ORIGINAL ARTICLE Prediction of the thermodynamic properties of metal–arsenate and metal–arsenite aqueous complexes to high temperatures and pressures and some geological consequences Luigi Marini Æ Marina Accornero Received: 4 September 2006 / Accepted: 1 November 2006 / Published online: 20 December 2006 Ó Springer-Verlag 2006 Abstract The standard thermodynamic properties at 25°C, 1 bar (DG f o , DH f o , S o , C P o , V o , x) and the coeffi- cients 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 corrobo- rated 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°C, 1 bar are in fair agreement with those of Whiting (MS The- sis, Colorado School of Mines, Golden, CO, 1992). Moreover, the equilibrium constants evaluated in this study are in good to fair agreement with those deter- mined experimentally for the Ca–dihydroarsenate and Ca–hydroarsenate complexes at 40°C (Mironov et al., Russ J Inorg Chem 40:1690, 1995) and for Fe(III)– hydroarsenate complex at 25°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°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 be- tween dissolved metals and arsenate/arsenite ions were investigated for seawater, high-temperature geother- mal liquids and acid mine drainage and aqueous solu- tions deriving from mixing of acid mine waters and surface waters. Introduction Today arsenic requires a high level of concern owing to its distribution in different geo-environmental matrices (e.g., surface waters, groundwaters, soils and stream sediments) as a result of both natural processes and anthropogenic pollution. The latter is chiefly caused by (1) widespread use of As compounds as parasiticides in agriculture and for wood preservation, (2) disposal of As-bearing wastes from mining, ore processing and metallurgy and (3) other industrial activities. Not surprisingly, a huge number of geochemical researches devoted to As have been carried out in the last decades, especially during the last one [see reviews by Cullen and Reimier (1989) and Smedley and Kinniburgh (2002) and references therein]. Particular attention has been paid to the abundance and the re- dox state of dissolved As in natural waters, especially those intended for human consumption. Indeed, As- rich groundwaters have been identified in several countries, including Argentina, Chile, Mexico, Hun- gary, China, Vietnam, Bangladesh and West Bengal Electronic Supplementary Material The online version of this article (http://dx.doi.org/10.1007/s00254-006-0578-2) contains supplementary material, which is available to authorized users. L. Marini (&)  M. Accornero Laboratory of Geochemistry, Dip. Te. Ris, University of Genova, Corso Europa 26, 16132 Genova, Italy e-mail: Imarini@dipteris.unige.it M. Accornero e-mail: m.accornero@unige.it 123 Environ Geol (2007) 52:1343–1363 DOI 10.1007/s00254-006-0578-5