Thermodynamic Behavior of FeCl3-H2O and HCl-FeCl3-H2O Systems – A Pitzer Model at 25 °C

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Most of the speciation models of aqueous electrolyte systems are based on the “ion pairing and complexing approach”. They are only suitable for aqueous solutions with ionic strengths lower than about 1 eq.kgw-1. For highly saline solutions, an approach based on “specific-ion interactions” (the so-called “Pitzer approach”) provides much more accurate results.Various thermodynamic databases supporting the “Pitzer approach” exist, but they are often inconsistent and poorly documented. The number of chemical elements is limited because of the difficulties inherent to producing relevant experimental data for wide ranges of temperature, ionic strength, pH, and for low solubility metals and hydroxides, and also because of the difficulty in evaluation of consistent and accurate values for interaction parameters.This study presents a new set of parameters using the standard Pitzer formalism that is able to model the behavior of Fe-rich solutions and solid-liquid equilibria in binary FeCl3-H2O and mixed HCl-FeCl3-H2O systems at 25°C. The new binary and mixing interaction parameters are adjusted in order to fit various independent experimental activity (water activity, osmotic coefficients) and ferric chloride salts solubility data. This work allows updating and improving thermodynamic databases for Fe(III) systems in order to simulate the complex geochemical behaviour of solutions and gas-brine-rock interactions in various applications in the geosciences or industrial processes.