EFFECT OF MINERALOGY ON LOW FREQUENCY ELECTRICAL PROPERTIES OF CORRODED IRON COLUMNS: IMPLICATIONS FOR THE MONITORING OF PERMEABLE REACTIVE BARRIERS (PRBS)

We conducted column experiments to investigate electrical signatures associated with different mineralogies during iron corrosion and mineral precipitation using (a) Na₂SO₄ (0.05 M, dissolved oxygen (DO) = 5.9 ppm, non-buffered), (b) NaNO₃ (0.05 M, DO = 3 ppm, non-buffered), and (c) NaHCO₃ plus CaCl₂ mixture (NaHCO₃ = 0.048 M, CaCl₂ = 0.002 M, DO < 0.1 ppm, buffered, pH = 7.3) solutions. At the influent sand/iron interface channels where iron corrosion and mineral precipitation were initiated and most severe, an increase in interfacial polarization (~250% for sulfate, ~100% for nitrate column) and decrease in resistivity (~30% for sulfate, ~12% for nitrate) were observed in sulfate and nitrate columns. In contrast, the bicarbonate column shows a decrease in interfacial polarization (~30%) and an increase in resistivity (~7%). Scanning electron microscopy (SEM) and X-ray diffractometry (XRD) analysis revealed the formation of corrosion rinds and the increasing mineralogical complexity in corroded sections of the core compared to minimal/non corroded sections. We attribute the different electrical behavior of the bicarbonate relative to the sulfate/nitrate columns to mineralogical differences resulting from iron corrosion and mineral precipitation within the columns: While newly precipitated/transformed polarizable and conductive/semi-conductive iron (oxyhydr)oxides (mostly magnetite) causes the increase in interfacial polarization and decrease in resistivity in sulfate and nitrate columns due to increased polarizable surface area and mineralogical alteration, the decreased interfacial polarization and increased resistivity in the bicarbonate column are attributed to non-polarizable and non-conductive carbonate mineral precipitation coating the zero-valent iron surface and decreasing interfacial and electronic conduction. Our result shows that mineralogy is a critical factor that influences electrical properties, and this should be taken into account when considering the use of electrical methods to monitor PRB barrier corrosion processes in situ.