The emplacement of nano zerovalent iron (nZVI) for groundwater remediation is usually monitored by common measurements such as pH, total iron content, and oxidation-reduction potential (ORP) by potentiometry. However, the interpretation of such measurements can be misleading because of the complex interactions between the target materials (e.g., suspensions of highly reactive and variably aggregated nanoparticles) and aquifer materials, and multiple complications related to sampling and detection methods. With respect to ORP measurement, a fundamental issue is the lack of contact between the particle and electrode surfaces. This can be overcome using electron transfer mediators (ETMs), which is a powerful approach for laboratory studies, but difficult to apply in the field. Alternatively, substances related to ETMs, but selected for compatibility with field applications, can be used as chemical redox probes (CRPs). The response of several CRPs to nZVI synthesized with carboxymethylcelluose (CMC-nZVI) is of particular interest because CMC-nZVI 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 not 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). 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.

Approaches to characterizing the fate and effects of nano zerovalent iron

251st National Meeting (2016)

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