New Paper: Redox Probe for Characterizing nZVI Injection
PGT
Redox reactions on mineral surfaces are among the most important biogeochemical processes, and recently there has been renewed interest in their role in the fate of subsurface contaminants (i.e., “abiotic natural attenuation”). We have been funded by SERDP (ER-2308) to tackle some of the most pressing aspects of this problem: specifically, what measurable property is predictive of contaminant reduction rates by minerals and what are the relative contributions of the various iron oxides and sulfide reductants.
This paper not only describes the first direct method for quantifying effective reduction potentials of mineral suspensions that could be applied to real porous media, but it demonstrates that the results obtained with the method can be used to develop predictive models for contaminant reduction kinetics. It lays the foundation for our next manuscript from this project, which will show that the approach taken can provide some pretty dramatic results when applied to columns packed with real aquifer material.
Fan, D., S. Chen, R. L. Johnson, and P. G. Tratnyek. 2015. Field deployable chemical redox probe for quantitative characterization of carboxymethylcellulose modified nano zerovalent iron. Environ. Sci. Technol. 49(17): 10589-10597.
Nano zerovalent iron synthesized with carboxymethylcelluose (CMC-nZVI) is among the leading formulations of nZVI currently used for in-situ groundwater remediation. The main advantage of CMC-nZVI is that it forms stable suspensions, which are relatively mobile in porous media. Rapid contaminant reduction by CMC-nZVI is well documented, but the fate of the CMC-nZVI (including “aging” and “reductant demand”) is less well characterized. Improved understanding of CMC-nZVI fate requires methods with greater specificity for Fe(0), less vulnerability to sampling/recovery artifacts, and more practical application in the field. These criteria can be met with a simple and specific colorimetric approach using indigo-2,6-disulfonate (I2S) as a chemical redox probe (CRP). The measured stoichiometric ratio for reaction between I2S and nZVI is 1.45±0.03, suggesting complete oxidation of nZVI to Fe(III) species. However, near pH 7, reduction of I2S is diagnostic for Fe(0), because aqueous Fe(II) reduces I2S much more slowly than Fe(0). At that pH, adding Fe(II) increased I2S reduction rates by Fe(0), consistent with depassivation of nZVI, but did not affect the stoichiometry. Using the I2S assay to quantify changes in the Fe(0) content of CMC-nZVI, the rate of Fe(0) oxidation by water was found to be orders of magnitude faster than previously reported values for other types of nZVI.