Spectral induced polarization response to nanoparticles in a saturated sand matrix

Abstract

Abstract Nanoparticles have grown in importance over the last decade with significant consumer and industrial applications. Yet, the behavior (fate and transport) of nanoparticles in the environment is virtually unknown. Research is needed to identify, characterize, and monitor nanomaterials in the subsurface. Here, we investigate the spectral induced polarization (SIP) response of nanometallic powders (nZVI, nAg, nTiO 2 , nZnO, and nCeO 2 ) in porous geologic media. Our main objective is to determine the sensitivity of the SIP response (0.1–10,000 Hz) to the presence of nanoparticles (metals and metal oxides) in porous media. The SIP response was tested under various conditions: increasing particle concentration under constant solution chemistry; varying solution molarity (0.0 M–1.0 M), and varying solution valence (+ 1, + 2, + 3 valence) under constant particle volume. We examine the results in terms of phase shift and resistance magnitude. Our data suggest that the oxide nanoparticles do not show SIP responses to increasing particle concentration, solution valence, and molarity, while the metallic particles show a clear response to increasing particle concentration, and frequency. Silver was the only material to show any significant response to increasing solution molarity, valence, and frequency. Because of the high propensity of the nanoparticles to form aggregates, they essentially behave as colloidal and clay particles allowing us to apply conventional SIP theory to our interpretation. We suggest that the oxidation state of the metals diminishes their SIP response consistent with more recent studies that have documented that polarization decreases with oxidation of metallic particles. We infer from our results that nanoparticle crystalline composition and aggregation effects control the SIP response of nanoparticles in porous media.