Anaerobic Iron Corrosion Research Articles

Significance of temperature as a key driver in ZVI PRB applications for PCE degradation

We report on the potential of elevated groundwater temperatures and zero-valent iron permeable reactive barriers (ZVI PRBs), for example, through a combination with underground thermal energy storage (UTES), to achieve enhanced remediation of chlorinated hydrocarbon (CHC) contaminated groundwater. Building on earlier findings concerning deionized solutions, we created a database for mineralized groundwater based on temperature dependence of tetrachloroethylene (PCE) degradation using two popular ZVIs (i.e., Gotthart–Maier cast iron [GM] and ISPAT sponge iron [IS]) in column experiments at 25 °C–70 °C to establish a temperature-dependent ZVI PRB dimensioning approach. Scenario analysis revealed that a heated ZVI PRB system in a moderate temperature range up to 40 °C showed the greatest efficiency, with potential material savings of ~55% to 75%, compared to 10 °C, considering manageability and longevity. With a 25 °C–70 °C temperature increase, rate coefficients of PCE degradation increased from 0.4 ± 0.0 h−1 to 2.9 ± 2.2 h−1 (GM) and 0.1 ± 0.1 h−1 to 1.8 ± 0.0 h−1 (IS), while TCE rate coefficients increased from 0.6 ± 0.1 h−1 to 5.1 ± 3.9 h−1 at GM. Activation energies for PCE degradation yielded 32 kJ mol−1 (GM) and 56 kJ mol−1 (IS). Temperature-dependent anaerobic iron corrosion was key in regulating mineral precipitation and passivation of the iron surface as well as porosity reduction due to gas production.

Electrode hydrophilicity enhanced the rate of extracellular electron uptake in Desulfovibrio ferrophilus IS5

The determination of extracellular electron uptake (EEU) by sulfate-reducing bacteria is crucial for understanding the mechanism underlying anaerobic iron corrosion. Several studies have attempted to elucidate the mechanism underlying EEU using Desulfovibrio ferrophilus IS5 as the model bacterium. This study examined the impact of electrode wettability on EEU activity and the electron transfer mechanism at the interfaces between IS5 cells and electrodes. The wettability of indium-tin-doped oxide (ITO) electrodes was modified by dip-coating them with hydrophilic NH2- or SH- ligands or hydrophobic CH3- ligands. With increasing electrode hydrophilicity, current generation coupled with sulfate reduction increased by as much as nine-fold and displayed a positive linear correlation with the number of cells attached to the electrodes. Hence, IS5-mediated EEU most likely requires direct cell attachment to the electrode surfaces. Differential pulse voltammetry showed that electrode wettability altered the peak potential and intensity of the IS5 reductive signal possibly because of direct interactions between outer-membrane cytochromes (OMCs) in IS5 and the electrode surfaces. These results support the hypothesis that IS5 utilizes an OMC-mediated direct EEU mechanism, which has implications for the mechanism underlying corrosion and anti-biocorrosion material.

The Oligosaccharyltransferase AglB Supports Surface-Associated Growth and Iron Oxidation in Methanococcus maripaludis.

Most microbial organisms grow as surface-attached communities known as biofilms. However, the mechanisms whereby methanogenic archaea grow attached to surfaces have remained understudied. Here, we show that the oligosaccharyltransferase AglB is essential for growth of Methanococcus maripaludis strain JJ on glass or metal surfaces. AglB glycosylates several cellular structures, such as pili, archaella, and the cell surface layer (S-layer). We show that the S-layer of strain JJ, but not strain S2, is a glycoprotein, that only strain JJ was capable of growth on surfaces, and that deletion of aglB blocked S-layer glycosylation and abolished surface-associated growth. A strain JJ mutant lacking structural components of the type IV-like pilus did not have a growth defect under any conditions tested, while a mutant lacking the preflagellin peptidase (ΔflaK) was defective for surface growth only when formate was provided as the sole electron donor. Finally, for strains that are capable of Fe0 oxidation, we show that deletion of aglB decreases the rate of anaerobic Fe0 oxidation, presumably due to decreased association of biomass with the Fe0 surface. Together, these data provide an initial characterization of surface-associated growth in a member of the methanogenic archaea. IMPORTANCE Methanogenic archaea are responsible for producing the majority of methane on Earth and catalyze the terminal reactions in the degradation of organic matter in anoxic environments. Methanogens often grow as biofilms associated with surfaces or partner organisms; however, the molecular details of surface-associated growth remain uncharacterized. We have found evidence that glycosylation of the cell surface layer is essential for growth of M. maripaludis on surfaces and can enhance rates of anaerobic iron corrosion. These results provide insight into the physiology of surface-associated methanogenic organisms and highlight the importance of surface association for anaerobic iron corrosion.

Iron corrosion induced by the hyperthermophilic sulfate-reducing archaeon Archaeoglobus fulgidus at 70 °C

This work assessed the role of the hyperthermophilic sulfate-reducing archaeon, Archaeoglobus fulgidus in anaerobic iron corrosion, at 70 °C, in the presence and the absence of lactate as energy source. Experiments performed with A. fulgidus planktonic cells, displayed their capacity to form a biofilm on an iron coupon and exhibited their biocorrosive activity. A. fulgidus was shown to cause indirect corrosion by producing sulfide while oxidizing lactate. Furthermore, the archaeon could grow lithotrophically by using elemental iron as a mineral energy source. Under these latter conditions, A. fulgidus formed chimneys enhancing the direct corrosion process. Moreover, physiological modifications occurred under these conditions notably highlighting the probable use of CO2 instead of sulfate as a terminal electron acceptor to produce acetate similarly to homoacetogens. All together, these results illustrate the metabolic versatility and hence the importance of this hyperthermophilic archaeon in microbial induced corrosion (MIC).

Electrode Potential Dependency of Single-Cell Activity Identifies the Energetics of Slow Microbial Electron Uptake Process.

Electrochemical measurements have been widely applied to study microbial extracellular electron transport processes. However, because electrochemistry detects not only microbial electron transport but also other reactions, background signals comparable to or larger than microbial ones hamper the identification of microbial electrochemical properties. This problem is crucial especially for the detection of electron uptake processes by slow-growing microbes in low-energy subsurface sediments, as the environmental samples contain electrochemically active humus and mineral particles. In this study, we report a cell-specific stable isotope analysis to quantify the electrode potential dependency of anabolic activity in individual cells for identifying the electron uptake energetics of slow-growing bacteria. Followed by the incubation of Desulfovibrio ferrophilus IS5 cells with isotopic 15N-ammonium as the sole N source on electrodes poised at potentials of -0.2, -0.3, -0.4, and -0.5 V [vs. standard hydrogen electrode (SHE)], we conducted nanoscale secondary ion mass spectroscopy (NanoSIMS) to quantify 15N assimilation in more than 100 individual cells on the electrodes. We observed significant 15N assimilation at potentials of -0.4 and more 15N assimilation at -0.5 V, which is consistent with the onset potential for electron uptake via outer-membrane cytochromes (OMCs). The activation of cell energy metabolism was further examined by transcriptome analysis. Our results showed a novel methodology to study microbial electron uptake energetics. The results also serve as the first direct evidence that energy acquisition is coupled to the electron uptake process in sulfate-reducing bacteria that are ubiquitous in the subsurface environments, with implications on the electron-fueled subsurface biosphere hypothesis and other microbial processes, such as anaerobic iron corrosion and anaerobic methane oxidation.

Ochrobactrum anthropi used to control ammonium for nitrate removal by starch-stabilized nanoscale zero valent iron.

In this study, the hydrogenotrophic denitrifying bacterium Ochrobactrum anthropi was added in to the process of nitrate removal by starch-stabilized nanoscale zero valent iron (nZVI) to minimize undesirable ammonium. The ammonium control performance and cooperative mechanism of this combined process were investigated, and batch experiments were conducted to discuss the effects of starch-stabilized nZVI dose, biomass, and pH on nitrate reduction and ammonium control of this system. The combined system achieved satisfactory performance because the anaerobic iron corrosion process generates H2, which is used as an electron donor for the autohydrogenotrophic bacterium Ochrobactrum anthropi to achieve the autohydrogenotrophic denitrification process converting nitrate to N2. When starch-stabilized nZVI dose was increased from 0.5 to 2.0 g/L, nitrate reduction rate gradually increased, and ammonium yield also increased from 9.40 to 60.51 mg/L. Nitrate removal rate gradually decreased and ammonium yield decreased from 14.93 to 2.61 mg/L with initial OD600 increasing from 0.015 to 0.080. The abiotic Fe0 reduction process played a key role in nitrate removal in an acidic environment and generated large amounts of ammonium. Meanwhile, the nitrate removal rate decreased and ammonium yield also reduced in an alkaline environment.

Iron Corrosive Sulfate Reducing Bacteria Uptake Extracellular Electrons Via Outer Membrane C-Type Cytochromes

Sulfate reducing bacteria (SRB) can oxidize a large variety of soluble organics or H2 and reduce sulfate to sulfide which is corrosive towards various materials, therefore have been recognized to play an important role in anaerobic iron corrosion which causes enormous economic losses through chemical reaction (Fe + H2S → FeS + H2). Recently, several novel SRB strains including Desulfovibrio ferrophilus IS5 were isolated using solid iron as a sole electron donor, and thus a new corrosion mechanism for SRB to promote corrosion via direct electron uptake from iron surface was proposed1,2. Previously, we showed that D. ferrophilus IS5 can extract electrons from an –0.4 V (vs. SHE)-poised indium tin-doped oxide (ITO) electrode surface without using H2 as an electron carrier3. However, unlike well-studied EET mechanism in iron-reducing bacteria, the EET mechanism for SRB remains ambiguous despite of its emerging importance. Here we examined our hypothesis that D. ferrophilus IS5 extracts electrons from an ITO electrode via outer-membrane (OM) c-type cytochromes by a combination of electrochemical, biochemical, and gene analysis. Electrochemical measurements using intact microbes were conducted in a single-chamber anaerobic 3-electrode system with negative potential poised-ITO electrode served as a sole electron donor. By measuring differential pulse (DP) voltammetry, we detected biogenic reduction signals including one with a peak potential of –0.26 V (vs. SHE), which negatively shifted for more than 120 mV when the reactor was heated from 30 °C to 60 °C. This strong temperature dependency suggests that its assignment is not small redox molecule but redox protein localized at the cell/electrode interface, as the temperature change may alter the protein structure to affect its redox potential. To directly identify the genes coding for detected OM redox active proteins in D. ferrophilus IS5, we analyzed the OM extracted from cells. Diffuse-transmission UV/Vis absorption spectrum of OM fraction showed characteristic absorption peaks of oxidized c-type cytochromes. Heme staining analysis of OM proteins showed five significant bands at 71, 31, 24, 15, 13 kDa, which amino acid sequences were sequenced and identified as c-type cytochromes in the genome of D. ferrophilus IS5. Notably, it contained 95 genes coding for cytochromes, including 2 extracellular cytochromes. Next to these 2 genes, there were 5 periplasmic cytochromes, with 2 were detected in the OM heme staining gel. Therefore, we proposed an EET pathway of D. ferrophilus IS5 composing of OM-spanning c-type cytochromes. Furthermore, based on blastp search in the NCBI non-redundant protein database, microbial species of a wide range, especially the sulfur-metabolizing microbes, turned out to have proteins identical to the detected OM c-type cytochromes of D. ferrophilus IS5, suggesting that the electron extraction via OM c-type cytochromes may also be the corrosion mechanism by other SRB. We will present our other biofilm voltammetric data, the whole genome analysis results, and transmission electron microscopic (TEM) visualization of cytochrome-stained bacterial OM. Our data suggest that microbial electron uptake was mediated via the c-type cytochromes located at cell surface of D. ferrophilus IS5. 1. Dinh H.T., Kuever J., Muβmann M., Hassel A. W., Stratmann M., and Widdel F. (2004) Iron Corrosion by Novel Anaerobic Microorganisms. Nature, 427, 829-832. 2. Enning D. and Garrelfs J. (2014) Corrosion of Iron by Sulfate-Reducing Bacteria: New Views of an Old Problem. Appl. Environ. Microbiol., 80 (4): 1226-1236. 3. Deng X., Nakamura R., Okamoto A., and Hashimoto K. (2015) Electron Extraction from an Extracellular Electrode by Desulfovibrio ferrophilus Strain IS5 Without Using Hydrogen as an Electron Carrier. Electrochemistry, 83 (7):529-531. Figure 1

Characterization of the enhancement of zero valent iron on microbial azo reduction.

BackgroundThe microbial method for the treatment of azo dye is promising, but the reduction of azo dye is the rate-limiting step. Zero valent iron (Fe0) can enhance microbial azo reduction, but the interactions between microbes and Fe0 and the potential mechanisms of enhancement remain unclear. Here, Shewanella decolorationis S12, a typical azo-reducing bacterium, was used to characterize the enhancement of Fe0 on microbial decolorization.ResultsThe results indicated that anaerobic iron corrosion was a key inorganic chemical process for the enhancement of Fe0 on microbial azo reduction, in which OH−, H2, and Fe2+ were produced. Once Fe0 was added to the microbial azo reduction system, the proper pH for microbial azo reduction was maintained by OH−, and H2 served as the favored electron donor for azo respiration. Subsequently, the bacterial biomass yield and viability significantly increased. Following the corrosion of Fe0, nanometer-scale Fe precipitates were adsorbed onto cell surfaces and even accumulated inside cells as observed by transmission electron microscope energy dispersive spectroscopy (TEM-EDS).ConclusionsA conceptual model for Fe0-assisted azo dye reduction by strain S12 was established to explain the interactions between microbes and Fe0 and the potential mechanisms of enhancement. This model indicates that the enhancement of microbial azo reduction in the presence of Fe0 is mainly due to the stimulation of microbial growth and activity by supplementation with elemental iron and H2 as an additional electron donor. This study has expanded our knowledge of the enhancement of microbial azo reduction by Fe0 and laid a foundation for the development of Fe0-microbial integrated azo dye wastewater treatment technology.Electronic supplementary materialThe online version of this article (doi:10.1186/s12866-015-0419-3) contains supplementary material, which is available to authorized users.

Electron Extraction from an Extracellular Electrode by <i>Desulfovibrio ferrophilus</i> Strain IS5 Without Using Hydrogen as an Electron Carrier

We report that an intensely iron-corroding microbe, Desulfovibrio ferrophilus strain IS5, is capable of extracting electrons from an indium tin-doped oxide electrode without consuming electrochemically generated hydrogen as an electron carrier. When sulfate was presented as a metabolic electron acceptor, significant cathodic current production was observed at an onset potential of−200mV vs. SHE, which was approximately 750mVmore positive than the onset for hydrogen evolution in our experimental condition. This finding indicates that hydrogen is not required for the cathodic reaction of IS5, suggesting that IS5 accelerates anaerobic iron corrosion through direct electron uptake. © The Electrochemical Society of Japan, All rights reserved.

Corrosion of Iron by Sulfate-Reducing Bacteria: New Views of an Old Problem

About a century ago, researchers first recognized a connection between the activity of environmental microorganisms and cases of anaerobic iron corrosion. Since then, such microbially influenced corrosion (MIC) has gained prominence and its technical and economic implications are now widely recognized. Under anoxic conditions (e.g., in oil and gas pipelines), sulfate-reducing bacteria (SRB) are commonly considered the main culprits of MIC. This perception largely stems from three recurrent observations. First, anoxic sulfate-rich environments (e.g., anoxic seawater) are particularly corrosive. Second, SRB and their characteristic corrosion product iron sulfide are ubiquitously associated with anaerobic corrosion damage, and third, no other physiological group produces comparably severe corrosion damage in laboratory-grown pure cultures. However, there remain many open questions as to the underlying mechanisms and their relative contributions to corrosion. On the one hand, SRB damage iron constructions indirectly through a corrosive chemical agent, hydrogen sulfide, formed by the organisms as a dissimilatory product from sulfate reduction with organic compounds or hydrogen ("chemical microbially influenced corrosion"; CMIC). On the other hand, certain SRB can also attack iron via withdrawal of electrons ("electrical microbially influenced corrosion"; EMIC), viz., directly by metabolic coupling. Corrosion of iron by SRB is typically associated with the formation of iron sulfides (FeS) which, paradoxically, may reduce corrosion in some cases while they increase it in others. This brief review traces the historical twists in the perception of SRB-induced corrosion, considering the presently most plausible explanations as well as possible early misconceptions in the understanding of severe corrosion in anoxic, sulfate-rich environments.

Investigating dominant processes in ZVI permeable reactive barriers using reactive transport modeling

The reactive and hydraulic efficacy of zero valent iron permeable reactive barriers (ZVI PRBs) is strongly affected by geochemical composition of the groundwater treated. An enhanced version of the geochemical simulation code MIN3P was applied to simulate dominating processes in chlorinated hydrocarbons (CHCs) treating ZVI PRBs including geochemical dependency of ZVI reactivity, gas phase formation and a basic formulation of degassing. Results of target oriented column experiments with distinct chemical conditions (carbonate, calcium, sulfate, CHCs) were simulated to parameterize the model. The simulations demonstrate the initial enhancement of anaerobic iron corrosion due to carbonate and long term inhibition by precipitates (chukanovite, siderite, iron sulfide). Calcium was shown to enhance long term corrosion due to competition for carbonate between siderite, chukanovite, and aragonite, with less inhibition of iron corrosion by the needle like aragonite crystals. Application of the parameterized model to a field site (Bernau, Germany) demonstrated that temporarily enhanced groundwater carbonate concentrations caused an increase in gas phase formation due to the acceleration of anaerobic iron corrosion.

Assessment of Long-Term Performance and Chromate Reduction Mechanisms in a Field Scale Permeable Reactive Barrier

An innovative full-scale implementation of a permeable reactive barrier, consisting of a double-row of cylinders filled with zerovalent iron shavings, for chromate remediation was monitored over four years. Solid samples were analyzed to elucidate (i) the relevant corrosion mechanisms and products, (ii) the pathways of chromate reduction and immobilization, and (iii) the long-term performance of the barrier situated in a hydrological and geochemical complex groundwater regime. Sampling and analysis of groundwater and reactive material revealed an oxidative iron corrosion zone evolving in the inflow and a zone of anaerobic iron corrosion in the center and outflow of the barrier. Chromate reduction was mainly confined to the inflow region. The formation and thickness of corrosion rinds depended on sampling time, position, and depth, as well as on the size, shape, and graphite content In the inflow, the corrosion rinds mostly consisted of goethite and ferrihydrite. X-ray absorption fine structure spectroscopy revealed two distinct Cr(III) species, most likely resulting from homogeneous and heterogeneous redox reaction pathways, respectively. The longevity and long-term effectiveness of the PRB appears to be primarily limited by reduced corrosion rates of the ZVI-shavings because of the thick layers of Fe-hydroxides.

Microbial reduction of nitrate in the presence of nanoscale zero-valent iron

Microbial reduction of nitrate in the presence of nanoscale zero-valent iron (NZVI) was evaluated to assess the feasibility of employing NZVI in the biological nitrate treatment. Nitrate was completely reduced within 3 d in a nanoscale Fe(0)-cell reactor, while only 50% of the nitrate was abiotically reduced over 7 d at 25 °C. The removal rate of nitrate in the integrated NZVI-cell system was unaffected by the presence of high amounts of sulfate. Efficient removal of nitrate by Fe(II)-supported anaerobic culture in 14 d indicated that Fe(II), which is produced during anaerobic iron corrosion in the Fe(0)-cell system, might act as an electron donor for nitrate. Unlike abiotic reduction, microbial reduction of nitrate was not significantly affected by low temperature conditions. This study demonstrated the potential applicability of employing NZVI iron as a source of electrons for biological nitrate reduction. Use of NZVI for microbial nitrate reduction can obviate the disadvantages associated with traditional biological denitrification, that relies on the use of organic substrates or explosive hydrogen gas, and maintain the advantages offered by nano-particle technology such as higher surface reactivity and functionality in suspensions.

Remediation of Trichloroethylene and Monochlorobenzene-Contaminated Aquifers Using the ORC-GAC-Fe 0CaCO 3 System: Volatilization, Precipitation, and Porosity Losses

The objectives of this study were to illustrate the reaction processes, to identify and quantify the precipitates formed, and to estimate the porosity losses in order to eliminate drawbacks during remediating monochlorobenzene (MCB) and trichloroethylene (TCE)-contaminated aquifers using the ORC-GAC-Fe 0-CaCO 3 system. The system consisted of four columns (112 cm long and 10 cm in diameter) with oxygen-releasing compound (ORC), granular activated carbon (GAC), zero-valent iron (Fe 0), and calcite used sequentially as the reactive media. The concentrations of MCB in the GAC column effluent and TCE in the Fe 0 column effluent were below the detection limit. However, the concentrations of MCB and TCE in the final calcite column exceeded the maximum contaminant level (MCL) under the Safe Drinking Water Act of the US Environmental Protection Agency (US EPA) that protects human health and environment. These results suggested that partitioning of MCB and TCE into the gas phase could occur, and also that transportation of volatile organic pollutants in the gas phase was important. Three main precipitates formed in the ORC-GAC-Fe 0-CaCO 3 system: CaCO 3 in the ORC column along with Fe(OH) 2 and FeCO 3 in the Fe 0 column. The total porosity losses caused by mineral precipitation corresponded to about 0.24% porosity in the ORC column, and 1% in the Fe 0 column. The most important cause of porosity losses was anaerobic corrosion of iron. The porosity losses caused by gas because of the production and entrapment of oxygen in the ORC column and hydrogen in the Fe 0 column should not be ignored. Volatilization, precipitation and porosity losses were considered to be the main drawbacks of the ORC-GAC-Fe 0-CaCO 3 system in remediating the MCB and TCE-contaminated aquifers. Thus, measurements such as using a suitable oxygen-releasing compound, weakening the increase in pH using a buffer material such as soil, stimulating biodegradation rates and minimizing the plugging caused by the relatively high dissolved oxygen levels should be taken to eliminate the drawbacks and to improve the efficiency of the ORC-GAC-Fe 0-CaCO 3 system.

Transport of Atomic Hydrogen through Graphite and Its Reaction with Azoaromatic Compounds

Graphite is a major non-iron component in commercial iron granules that are typically used for groundwater remediation. Recent studies suggest graphite inclusions in commercial iron may serve as both adsorption and reaction sites for nitrogenous pollutants such as nitroaromatics, nitrate esters, and heterocyclic nitramines. In this study, we investigated graphite-mediated reduction of azoaromatic compounds with elemental iron in dialysis cells, where azo compounds and iron were physically separated by graphite foil. Both the nonpolar azobenzene and the water-soluble orange G were reduced to aniline, suggesting that exposed graphite in granular iron may mediate reduction of both polar and nonpolar compounds. Orange G reduction was zero-order and commenced after a long initial lag. Both the lag time and the zero-order rate constant varied with graphite thickness, consistent with the explanation that orange G reduction was limited by atomic hydrogen, which was formed via anaerobic iron corrosion and spilled over to graphite. Involvement of atomic hydrogen was confirmed by detection of deuterated aniline when iron was placed in a D2O-based buffer. Our results indicate that atomic hydrogen is mobile in graphite at room temperature, is reactive toward azoaromatic compounds, and may be consumed during transport in graphite.

Further Studies of the Anaerobic Corrosion of Steel in Bentonite

ABSTRACTIn the horizontal emplacement concept (KBS-3H) for the disposal of radioactive waste, which is being developed in Sweden and Finland, copper canisters with cast iron inserts will be surrounded by bentonite buffer and mounted in perforated carbon steel support structures in boreholes within the bedrock. The groundwater will be reducing, leading to anaerobic corrosion of the ferrous material. It is important to understand both the effect of bentonite on the corrosion behaviour of the steel and the effect of the corrosion products on the performance of the bentonite. Previous work on the corrosion of steel in bentonite was extended to investigate a wider range of conditions, including the possible effects of alkaline plumes released from concrete support structures and the effect of chloride concentration and temperature on the corrosion rate of steel in bentonite. Corrosion rates were measured by collecting hydrogen produced by the anaerobic corrosion of iron. In addition, a range of analytical techniques was applied to study the composition and morphology of the corrosion products and the distribution and chemical state of the iron released into the bentonite. Comparison was also made between corrosion in compacted bentonite and artificial bentonite porewater. In the presence of bentonite, the corrosion product layer was relatively thin compared to fully aqueous conditions, probably because the ferrous ions released by corrosion exchanged with the bentonite interlayer or attached to the surface of the bentonite grains, rather than forming a separate iron oxide phase.

Modeling porosity reductions caused by mineral fouling in continuous-wall permeable reactive barriers

A study was conducted to assess key factors to include when modeling porosity reductions caused by mineral fouling in permeable reactive barriers (PRBs) containing granular zero valent iron. The public domain codes MODFLOW and RT3D were used and a geochemical algorithm was developed for RT3D to simulate geochemical reactions occurring in PRBs. Results of simulations conducted with the model show that the largest porosity reductions occur between the entrance and mid-plane of the PRB as a result of precipitation of carbonate minerals and that smaller porosity reductions occur between the mid-plane and exit face due to precipitation of ferrous hydroxide. These findings are consistent with field and laboratory observations, as well as modeling predictions made by others. Parametric studies were conducted to identify the most important variables to include in a model evaluating porosity reduction. These studies showed that three minerals (CaCO 3, FeCO 3, and Fe(OH) 2 (am)) account for more than 99% of the porosity reductions that were predicted. The porosity reduction is sensitive to influent concentrations of HCO 3 −, Ca 2+, CO 3 2−, and dissolved oxygen, the anaerobic iron corrosion rate, and the rates of CaCO 3 and FeCO 3 formation. The predictions also show that porosity reductions in PRBs can be spatially variable and mineral forming ions penetrate deeper into the PRB as a result of flow heterogeneities, which reflects the balance between the rate of mass transport and geochemical reaction rates. Level of aquifer heterogeneity and the contrast in hydraulic conductivity between the aquifer and PRB are the most important hydraulic variables affecting porosity reduction. Spatial continuity of aquifer hydraulic conductivity is less significant.

Reductive Dehalogenation of Chlorinated Ethenes with Elemental Iron: The Role of Microorganisms

Trichloroethne (TCE) transformation and the product distribution in an aqueous medium containing zero-valent iron (Fe(0)) was investigated in the presence of an anaerobic mixed culture to assess the potential role of microorganisms in permeable iron barriers. The presence of the culture increased the rate of TCE disappearance and changed the product distribution. Rapid formation and degradation of cis-dichloroethene ( cis-DCE) was observed in reactors containing cells plus Fe(0) or H 2 as a bulk reducing agent. High levels of vinyl chloride (VC) were formed and very similar profiles were obtained in the Fe(0) plus cell and H 2 plus cell reactors, but not in Fe(0)-only reactors. The similar trends observed in Fe(0)–cell and H 2–cell reactors suggest that most cis-DCE and VC in the Fe(0)–cell reactors were produced and transformed biologically rather than abiotically. Accumulation of methane in the Fe(0)–cell system indicates that hydrogen gas generated during anaerobic iron corrosion could support a methanogenic culture. Digital confocal images showed that the microorganisms were able to colonize the iron surface. The results suggest that potential development of dechlorinating populations in Fe(0) barriers may alter the TCE reduction pathway and produce VC, which would have significant impact on the performance of Fe(0) barriers.

Research on the mechanisms of anaerobic corrosion

Investigations of anaerobic corrosion of iron over the past years have led to a number of conclusions regarding the mechanisms. In addition several unusual observations related to these corrosion studies were noted. The cathodic depolarization mechanism does not appear too significant. The primary cause appears to be the production of a volatile phosphorous compound by SRB which reacts with iron to form a black precipitate in the medium. The highly reactive phosphorous compound is produced from an organic compound, inositol hexaphosphate, the major source of phosphorous in plants. Other findings include “electrochemical noise”, “hollow whiskers”, and “rusticles”, FeS and Fe 3P in tektites and a most unusual attack on glass.

Mechanism of oxide film formation on iron in simulating groundwater solutions: Raman spectroscopic studies

In the use of iron for reductive dehalogenation of chlorinated solvents in ground water, the formation of surface films may cause long-term problems by reducing the activity of the metal surfaces or by causing the clogging of pores. Normal (NRS) and enhanced (SERS) Raman spectroscopy was used to identify the surface film(s) formed during contact of iron particles with solvent-free simulated ground water solutions in column experiments. It was found that anaerobic corrosion of iron leads to the initial formation of ferrous hydroxide at the beginning of the reaction. Independent of the ground water composition, however, the final corrosion product is magnetite. The spontaneous (no current applied) formation of magnetite takes place by a dissolution/precipitation mechanism with the separation of anodic and cathodic sites across the surface film. The cathodic reaction, which takes place at the porous film/solution interface, requires the film to be electron conducting.