Electrical properties of iron-sand columns: Implications for induced polarization investigation and performance monitoring of iron-wall barriers

Abstract

Abstract We investigate the electrical response (0.1–1000 Hz) of reactive iron barriers by making measurements on zero valent iron (Fe0)-sand columns under the following conditions: (1) variable Fe0 surface area (0.1–100% by volume Fe0) under constant electrolyte chemistry; (2) variable electrolyte activity (0.01–1 mol/liter), valence (mono trivalent), and pH under constant Fe0-sand composition; and (3) forced precipitation of iron hydroxides and iron carbonates on the Fe0 surface. We model the measurements in terms of conduction magnitude, polarization magnitude, and polarization relaxation time. Our key findings are: Polarization magnitude exhibits a linear relation to the surface area of Fe0, whereas conduction magnitude is only weakly dependent on the Fe0 concentration below 30% by volume Fe0.Polarization magnitude shows a power law relation to electrolyte activity, with exponents decreasing from 0.9 for monovalent solutions to 0.7 for trivalent solutions.The relaxation time parameter depends on activity and valence in a manner that is partly consistent with the variation in double layer thickness predicted from theory.pH exerts minor control on the electrical parameters.Polarization magnitude and relaxation time both increase as a result of precipitation induced on the surface of Fe0. Our results show that induced polarization parameters systematically change in response to changes in the Fe0-electrolyte interfacial chemistry.