Thermodynamic properties in the Fe(II)-Fe(III)-As(V)-HClO4–H2O and Fe(II)-Fe(III)-As(V)-HCl–H2O systems from 5 to 90 °C

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)-HClO₄-H₂O and Fe(II)-Fe(III)-As(V)-HCl-H₂O systems using redox potential measurements from 5 to 90 °C. The association constants for FeHAsO₄⁺ and FeH₂AsO₄^2 + 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 ⁻¹mol^− 1, respectively. Activity coefficients for H⁺, ClO₄⁻, Fe^2 +, Fe^3 +, HAsO₄^2 −, and H₂AsO₄⁻ at 25 °C in the form of the Hückel 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%.