Batch Dissolution

The evolution of dissolved concentrations of uranium, phosphorus and calcium in duplicate batch experiments is presented in Figure 7.5. The dissolved concentrations in the stock suspension were measured immediately before the stock aliquots were added to batch reactors, and can be used to calculate instantaneous dissolved concentrations at the beginning of the experiments of 0.13 µM uranium, 10.5 µM phosphorus, and 0.67 µM calcium. The dissolved concentrations then evolved from these initial values. Uranium release to solution was small with dissolved concentrations varying over the course of the experiment from 0.04 to 1.58 µM. In both experiments, the highest uranium

concentrations occurred after 8 days of dissolution and decreased from there. The final dissolved uranium concentration in the long-term (56 day) reactor was 1.02 µM.

Calcium was completely released over the duration of the experiments, culminating at a dissolved concentration of 57.8 µM after 1350 hours. The final dissolved calcium concentration was approximately equal to the total concentration of 55.3 µM, which was estimated from the measured calcium content of the stock

suspension. Coincident with the release of calcium from the solid was the uptake of phosphorus by the solid. The dissolved concentration of phosphorus was highest (13-14 µM) in the first hour of the experiment and gradually decreased until a stable value of approximately 1 µM (0.8-1.3 µM) was reached after 826 hours. The initial pH was buffered at pH 6, but was not subsequently measured or adjusted. In order to maintain electroneutrality, protons were probably removed from solution as calcium was released to and phosphate (H2PO4- is the dominant species at pH 6) was removed from solution, thereby resulting in an increase in the pH.

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0

200

400

600

8

12

16

20

24

28

2Θ (Degrees)

Intensity

510 h

124 h

12 h

1 h

0.08 h

stock

Figure 7.6: Time-series of X-ray diffraction patterns of filtered solids collected during the batch dissolution process and a reference pattern for hydrogen uranyl phosphate. The broad peak from 17-19º is due to the polycarbonate filter membranes.

Remarkably, the dramatic changes in the dissolved phosphorus and calcium changes were not associated with major changes in the morphology of the solid. Scanning electron micrographs of the material subjected to dissolution (Fig. 7.3) for varying amounts of time show no changes in the general shape or length of the needle- like crystals. Although the resolution is poor for the image of the material that had

undergone dissolution for 1350 h, this is not likely to be the result of structural changes in the material.

A time-series of X-ray diffraction patterns (Fig. 7.6) of the solids collected from the batch reactors displays only minor changes. Dominant peaks at 8.44º (10.37 Å), 17.01º (5.21 Å), and 20.89º (4.24 Å) persisted throughout the dissolution process. The broad peak centered at 17.4º is generated by the polycarbonate filter membranes upon

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which the analyzed solids are loaded. The most significant change is the shift and growth of a peak from 10.4º (8.50 Å) to 10.2º (8.63 Å), which is quite apparent in the 510 hour sample. This shift is even more pronounced in an XRD pattern of residual solids from the batch reactors analyzed after 9 months of storage in a concentrated suspension. Other changes are the disappearances of the small shoulder peaks in the stock suspension at 10.09º (8.76 Å) and 21.24º (4.18 Å) upon initiation of the dissolution experiment.

In contrast to the SEM and XRD data, the Raman spectrum of residual solids from the experiment is dramatically different from the spectrum of the stock suspension. The symmetric stretch of the uranyl moiety shifts from 862.3 to 832.6 cm-1, and the two discrete phosphate bands around 1000 cm-1 in the stock suspension appear to have merged into a single broad peak at 990 cm-1. The position and width of the peak at 990 cm-1 suggest that it might also be due to the antisymmetric stretch of the uranyl moiety, which could have become activated by symmetry lowering as the solid phase

transformed. It should be noted that the Raman spectrum of the solids from the batch reactor was collected six months after the conclusion of the batch experiments, and may reflect structural changes during that period that were not probed by SEM or XRD.

7.3.3 Flow-through Dissolution

The objective of flow-through experiments was to quantify dissolution rates under steady-state conditions. In all five flow-through experiments, the dissolved

concentrations of uranium and phosphorus stabilized at constant values within 10-20 reactor volumes (Fig. 7.7). While the pH and ionic strength in the reactors were the same for all five reactors, the influent calcium concentration and the reactor residence time

7-18 0.0 0.5 1.0 1.5 0 20 40 60 80 100 Co n c . ( µ M) 0.0 0.5 1.0 1.5 0 20 40 60 80 100 C o n c . ( µ M) 0.0 0.5 1.0 1.5 0 20 40 60 80 100 Co n c . ( µ M) 0.0 0.5 1.0 1.5 0 20 40 60 80 100 Co n c . ( µ M) a) b) d) c) 0.0 0.5 1.0 1.5 0 20 40 60 80 100 R esidence Times C onc . ( M) e)

Figure 7.7: Effluent uranium (z) and phosphorus ({) concentrations from flow-through dissolution reactors. Influents to all reactors contained 0.01 M NaNO3 and were buffered at pH 6 with 5 mM MES buffer. The influent calcium concentration and reactor residence time varied for each experiment (numbered as in Table 7.4): a) Expt. 3: [Ca]inf = 0 µM, tres = 1 h; b) Expt. 4: [Ca]inf = 10 µM, tres = 1 h; c) Expt. 5: [Ca]inf = 100 µM, tres = 1 h; d) Expt. 6: [Ca]inf = 0 µM, tres = 0.5 h; e) Expt. 7: [Ca]inf = 100 µM, tres = 0.5 h.

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Table 7.4: Conditions and calculated dissolution rates of flow-through dissolution experiments. All reactors were operated at pH 6.00 and with an influent containing 0.01 M NaNO3 and 5 mM MES buffer. The total uranium concentration in each reactor was 1.26-1.36 mM, primarily associated with the solid phase.

Avg. Steady-state Conc. Avg. Release Rates

Expt. [Ca]inf tres [U]eff [P]eff U P

(#) (µM) (h) (µM) (µM) (µmol m-2 _h-1₎ 3 0 1.01-1.04 0.27-0.40 0.30-0.78 0.34 ± 0.03 0.44 ± 0.06 4 10 0.98-1.01 0.33-0.50 0.44-0.61 0.42 ± 0.05 0.49 ± 0.04 5 100 1.02-1.05 0.22-0.39 0.28-0.50 0.26 ± 0.05 0.38 ± 0.06 6a _{0 0.51-0.53 0.17-0.27 0.39-0.42 0.46 ± 0.06 0.81 ± 0.03} 7b 100 0.50-0.51 0.18-0.22 0.00-0.00 0.43 ± 0.03 ≤ 0.40 a _{continuation of experiment #3;} b _{continuation of experiment #5}

varied. The conditions for the five experiments and the corresponding steady-state effluent concentrations and dissolution rates are given in Table 7.4. The dissolved phosphorus concentration in experiment #7 (Fig 7.7) fell below the detection limit (0.2 µM) after ten residence times. Calcium concentrations were also measured and matched influent concentrations (data not shown). For experiment #3 (Fig. 7a; 0 µM influent calcium), the effluent calcium concentration had dropped below the detection limit (1 µM) within 4 residence times.

Dissolution rates are calculated from the steady-state reactor conditions using the following equation: A S 1 t C Rate res ⋅ = , (1)

where C is the effluent uranium concentration (µM), tres is the hydraulic residence time of the reactor (h), S is the mass concentration of solid in the reactor (g L-1_{), and A is the} specific surface area of the solid (m2 g-1). The dissolution rate was calculated at each sampling point, and the average value and standard deviation in each experiment for the period corresponding to steady-state are reported.

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The uranium release rates (for corresponding experiments) increase as the residence time is decreased from 1 to 0.5 h, suggesting that the dissolution reaction is inhibited by the accumulation of dissolution products in the reactor for experiments with a 1-h residence time (and probably for those with a 0.5 h residence time as well). No systematic effect of calcium in the reactor influent on dissolution rates was observed. With a 1-h residence time, dissolution rates (based either on U or P release) were higher with 10 mM Ca than without Ca in the influent but lower with 100 mM Ca. With a 0.5-h residence time, the U release rate was the same with 100 mM Ca as without Ca while the P release rate was lower with 100 mM Ca. The release rates of P (which ranged from 0.38 to 0.81 µmol m-2 h-1) were more variable than those of U (which ranged from 0.26 to 0.46 µmol m-2 h-1). Roughly congruent dissolution was observed in experiments 3-5 but, in experiment 6, the release rate of P was approximately double that of U.

7.4 Discussion