Chemical Reactivity Probes for Assessing Abiotic Natural Attenuation by Reducing Iron Minerals

Increasing recognition that abiotic natural attenuation (NA) of chlorinated solvents can be important has created demand for improved methods to characterize the redox properties of the aquifer materials that are responsible for abiotic NA. This study explores one promising approach: using chemical reactivity probes (CRPs) to characterize the thermodynamic and kinetic aspects of contaminant reduction by reducing iron minerals. Assays of thermodynamic CRPs were developed to determine the reduction potentials (E_CRP) of suspended minerals by spectrophotometric determination of equilibrium CRP speciation and calculations using the Nernst equation. E_CRP varied as expected with mineral type, mineral loading, and Fe(II) concentration. Comparison of E_CRP with reduction potentials measured potentiometrically using a Pt electrode (E_Pt) showed that E_CRP was 100-150 mV more negative than E_Pt. When E_Pt was measured with small additions of CRPs, the systematic difference between E_Pt and E_CRP was eliminated, suggesting that these CRPs are effective mediators of electron transfer between mineral and electrode surfaces. Model contaminants (4-chloronitrobenzene, 2-chloroacetophenone, and carbon tetrachloride) were used as kinetic CRPs. The reduction rate constants of kinetic CRPs correlated well with the E_CRP for mineral suspensions. Using the rate constants compiled from literature for contaminants and relative mineral reduction potentials based on E_CRP measurements, qualitatively consistent trends were obtained, suggesting that CRP-based assays may be useful for estimating abiotic NA rates of contaminants in groundwater.