PNNL

Abstract

Spectrophotometric methods were used to investigate the interaction of the NpO22+ ion with orthosilicic acid, Si(OH)4, and polymeric silicic acids (PSA) in aqueous solutions. At ?? = 4.5, the interaction is described by the reaction NpO22+ + Si(OH)4 = NpO2OSi(OH)3+ + H+ with an equilibrium constant of lg K = -2.88 ± 0.12 (ionic strength I = 0.1-0.2). Recalculation to I = 0 gives lg K0 =-2.61 ± 0.12; the stability constant of the complex NpO2OSi(OH)3+ is lg ß0 = 7.20 ± 0.12. Polymerization of Si(OH)4 does not lead to an apparent increase in the constant K. In solutions with ?? higher than 5, and with silicate concentration more than 0.02 mole/l, leads to formation of a second complex, whose structure probably is described by the formula NpO2(=SiO)2(=SiOH)m-2, where (=SiOH)m designates the PSA molecule with superficial Si-OH groups. The factors influencing the apparent equilibrium constants in forming of complexes NpO2OSi(OH)3+ and NpO2(=SiO)2(=SiOH)m-2 are considered upon polymerization of Si(OH)4. Absorption spectra of complexes NpO2OSi(OH)3+ and NpO2(=SiO)2(=SiOH)m-2 were obtained. Molar extinction coefficients at the maxima (500-600 nanometers) are much higher than those of the Np(VI) aquo ion and are about 25-30 l/mol·cm. The stabilities of silicate complexes of all types - MO2OSi(OH)3+, MO2(=SiO)2(=SiOH)m-2, or MO2SiO3 (M = U, Np, or Pu) - decrease in order U> Np> Pu with the greatest difference occurring between Np and Pu.