Thermodynamic properties in the Fe(II)-Fe(III)-As(V)-HClO 4 –H 2 O and Fe(II)-Fe(III)-As(V)-HCl–H 2 O systems from 5 to 90°C

Abstract Fe-As mineral solubility and associated aqueous species have been intensively studied because of the environmental need to immobilize arsenic. The thermodynamic data for aqueous iron-arsenic species are inadequately characterized, however. The Gibbs free energy, enthalpy, entropy, and heat capacity and activity coefficients were refined in the Fe(II)-Fe(III)-As(V)-HClO 4 -H 2 O and Fe(II)-Fe(III)-As(V)-HCl-H 2 O systems using redox potential measurements from 5 to 90 °C. The association constants for FeHAsO 4 + and FeH 2 AsO 4 2 + at 25 °C were 10 10.28 and 10 4.02 and the corresponding association reaction enthalpies and heat capacities were 25.74 and 8.73 kJ mol − 1 and 843.1 and − 529.6 J K  − 1 mol − 1 , respectively. Activity coefficients for H + , ClO 4 − , Fe 2 + , Fe 3 + , HAsO 4 2 − , and H 2 AsO 4 − at 25 °C in the form of the Huckel equation were derived for ionic strengths up to 1 mol − 1  kg − 1 . Newly derived activity coefficients and thermodynamic data were incorporated into PHREEQCI to calculate the Eh of laboratory solutions. The differences between calculated and measured Eh were all within 10 mV and relative differences were all lower than 1.5%.