Multi-approach assessment of groundwater biogeochemistry: Implications for the site characterization of prospective spent nuclear fuel repository sites

The disposal of spent nuclear fuel in deep subsurface repositories using multi-barrier systems is considered to be the most promising method for preventing radionuclide leakage. However, the stability of the barriers can be affected by the activities of diverse microbes in subsurface environments. Therefore, this study investigated groundwater geochemistry and microbial populations, activities, and community structures at three potential spent nuclear fuel repository construction sites. The microbial analysis involved a multi-approach including both culture-dependent, culture-independent, and sequence-based methods for a comprehensive understanding of groundwater biogeochemistry. The results from all three sites showed that geochemical properties were closely related to microbial population and activities. Total number of cells estimates were strongly correlated to high dissolved organic carbon; while the ratio of adenosine-triphosphate:total number of cells indicated substantial activities of sulfate reducing bacteria. The 16S rRNA gene sequencing revealed that the microbial communities differed across the three sites, with each featuring microbes performing distinctive functions. In addition, our multi-approach provided some intriguing findings: a site with a low relative abundance of sulfate reducing bacteria based on the 16S rRNA gene sequencing showed high populations during most probable number incubation, implying that despite their low abundance, sulfate reducing bacteria still played an important role in sulfate reduction within the groundwater. Moreover, a redundancy analysis indicated a significant correlation between uranium concentrations and microbial community compositions, which suggests a potential impact of uranium on microbial community. These findings together highlight the importance of multi-methodological assessments in better characterizing groundwater biogeochemical properties for the selection of potential spent nuclear fuel disposal sites.

Conductive magnetic nanowires accelerated electron transfer between C1020 carbon steel and Desulfovibrio vulgaris biofilm

Microbial biofilms are behind microbiologically influenced corrosion (MIC). Sessile cells in biofilms are many times more concentrated volumetrically than planktonic cells in the bulk fluids, thus providing locally high concentrations of chemicals. More importantly, "electroactive" sessile cells in biofilms are capable of utilizing extracellularly supplied electrons (e.g., from elemental Fe) for intracellular reduction of an oxidant such as sulfate in energy metabolism. MIC directly caused by anaerobic biofilms is classified into two main types based on their mechanisms: extracellular electron transfer MIC (EET-MIC) and metabolite MIC (M-MIC). Sulfate-reducing bacteria (SRB) are notorious for their corrosivity. They can cause EET-MIC in carbon steel, but they can also secrete biogenic H2S to corrode other metals such as Cu directly via M-MIC. This study investigated the use of conductive magnetic nanowires as electron mediators to accelerate and thus identify EET-MIC of C1020 by Desulfovibrio vulgaris. The presence of 40 ppm (w/w) nanowires in ATCC 1249 culture medium at 37 °C resulted in 45 % higher weight loss and 57 % deeper corrosion pits after 7-day incubation. Electrochemical tests using linear polarization resistance and potentiodynamic polarization supported the weight loss data trend. These findings suggest that conductive magnetic nanowires can be employed to identify EET-MIC. The use of insoluble 2 μm long nanowires proved that the extracellular section of the electron transfer process is a bottleneck in SRB MIC of carbon steel.

Susceptibility of Cd availability in microplastics contaminated paddy soil: Influence of ferric minerals and sulfate reduction

The combined contamination of cadmium (Cd) and microplastics (MPs) in paddy soil always occurred, while its influence on Cd availability remained unclear. This study investigated the Cd availability in Cd-MPs co-contaminated paddy soil in consideration of both ferric minerals and sulfate reduction under flooding conditions. The presence of MPs resulted in a higher Cd releasing risk, as represented by the increase in the available Cd and decrease in Fe-Mn oxide-bound Cd contents, especially on the 7th and 14th days based on the sequential extraction results. MPs facilitated the formation of Fe-organic ligands, which accelerated the reductive dissolution of iron minerals but decreased the amounts of amorphous iron minerals due to the release of dissolved organic substances into pore water. Furthermore, MPs promoted the relative abundance of sulfate-reducing bacteria (such as Streptomyces and Desulfovibrio genera), thus increasing the contents of reductive S species, which was advantageous to the co-precipitation of Fe, S, and Cd on the surface of MPs based on our experimental and statistical results. Taken together, both iron and sulfate reduction under anaerobic conditions played a critical role in Cd mobilization in Cd-MPs co-contaminated paddy fields.

Integrated biological system for remediation and valorization of tannery wastewater: Focus on microbial communities responsible for methanogenesis and sulfidogenesis

Microbial communities in hybrid linear flow channel reactors and anaerobic sequencing batch reactors operated in series for remediation and beneficiation of tannery wastewater were assessed. Despite concurrent sulfidogenesis, more intensive pre-treatment in hybrid linear flow channel reactors reduced methanogenic inhibition usually associated with anaerobic digestion of tannery effluent and promoted efficiency (max 321 mLCH4/gCODconsumed, 59% biogas CH4). Nitrification and biological sulfate reduction were key metabolic pathways involved in overall and sulfate reducing bacterial community selection, respectively, during pre-treatment. Taxonomic selection could be explained by the proteinaceous and saline character of tannery effluent, with dominant genera being protein and/or amino acid degrading, halotolerant and/or ammonia tolerant. Complete oxidizers dominated the sulfidogenic populations during pre-treatment, while aceticlastic genera dominated the methanogenic populations during anaerobic digestion. With more intensive pre-treatment, the system shows promise for remediation and recovery of biogas and sulfur from tannery wastewater in support of a bio-circular economy.

Effect of alkaline leaching of phosphogypsum on sulfate reduction activity and bacterial community composition using different sources of anaerobic microbial inoculum

Phosphogypsum (PG), a by-product of the phosphate industry, is high in sulfate, (SO42-), which makes it an excellent substrate for sulfate-reducing bacteria (SRB) to produce hydrogen sulfide. This work aimed to optimize SO42- leaching from PG to achieve a high biological reduction of SO42- and generate high sulfide concentrations for subsequent use in the biological recovery of elemental sulfur. Five SRB consortia were isolated and enriched from: IS (Industrial sludges), MS (Marine sediments), WC (Winogradsky column), SNV (petroleum industry sediments) and PG (stored Phosphogypsum). The five consortia showed reduction activity when using PG leachate (with water) as source of SO42- and lactate, acetate, or glucose as the electron donor. The highest reduction rate (81.5 %) was registered using lactate and the IS consortium (81.5 %) followed by MS (79 %) and PG (71 %). To enhance the concentration of leached SO42- from PG for future utilization with the isolated consortia, PG was treated with NaOH solutions (2 % and 5 %). SO42- release of 97 % was achieved with a 5 % concentration and the resulting leachate was further diluted to target a SO42- concentration of 12.4 g·L-1 for utilization with the isolated consortia. Compared to water leachate, a significantly higher reduction rate was registered (2 g·L-1 of SO42) using the IS consortium, demonstrating limited inhibition effect of sulfide- concentration on SRB functionalities. Moreover, metagenomic analysis of the consortia revealed that using PG as a source of SO42- increased the abundance of Deltaproteobacteria, including known SRB like Desulfovibrio, Desulfomicrobium, and Desulfosporosinus, as well as novel SRB genera (Cupidesulfovibrio, Desulfocurvus, Desulfococcus) that showed, for the first time, significant potential as novel sulfate-reducers using PG as a SO42- source.

d-tyrosine enhances disoctyl dimethyl ammonium chloride on alleviating SRB corrosion

Microbiologically influenced corrosion (MIC) caused by sulfate reducing bacteria (SRB) is a serious challenge in many industries, but biofilm greatly decreases the toxicity of bactericides to cell inside. d-amino acids are potential enhancers for bactericides due to their excellent performance on biofilm inhibition. However, the mechanism of d-amino acid cooperating with bactericides for MIC inhibition is still unknown. In this study, d-tyrosine（D-Tyr）and disoctyl dimethyl ammonium chloride (DDAC) were selected as the typical d-amino acid and bactericide, respectively, to evaluate their synergetic inhibition on the corrosion caused by Desulfovibrio vulgaris. D-Tyr obviously enhanced the role of DDAC in inhibiting corrosion with high corrosion inhibition efficiency at 77.23 %. The attachment of EPS and live cells on the coupon surface decreased in the presence of D-Try, leading to more cells directly exposed to DDAC. Besides, D-Try decreased the amount of live cells on the surface and thus reduced the utilization of Fe by SRB and corrosion current. Moreover, dead cells settling to the coupon surface may form a protective lay to retard the contact between live SRB and Fe, leading to slow cathode reaction and less corrosion. Therefore, D-Tyr can reduce the coverage of biofilm, thereby reducing its protective effect on SRB and achieving better corrosion inhibition effect. This work provides a new strategy for improving bactericides and inhibiting MIC.

Revealing impacts of operational modes on anaerobic digestion systems coupling with sulfate reduction

Anaerobic digestion (AD) is promising for treating high-strength wastewater. However, the effect of operational parameters on microbial communities of AD with sulfate is not yet fully understood. To explore this, four reactors were operated under rapid- and slow-filling modes with different organic carbons. Reactors in the rapid-filling mode generally exhibited a fast kinetic property. For example, the degradation of ethanol was 4.6 times faster in ASBR_ER than in ASBR_ES, and the degradation of acetate was 11.2 times faster in ASBR_AR than in ASBR_AS. Nevertheless, reactors in the slow-filling mode could mitigate propionate accumulation when using ethanol as organic carbon. Taxonomic and functional analysis further supported that rapid- and slow-filling modes were suitable for the growth of r-strategists (e.g., Desulfomicrobium) and K-strategists (e.g., Geobacter), respectively. Overall, this study provides valuable insights into microbial interactions of AD processes with sulfate through the application of the r/K selection theory.

Effect of alloying element content on anaerobic microbiologically influenced corrosion sensitivity of stainless steels in enriched artificial seawater

Stainless steels (SS) are not immune to microbiologically influenced corrosion (MIC) especially in the presence of sulfate reducing bacteria (SRB). It is necessary to study the influence of alloying elements on the MIC. SRB MIC behaviors of four stainless steels (2205 SS, 316L SS, 304 SS, and 410 SS), with different alloying element compositions were compared after 14 days of incubation at 37°C in enriched artificial seawater inoculated with Desulfovibrio sp. The sessile cell sequence was 410 SS > 316L SS > 304 SS > 2205 SS, inversely proportional to Cr content. The uniform corrosion rate (based on weight loss) sequence was 410 SS > 304 SS > 316L SS > 2205 SS, which matches the pitting resistance equivalent number (PREN) sequence inversely. 410 SS with the lowest Cr and Mo contents suffered the most severe pitting, with pit depth of 35 μm and weight loss of 0.75 mg/cm² (0.91 mm/a pitting rate and 25 μm/a uniform corrosion rate). The other three stainless steels with higher Cr and Mo contents suffered only metastable pits. The semiconductor characteristics and the re-passivation abilities of the passive films were found to be affected by Cr and Mo contents.

Use of propionic acid additions to enhance zinc removal from mine drainage in short residence time, flow-through sulfate-reducing bioreactors

The effectiveness of liquid carbon additions to enhance zinc removal in laboratory-scale short hydraulic residence time (19 h) compost bioreactors receiving synthetic mine water with a high influent zinc concentration (45 mg/L) was investigated. Effective removal of such elevated zinc concentrations could not be sustained by sulfate reduction and/or other attenuation processes without carbon supplementation. Propionic acid addition resulted in improved and sustained performance by promoting the activities of sulfate reducing bacteria, leading to efficient zinc removal (mean 99%) via bacterial sulfate reduction. In contrast, cessation of propionic acid addition led to carbon limitation and the growth of sulfur oxidising bacteria, compromising zinc removal by bacterial sulfate reduction. These research findings demonstrate the potential for modest liquid carbon additions to compost-based passive treatment systems to engineer microbial responses which enhance rates of zinc attenuation in a short hydraulic residence time, enabling remediation of highly polluting mine drainage at sites with limited land availability.

Adaptive response mechanisms of granular and flocculent sulfate-reducing sludge toward acidic multi-metal-laden wastewater

Dissimilatory sulfate reduction-based processes have long been a viable option for treating acidic metal-laden wastewater (AMW). Such processes can be optimized through enhancing sulfidogenic activity and the microbial consortia's resilience against a harsh environment. This study investigated how granular and flocculent sulfate-reducing bacteria (SRB) sludge respond to AMW as well as the mechanisms through which they adapt to the wastewater, with particular focuses on the stability of the sulfidogenic activities, metal removal, and the bacteria's resistance over the long-term: the flocculent SRB lost more than 50% of their treatment capacity after 35 days of treating AMW with the presence of Cd²⁺, Cu²⁺, Zn²⁺, and Ni²⁺ at 30 mg/L each, under pH = 4.5. In contrast, the granular SRB maintained its metal removal rate at 91% throughout the 161-day trial. Despite the SRB abundance remaining at approximate 40%, organics-partial oxidizing genera (Desulfobulbus and Desulfobacter) began to dominate due to their kinetic advantage. The extracellular glycosyl compositions were revealed to be critical for the stability of the granular structure and microbial activity as the extracellular proteins disintegrated irreversible. Usage the molecular dynamic simulation, the mobility of the metal ions in the SRB granular system was suppressed by the presence of a more diverse glycosyl composition compared with the flocculent system (10-50% diffusion coefficients differences). All of the identified glycosyls (especially xylose and rhamnose) exhibited strong interactions with Cu²⁺ (-470 kJ mol^-1), while the maximum binding strength of Cd²⁺ to glycosyls was greater than -40 kJ mol^-1, suggesting a low Cd²⁺complexation efficiency. The findings of this study shed light on the defensive mechanisms of SRB granules against multi-metal stress, and provide clues for efficient AMW treatment.

Algae, biochar and bacteria for acid mine drainage (AMD) remediation: A review

Acid mine drainage (AMD) is a global issue and causes harmful environmental impacts. AMD has high acidity and contains a high concentration of heavy metals and metalloids, making it toxic to plants, animals, and humans. Traditional treatments for AMD have been widely used for a long time. Nevertheless, some limitations, such as low efficacy and secondary contamination, have led them to be replaced by other methods such as bio-based AMD treatments. This study reviewed three bio-based treatment methods using algae, biochar, and bacteria that can be used separately and potentially in combination for effective and sustainable AMD treatment to identify the removal mechanisms and essential parameters affecting AMD treatment. All bio-based methods, when applied as a single process and in combination (e.g. algae-biochar and algae-bacteria), were identified as effective treatments for AMD. Also, all these bio-based methods were found to be affected by some parameters (e.g. pH, temperature, biomass concentration and initial metal concentration) when removing heavy metals from AMD. However, we did not identify any research focusing on the combination of algae-biochar-bacteria as a consortium for AMD treatment. Therefore, due to the excellent performance in AMD treatment of algae, biochar and bacteria and the potential synergism among them, this review provides new insight and discusses the feasibility of a combination of algae-biochar-bacteria for AMD treatment.

Physiological suitability of sulfate reducing granules for the development of bioconcrete

Regular monitoring and timely repair of concrete cracks are required to minimise further deterioration. Self-healing of cracks has been proposed as an alternative to the crack maintenance procedures. One of the proposed techniques is to use axenic cultures to exploit microbial induced calcite precipitation (MICP). However, such healing agents are not cost-effective for in situ use. As the market for bio-based self-healing concrete necessitates a low-cost bio-agent, non-axenic sulfate reducing bacterial (SRB) granules were investigated in this study through cultivation in an upflow anaerobic sludge blanket (UASB) reactor. The compact granules can protect the bacteria from adverse conditions without encapsulation. This study investigated the microbial activities of SRB granules at different temperatures, pH, and COD concentrations which the microbes would experience during the concrete casting and curing process. The attenuation and recovery of microbial activities were measured before and after the exposure. Moreover, the MICP yield was also tested for a possible use in self-healing bioconcrete. The results consistently showed that SRB granules were able to survive starvation, high temperature (50-60 ^o C), and high pH (12), together with SEM/EDS/XRD evidence. Microbial staining analysis demonstrated the formation of spores in the granules during their exposure to the harsh conditions. SRB granule was thus demonstrated to be a viable self-healing non-axenic agent for low-cost bioconcrete. This article is protected by copyright. All rights reserved.

Extracellular electron transfer-dependent Cr(VI)/sulfate reduction mediated by iron sulfide nanoparticles

The slow electron transfer rate is a bottleneck to the biological wastewater treatment. This study evaluated the concomitant biotransformation and nonenzymatic reduction of Cr(VI) mediated by sulfate reducing bacteria (SRB), especially for the reinforcing Cr(VI) reduction via accelerating the electron transfer by the in-situ biosynthesized iron sulfide nanoparticles (FeS NPs). The kinetic results showed that 10 mg/L Cr(VI) was completely removed by pre-cultured FeS NPs within 7 h with k_Cr(VI) of 2.6 × 10^-4 s^-1, one magnitude higher than that without FeS NPs. Despite its competing electron to postpone sulfate reduction, the reduction of Cr(VI) was markedly improved via nonenzymatic reactions by the sulfide, the product of sulfate reduction. In the reinforcing system (bio-FeS NP@SRB), the bio-FeS NPs served as an electronic bypass conduit for CoQ could significantly amplify the electron flux, and switch the Cr(VI) reduction from intracellular space to extracellular environment, which had a great detoxification effect on the microorganisms, eventually markedly promoted electron transfer extracellularly and the reduction of Cr(VI). After the long-term acclimatization, Desulfovibrio became the dominant bacteria at the genus level and accounted for the relative abundance of 32%. This study provides an alternative to use biogenic FeS NPs for Cr(VI) remediation.

The symbiotic system of sulfate-reducing bacteria and clay-sized fraction of purplish soil strengthens cadmium fixation through iron-bearing minerals

The microbe-clay mineral system is widely known to reduce the fluidity of heavy metals through biomineralization, thus mitigating soil pollution stemming from heavy metals. Here, we investigated the effect of mineral distinction on the solidification of cadmium (Cd) using sulfate-reducing bacteria (SRB) to construct symbiotic systems with purplish soil, clay-sized fraction of purple soil (Clay_-csp), clay particles of amorphous iron (Fe) oxide (Clay_-ox), clay particles removing crystalline Fe oxide (Clay_-CBD), and residues of Clay_-CBD treated by hydrochloric acid (Clay_-HCl). The difference in Cd morphology among purplish soil, Clay_-csp, and Clay_-ox indicated that the fixation of Cd in soil was largely determined by Fe oxides. The content of Cd in Clay_-csp decreased by 66.7% after the removal of amorphous Fe, confirming that clay easily adsorbed infinitive Fe oxides in purple soil. In the system of SRB and Clay_-ox, carbonate-bound Cd (F2) decreased by 14.85% and residual Cd (F5) increased by 14% from the retardation to late decline phase, eventually forming iron-sulfur (Fe-S) compounds. Based on the correlation analyses of Cd and Fe in amorphous-bound state and Fe-manganese (Mn) oxidation state in simulation experiments, it is demonstrated that Fe-Mn oxides control the behavior of Cd in soil clay, and SRB-mediated Fe-bearing minerals promote the transformation of Cd from activated to stable state.

The role of organic acid metabolites in geo-energy pipeline corrosion in a sulfate reducing bacteria environment

The dominant factors in Microbial Influenced Corrosion (MIC) are hard to determine because normally several individual species and their metabolites are involved, and, moreover, different metabolites may cause opposing effects. To address this problem, the effects of individual metabolites from different species should be elucidated when at the same time other metabolites are held constant. In this study, the role is investigated of simulated organic acid metabolites, namely, acetic and L–ascorbic acids, on corrosion of geo-energy pipelines (carbon steel) in a simulated Sulfate Reducing Bacteria (SRB) environment. The SRB environment is simulated using a calcium alginate biofilm, abiotic sulfide, CO₂, and NaCl brine. The electrochemical results show that both simulated organic acid metabolites accelerate corrosion in a simulated SRB environment. The results are further supported by electrochemical weight losses, kinetic corrosion activation parameters, multiple linear regression, ICP-OES, pH, and XRD. However, a comparison of electrochemical results with those published in the literature for a simulated SRB environment without acetic or L-ascorbic acid under similar experimental conditions shows that the presence of acetic in this study results in lower corrosion current densities while in presence of L-ascorbic acid results into higher corrosion current densities. This implies that acetic and L-ascorbic acids inhibit and accelerate corrosion, respectively. In addition, the results highlight that H₂S is a key role of corrosion in the presence of organic acid. The results of this study are important new and novel information on the role of acetic and L-ascorbic acids in corrosion of geo-energy pipelines in the SRB environment.

Antimony reduction by a non-conventional sulfate reducer with simultaneous bioenergy production in microbial fuel cells

Environmental toxicity of antimony (Sb) is significantly increased through the widespread industrial application. The extended release of Sb above the regulatory level became a risk to humans habituated in the ecosystem. Conventional methods to remediate Sb demand high energy or resource input, which further leads to secondary pollution. The bio-electrochemical system offers a promising bioremediation strategy to remove or reduce toxic heavy metals. Thus, this research explores the possibilities of simultaneous metal sulfide (MeS) precipitation and electricity production using a full biological Microbial fuel cell (MFC). A non-conventional sulfate-reducing bacteria (SRB) Citrobacter freundii SR10 was used for this investigation, where the MFC was operated for lactate utilization in the bio-anode and Sb reduction at the bio-cathode. This study observed 81% of coulombic efficiency (bio-anode) and 97% of sulfate reduction with 99.3% Sb (V) reduction (bio-cathode), and it was concluded that the MeS precipitation entirely depends on sulfide concentration via SR10 sulfate reduction. The MFC-SR10 offers a maximum power density of 1652.9 ± 32.1 mW/m^3, and their performance was depicted using cyclic voltammetry and electrochemical impedance spectroscopy. The Sb reduction was evaluated through fluorescence spectroscopy, and the Sb (V) MeS precipitation was confirmed as stibnite (Sb₂S₃) by Raman spectroscopy and X-ray photoelectron spectroscopy. Furthermore, the matured anodic and cathodic biofilm formation was confirmed by Scanning electron microscopy with Energy-dispersive X-ray spectroscopy. Thus the MFC with SRB bio-cathode can be used as an alternative to simultaneously remove sulfate and Sb from the wastewater with electricity production.

Evaluation of trehalase as an enhancer for a green biocide in the mitigation of Desulfovibrio vulgaris biocorrosion of carbon steel

Trehalase can biocatalyze the conversion of trehalose to glucose. It is an enzyme that plays an important role in biofilm formation. Thus, trehalase has been patented as a chemical for preventing and treating biofilms. Sulfate-reducing bacteria (SRB) biofilms are often found responsible for biocorrosion, also known as microbiologically infuenced corrosion (MIC), especially in the oil and gas industries and in water utilities. The MIC treatment process typically requires biocide treatment of biofilms, sometimes together with scrubbing. Owing to environmental concerns, a lower biocide dosage is desired in the treatment process. In this work, trehalase was tested as a green biocide enhancer to enhance tetrakis hydroxymethyl phosphonium sulfate (THPS) in the prevention of Desulfovibrio vulgaris MIC of C1018 carbon steel in ATCC 1249 culture medium at 37 °C. THPS is one of the most popular industrial biocides owing to its broad-spectrum efficacy and green chemical status. After 7 days of incubation in 50 mL anaerobic vials containing 40 mL culture medium at pH 7.0, the sessile cell counts indicated that 50 ppm (w/w) THPS + 30 ppm (w/w) trehalase led to an extra 5.7-fold sessile cell reduction when compared with the 50 ppm THPS alone treatment. As a consequence, the combination treatment also resulted in an extra 54% in pit depth reduction and 30% in weight loss reduction when compared with the 50 ppm THPS alone treatment (with 9.0 μm and 1.0 mg/cm²). The biofilm images corroborated the decreased sessile cell count and pitting corrosion.

Investigation of mixed species biofilm on corrosion of X65 steel in seawater environment

Microbiologically influenced corrosion (MIC) is a complex process involving the cooperative effect of different bacterial species that coexist in the biofilm. Early studies focused on the MIC of single bacterial communities. However, in natural and industrial fields, biofilms are mostly composed of a variety of species. In this work, the effect of interspecific interaction on corrosion of X65 steel was investigated through the mixed culture of sulfate reducing bacteria (SRB) and iron oxidizing bacteria (IOB). Results demonstrated that the mixed microbial consortia created a cooperative effect to aggravate the local corrosion of X65 steel. Compared with the single species, the presence of IOB increased the growth activity of SRB cells and promoted the role of SRB in steel corrosion. The corrosion form on the surface of X65 steel gradually changed to annular pits induced by anaerobic SRB. The succession of dominant bacteria and the development of mixed species biofilm led to an increase in corrosion rate and local corrosion. The corrosion mechanism of X65 steel by mixed species biofilm at different stages was carefully elucidated.

The role of pH on sewer corrosion processes and control methods: A review

The production and emission of hydrogen sulfide (H₂S) in sewer systems is associated with the corrosion of sewer structures and harmful odour. Numerous studies have been conducted to find the best solution to overcome this issue. The pH plays a critical role not only on microbial and chemical processes that are responsible for all processes of corrosion but also on the efficiency of several control methods. This paper first critically reviews the literature on the interplay between pH and various chemical and microbial in-sewer processes, followed by a review of the control methods that depend on pH or indirectly alter pH. The paper argues that proper evaluation of each method should include the impact the control method has on downstream processes. This paper concludes the raising of pH has several benefits but is operationally difficult to implement. It also emphasises single control method may not be as efficient as combination of one or two methods in controlling the production and emission of H₂S. Finally, the research requirements and future directions in relation to emerging and potential methods that are not heavily reliant on pH control are discussed.

Aggressive corrosion of carbon steel by Desulfovibrio ferrophilus IS5 biofilm was further accelerated by riboflavin

EET (extracellular electron transfer) is behind MIC (microbiologically influenced corrosion) of carbon steel by SRB (sulfate reducing bacteria). This work evaluated 20 ppm (w/w) riboflavin (an electron mediator) acceleration of C1018 carbon steel MIC by Desulfovibrio ferrophilus IS5 in enriched artificial seawater (EASW) after 7-d incubation in anaerobic vials at 28 °C. Twenty ppm riboflavin did not significantly change cell growth or alter the corrosion product varieties, but it led to 52% increase in weight loss and 105% increase in pit depth, compared to the control without 20 ppm riboflavin. With 20 ppm riboflavin supplement in EASW, D. ferrophilus yielded weight loss-based corrosion rate of 1.57 mm/y (61.8 mpy), and pit depth growth rate of 2.88 mm/y (113 mpy), highest reported for short-term pure-strain SRB MIC of carbon steel. Electrochemical tests in 450 mL glass cells indicated that the biofilm responded rather quickly to the riboflavin injection (20 ppm in broth) to the culture medium. Polarization resistance (R_p) began to decrease within minutes after injection. Within 2 h, the riboflavin injection led to 31% decrease in R_p and 35% decrease in R_ct + R_f from electrochemical impedance spectroscopy (EIS). The Tafel corrosion current density increased 63% 2 h after the injection.

Generation of electrical energy in a microbial fuel cell coupling acetate oxidation to Fe3+ reduction and isolation of the involved bacteria

An iron reducing enrichment was obtained from sulfate reducing sludge and was evaluated on the capability of reducing Fe³⁺ coupled to acetate oxidation in a microbial fuel cell (MFC). Three molar ratios for acetate/Fe³⁺ were evaluated (2/16, 3.4/27 and 6.9/55 mM). The percentages of Fe³⁺ reduction were in a range of 80-90, 60-70 and 40-50% for the MFCs at closed circuit for the molar ratios of 2/16, 3.4/27 and 6.9/55 mM, respectively. Acetate consumption was in a range of 80-90% in all cases. The results obtained at closed circuit for current density were: 11.37 mA/m², 4.5 mA/m² and 7.37 mA/m² for the molar ratios of 2/16, 3.4/27 and 6.9/55 mM, respectively. Some microorganisms that were isolated and identified in the MFCs were Azospira oryzae, Cupriavidus metallidurans CH34, Enterobacter bugandensis 247BMC, Citrobacter freundii ATCC8090 and Citrobacter murliniae CDC2970-59, these bacteria have been reported as exoelectrogens in MFC and in MFC involving metals removal but not all of them have been reported to utilize acetate as preferred substrate. The results demonstrate that the isolates can utilize acetate as the sole source of carbon and suggest that Fe³⁺ reduction was carried out by a combination of different mechanisms (direct contact and redox mediators) utilized by the bacteria identified in the MFC. Storage of the energy generated from the 2/16 mM MFC system arranged in a series of three demonstrated that it is possible to utilize the energy to charge a battery.

Influence of environmental factors on benthic nitrogen fixation and role of sulfur reducing diazotrophs in a eutrophic tropical estuary

Benthic nitrogen fixation in the tropical estuaries plays a major role in marine nitrogen cycle, its contribution to nitrogen budget and players behind process is not well understood. The present study was estimated the benthic nitrogen fixation rate in a tropical estuary (Cochin) and also evaluated the contribution of various diazotrophic bacterial communities. Nitrogen fixation was detected throughout year (0.1-1.11 nmol N g^-1 h^-1); higher activity was observed in post-monsoon. The nifH gene abundance was varied from 0.8 × 10⁴ to 0.6 × 10⁸ copies g^-1dry sediment; highest was detected in post-monsoon. The Cluster I and Cluster III were the dominant diazotrophs. Sulfur reducing bacterial phylotypes (Deltaproteobacteria) contributed up to 2-72% of total nitrogen fixation. These bacteria may provide new nitrogen to these systems, counteracting nitrogen loss via denitrification and anammox. Overall, the study explained the importance of benthic nitrogen fixation and role of diazotrophs in a monsoon influenced tropical estuarine environments.

Integrated remediation of sulfate reducing bacteria and nano zero valent iron on cadmium contaminated sediments

Integrated-remediation technologies on heavy metal polluted sediments have received much attention. In this study, Cd contaminated sediments were treated with various conditions: sulfate reducing bacteria (SRB) only and SRB combined with different dosages of nano zero valent iron (nZVI (0.5-10 mg/g)). The immobilization of Cd was found in all remediation treatments according to the decreases of mobile Cd and the increases of more stable Cd compared with control. Five typical SRBs (Desulfobulbaceae, Desulfobacteraceae, Syntrophobacteraceae, Desulfovibrionaceae and Desulfomicrobiaceae) were identified having significant influences on Cd speciation transformation and they could stabilize Cd into sulfide precipitation through dissimilatory sulfate reduction (DSR). The ANOVA results of mobilization index and Cd concentration in overlying water both demonstrated that integrated-remediation systems with 5 mg/g and 10 mg/g of nZVI (Fe5 and Fe10 systems, respectively) presented better immobilization performance than conventional SRB only system (P < 0.05). It is confirmed that nZVI could stimulate the SRB bio-immobilization possibily through providing electrons and enhancing enzyme activities during DSR. The XPS analyses and Pourbaix diagrams revealed that mackinawite may be produced in the Fe10, resulting in the possible formation of Cd-S-Fe. This study indicates that integrated-remediation of SRB and nZVI have great potential in Cd immobilization of sediments, especially with higher addition of nZVI.

Recent advances in heavy metal recovery from wastewater by biogenic sulfide precipitation

Biological sulfide precipitation by sulfate reducing bacteria (SRB) is an emerging technique for the recovery of heavy metals from metal contaminated wastewater. Advantages of this technique include low capital cost, ability to form highly insoluble salts, and capability to remove and recover heavy metals even at very low concentrations. Therefore, sulfate reduction under anaerobic conditions has become a suitable alternative for the treatment of wastewaters that contain metals. However, bioreactor configurations for recovery of metals from sulfate rich metallic wastewater have not been explored widely. Moreover, the recovered metal sulfide nanoparticles could be applied in various fields such as solar cells, dye degradation, electroplating, etc. Hence, metal recovery in the form of nanoparticles from wastewater could serve as an incentive for industries. The simultaneous metal removal and recovery can be achieved in either a single-stage or multistage systems. This paper aims to present an overview of the different bioreactor configurations for the treatment of wastewater containing sulfate and metal along with their advantages and drawbacks for metal recovery. Currently followed biological strategies to mitigate sulfate and metal rich wastewater are evaluated in detail in this review.

Green mitigation of microbial corrosion by copper nanoparticles doped carbon quantum dots nanohybrid

Recently, nanomaterials have been introduced as a new generation of inhibitors to control the microbiologically influenced corrosion (MIC). In this study, copper nanoparticles doped carbon quantum dots (Cu/CQDs) nanohybrid was used as an inhibitor to reduce the MIC. FESEM, EDS, FTIR, and XRD were used to characterize the nanohybrid. The dose-response test was performed to evaluate the inhibitory effect of Cu/CQDs against SRB. Design-Expert software was used to design the matrix of experiment and analyze the result. Cu/CQDs showed significant inhibitory effect against SRB compared to the copper nanoparticles (CuNPs) and carbon quantum dots (CQDs), at 50 ppm. Moreover, corrosion behavior of X60 steel was evaluated via electrochemical impedance spectroscopy (EIS) and Tafel polarization techniques in the presence of SRB and Cu/CQDs. The fitted result of EIS showed that the charge transfer resistance (R_ct) value increased in the presence of Cu/CQDs owing to the enhancement in the thickness of the electrical double layer, indicating that Cu/CQDs is able to provide significant corrosion protection to X60 steel in the presence of SRB. In addition, FESEM, EDS, and XRD were used to study the formed corrosion products and biofilm on the surface of X60 steel. Corrosion test results indicated that the addition of the Cu/CQDs reduced the surface damage of X60 steel in the presence of SRB. It is attributed to the carbon dots adsorption film formation, which possessed a significant protective ability to inhibit the corrosion of steel in the presence of SRB.

Reaction mechanism catalyzed by the dissimilatory adenosine 5'-phosphosulfate reductase. Adenosine 5'-monophosphate inhibitor and key role of arginine 317 in switching the course of catalysis

The present research is a continuation of our work on dissimilatory reduction pathway of sulfate - involved in biogeochemical sulfur turnover. Adenosine 5'-phosphosulfate reductase (APSR) is the second enzyme in the dissimilatory pathway of the sulfate to sulfide reduction. It reversibly catalyzes formation of the sulfite anion (HSO₃^-) from adenosine 5'-phosphosulfate (APS) - the activated form of sulfate provided by ATP sulfurylase (ATPS). Two electrons required for this redox reaction derive from reduced FAD cofactor, which is suggested to be involved directly in the catalysis by formation of FADH-SO₃^- intermediate. The present work covers quantum-mechanical (QM) studies on APSR reaction performed for eight models of APSR active site. The cluster models were constructed based on two crystal structures (PDB codes: 2FJA and 2FJB), differing in conformation of Arg317 active site residue. The described results indicated the most feasible mechanism of APSR forward reaction, including formation of FADH_N-SO₃^- adduct (with proton on N5 atom of isoalloxazine), tautomerization of FADH_N-SO₃^- to FADH_O-SO₃^- (with proton on CO moiety of isoalloxazine), and its reductive cleavage to oxidized FAD and sulfite anion. The reverse reaction proceeds in the backward direction. It is suggested that it requires two AMP molecules, one acting as a substrate and another as an inhibitor of forward reaction, which forces change of Arg317 conformation from "arginine in" (2FJA) to "arginine out" (2FJB). Important role of Arg317 in switching the course of the APSR catalytic reaction is revealed by changing the direction of thermodynamic driving force. The presented research also shows the importance of the protonation pattern of the reduced FAD cofactor and protein residues within the active site.

Reduction mechanism of Cd accumulation in rice grain by Chinese milk vetch residue: Insight into microbial community

Chinese milk vetch is an efficient approach to reduce Cd accumulation in rice, nevertheless, its reduction mechanism is not well understood. In this study, we investigated the rice grain Cd, soil properties and microbial community in a Cd-polluted paddy field amended with milk vetch residue (MV) or without (CK) during rice growth period. We found that milk vetch residue averagely decreased the Cd content in rice grain by 45%. Decrease of Cd in rice mainly attributed to the inhibition of Cd activation by milk vetch residue at heading stage probably by the formation of HA-Cd (Humic Acid) and CdS. Increased pH and organic matter (OM) promoted the reduction of available Cd. In addition, nonmetric multidimensional scaling (NMDS) analysis revealed that microbial community structure was significantly different between MV and CK treatment (r = 0.187, p = 0.002), and the core functions of differentially abundant genera were mainly associated with N-cycling, organic matter degradation and sulfate-reducing. The application of milk vetch residue increased the abundance of sulfate-reducing bacteria (SRB) by 8-112% during the rice growth period, which may involve in promoting the transformation of Cd to a more stably residual Cd (CdS). Canonical correspondence analysis (CCA) and mantel test analysis indicated that available K (p = 0.004) and available N (p = 0.005) were the key environmental factors of shaping the SRB. Altogether, changes in soil properties affected microbial structure and functional characteristics, especially the response of SRB in MV treatment would provide valuable insights into reducing the bioavailability of Cd in soil.

Effects of adsorbed phosphate on jarosite reduction by a sulfate reducing bacterium and associated mineralogical transformation

Jarosite is one of the iron oxyhydroxysulfate minerals that are commonly found in acid mine drainage (AMD) systems. In natural environments, phosphate and sulfate reducing bacteria (SRB) may be coupled to jarosite reduction and transformation. In this research, the effect of phosphate on jarosite reduction by SRB and the associated secondary mineral formation was studied using batch experiments. The results indicated that Fe³⁺ is mainly reduced by biogenic S^2- in this experiment. The effect of PO₄^3- on jarosite reduction by SRB involved not only a physico-chemical factor but also a microbial factor. Phosphate is an essential nutrient, which can support the activity of SRB. In the low PO₄^3- treatment, the production of total Fe²⁺ was found to be slightly larger than that in the zero PO₄^3- treatment. Sorption of PO₄^3- effectively elevated jarosite stability via the formation of inner sphere complexes, which, therefore, inhibited the reductive dissolution of jarosite. At the end of the experiment, the amounts of total Fe²⁺ accumulation were determined to be 4.54 ± 0.17^a mM, 4.66 ± 0.22^a mM, 3.91 ± 0.04^b mM and 2.51 ± 0.10^c mM (p < 0.05) in the zero, low, medium and high PO₄^3- treatments, respectively, following the order of low PO₄^3- treatment > zero PO₄^3- treatment > medium PO₄^3- treatment > high PO₄^3- treatment. PO₄^3- loading modified the transformation pathways for the jarosite mineral, as well. In the zero PO₄^3- treatment, the jarosite diffraction lines disappeared, and mackinawite dominated at the end of the experiment. Compared to PO₄^3--free conditions, vivianite was found to become increasingly important at higher PO₄^3- loading conditions. These findings indicate that PO₄^3- loading can influence the broader biogeochemical functioning of AMD systems by impacting the reactivity and mineralization of jarosite mineral.

Uncommon isolation of Desulfovibrio vulgaris from a depressed fracture wound on the forehead

Desulfovibrio spp. are gram negative, obligate anaerobes capable of reducing sulfate. They have caused infections in humans, but very rarely. They are slow growers and difficult to identify. Hence, they are often overlooked and their actual presence goes unnoticed. Here, we describe a case of a 15- year old boy who was involved in a road traffic accident and he presented with seropurulent discharge from a depressed fracture wound on the forehead. Desulfovibrio vulgaris (D.vulgaris), was isolated from the pus discharge, the first to be reported. The characteristic desulfoviridin pigment production in the organism aided in the identification. The infection was successfully managed with pain reliever and course of amoxicillin - clavulanic acid and linezolid.

Assessment of the tolerance to Fe, Cu and Zn of a sulfidogenic sludge generated from hydrothermal vents sediments as a basis for its application on metals precipitation

A paramour factor limiting metal-microorganism interaction is the metal ion concentration, and the metal precipitation efficiency driven by microorganisms is sensitive to metal ion concentration. The aim of the work was to determine the tolerance of the sulfidogenic sludge generated from hydrothermal vent sediments at microcosms level to different concentrations of Fe, Cu and Zn and the effect on the microbial community. In this study the chemical oxygen demand (COD) removal, sulfate-reducing activity (SRA) determination, inhibition effect through the determination of IC₅₀, and the characterization of the bacterial community´s diversity were conducted. The IC₅₀ on SRA was 34 and 81 mg/L for Zn and Cu, respectively. The highest sulfide concentration (H₂S mg/L) and % of sulfate reduction obtained were: 511.30 ± 0.75 and 35.34 ± 0.51 for 50 mg/L of Fe, 482.48 ± 6.40 and 33.35 ± 0.44 for 10 mg/L of Cu, 442.26 ± 17.1 and 30.57 ± 1.18 for 10 mg/L of Zn, respectively. The COD removal rates were of 71.81 ± 7.6, 53.92 ± 1.07 and 57.68 ± 10.2 mg COD/ L d for Fe (50 mg/L), Cu (40 mg/L) and Zn (20 mg/L), respectively. Proteobacteria, Firmicutes, Chloroflexi and Actinobacteria were common phyla to four microcosms (stabilized sulfidogenic and added with Fe, Cu or Zn). The dsrA genes of Desulfotomaculum acetoxidans, Desulfotomaculum gibsoniae and Desulfovibrio desulfuricans were expressed in the microcosms supporting the SRA results. The consortia could be explored for ex-situ bioremediation purposes in the presence of the metals tested in this work.

Enhanced production of volatile fatty acids by adding a kind of sulfate reducing bacteria under alkaline pH

Anaerobic digestion could make sludge stable and harmless, and the volatile fatty acids (VFAs) produced from it. The objective of this study was to reduced sludge production and realize the resource utilization of VFAs through enhance anaerobic sludge fermentation by adding sulfate reducing bacteria (SRB) under alkaline pH. Under the neutral and alkaline pH, SRB was added into the sludge fermentation liquid with sole stock solution and sterilization treatment respectively, while the liquid without any additives was used as control. The results indicated that obvious increase of the production of VFAs was observed after adding SRB under alkaline pH. And, more protein and polysaccharide were obtained which were the main substrates for the production of VFAs. The concentration of ammonia nitrogen (NH₄⁺-N) and phosphate (PO₄^3--P) were also increased with the addition of SRB. So, a high yield production of VFAs could be achieved through the addition of SRB + alkaline pH.

Shallow floating treatment wetland capable of sulfate reduction in acid mine drainage impacted waters in a northern climate

Floating treatment wetlands (FTW)s that can uptake nutrients and metals from water, and/or trap suspended solids in their roots, are becoming viable options to treat urban, agriculture and sewage runoffs. However, current FTW designs favor aerobic processes and short-term storage of metals, which are ineffective in acid mine drainage (AMD) environments. Many also function poorly in northern latitudes with strong seasonality and several months of sub-zero temperatures. In this study, we designed a novel FTWs with 20 cm soil profile to test its ability to sustain anaerobic microbial processes, such as iron and sulfate reduction and remain functional after freezing conditions of winter months. Three different plants, Carex lacustris, Typha latifolia, and Juncus canadensis were used to test in our FTWs, which were deployed in a mining-impacted water in Sudbury, ON, Canada. Porewater samples were acquired using built-in porewater peepers. Low to moderately reducing conditions, along with presence of ferrous iron and hydrogen sulfide in the porewater of all FTWs was prevalent, irrespective of the constituent vegetation type. Moreover, as well as a ~30% increase in sulfate-reducing bacteria (SRB) richness and ~100% increase in SRB abundance between years, was the evidence that anaerobic processes were occurring in these shallow FTWs. From this study we estimated that during its lifetime, one shallow FTW can treat ~61 m³ of sulfate-rich water, thus offering an alternative way to capture sulfate and other metals from mining-impacted waters.

Effect of sulfate addition on carbon flow and microbial community composition during thermophilic digestion of cellulose

Substrates with high sulfate levels pose problems for biogas production as they allow sulfate reducing bacteria to compete with syntrophic and methanogenic members of the community. In addition, the end product of sulfate reduction, hydrogen sulfide, is toxic and corrosive. Here we show how sulfate addition affects physiological processes in a thermophilic methanogenic system by analyzing the carbon flow and the microbial community with quantitative PCR and amplicon sequencing of the 16s rRNA gene. A sulfate addition of 0.5 to 3 g/L caused a decline in methane production by 73-92%, while higher sulfate concentrations had no additional inhibitory effect. Generally, sulfate addition induced a shift in the composition of the microbial community towards a higher dominance of Firmicutes and decreasing abundances of Bacteroidetes and Euryarchaeota. The abundance of methanogens (e.g., Methanoculleus and Methanosarcina) was reduced, while sulfate reducing bacteria (especially Candidatus Desulforudis and Desulfotomaculum) increased significantly in presence of sulfate. The sulfate addition had a significant impact on the carbon flow within the system, shifting the end product from methane and carbon dioxide to acetate and carbon dioxide. Interestingly, methane production quickly resumed, when sulfate was no longer present in the system. Despite the strong impact of sulfate addition on the carbon flow and the microbial community structure during thermophilic biogas production, short-term process disturbances caused by unexpected introduction of sulfate may be overcome due to the high resilience of the engaged microorganisms.

The influence of upflow velocity and hydraulic retention time changes on taxonomic and functional characterization in Fluidized Bed Reactor treating commercial laundry wastewater in co-digestion with domestic sewage

A large-scale (19.8L) Fluidized Bed Reactor (FBR) operated for 592 days was used to assess the removal performance of linear alkylbenzene sulfonate (LAS). Adjustments in hydraulic retention time (HRT) (18 and 30 h), ethanol (50, 100, 200 mg L^-1) and linear alkylbenzene sulfonate (LAS) concentration (6.3-24.7 mg L^-1) with taxonomic and functional characterization of biomass using Whole Genome Shotgun Metagenomic (WGSM) represented a major step forward for optimizing biological treatments of LAS. In addition, the variation of the upflow velocity (0.5, 0.7 and 0.9 cm s^-1) was investigated, which is a parameter that had not yet been correlated with the possibilities of LAS removal in FBR. Lower Vup (0.5 cm s^-1) allied to higher ethanol concentration (200 mg L^-1) resulted in lower LAS removal (29%) with predominance of methanogenic archaea and genes related to methanogenesis, while higher Vup (0.9 cm s^-1) led to aerobic organisms and oxidative phosphorylation genes. An intermediate Vup (0.7 cm s^-1) and higher HRT (30 h) favored sulfate reducing bacteria and genes related to sulfur metabolism, which resulted in the highest LAS (83%) and COD (77%) removal efficiency.

Biosafety of cadmium contaminated sediments after treated by indigenous sulfate reducing bacteria: Based on biotic experiments and DGT technique

Sulfate reducing bacteria (SRB) biostabilization has attracted particular attention due to its ability to prevent and control heavy metal pollution. In this study, biotic experiments (immobilisation test of Daphnia (D.) magna, germination experiment of cucumber seeds, and in vitro experiment using gut juices of Sipunculus (S.) nudus) and diffusive gradients in thin films (DGT) technique were performed to investigate the biosafety of cadmium (Cd) contaminated sediments after being treated by indigenous SRB. Results showed that SRB treatment reduced Cd bioaccessibility of sediment to S. nudus, Cd levels in the overlying water and Cd bioavailability to D. magna. However, the treatment increased the biotoxicity of overlying water due to significant reduction in the root length and germination index of cucumber seeds. DGT results confirmed that SRB treatment increased Cd stability in sediment, and reduced its release from the sediment into the overlying water. The biotoxicity of overlying water was not caused by Cd, but possibly by the added culture medium, SRB itself, or its metabolites. More attention is required to assess the safety of SRB treatment when it is used to remediate environmental matrix contaminated by heavy metals.

Microbial interactions regulated by the dosage of ferroferric oxide in the co-metabolism of organic carbon and sulfate

Effects of ferroferric oxide (Fe₃O₄) and organic carbon on co-metabolism of sulfate and organic carbon were investigated. With Fe₃O₄, the degradation of acetate and sulfate was inhibited when fed with acetate, while the degradation of acetate and propionate produced from ethanol was promoted when fed with ethanol. The dominant sulfate reducing bacteria (SRB) of acetate-fed reactors were Desulfobacteraceae (complete oxidizing SRB, CO-SRB) and Desulfurmonas (incomplete oxidizing SRB, IO-SRB). IO-SRBs of Desulfobulbus and Desulfomicrobium were dominant in ethanol-fed reactors. CO-SRB had higher competitiveness than methanogens to utilize acetate, while IO-SRBs might cooperate with methanogens to produce methane when dosed with ethanol and Fe₃O₄. The dosage of Fe₃O₄ changed the dominant methanogen from Methanosarcina to Methanosaeta with acetate as the organic carbon, while increased the relative abundance of Methanosaeta with ethanol as the organic carbon.

Towards sulfide removal and sulfate reducing bacteria inhibition: Function of biosurfactants produced by indigenous isolated nitrate reducing bacteria

The effectiveness of nitrate-mediated souring control highly depends on the interactions of sulfate reducing bacteria (SRB) and nitrate reducing bacteria (NRB). Biosurfactants produced by natural NRB are promising bio-agents for enhancing NRB competence towards SRB. However, the function of NRB-produced biosurfactants in NRB-SRB interactions remains unexplored due to the rarely successful isolation of natural biosurfactant-producing NRB. Hereby, biosurfactant-aided inhibitory control of SRB strain Desulfomicrobium escambiense ATCC 51164 by biosurfactant-producing NRB strain Pseudomonas stutzeri CX3, reported in our previous work, was investigated. Under non-sour conditions, insufficient nitrate injection resulted in limited SRB inhibition. Phospholipid fatty acid (PLFA) biomarkers traced the overall bacterial responses. Compositional PLFA patterns revealed biosurfactant addition benefitted both SRB and NRB towards stressful conditions. Under sour conditions, nitrite oxidation of sulfide proved to be the primary mechanism for sulfide removal. The subsequent elevation of redox potential and pH inhibited SRB activities. NRB-produced biosurfactants significantly enhanced SRB inhibition by NRB through more efficient sulfide removal and effective duration of nitrate in the microcosms. Biosurfactants specially produced by the NRB strain are for the first time reported to significantly strengthen SRB inhibition by NRB via reduced nitrate usage and prolonged effective duration of nitrate, which has encouraging potential in nitrate-dependent souring control.

Anaerobic methane oxidation coupled to sulfate reduction in a biotrickling filter: Reactor performance and microbial community analysis

The aim of this work was to evaluate the performance of a biotrickling filter (BTF) packed with polyurethane foam and pall rings for the enrichment of microorganisms mediating anaerobic oxidation of methane (AOM) coupled to sulfate reduction (SR) by activity tests and microbial community analysis. A BTF was inoculated with microorganisms from a known AOM active deep sea sediment collected at a depth of 528 m below the sea level (Alpha Mound, Gulf of Cadiz). The microbial community analysis was performed by catalyzed reporter deposition - fluorescence in situ hybridization (CARD-FISH) and 16S rRNA sequence analysis. The AOM occurrence and rates in the BTF were assessed by performing batch activity assays using ¹³C-labelled methane (¹³CH₄). After an estimated start-up time of ∼20 days, AOM rates of ∼0.3 mmol l^-1 day^-1 were observed in the BTF, values almost 20 times higher than previously reported in a polyurethane foam packed BTF. The microbial community consisted mainly of anaerobic methanotrophs (ANME-2, 22% of the total number of cells) and sulfate reducing bacteria (SRB, 47% of the total number of cells). This study showed that the BTF is a suitable reactor configuration for the enrichment of microbial communities involved in AOM coupled to SR at ambient pressure and temperature with a relatively short start-up time.

Inhibition of anaerobic biological sulfate reduction process by copper precipitates

The single-stage biological sulfate reduction process for treatment of heavy metal laden wastewater is a promising treatment method, but the formation of metal precipitates has been suggested to be inhibitory to the activity of sulfate reducers. The present study examined the impact of copper (Cu) precipitates on anaerobic biological sulfate reduction in semi continuous stirred tank reactors (SCSTRs) at 35 ± 2 °C. The results show that Cu precipitates significantly affected the sulfate reduction process. At an HRT of 50 days, steady-state sulfate reduction was approximately 55% at influent Cu concentrations of 0 (control) and 200 mg/L, which reduced to approximately 39% at influent Cu concentration of 400 mg/L. Microbial population (measured as volatile suspended solids) and rate of sulfate reduction were also affected, with reductions observed even at influent Cu of 200 mg/L. Copper precipitates also affected the microbial community diversity distribution.

The microbial zonation of SRB and soNRB enhanced the performance of SR-DSR process under the micro-aerobic condition

The micro-aerobic condition has proven to effectively enhance the COD removal and elemental sulfur (S⁰) transformation rate in the sulfate reduction-denitrifying sulfide removal (SR-DSR) process. However, the mechanisms of how micro-aerobic condition enhances S⁰ transformation remain largely unknown. Therefore in this work an integrated investigation was performed to document the mechanisms and the effect of different startup modes (micro-aerobic startup (termed as mSR-DSR) and anaerobic startup (termed as aSR-DSR)) on bioreactor performance and microbial community dynamics. The results showed that micro-aerobic startup achieved a shorter period to reach a stable performance for SR-DSR, which could be one of the factors affecting the choice of the bioreactor startup mode considering engineering application. For all the tested conditions, removal of nitrate, sulfate and lactate were 100%, >80% and 100%, respectively. The maximum transformation rate of elemental sulfur in mSR-DSR was 57%, which was higher than that in aSR-DSR. The mechanism explorations revealed that micro-aerobic condition not only particularly enriched the sulfide-oxidizing, nitrate-reducing bacteria (soNRB) but also promoted the microbial zonation of sulfate-reducing bacteria (SRB) and soNRB, thereby achieving more S⁰ transformation in the effluent. Under micro-aerobic condition, SRB were mainly distributed in the bottom and middle part of the reactor, while soNRB were assembled in the top. The relative abundance of soNRB in both aSR-DSR and mSR-DSR notably increased to 41.5% and 23.7% at the top when 5 mL air min^-1 L_reactor^-1 was applied. Furthermore, the degradation of organic carbon was also accelerated under micro-aerobic condition, possibly due to the enrichment of organic compounds degrading bacteria Bacteroidetes_vadin HA17.

Genome insights of mercury methylation among Desulfovibrio and Pseudodesulfovibrio strains

Mercury methylation converts inorganic mercury into the toxic methylmercury, and the consequences of this transformation are worrisome for human health and the environment. This process is performed by anaerobic microorganisms, such as several strains related to Pseudodesulfovibrio and Desulfovibrio genera. In order to provide new insights into the molecular mechanisms of mercury methylation, we performed a comparative genomic analysis on mercury methylators and non-methylators from (Pseudo)Desulfovibrio strains. Our results showed that (Pseudo)Desulfovibrio species are phylogenetically and metabolically distant and consequently, these genera should be divided into various genera. Strains able to perform methylation are affiliated with one branch of the phylogenetic tree, but, except for hgcA and hgcB genes, no other specific genetic markers were found among methylating strains. hgcA and hgcB genes can be found adjacent or separated, but proximity between those genes does not promote higher mercury methylation. In addition, close examination of the non-methylator Pseudodesulfovibrio piezophilus C1TLV30 strain, showed a syntenic structure that suggests a recombination event and may have led to hgcB depletion. The genomic analyses identify also arsR gene coding for a putative regulator upstream hgcA. Both genes are cotranscribed suggesting a role of ArsR in hgcA expression and probably a role in mercury methylation.

Persistence times of refractory materials in landfills: A review of rate limiting conditions by mass transfer and reaction kinetics

Monitoring programs at closed landfills show that transformation of plastics, wood, and metals continue long after the active decomposition of the waste fractions are considered as complete. Studies conducted in natural anaerobic environments (e.g., marine sediments and rocks) provide insight for slow degradation mechanisms involving coupling of thermodynamically favorable and unfavorable reactions and biochemical transformations by microbial consortia. These transformations occur at much slower rates through more complex and less obvious mechanisms and are not evident until after the primary decomposition mechanisms become less significant. This study presents a review of the conditions that limit the mass transfer and reaction kinetics for anaerobic transformations in landfills and provides new insights for reaction mechanisms (e.g., anaerobic oxidation and anaerobic corrosion) that occur at relatively slow rates in mature landfills. Conditions and mechanisms of slow transformations by microbial and chemical activities with relatively small energy yields and availability of electron acceptors (e.g., inorganics, plastics) and/or diffusion of gas and moisture into the previously isolated areas in waste deposits were discussed. Time scales for mass transfer and reaction kinetics were compared under anaerobic conditions for different waste components deposited at municipal solid waste landfills. Half-lives of different materials under anaerobic conditions were estimated and compared. Emergence of syntrophic methanogenic communities and conditions for triboelectric effects were evaluated as possible electron transfer mechanisms between waste layers for occurrence of extremely slow transformations of wastes deposited in landfills.

Inhibition mitigation and ecological mechanism of mesophilic methanogenesis triggered by supplement of ferroferric oxide in sulfate-containing systems

Methanogenesis can be inhibited by volatile fatty acids (VFAs) accumulation and sulfate during anaerobic wastewater treatment. In this study, effects of ferroferric oxide (Fe₃O₄) on VFAs degradation and methanogenesis in sulfate-containing environment were investigated. Methanogenesis in reactors with or without sulfate were both favored through the addition of Fe₃O₄. In reactors without sulfate, the dosage of Fe₃O₄ increased the maximum methane production rate by 21.7% accompanied with faster acetate and propionate degradation. Metagenomic analysis showed that Fe₃O₄ mainly promoted electron exchange between Mesotoga, Syntrophobacter, Smithella and Methanosaeta without altering the syntrophic patterns. However, in the sulfate-containing reactor with low methanogenic efficiency, syntrophic ethanol users and Methanosaeta were replaced by sulfate-reducing bacteria and Methanosarcina, respectively. The supplement of Fe₃O₄ re-enriched the syntrophic partners inhibited by sulfate and rebuilt a new syntrophic interaction with high efficiency similar to that in sulfate-free environment, leading to better methanogenic performance in sulfate-containing environment.

Competitive dynamics of anaerobes during long-term biological sulfate reduction process in a UASB reactor

To reveal the long-term competitive dynamics of anaerobes in anaerobic bioreactors with sulfate reduction, a comprehensive structured mathematical model was designed for an extension of the Anaerobic Digestion Model No. 1 (ADM1). Sulfate reduction bacteria (SRB) were categorized into acetogenic-likewise SRB (ASRB) and methanogenic-likewise SRB (MSRB). Experimental data from 329 days of continuous operation of a laboratory-scale upflow anaerobic sludge bed (UASB) reactor was used for model calibration and validation. Results show that the model has a good agreement with experimental data and that three stages including the MPA dominant, stalemate and SRB dominant stages were clearly appeared throughout the whole competition period. The model was capable of predicting the long-term dynamic competition of sulfidogens and methanogens for electrons. This could explain a long-term of over 200 days needed for the SRB out-competing the MPA, and support speculation that the SRB could finally out-compete both the AcB and the MPA.

Integration of nanoscale zero-valent iron and functional anaerobic bacteria for groundwater remediation: A review

The technology of integrating nanoscale zero-valent iron (nZVI) and functional anaerobic bacteria has broad prospects for groundwater remediation. This review focuses on the interactions between nZVI and three kinds of functional anaerobic bacteria: organohalide-respiring bacteria (OHRB), sulfate reducing bacteria (SRB) and iron reducing bacteria (IRB), which are commonly used in the anaerobic bioremediation. The coupling effects of nZVI and the functional bacteria on the contaminant removal in the integrated system are summarized. Generally, nZVI could create a suitable living condition for the growth and activity of anaerobic bacteria. OHRB and SRB could synergistically degrade organic halides and remove heavy metals with nZVI, and IRB could reactive the passivated nZVI by reducing the iron (hydr)oxides on the surface of nZVI. Moreover, the roles of these anaerobic bacteria in contaminant removal coupling with nZVI and the degradation mechanisms are illustrated. In addition, this review also discusses the main factors influencing the removal efficiency of contaminants in the integrated treatment system, including nZVI species and dosage, inorganic ions, organic matters, pH, type of pollutants, temperature, and carbon/energy sources, etc. Among these factors, the nZVI species and dosage play a fundamental role due to the potential cytotoxicity of nZVI, which might exert a negative impact on the performance of this integrated system. Lastly, the future research needs are proposed to better understand this integrated technology and effectively apply it in groundwater remediation.

A two-compartment upflow pilot scale bioreactor system for microbial sulfate reduction control studies

Souring in oil fields occurs mainly due to the activity of sulfate reducing bacteria (SRB). Most of the studies on SRB are performed using upflow packed-bed reactors that have a limitation to describe the region close to the injection wells in oil fields, which is characterized by void and saturated porous bed regions. Here, it is described the design and operation of a pilot scale system to investigate the SRB activity, inhibition and control in oil fields. •The bioreactor is composed by two-compartments (empty and packed-bed).•The reactor system has two parallel bioreactors that can be supplied with the same source of nutrients through a single pump or can be supplied separately with different solutions using two pumps.•The hydrodynamics for conventional packed bed bioreactors has a mixing behavior dependent of the flow rate and has a significant by-pass. In contrast, the two-compartment system presented here has a mixing behavior almost independent of the flow rate.

Comparison of the efficiency of chitinous and ligneous substrates in metal and sulfate removal from mining-influenced water

Mining-influenced water (MIW) remediation is challenging, not only due to its acidity and high metal content, but also due to its presence in remotely located mine sites with difficult surrounding environments. An alternative to common remediation technologies, is the use of sulfate-reducing bacteria (SRB) to achieve simultaneous sulfate reduction and metal removal in on-site anaerobic passive systems. In these systems, the organic carbon source (substrate) selection is critical to obtaining the desired effluent water quality and a reasonable treated volume. In this study, we evaluated the use of two different substrates: a chitinous product obtained from crushed crab shells, and a more traditional ligneous substrate. We put the substrates, both with and without water pretreatment consisting of aeration and pH adjustment, in anaerobic experimental columns. The treatment with the chitinous substrate was more effective in removing metals (Al, Cu, Fe, Cd, Mn, Zn) and sulfate for a longer period (458 days) than the ligneous substrate (78 days) before suffering Zn breakthrough. The reactors fed with pretreated water had longer operational periods and lower metals and sulfate concentrations in the effluent than those with untreated influent water. Zn was consistently removed to levels <0.3 mg/L for 513 days in the chitinous substrate columns, while levels <0.3 mg/L were maintained for only 140 days in the ligneous substrate pretreated column. The highest sulfate removal rates achieved in this study were in the range of 5-6 mol/m³/d for the chitinous substrate and 1-2 mol/m³/d for the ligneous substrate. Overall, the chitinous substrate proved to be more efficient in the removal of all the aforementioned metals and for sulfate when compared to the ligneous substrate. This could be the determinant when selecting a substrate for passive systems treating acidic MIW, particularly when Zn and Mn removal is necessary.

Performance of two differently designed permeable reactive barriers with sulfate and zinc solutions

For the first time, this laboratory-scale study evaluates the feasibility of incorporating diffusive exchange in permeable reactive barriers. In order to do this, the performance of two permeable reactive barriers (PRB) with different internal substrate arrangements were compared during the administration of a sulfate solution without metals (for 163 days) and with metals (for 60 days), simulating groundwater contaminated with acid mine drainage (AMD). In order to simulate a traditional PRB, a homogeneous distribution was implemented in the first reactor and the other PRB reactor utilized diffusion-active technology (DAPRB). In the DAPRB, the distribution of the reactive material was interspersed with the conductive material. The measurements in the internal ports showed that transverse gradients of sulfide formed in the DAPRB, causing the diffusion of sulfide from the substrate toward the layer interface, which is where the sulfide reacts by forming complexes with the metal. The DAPRB prevents the microorganisms from direct contact with AMD. This protection caused greater activity (sulfide production).

Schwertmannite transformation via direct or indirect electron transfer by a sulfate reducing enrichment culture

Understanding the mechanism of the microbial transformation of Fe(III)-oxyhydroxysulfate minerals is of considerable interest, because this transformation plays an important role in controlling the behaviour of toxic metals from acid mine drainage (AMD). In this study, we examined a sulfate reducing enrichment culture from AMD-contaminated sediments and predicted the possible pathway of electron transfer when incubated with schwertmannite, a common Fe(III)-oxyhydroxysulfate occurring in the AMD environment. Experiments were designed to distinguish the mechanisms by which bacteria facilitate direct (i.e., bacteria allowed to adhere to the mineral) or indirect (i.e., bacteria separated from the mineral by dialysis bag) electron transfer to reduce the mineral. The effects of adding anthraquinone-2,6-disulfonate (AQDS) as an exogenous electron shuttle were also investigated. Vivianite was detected as the main product of schwertmannite transformation. Reduction of sulfate and iron were more pronounced in direct treatments, while more non-reductive dissolution were observed in indirect treatments. The addition of AQDS lead to the production of more dissolved Fe²⁺ over 20 d than in the absence of AQDS. Microbial community composition differed in direct and indirect treatments, while the addition of AQDS did not significantly affect the community structure in each treatment. After incubation for 20 d, the growth of Desulfovibrio exceeded that of the originally dominant Citrobacter in direct treatments, while an unknown genus most closely related to Citrobacter within Enterobacteriaceae was predominant in indirect treatments. This monodominant community in indirect treatments was assumed not to transfer electron directly to schwertmannite but to rely on shuttling mechanism. PICRUSt results implied that bacteria in indirect treatment have potential to produce shuttling compounds or complexing agents. The absence of dsr genes and the putative fermentative process suggested that the Enterobacteriaceae might indirectly facilitate the dissolution and transformation of schwertmannite.

Producing desulfurized biogas using two-stage domesticated shear-loop anaerobic contact stabilization system

In this study, a two-stage domesticated shear-loop anaerobic contact stabilization (SLACS) system is introduced as a new reactor design to enhance methane productivity with significant reduction in hydrogen sulphide (H₂S) synthesis. Due to the rich sulfate content in industrial wastewaters, the initial fermentation phase of anaerobic digestion is highly acidifying and often leads to severe performance losses, digester's instability, and even culture crash. The SLACS system functions as a dissimilatory sulfate reduction - methanogenic reactor consisting of two compartments, a shear-loop anaerobic bed (SLAB) unit and an anaerobic plug flow (APF) unit. The functional role of the SLAB unit is not limited to acidogenesis but also sulfidogenic processes, which curtails H₂S generation in the APF unit (methanogenic stage). Experimental observations indicated that pH serves a critical role in the cohabitation of acidogenic and sulfidogenic microbes in the SLAB unit. Although acidogenesis was not influenced by pH within the range of 4.5-6.0, it is vital to stabilize the pH of this unit at 5.4 to establish a steady sulfate reduction of above 75%. The highest desulfurization achieved in this compartment was 88% under a hydraulic retention time (HRT) of 4 h. With an average methane productivity of 256 mL g^-1 VS, the methanogenic performance of the two-stage domesticated SLACS system shows a 32% methanogenic proficiency higher than that of the one-stage digestion system. Microbial community structure within the system carried out via Next Generation Sequencing (NGS) provided qualitative data on the sludge's sulfidogenic and methanogenic performance.