Sensing Cr(VI) retention with spectral induced polarization (SIP) in a magnetite-coated sand pack

Removing Cr from contaminated (ground)water is often attempted via active remediation using easily deployable permeable reactive (barrier) materials, such as iron oxide mineral coatings. In particular, magnetite has been shown to be a highly effective and low-cost option for removing redox-active Cr from solution. Magnetite not only binds Cr, but it also reduces Cr(VI) to the less toxic and immobile Cr(III). Monitoring the extent of Cr retention in remediation schemes, however, relies on down-flow concentration sampling. Consequently, detectable levels of Cr must exit remediation barriers in order to detect the decreasing remediation efficiency of reactive materials with the progression of immobilization. Spectral induced polarization (SIP), a non-invasive geophysical technique sensitive to sorption-induced changes in the surface charging properties of mineral surfaces in porous media, offers a potentially powerful monitoring alternative to detect changes in remediation efficiency in situ without the need for down-flow monitoring and contamination hazard. Here, we apply SIP, as a proxy to monitor the extent of Cr retention in a flow-through column experiment, packed with magnetite-coated sand. We observed a rapid increase in polarization upon Cr(VI) adsorption on magnetite coated sand, followed by a strong continuous decrease. Our joint reactive transport modeling and post-column geochemical measurements highlighted a drop in the remaining sorption capacity of the coated sand, thereby linking the reduced sorption capacity to the drop in SIP signal. The excellent agreement between concentration breakthrough curves, our model and SIP measurements suggests that SIP signals can be used as an early warning tool to detect the approaching saturation of reactive materials deployed in remediation schemes.