Mechanistic and kinetic investigation of crystalline UO(2) dissolution.

摘要

To evaluate the efficacy of REDOX permeable reactive barriers for remediation of uranium contaminated groundwater, knowledge of the fundamental reaction kinetics associated with the dissolution of reduced uranium(IV) dioxide is necessary. Once, oxidizing conditions prevail within the treatment zone uranium can be remobilized. Therefore, the fundamental hypothesis of this dissertation is that under oxic conditions, an oxidative dissolution process controls the remobilization of UO₂. The objective is to elucidate the rates of reaction and propose the mechanism(s) associated with this oxidative dissolution process.; Over four decades of existing UO₂ dissolution studies, focusing on spent nuclear fuel disposal, have generated numerous inconsistencies and unanswered questions. A cardinal gap in the published literature is the relationship between the activity of carbonate and pH on UO₂ (cr) dissolution, as well as the controlling mechanism of dissolution. To resolve these inconsistencies, several UO₂ dissolution experiments have been conducted under oxic conditions in the presence of total dissolved carbonate ([CO₃ −2]_T) from 0.001 to 0.1 M; pH from 7.5 to 10.7; flow rates (q) from 6 to 120 mL d−1: B.E.T. surface area (S) of 1.2 m2 g−1; and temperatures (T) from 25° to 90°C utilizing both powder and pellet specimens in a single pass flow-through apparatus.; Steady state conditions were achieved in each experiment after approximately seven reactor volumes. The elemental release rate of uranium from the UO ₂ specimen increases by an order of magnitude with each 30-degree increase in temperature. Measured log₁₀ rates (mol m−2 s−1) at 30° and 60°C at a flow rate of 120 mL d−1 for the powder specimens were −9.84 ± 0.08 and −8.88 ± 0.05, respectively. Additionally, these results show that UO₂ dissolution rate varies as a function of the ratio of flow rate, q, to sample surface area, S, which indicates reaction affinity control. At values of log₁₀ q/S greater than −9.2, UO ₂ dissolution becomes invariant with respect to q/S, which is evidence for dissolution at the forward rate of reaction. Other factors that affect the rate of dissolution include [CO₃−2 ]_T, pH, and carbonate activity {lcub}CO₃2− {rcub}.; From the results of these experiments at 30°C the following rate equations were derived, where [] and {lcub}{rcub} denotes the concentration (in molar) and activity (in molar) of a particular species in solution, respectively.; cwl="83:114" wdm="100"> row> cell>(0.001 M [CO₃−2]_T 0.1 M) cell>Log₁₀ Rate = 0.7375 Log₁₀ [CO₃ −2]_T − 6.78 row rht="0.00px"> cell>(7.5 pH 9.7) cell>Log₁₀ Rate = 0.2720 Log₁₀ [H+] − 12.01 row> cell>(0.0003 M {lcub}CO₃2−{rcub} 0.03 M) cell>Log₁₀ Rate = 0.3023 Log₁₀ {lcub}CO₃ −2{rcub} −8.92 row rht="0.00px"> cell spn="2"> (Abstract shortened by UMI.)