CORRELATION BETWEEN STRUCTURE AND THERMODYNAMIC PROPERTIES OF AQUEOUS SULFUR SPECIES

The thermodynamic properties of aqueous sulfur species can be estimated using a structure-based, group contribution additivity method that is based upon first-order approximations. Free energy and enthalpy data used to calibrate the additivity model were critically selected from a variety of sources. Structural groups were chosen so as to minimize subjectivity and maximize the ease of recognition. We have chosen five fundamental groups to represent the major structural components: (1) S(n)*, polymeric sulfur (as 1/n, where n is the length of the longest continuous sulfur chain in the species), (2) O3SIV; (3) O3SVI; (4) O2SIII and (5) bridging oxygen. In addition, a modified "charge-to-size" ratio (C) is used to model the coulombic interaction between ions and the solvent. Thermodynamic data were broken into two sets, one for monovalent species and another for divalent species. Multiple linear regressions of these thermodynamic data were performed in terms of structural groups to yield the following relationships: DELTAG(f divalent)0 = -176.7O3SIV - 1270.4O3SVI - 691.8O2SIII + 1954.5S(n)* + 757.3O - 1182.2C + 55.0 DELTAH(f divalent)0 = -231.3O3SIV - 1316.9O3SVI - 734.1O2SIII + 2006.5S(n)* + 734.7O - 1212.5C - 33.2 DELTAG(f monovalent)0 = -536-9O3SIV - 635.1O3SVI - 351.7O2SIII - 128.OS(n)* - 140.3O - 828.0C + 64.1 DELTAH(f monovalent)0 = -105.4O3SIV - 366.5O3SII - 0.0O2SIII + 505.8S(n)* + 0.0O + 0.0O - 520.8. R2 Values for regressions exceeded 0.9999 and all residuals were normally distributed. Large values for the regression coefficients indicate that the choice of structural parameters is significant. These coefficients were used to estimate thermodynamic properties of a number of aqueous species for which no experimental data currently exist. Model-derived thermodynamic data were used to find the thermodynamic stability of intermediate sulfur species that occur during the aqueous oxidation of sulfide minerals. This exercise identified at least one thermodynamically feasible pathway for the overall reaction. The data were also used to construct an Eh-pH diagram for aqueous sulfur species with average sulfur oxidation states less than sulfate (VI). The structure-thermodynamic correlation was used to determine the likely structure of the aqueous S2O52- ion, which has been debated over the past 50 years.