Precipitation equilibria of the chromium(VI)/iron(III) system and spectroscopic characterization of the precipitates

Precipitation equilibria and spectroscopic studies of the Cr-(VI)/Fe(III)/H2O system have been performed to identify the aqueous and solid speciation of the system in the pH range 1-9. The results suggest that three different precipitates FeOHCrO4, FeOHCrO4 . 2Fe(OH)(3), and Fe(OH)(3) exist depending on the pH range. The last two precipitates have been reported in the literature before, and they are confirmed in this study. The formation of a new precipitate, FeOHCrO4, that predominates in the pH range 1.5-2.5 is being suggested, and its thermodynamic solubility constant was estimated to be pK(so) = 22.50 +/- 0.07. The mixed precipitate predominated in the 2.5-3.5 pH range and the pure ferric iron hydroxide predominated in the pH range grater than 3.5. All three precipitates were analyzed using X-ray photoelectron spectroscopy (XPS). The spectroscopic results suggest that the first and second precipitates contain one type of chromium species which corresponds to Cr(VI) type and one type of Fe species. The third precipitate exhibited two peaks with width greater than 5 eV, indicating the presence of two chromium species, one of which is closely related to CrOOH type of binding and the other may be attributed to metal-chromate binding. We postulate that in this case chromate was adsorbed onto the iron hydroxide forming two surface complexes. Since iron is abundant in nature, the results of this study are important for the understanding of the mobility and reactivity of hexavalent chromium in contaminated groundwater sites.