Broad-Scale Evidence That pH Influences the Balance Between Microbial Iron and Sulfate Reduction

Biostimulation of sulfate reduction for in-situ metal(loid) precipitation at an industrial site in Flanders, Belgium

A 30-month pilot study was conducted to evaluate the potential of in-situ metal(loid) removal through biostimulation of sulfate-reducing processes. The study took place at an industrial site in Flanders, Belgium, known for metal(loid) contamination in soil and groundwater. Biostimulation involved two incorporations of an organic substrate (emulsified vegetable oil) as electron donor and potassium bicarbonate to raise the pH of the groundwater by 1-1.5 units. The study focused on the most impacted permeable fine sand aquifer (8-9 m below groundwater level) confined by layers of non-permeable clay. The fine sands exhibited initially oxic conditions (50-200 mV), an acidic pH of 4.5 and sulfate concentrations ranging from 600 to 800 mg/L. At the central monitoring well, anoxic conditions (-200 to -400 mV) and a pH of 5.9 established shortly after the second substrate and reagent injection. Over the course of 12 months, there was a significant decrease in the concentration of arsenic (from 2500 to 12 μg/L), nickel (from 360 to <2 μg/L), zinc (from 78,000 to <2 μg/L), and sulfate (from 930 to 450 mg/L). Low levels of metal(loid)s were still present after 34 months (end of study). Mineralogical analysis indicated that the precipitates formed were amorphous in nature. Evidence for biologically driven metal(loid) precipitation was provided by compound specific stable isotope analysis of sulfate. In addition, changes in microbial populations were assessed using next-generation sequencing, revealing stimulation of native sulfate-reducing bacteria. These results highlight the potential of biostimulation for long-term in situ metal(loid) plume treatment/containment.

Influencing of hydrogeochemical conditions and engineering parameters on phase behaviors and remediation performance of in-situ microemulsion for residual PCE in aquifers

In-situ microemulsion has great potential for remediation of chlorinated solvent contaminated aquifers due to its efficient solubilization, and the in-situ formation and phase behaviors of microemulsion is a key factor in determining the remediation performance. However, the role of aquifer properties and engineering parameters on microemulsion in-situ formation and phase transition has been rarely attended. In this work, the influences of hydrogeochemical conditions on in-situ microemulsion phase transition and solubilization ability for tetrachloroethylene (PCE) were explored, and the formation condition, phase transition and removal efficiency for in-situ microemulsion flushing under various flushing conditions were investigated. The results indicated that the cations (Na⁺, K⁺, Ca²⁺) all facilitated the microemulsion phase altering from Winsor I → III → II, whereas the anions (Cl^-, SO₄^2-, CO₃^2-) and pH variation (5-9) had no profound influence on phase transition. Besides, the solubilization capacity of microemulsion was enhanced by the pH variation and the cations, which was proportional to the cation concentration of groundwater. The column experiments demonstrated that PCE underwent the phase transition from emulsion to microemulsion and then to micellar solution during the flushing process. The formation and phase transition of microemulsion were mainly related to injection velocity and PCE residual saturation in aquifers. The slower injection velocity and higher residual saturation were profitable to the in-situ formation of microemulsion. In addition, the removal efficiency could achieve 99.29 % for residual PCE at 12 °C, enhancing with the finer porous medium, lower injection velocity and intermittent injection. Furthermore, the flushing system exhibited high biodegradability and weak reagent adsorption onto the aquifer media, presenting a low environmental risk. This study provides valuable information on the in-situ microemulsion phase behaviors and the optimal reagent parameters, facilitating the application of in-situ microemulsion flushing.

In-situ formation and self-immobilization of biogenic Fe oxides in anaerobic granular sludge for enhanced performance of acidogenesis and methanogenesis

Addition of ferric oxides into flocculent anaerobic sludge was reported to enhance methanogenesis due to accelerated direct interspecies electron transfer (DIET) between syntrophic microbial communities. However, it is generally hard to incorporate Fe oxides into already matured anaerobic granular sludge (AGS) due to its special aggregated structure. In this study, a novel method was attempted to fast incorporate Fe oxides into AGS through in-situ microbial formation and immobilization of biogenic Fe oxides. Factors influencing the formation of Fe oxides were investigated and effects of Fe oxides on the acidogenic and methanogenic performance of AGS were assessed. Results showed that AGS could form Fe oxides mainly in the form of magnetite and hematite through biological reduction of Fe(III) oxyhydroxide. A maximum loading amount of 83.9 mg Fe/g MLVSS was obtained at pH 7 after contacting with 60 mM Fe(III) oxyhydroxide. The efficiency of electron donors which supported Fe(III) reduction followed the order of pyruvate > propionate > glucose > acetate > lactate > formate. Addition of electron transfer mediators (ETMs) promoted the formation of Fe oxides and their performance followed the order of AQDS > AQC > humics > FMN > riboflavin. Presence of Fe oxides in AGS (134.6 Fe/g VSS) increased the production of volatile fatty acids (VFAs) and methane by 16.28% and 41.94% respectively, comparing to the control. The enhancement may be attributed to increased conductivity and stimulated growth of exoelectrogens (Clostridium and Anaerolinea) and methanogenic endoelectrogens Methanosaeta in granular sludge which may strengthen direct interspecies electron transfer between syntrophic microbial communities. Overall, this study provides an alternative strategy to improve the digestion performance of AGS through in-situ formation and immobilization of biogenic Fe oxides.

Influences of pH and substrate supply on the ratio of iron to sulfate reduction

Iron reduction and sulfate reduction often occur simultaneously in anoxic systems, and where that is the case, the molar ratio between the reactions (i.e., Fe/SO₄^2- reduced) influences their impact on water quality and carbon storage. Previous research has shown that pH and the supply of electron donors and acceptors affect that ratio, but it is unclear how their influences compare and affect one another. This study examines impacts of pH and the supply of acetate, sulfate, and goethite on the ratio of iron to sulfate reduction in semi-continuous sediment bioreactors. We examined which parameter had the greatest impact on that ratio and whether the parameter influences depended on the state of each other. Results show that pH had a greater influence than acetate supply on the ratio of iron to sulfate reduction, and that the impact of acetate supply on the ratio depended on pH. In acidic reactors (pH 6.0 media), the ratio of iron to sulfate reduction decreased from 3:1 to 2:1 as acetate supply increased (0-1 mM). In alkaline reactors (pH 7.5 media), iron and sulfate were reduced in equal proportions, regardless of acetate supply. Secondly, a comparison of experiments with and without sulfate shows that the extent of iron reduction was greater if sulfate reduction was occurring and that the effect was larger in alkaline reactors than acidic reactors. Thus, the influence of sulfate supply on iron reduction extent also depended on pH and suggests that iron reduction grows more dependent on sulfate reduction as pH increases. Our results compare well to trends in groundwater geochemistry and provide further evidence that pH is a major control on iron and sulfate reduction in systems with crystalline (oxyhydr)oxides. pH not only affects the ratio between the reactions but also the influences of other parameters on that ratio.

Occurrence and Geochemistry of Lead-210 and Polonium-210 Radionuclides in Public-Drinking-Water Supplies from Principal Aquifers of the United States

On the basis of lifetime cancer risks, lead-210 (²¹⁰Pb) and polonium-210 (²¹⁰Po) ≥ 1.0 and 0.7 pCi/L (picocuries per liter), respectively, in drinking-water supplies may pose human-health concerns. ²¹⁰Pb and ²¹⁰Po were detected at concentrations greater than these thresholds at 3.7 and 1.5%, respectively, of filtered untreated groundwater samples from 1263 public-supply wells in 19 principal aquifers across the United States. Nationally, 72% of samples with radon-222 (²²²Rn) concentrations > 4000 pCi/L had ²¹⁰Pb ≥ 1.0 pCi/L. ²¹⁰Pb is mobilized by alpha recoil associated with the decay of ²²²Rn and short-lived progeny. ²¹⁰Pb concentrations ≥ 1.0 pCi/L occurred most frequently where acidic groundwaters inhibited ²¹⁰Pb readsorption (felsic-crystalline rocks) and where reducing alkaline conditions favored dissolution of iron-manganese- (Fe-Mn-) oxyhydroxides (which adsorb ²¹⁰Pb) and formation of lead-carbonate complexes (enhancing lead (Pb) mobility). ²¹⁰Po concentrations ≥ 0.7 pCi/L occurred almost exclusively in confined Coastal Plain aquifers where old (low percent-modern carbon-14) groundwaters were reducing, with high pH (>7.5) and high sodium/chloride (Na/Cl) ratios resulting from cation exchange. In high-pH environments, aqueous polonium (Po) is poorly sorbed, occurring as dihydrogen polonate (H₂PoO₃(aq)) or, under strongly reducing conditions, as a hydrogen-polonide anion (HPo^-). Fe-Mn- and sulfate-reduction and cation-exchange processes may mobilize polonium from mineral surfaces. Po²⁺ occurrence in low-to-neutral-pH waters is attenuated by adsorption.

Influence of pH on the balance between methanogenesis and iron reduction

Methanogenesis and iron reduction play major roles in determining global fluxes of greenhouse gases. Despite their importance, environmental factors that influence their interactions are poorly known. Here, we present evidence that pH significantly influences the balance between each reaction in anoxic environments that contain ferric (oxyhydr)oxide minerals. In sediment bioreactors that contained goethite as a source of ferric iron, both iron reduction and methanogenesis occurred but the balance between them varied significantly with pH. Compared to bioreactors receiving acidic media (pH 6), electron donor oxidation was 85% lower for iron reduction and 61% higher for methanogenesis in bioreactors receiving alkaline media (pH 7.5). Thus, methanogenesis displaced iron reduction considerably at alkaline pH. Geochemistry data collected from U.S. aquifers demonstrate that a similar pattern also exists on a broad spatial scale in natural settings. In contrast, in bioreactors that were not augmented with goethite, clay minerals served as the source of ferric iron and the balance between each reaction did not vary significantly with pH. We therefore conclude that pH can regulate the relative contributions of microbial iron reduction and methanogenesis to carbon fluxes from terrestrial environments. We further propose that the availability of ferric (oxyhydr)oxide minerals influences the extent to which the balance between each reaction is sensitive to pH. The results of this study advance our understanding of environmental controls on microbial methane generation and provide a basis for using pH and the occurrence of ferric minerals to refine predictions of greenhouse gas fluxes.

Independent and interactive effects of reduced seawater pH and oil contamination on subsurface sediment bacterial communities

Ocean acidification may exacerbate the environmental impact of oil hydrocarbon pollution by disrupting the core composition of the superficial (0-1 cm) benthic bacterial communities. However, at the subsurface sediments (approximately 5 cm below sea floor), the local biochemical characteristics and the superjacent sediment barrier may buffer these environmental changes. In this study, we used a microcosm experimental approach to access the independent and interactive effects of reduced seawater pH and oil contamination on the composition of subsurface benthic bacterial communities, at two time points, by 16S rRNA gene-based high-throughput sequencing. An in-depth taxa-specific variance analysis revealed that the independent effects of reduced seawater pH and oil contamination were significant predictors of changes in the relative abundance of some specific bacterial groups (e.g., Firmicutes, Rhizobiales, and Desulfobulbaceae). However, our results indicated that the overall microbial community structure was not affected by independent and interactive effects of reduced pH and oil contamination. This study provides evidence that bacterial communities inhabiting subsurface sediment may be less susceptible to the effects of oil contamination in a scenario of reduced seawater pH.

Effects of Fe-S-As coupled redox processes on arsenic mobilization in shallow aquifers of Datong Basin, northern China

High arsenic groundwater generally coexists with elevated Fe²⁺ concentrations (mg L^-1 levels) under reducing conditions, but an explanation for the extremely high arsenic (up to ∼2690) concentrations at very low Fe²⁺ (i.e., μg L^-1 levels) in groundwater of Datong Basin remains elusive. Field groundwater investigation and laboratory microcosm experiments were implemented in this study. The field groundwater was characterized by weakly alkaline (pH 7.69 to 8.34) and reducing conditions (Eh -221.7 to -31.9 mV) and arsenic concentration averages at 697 μg L^-1. Acinetobacter (5.9-51.3%), Desulfosporosinus (4.6-30.2%), Brevundimonas (3.9-19%) and Pseudomonas (3.2-14.6%) were identified as the dominant genera in the bacterial communities. Bacterially mediated arsenate reduction, Fe(III) reduction, and sulfate reduction are processes occurring (or having previously occurred) in the groundwater. Results from incubation experiment (27 d) revealed that nitrate, arsenate, and Fe(III)/sulfate reduced sequentially with time under anoxic conditions, while Fe(III) and sulfate reduction processes had no obvious differences, occurring almost simultaneously. Moreover, low Fe²⁺ concentrations were attributed to initially high pH conditions, which relatively retarded Fe(III) reduction. In addition, arsenic behavior in relation to groundwater redox conditions, matrices, and solution chemistry were elaborated. Bacterial arsenate reduction process proceeded before Fe(III) and sulfate reduction in the incubation experiment, and the total arsenic concentration (dominated by arsenite) gradually increased from ∼7 to 115 μg L^-1 as arsenate was reduced. Accordingly, bacterially mediated reductive desorption of arsenate is identified as the main process controlling arsenic mobility, while Fe(III) reduction coupled with sulfate reduction are secondary processes that have also contributed to arsenic enrichment in the study site. Overall, this study provide important insights into the mechanism controlling arsenic mobility under weakly alkaline and reducing conditions, and furnishes that arsenate reduction by bacteria play a major role leading to high accumulation of desorbed arsenite in groundwater.

Thermodynamic and Kinetic Response of Microbial Reactions to High CO2

Geological carbon sequestration captures CO₂ from industrial sources and stores the CO₂ in subsurface reservoirs, a viable strategy for mitigating global climate change. In assessing the environmental impact of the strategy, a key question is how microbial reactions respond to the elevated CO₂ concentration. This study uses biogeochemical modeling to explore the influence of CO₂ on the thermodynamics and kinetics of common microbial reactions in subsurface environments, including syntrophic oxidation, iron reduction, sulfate reduction, and methanogenesis. The results show that increasing CO₂ levels decreases groundwater pH and modulates chemical speciation of weak acids in groundwater, which in turn affect microbial reactions in different ways and to different extents. Specifically, a thermodynamic analysis shows that increasing CO₂ partial pressure lowers the energy available from syntrophic oxidation and acetoclastic methanogenesis, but raises the available energy of microbial iron reduction, hydrogenotrophic sulfate reduction and methanogenesis. Kinetic modeling suggests that high CO₂ has the potential of inhibiting microbial sulfate reduction while promoting iron reduction. These results are consistent with the observations of previous laboratory and field studies, and highlight the complexity in microbiological responses to elevated CO₂ abundance, and the potential power of biogeochemical modeling in evaluating and quantifying these responses.