The pH dependence of spectral induced polarization of silica sands: Experiment and modeling

Abstract

6p.International audienceIn electrolyte-saturated sands, the storage of electrical charges under an alternating electrical field (called "induced polarization") is responsible for a phase lag between the applied current and the resulting electrical field. Because a variety of polarization mechanisms exists in porous materials, the underlying physics of induced polarization is somehow unclear and the field data difficult to interpret quantitatively. Measurements at various pHs and salinities can be used to discriminate between different competing mechanisms at low frequencies (1 mHz-1 kHz) in porous media in the absence of electronic conductors. New experimental data point out that, in addition to the polarization of the Stern layer (the inner part of the electrical double layer coating the surface of the silica grains), there is another polarization mechanism possibly associated with a hopping process of the protons on the silica surface. We propose that such a process could follow a Grotthuss cooperation mechanism (as in ice) involving the bound water of the silica surface. Our data also rule out a mechanism based on the diffuse layer. The new polarization mechanism may be applied to quantifying induced-polarization data collected over acidic contaminant plumes

Salinity dependence of complex conductivity of unconsolidated and consolidated materials: Comparisons with electrical double layer models

“…Equation 18 can be directly compared with equation 12. Skold et al (2011) recently presented an extension of this model that accounts for an additional polarization mechanism beyond the Stern layer polarization that these authors attribute to hopping of the protons on the mineral surface. This extension to the SLP model is used to explain the pH dependence of the imaginary conductivity observed for silica sands.…”

“…To account for this effect, Skold et al (2011) include an additional contribution to the specific surface conductivity,…”

“…However, the contribution of the pore fluid chemical composition to the electrical double layer (EDL) polarization recorded with IP is less well defined, although several researchers have previously studied the controls of pore fluid salinity (Klein and Sill, 1982;Vinegar and Waxman, 1984;Lesmes and Frye, 2001;Revil and Skold, 2011;Weller et al, 2011), in addition to pH (Lesmes and Frye, 2001;Skold et al, 2011), and valence (Börner, 1992;Revil and Skold, 2011;Weller et al, 2011). A notable advance in the mechanistic understanding of fluid chemistry effects is the work of Vaudelet et al (2011), who combined a mechanistic model (discussed later) with a surface complexation model to predict the effect of differing sorption properties for the mineral surface for different cations.…”

Laboratory study of spectral induced polarization responses of magnetite — Fe2+ redox reactions in porous media

“…However, hematite is much more conductive than goethite, so whereas hematite is likely to exhibit similar SIP behavior to magnetite, we predict that goethite will have a much smaller phase response that predominantly reflects pH dependent surface charge from iron hydroxyl groups, rather than the migration of mobile charges within the mineral itself. This dominant SIP polarization mechanism (related to polarization of an EDL defined by pH dependent surface charge) has previously been demonstrated by Skold et al (2011) for silica. We also note that surface charges might however be more important in field SIP measurements, where smaller iron (oxyhydr)oxide mineral particles and smaller applied voltage gradients than those used in this study may be present.…”

Spectral induced polarization measurements for predicting the hydraulic conductivity in sandy aquifers

“…Previous workers have shown that the IP mechanisms can be very sensitive to changes in lithology and pore fluid chemistry (e.g., Pelton et al, 1978;Weller et al, 2010b;and Skold et al, 2011). Environmental examples for the successful use of SIP include the detection of clay units (e.g., Hördt et al, 2009;Attwa et al, 2011;and Attwa and Günther, 2012), the detection of both organic and inorganic contaminants (e.g., Olhoeft, 1984Olhoeft, , 1985Olhoeft, , 1992Chen et al, 2008Chen et al, , 2012andAbdel Aal et al, 2009, 2010) and hydraulic conductivity estimation (e.g., Tong and Tao, 2007).…”