Rasmus Jakobsen

Characterization of redox conditions in groundwater contaminant plumes.

Evaluation of redox conditions in groundwater pollution plumes is often a prerequisite for understanding the behaviour of the pollutants in the plume and for selecting remediation approaches. Measuring of redox conditions in pollution plumes is, however, a fairly recent issue and yet relative few cases have been reported. No standardised or generally accepted approach exists. Slow electrode kinetics and the common lack of internal equilibrium of redox processes in pollution plumes make, with a few exceptions, direct electrochemical measurement and rigorous interpretation of redox potentials dubious, if not erroneous. Several other approaches have been used in addressing redox conditions in pollution plumes: redox-sensitive compounds in groundwater samples, hydrogen concentrations in groundwater, concentrations of volatile fatty acids in groundwater, sediment characteristics and microbial tools, such as MPN counts, PLFA biomarkers and redox bioassays. This paper reviews the principles behind the different approaches, summarizes methods used and evaluates the approaches based on the experience from the reported applications.

Redox zonation: Equilibrium constraints on the Fe(III)/SO4-reduction interface

Abstract The concept of redox zonation during degradation of organic matter, which is usually explained by the overall energy yield of different reactions, has been reevaluated. At least for reduction of Mn(IV), Fe(III), sulfate, and methanogenesis, the sequential occurrence of these processes is much easier explained by a partial equilibrium approach where the fermentive step is overall rate limiting, while the electron accepting processes are considered to be close to equilibrium. Using the partial equilibrium approach, an explanation is sought for the simultaneous occurrence of Fe(III) and sulfate reduction, observed in several field studies. Calculations of conditions for equilibrium between Fe(III) and sulfate reduction indicate that, depending on the stability of the iron oxide, simultaneous reduction of Fe(III) and sulfate is thermodynamically possible under a wide range of sedimentary conditions and sulfate reduction may even occur before Fe(III) reduction. In Fe2+-rich environments, the pH of the porewater has in addition a strong influence on whether Fe(III) or sulfate reduction is favored. In natural sediments, the presence of a wide range of iron oxide stabilities is likely to cause considerable overlap between zones of Fe(III) and sulfate reduction, while a better confined stability range of iron oxides should cause more distinct zones of Fe(III) and sulfate reduction.

REDOX ZONING, RATES OF SULFATE REDUCTION AND INTERACTIONS WITH FE-REDUCTION AND METHANOGENESIS IN A SHALLOW SANDY AQUIFER, ROMO, DENMARK

Abstract Fe-oxide reduction, sulfate reduction and methanogenesis, have been studied in a shallow aquifer with the main focus on sulfate reduction. Direct measurements of sulfate reduction rates have for the first time been applied in an aquifer system. Rates were much lower than reported in other anoxic environments - three orders of magnitude lower than in marine settings and one order of magnitude lower than in lacustrine environments, and varied substantially mainly due to differences in the reactivity of the organic matter. At the extremely low substrate levels in the aquifer, sulfate reduction rates are not primarily limited by low sulfate concentrations. The produced sulfide forms framboidal pyrite via a FeS precursor, with elemental sulfur as an intermediate. Fe-oxide reduction rates were comparable to sulfate reduction rates, but appeared to depend more on Fe-oxide reactivity than organic matter reactivity. Low sulfate concentrations, combined with low-reactivity Fe(III), in the aquifer sediment, has led to an increased appreciation of the existence of concomitant redox processes. This indicates that competitive exclusion is not always effective, and raises questions as to what H 2 data reflect in such a system. Calculations of the in situ energy yield for Fe- and sulfate reduction via H 2 oxidation are ∼2.5 kcal/mol H 2 , indicating that thermodynamic equilibrium is approached. The calculated available energy yield for methanogenesis was very low indicating that CH 4 production must occur in micro-environments where higher H 2 concentrations prevail. The system may be described as being in a state of partial equilibrium, where the overall rate of organic matter oxidation is controlled by the rate of fermentation of the organic matter, and terminal electron acceptor processes occur at close to equilibrium conditions . This partial equilibrium depends on other processes in the system, in this case an increase in pH due to calcite dissolution appears to induce a shift from predominantly Fe-reducing to predominantly sulfate reducing conditions by changing the energy available to Fe-oxide reduction. Numerical modeling using the partial equilibrium approach was successful in modeling this complex of interacting processes.

Formation and solid solution behavior of Ca-rhodochrosites in marine muds of the Baltic deeps

Authigenic Ca-rhodochrosites are found in organic-rich sediments in the deep anoxic basins of the Baltic Sea. Rhodochrosite formation is apparently the result of organic matter degradation. The rhodochrosite contains 10 to 40 mol% CaCO3 and 2 to 5 mol% MgCO3, as determined by microprobe. The composition of the carbonates at a given depth varies by about 5 mol% MnCO3. SEM revealed the rhodochrosite to be present as globular aggregates consisting of microcrystallites. Dissolution features such as cavities within the aggregates are frequently observed and indicate that extensive recrystallization takes place. Pore waters are greatly supersaturated with respect to both rhodochrosite and calcite. Rhodochrosite is only found in sediments where pore waters show the highest degree of supersaturation. Due to ion exchange, Ca2+ diffuses from the underlying freshwater clays. This results in increasing aCa2+aMn2+ ratios in the pore waters from the sediment surface downward. In response to this, the maximum CaCO3MnCO3 ratio also increases with depth. The composition of rhodochrosite which forms under these conditions may be determined by both the porewater aCa2+aMn2+ and precipitation kinetics. However, if their composition is controlled solely by the aCa2+aMn2+ in solution, their behavior can be described by a regular solid solution model.

In situ rates of sulfate reduction in an aquifer (Rømø, Denmark) and implications for the reactivity of organic matter

Estimates of rates of organic matter degradation by bacterial sulfate reduction in aquifers are few, and all are obtained by indirect means. Here we present the first direct radiotracer measurements of in situ rates of sulfate reduction for an aquifer (Romo, Denmark). Sulfate reduction occurs in distinct reaction zones within the aquifer at rates that vary greatly over short lateral distances. In situ rates of sulfate reduction are significantly higher than previous indirect estimates. In contrast, rates of sulfate reduction in aquifers are orders of magnitude lower than in marine and limmic environments, emphasizing the low reactivity of natural organic matter in aquifers.

Methanogenesis in a shallow sandy aquifer, Rømø, Denmark

Abstract The degradation of organic matter and the formation of methane were investigated in a shallow sandy aquifer. The aquifer was found to be anoxic from the water table downward; the upper 2 m contained sulfate and was enriched in Fe(II). Methane was present in the groundwater from 2 to 3 m below the water table in concentrations of up to 0.4 mM. Fermentative metabolic intermediates such as acetate and formate were present at levels of a few micromoles, whereas hydrogen concentrations ranged from 0.1 to 8 nM. Radiotracer methods were used to quantify organic matter degradation rates. In the upper part of the aquifer, rates of acetate oxidation of up to 4 mM/yr were measured in the same zone where sulfate reduction and the reduction of iron oxides takes place. Total methane formation rates range from 0.1 to 4 mM/yr and proceeds through both the pathway of CO2 reduction and acetate fermentation. CO2 reduction was found to be the dominant pathway, although in some cases acetate fermentation contributed up to 50% of the total methane formation rate. High spatial variation, both vertically and horizontally, in methane formation rates are a characteristic feature of this aquifer sediment. Therefore the groundwater methane concentration is not a reliable indicator for the occurrence and intensity of methanogenesis at a detailed scale. Methane stable isotope data yielded values between −80 and −50‰ for δ13CCH4, and the few available measurements for δDCH4 are in the range of −320 to −300‰. The usual interpretation of the stable isotope data would then suggest acetate fermentation to be the dominant pathway for methanogenesis, in conflict with the radiotracer data. However, recent evidence suggests the deuterium content of the groundwater to have a dominant effect on the deuterium content of methane rather than the pathway of methane formation. Comparison of the depth distribution of the rates of sulfate reduction and methane formation with the H2 concentration shows that the latter is not a reliable indicator of the predominant terminal electron acceptor process. The free energy of reaction was calculated for different substrates and electron acceptors. The results indicate that the free energy gains are well constrained by bacterial metabolism and are close to the threshold for energy storage. However, for CO2 reduction, the free energy gain is below the energy storage threshold, which suggests methane formation predominantly occurs in microenvironments with higher H2 concentrations.

Review of reactive kinetic models describing reductive dechlorination of chlorinated ethenes in soil and groundwater.

Reductive dechlorination is a major degradation pathway of chlorinated ethenes in anaerobic subsurface environments, and reactive kinetic models describing the degradation process are needed in fate and transport models of these contaminants. However, reductive dechlorination is a complex biological process, where many microbial populations including dechlorinating, fermentative, methanogenic, iron and sulfate reducing, interact. In this article the modeling approaches and the experimental data needed to calibrate them are reviewed, classified, and discussed. Model approaches considered include first order kinetics, Monod kinetics to describe sequential reductive dechlorination and bacterial growth, and metabolic models which simulate fermentation and redox processes interacting with reductive dechlorination processes. The review shows that the estimated kinetic parameters reported vary over a wide range, and that experimental microbial data are scarce. Very few studies have been performed evaluating the influence of sulfate and iron reduction, and contradictory conclusions on the interaction of redox processes with reductive dechlorination have been reported. The modeling approaches for metabolic reductive dechlorination employing different descriptions of the interaction between redox and dechlorination processes and competition for hydrogen are classified. The current concepts lead to different results, suggesting a need for further investigations on the interactions between the microbial communities performing dechlorination and redox processes, including the establishment of biomarkers quantifying dechlorination, and on geochemical characterization. Finally, the relevance of laboratory data and the development of practical modeling tools for field applications are discussed. Biotechnol. Bioeng. 2013; 110: 1–23.

Methanosarcina spp. drive vinyl chloride dechlorination via interspecies hydrogen transfer

ABSTRACT Two highly enriched cultures containing Dehalococcoides spp. were used to study the effect of aceticlastic methanogens on reductive vinyl chloride (VC) dechlorination. In terms of aceticlastic methanogens, one culture was dominated by Methanosaeta, while the other culture was dominated by Methanosarcina, as determined by fluorescence in situ hybridization. Cultures amended with 2-bromoethanesulfonate (BES), an efficient inhibitor of methanogens, exhibited slow VC dechlorination when grown on acetate and VC. Methanogenic cultures dominated by Methanosaeta had no impact on dechlorination rates, compared to BES-amended controls. In contrast, methanogenic cultures dominated by Methanosarcina displayed up to sevenfold-higher rates of VC dechlorination than their BES-amended counterparts. Methanosarcina-dominated cultures converted a higher percentage of [2-14C]acetate to 14CO2 when concomitant VC dechlorination took place, compared to nondechlorinating controls. Respiratory indices increased from 0.12 in nondechlorinating cultures to 0.51 in actively dechlorinating cultures. During VC dechlorination, aqueous hydrogen (H2) concentrations dropped to 0.3 to 0.5 nM. However, upon complete VC consumption, H2 levels increased by a factor of 10 to 100, indicating active hydrogen production from acetate oxidation. This process was thermodynamically favorable by means of the extremely low H2 levels during dechlorination. VC degradation in nonmethanogenic cultures was not inhibited by BES but was limited by the availability of H2 as electron donor, in cultures both with and without BES. These findings all indicate that Methanosarcina (but not Methanosaeta), while cleaving acetate to methane, simultaneously oxidizes acetate to CO2 plus H2, driving hydrogenotrophic dehalorespiration of VC to ethene by Dehalococcoides.

Amendment of arsenic and chromium polluted soil from wood preservation by iron residues from water treatment.

An iron-rich water treatment residue (WTR) consisting mainly of ferrihydrite was used for immobilization of arsenic and chromium in a soil contaminated by wood preservatives. A leaching batch experiment was conducted using two soils, a highly contaminated soil (1033 mg kg(-1) As and 371 mg kg(-1) Cr) and slightly contaminated soil (22 5mg kg(-1) As and 27 mg kg(-1) Cr). Compared to an untreated reference soil, amendment with 5% WTR reduced leaching in the highly contaminated soil by 91% for Cr and 98% for As. No aging effect was observed after 103 d. In a small field experiment, soil was mixed with 2.5% WTR in situ. Pore water was extracted during 3 years from the amended soil and a control site. Pore water arsenic concentrations in the amended soil were more than two orders of magnitude lower than in the control for the upper samplers. An increased release of arsenic was observed during winter in both fields, mostly in the deepest samplers. This is likely due to the formation of a pseudo-gley because of precipitation surplus. Stabilization of arsenic and chromium contaminated soil using WTR is a promising method but the transformation of ferrihydrite in soil proves a concern in case of waterlogged soils. Still the amendment minimized the leaching of arsenic, even in cases of seasonal releases.

Hydrogeochemical and mineralogical effects of sustained CO2 contamination in a shallow sandy aquifer: A field‐scale controlled release experiment

A shallow aquifer CO2 contamination experiment was performed to investigate evolution of water chemistry and sediment alteration following leakage from geological storage by physically simulating a leak from a hypothetical storage site. In a carbonate-free aquifer, in western Denmark, a total of 1600 kg of gas phase CO2 was injected at 5 and 10 m depth over 72 days through four inclined injection wells into aeolian and glacial sands. Water chemistry was monitored for pH, EC, and dissolved element evolution through an extensive network of multilevel sampling points over 305 days. Sediment cores were taken pre and postinjection and analyzed to search for effects on mineralogy and sediment properties. Results showed the simulated leak to evolve in two distinct phases; an advective elevated ion pulse followed by increasing persistent acidification. Spatial and temporal differences in evolution of phases suggest separate chemical mechanisms and geochemical signatures. Dissolved element concentrations developed exhibiting four behaviors: (1) advective pulse (Ca, Mg, Na, Si, Ba, and Sr), (2) pH sensitive abundance dependent (Al and Zn), (3) decreasing (Mn and Fe), and (4) unaffected (K). Concentration behaviors were characterized by: (1) a maximal front moving with advective flow, (2) continual increase in close proximity to the injection plane, (3) removal from solution, and (4) no significant change. Only Al was observed to exceed WHO guidelines, however significantly so (10-fold excess). The data indicate that pH is controlled by equilibrium with gibbsite which is again coupled to cation exchange processes. Pre and postinjection sediment analysis indicated alteration of sediment composition and properties including depletion of reactive mineral species.