Thermodynamic behavior of FeCl3-H2O and HCl-FeCl3-H2O Geo Eco Consulting 2010 Thermodynamic behavior of FeCl3-H2O and HCl-FeCl3-H2O systems - a Pitzer model at 25C Laurent ANDRE1, Christomir CHRISTOV2, Arnault LASSIN1, Mohamed AZAROUAL1 1 BRGM, Water, Environment and Ecotechnology Division, 3 Avenue Claude Guillemin 45060 Orléans Cedex 1, France 2 GeoEco Consulting 2010, San Diego, California, USA, and Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria. INTRODUCTION OBJECTIVES OF THE STUDY Knowledge of the thermodynamic behavior of saline multicomponent aqueous systems is important in industrial processes as well as in geosciences. Increasing use of such non-conventional water resources needs development of robust geochemical models for describing and predicting chemical reactions within these particular media (dissolution/ precipitation of salts, evaporation…). The Pitzer approach is suitable to deal with major elements in brines. But, some systems are poorly documented and the Pitzer databases have to be continuously improved to integrate new chemical species and interaction parameters. Listing of observable/measured data and state-of-the-art concerning the ternary H-Fe-Cl-H2O system at 25°C; Determination of a new set of pure electrolyte parameters for Fe(III)-Cl interactions valid up to a very high molality FeCl3-H2O solutions (over the saturation with respect to FeCl3:6 H2O); Application of the database, including new interaction parameters, to reproduce the thermodynamic behavior of more complex systems (industrial and/or natural brines) with the geochemical code PHREEQC [1]. OVERALL METHODOLOGY OF THE APPROACH Iso-activity curves in the FeCl3-HCl- H2O ternary system Compilation of literature data on the FeCl3-H2O binary system (water activity, osmotic coefficients, salt solubility in pure water …) and FeCl3-HCl-H2O ternary system (water activity, brine density, pH, salt solubility in acidic solutions…) Solid phases Experimental m(sat) (mol/kgw) FeCl3:6 H2O 6.05 FeCl3:3.5 H2O 13.04 FeCl3:2.5 H2O 16.10 Water activity in the FeCl3-H2O binary system Calculations of interaction parameters (binary : b0, b1, b2 cF between Fe(III) and Cl and ternary: QFe(III)-H, YFe(III)-H-Cl) able to reproduce observable data (water activity, iso-activity curves…) Geochemical code Measured osmotic coefficient in the FeCl3-H2O binary system Comparison between measured pH (See, 2006) and calculated pH for solutions containing H, Fe and Cl compounds Validation of interaction parameters with independent measured data not already used in the parameterization process Optimization code Selection and format of the measured data to the norms of the geochemical code Implementation of new interaction parameters in the specific database and applications CONCLUSIONS The set of parameters established in this work is suitable to reproduce water characteristics of ferric chloride solutions both at low and high ionic strengths. This new set of parameters can represent the composition of highly saline solutions up to the eutonic points of hydrated ferric chloride salts in the mixed HCl-FeCl3-H2O system. All binary and mixed solution interaction parameters and equilibrium constants of Fe(III) minerals are coherent, and therefore can be used to determine the activities of dissolved species up to the salt solubility concentrations. The models have been developed in order to update the PHREEQC code. The presented parameterization can be used without any additional adjustments to determine the solution behavior of more complex Fe(III) systems, as well as to develop a solubility model for a very important geochemical Fe(III) hydroxides minerals in high salinity solutions. This work has been carried out thanks to the financial support of the Research Direction of the BRGM. Тhe work of Christomir Christov has been supported in a part by National Science Fund of the Bulgarian Ministry of Science and Education (Grant No. DO 02-243). REFERENCES [1] Parkhurst D.L., Appelo C.A.J., 1999. User's guide to PHREEQC (version 2) - A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations. U.S. Geological Survey Water-Resources Investigations Report 99-4259, 1999. [2] Mikulin G. 1968. Voprosy Fizicheskoi Khimii Electrolytov. Izd. Khimiya. [3] Kangro W., Groeneveld A., 1962. Konzentrierte wäßrige Lösungen, I. Z. Phys. Chem. (Frankfurt am Main), 32,110-126. [4] Liu X., Millero F.J., 1999. The solubility of iron in sodium chloride solutions. Geochim. Cosmochim. Acta, 63, 3487–3497. [5] See M.S., 2006. Use of the Bromley equation for the analysis of ionic equilibria in mixed ferric and ferrous chloride solutions at 25°C. Metallurgical and Materials Transactions B, 37B, 173-179.