Solubility of Selenium at High Ionic Strength under Anoxic Conditions

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, Se4 2-, 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− = Se4 2- log K0 = −16:67 ± 0:03 (9 < pH < 13) and ion interaction coefficients for HSe− and Se4 2- versus Na+, ε(HSe−, Na+) = −0:01 ± 0:10 and ε(Se4 2-, 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 Δf G m 0(Se(am)) = 6:0 ± 2:2 kJ mol−1, Δf G m 0(Se2 2-) = 111:64 ± 1:61 kJ mol−1, Δf G m 0(Se3 2-) = 98:68 ± 1:02 kJ mol−1, and Δf G m 0(Se4 2-) = 95:14 ± 0:17 kJ mol−1.