 1000 nM) and the pH (7.00–8.01) was investigated in seawater. Major seawater ions interactions with both GA and Fe(III) were also considered using 0.7 M NaCl +2 mM NaHCO3 solutions with each major ions (Mg2+, Ca2+, SO42−, K+, F−, Sr2+) at the same ratio than that in a 35 salinity seawater. The Fe(III)-GA complex in solution was able to produce Fe(II) in a pH-dependent process that was faster in seawater than in NaCl-NaHCO3 solution. The addition of each major ion affected the Fe(III) reduction process. At pH 7.5, the presence of Mg2+ and Ca2+ contributes to accelerate the reduction rate constant of Fe(III) to Fe(II). The K+, F−, Sr2+, and SO42− ions slow down the Fe(III) reduction rate constant. At pH 7.8, the effect of the ions is counteracted. Increasing the pH to 8, the effect on the solubility of Fe(III) is more important, being greater in NaCl and no reduction of Fe(III) was detected, compared with seawater.In this study, the formation and dissociation of the Fe(III)-GA complex in seawater was studied, resulting in kf = 1.19(±0.18)•104 (M−1 s−1), kd = 1.86 (±0.53)•10−4 (s−1), and conditional complexation constant of logKFe3+LCond = 17.81 ± 0.05 in seawater.Accordingly, phenolic compounds like GA influence the Fe marine biogeochemical cycles promoting the formation of the bioavailable Fe(II) in solution. Taking this into account, the Fe(III) reactions with phenolic compounds have to be considered to improve our understanding of the global iron cycle.

Microorganisms maintain C:N stoichiometric balance by regulating the priming effect in long-term fertilized soils

Labile carbon (C) inputs affect the soil carbon:nitrogen (C:N) ratio and microbial stoichiometric homeostasis, which control the intensity and direction of the priming effect (PE). Here, we clarified how soil microorganisms regulate enzyme production and PE to maintain the C:N stoichiometric balance. Specifically, we conducted an incubation experiment by adding 13C-labeled glucose to four long-term fertilized paddy soils: no fertilization; fertilization with mineral nitrogen, phosphorus, and potassium (NPK); NPK combined with straw; and NPK with manure (NPKM). After glucose addition, the dissolved organic carbon-to-ammonium (DOC:NH4+) ratio (24–39) initially increased, but subsequently decreased after day 2 following glucose exhaustion. In parallel, the microbial C:N imbalance [(DOC:NH4+):(microbial biomass C:microbial biomass N)] rapidly decreased from day 2 (4.6–7.2) to day 20 (<0.5). Thus, microorganisms became C limited after 20 days of incubation. Excess C, resulting from glucose addition, increased N-hydrolase (chitinase) production and N mining from soil organic matter (SOM) through positive PEs. However, C hydrolase (β-1,4-glucosidase and β-xylosidase) activity increased, while that of N hydrolase (chitinase) decreased, following glucose exhaustion. Consequently, the C:N microbial biomass ratio increased as the DOC:NH4+ ratio decreased, leading to negative PEs. NPKM-fertilized soil had the largest cumulative PE (2.3% of soil organic carbon) because it had the highest microbial biomass and iron (Fe) reduction rate. Thus, this increased N mining from SOM maintained the microbial C:N stoichiometric balance. We concluded that soil microorganisms regulate C- and N-hydrolase production to control the intensity and direction of PE, maintaining the C:N stoichiometric balance in response to labile C inputs.