URANYL PHOSPHATE DISSOLUTION AND TRANSFORMATION
7.2 Experimental
7.4.2 Dissolution Rate Quantification
Flow-through dissolution experiments are complementary to batch experiments and allow a better quantification of dissolution rates when undersaturated conditions can not be maintained in batch experiments. The calcium release and phosphate uptake are well resolved in the batch dissolution experiments, but the changes in the dissolved uranium concentrations with time are far too small to allow for rate calculations. In flow- through experiments, the dissolution rate of the solid was calculated using the release of either uranium or phosphorus from the solid phase. For congruent dissolution of
hydrogen uranyl phosphate hydrate, the release rates of uranium and phosphorus would be identical, and for uranyl phosphate hydrate the uranium release rate would be 1.5 times that of phosphorus. In all but one of the flow-through dissolution experiments, the phosphorus release rate is actually higher than that of uranium. This observation can not be explained by any form of congruent dissolution; however, it should be emphasized that in several cases the uranium and phosphorus release rates were in agreement when considering the error estimates for the release rates. No simple explanation is available for the phosphorus release rate that is nearly double the uranium release rate in
experiment #6. It is possible that the phosphate was released from the solid and that the uranium was retained in a uranyl oxide hydrate phase; however, the uranyl
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Little or no effect of the influent calcium concentration on the uranium release rate was observed. This observation is consistent with the identification of the dissolving solid as a calcium-free uranyl phosphate or hydrogen uranyl phosphate phase. There is also no effect of calcium on the phosphate release rate for one hour residence
experiments. The detection limit for phosphorus makes it difficult to determine phosphorus release rates at the shorter half hour residence time.
The maximum dissolution rate as measured by uranium release is 0.46 µmol m-2 h-1, but this may only be a lower limit. The accumulation of dissolution products in the flow-through reactors decreases the driving force of the dissolution reaction, and consequently decreases the dissolution rate. If dissolution were occurring far from equilibrium and dissolution were surface-controlled, then the dissolution rates would be equal for the two different reactor residence times. Using the average values for
experiment #6 ([U]diss = 0.235 µM; [P]diss = 0.411 µM), the reaction quotient for reaction 4 is 10-27.79. Relating this reaction quotient to the equilibrium solubility constant
determined in batch dissolution experiments yields a free energy for the dissolution reaction of only -5.8 kJ mol-1. In a similar calculation, the free energy of the dissolution reaction in experiment #3 (rate = 0.34 µmol m-2 h-1) is only -4.4 kJ mol-1. For conditions further from saturation than those of experiment #6, the free energy of the reaction would be greater than –5.8 kJ mol-1 and the dissolution rate would correspondingly be higher. The current experimental approach is limited by the minimum flow-through reactor residence time of 30 minutes.
Despite providing only a conservative estimate for the dissolution rate, the rate of uranyl phosphate dissolution is convincingly slower than for a uranyl silicate and a uranyl
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oxide hydrate studied in related experiments. On a mass basis, the maximum uranyl phosphate dissolution rate of 0.97 µmol g-1 h-1 is significantly lower than that of the uranyl silicate soddyite (2.07 µmol g-1 h-1) or uranyl oxide hydrate schoepite (rate > 9 µmol g-1 h-1). When normalized to surface area, the difference among the minerals is not as striking but the dissolution rate of uranyl phosphate is still the lowest of the three phases.
7.4.3. Environmental Implications
Uranyl phosphates have both low solubilities and low dissolution rates as compared to other uranyl minerals. These two properties make uranyl phosphates important phases for sequestering uranium when sufficient phosphate concentrations are present. The formation of uranyl phosphates at contaminated sites may maintain both the uranium concentration in the porewater and the rate of uranium release to the porewater at values that are low enough to mitigate the problem of uranium contamination. The removal of uranium from contaminated soils by soil-washing processes is expected to be considerably slower in soils with high phosphate levels than in soils with low phosphate levels. Nonetheless, bicarbonate has been demonstrated as an effective extractant for both contaminated soils (Francis et al., 1999) and a synthetic meta-autunite (Sowder, 1998).
As expected, the solution chemistry dictates the formation of specific phases; however, the formation of certain phases may be kinetically hindered. Despite the exposure of the originally synthesized hydrogen uranyl phosphate hydrate to high calcium concentrations at elevated temperatures, only a small portion of the solid was
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converted to an autunite-like calcium uranyl phosphate phase. The transition between hydrogen uranyl phosphate hydrate and uranyl phosphate hydrate phases is also ill- defined. It is likely that uranyl phosphates in contaminated environments may be transformed from one uranyl phosphate phase to another depending on the solution pH and cation composition. The distinction between phases may not be important if different uranyl phosphate phases have comparable solubilities and are all relatively insoluble compared with other uranyl minerals.