Growth of calcium carbonate in the presence of Se(VI) in silica hydrogel

The element selenium has become a considerable environmental concern due its accumulation in aquifers on the one hand and the high radiotoxicity of its long-lived isotope (super 79) Se on the other hand. Se(VI) is the most mobile of the various Se cations. This study deals with the interaction of Se(VI) with rock-forming minerals (carbonates) to better understand how to mitigate the potential environmental hazards of Se. The effect of Se(VI) on the cystallization of CaCO (sub 3) at room temperature was studied using the silica hydrogel method. The CaCO (sub 3) crystals obtained were characterized by X-ray powder diffraction, scanning electron microscopy and electron microprobe. The presence of Se(VI) in the growth medium has a clear effect on the polymorph selection of CaCO (sub 3) , promoting the formation of vaterite and inhibiting that of aragonite. Se(VI) also affects the characteristics of calcite crystals, which show progressively more elongated habits and smaller sizes with increasing Se(VI) concentration in the growth medium. The effect of Se(VI) on both the polymorphic crystallization of CaCO (sub 3) and the characteristics of calcite crystals shows features strikingly similar to those of other tetrahedral anionic groups like S(VI) and Cr(VI). This similarity extends to the amount of Se incorporated into the structure of the different CaCO (sub 3) polymorphs, with calcite having Se contents up to 1200 ppm, vaterite up to 500 ppm, and aragonite growing virtually Se-free. The role of Se(VI) on the crystallization of CaCO (sub 3) is discussed taking into consideration the physicochemical conditions in the growth medium at nucleation, which were modeled using the PHREEQ code for low-temperature aqueous geochemistry. The possible effect of the incorporation of the Se(VI) on the relative stability and, by extension, on the solubility of CaCO (sub 3) polymorphs is also discussed.