Modeling Kinetic Processes Controlling Hydrogen and Acetate Concentrations in an Aquifer-Derived Microcosm

Abstract

Groundwater contaminants may degrade via fermentation to intermediate species, which are subsequently consumed by terminal electron-accepting processes (TEAPs). A numerical model of an aquifer-derived laboratory microcosm is developed to simulate the dynamic behavior of fermentation and respiration in groundwater by coupling microbial growth and substrate utilization kinetics with a formulation that also includes aqueous speciation and other geochemical reactions including surface complexation, mineral dissolution, and precipitation. The model is used to test approaches that currently make use of H2(aq) to diagnose prevalent TEAPs in groundwater. Competition between TEAPs is integral to the conceptual model of the simulation, and the results indicate that competitive exclusion is significant but with some overlap found in the temporal sequence of TEAPs. Steady-state H2(aq) concentrations observed during different TEAPs do not differ significantly. The results are not consistent with previous applications of the partial equilibrium approach since most TEAP redox pairs exhibit free energies that suggest a particular process is able to proceed, yet observations here show that this process does not proceed.

Microbiological Controls on Geochemical Kinetics 1: Fundamentals and Case Study on Microbial Fe(III) Oxide Reduction

“…From a mechanistic standpoint, it makes sense that a kinetic model that includes microbial reactions should include the morgs themselves together with the factors that determine their abundance and metabolic rates (Rittmann and VanBriesen, 1996); otherwise rates of energy metabolism and associated geochemical processes (e.g., mineral dissolution and precipitation) are functionally disconnected from the temporal/spatial evolution of the microbial communities responsible for those processes. Since the next wave of microbial reaction models is likely to include multiple groups of interacting populations with different physiological properties (Rittmann and McCarty (2001); see Watson et al (2003), Wirtz (2003), and Maurer and Rittmann (2004) for recent examples), we examine the basic framework for depicting competition and population dynamics in some detail below, using sediment TEAPs as an example. In this case, the kinetic expressions depict utilization of dissolved monomeric substrates such as acetate or H 2 , which are in fact the major substrates for TEAPs in anaerobic sediments (Christensen, 1984;Lovley and Klug, 1982;Lovley and Phillips, 1989;Sorensen et al, 1981).…”

Hydrogen as an Indicator to Assess Biological Activity During Trace-Metal Bioremediation

“…Simultaneous iron and sulfate reduction has also been measured in a long-term batch experiment using aquifer sediments (Watson et al 2003) in which H 2 concentrations during iron reduction were not always in the range typically reported for iron reducing conditions. Though amorphous iron has been shown to inhibit sulfate reduction and methanogenesis (Lovley & Phillips 1987), significant increases in spatial and temporal Fe(II) concentrations are measured in aquifers where other TEAPs that are less energetically favorable (sulfate reduction, methanogenesis) are active and considered to be the dominant TEAP due to higher H 2 concentrations Chapelle et al 1995;Chapelle et al 1996;Chapelle et al 1997).…”

“…For example, the presence of sulfate (> 6-9 mg/l) has been shown to inhibit methanogenesis (Chapelle et al 1995) and the presence of amorphous Fe(III) has been documented to inhibit both sulfate reduction and methanogenesis (Lovley & Phillips 1987). Contrary to this, research has shown that iron reduction can occur simultaneously with sulfate reduction (Watson et al 2003) even though more energy is gained from iron reduction. One possible explanation for both processes to occur simultaneously is that the limited availability of the iron to the iron reducing microorganisms (referred throughout as "bioavailability") decreases the potential rate of iron reduction, and even though iron is present, it cannot be reduced fast enough to outcompete lower energy yielding TEAPs.…”

Microbiological Controls on Geochemical Kinetics 2: Case Study on Microbial Oxidation of Metal Sulfide Minerals and Future Prospects

“…Within the past few years, several groups have introduced modeling frameworks that are capable of handling the full spectrum of aqueous/solid-phase geochemistry and microbial population dynamics (Aguilera et al, 2005;Fang et al, 2003;Islam and Singhal, 2002;Jin and Bethke, 2003;Krautle and Knabner, 2005;Maurer and Rittmann, 2004;Mayer et al, 2002;Phanikumar and McGuire, 2004;Regnier et al, 2002;Roden et al, 2005;Wirtz, 2003;Yeh et al, 1998), and these models have been applied to a limited number of laboratory- (Roden et al, 2005;Thullner et al, 2005;Watson et al, 2003) and field-scale (Scheibe et al, 2006;Thullner et al, 2005;Watson et al, 2005) systems. The application of multispecies modeling to geochemical systems is bound to increase rapidly in the coming years, and will in due course catch-up with the sophisticated multispecies models that exist for activated sludge, anaerobic digestion, and various biofilm processes (Rittmann and McCarty, 2001).…”