The Relative Importance of Saturated Silica Sand Interfacial and Pore Fluid Geochemistry on the Spectral Induced Polarization Response

Abstract

Adsorption at the solid‐pore fluid interface is a key mechanism controlling the mobility of nutrients and contaminants in subsurface soils and sediments. The spectral induced polarization (SIP) method has been shown to be sensitive to the quantity and type of adsorbed ions. Extending previous results, we investigated the relevance of pH, solution conductivity, and ion type on the SIP response of saturated silica sand. We also performed adsorption experiments to evaluate whether adsorption plays a relevant role on the effect of saturating solution conductivity and pH. Given their environmental relevance and different electrochemical characteristics, we focused on exploring the influence of Cu2+ and Na+ adsorption on the SIP signature. The adsorption results confirm the expected and modeled pH influence on the adsorption of both Cu2+ and Na+. The measured quadrature conductivity spectra indicate that pH and solution conductivity control the electrical double layer electrochemical state and its capacitive behavior. On the contrary, no appreciable SIP signal changes are associated with ion substitution. The adsorption experiments highlight low values of site occupancy for Na and Cu, which suggests that the effects of pH and fluid conductivity are unrelated to their control on the ion adsorption. We interpret the solution conductivity as a proxy for ionic strength. The relative importance of pH and solution conductivity over ion type helps to further constrain the interpretation of SIP results in field geochemical and biogeochemical characterization and monitoring.

Effects of salinity and pH on the spectral induced polarization signals of graphite particles

“…Because the fluid conductivity is kept constant during the pH variation experiments, the possibility of fluid conductivity variation as the underlying cause of the observed polarization behaviours can be excluded. Previous experimental and modelling studies on nonconductive media, such as sandstones, suggest that pH effect on SIP is only secondary (Lesmes & Frye 2001;Skold et al 2011;Peruzzo et al 2018) and is primarily realized through its impact on charge density in the EDL. In the case of graphite, because there is limited understanding of the EDL properties of graphite, such an effect is difficult to evaluate.…”

Induced Polarization of Clayey Rocks and Soils: Non‐Linear Complex Conductivity Models

“…EDL almost exists at the interface between any material and interstitial fluid, even for silica (Hao et al., 2015; Peruzzo et al., 2018). Although our non‐linear models are presented for clayey rocks and soils, it stands to reason that they also apply to other ionically conductive materials in the IP realm as long as the EDL and geometry change negligibly.…”

Three‐channel electrical impedance spectroscopy for field‐scale root phenotyping

“…Without the intrinsic capacitive behavior of the singular grain, the capacitive response of the soil derives from the impedance properties of the interface between pore fluid and mineral surfaces (Binley et al., 2005; Revil et al., 2015). Consequently, specific surface area and specific surface charge are the key parameters controlling the intensity of the polarization (Peruzzo et al., 2018; Weller & Slater, 2012). Under these conditions, the interfacial polarization in soil is typically significantly smaller than the cell‐controlled polarization of biological materials.…”