Chromium(III) Hydroxide Solubility in The Aqueous Na+-OH?-H2PO?4-HPO2?4-PO3?4-H2O System: A Thermodynamic Model

Abstract

Chromium(III)-phosphate reactions are expected to be important in managing high-level radioactive wastes stored in tanks at many DOE sites. Extensive studies on the solubility of amorphous Cr(III) solids in a wide range of pH (2.8–14) and phosphate concentrations (10−4 to 1.0 m) at room temperature (22±2)°C were carried out to obtain reliable thermodynamic data for important Cr(III)-phosphate reactions. A combination of techniques (XRD, XANES, EXAFS, Raman spectroscopy, total chemical composition, and thermodynamic analyses of solubility data) was used to characterize solid and aqueous species. Contrary to the data recently reported in the literature,(1) only a limited number of aqueous species [Cr(OH)3H2PO−4, Cr(OH)3(H2PO4)2−2), and Cr(OH)3HPO2−4] with up to about four orders of magnitude lower values for the formation constants of these species are required to explain Cr(III)-phosphate reactions in a wide range of pH and phosphate concentrations. The log Ko values of reactions involving these species [Cr(OH)3(aq)+H2PO−4⇌Cr(OH)3H2PO−4; Cr(OH)3(aq)+2H2PO−4⇌Cr(OH)3(H2PO4)2−2; Cr(OH)3(aq)+HPO2−4⇌Cr(OH)3HPO2−4] were found to be 2.78±0.3, 3.48±0.3, and 1.97±0.3, respectively.