Geochemical features of the behavior of Terbium, Dysprosium, and Holmium in aquatic medium in the presence of humic acids (experimental data)

Behavior of rare earth elements in model solutions has been studied in this article using the example of dysprosium, holmium and terbium in the presence of humic acids. The research has been carried out using ultrafiltration, inductively coupled plasma mass spectrometry, infrared spectrometry methods, together with thermodynamic calculations. The solutions of different pH values and different content of matrix components at constant values of the concentration of lanthanides were prepared to study the geochemical features in the behavior of rare-earth elements. The value of the activity of complex compounds of rare-earth elements with chloride and nitrate ions is relatively low over the entire pH range and the content of matrix components. The considered elements in an acidic medium with the pH value 4 are mainly present in the form of aqua ions Ln3+. The pH value 6 leads to the dominance of the fraction of their humic associate HA-Ln with the maximum content in terbium in the solution. When the solution is made alkaline, carbonate complexes of Ln(CO3)2- lanthanides are formed in significant amounts with their maximum content at the pH value 11. The ratio between organic and inorganic compounds for each element is different. Thermodynamic calculations prove that the limitation of mobility in a solution of dysprosium, holmium and terbium occurs due to their sorption by humic acids, the formation of poorly soluble hydroxides Ln(OH)3 and their precipitation. With the help of thermodynamic calculations, anomalously low concentrations of rare earth elements of the MREE are explained in comparison with the content of elements of LREE and HREE subgroups in the water column of Lake Doroninskoye. The calculation of the speciation of lanthanides shows that the main form of migration in water is complex organic compounds of lanthanides, the proportion of which in the La-Lu series increases with an insignificant decrease in the Gd concentration. The MREE downward concavity displayed by REE-HA complexation pattern determined in this study compares well with results from REE-fulvic acid (FA) and REE-acetic acid complexation studies. This similarity in the REE complexation pattern suggests that carboxylic groups are the main binding sites of REE in HA. The Ln(CO3)2- carbonate forms are present in incommensurably small amounts, and, on the contrary, decrease in the La-Lu series. The decrease in the concentration of lanthanides from the water surface to the bottom is explained by the possibility of water saturation relative to Ln(OH)3 with the highest saturation index for MREE elements; they are also sorbed in large quantities by humic acids. The results presented in this study show that, before running speciation models, it is essential to know the proportion of organics that is active in complexing REE in a given water sample. This requirement could severely complicate the use of models to assess the role of organics in controlling the speciation of REE in natural waters.

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