Many approaches to remediation of contaminated groundwater rely on contaminant degradation processes that are, directly or indirectly, based on redox reactions. Predicting and optimizing such processes requires reliable methods for characterizing in situ redox conditions, but the existing methods (usually direct oxidation-reduction potential [ORP] measurement with a Pt electrode) have repeatedly been shown to be inadequate.
This project took a “chemical probe reaction” approach, which involved adding chemical reactivity or redox probes (CRPs) that reacted with the medium, and the resulting change in speciation of the CRP was monitored either spectrophotometrically (i.e., by measuring the color change) or potentiometrically (i.e., with a Pt electrode). Variations on this method can be used to characterize thermodynamic, kinetics, and capacity aspects of the redox conditions in a sample. CRPs could be used at field sites in push-pull tests, or for ex situ characterization of preserved samples. Ultimately, the project aimed to relate CRP results to contaminant degradation under in situ conditions.
A variety of CRPs were identified that have potential application for characterizing redox conditions in environmental materials, with several offering the most promise: resazurin, indigo disulfonate, and anthraquinone sulfonate. These CRPs are non-toxic, readily available, and undergo rapid redox reactions with minimal sorption. Using these CRPs, with spectrophotometric or potentiometric detection, the reducing properties of minerals such as magnetite, with and without added Fe(II), were characterized. This information is not available using traditional direct ORP measurements done without CRPs to mediate electron transfer between the mineral surface and the electrode.
Using other CRPs, the kinetics of reduction by minerals such as magnetite were measured reliably in relatively rapid tests (less than one week), whereas the kinetics of reduction of chlorinated solvents by mineral is much slower, and therefore, much harder to measure. Serial application of CRPs could also be done to provide a kind of redox titration, thereby characterizing the reducing capacity of aquifer materials.
Using a combination of thermodynamic and kinetic CRPs measurements, and data from prior work on abiotic reduction of contaminants, a correlation was obtained that predicts the kinetics of contaminant reduction from the reducing potentials of iron oxide minerals. This correlation includes the effect of Fe(II) sorbed to Fe(III) oxide minerals through the reduction potential of the resulting phase.
The use of CRPs to characterize the redox properties of environmental materials—including aquifer solids, but also sediments, soils, and sludges—can overcome many of the fundamental and practical limitations to traditional Pt electrode (only) measurements. Probably, the most significant advantage is the ability of CRPs to detect redox conditions on mineral surfaces, which can be much more strongly reducing than the associated pore water, and are primarily responsible for abiotic natural attenuation.
The CRP-based methods developed here were also flexible so that they can be adapted to characterize not only redox potential, but also kinetic and capacity characteristics of environmental media. Therefore, only CRP-based methods are likely to be able to fully characterize all of the aspects of in situ redox conditions necessary to determine if abiotic natural attenuation (or other redox-based remediation strategies, both chemical and microbiological) is likely to be successful as a remedy at particular sites. However, the methods were validated and optimized mainly with laboratory-based measurements, so some aspects of their implementation under field conditions remain to be explored. Some of this technology transfer is continuing through other SERDP projects (especially ER-2621), and other efforts are under development through collaborations with a variety of practitioners.