Bridging spatially segregated redox zones with a microbial electrochemical snorkel triggers biogeochemical cycles in oil-contaminated River Tyne (UK) sediments

Microbial remediation and deteriorated corrosion in marine oil pollution remediation engineering: A critical review

Research on mechanism of microbial deteriorated corrosion in oil-pollution remediation is limited. This paper discusses principles and technical methods of the cost-effective and environmental-friendly bioremediation in marine oil pollution control including the highly efficient microbial resources and bioenhancement technology. Deteriorated corrosion is creatively put forward to interpret the corrosion phenomenon under pollutant-degrading conditions, primarily induced by anaerobic electroactive microorganisms via electron transfer. It summarizes the potential link of microorganisms between oil pollutant degradation and corrosion destruction and illustrates the importance of screening microorganisms with hydrocarbon degradation and corrosion inhibition functions. We critically point out that the severe damage of metal materials in the oil-containing environment is related to the service environment and the interactions between microbial interspecies. The study of the material failure mechanism and the microbial protection technology in the oil-contaminated environment contributes to the sustainability of safe and clean marine ecological restoration engineering.

A. Jorand F, Barrière F, Etienne M. Challenges and Applications of Nitrate-Reducing Microbial Biocathodes

Bioelectrochemical systems which employ microbes as electrode catalysts to convert chemical energy into electrical energy (or conversely), have emerged in recent years for water sanitation and energy recovery. Microbial biocathodes, and especially those reducing nitrate are gaining more and more attention. The nitrate-reducing biocathodes can efficiently treat nitrate-polluted wastewater. However, they require specific conditions and they have not yet been applied on a large scale. In this review, the current knowledge on nitrate-reducing biocathodes will be summarized. The fundamentals of microbial biocathodes will be discussed, as well as the progress towards applications for nitrate reduction in the context of water treatment. Nitrate-reducing biocathodes will be compared with other nitrate-removal techniques and the challenges and opportunities of this approach will be identified.

Passive electrobioremediation approaches for enhancing hydrocarbons biodegradation in contaminated soils

Electrobioremediation technologies hold considerable potential for the treatment of soils contaminated by petroleum hydrocarbons (PH), since they allow stimulating biodegradation processes with no need for subsurface chemicals injection and with little to no energy consumption. Here, a microbial electrochemical snorkel (MES) was applied for the treatment of a soil contaminated by hydrocarbons. The MES consists of direct coupling of a microbial anode with a cathode, being a single conductive, non-polarized material positioned suitably to create an electrochemical connection between the anoxic zone (the contaminated soil) and the oxic zone (the overlying oxygenated water). Soil was also supplemented with electrically conductive particles of biochar as a strategy to construct a conductive network with microbes in the soil matrix, thus extending the radius of influence of the snorkel. The results of a comprehensive suite of chemical, microbiological and ecotoxicological analyses evidenced that biochar addition, rather than the presence of a snorkel, was the determining factor in accelerating PH removal from contaminated soils, possibly accelerating syntrophic and/or cooperative metabolisms involved in the degradation of PH. The enhancement of biodegradation was mirrored by an increased abundance of anaerobic and aerobic microorganisms known to be involved in the degradation of PH and related functional genes. Plant ecotoxicity assays confirmed a reduction of soils toxicity in treatments receiving electrically conductive biochar.

Snorkels enhance alkanes respiration at ambient and increased hydrostatic pressure (10 MPa) by either supporting the TCA cycle or limiting alternative routes for acetyl-CoA metabolism

The impact of piezosensitive microorganisms is generally underestimated in the ecology of underwater environments exposed to increasing hydrostatic pressure (HP), including the biodegradation of crude oil components. Yet, no isolated pressure-loving (piezophile) microorganism grows optimally on hydrocarbons, and no isolated piezophile at all has a HP optimum <10 MPa (e.g. 1000 m below sea water level). Piezosensitive heterotrophs are thus largely accountable for oil clean up < 10 MPa, however, they are affected by such a mild HP increase in ways which are not completely clear. In a first study, the application of a bioelectrochemical system (called "oil-spill snorkel") enhanced the alkane oxidation capacity in sediments collected at surface water but tested up to 10 MPa. Here, the fingerprint left on transcript abundance was studied to explore which metabolic routes are 1) supported by snorkels application and 2) negatively impacted by HP increase. Transcript abundance was comparable for beta-oxidation across all treatments (also at a taxonomical level), while the metabolism of acetyl-CoA was highly impacted: at either 0.1 or 10 MPa, snorkels supported acetyl-CoA oxidation within the TCA cycle, while in negative controls using non-conductive rods several alternative routes for acetyl-CoA were stimulated (including those leading to internal carbon reserves e.g. 2,3 butanediol and dihydroxyacetone). In general, increased HP had opposite effects as compared to snorkels, thus indicating that snorkels could enhance hydrocarbons oxidation by alleviating in part the stressing effects imposed by increased HP on the anaerobic, respiratory electron transport chain. 16S rRNA gene analysis of sediments and biofilms on snorkels suggest a crosstalk between oil-degrading, sulfate-reducing microorganisms and sulfur oxidizers. In fact, no sulfur was deposited on snorkels, however, iron, aluminum and phosphorous were found to preferentially deposit on snorkels at 10 MPa. This data indicates that a passive BES such as the oil-spill snorkel can mitigate the stress imposed by increased HP on piezosensitive microorganisms (up to 10 MPa) without being subjected to passivation. An improved setup applying these principles can further support this deep-sea bioremediation strategy.

Enhancement of wastewater treatment under low temperature using novel electrochemical active biofilms constructed wetland

A novel electrochemical active biofilms constructed wetland (NEAB-CW) was built to enhance the treatment efficiency for domestic sewage under low temperature environment (0-15 °C). In NEAB-CW, the traditional matrixes were replaced with conductive layer, in which laid stainless steel mesh tubes (SSMT) and added slow-release oxygen matrixes (SROM) and zero-valent iron rod (IR) were used to build a bioelectrochemical activity biofilms system. According to the results of 180 d experiment, the removal efficiencies of COD, NH₄⁺-N and TP of NEAB-CW were 1.52 and 2.21, 2.97 and 1.68, 3.95 and 1.76 times higher than the CW without SROM and IR at 10-20 and 0-10 °C, respectively. The transverse and longitudinal electric potential (EP) variations in NEAB-CW improved microbial activities under low temperature by enhancing the electron transfer efficiency, resulting in higher and stable EP and electron currents density, as well as protein-like contents secreted from biofilms. The pollutant-degrading microorganisms (e.g., Clostridia, Simplicispira), low temperature-resistant microorganisms (e.g., Psychrobacter, Acinetobacter), and electrochemical active microorganisms (e.g., Negativicutes, Gammaproteobacteria) obviously accumulated in NEAB-CW under low temperature environment to generate electricity and degrade pollutants. The results provided a good choice to treat domestic sewage at 0-15 °C by using NEAB-CW.

Microbial Electrochemical Fluidized Bed Reactor: A Promising Solution for Removing Pollutants From Pharmaceutical Industrial Wastewater

The capacity of electroactive bacteria to exchange electrons with electroconductive materials has been explored during the last two decades as part of a new field called electromicrobiology. Such microbial metabolism has been validated to enhance the bioremediation of wastewater pollutants. In contrast with standard materials like rods, plates, or felts made of graphite, we have explored the use of an alternative strategy using a fluid-like electrode as part of a microbial electrochemical fluidized bed reactor (ME-FBR). After verifying the low adsorption capacity of the pharmaceutical pollutants on the fluid-bed electrode [7.92 ± 0.05% carbamazepine (CBZ) and 9.42 ± 0.09% sulfamethoxazole (SMX)], our system showed a remarkable capacity to outperform classical solutions for removing pollutants (more than 80%) from the pharmaceutical industry like CBZ and SMX. Moreover, the ME-FBR performance revealed the impact of selecting an anode potential by efficiently removing both pollutants at + 200 mV. The high TOC removal efficiency also demonstrated that electrostimulation of electroactive bacteria in ME-FBR could overcome the expected microbial inhibition due to the presence of CBZ and SMX. Cyclic voltammograms revealed the successful electron transfer between microbial biofilm and the fluid-like electrode bed throughout the polarization tests. Finally, Vibrio fischeri-based ecotoxicity showed a 70% reduction after treating wastewater with a fluid-like anode (+ 400 mV), revealing the promising performance of this bioelectrochemical approach.

Anaerobic-petroleum degrading bacteria: Diversity and biotechnological applications for improving coastal soil

Due to the industrial emissions and accidental spills, the critical material for modern industrial society petroleum pollution causes severe ecological damage. The prosperous oil exploitation and transportation causes the recalcitrant, hazardous, and carcinogenic sludge widespread in the coastal wetlands. The costly physicochemical-based remediation remains the secondary and inadequate treatment for the derivatives along with the tailings. Anaerobic microbial petroleum degrading biotechnology has received extensive attention for its cost acceptable, eco-friendly, and fewer health hazards. As a result of the advances in biotechnology and microbiology, the anaerobic oil-degrading bacteria have been well developing to achieve the same remediation effects with lower operating costs. This review summarizes the advantages and potential scenarios of the anaerobic degrading bacteria, such as sulfate-reducing bacteria, denitrifying bacteria, and metal-reducing bacteria in the coastal area decomposing the alkanes, alkenes, aromatic hydrocarbons, polycyclic aromatic, and related derivatives. In the future, a complete theoretical basis of microbiological biotechnology, molecular biology, and electrochemistry is necessary to make efficient and environmental-friendly use of anaerobic degradation bacteria to mineralize oil sludge organic wastes.

Electrochemical analysis of a microbial electrochemical snorkel in laboratory and constructed wetlands

Microbial electrochemical snorkel (MES) is a short-circuited microbial fuel cell applicable to water treatment that does not produce energy but requires lower cost for its implementation. Few reports have already described its water treatment capabilities but no deeper electrochemical analysis were yet performed. We tested various materials (iron, stainless steel and porous graphite) and configurations of snorkel in order to better understand the rules that will control in a wetland the mixed potential of this self-powered system. We designed a model snorkel that was studied in laboratory and on the field. We confirmed the development of MES by identifying anodic and cathodic parts, by measuring the current between them and by analyzing microbial ecology in laboratory and field experiments. An important application is denitrification of surface water. Here we discuss the influence of nitrate on its electrochemical response and denitrification performances. Introducing nitrate caused the increase of the mixed potential of MES and of current at a potential value relatively more positive than for nitrate-reducing biocathodes described in the literature. The major criteria for promoting application of MES in artificial wetland dedicated to mitigation of non-point source nitrate pollution from agricultural water are considered.

Enhanced Hydrocarbons Biodegradation at Deep-Sea Hydrostatic Pressure with Microbial Electrochemical Snorkels

In anaerobic sediments, microbial degradation of petroleum hydrocarbons is limited by the rapid depletion of electron acceptors (e.g., ferric oxide, sulfate) and accumulation of toxic metabolites (e.g., sulfide, following sulfate reduction). Deep-sea sediments are increasingly impacted by oil contamination, and the elevated hydrostatic pressure (HP) they are subjected to represents an additional limitation for microbial metabolism. While the use of electrodes to support electrobioremediation in oil-contaminated sediments has been described, there is no evidence on their applicability for deep-sea sediments. Here, we tested a passive bioelectrochemical system named ”oil-spill snorkel” with two crude oils carrying different alkane contents (4 vs. 15%), at increased or ambient HP (10 vs. 0.1 MPa). Snorkels enhanced alkanes biodegradation at both 10 and 0.1 MPa within only seven weeks, as compared to nonconductive glass controls. Microprofiles in anaerobic, contaminated sediments indicated that snorkels kept sulfide concentration to low titers. Bulk-sediment analysis confirmed that sulfide oxidation by snorkels largely regenerated sulfate. Hence, the sole application of snorkels could eliminate a toxicity factor and replenish a spent electron acceptor at increased HP. Both aspects are crucial for petroleum decontamination of the deep sea, a remote environment featured by low metabolic activity.

Biofilm structure, dynamics, and ecology of an upscaled biocathode wastewater microbial fuel cell

A microbial fuel cell (MFC) system containing modular half-submerged biocathode was operated for 6 months in an 800 L flow-through system with domestic wastewater. For the first time, spatial and temporal differences in biofilm communities were examined on large three-dimensional electrodes in a wastewater MFC. Biocathode microbial community analysis showed a specialized biofilm community with electrogenic and electrotrophic taxa forming during operation, suggesting potentially opposing electrode reactions. The anodic community structure shifted during operation, but no spatial differences were observed along the length of the electrode. Power output from the system was most strongly influenced by pH. Higher power densities were associated with the use of solids-dewatering filtrate with increased organic matter, conductivity, and pH. The results show that the biocathode was the rate-limiting step and that future MFC design should consider the effect of size, shape, and orientation of biocathodes on their community assembly and electrotrophic ability.

Assessing the efficiency and eco-sustainability of bioremediation strategies for the reclamation of highly contaminated marine sediments

Coastal sediments subjected to high anthropogenic impacts can accumulate large amounts of polycyclic aromatic hydrocarbons (PAHs) and metals, demanding effective and eco-sustainable remediation solutions. In this study, we carried out bioremediation experiments on marine sediments highly contaminated with PAHs and metals. In particular, we investigated the effects of biostimulation (by the addition of inorganic nutrients), bioaugmentation (by the addition of fungi belonging to Aspergillus sp.) and microbial fuel cell-based strategies on PAH degradation and on changes in metal partitioning. Results reported here indicate that all biotreatments determined a significant decrease of PAH concentrations (at least 60%) in a relatively short time interval (few weeks) and that biostimulation was the most effective approach (>90%). Biostimulation determined a faster degradation rate of high than low molecular weight PAHs, indicating a preferential biodegradation of specific PAH congeners. At the same time, the biotreatments changed the partitioning of metals, including their solubilization, suggesting the need of parallel environmental risk assessment. Our findings also suggest that ex situ biotreatments can have a lower carbon footprint than current management options of contaminated sediments (i.e., landfill disposal and/or disposal in confined aquatic facilities), but integration with other strategies for metal removal (e.g. through bioleaching) from sediments is needed for their safe re-use. Overall, results presented here provide new insights into the development of effective and eco-sustainable bioremediation strategies for the reclamation of highly contaminated marine sediments.

Sediment microbial fuel cells as a barrier to sulfide accumulation and their potential for sediment remediation beneath aquaculture pens

Sediment microbial fuel cells (SMFCs) generate electricity through the oxidation of reduced compounds, such as sulfide or organic carbon compounds, buried in anoxic sediments. The ability to remove sulfide suggests their use in the remediation of sediments impacted by point source organic matter loading, such as occurs beneath open pen aquaculture farms. However, for SMFCs to be a viable technology they must remove sulfide at a scale relevant to the environmental contamination and their impact on the sediment geochemistry as a whole must be evaluated. Here we address these issues through a laboratory microcosm experiment. Two SMFCs placed in high organic matter sediments were operated for 96 days and compared to open circuit and sediment only controls. The impact on sediment geochemistry was evaluated with microsensor profiling for oxygen, sulfide, and pH. The SMFCs had no discernable effect on oxygen profiles, however porewater sulfide was significantly lower in the sediment microcosms with functioning SMFCs than those without. Depth integrated sulfide inventories in the SMFCs were only 20% that of the controls. However, the SMFCs also lowered pH in the sediments and the consequences of this acidification on sediment geochemistry should be considered if developing SMFCs for remediation. The data presented here indicate that SMFCs have potential for the remediation of sulfidic sediments around aquaculture operations.

Centimeter-Long Microbial Electron Transport for Bioremediation Applications

Microbial bioremediation based on nano- to micrometer-scale electron transport has been intensively studied during the past decade, but its application can be hindered by a deficiency of suitable electron acceptors or slow mass transportation at contaminated sites. Microbial long-distance electron transport (LDET), which can couple spatially separated redox reactions across distances in natural environments, has recently emerged at centimeter-length scales. LDET explains a range of globally important biogeochemical phenomena and overcomes the drawbacks of conventional bioremediation by directly linking distant electron donors and acceptors. Here, we highlight recent research outcomes in examining, characterizing, and engineering LDET, and describe how LDET can be exploited to develop advanced technologies for the bioremediation of soils and sediments.

Parallel artificial and biological electric circuits power petroleum decontamination: The case of snorkel and cable bacteria

Degradation of petroleum hydrocarbons (HC) in sediments is often limited by the availability of electron acceptors. By allowing long-distance electron transport (LDET) between anoxic sediments and oxic overlying water, bioelectrochemical snorkels may stimulate the regeneration of sulphate in the anoxic sediment thereby accelerating petroleum HC degradation. Cable bacteria can also mediate LDET between anoxic and oxic sediment layers and thus theoretically stimulate petroleum HC degradation. Here, we quantitatively assessed the impact of cable bacteria and snorkels on the degradation of alkanes in marine sediment from Aarhus Bay (Denmark). After seven weeks, cable bacteria and snorkels accelerated alkanes degradation by +24 and +25%, respectively, compared to control sediment with no cable bacteria nor snorkel. The combination of snorkels and cable bacteria further enhanced alkanes degradation (+46%). Higher degradation rates were sustained by LDET-induced sulphide removal rather than, as initially hypothesized, sulphate regeneration. Cable bacteria are thus overlooked players in the self-healing capacity of crude-oil contaminated sediments, and may inspire novel remediation treatments upon hydrocarbon spillage.

In situ groundwater remediation with bioelectrochemical systems: A critical review and future perspectives

Groundwater contamination is an ever-growing environmental issue that has attracted much and undiminished attention for the past half century. Groundwater contamination may originate from both anthropogenic (e.g., hydrocarbons) and natural compounds (e.g., nitrate and arsenic); to tackle the removal of these contaminants, different technologies have been developed and implemented. Recently, bioelectrochemical systems (BES) have emerged as a potential treatment for groundwater contamination, with reported in situ applications that showed promising results. Nitrate and hydrocarbons (toluene, phenanthrene, benzene, BTEX and light PAHs) have been successfully removed, due to the interaction of microbial metabolism with poised electrodes, in addition to physical migration due to the electric field generated in a BES. The selection of proper BESs relies on several factors and problems, such as the complexity of groundwater and subsoil environment, scale-up issues, and energy requirements that need to be accounted for. Modeling efforts could help predict case scenarios and select a proper design and approach, while BES-based biosensing could help monitoring remediation processes. In this review, we critically analyze in situ BES applications for groundwater remediation, focusing in particular on different proposed setups, and we identify and discuss the existing research gaps in the field.

Increased power production and removal efficiency of polycyclic aromatic hydrocarbons by plant pumps in sediment microbial electrochemical systems: A preliminary study

The low mass transfer of sediment substrates has limited the efficiency and application of a sediment microbial electrochemical system (SMES) as a power generator and as a practical bioremediation technology. In this study, we designed a new plant-driven SMES (New-PSMES) with a separated sand-filled anode column in order to improve the mass transfer and thereby enhance the microorganism activity, power generation and bioremediation range and efficiency for polycyclic aromatic hydrocarbons (PAHs). Because of the mass flow driven by the plants, the New-PSMESs started up approximately 7 d earlier and produced voltages 30-70 mV higher than the planted SMESs, and had greater enzyme activities and residual organic carbon than the unplanted and planted SMESs. In the New-PSMES, the total mass removal rates of phenanthrene and pyrene were 62.98% and 57.02% after 82 d, and these values were 1.5-2 times higher than those of the unplanted and planted SMESs. The removal of PAHs in the sediment was primarily attributed to nonelectrochemical biodegradation at sites far from the anode and to electrochemical reactions on the anode. The top three most abundant phyla in all samples were Proteobacteria, Chloroflexi, and Bacteroidetes. Aerobic bacteria, such as Nautella, were enriched in the biofilms of the New-PSMESs.

Removal of Hepatitis B virus surface HBsAg and core HBcAg antigens using microbial fuel cells producing electricity from human urine

Microbial electrochemical technology is emerging as an alternative way of treating waste and converting this directly to electricity. Intensive research on these systems is ongoing but it currently lacks the evaluation of possible environmental transmission of enteric viruses originating from the waste stream. In this study, for the first time we investigated this aspect by assessing the removal efficiency of hepatitis B core and surface antigens in cascades of continuous flow microbial fuel cells. The log-reduction (LR) of surface antigen (HBsAg) reached a maximum value of 1.86 ± 0.20 (98.6% reduction), which was similar to the open circuit control and degraded regardless of the recorded current. Core antigen (HBcAg) was much more resistant to treatment and the maximal LR was equal to 0.229 ± 0.028 (41.0% reduction). The highest LR rate observed for HBsAg was 4.66 ± 0.19 h⁻¹ and for HBcAg 0.10 ± 0.01 h⁻¹. Regression analysis revealed correlation between hydraulic retention time, power and redox potential on inactivation efficiency, also indicating electroactive behaviour of biofilm in open circuit control through the snorkel-effect. The results indicate that microbial electrochemical technologies may be successfully applied to reduce the risk of environmental transmission of hepatitis B virus but also open up the possibility of testing other viruses for wider implementation.