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

We investigate the electrical response (0.1-1000 Hz) of reactive iron barriers by making measurements on zero valent iron (Fe-0)-sand columns under the following conditions: (1) variable Fe-0 surface area (0.1-100% by volume Fe-0) under constant electrolyte chemistry; (2) variable electrolyte activity (0.01-1 mol/liter), valence (mono trivalent), and pH under constant Fe-0-sand composition; and (3) forced precipitation of iron hydroxides and iron carbonates on the Fe-0 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 Fe-0, whereas conduction magnitude is only weakly dependent on the Fe-0 concentration below 30% by volume Fe-0. 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 Fe-0. Our results show that induced polarization parameters systematically change in response to changes in the Fe-0-electrolyte interfacial chemistry.