X-ray absorption spectroscopic study of the sorption of Cr(III) at the oxide-water interface: II. Adsorption, coprecipitation, and surface precipitation on hydrous ferric oxide

Abstract

The sorption of Cr(III) by hydrous Fe oxides involves adsorption, surface precipitation, and coprecipitation phenomena. These phenomena lead to different phases which are here compared with regard to both their Cr solubility and their local structure. The former is simulated by the “surface precipitation model” and the latter is derived from X-ray absorption fine-structure (EXAFS) spectroscopic data interpreted by the “polyhedral approach method.” The adsorption of a Cr(III) atom onto goethite or hydrous ferric oxide (HFO) occurs via the formation of inner-sphere surface complexes. In such complexes, Cr atoms are never isolated, but present as small surface hydroxy polymers. This polymerization has been catalyzed by the surface, and it occurs when Cr(III) bonds only 10% of the surface “active” sites (i.e., when it covers 1% of the BET surface area). In these surface polymers, Cr(III) atoms are surrounded by three metal (Fe or Cr) shells at 3.00–3.05, 3.40–3.46, and 3.94–4.03 Å, as in a mixed α- and γ-MeOOH local structure (Me = Fe or Cr). These polymers act as nuclei for the precipitation of a surface hydrous Cr oxide, a precipitation that starts under conditions undersaturated with respect to the homogeneous precipitation of the same oxide, a phenomenon well described by the surface precipitation model. This surface precipitate and the pure hydrous chromium oxide have identical solubility products and the same local structure (γ-CrOOH). On the contrary, when Cr(III) is coprecipitated with Fe(III), only two cation shells are detected around Cr(III) at 2.99 and 3.40 Å, indicating that Cr substitutes for Fe in an α-(Fe, Cr)OOH framework. A difference in local structure (α-CrOOH versus γ-CrOOH) therefore accounts for the difference in solubility products of the surface-precipitated and coprecipitated Cr hydroxides.