Prediction of the thermodynamic properties of metal-arsenate and metal-arsenite aqueous complexes to high temperatures and pressures and some geological consequences

The standard thermodynamic properties at 25 degrees C, 1 bar (Delta G(f)(o), Delta H(f)(o), S(o), C(P)(o), V(o), omega) and the coefficients of the revised Helgeson-Kirkham-Flowers equations of state were evaluated for several aqueous complexes formed by dissolved metals and either arsenate or arsenite ions. The guidelines of Shock and Helgeson (Geochim Cosmochim Acta 52: 2009-2036, 1988) and Sverjensky et al. (Geochim Cosmochim Acta 61:1359-1412, 1997) were followed and corroborated with alternative approaches, whenever possible. The SUPCRT92 computer code was used to generate the log K of the destruction reactions of these metal arsenate and metal-arsenite aqueous complexes at pressures and temperatures required by the EQ3/6 software package, version 7.2b. Apart from the Al-AsO(4)(o) and FeAsO(4)(o) complexes, our log K at 25 degrees C, 1 bar are in fair agreement with those of Whiting (MS Thesis, Colorado School of Mines, Golden, CO, 1992). Moreover, the equilibrium constants evaluated in this study are in good to fair agreement with those determined experimentally for the Ca-dihydroarsenate and Ca-hydroarsenate complexes at 40 degrees C (Mironov et al., Russ J Inorg Chem 40:1690, 1995) and for Fe(III)-hydroarsenate complex at 25 degrees C (Raposo et al., J Sol Chem 35:79-94, 2006), whereas the disagreement with the log K measured for the Ca-arsenate complex at 40 degrees C (Mironov et al., Russ J Inorg Chem 40:1690, 1995) might be due to uncertainties in this measured value. The implications of aqueous complexing between dissolved metals and arsenate/arsenite ions were investigated for seawater, high-temperature geothermal liquids and acid mine drainage and aqueous solutions deriving from mixing of acid mine waters and surface waters.