Thermodynamic properties of FeAsO4·0.75H2O - a more favorable disposable product of low As solubility

and So for the ferric orthoarsenate of 154.2 ± 0.5 J mol− 1 K− 1, respectively. These data give ΔfGo(FeAsO4·0.75H2O) = − 993.15 ± 2.57 kJ mol− 1 and ΔfHo(FeAsO4·0.75H2O) = − 1140.38 ± 2.59 kJ mol− 1. The calculated enthalpy was checked by thermodynamic considerations of the system FeAsO4-H2O and found in a good agreement with the expected value. Heat capacity at temperatures higher than the ambient ones was fit by a Berman-Brown polynomial Cp = k0 + k1T-0.5 + k2T− 2 + k3T− 3 with the coefficients k0 = 1.774 × 102, k1 = 6.531 × 102, k2 = − 1.265 × 107, and k3 = 1.754 × 109 (fit valid between 280 and 465 K). Ferric orthoarsenate is less soluble than scorodite (FeAsO4·2H2O) and could be thus an interesting alternative for the disposal of arsenical waste. Its use will be certainly hampered by the high temperature required for its synthesis (> 200 °C). One could, however, attempt to tweak the pressure oxidation technology such that the formation of ferric orthoarsenate would be enhanced, hence producing a waste form with relatively low arsenic solubility.