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Experimental Equilibrium Data for the Uranium(IV) Thiocyanate System

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Experimental Equilibrium Data for the Uranium(IV) Thiocyanate System Group 14 compounds and complexes 333 Table V.45: Experimental equilibrium data for the uranium(IV) thiocyanate system. t β(a) β◦(b) Method Ionic log10 q log10 q Reference Medium (◦C) U4+ +SCN− USCN3+ emf 0.6 M HClO4 20 1.49 ± 0.03 2.97 ± 0.06 [54AHR/LAR2] +0.4 MNaClO4 dis 1.00 M HClO4 10 1.78 [55DAY/WIL] (c) 1.00 M NaClO4 25 1.49 ± 0.20 2.97 ± 0.21 1.00 M NaClO4 40 1.30 ............................. 4+ − 2+ U + 2SCN U(SCN)2 emf 0.6 M HClO4 20 1.95 ± 0.17 4.20 ± 0.22 [54AHR/LAR2] +0.4 MNaClO4 dis 1.00 M HClO4 10 2.30 [55DAY/WIL] (c) 1.00 M NaClO4 25 2.11 ± 0.30 4.36 ± 0.30 1.00 M NaClO4 40 1.98 ............................. 4+ − + U + 3SCN U(SCN)3 emf 0.6 M HClO4 20 2.2 [54AHR/LAR2] +0.4 MNaClO4 β (a) log10 q refers to the equilibrium and the ionic strength given in the table. β◦ I (b) log10 q is the corresponding value corrected to = 0 at 298.15 K. (c) Uncertainty estimated by this review. H◦ q − ± · −1 ∆r m(V.176, = 1, 298.15 K) = (27 8) kJ mol H◦ q − ± · −1 ∆r m(V.176, = 2, 298.15 K) = (18 4) kJ mol These values are used to derive the enthalpies of formation of the two complexes 3+ 2+ USCN and U(SCN)2 : H◦ 3+, , . − . ± . · −1 ∆f m(USCN aq 298 15 K) = (541 8 9 5) kJ mol H◦ 2+, , . − . ± . · −1 ∆f m(U(SCN)2 aq 298 15 K) = (456 4 9 5) kJ mol . S◦ Using the selected reaction data, the entropies are calculated via ∆r m. S◦ 3+, , . − ± · −1 · −1 m(USCN aq 298 15 K) = (306 35) J K mol S◦ 2+, , . − ± · −1 · −1 m(U(SCN)2 aq 298 15 K) = (107 42) J K mol 334 Discussion of data selection V.7.1.4.2. Solid uranium thiocyanates No thermodynamic data are available on any solid uranium thiocyanate compounds. V.7.2. Silicon compounds and complexes V.7.2.1. Aqueous uranium silicates The only experimental information on uranium silicate complexes refers to the reac- tion 2+ + + UO2 +Si(OH)4(aq) UO2SiO(OH)3 +H (V.177) ∗K . − . ± . ◦ with log10 (V 177) = (1 98 0 13) at 25 C and in 0.2 M NaClO4 solution [71POR/WEB]. The experimental data clearly indicate that a complex formation takes place. However, it was not demonstrated by Porter and Weber [71POR/WEB] − that the ligand really is SiO(OH)3 . The method of preparing the ligand solution indicates that considerable quantities of polysilicates might be present. From the known value of the first dissociation constant of Si(OH)4(aq), cf. Chapter VI, it can K . − . be calculated that log10 (V 178) = 1 98 + 9 55 = 7 57. 2+ − + UO2 +SiO(OH)3 UO2SiO(OH)3 (V.178) This value is unexpectedly large and indicates that the ligand must have a larger negative charge and that a chelate complex must be formed. Both these facts sug- gest that the complex contains a polynuclear silicate ligand. The experimental in- formation on aqueous uranium silicate species is not sufficiently precise to define a stoichiometry and to evaluate an equilibrium constant. However, aqueous uranium silicate complexes may well exist in groundwater systems, and additional experimen- tal investigations are necessary to decide the type of complexes formed and their stabilities. V.7.2.2. Solid uranium silicates V.7.2.2.1. (UO2 )2 SiO4 · 2H2 O(cr) In the uranium(VI)-silicate mineral group, (UO2)2SiO4·2H2O(cr), soddyite, is the only compound reported [81STO/SMI]. Thermodynamic data for this solid phase have not been reported. V.7.2.2.2. USiO4 (cr) Coffinite, USiO4(cr), is a relatively abundant mineral in reduced sedimentary ura- nium ore deposits. This mineral generally forms small crystals (< 5µm), and is almost always associated with amorphous USiO4, uraninite and auxiliary miner- als. Coffinite minerals have been synthesized only with difficulty because many Group 14 compounds and complexes 335 particular conditions are necessary: reducing media, basic pH (7 < pH < 10), solu- tions rich in dissolved silicate. Coffinite minerals are always obtained in association with UO2(cr) and SiO2(cr) [59FUC/HOE]. Therefore, it is very difficult to deter- mine thermodynamic data for pure coffinite experimentally. Brookins [75BRO] was the first author to estimate a value for the Gibbs energy of formation of coffinite, G◦ , , . − . · −1 ∆f m(USiO4 cr 298 15 K) = 1907 9kJ mol . From this value the aqueous equi- librium between coffinite and uraninite according to Eq. (V.179) USiO4(cr) + 2H2O(l) UO2(cr) + Si(OH)4(aq) (V.179) −6.9 should be established at 10 M dissolved silica (8 ppb as SiO2). This value is much lower than the average value (17 ppm) found in most groundwaters. Con- sequently, according to Brookin’s estimate, coffinite should be more stable than uraninite in natural waters. This result seems to be unrealistic because uraninite is more abundant than coffinite. Langmuir [78LAN] assumed an average silica con- −3 centration of 10 M (60 ppm as SiO2) for the coffinite-uraninite equilibrium ac- cording to Eq. (V.179). This assumption (with auxiliary data from Ref. [78LAN]) G◦ , , . − . · −1 leads to ∆f m(USiO4 cr 298 15 K) = 1882 4kJ mol [80LAN/CHA] (the value of −1891.2kJ· mol−1 reported by Langmuir [78LAN] was in error). This estimated value [80LAN/CHA] seems to be more reasonable than that suggested by Brookins [75BRO]. Therefore, this review accepts Langmuir’s assumption, and calculates G◦ , , . − . ± . · −1 ∆f m(USiO4 cr 298 15 K) = (1883 6 4 0) kJ mol based on auxiliary data presented in Tables III.1 and IV.1. The uncertainty is esti- mated by this review. Langmuir [78LAN] estimated the entropy of this compound as the sum of the values for quartz and UO2(cr). Using data for these two compounds as selected in this review, this entropy value is reestimated and an uncertainty of ±10% is assigned to it. S◦ , , . ± · −1 · −1 m(USiO4 cr 298 15 K) = (118 12) J K mol The enthalpy of formation is calculated from the selected Gibbs energy of formation and entropy. H◦ , , . − . ± . · −1 ∆f m(USiO4 cr 298 15 K) = (1991 3 5 4) kJ mol V.7.2.2.3. USiO4 (am) Amorphous USiO4 is often observed together with crystalline coffinite and uraninite in several reduced sedimentary uranium ore deposits. No experimental thermody- namic data of amorphous USiO4 appeared to be reported. Galloway and Kaiser [80GAL/KAI] estimated the Gibbs energy difference between crystalline and amor- phous USiO4 to be similar to that between crystalline and amorphous UO2.Inthis G◦ , , . − G◦ , , . − . · −1 way, using ∆f m(UO2 cr 298 15 K) ∆f m(UO2 am 298 15 K) = 31 8kJ mol , G◦ − . · −1 they calculated [80GAL/KAI] ∆f m(USiO4, am, 298.15 K) = 1850 6kJ mol . 336 Discussion of data selection As discussed in Section V.3.3.2.2, the value of the Gibbs energy of formation of an “amorphous” form of UO2 is specific to a particular sample, and may be 20 to −1 50 kJ·mol less stable than UO2(cr). However, the evidence for such a large difference in Gibbs energies between crystalline and amorphous USiO4 is not conclusive, and in this review no chemical thermodynamic parameters for USiO4(am) are selected. V.7.3. Lead compounds In the lead-uranium(VI)-silicate group, the only mineral identified is kasolite, PbUO2SiO4·H2O(cr). The exact formula of this compound was determined by Rosen- zweig and Ryan [77ROS/RYA]. Thermodynamic data of this mineral have not been reported. V.8. Actinide complexes V.8.1. Actinide-actinide interactions + The tendency of actinide(V) cations, MO2 , to interact with certain cations (mostly multicharged) was first identified by Sullivan, Hindman and Zielen [61SUL/HIN], + 2+ who found that, in acid solution, NpO2 ions formed complexes with UO2 ions. In the years following this discovery, many publications appeared and cation-cation interactions of the actinides were studied by an array of techniques, mainly absorp- tion spectrophotometry, proton spin relaxation, potentiometric techniques and later Raman spectroscopy. All relevant studies before 1977 were extensively reviewed by [79FRO/RYK]. + 2+ Interactions of UO2 -UO2 (1:1 complexes) were reported [65NEW/BAK] using spectrophotometry over the temperature range 273 to 298.15 K at I = 2 M (NaClO4/ HClO4), and these results yielded values for the enthalpy and entropy changes due to the association. + The interaction of NpO2 with cations was studied very extensively, and quan- + 2+ titative data appeared, among others, on the NpO2 -UO2 system [61SUL/HIN, 79MAD/GUI, 82GUI/BEG]. All these studies, carried out over a range of ionic strengths and media, concluded that weak complexes are formed, with no reported β . value above log10 1(V 180) = 0 4. + 2+ 3+ MO2 +MO2 MM O4 (V.180) However, even restricted to perchlorate media, the data are too scattered to warrant + 2+ + 2+ + 2+ generalization. Even comparing UO2 -UO2 with NpO2 -UO2 or NpO2 -NpO2 is I β . , + 2+ . ± . difficult. For = 2 M (NaClO4/HClO4), log10 1(V 180 UO2 -UO2 )=(122 0 02) β + 2+ was reported [65NEW/BAK], while other authors found log10 1(V.180, NpO2 -UO2 , − . ± . β + 3 M (Na, Mg, H)ClO4)= (0 16 0 01) [61SUL/HIN] and log10 1(V.180, NpO2 - 2+ . ± . UO2 , 7 M (Mg, H)ClO4)=(048 0 02) [79MAD/GUI]. It is interesting to note + + β − .
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