Pore fluid chemistry and spectral induced polarization signatures of calcium carbonate

Calcium carbonate (CaCO3) minerals are a key family of compounds that frequently precipitate during natural and engineered subsurface processes. They play important roles in elemental cycling within geosystems and can be utilized in the context of environmental remediation (such as metal sequestration through co-precipitation) and in geotechnical engineering (such as improving soil strength or decreasing rock permeability). Characterizing the spatial extent and temporal dynamics of carbonate mineral precipitation is critical for these studies. Our previous research has indicated the potential of geophysical methods, particularly spectral induced polarization (SIP) for tracking the onset and evolution of mineral precipitates, including calcite. Here, we experimentally document the significant role of pore fluid chemistry and surface charge structure on the SIP signature of calcium carbonates. Our column studies revealed that the SIP signature of calcium carbonate is dictated by surface charge structure that relies heavily on surface complexation properties, such as charge density and speciation. For calcium carbonate, the primary potential determining ions (PDIs) are calcium and carbonate ions and the SIP signatures of calcium carbonate are primarily controlled by the concentrations of these species. Our data show that calcium carbonates in thermodynamic equilibrium with pore fluid produce a negligible SIP response due to very small (if any) surface charges. In contrast, systems that are over saturated with respect to calcium carbonate (i.e., far from equilibrium) produce significant SIP responses, which is consistent with high surface charge densities shown by high zeta potential values in previous studies. Our studies reveal that a closed system that transitions from over-saturation to equilibrium conditions is accompanied by significant decrease of SIP signals (and vice-versa). The studies also show that the effect of pH on SIP signature of calcium carbonate is manifested through its impact on calcite solubility and carbonate speciation and that the effects of other indifferent ions (such as Na and Cl) are minor. Our study illustrates the critical role of surface charge structure on SIP signatures of calcium carbonate minerals, which provides a fundamental basis for understanding the physiochemical mechanisms of SIP responses associated with natural and engineered processes.