J. MATER. CHEM., 1991, 1(4), 637-642 Transformation of Schoepite into Uranyl Oxide Hydrates of the Bivalent Cations Mg2+,Mn2+ and Ni2+ Renaud Vochten," Laurent Van Haverbekeb and Roger Sobry" a Laboratorium voor chemische en fysische mineralogie, Universiteit Antwerpen (RUCA), Middelheimlaan I, B 2020 Antwerpen, Belgium Laboratorium voor anorganische scheikunde, Universiteit Antwerpen (RUCA), Groenenborgerlaan 171, 82020 Antwerpen, Belgium lnstitut de physique, Universite de I'etat de Liege, Sart Tilman, B 4001 Liege, Belgium The magnesium, manganese and nickel uranyl oxide hydrates have been synthesized by the reaction of the appropriate metal salts with schoepite at elevated temperature. After separation from the reaction mixture, the chemical composition of the different compounds was determined, and the chemical formulae were calculated based on different structural hypotheses.X-Ray powder diffraction data were obtained from each of the three species. After indexing and unit-cell parameter determination, the most plausible space groups were proposed, and compared with the known space groups of existing uranyl oxide hydrates. Thermal analysis was performed to determine the character and the number of water molecules in the structures. The solubility and solubility products were determined, and the influence of the different uranyl species was evaluated. Finally, to examine the presence of a charged double layer, the zeta potential was determined. Keywords: Uranyl oxide hydrate; Schoepite; X-Ray diffraction; Powder diffraction For the past few years, we have been studying the formation and the transformation of secondary uranium minerals in nature. In order to understand these phenomena more clearly, we have performed several syntheses of these minerals in the laboratory.Previously, a number of uranyl oxide hydrates (uranates) of the bivalent cations of calcium, barium, lead and strontium were synthesized in aqueous media at room temperature or elevated temperature. For these syntheses, the reader is referred to the papers of Potdevin and Brasseur,' Peters,2 Pr~tas,~ Bignand' and Brindley and Gillard and P~tdevin,~ Bastovanov.6 In a previous study,7 schoepite H30+[U020(OH)] was transformed directly into ca[(uo&o4(oH)6] 8H20 becquerelite, Ba[(U02),04(0H)6]*8H20 billietite and PbO*2U03 *2H20 wolsendorfite by treating with the appro- priate cations.In the present study, the uranyl oxide hydrates of Mg2+, Mn2+ and Ni2+ were synthesized in the same way by adding the cation solution to schoepite. The exchange of the H30+ ions by the bivalent cations was performed in order to give us a positive confirmation of the oxonium hypothesis of Brasseur.* It also indicates that this hypothesis is not restricted to naturally occurring minerals, but may be extended to chemical compounds that have not (yet) been found in natural formations. Experimental Schoepite was synthesized either by the reaction of C02,O2 and H20 on U3o8' or by the hydrolysis of uranyl acetate' in aqueous solution at 373 K.All other compounds were commercially available reagent-grade chemicals. The relative amounts of the bivalent cations were deter- mined by atomic absorption spectrometry. For this purpose, a Pye Unicam PU-9200 atomic absorption spectrometer was used, equipped with, respectively, an Mg, an Mn or an Ni lamp. Uranium was determined spectrophotometrically by means of Arsenazo 111. Measurements were performed with a Pye Unicam SP8- 100 UV-VIS absorption spectrometer at a wave- length of 662.5 nm. Thermal analysis was carried out using a DuPont 990 thermal analysis controller equipped with either a DSC-9 10 differential scanning calorimeter or a TG-95 1 thermogravi-metric analyser. A heating rate of 5 K min-' was used in combination with a nitrogen flow of 20 cm3 min-'.The densities were determined by measuring the buoyancy in toluene using a Cahn RG electrobalance. X-Ray powder diffraction data were obtained by means of a Guinier-Hagg camera, with a diameter of 100mm, using Cu-Kcr, radiation, 1= 1S406 A and silicon powder NBS-460 as an internal calibrant. Relative intensities were determined with a Zeiss MD-100 microdensitometer. Measurement of pH values was carried out with a Radi- ometer PHM-62 pH meter equipped with a standard com- bined glass electrode. The electrophoretic mobility of the particles in suspension was measured in a Rank Brothers Mark I1 cylindrical micro- electrophoretic cell with an applied magnification of 200.We used platinum electrodes, for which the polarity was reversed after each measurement. The calibration was carried out by means of a standard quartz suspension, with an accuracy of 1 mV. Results and Discussion The uranyl oxide hydrates were easily obtained by treating 1 g of schoepite with 100 cm3 of an 0.5 mol dm-3 solution of MgS04, MnSO, and NiS04, respectively, at 333 K over 2 weeks. The remaining solids were filtered off, and dried under atmospheric conditions; non-fluorescent powders were obtained for the magnesium (lemon-yellow), manganese (orange) and nickel (ochre) uranyl oxide hydrates. The results of the chemical analyses on the three compounds are given in Table 1, together with their formulae, their densities and the number of molecules in the unit cell. The formulae have been deduced from the oxidic composition, J. MATER.CHEM., 1991, VOL. 1 Table 1 Chemical composition of synthetic Mg, Mn and Ni uranyl oxide hydrates composition (wt.%) composition (XO :U03:H20) z value measured density X XO U03 H,O total calculated ideal cell volume/A3 (25 "C)/g ~rn-~ calculated ideal MgMn 2.10 6.40 86.00 83.00 11.60 10.50 99.70 99.90 1.03:5.98:12.79 0.94:3.02:6.03 1:0.6: 13 1 :3:6 260 1 .O 2593 5.128 5.298 4.03 7.98 4 8 Ni 6.70 82.96 9.63 99.29 0.94:3.02: 5.60 1 :3:6 2555.8 5.310 6.16 6 taking into account the structural formulae of protasiteg and that can be represented as [(U02)604(OH),]~n- and of billietite" and becquerelite.' [(U02)303(OH)2]in-.Brasseur' postulated the presence of According to Brasseur' and Sobry,12 the general oxidic oxonium and hydroxyl ions in uranates, and proposed a formula of the uranates can be expressed as: general formula for uranates mXO-2U03-(4-2m)H20 Oimi 2 X2+(H30+)2 -mC(UO2)202+,(OH), -mI 0 5m 5 2 Based on the oxidic composition, the studied uranyl oxide in which the H30+ ions are balanced by OH ions. hydrates can be represented as The presence of oxonium ions in the structure of hydrated +XO-2U03 *23H20 (X =Mn, Ni) uranates was clearly demonstrated uia NMR spectroscopy by Sobry.l2 Simple ion-exchange experiments proved that biva- and lent cations of uranates can easily be substituted by univalent fMgO.2U03 *3+H2O cations.Taking into account the structure of schoepite proposed by which results in the overall formulae Sobry,14 the formation of uranates of bivalent cations can be XO-3U03*4H20 (X=Mn, Ni) written as and MgO*6U03 10H20 The results from the chemical analyses in Table I show a higher water content. This may be regarded as zeolitic water. The results of the X-ray powder diffraction data show the The Mn and Ni compounds can be represented as remarkable fact that the nickel uranyl oxide hydrate is charac- X2 [(U02)303(OH)2]2-5H20 (X2+ =Mn2+,Ni2+) terized by fewer reflections than the magnesium and manga- + nese homologues. The first 20 diffraction lines of the Mn, Ni whereas the Mg compound can be represented as and Mg uranyl oxide hydrates are given in Table 2, together with their relative intensities and hkl values.From these data Mg2f[(U02)604(OH)6]2-10H20 it is clear that the three synthetic compounds can be easily These formulae meet the prediction of Evans13 who suggested distinguished from one another. that the uranyl oxide hydrates would show mainly pentagonal Since no single-crystal measurements could be carried out, co-ordination around the uranyl ion, with structures the unit-cell parameters were calculated from the powder [(UO2)O2(0H),] and [(UO2)O3(0H),]. These polyhedra give diffraction data using the method of Cox,l5 resulting in a rise to the formation of sheets" with a polymeric structure pseudohexagonal orthorhombic sublattice with approximate Table 2 X-Ray powder data of synthetic uranyl oxide hydrates of bivalent cations (Mn, Ni, Mg)" +Mn0.2U03 .23H20 +Ni0.2UO3 .25H20 3Mg0 .2U03 .3+H20 dabs hkl 1/10 dabs hkl 1/10 dabs hkl 1/10 7.5 1 002 65 7.5 1 002 65 7.5 1 002 50 6.16 210 15 3.872 031 10 6.22 020 10 5.76 112 25 3.746 131 65 5.7 1 21 1 50 4.718 103 10 3.606 104 25 4.790 212 10 4.6 13 013 5 3.487 230 90 3.872 213 10 3.872 213 20 3.237 402 30 3.734 123 30 3.734 004 50 3.156 232 100 3.585 032 100 3.565 400 95 2.553 234 100 3.486 132 10 3.536 230 100 2.494 225 40 3.220 402 100 3.393 223 25 2.402 342 15 3.095 420 10 3.220 313 100 2.333 610 10 3.021 42 1 50 3.188 024 100 2.03 1 515 80 2.846 3 14 40 3.065 420 15 2.019 623 85 2.797 240 10 3.036 040 40 1.977 451 I0 2.773 510 10 2.846 115 35 1.946 254 75 2.636 242 10 2.821 042 35 1.876 055 30 2.583 520 50 2.76 1 205 10 1.775 346 65 2.533 34 1 10 2.625 333 10 1.746 643 60 2.475 006 10 2.573 404 85 1.697 536 60 2.384 151 10 2.489 006 25 1.652 626 60 2.34 1 530 30 +67 additional lines +I7 additional lines +80 additional lines a Cu-Ka, radiation (A= 1.5406 A), 40 kV,20 mA.J. MATER. CHEM., 1991, VOL. 1 values of 0.701, 0.405 and 0.745 nm for a', b' and c', respect-ively, (a'xb'J3). The real lattice parameters a, b and c, corresponding to a larger orthorhombic cell, can be obtained by multiplying a' by 2, b' by 3 and c' by 2. The obtained values are summarized in Table 3. These cell parameters are in good agreement with the values obtained for other uranates by Sobry.I2 When these parameters were used in the computer program of Visser,16 all lines could be indexed with an accuracy of 0.5 pm.The obtained hkl values for the three species agree with the reflection conditions of the space groups Pnma for the manga- nese and nickel compounds and Pbn2, or Pb21 for the magnesium uranyl oxide hydrate. Based on these results, we assume that the compounds belong to these space groups. The dehydration of the three compounds was studied by thermogravimetry and differential scanning calorimetry. The combination of the two techniques shows for the three com- pounds four well separated endothermic dehydration steps. Table4 lists the four temperature ranges and the number of water molecules lost in each range for the three species.The dehydratation mechanism is extensively discussed by Sobry.12 Taking into account the general formula mXO .2U03 '(4 -2rn)H20 and the total amount of water molecules lost, it is obvious that some water molecules are not structurally bound. They may be regarded as surface- bound or zeolitic-bound water molecules. This zeolitic water in the magnesium and manganese compounds is lost below 400 K and in the nickel compound below 425 K. The remain- ing dehydration steps agree within 25% with the temperature intervals proposed by Sobry.12 The solubility of the three compounds in aqueous medium was measured after equilibration of the solid phases at different pH values at 303 K over 2 weeks. The pH was adjusted to the desired value with nitric acid or sodium hydroxide.After equilibration, the suspensions were centrifuged, the pH meas- ured accurately and the metal-ion concentrations determined. From the metal-ion concentrations, the solubilities of the three compounds were calculated. The results are given in Table 5, together with the pH values of the solutions. Taking into account the oxidic formulae of the compounds, the dissolution can be expressed as +nX, + +pUO:+ +2(n +p)OH- leading to a solubility product which can be expressed as In order to calculate the solubility product, the exact concen- tration of each ion must be known accurately. There are no problems for the metal ions because they exist only as free ions in solution. This is also the case for the hydroxyl ions, since the pH value is known precisely.However, this is difficult for the uranyl ions, because of the various complexes that are formed between uranyl and hydroxyl ions. The formation of these complexes can be described generally as pUO$++qH20-+(U02)p(OH)~P-4)++4H+ These reactions have an associated formation constant which can be expressed as [(U02)p(OH)fP-q)+][H 'Iq PP, = [uo;'3" The most relevant uranyl-hydroxyl formation constants have been subtracted from Sillen and Martell," Perrin" and Hogfeldt." Using these values, we can express the total uranyl concentration as follows 843[U0,]=3 +4+ 853 [uof+]3i -1 CH 1 CH+I5 Table 3 Unit cell parameters in %i of synthetic uranyl oxide hydrates of Mn, Ni and Mg" m xo n a Aa b Ab C Ac unit cell volume/A3 3 MnO 84 14.2 198 0.0078 12.1880 0.0075 14.9623 0.0085 2593.1 3 NiO 37 14.2860 0.0 165 12.0283 0.0 154 14.8879 0.01 70 2555.8 3 MgO 88 14.2818 0.01 15 12.2647 0.0107 14.8610 0.01 17 2603.0 Aa, Ab and Ac represent the maximum deviations of the cell parameters; n =number of reflections used in lattice parameter calculations.Table 4 Dehydration data of synthetic Mg, Mn and Ni uranyl oxide hydrates Mg uranate Mn uranate Ni uranate temp. range/ "C moles H20 lost temp. range/ "C moles H20 lost temp. range/ "C moles H20 lost <125 3.06 <125 1.78 <175 1.92 125-250 4.38 125-150 2.45 175-250 2.02 2 50- 3 60 2.74 150-450 1.53 250-400 1.38 350-600 2.6 1 450-600 0.27 400-600 0.28 Table 5 Solubilities at 25 "C of Mg, Mn and Ni uranyl oxide hydrate at different pH values Mg uranyl oxide hydrate Mn uranyl oxide hydrate Ni uranyl oxide hydrate PH solubility/10-5 mol dm-3 PH solubility/10-5 mol dm-j PH solubility/10-5 mol dm-3 3.91 254.00 4.25 78.23 4.10 160.00 4.40 43.13 4.35 30.20 4.27 64.70 4.73 4.93 5.30 6.98 5.88 9.20 5.52 2.46 6.19 2.00 5.83 2.2 1 6.07 1.06 6.5 1 0.78 6.28 0.50 6.43 0.20 6.61 0.47 6.60 0.20 Since the total uranyl concentration can be derived from the metal-ion concentration, the concentration of the free uranyl ions can be determined at a given pH.After introducing these values in the expressions of the solubility product, we obtained 156.3, 86.2 and 87.7 for the pK,, values of the magnesium, manganese and nickel uranyl oxide hydrate, respectively, with an accuracy of 4.Since the number of ions in the expression of the solubility is very large, it is clear that the pK,, values must be interpreted with some caution. For example, the magnesium uranyl oxide hydrate has a solubility product which can be expressed as K,, =[Mg' '1 [UO? l6 [OH-]'4+ J. MATER. CHEM., 1991, VOL. 1 Indeed the solubility product is largely influenced by the factor [OH-]. If one assumes pH 7, it is clear that in this case the solubility product is influenced by since [OH-] must be written as [OH]'4. Therefore, these extremely low solubility products may well be responsible for relatively moderate solubilities. Inverse calculations were also performed.Given the pK,, of the compound, the relative contribution of the different uranyl species to the total uranyl content is determined in the pH range 2-12. Fig. l(a), (b)and (c) show these contri- butions for the magnesium, manganese and nickel uranyl oxide hydrates, respectively. All three graphs show the same 7 14 PH 7 14 PH Fig. 1 J. MATER. CHEM., 1991, VOL. 1 641 0 7 14 PH Fig. 1 Distribution of the different uranyl species between pH 2 and 12 for a saturated solution at 25 "C for (a) magnesium, (b)manganese and (c) nickel uranyl oxide hydrates tendencies. Above pH 9, all three compounds show a domi- nant presence of (UO,)(OH)+. In the pH range 2-8, the presence of UO; +,(UO,),(OH);+ and (UO,),(OH); varies according to the sequence Mg-Ni-Mn.From these graphs, it is clear that the large difference in solubility product does not cause large differences in the uranyl species distribution, but is mainly a consequence of the chemical composition. In order to determine the variation of the surface charge of these compounds us. the pH, the zeta potential was meas- ured. In order to avoid the formation of negatively charged uranyl carbonato complexes, the suspensions of very-fine powder particles (35-75 pm) were shaken over a period of 1 week in a nitrogen atmosphere. The measurements of the zeta potential were carried out also in a nitrogen atmosphere. The zeta potential (expressed in mV) of an electrical double layer is 112890 D +Iu with D the relative permittivity of the solvent, +I the viscosity (in lo-' Pa s) of the solvent and u the electrophoretic mobility of the particles in the suspension (in pm cm s-' V-I).Fig. 2 shows the variation of the zeta potentials of the three species us. the pH. It is clear that the surface of the uranates becomes more negatively charged with increasing pH, prob- ably caused by an increased static presence of hydroxyl ions at the surface of the particles. The variation of the zeta potential as function of pH is more pronounced for the nickel and manganese compounds than for the magnesium one. The points of zero charge have been determined at pH values 4.10, 4.25 and 4.40 for the magnesium, nickel and manganese compounds, respectively. Conclusions The fact that the magnesium, nickel and manganese uranyl oxide hydrates are synthesized by simple exchange reactions between schoepite and the appropriate metal ions indicates that in schoepite a positively charged ion other than uranyl -30--10-0-L PH Fig.2 Variation of the zeta potential as function of the pH at 25 "C for (a)nickel, (b)manganese and (c) magnesium uranyl oxide hydrates is present. This can be seen as a confirmation of the oxonium hypothesis of Brassew.* Because the chemical composition, the unit-cell parameters, the space groups and the thermal behaviour all agree very well with bivalent uranyl oxide hydrates occurring in natural environments, it may be concluded that they are structurally comparable. The solubility product of the magnesium com- pound is of the same order of magnitude as those of the calcium and barium uranyl oxide hydrates. The solubility products of the transition metals nickel and manganese are much higher.This can be explained by the different chemical composition of the latter compounds. Because of the anal- ogous uranyl distribution pattern, it may be concluded that this is not an indication of different behaviour. There is a striking similarity between all uranyl oxide 642 J. MATER. CHEM., 1991, VOL. 1 hydrates studied in this and previous papers. Therefore, it is very remarkable that in the series of uranyl oxide hydrates that we synthesized, the magnesium, manganese and nickel uranyl oxide hydrates have not yet been found in nature.Our experiments do not indicate any reason why these compounds 4 5 6 7 S. Gillard and H. Potdevin, Bull. SOC. R. Sci. Liige, 1959, 28, 222. C. Bignand, Bull. SOC.Fr. Mineral. Crist., 1955, 78, 1. G. W. Brindley and M. Bastovanov, Clay Clay Minerals, 1982, 30, 135. R. Vochten, E. De Grave and H. Lauwer, Mineral. Petrol., 1990, do not occur in a natural environment. Since magnesium ions in particular are present in a number of uranium deposits, the question why these uranyl oxide hydrates do not occur remains open. 8 9 10 41, 247. H. Brasseur, Bull. SOC.Fr. Mineral. Crist., 1962, 85, 242. M. K. Papagoa, D. E. Apleman and J. M. Stewart, Mineral. Magaz., 1986, 50, 125. J. Piret-Meunier and P. Piret, Bull. Mineral., 1982, 105, 606.11 M. K. Papagoa, D. E. Appleman and J. M. Stewart, Am. Mineral., The authors wish to thank the National Fund for Scientific Research (Belgium) for financial support. They are grateful 12 13 1987, 72, 1230. R. Sobry, J. Znorg. Nucl. Chem., 1973, 35, 1515. H. T. Evans Jr., Science, 1963, 141, 154. for the technical assistance of Mr. K. Van Springe1 and Miss A. Vanysacker. 14 15 R. Sobry, Am. Mineral., 1971, 56, 1065. A. A. Cox, A Program for Least-Squares Rejnement of Unit Cell Dimensions, The City University of London, 1967. 16 J. W. Visser, J. Appl. Crystallogr., 1969, 2, 89. 17 L. G. Sillen and A. E. Martell, in Stability Constants of Metal- References 18 ion Complexes, The Chemical Society, London, 1964, pp. 50-51. D. D. Perrin, in Ionisation Constants of Inorganic Acids and Bases 1 H. Potdevin and H. Brasseur, Bull. Acad. R. Belg. Cl. Sci., 1958, 44, 874. 19 in Aqueous Solution, Pergamon Press, Oxford, 1982, pp. 122-124. E. Hogfeldt, in Stability Constants of Metal Zon Complexes- 2 J. M. Peters, Bull. SOC.Sci. Liige, 1965, 34, 308. 3 J. Protas, Bull. SOC. Fr. Mineral. Crist., 1956, 79, 350; J. Protas, C.R. Acad. Sci. Paris, 1957, 244, 91; J. Protas, Bull. SOC. Fr. Part A: Inorganic Ligands, Pergamon Press, Oxford, 1982, pp. 40-4 1. Mineral. Crist., 1959, 82, 239. Paper 1/01897C; Received 25th February, 1991