Considering a Threshold Energy in Reactive Transport Modeling of Microbially Mediated Redox Reactions in an Arsenic-Affected Aquifer

Abstract

Abstract:The reductive dissolution of Fe-oxide driven by organic matter oxidation is the primary mechanism accepted for As mobilization in several alluvial aquifers. These processes are often mediated by microorganisms that require a minimum Gibbs energy available to conduct the reaction in order to sustain their life functions. Implementing this threshold energy in reactive transport modeling is rarely used in the existing literature. This work presents a 1D reactive transport modeling of As mobilization by the reductive dissolution of Fe-oxide and subsequent immobilization by co-precipitation in iron sulfides considering a threshold energy for the following terminal electron accepting processes: (a) Fe-oxide reduction, (b) sulfate reduction, and (c) methanogenesis. The model is then extended by implementing a threshold energy on both reaction directions for the redox reaction pairs Fe(III) reduction/Fe(II) oxidation and methanogenesis/methane oxidation. The optimal threshold energy fitted in 4.50, 3.76, and 1.60 kJ/mol e − for sulfate reduction, Fe(III) reduction/Fe(II) oxidation, and methanogenesis/methane oxidation, respectively. The use of models implementing bidirectional threshold energy is needed when a redox reaction pair can be transported between domains with different redox potentials. This may often occur in 2D or 3D simulations.

Overlapping redox zones control arsenic pollution in Pleistocene multi-layer aquifers, the Po Plain (Italy)

“…S3) for groundwaters with Feoxide reduction and methanogenesis. Fe-oxide reduction and methanogenesis were previously reported to occur concomitantly near equilibrium in reducing groundwaters worldwide (Jakobsen and Cold, 2007;Postma et al, 2007;Zhou et al, 2014) and simulated through modelling (Jakobsen, 2007;Rotiroti et al, 2018). Their simultaneous occurrence could be the result of a mutualistic relationship between Fe-reducing bacteria and methanogens aided by the precipitation of siderite (Jakobsen and Cold, 2007).…”

“…Fig. 2b shows computed values of Gibbs free energy of reaction at our system conditions (ΔG r ) for Fe-oxide reduction (considering the various oxides at different stabilities listed in Table S2), sulfate reduction and methanogenesis, compared to a range of threshold energy values (ΔG min ; representing the energy level maintained by the microbes) taken from literature (Hoehler, 2004;Jakobsen and Cold, 2007;Rotiroti et al, 2018), so that an estimation of the usable energy (ΔG r − ΔG min ) can be given. The usable energy for all the three TEAPs (considering the hypothetical medium-stable oxide for Fe-reduction) results in the order of a few kJ/mol per H 2 (Fig.…”

Natural attenuation of pools and plumes of carbon tetrachloride and chloroform in the transition zone to bottom aquitards and the microorganisms involved in their degradation

“…To assess the dominant redox conditions in the subsurface of the studied site, the guidelines established by Christensen et al (2000) and subsequently implemented by many other authors (Rotiroti et al, 2018;Weatherill et al, 2018) were followed in the current work. They consist in characterizing the dominant redox processes in groundwater, which allowed identification of the dominant redox conditions under which these processes occurred.…”

Geochemical Effects of Millimolar Hydrogen Concentrations in Groundwater: An Experimental Study in the Context of Subsurface Hydrogen Storage

“…In pristine aquifers, fermentative degradation of peptides, saccharides, and further organic substances produces hydrogen, while other microorganisms consume it, usually resulting in nanomolar concentrations of dissolved hydrogen in a steady state. − A competition for hydrogen may then result in the development of a typical redox sequence (i.e., decreasing redox potential with increasing time or flow path length) because the electron accepting processes coupled to hydrogen oxidation with the higher energy yield (e.g., sulfate reduction) outcompete the others (e.g., acetogenesis) by keeping the hydrogen concentrations below a threshold concentration required for that process (Table ). , Among other possibilities, such reactive environments may be numerically simulated assuming a dual substrate limitation by hydrogen concentration on the one hand and the concentration of sulfate or total dissolved inorganic carbon species on the other. Furthermore, not only the occurrence but also the kinetics of the redox reactions is usually dependent on the concentration of dissolved hydrogen in those hydrogen-limited reactive environments, and such processes are often modeled using Monod kinetics. − …”