Changing Cr-Retention Capacity of Magnetite-Coated Sand Captured by Spectral Induced Polarization (SIP)

Monitoring the extent of heavy metal remediation on reactive materials employed in remediation schemes relies on downflow concentration sampling. The latter implies that a decreasing barrier remediation efficiency only becomes apparent once a contaminant breakthrough has occurred and thus contamination past the barrier. Spectral induced polarization (SIP), a noninvasive geophysical technique sensitive to sorption-induced changes in the surface charging properties of mineral surfaces in porous media, offers a potentially powerful monitoring alternative. We conducted a flow-through column experiment fitted with retracted electrodes and packed with magnetite-coated sand as a bench-scale reactive barrier analogue for monitoring Cr(VI) sorption using SIP. Our SIP responses measured during the Cr(VI)-injection pulse highlight that adsorption of Cr(VI) onto the magnetite coating led to a short-lived increase in imaginary conductivity, followed by a strong continuous decrease. Via reactive transport modeling, we simulated the redox-dependent sorption of Cr(VI) and linked the decrease in the SIP-derived imaginary conductivity to the gradual reduction in the remaining sorption capacity on the magnetite coating. The excellent agreement between our geochemical results and SIP signals suggests that noninvasive geophysical methods can function as an early warning tool for monitoring the finite lifetime of reactive barriers in situ.