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

Abstract

In this work, we determined the full set of thermodynamic properties for the ferric orthoarsenate, FeAsO4·0.75H2O. This phase could be expected in metallurgical waste forms from pressure oxidation of arsenopyrite-rich ores and its solubility is therefore of interest for the understanding of arsenic mobility in such waste forms. Using a combination of solubility experiments at low pH (<2) and relaxation calorimetry, we have determined log Ksp of −26.54±0.35 for the reactionFeAsO4·0.75H2O=Fe3++AsO43–+0.75H2Oand So for the ferric orthoarsenate of 154.2±0.5Jmol−1K−1, respectively. These data give ΔfGo(FeAsO4·0.75H2O)=−993.15±2.57kJmol−1 and ΔfHo(FeAsO4·0.75H2O)=−1140.38±2.59kJmol−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 465K). 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.