Sodium Sulfate Separation from Aqueous Alkaline Solutions
via Crystalline Urea-Functionalized Capsules: Thermodynamics and Kinetics
of Crystallization
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Abstract
The
thermodynamics and kinetics of crystallization of sodium sulfate
with a tripodal tris-urea receptor (L1) from aqueous alkaline solutions
have been measured in the 15–55 °C temperature range for
a fundamental understanding of the elementary steps involved in this
sulfate separation method. The use of radiolabeled Na<sub>2</sub><sup>35</sup>SO<sub>4</sub> provided a practical way to monitor the sulfate
concentration in solution by β liquid scintillation counting.
Our results are consistent with a two-step crystallization mechanism,
involving relatively quick dissolution of crystalline L1 followed
by the rate-limiting crystallization of the Na<sub>2</sub>SO<sub>4</sub>(L1)<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub> capsules. We found
that temperature exerted relatively little influence over the equilibrium
sulfate concentration, which ranged between 0.004 and 0.011 M. This
corresponds to 77–91% removal of sulfate from a solution containing
0.0475 M initial sulfate concentration, as found in a typical Hanford
waste tank. The apparent pseudo-first-order rate constant for sulfate
removal increased 20-fold from 15 to 55 °C, corresponding to
an activation energy of 14.1 kcal/mol. At the highest measured temperature
of 55 °C, 63% and 75% of sulfate was removed from solution within
8 and 24 h, respectively. These results indicate the capsule crystallization
method is a viable approach to sulfate separation from nuclear wastes