S. Sevinç Şengör

Reoxidation of Biogenic Reduced Uranium: A Challenge Toward Bioremediation

Uraninite (UO2) is the most desirable end product of in situ bioreduction because of its low solubility under reducing conditions. For effective long-term immobilization of uranium (U), there should be no biotic or abiotic reoxidation of the insoluble biogenic U(IV). It is therefore critical to understand the long-term stability of U(IV) under oxic- and nutrient-limited conditions at U-contaminated subsurface sites. It has now been established that following in situ bioremediation of U(VI) via nutrient addition in the subsurface, a range of physical, chemical, and biological factors control the rate and extent of long-term stability of U(IV). Some of these factors are tied to site specific conditions including existence of oxidants such as Fe(III)(hydr)oxides, Mn(IV) oxides, oxygen, and nitrate; the presence of organic carbon and the reduced forms of U (e.g., mononuclear U(IV) or nanometer-sized uraninite particles); and the carbonate concentration and pH of groundwater. This review analyzes the contribution of these factors in controlling U(IV)-reoxidation, and highlights the competition among U(IV) and other electron acceptors and possible mechanisms of reoxidation of various forms of U(IV).

A reactive transport benchmark on heavy metal cycling in lake sediments

Sediments are active recipients of anthropogenic inputs, including heavy metals, but may be difficult to interpret without the use of numerical models that capture sediment-metal interactions and provide an accurate representation of the intricately coupled sedimentological, geochemical, and biological processes. The focus of this study is to present a benchmark problem on heavy metal cycling in lake sediments and to compare reactive transport models (RTMs) in their treatment of the local-scale physical and biogeochemical processes. This benchmark problem has been developed based on a previously published reactive-diffusive model of metal transport in the sediments of Lake Coeur d’Alene, Idaho. Key processes included in this model are microbial reductive dissolution of iron hydroxides (i.e., ferrihydrite), the release of sorbed metals into pore water, reaction of these metals with biogenic sulfide to form sulfide minerals, and sedimentation driving the burial of ferrihydrite and other minerals. This benchmark thus considers a multicomponent biotic reaction network with multiple terminal electron acceptors (TEAs), Fickian diffusive transport, kinetic and equilibrium mineral precipitation and dissolution, aqueous and surface complexation, as well as (optionally) sedimentation. To test the accuracy of the reactive transport problem solution, four RTMs—TOUGHREACT (TR), CrunchFlow (CF), PHREEQC, and PHT3D—have been used. Without sedimentation, all four models are able to predict similar trends of TEAs and dissolved metal concentrations, as well as mineral abundances. TR and CF are further used to compare sedimentation and compaction test cases. Results with different sedimentation rates are captured by both models, but since the codes do not use the same formulation for compaction, the results differ for this test case. Although, both TR and CF adequately capture the trends of aqueous concentrations and mineral abundances, the difference in results highlights the need to consider further the conceptual and numerical models that link transport, biogeochemical reactions, and sedimentation.

A reactive transport benchmark on modeling biogenic uraninite re-oxidation by Fe(III)-(hydr)oxides

A reactive transport benchmark on uranium (U) bioreduction and concomitant reoxidation has been developed based on the multicomponent biogeochemical reaction network presented by Spycher et al. (Geochim Cosmochim Acta 75:4426–4440, 2011). The benchmark problem consists of a model inter-comparison starting with the numerical simulations of the original batch experiments of Sani et al. (Geochim Cosmochim Acta 68:2639–2648, 2004). The batch model is then extended to 1D and 2D reactive transport models, designed to evaluate the model results for the key biogeochemical reaction processes and their coupling with physical transport. Simulations are performed with four different reactive transport simulators: PHREEQC, PHT3D, MIN3P, and TOUGHREACT. All of the simulators are able to capture the complex biogeochemical reaction kinetics and the coupling between transport and kinetic reaction network successfully in the same manner. For the dispersion-free variant of the problem, a 1D-reference solution was obtained by PHREEQC, which is not affected by numerical dispersion. PHT3D using the sequential non-iterative approach (SNIA) with an explicit TVD scheme and MIN3P using the global implicit method (GIM) with an implicit van Leer flux limiter provided the closest approximation to the PHREEQC results. Since the spatial weighting schemes for the advection term and numerical dispersion played an important role for the accuracy of the results, the simulators were further compared using different solution schemes. When all codes used the same spatial weighting scheme with finite-difference approximation, the simulation results agreed very well among all four codes. The model intercomparison for the 2D-case demonstrated a high level of sensitivity to the mixing of different waters at the dispersive front. Therefore this benchmark problem is well-suited to assess code performance for mixing-controlled reactive transport models in conjunction with complex reaction kinetics.

A uranium bioremediation reactive transport benchmark

A reactive transport benchmark problem set has been developed based on in situ uranium bio-immobilization experiments that have been performed at a former uranium mill tailing site in Rifle, CO, USA. Acetate-amended groundwater stimulates indigenous microorganisms to catalyze the reduction of U(VI) to a sparingly soluble U(IV) mineral. The interplay between the flow, acetate loading periods and rates, and microbially mediated and geochemical reactions leads to dynamic behavior in metal- and sulfate-reducing bacteria, pH, alkalinity, and reactive mineral surfaces. The benchmark is based on an 8.5 m long one-dimensional model domain with constant saturated flow and uniform porosity. The 159-day simulation introduces acetate and bromide through the upgradient boundary in 14- and 85-day pulses separated by a 10 day interruption. Acetate loading is tripled during the second pulse, which is followed by a 50 day recovery period. Terminal electron-accepting processes for goethite, phyllosilicate Fe(III), U(VI), and sulfate are modeled using Monod-type rate laws. Major ion geochemistry modeled includes mineral reactions as well as aqueous and surface complexation reactions for UO22+

Synthesis of graphene oxide/magnesium oxide nanocomposites with high-rate adsorption of methylene blue

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Biogeochemical reactive–diffusive transport of heavy metals in Lake Coeur d’Alene sediments

, Fe2+, and H+. In addition to the dynamics imparted by the transport of the acetate pulses, U(VI) behavior involves the interplay between bioreduction, which is dependent on acetate availability, and speciation-controlled surface complexation, which is dependent on pH, alkalinity, and available surface complexation sites. The general difficulty of this benchmark is the large number of reactions (74), multiple rate law formulations, a multisite uranium surface complexation model, and the strong interdependency and sensitivity of the reaction processes. Results are presented for three simulators: HYDROGEOCHEM, PHT3D, and PHREEQC.