Spatiotemporal geo-electrical sensing of a Pluronic-coated cobalt ferrite nanoparticle slug in natural sand flow-through columns

Highlights: • Non-invasive geophysical techniques can capture nanoparticle (NP) transport and fate in the subsurface. • Spectral Induced Polarization (SIP) provides real-time signals related to coated-NP concentrations. • SIP dynamics suggest that NP aggregates can rupture during straining and release smaller NPs. • Attached (remaining) NPs (8.8 mg kg⁻¹) yielded SIP signals exceeding pre-injection conductivities. • Cole-Cole relaxation times highlight the contribution of smaller diameter NPs to overall polarization. Rising industrial interest in the application of nanomaterials for the remediation of contaminated sites has led to concern over the environmental fate of the nanoremediation agents used. A critical requirement in evaluating and understanding nanoparticle (NP) behaviour in porous media is the development of analytical methods capable of in situ monitoring of complex NP transport dynamics. Spectral induced polarization (SIP), a non-invasive geo-electrical technique, offers a promising tool for detecting and quantifying NPs in soil and aquifer media. However, its application for monitoring the spatial migration and attachment behaviour of NPs remains uninvestigated. Here, we present results from flow-through experiments where we monitored the transport of cobalt ferrite nanoparticles (CoFe-NPs) coated with Pluronic, an amphiphilic polymer, in natural aquifer sand columns. We coupled concentration breakthrough curve analysis with SIP monitoring and reactive transport modeling to relate spatiotemporal NP concentration distributions to geo-electrical signals. Changes in the real (σ′) conductivity at three different locations along the columns closely correlated with model-computed total (solid plus aqueous phase) NP concentrations during the propagation of a NP slug. The imaginary conductivity (σ″) correlated closely with the arrival of the NP-slug. However, during the receding front, bimodal σ″-signal peak behaviour was observed propagating through the columns, indicating the existence of complex in situ NP transport dynamics, potentially revealing the rupture of nanoclusters upon straining and their effect on bulk charge storage that may not be obvious from breakthrough curve data alone. Fitting of a double Cole-Cole relaxation model yielded distinct shifts in relaxation time (τ) associated with the polarization of smaller length-scale particles. Post-NP pulse τ and σ″ did not return to pre-injection values; these lingering signals were caused by retained NP concentrations as low as 8.8 mg kg⁻¹. Our results support the applicability of SIP for spatial and temporal monitoring of NP distributions, with implications for the investigation of NP transport and nanoremediation strategies.