Wässrige Löslichkeit von Vanadat substituiert für Phosphat in Apatit
Zusammenfassung der Projektergebnisse
Elevated vanadium (V) concentrations have been found during aquifer thermal energy storage (ATES), but which is not an effect of thermal desorption. Elevated V concentrations have also been found in volcanic aquifers. As a V source in aquifers of the German Eifel area, tiny grains of the V-bearing fluorapatite mineral Ca5[(V,P)O4]3F have been found previously by us using an electron microprobe. Solid solution formation is often disregarded in water/rock interaction geochemistry, which focuses primarily on surface adsorption reactions. An understanding of the factors controlling geogenic vanadium mobility during water/rock interactions requires more fundamental research on the thermodynamics and kinetics of such solid solution formation. In fact, apatite is a versatile mineral because of its structure with both cation and anion exchange sites that can incorporate more than half of the stable elements in the periodic table, leading to an apatite supergroup with over 40 individually named minerals. Apatite-type minerals are therefore of quite variable composition, A5B3X, where A stands for a divalent cation such as Ca or Sr, B for a trivalent oxyanion such as PO43−, AsO43−, or VO43−, and X for a monovalent halogen anion such as F−, Cl−, or the hydroxyl OH− anion. In fact, the crystal structure of these apatite supergroup minerals allows for numerous cationic and anionic substitutions on three different sublattice sites. These minerals are applied for water and soil pollution control due to their extensive ion-exchange capabilities. Most papers on the apatite supergroup have considered the variability in cation substitution only, few have yet considered oxyanion substitution. The aim of this project was to understand the elevated vanadate concentrations in oxic aquifers of volcanic areas triggered by apatite minerals. Therefore, the solubility products of the solid solutions between phosphate and vanadate apatite phases in the binary subsystems Ca5(PXV1-XO4)3(OH,F) and Sr5(PXV1-XO4)3(OH,F) were investigated in our laboratory. Different binary apatite solid solution (SS) series were prepared and characterized by using X-ray diffraction, which indicated fully and ideal SS formation according to Vegard’s law. While synthesis had been performed under hydrotherla conditions to yield in the hexagonal apatite structure, the dissolution of the synthetic solids was studied at 25°C in a series of batch equilibrium experiments over a couple of weeks. The pH values increased rapidly at the beginning of dissolution and remained constant after 24–36 h, indicating steady state formation. The solubility of the solid solutions increased with an increase in the mole fractions of VO4 and Sr, with orders of magnitude higher solubilities of the respective apatite endmembers. The endmember thermodynamic constants thus derived were justified by theoretical evaluation based on the simple salt approximation approach, once some calculation errors have been corrected upon discussion with the developer. Lippmann diagrams show that even a trace phosphate substitution in the vanadate apatite endmembers decreased the solubility of the resulting solid solution and hence V mobility. A theoretical interpretation of the data was further performed for the first time based on an advanced sublattice solid-solution aqueous-solution (SSAS) equilibrium thermodynamic model for the multicomponent apatite solid solution (Ca,Sr)5[(P,V)O4]3(OH,F) with eight endmembers. Solution calorimetry measurements are yet pending. This will allow for finalizing the manuscripts for predicting solubilities in any of the possible solid solution subsystems in this apatite supergroup subsystem.
