Low-Frequency Electrical Properties of Zero Vvalent Iron-Sand Columns: Implications for Monitoring the Performance of Reactive Iron Wall Barriers

The reactive iron barrier is an in-situ technology for passive remediation of chlorinated solvents and heavy metals. Redox reactions occurring on the iron surface effectively remove these contaminants from groundwater. The effectiveness of this redox reaction diminishes with time due to oxidation and precipitation occurring on the metal surface, such that the long-term performance of reactive barriers is uncertain. Non-invasive measurement methods for evaluating reactive barrier performance are thus required to support remedial strategies at reactive barrier installations. Low-frequency (0.1-1000 Hz) electrical measurements are sensitive to the electrochemistry of the metal surface-pore fluid interface. We are conducting a series of laboratory experiments to assess the sensitivity of electrical methods (induced polarization and resistivity) to changes in the physicochemical properties of the metal-fluid interface that occur over time. In this paper we present the results of baseline studies on zero-valent iron-sand columns as a function of (a) reactive iron concentration (b) saturating fluid chemistry, and (c) degree of surface oxidation. The sensitivity of low-frequency electrical parameters to total zero-valent iron (Fe0) surface area was investigated by synthesizing Fe-Ottawa sand samples with varying Fe0 concentration from 0-10 percent. The dependence on ionic strength and electrolyte activity was investigated by making measurements on samples saturated with 0.001-1.0 for NaNO3, NaCl and CaCl2 solutions. The effect of pH was evaluated at constant electrolyte activity. As a first step towards evaluating the sensitivity of electrical measurements to reduction in reactive iron performance, measurements were made over a three month period of ageing and correlated with geochemical indicators (pH, Eh, electrical conductivity, iron concentrations) of Fe surface oxidation and precipitation. We find that induced polarization (IP) parameters are highly sensitive to Fe0 surface area whereas conduction parameters measured with the resistivity method are insensitive to Fe0 concentration over the investigated range. Polarization at the iron-electrolyte interface shows a power law relationship with electrolyte activity for all solutions and is consistent with Warburg impedance theory. Power-law exponents are slightly higher than that predicted for the active ion species based on Warburg impedance theory. Polarization magnitude depends on ionic composition of the electrolyte with the magnitude following the order CaCl2: NaCl: NaNO3. Conduction parameters are insensitive to ionic composition at constant electrolyte activity. Electrolyte activity exerts a strong control on the polarization relaxation length-scale, with time constant of the relaxation decreasing with increasing electrolyte activity. Polarization parameters measured during three months of ageing are clearly correlated with time and suggest that electrical measurements are sensitive to reduction in reactive iron performance.