Europium LIII-edge X-ray absorption near-edge structure (XANES) was employed to determine the Eu(II)/Eu(III) ratios in minerals. This ratio can be determined based on the peak-area ratio of white lines, the resonance peak, in normalized XANES spectra for Eu(II) and Eu(III) species. For precise determination of the Eu(II)/Eu(III) ratios, however, it was revealed that the transition probabilities for each individual Eu(II) and Eu(III) species in the system must be quantified, because we found that the peak area in normalized XANES spectra is different in each Eu(II) and Eu(III) species. Despite this ambiguity, the method was applied to Eu in natural hydrothermal apatites (Eu = 39 and 64 ppm) and fluocerite (Eu = 282 ppm). The relationship between the Eu(II)/Eu(III) ratio in these hydrothermal minerals, and the distribution coefficients of Eu(II) and Eu(III) were discussed, taking into account Eu anomalies in their REE patterns. It is considered that by combining the Eu(II)/Eu(III) ratios determined by XANES and the degree of Eu anomaly in REE patterns, we can provide new information on the distribution of Eu(II) and Eu(III) in various geochemical studies.

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Approaches to the construction of thermodynamic models for sorption of trace-element cations on carbonates are considered. To calculate thermodynamic equilibria by the method of Gibbs free-energy minimization, the existing database of reaction constants and thermodynamic potentials was extended. Different types of models are illustrated by the example of precipitation of Cd, Cu, Pb, and Zn from the water of a drainage stream flowing out of the impoundment of barite-polymetallic ore-dressing wastes. It is shown that the mobility of metals in such cases can be controlled by their sorption by calcite present in bottom sediments and suspension. Any approach can be successfully applied to both the modeling of experimental data on cation sorption and the prediction of the ecologo-geochemical situation in the districts of dressing works.

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We have studied the hydrolytic behavior of Y3+ and trivalent ions of rare earth elements in aqueous solutions at 25 ºC. The stepwise stability constants of hydroxide complexes were measured by spectrophotometry, using m-cresol purple and 1-(2-pyridylazo)-2-naphthol as pH indicators at an ionic strength no more than 0.0005. The results showed that at pH ranging between 6.0 and 11.0 in freshly prepared solutions of REE trichlorides, lanthanides are presented as Ln3+, Ln(OH)2+, Ln(OH)2+, and Ln(OH)30. The plots of the formation constants of monohydroxo complexes of 4f n ions M3+ versus atomic number Z deviate from smooth ones and consist of four convex curves. This phenomenon is also observed in normalized spectra of REE concentrations in natural objects and is known as the tetrad effect. The obtained data give an insight into the relationship between REE complex formation and REE fractionation in geochemical processes and can be used for physicochemical modeling of geochemical systems.