Thermodynamics of arsenates, selenites, and sulfates in the oxidation zone of sulfide ores: IV. Eh-pH diagrams of the Me-Se-H₂O systems (Me = Co, Ni, Fe, Cu, Zn, Pb) at 25°C

Abstract

The behavior of selenium at the Earth’s surface and nearby at low temperatures and pressures is controlled by variations of the redox potential and the acidity of solutions. These parameters determine the migration of selenium and its precipitation as various solid phases. Understanding the mechanism of selenium’s behavior under surface conditions, which is important for solving environmental problems, is an urgent task of contemporary mineralogy and geochemistry. The activities of components in natural waters beyond the zones of natural (oxidation zones) and man-made contamination with selenium (a _ΣSe = 10⁻⁹, a _ΣFe = 10⁻⁵, a _ΣCu = 10⁻⁷, a _ΣZn = 5 × 10⁻⁷, a _ΣCo = 10⁻⁸, a _ΣNi = 6 × 10⁻⁸, a _ΣPb = 10⁻⁸) and in waters formed in the oxidation zone (a _ΣSe = 10⁻⁵–10⁻⁴, a _ΣFe = 10⁻², a _ΣCu = 10⁻², a _ΣZn = 5 × 10⁻², a _ΣCo = 10⁻³, a _ΣNi = 10⁻², a _ΣPb = 10⁻⁴) have been estimated. Eh-pH diagrams were calculated and plotted using the Geochemist’s Workbench (GMB 7.0) software package. The database comprises the thermodynamic parameters of 46 elements, 47 main particles, 48 redox pairs, 551 particles in solution, 624 solid phases, and 10 gases. The Eh-pH diagrams of the Me-Se-H₂O systems (Me = Co, Ni, Fe, Cu, Zn, Pb) were plotted for the average contents of these elements in underground water and for their contents in oxidation zones of sulfide deposits. The formation of Co, Ni, Fe, Cu, Zn, and Pb selenites and selenates at the surface is discussed.