Journal of Solution Chemistry

, Volume 33, Issue 10, pp 1213-1242

First online:

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

  • Dhanpat RaiAffiliated withPacific Northwest National Laboratory Email author 
  • , Dean A. MooreAffiliated withPacific Northwest National Laboratory
  • , Nancy J. HessAffiliated withPacific Northwest National Laboratory
  • , Linfeng RaoAffiliated withLawrence Berkeley National Laboratory
  • , Sue B. ClarkAffiliated withWashington State University

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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)3H2PO4, 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 K o values of reactions involving these species [Cr(OH)3(aq)+H2PO4⇌Cr(OH)3H2PO4; Cr(OH)3(aq)+2H2PO4⇌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.

Thermodynamics Cr(OH)3(am) solubility hydrolysis constants ion-interaction parameters Cr(III)-phosphate complexes Cr(OH)3H2PO4, Cr(OH)3(H2PO4)2−2, Cr(OH)3HPO2−4