Synthesis, characterization and solubility of Bayleyite Mg2[UO2(CO3)3].18H2O

The intensive uranium mining and milling activities in the southeast of Germany (Saxony and Thuringia) have caused severe environmental pollution in this region. The uranium intrusion into the biosphere occurs most likely through aqueous transport. The prediction of the uranium transport behavior is essential for safety assessment studies and for the development of efficient remediation methods.
The seepage water of the rock piles and the flooding water of the mines contain relatively high amounts of magnesium, calcium, hydrogen carbonate/carbonate and sulfate ions. In these waters the pH values range from 7.0 to 8.0 and the uranium content is about 0.02 mmol/L /1/. In a former study /2/ we have found that Ca2[UO2(CO3)3]aq. is the dominating uranium species in mining related waters. Under these conditions the precipitation of the secondary uranium mineral - liebigite, Ca2[UO2(CO3)3] x 10H2O - is possible. In oxidizing water which contains uranyl, magnesium and hydrogen carbonate/carbonate ions, the formation of magnesium uranyl carbonate as solid or dissolved species could be expected. In nature the mineral bayleyite - Mg2[UO2(CO3)3] x 18H2O - is a comparatively common secondary mineral in uranium ore deposits set within an alkaline environment and associated with gypsum, carnotite and metaautunite.

The aim of this work was to synthesize and to characterize the bayleyite using several analytical methods. This well characterized substance was used for the determination of its solubility and for speciation experiments.

Synthesis and characterization of Mg2[UO2(CO3)3] x 18H2O:
Several methods for preparation of bayleyite are described in the literature /3,4/. By variation of the preparation conditions, we could increase the reproducibility and the yield (95±3%) of the product. Bayleyite was prepared by mixing stoichiometric amounts of dissolved uranyl nitrate (Merck p.a.), mag-nesium carbonate (-Merck p.a.) and sodium carbonate (-Merck p.a.) at ambient temperature. During this process, carbon dioxide was passed through the solution until dissolution of all reactants. Then bayleyite was p--recipitated by s-low evaporation of the solution at room temperature. The product was purified by recrystallization. The obtained transparent green-ish yellow p-risms were dried on air.
The results of the chemical analysis are represented in Tab. 1. The determined chemical composition was in good agree-ment with the calculated values.

Tab. 1: Chemical analysis of Mg2[UO2(CO3)3] x18H2O

Element U Mg C H
Found (%) 28.92 ± 0.60 5.90 ± 0.70 4.34 ± 0.01 3.98 ± 0.28
Calculated (%) 28.92 5.91 4.38 4.41
Method ICP-MS AAS EA. EA.

The thermoanalytical measurements showed that the water is released in a temperature range from 22 to 338oC. The CO2 release occurred from 338 to 800oC. As a result from these measurements, the content of water and CO2 was determined to be 17.36±0.52 and 3.17±0.10 moles, respectively.

The synthesized bayleyite was identified and characterized by X-ray Powder Diffraction (XRD), Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS), and Time-resolved Laser-induced Fluorescence Spectroscopy (TRLFS).
The indexing of the diffraction pattern peaks observed in the X-ray diffraction diagram showed that the synthesized substance has monoclinic structure as described in the literature /3,5/. The lattice parameters (Tab. 2), which were calculated from all clearly identifiable reflections, are in good agree-ment with those found by Mayer et al. /5/ and described in PDF /6/.

Tab. 2: Lattice constants of Mg2[UO2(CO3)3] x18H2O

This work H. Mayer /5/ PDF /6/
a (Å ) 26.593 ± 0.101 26.560 (3) 26.65 (5)
b (Å) 15.263 ± 0.052 15.256 (2) 15.31 (5)
c (Å) 6.503 ± 0.026 6.505 (1) 6.53 (2)

We performed EXAFS measurements of the solid bayleyite and its solution (pH 8.0, Uconc. 0.182 mol/L) in order to compare the obtained structural parameters with the XRD data and to detect agreements or dif...