James E. Amonette

Identification of noncrystalline (Fe,Cr)(OH) 3 by infrared spectroscopy

Iron-chromium hydroxides are important solid phases governing the aqueous concentrations of Cr(III) in soils and fly ashes. Although direct identification of noncrystalline (Fe,Cr)(OH)3 is difficult, the infrared spectra of noncrystalline Fe(OH)3 and Cr(OH)3, coprecipitated (Fe,Cr)(OH)3, and physical mixtures of Fe(OH)3 and Cr(OH)3 can be distinguished on the basis of the asymmetric stretching doublet (ν3) of structural carbonate anions. As the Cr mole fraction of the coprecipitated (Fe,Cr)(OH)3 increases, the position of the low-frequency ν3 peak (ν3′) changes progressively to higher frequencies, and the carbonate ν3 splitting decreases. No change in carbonate ν3 splitting or v3′ location was observed for physical mixtures of Fe(OH)3 and Cr(OH)3. The changes in ν3 splitting are believed to be caused by different degrees of polarization of the carbonate ligand by the Fe and Cr cations.Pure Cr(OH)3 exhibits a strong affinity for carbonate and H2O and tends to remain noncrystalline even at very high pHs. In contrast, pure Fe(OH)3 gradually converts to crystalline goethite at high pH, to the exclusion of much of the H2O and carbonate. The presence of Cr in (Fe,Cr)(OH)3 solid solutions seems to inhibit the transformation to crystalline goethite. The strong association of carbonate with Cr and the kinetic inertness of Cr(III) inner-sphere complexes in general may account for the maintenance of noncrystalline solid-solution materials in lieu of transformation to a crystalline end product.

The solubility of barite and celestite in sodium sulfate: Evaluation of thermodynamic data

The solubilities of barite [BaSO4(c)] and celestite [SrSO4(c)] in Na2SO4 were studied and found to be significantly lower than the experimental values reported in the literature. Our new solubility data are in excellent agreement with the predictions of ion interaction models, which have previously been parameterized primarily from solubility data obtained in chloride media. Our solubility data were analyzed both in terms of aqueous thermodynamic models that included ion association species and in terms of ion interaction models that did not require the explicit recognition of such species. In the case of SrSO4, although both ion association and ion interaction models can accurately model our solubility data, the ion interaction approach is preferred because it is easier to extend to higher concentrations. In the case of BaSO4, the aqueous ion interactions appear to be stronger than those for SrSO4, and so the explicit recognition of a BaSO4(aq) ion association species is preferred. The logarithms of the thermodynamic solubility products (log Ksp) for celestite and barite were −6.62±0.02 and −10.05±0.05, respectively. When the data were analyzed using models that include ion association species, the logarithms of the thermodynamic equilibrium constants for the SrSO4(aq) and BaSO4(aq) association reactions were 1.86±0.03 and 2.72±0.09, respectively.