Electrodic voltages in the presence of dissolved sulfide: Implications for monitoring natural microbial activity

Abstract

There is growing interest in the development of new monitoring strategies for obtaining spatially extensive data diagnostic of microbial processes occurring in the earth. Open-circuit potentials arising from variable redox conditions in the fluid local-toelectrode surfaces ͑electrodic potentials͒ were recorded for a pair of silver-silver chloride electrodes in a column experiment, whereby a natural wetland soil containing a known community of sulfate reducers was continuously fed with a sulfate-rich nutrient medium. Measurements were made between five electrodes equally spaced along the column and a reference electrode placed on the column inflow. The presence of a sulfate reducing microbial population, coupled with observations of decreasing sulfate levels, formation of black precipitate ͑likely iron sulfide͒, elevated solid phase sulfide, and a characteristic sulfurous smell, suggest microbial-driven sulfate reduction ͑sulfide generation͒ in our column. Based on the known sensitivity of a silver electrode to dissolved sulfide concentration, we interpret the electrodic potentials approaching 700 mV recorded in this experiment as an indicator of the bisulfide ͑HS ‫מ‬ ͒ concentration gradients in the column. The measurement of the spatial and temporal variation in these electrodic potentials provides a simple and rapid method for monitoring patterns of relative HS ‫מ‬ concentration that are indicative of the activity of sulfate-reducing bacteria. Our measurements have implications both for the autonomous monitoring of anaerobic microbial processes in the subsurface and the performance of self-potential electrodes, where it is critical to isolate, and perhaps quantify, electrochemical interfaces contributing to observed potentials.

Monitoring microbial sulfate reduction in porous media using multipurpose electrodes

“…Biogeophysics is a rapidly evolving Earth science discipline concerned with the links between dynamic subsurface microbial processes, microbial‐induced alterations to geologic materials, and geophysical signatures [ Atekwana and Slater , 2009]. There is growing interest in applying electrical geophysical techniques (resistivity, complex conductivity, and self potential (SP)) for biogeophysical projects as they have repeatedly been shown to be sensitive to bacterial cells, microbial growth and microbe‐mineral alterations [ Atekwana et al , 2004a, 2004b; Naudet et al , 2004; Personna et al , 2008; Slater et al , 2008]. Previous studies have confirmed the potential of electrical geophysics for characterizing microbial production of metabolic byproducts and resulting alteration of mineral surfaces [ Werkema et al , 2003; Atekwana et al , 2004a, 2004b; Allen et al , 2007; Che‐Alota et al , 2009], evaluating hydrological and biotransformations due to microbial reduction of heavy metals [ Hubbard et al , 2008; Williams et al , 2009], improving understanding of metal and nutrient cycling driven by microbe‐mineral transformations [ Ntarlagiannis et al , 2005b; Williams et al , 2005; Personna et al , 2008], and detection of microbes, microbial growth and biofilm formation [ Abdel Aal et al , 2004; Prodan et al , 2004; Ntarlagiannis et al , 2005a; Davis et al , 2006; Abdel Aal et al , 2009; Ntarlagiannis and Ferguson , 2009; Slater et al , 2009].…”

“…Recent studies have shown that electrodic potential (EP) measurements, using simple in construction and inexpensive electrodes, can be utilized to capture temporal and spatial variability in sulfide chemistry associated with microbial sulfate reduction under anaerobic conditions [ Williams et al , 2007; Personna et al , 2008; Slater et al , 2008]. The EP measurements record the tendency for spontaneous redox reactions to occur on the surfaces of metal and/or metal‐metal salt electrodes.…”

“…The EP measurements record the tendency for spontaneous redox reactions to occur on the surfaces of metal and/or metal‐metal salt electrodes. Williams et al [2007], Personna et al [2008] and Slater et al [2008] showed that, in the presence of a bisulfide (HS − ) concentration gradient across two Ag‐AgCl metal electrodes, a galvanic cell (GC) potential is recorded, being the potential of an electrochemical cell with a known reactivity between the target compound and the electrodes used. We stress here that this EP method may provide only an approximation of HS − concentration, as the accurate interpretation of HS − may be limited due to possible complex electrochemical reactions in the system.…”

“…In this paper, we investigate the application of simultaneous EP/electrical geophysical measurements for monitoring microbial sulfate reduction. Unlike earlier studies [ Williams et al , 2007; Personna et al , 2008; Slater et al , 2008], we employ a dual electrode (Ag‐AgCl metal as sensing/EP electrode; Ag‐AgCl metal in KCl gel as reference/SP electrode) technique that constrains EP interpretation of HS − concentration by using a non polarizing (SP) electrode as the reference cathode. This dual electrode strategy results in EP measurements with a constant stable reference potential, facilitating HS − concentration predictions at the active electrode locations.…”

Electrodic voltages accompanying stimulated bioremediation of a uranium‐contaminated aquifer

“…Under conditions where the measurement and reference electrodes are (1) located in geochemically distinct redox environments and (2) exposed to their local geochemical environment, a subset of the SP method, referred to as the electrodic potential (EP) method, has been shown to be sensitive to electrochemical reactions involving the electrodes themselves [ Corry , 1985; Nyquist and Corry , 2002; Williams et al , 2007; Slater et al , 2008]. When bridged through a measuring voltmeter, electrodes located in the electrochemically distinct regions constitute a galvanic cell, generating a voltage potential that persists as long as the concentration gradient is maintained.…”