Solubility of Selenium at High Ionic Strength under Anoxic Conditions

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Solubility experiments for selenium were performed at high ionic strengths (1.0 and 2.0 mol dm−3 NaCl) under anoxic conditions (pO2<10−6 atm) from undersaturation and oversaturation directions. Equilibrium was attained in 40 days. The dominant aqueous selenium species were identified as hydrogen selenide, HSe−, at pH between 5 and 8, and polyselenide, Se42−, at pH between 9 and 13 by UV-Vis absorption spectrometry. A solubility-limiting solid was determined to be amorphous selenium at pH between 5 and 8, and crystalline selenium at pH between 9 and 13. The equilibrium constants of Se(am)+H++2e− = HSe− log K0 = −6.57±0.15 (5<pH<8) 4Se(cr)+2e− = Se42− log K0 = −16.67±0.03 (9<pH<13) and ion interaction coefficients for HSe− and Se42− versus Na+, ε(HSe−, Na+) = −0.01±0.10 and ε(Se42−, Na+) = −0.03±0.02, were determined with the specific ion interaction theory. The standard molar free energies of formation of amorphous selenium and polyselenide species were determined to be ΔfGm0(Se(am)) = 6.0±2.2 kJ mol−1, ΔfGm0(Se22−) = 111.64±1.61 kJ mol−1, ΔfGm0(Se32−) = 98.68±1.02 kJ mol−1, and ΔfGm0(Se42−) = 95.14±0.17 kJ mol−1.
2010-05-01