Electrodic voltages accompanying stimulated bioremediation of a uranium‐contaminated aquifer

Abstract

[1] The inability to track the products of subsurface microbial activity during stimulated bioremediation has limited its implementation. We used spatiotemporal changes in electrodic potentials (EP) to track the onset and persistence of stimulated sulfate-reducing bacteria in a uranium-contaminated aquifer undergoing acetate amendment. Following acetate injection, anomalous voltages approaching −900 mV were measured between copper electrodes within the aquifer sediments and a single reference electrode at the ground surface. Onset of EP anomalies correlated in time with both the accumulation of dissolved sulfide and the removal of uranium from groundwater. The anomalies persisted for 45 days after halting acetate injection. Current-voltage and current-power relationships between measurement and reference electrodes exhibited a galvanic response, with a maximum power density of 10 mW/m 2 during sulfate reduction. We infer that the EP anomalies resulted from electrochemical differences between geochemically reduced regions and areas having higher oxidation potential. Following the period of sulfate reduction, EP values ranged from −500 to −600 mV and were associated with elevated concentrations of ferrous iron. Within 10 days of the voltage decrease, uranium concentrations rebounded from 0.2 to 0.8 mM, a level still below the background value of 1.5 mM. These findings demonstrate that EP measurements provide an inexpensive and minimally invasive means for monitoring the products of stimulated microbial activity within aquifer sediments and are capable of verifying maintenance of redox conditions favorable for the stability of bioreduced contaminants, such as uranium.

Review: Some low-frequency electrical methods for subsurface characterization and monitoring in hydrogeology

“…The relationship between the current density and the gradient of the redox potential is found to be linear (Revil et al 2010a). The effects should not be confused with electrodic voltages associated with changes in the surface chemistry of the electrodes which are not self-potential signals Williams et al 2010). …”

An Overview of Electron Acceptors in Microbial Fuel Cells

“…Leachate in uranium processing areas have low but stable uranium concentrations and can contaminate water resources, groundwater and sediment (Williams et al, 2010 ). To solve this problem in situ, metal reducing bacteria are usually used together with the acetate feed (Vrionis et al, 2005 ).…”

Coupling anaerobic bacteria and microbial fuel cells as whole-cell environmental biosensors

“…This spike in activity also allowed monitoring of changing subsurface water flow through the use of microbial activity measurements, in a grid pattern well set up. Thus this multiplexed sensor allows for monitoring of uranium reduction, while acting as an on-site bioreactor, removing the need for continuous sampling and off-site testing 7 . The use of a cathode alone can provide the redox potential required for promoting microbial activity, while simplifying the system.…”

“…Anodes were installed down-gradient of the site of bioremediation and a cathode was embedded on the soil surface ( Figure 1b). This allowed a correlation to be determined between the injections of acetate at the site of contamination, with a significant increase in current 7 . This spike in activity also allowed monitoring of changing subsurface water flow through the use of microbial activity measurements, in a grid pattern well set up.…”

“…during metabolic processes 7 , to drive reduction of soluble U( VI ) to less-soluble U( IV ). Anodes were installed down-gradient of the site of bioremediation and a cathode was embedded on the soil surface ( Figure 1b).…”