Formatted Title
Developing a Field-Deployable ORP Kit to Measure Redox Potential of Aquifer Reactive Solids for Assessment of Abiotic Natural Attenuation
Background/Objectives
There are major challenges associated with quantitative assessment of abiotic natural attenuation (ANA), an important element of passive management of contaminated groundwater sites. The existing tools for ANA assessment are largely qualitative, which are not easy to obtain quantitative rate information to assist with the transition assessment. This work aims to develop a novel oxidation-reduction potential (ORP) Kit for laboratory and field measurements of redox potentials (i.e., thermodynamics) of reactive aquifer solids, which are primarily responsible for ANA in the subsurface. The redox potential can be further related to the rates (i.e., kinetics) of ANA mediated by these solid phases.
Approach/Activities
The fundamental principle of the proposed ORP Kit is built upon well-established science that electron shuttles accelerate electron transfer involved in redox processes. Here, electron shuttles serve to mediate electron transfer between reactive solids and an electrode, thereby improving the electrode sensitivity to the reactive solids, resulting in more accurate potentiometric measurement. The designed ORP Kit contains a mixture of electron shuttles that have been used in our prior projects to cover a wide range of redox potentials relevant to ANA. Laboratory tests were conducted to refine and optimize the composition and protocol of the ORP Kit using reference materials, including model reactive minerals and the site materials that have been well characterized in prior work. The Kit was then be applied to aquifer materials collected from other SERDP/ESTCP projects where ANA has been shown or suggested to be relevant, to generate additional ORP measurement data points.
Results/Lessons Learned
The initial test using model minerals exhibited various responses to the ORP Kit addition. Two prominent features were observed: (i) pH change during measurement can result in ORP change, which complicates data interpretation; and (ii) Kit addition can result in increase in ORP values. The first observation led to the conclusion that stable pH needs to be maintained during the potentiometric measurement and the ORP Kit should include a buffer in its composition. The second feature, upon more systematic investigation under anaerobic conditions, was due to certain electron shuttles in the ORP Kit serving as oxidants to oxidize the reducing materials. Lowering ES concentrations in the ORP Kit was proven to be effective in mitigating the oxidation titration effects. The ORP Kit was further interrogated with aquifer solids amended with aged nZVI samples recovered from a column treatability study under anaerobic conditions either in glovebox or under N2 headspace on benchtop. The results showed that the OPR Kit improved electrode responses to the solids across the potential ranges that are relevant to ANA. We evaluated the effects of different field operational variables and concluded that liquid to solid ratio was not important but mixing and headspace control are needed. The benchtop ORP values were slightly more positive than the glovebox values. The deviation might be caused by brief oxygen exposure during sample transfer process, which is undergoing continuous improvement. Finally, the ORP Kit was applied to prior collected cryogenic samples and an ORP library for these samples was established. We also expect to generate mediated ORP for additional samples from external laboratories.