New Paper: ORP of nZVI

[PGT]

This paper clarifies the interpretation of oxidation-reduction potential (ORP) measurements made on solutions of nano zerovalent iron (nZVI). The primary motivation is to assess the use of ORP as in indicator of nZVI transport during injection into the subsurface. The paper also addresses the more general issue of how electrodes respond to redox active nano particles in suspension.

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Shi, Z., J. T. Nurmi, and P. G. Tratnyek. 2011. Effects of nano zero-valent Iron (nZVI) on oxidation-reduction potential (ORP). Environ. Sci. Technol.  45(5): 1586-1592.

Oxidation-reduction potential (ORP) measurements have been widely used to assess the results of injection of nano zero-valent iron (nZVI) for groundwater remediation, but the significance of these measurements has never been established. Using a rotating disk electrodes (RDE) in suspensions of nZVI, we found the electrode response to be highly complex, but also a very sensitive probe for a range of fundamentally significant processes. The time dependence of the electrode response reflects both a primary effect (attachment of nZVI onto the electrode surface) and several secondary effects (esp., oxidation of iron and variations in dissolved H2 concentration). At nZVI concentrations above ~200 mg/L, attachment of nZVI to the electrode is sufficient to give it the electrochemical characteristics of an Fe0 electrode, making the electrode relatively insensitive to changes in solution chemistry. Lower nZVI concentrations give a proportional response in ORP, but much of this effect is mediated by the secondary effects noted above. Coating the nZVI with natural organic matter (NOM), or the organic polymers used to make stabile suspensions of nZVI, moderates its effect on ORP measurments. Our results provide the basis for interpretating ORP measurements used to characterize the results of injecting nZVI into groundwater.

DOI: http://dx.doi.org/10.1021/es103185t