Aqueous species equilibrium constants and activity models form the foundation of the complex speciation codes used to model the geochemistry of geothermal energy production, extremophilic ecosystems, ore deposition, and a variety of other processes. Researchers have shown that a simple three species model (i.e., Na+, Cl-, and NaCl(aq)) can accurately describe conductivity measurements of concentrated NaCl and KCl solutions at elevated temperatures and pressures (Sharygin et al., 2002). In this model, activity coefficients of the charged species (e.g., Na+, K+, Cl-) become sufficiently low that the complexes must redisocciate with increasing salt concentration in order to meet equilibrium constant constraints. Redissociation decreases the proportion of the elements bound up as neutral complexes, and thereby increases the true ionic strength of the solution. In this contribution, we explore the consequences of the redissociation phenomenon in albite-paragonite-quartz (APQ) buffered systems. We focus on the implications of the redissociation phenomenon for mineral solubilities, particularly the observation that, at certain temperatures and pressures, calculated activities of charged ions in solution remain practically constant even as element concentrations increase from <1molal to 4.5molal. Finally, we note that redissociation has a similar effect on pH, and therefore aqueous speciation, in APQ-hosted systems. The calculations and discussion presented here are not limited to APQ-hosted systems, but additionally apply to many others in which the dominant cations and anions can form neutral complexes.
Bibliographical noteFunding Information:
We gratefully acknowledge support for this publication and related research from the National Science Foundation (NSF) MGG-OCE program under Grant Nos. 0927615 and 1232704 , from the U.S. Department of Energy (DOE) under Grant No. DE-EE0002764 , and by a grant from the Initiative for Renewable Energy and the Environment (IREE), a signature program of the Institute on the Environment (IonE) at the University of Minnesota (UMN). Any opinions, findings, conclusions and/or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF, DOE, IREE, IonE, or UMN. M.O.S. also thanks the George and Orpha Gibson endowment for its generous support of the Hydrogeology and Geofluids Research group in the Department of Earth Sciences, UMN. We additionally thank the guest editors of this Special Issue of Applied Geochemistry, “Geochemical Speciation Codes and Databases: Present Status and Future Needs” for inviting us to contribute this manuscript, which benefited considerably from the intelligent and thorough review of an anonymous reviewer.
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