FREZCHEM: A geochemical model for cold aqueous solutions

FREZCHEM is an equilibrium chemical thermodynamic model parameterized for concentrated electrolyte solutions [up to ≈20 moles/kg(H2O) (molality)] using the Pitzer approach for the temperature range from <−70 to 25 °C and the pressure range from 1 to 1000 bars. This subzero temperature model has been extensively used to explore cold geochemical processes and/or limits for life in the Arctic, Antarctica, Europa, and Mars. FREZCHEM contains environmental pathways for simulating temperature change, evaporation, and pressure change. The objectives of this work were to (1) adopt a better mathematical algorithm to calculate chemical equilibrium, (2) develop a more user-friendly Internet version, and (3) develop an archive for various model versions. We adopted a Gibbs free energy mathematical algorithm that significantly reduced convergence problems that existed with older versions of this model. An Internet site was developed at http://frezchem.dri.edu that simplifies model access and is more user-friendly than pre-existing FREZCHEM versions. At this Internet site, FORTRAN codes and instructions for using various FREZCHEM versions are archived. This effort will make this unique theoretical research tool readily and easily available, even to occasional users, now and in the future.