A critical review of thermodynamic data for selenium species at 25 degreesC

The accuracy of published selenium thermodynamic data was evaluated to obta in a reliable basis for modelling Se migration behaviour in contaminated ar eas. The Modified Bromley's Methodology was used to correct equilibrium con stant to the standard state (zero ionic strength, 25 degreesC and 1 bar pre ssure). Since redox reactions control the migration of selenium, equilibria between the various Se oxidation states and the associated reactions - aci d-base, precipitation, and complexation - were studied. Although pK(a) values for H2SeO3 and H2SeO4 acid-base reactions are well kn own, an ambiguity exists for the second dissociation of H2Se. A value of 14 appears to be more suitable than a value close to 11.5. The most adequate standard potentials for the redox reactions of selenium are proposed. Becau se of the very low solubility of metal-selenides, most of their solubility product constants are derived from combination of formation thermodynamic d ata selected in the Literature or extrapolated from those of the correspond ing sulfides. Several values are reported for Se(IV) and Se(VI), but they a re not always in agreement. In generally, few values have been determined r ecently and noticeable differences can be observed between old and recent v alues. For complexation reactions, there is a lack of data and for many equ ilibria only one constant is available. (C) 2001 Elsevier Science B.V. All rights reserved.