Growth of Calcium Carbonate in the Presence of Cr(VI)

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The extended use of hexavalent chromium Cr(VI) compounds in industrial processes caused a significant increase of the concentration of this highly toxic heavy metal in natural environments. In order to investigate the influence of Cr(VI) in the formation of CaCO3, crystallization experiments were carried out in a double diffusion system, using silica hydrogel with different Cr(VI) contents as the growth medium. Crystalline products were examined by scanning electron microscopy, Raman spectroscopy, electron microprobe analysis, and single crystal X-ray diffraction. Increasing Cr(VI) concentration caused inhibition of the nucleation and growth of calcite and promoted the formation of the metastable polymorphs aragonite and vaterite. This effect correlated with a decrease of crystal size. Furthermore, the habit of calcite crystals changed with increasing Cr(VI) concentrations from {104} to forms increasingly elongated parallel to the c-axis. Raman spectroscopy, single crystal X-ray diffraction (XRD), and electron microprobe analysis (EMPA) gave strong indications of an isomorphic anion substitution of trigonal planar carbonate by the tetrahedral chromate within the calcite lattice. The apparent partitioning coefficients of Cr(VI) into calcite determined in this work suggest that the fate of this pollutant in natural environments can be significantly influenced by CaCO3 precipitation processes.
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