Precipitation Equilibria of the Chromium(VI)/Iron(III) System and Spectrospcopic Characterization of the Precipitates

Abstract

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 pKso = 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 prec...