Hydroxo and Chloro Complexes/Ion Interactions of Hf4+ and the Solubility Product of HfO2(am) |
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Authors: | Dhanpat Rai Yuanxian Xia Nancy J Hess Denis M Strachan Bernard P McGrail |
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Institution: | (1) Pacific Northwest National Laboratory, Richland, WA 99352, USA;(2) Japan Nuclear Cycle Development Institute Tokai Works, Japan |
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Abstract: | The solubility of HfO2(am) was determined at different equilibration periods from the over- and undersaturation directions, in very acidic to basic solutions (0.1 m HCl to 3.2 m NaOH), and in NaCl solutions ranging in concentrations from very dilute to as high as 5.59 m and in a ${\text{p}}C_{{\text{H}} + }$ range from 1 to 4 to obtain reliable thermodynamic data for the Hf4+–Cl?–Na+–H+–OH?–H2O system. The studies indicate that equilibrium is reached rapidly (<5 days) and that HfO2(am) solubility shows amphoteric behavior. The solubility data obtained in this study, along with the data reported in the literature, at NaOH molalities as high as 21.7 m were interpreted using the ion-interaction model of Pitzer. The log K 0 for the solubility of HfO2(am) HfO2(am) + 2H2O ? Hf4+ + 4OH?] was determined to be ?55.1 ± 0.7. The log K 0 values for the formation of HfOH3+, Hf(OH)0 4, Hf(OH)5 ?, and Hf(OH)6 2? according to the reaction (Hf4+ + xOH? ? Hf(OH)4?x x) were determined to be 13.8, <44.8, 49.7 ± 0.2, and 51.2 ± 0.2, respectively. The thermodynamic model developed in this study is valid for a wide range of conditions (as high as 0.1 m HCl, 21.7 m NaOH, and 5.59 m NaCl). The binary ion-interaction parameters for Hf4+–Cl?, HfOH3+–Cl?, and Hf(OH)2? 6–Na+ were determined in this study to accurately define the observed solubility behavior of hafnium in various systems. |
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