Effect of Ionic Strength on H2O and Si-Species Stability Field Geometry in pH-Eh Space

Abstract

Speciation diagrams in pH-Eh space at some pressure, temperature, and composition are highly useful in indicating the thermodynamic behavior expected in a system. While these diagrams are found frequently throughout geochemical literature, they often overlook the geometric behavior of stability fields and fail to address the specific impacts of ionic strength and fluid composition. Modeling results from the Salton Sea and Krafla geothermal sites suggest a positive correlation between ionic strength and the H₂O_(l) stability field area or reactivity domain. In addition, the Si-bearing species (selected due to the propensity of adverse scale occurrence) within the Salton Sea brine are shifted farther apart than the centroids of the same Si species in the comparatively dilute Krafla. These modeling observations imply that geothermal brines at equilibrium accommodate a larger H₂O reactive field in pH-Eh space, allowing for the possibility that water–rock interactions could occur at pH-Eh conditions that would conventionally be unexpected if ionic strength is not considered.