Evaluation of Surface Sorption Processes Using Spectral Induced Polarization and a ²²Na Tracer

We investigate mechanisms controlling the complex electrical conductivity of a porous media using noninvasive spectral induced polarization (SIP) measurements of a silica gel during a pH dependent surface adsorption experiment. Sorption of sodium on silica gel surfaces was monitored as the pH of a column was equilibrated at 5.0 and then successively raised to 6.5 and 8.0, but the composition of the 0.01 M NaCl solution was otherwise unchanged. SIP measurements show an increase in the imaginary conductivity of the sample (17.82 ± 0.07 μS/cm) in response to the pH change, interpreted as deprotonation of silanol groups on the silica gel surface followed by sorption of sodium cations. Independent measurements of Na⁺ accumulation on grain surfaces performed using a radioactive ²²Na tracer support the interpretation of pH-dependent sorption as a dominant process controlling the electrical properties of the silica gel (R² = 0.99) and confirms the importance of grain polarization (versus membrane polarization) in influencing SIP measurements of silicate minerals. The number of surface sorption sites estimated by fitting a mechanistic, triple-layer model for the complex conductivity to the SIP data (13.22 × 10¹⁶ sites/m²) was 2.8 times larger than that estimated directly by a ²²Na mass balance (5.13 × 10¹⁶ sites/m²), suggesting additional contributions to polarization exist.