A MOLAL-BASED MODEL FOR STRONG ACID CHEMISTRY AT LOW TEMPERATURES (<200 TO 298 K)

Abstract

Geochemical processes occurring in cold environments on Earth, Mars, and Europa have elicited considerable interest in the application of geochemical models to subzero temperatures. Few existing geochemical models explicitly include acid chemistry and those that do are largely restricted to temperatures =<0°C or rely on the mole-fraction scale rather than the more common molal scale. This paper describes (1) use of the Clegg mole-fraction acid models to develop a molal-based model for hydrochloric, nitric, and sulfuric acids at low temperatures; (2) incorporation of acid chemistry and nitrate minerals into the FREZCHEM model; (3) validation and limitations of the derived acid model; and (4) simulation of hypothetical acidic brines for Europa.The Clegg mole-fraction acid models were used to estimate activities of water and mean ionic activity coefficients that serve as the database for estimating molal Pitzer-equation parameters for HCl (188 to 298 K), HNO3 (228 to 298 K), and H2SO4 (208 to 298 K). Model eutectics for HNO3 and H2SO4 agree with experimental measurements to within +/- 0.2°C. In agreement with previous work, the experimental freezing point depression (fpd) data for pure HCl at subzero temperatures were judged to be flawed and unreliable. Three alternatives are discussed for handling HCl chemistry at subzero temperatures. In addition to defining the solubility of solid-phase acids, this work also adds three new nitrate minerals and six new acid salts to the FREZCHEM model and refines equilibria among water ice, liquid water, and water vapor over the temperature range from 180 to 298 K. The final system is parameterized for Na-K-Mg-Ca-H-Cl-SO4-NO3-OH-HCO3-CO3-CO2-H2O. Simulations of hypothetical MgSO4-H2SO4-H2O and Na2SO4-MgSO4-H2SO4-H2O brines for Europa demonstrate how freezing can convert a predominantly salt solution into a predominantly acid solution at subzero temperatures. This result has consequences for the effects of salinity, acidity, and temperature as limiting factors for potential life on Europa. Strong acidity would limit life-forms to highly acidophilic organisms.