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 (P-O2 < 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, Se-4(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 K-0 = -6.57 +/- 0.15 (5 < pH < 8) 4Se(cr) + 2e(-) = Se-4(2-) log K-0 = -16.67 +/- 0.03 (9 < pH < 13) and ion interaction coefficients for HSe- and Se42- versus Na+, epsilon(HSe-, Na+) = -0.01 +/- 0.10 and epsilon(Se-4(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 Delta(f)G(m)(0)(Se(am)) = 6.0 +/- 2.2 kJ mol(-1), Delta(f)G(m)(0)(Se-2(2-)) = 111.64 +/- 1.61 kJ mol(-1), Delta(f)G(m)(0)(Se-3(2-)) = 98.68 +/- 1.02 kJ mol(-1) , and Delta(f)G(m)(0)(Se-4(2-)) = 95.14 +/- 0.17 kJ mol(-1).