A Spectrophotometric Study of the Interaction of Uranyl Ions 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 = UO 2 OSi(OH) 3 + + H + in solutions with the ionic strength I = 0.13 0.2 is log K = 32.56 + 0.09 (log K 0 = 32.29 + 0.09 recalculated to I = 0); the stability constant of the complex UO 2 OSi(OH) 3 + (I = 0) is log b 0 = 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 233, and the [true] equilibrium constant recalculated to the actual concentration of silanol groups increases by a factor of 33 4. The absorption spectrum of the complex UO 2 OSi(OH) 3 + was obtained by treatment of the experimental spectra; it has an absorption maximum in the visible range at 422.5 nm, e 422.5 = 35 + 2 l mol !1 cm !1 . At pH higher than 53 6, complexes of UO 2 2+ with PSAs of the composition UO 2 (=SiO) 2 (=SiOH) m !2 are formed. The absorption spectrum of such a complex was obtained.Much attention is given today to the interaction of f-element ions with orthosilicic acid Si(OH) 4 and other silicate species. It has been shown that silicate ions appreciably affect the state and behavior of UO 2 2+ and other actinide ions under widely varied conditions [138]. The interaction of uranyl ions with orthosilicic acid at pH 33 4.5 is described by the equation [13 6, 9] UO 2 2+ + Si(OH) 4 = UO 2 OSi(OH) 3 + + H + (1) with the equilibrium constant [UO 2 OSi(OH) 3 + ][H + ] K = ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ. (2) [UO 2 2+ ][Si(OH) 4 ] The values given in the literature for log K range from 31.37 [5] to 32.92 [3] in 0.13 0.2 M NaClO 4 . Jensen and Choppin [3] believe that the major cause of this inconsistency is the polymerization of Si(OH) 4 , which took place in some studies. Polymeric silicic acids (PSAs) were believed, without due proof, to interact with the UO 2 2+ ion more strongly. The value recommended by the thermodynamic database of the year 2003 [9] is log K = 32.11 + 0.10 at the ionic strength I = 0.13 0.2, whence log K 0 = 31.84 + 0.10 at I = 0 (T = 293.15 K). The interaction of Si(OH) 4 with the PuO 2 2+ ion [10] appeared to be considerably weaker: log K = 33.91 + 0.17 at I = 0.2. Relevant data for the NpO 2 2+ ion are lacking.Thus, the following questions concerning the interaction of hexavalent actinides with Si(OH) 4 remain to be answered. (1) What is the cause of major disagreement between the published values of the constant K (without understanding these causes, the choice of the constant recommended in [9] seems rather arbitrary)?(2) How does the polymerization of Si(OH) 4 affect K?(3) What is the trend in variation of K in the series U(VI)3Np(VI)3Pu(VI)?Here and in the subsequent paper [11] we report on a direct spectrophotometric study of the interaction of Si(OH) 4 and PSAs with the UO 2 2+ and NpO 2 2+ ions.EXPERIMENTAL Uranyl solutions were prepared by dilution of...

Uranyl Adsorption and Speciation at the Fused Silica/Water Interface Studied by Resonantly Enhanced Second Harmonic Generation and the χ(3) Method

“…X-ray microprobe studies are notable exceptions, but imaging the spatial and temporal evolution of U(VI)-surface interactions in real time and under bulk aqueous solution and flow conditions has not yet been achieved. In addition, these experiments are often performed at either acidic or alkaline conditions ,,,, or under carbonate-free conditions ,, to simplify the bulk aqueous speciation. Nonlinear optical methods can overcome these challenges and yield experimental surface-specific thermodynamic, spectroscopic, charge state, speciation, and kinetic data complementary to synchrotron studies.…”

“…The SHG studies performed in this work utilize the pH- and ligand-dependent aqueous uranyl absorption bands centered at 306 nm as an intrinsic label for adsorbed uranyl species (see Figure ). ,,, Second harmonic generation (SHG) is a surface-specific spectroscopy applicable to studying buried interfaces. Within the electric dipole approximation, the SHG process is symmetry forbidden in centrosymmetric media, − therefore the SHG response from the silica/water interface studied here only results from the interfacial region between the bulk phases where centrosymmetry is broken.…”

Characterizing solution and solid-phase amorphous uranyl silicates

“…Available mineral surfaces and dissolved anions, in combination with groundwater Eh and pH are known to play a central role in uranyl-ion transport. A well studied example is the interaction of uranyl with dissolved silica (Porter and Weber, 1971;Satoh and Choppin, 1992;Jensen and Choppin, 1998;Moll et al, 1998;Hrnecek and Irlweck, 1999;Yusov and Fedoseev, 2005), silica gels (Reich et al, 1998;Allard et al, 1999;Gabriel et al, 2001) and silicate-mineral surfaces (Chisholm-Brause et al, 1994;Hudson et al, 1999;Waite et al, 2000;Froideval et al, 2003;Walter et al, 2005;Arai et al, 2006). Although the mineral geochemistry of uranyl silicates 0016-7037/$ -see front matter Ó 2007 Elsevier Ltd. All rights reserved.…”

“…Whereas this technique has been often applied to uranyl silicates, (ICSD, 2006) it is only effective for samples exhibiting long-range order. Structural and chemical information has been sought for colloidal and amorphous samples through indirect probes such as IR/Raman (Plesko et al, 1992;Frost et al, 2006a,b), NMR (Harris and Newman, 1977;Harris et al, 1980;Cho et al, 2006) optical, (Porter and Weber, 1971;Yusov and Fedoseev, 2005) and fluorescence spectroscopies Wang et al, 2005) by comparing spectra to those obtained from known phases. X-ray absorption spectroscopy (XAS), a single-ion probe that provides direct information about structural correlations has been applied liberally to solution and solid-phase samples (Dent et al, 1992;Reich et al, 1996;Giaquinta et al, 1997;Thompson et al, 1997;Hudson et al, 1999;Sylwester et al, 2000;Antonio and Soderholm, 2006).…”

U(VI) Adsorption and Speciation at the Acidic Silica/Water Interface Studied by Resonant and Nonresonant Second Harmonic Generation

“…The surface SHG spectrum revealed a plateau region of weak enhancement between 310 and 330 nm and then increasing enhancement as you move toward shorter wavelengths in the UV. In the bulk, the UV–vis absorption in the plateau region has been reported to be due to the uranyl ion complexation by carbonate and hydroxyl ions and decreases with decreasing pH. It is, therefore, not surprising that the observed enhancement in this region is weak at pH 4.…”