A Spectrophotometric Study of the Interaction of Np(VI) with Orthosilicic Acid and Polymeric Silicic Acids in Aqueous Solutions

Abstract

The interaction of UO 2 2+ ions with orthosilicic acid Si(OH)4 and polymeric silicic acids (PSAs) was studied spectrophotometrically. The equilibrium constant of the reaction UO 2 2+ + Si(OH)4 = UO2OSi(OH) 3 + + H+ in solutions with the ionic strength I = 0.1–0.2 is log K = −2.56±0.09 (log K 0 = −2.29±0.09 recalculated to I = 0); the stability constant of the complex UO2OSi(OH) 3 + (I = 0) is log β0 = 7.52±0.09. Formation of small oligomers (degree of polymerization n ≤ 4) has virtually no effect on the apparent constant K. When high polymers are formed (n > 100), the apparent equilibrium constant decreases by a factor of 2–3, and the “true” equilibrium constant recalculated to the actual concentration of silanol groups increases by a factor of 3–4. The absorption spectrum of the complex UO2OSi(OH) 3 + was obtained by treatment of the experimental spectra; it has an absorption maximum in the visible range at 422.5 nm, ɛ422.5 = 35±2 l mol−1 cm− 1. At pH higher than 5–6, complexes of UO 2 2+ with PSAs of the composition UO2(≡SiO)2(≡ SiOH) m − 2 are formed. The absorption spectrum of such a complex was obtained.