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U.S. Department of Energy

Unraveling Biogeochemical Pathways Challenge

Synopsis

This challenge is motivated by the recognition that sustainable bioremediation requires a predictive understanding of the governing complex biogeochemical pathways. Research in this challenge will focus on characterizing critical and interrelated microbial metabolic and geochemical mechanisms associated with Chromium biostimulation and reoxidation from the molecular to the local (push-pull) field scale, as needed to assess long-term sustainability of Chromium biostimulation.
Chromium Reduction and Oxidation - click for complete image

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Background

Chromium contamination in groundwater is widespread within the DOE complex, including in the subsurface at Hanford, Idaho National Laboratory, Savannah River, Pantex Plant, and Lawrence Livermore National Laboratory sites (DOE Groundwater Data Base, 2003). The total volume of DOE Cr-contaminated groundwater is estimated to be 1.3x1010 gal, and the maximum mass of Cr is estimated to be 2.8x105 kg (Hazen et al., 2008). At the Hanford 100 Area, the volume of Cr-contaminated groundwater is estimated to be 1.5x109 gal, and the maximum mass of Cr is estimated to be 1x104 kg.

Hanford 100

Map of Cr concentration in groundwater
at Hanford 100H and 100D areas, both located
near the Columbia River (from PNNL, 2007).

The 100-D Area aquifer has the highest concentrations of chromium at Hanford (>2,000 ppb). Cr(VI) concentrations in groundwater samples near the Columbia River shore are at levels greater than 200 ppb, which is more than 20 times the aquatic standard, and more than 10 times the remedial action objective of 20 ppb. Hanford's goal is to decrease the Cr concentration to 20 ppb in the monitoring wells located 40 feet from the river. The Hanford 100-H area is located along the Cr(VI)-contaminated groundwater pathway from the Hanford 100-D site to the Columbia River. Over the last several years, LBNL has led an experiment at the 100H Area to evaluate the ability of a polylactate electron donor called Hydrogen Release Compound (HRC) to stimulate microbiological activity, thereby creating a reducing environment that will stimulate the transformation of Cr(VI) to Cr(III). The results of in-situ bioimmobilization of Cr(VI) in groundwater showed that the injected HRC reduced Cr(VI) concentrations in the monitoring well decreased to below the drinking-water MCL and remained at this level for about 2 years. Details of the biostimulation experiment were described by Faybishenko et al. (2008) and of the biostimulation monitoring using integrated geophysical and geochemical measurements were described by Hubbard et al. (2008).

Hanford 100

Chromium exists in the environment primarily in one of two oxidation states, Cr(III) or Cr(VI). Cr(III) is usually found as (hydr)oxides that are sparingly soluble, and therefore immobile and relatively non-toxic. In contrast, Cr(VI) exists as the chromate oxyanion (CrO42-), which is highly soluble, mobile, toxic, and carcinogenic. Cr(VI) can be reduced to Cr(III) by Fe2+, S2-, and organic carbon, but of these reactants, Fe2+ is the kinetically favored reductant above pH ~5.5. The products of Cr(VI) reduction are generally Cr (hydr)oxides in the case of reduction by organic matter and S2-, or mixed Cr and Fe (hydr)oxides in the case of reduction by Fe2+. Reduction by Fe2+ has been shown to be significantly enhanced by adsorption onto the Fe(III) mineral surfaces. Similarly, Cr(VI) is rapidly reduced at the surface of amorphous FeS, mackinawite (FeS), pyrite (FeS2), and Fe(II)-containing minerals such as siderite and green rust. Interestingly, at least in the case of pyrite, it appears to be only the Fe(II) as opposed to the disulfide centers that act as a direct reductant of Cr(VI). It is not clear whether this is also the case with the FeS minerals. The importance of surface-based as opposed to aqueous phase reduction is difficult to estimate given the uncertainty in estimating the effective near-surface concentrations of the reactants, i.e., chromate adsorption capacity, and the tendency for surfaces to become passivated. All mineral surface reactions are subject to some degree of passivation by precipitation of CrxFe3-x(OH)3 after reduction of Cr(VI). Surface passivation tends to decrease with decreasing pH and decreasing Cr(VI)/mineral surface area ratios. Similarly the point of zero charge will impact the importance of surface-based Cr(VI) reduction by controlling the near-surface concentrations of Cr(VI). The reported zero point of charge is quite low for pyrite, pH =1.2-1.4, but is significantly higher for disordered mackinawite (pH = 7.5). Therefore, FeS is likely to be a more significant sink for the negatively charged chromate at moderately acid pH values.

Hanford 100

Once reduced to Cr(III), the re-oxidation of Cr(III) to Cr(VI) by molecular O2 and other common oxidants is kinetically limited, in spite of the fact that chromate is the thermodynamically stable form of Cr in oxidized environments. This kinetic limitation on oxidation is crucial from a remediation perspective because it means that, unlike less kinetically hindered elements like U, it is feasible to maintain the vast majority of Cr in the reduced Cr(III) state even under transient or predominantly aerobic conditions. In fact, under moderate pH conditions, manganese oxides appear to be the only significant oxidizers of Cr(III), making control of Mn geochemistry, as opposed to control of overall redox potential, the key to limiting re-oxidation of Cr(III). Although many different manganese oxides have been shown to oxidize Cr(III), the most common natural oxides are of mixed valence, containing both Mn(III) and Mn(IV).

Hypotheses

Although the LBNL biostimulation at the 100H site demonstrated that Cr(VI) reduction is attainable, many questions about the mechanisms and sustainability of the treatment remain; these questions motivate the research in this challenge. In order to implement sustained chromium reductive immobilization in the Hanford 100H aquifer, it is necessary to develop a conceptual model that integrates flow and transport mechanisms with the effects of complex and interrelated microbial metabolic and geochemical processes, including direct (enzymatic) and indirect (secondary abiotic) Cr(VI) reduction, as well as re-oxidation under aerobic or anaerobic conditions. This challenge will combine a variety of state-of-the art approaches, including a new systems biology technique to assess whole-community gene expression, and the novel use of biomolecular, spectroscopic and isotopic signatures in reactive transport models to study key processes at scales encompassing the molecular scale, the pore scale, and the local field scale. The initial model within the Systems Framework is defined by the following three hypotheses:

  • Hypothesis 1 - Microbial processes mediate both direct (enzymatic) and indirect Cr(VI) reduction at Hanford 100H, but indirect pathways dominate sustained reduction. Furthermore, sulfate reduction is the electron-accepting process ultimately driving sustained Cr(VI) reduction at Hanford 100H.
  • Hypothesis 2 – The rate and extent of Cr(III) (re)oxidation will be controlled by the abundance and mineral form of Mn (III/IV) oxides in the sediment.
  • Hypothesis 3 - Fermentative/acetogenic versus respiratory metabolism will promote retention of organic carbon in the aquifer.

Expected Outcomes

Through research conducted in this challenge, we expect to greatly improve our understanding of the complex interactions between microbiological and geochemical processes mediating chromium reduction and re-oxidation under conditions representative of those in the Hanford 100H subsurface environment. Specific examples of expected outcomes are given below.

LBNL Research Team & Collaborators

The LBNL challenge team consists of the following interdisciplinary group of researchers with expertise in microbiology, isotope geochemistry, hydrology, surface chemistry and synchrotron methods, and reactive transport modeling.

This challenge will significantly leverage on activities conducted by the Arkin/Hazen Genomics:GtL project, which is using a systems biology approach to investigating the responses of model bacteria to environmental stressors (e.g., nitrate, oxygen) during push-pull tests at the Hanford 100H site. Phil Long at PNNL collaborates with the LBNL challenge team, and the team will also communicate regularly with Mike Truex and his colleagues at PNNL who have been testing the longevity and radius of influence of other electron donors (molasses, vegetable oil) for reducing Cr(III) at the Hanford 100D site.