Influence of Surface Sorption Processes on Spectral Induced Polarization Evaluated Using in-Situ Monitoring of a Na-22 Tracer

Abstract:

Spectral Induced Polarization (SIP) has been used to monitor subsurface biogeochemical processes in a variety of lab and field studies. However, there are several competing mechanisms that have been proposed to explain the SIP effect. This work targets the influence of ion sorption to mineral surfaces as a controlling factor on SIP utilizing a pH dependent surface adsorption experiment. In this experiment we use silica gel as an idealized medium where the number of available sites for cation sorption, which in this case is limited to Na⁺ and H⁺ ions, is influenced by changes in pH via protonation/deprotonation of silanol groups. The experiment uses ²²Na as an in situ tracer as the radioactive decay of this nuclide can be continuously and non-invasively monitored using sensors placed outside of a column. The experiment was conducted by continuously pumping a 0.01M NaCl solution spiked with of 1µCi/L ²²Na in to the column under three pH conditions (pH 5.0, 6.0 and 8.0). In the experiment, we observed an increasing number of gamma counts caused by the accumulation of sorbed ²²Na in the column as we increased the pH from 5.0 to 6.5, and finally to 8.0. Simultaneously, we observed a linearly correlated (R² = 0.99) rise in the imaginary conductivity response of the SIP measurements. Using the triple layer electrochemical polarization model for grain polarization to simulate our experimental SIP responses, we found that the estimated surface site density is within a factor of two of that estimated from the mass accumulation of sodium. Since the accumulation of sodium on the silica gel surface is responsible for both the increase in gamma radiation and the change in the electrical response, these observations support the theory that mobile ions in the Stern layer of mineral surfaces provide the primary control on SIP signals in silicate materials.