A calorimetric and thermodynamic investigation of zinc and cadmium hydrous selenites

A calorimetric and thermodynamic investigation of three hydrous selenites, ZnSeO3·2H2O, ZnSeO3·H2O and CdSeO3·H2O, was undertaken. All three phases were synthesized by mixing of aqueous solutions of zinc and cadmium nitrates, accordingly, and sodium selenite and characterized by XRD powder diffraction, energy dispersive X-ray microanalysis, inductively coupled plasma mass spectrometry and thermal analisys methods. The low-temperature heat capacity of ZnSeO3·2H2O, ZnSeO3·H2O and CdSeO3·H2O were measured using adiabatic calorimetry between 5 and 324 K, and the third-law entropies were determined. Values of molar third law entropy Sm°(298 K, ZnSeO3·2H2O, cr.) = 179 ± 1 J·K−1·mol−1, Sm°(298 K, ZnSeO3·H2O, cr.) = 150 ± 1 J·K−1·mol−1, Sm°(298 K, CdSeO3·H2O, cr.) = 171 ± 1 J·K−1·mol−1 were calculated. The expanded uncertainties for Sm° are given at the 0.95 confidence level (k ≈ 2). The enthalpies of formation for ZnSeO3·H2O, ZnSeO3·2H2O and CdSeO3·H2O were determined using H2SO4-solution calorimetry giving ΔfHm° (298 K, ZnSeO3·2H2O, cr) = −1238 ± 2 kJ·mol−1, ΔfHm°(298 K, ZnSeO3·H2O, cr) = −931 ± 2 kJ·mol−1, ΔfHm°(298 K, CdSeO3·H2O, cr) = −906 ± 2 kJ·mol−1. The expanded uncertainties for ΔfHm° are given at the 0.95 confidence level (k ≈ 2). The Gibbs energy of formation for ZnSeO3·2H2O, ZnSeO3·H2O and CdSeO3·H2O at T = 298 K, 1 atm have been calculated on the basis on ΔfHm° and ΔfSm°: ΔfGm°(298 K, ZnSeO3·2H2O, cr.) = −1035 ± 2 kJ·mol−1, ΔfGm°(298 K, ZnSeO3·H2O, cr.) = −790 ± 2 kJ·mol−1 and ΔfGm°(298 K, CdSeO3·2H2O, cr.) = −768 ± 2 kJ·mol−1. The expanded uncertainties for ΔfGm° are given at the 0.95 confidence level (k ≈ 2). Smoothed Cp,m°(T) values between T = 0 K and T = 320 K for ZnSeO3·2H2O(cr.), ZnSeO3·H2O(cr.) and CdSeO3·H2O(cr.) are presented along with values for Sm° and the functions [Hm°(T) − Hm°(0)] and [Gm°(T) − Hm°(0)]. These results motivate a re-evaluation of the natural conditions under which selenites, and selenates replace selenides, and sulfides in the oxidation zones of sulfide ore deposits or upon weathering of technologic waste. The values of ΔfG° for ZnSeO3·2H2O, ZnSeO3·H2O and CdSeO3·H2O were used to calculate the Eh-pH diagrams of the Zn-Se-H2O and Cd-Se-H2O, systems. These diagrams have been constructed for the average contents of these elements in acidic waters of the oxidation zones of sulfide deposits. The behavior of selenium, zinc, and cadmium in the surface environment have been quantitatively explained by variations of the redox potential and the acidity-basicity of the mineral-forming medium. Precisely these parameters determine the migration ability of selenium compounds and its precipitation in the form of various solid phases.