Reaction of Plutonium(VI) with Orthosilicic Acid Si(OH)4

Abstract

Reaction of Pu(VI) with Si(OH)4 (at concentration 0.004–0.025 mol l–1) in a 0.2 M NaClO4 solution at pH 3–8 is studied by spectrophotometric method. In the range of pH 4.5–5.5, PuO2(H2O)4OSi(OH)3+ complex is formed, while at pH > 6, PuO2(H2O)3O2Si(OH)2 or hydroxosilicate complex PuO2(H2O)3(OH)OSi(OH)3 is recorded. The equilibrium constants are calculated for the reactions of formation of PuO2(H2O)4OSi(OH)3+ and PuO2(H2O)3O2Si(OH)2 and their concentration stability constants: log K1 = –3.91 ± 0.17 and log K2≈ –10.5; log β1= 5.90 ± 0.17 and log β2≈ 12.6. The PuO2(H2O)4OSi(OH)3+ complex is significantly less stable than analogous complex of U(VI). Calculations of the forms of Pu(VI) occurrence at the Si(OH)4 concentration equal to 0.002 mol l–1 showed that the maximum fraction of the PuO2(H2O)4OSi(OH)3+ complex is ∼10% (pH 6.5), while the fraction of PuO2(H2O)3O2Si(OH)2 is almost 40% (pH 8).