Effects of Precipitation on the Low-Frequency Electrical Properties of PRB: Implications for Monitoring PRBs

The ability of induced polarization (IP) to monitor reduction in reactive iron performance is being investigated in the laboratory. Low frequency (0.1-1000 Hz) electrical properties are sensitive to metal-aqueous solution interface chemistry. Measurements have focused on sensitivity of IP to changes in Fe0 surface chemistry with aging due to mineral precipitation and aqueous electrochemical controls. High sensitivity of parameters defining polarization magnitude at/near the metal surface to total Fe0 surface area is observed. Polarization magnitude and dominant relaxation time correlate with electrolyte activity for 0.001-1.0 M for NaNO3, NaCl and CaCl2. Both parameters depend also on valence. Observations are consistent with double-layer (EDL) theory for the thickness of the EDL, although the electrochemical polarization mechanism observed with IP is uncertain. Polarization magnitude shows no relationship to pH, indicating that the fixed charge does not contribute to IP. Th e effects of Fe-precipitation by OH, SO4, PO4 and CO3 on electrical parameters was investigated for Fe0-sand samples (10% Fe0) over a period of induced precipitation. Aqueous chemistry was monitored and Fe0 surface precipitation verified by x-ray diffraction/scanning electron microscopy. Changes in electrical parameters provide insight into the sensitivity of the low-frequency electrical method for monitoring PRBs.