Uranyl carbonato complexes

Uranyl carbonato complexes are important and much studied since they occur in Nature and are of environmental concern this is also the case for other actinide carbonate complexes.40 The C03 ion is exceptionally strongly bound to UO + and similar actinide ions. There are several naturally occurring minerals such as U02C03 while the anion [U02(C03)3]4 has importance in the extraction of U from ores and is responsible for the migration of U02+ in ground waters. Interaction of the 4-ion with HC104 proceeds as follows ... 

A more convincing experiment performed by Crookes [34] involved Fe(OH)3 as a carrier of uranium X (having the more convenient ti/2 = 24 days) precipitated from the soluble uranyl carbonato complex in excess (NH4)2C03. These observations show an insoluble hydroxide, but they did not establish the thorium(IV) chemistry of this Th isotope, the first descendant of Sir Ernest Rutherford (1871-1937) and Frederick Soddy (1877-1956) then showed [35] that ti 2 = 3.6 days of thorium X ( Ra), soluble in aqueous ammonia, is replaced by radio-elements precipitating as hydroxides. In 1907 Rutherford demonstrated [36] that ionium ( °Th with tj j = 80000 years, the immediate ancestor of Ra with t j = 1600 years in uranium minerals), Th (tiy2 = 14100 million years), and radiothorium Th (ti/2 = 1-9 years) cannot be separated by chemical means. 

Ratios of U and U to Th and Ra daughters, combined with differences in chemical reactivity have been used to investigate the formation and weathering of limestone in karst soils of the Jura Mountains, and of the mountains in the central part of Switzerland. Uranium contained within calcite is released during weathering, and migrates as stable uranyl(VI) carbonato complexes through the soil. In contrast, the uranium decay products, Th and Ra,... 

Ion Excha.nge, The recovery of uranium from leach solutions using ion exchange is a very important process (42). The uranium(VI) is selectively adsorbed to an anion-exchange resin as either the anionic sulfato or carbonato complexes. In carbonate solutions, the uranyl species is thought to be the tris carbonato complex, U02(C03) 3 [24646-13-7] and from sulfate solutions the anion is likely to be U02(S0 , where nis ) [56959-61-6] or 2 [27190-85-8], The uranium is eluted from the resin with a salt or acid solution of 1 AfMCl or MNO (M = H", Na", The sulfate solution is... 

Uranyl ion forms complexes with many oxy anions. Both U(VI) and U(IV) compounds dissolve in alkali carbonate solutions with formation of carbonato complexes. Those of the larger alkali cations are only slightly soluble KSp = 6 x 10-5 for both K4[U0 tC03)3] and iNHu tUOitCO, ] 2H 0. 

Uranium is present in seawater as uranyl carbonate or carbonato-complexes. 

BAN/GLA] Banyai, I., Glaser, J., Micskei, K., Toth, I., Zekany, L., Kinetic behavior of carbonate ligands with different coordination modes Equilibrium dynamics for uranyl(2+) carbonato complexes in aqueous solution. A C and 0 NMR study, Inorg. Chem., 34, (1995), 3785-3796. Cited on page 349. 

In sununaiy, the aqueous speciation of U in solutions that are believed to be representative of the relatively oxidizing and reducing groundwaters of the Tono region is dominated by U(OH)4(aq) under reducing conditions, and by the uranyl mono-, di- and tri-carbonato complexes, and (or) U02(0H)3, under relatively oxidizing conditions. Uranous fluoride or phosphate complexes, or their uranyl counterparts, are important in these respective solutions only if pH is less than 6 or 7. 

Of all uranium species existing in solutions those containing uranium in the oxidation state d-5 (normally in the form of U02 moieties) are the least studied. Aqueous solutions of U02 are prone to disproportionation, with the carbonato-complex 002(003)3 being the best known. A number of uranium(V) complexes were characterised in organic solvents [1]. In solutions uranium(V) species are most stable in ionic solvents including molten salts. In the chloride-based melts uranyl(V) chloro-complexes were reported over half of a century ago [2, 3] and their electrochemical, thermodynamic and structural properties have subsequently been studied to a certain extent [4-9]. 

Ikeda A, Hennig C, Tsushima S, Takao K, Dceda Y, Scheinost AC and Bernhard G 2007 Comparative study of uranyl (vi) and-(v) carbonato complexes in an aqueous solution. Inorganic chemistry 46(10), 4212-4219. 

Compared to studies in acidic media, studies on the electrochemical behavior of 1102 " in basic media are more limited. The report from Morris [52] describing voltammetry results for hydroxo and carbonato uranyl complexes is a recent example. Previous studies have been performed mostly in carbonate and bicarbonate solutions. Wester and Sullivan have studied the reduction of 1102 in these solutions to find an electrochemically irreversible process but disproportionation of U(V)02" " was evidenced only in the bicarbonate solutions [67]. 

---