PCT Experiments
The results of PCT type experiments of the CSXC-NaAlO2 and CSXC-BORAX are displayed in Figures 6.11 - 6.12, Figure 6.15 and Table 6.5. Figure 6.11 displays data obtained from PCT experiments on CSXC-NaAlO2 and CSXC-BORAX. This figure provides plots of NLi against time for each element from both CSXC-NaAlO2 and CSXC-BORAX, illustrating the relationship between the individual NLi values. Figure 6.12 provides plots of NLi against time for each element of interest for both CSXC-NaAlO2 and CSXC-BORAX. This allows comparison of the dissolution behaviour in the two wasteforms. The errors are set to ± 10 % based on RSD values of concentrations, as measurement using multiple ICP-MS measurements. The pH data is shown for the experiment in Figure 6.12 showing an initial rapid increase to approximately pH 9.2. Despite some minor variation the pH roughly stays constant at this value throughout the duration of testing. 28 days. Figure 6.15 provides the same comparison as Figure 6.12 with the addition of data from MCC-1 experiments, described later in this section, which allows comparison of NLi under different experimental methodologies. Table 6.5 displays both the NLi and NRi from PCT and MCC-1 experiments after 28 days sampling.
The behaviour of CSXC-NaAlO2 during PCT experiments shown in Figures 6.11 and 6.12 illustrates that, within the expected error margins, the majority of elements obtain a stable value of NLi during the testing period. The exceptions to this are Ba and Cs. NLBa decreases over time, which indicates that Ba is likely to be precipitating from solution as the experiment progresses. NLCs shows a decrease from the initial sampling to the 14 day sampling point, however it is then increased in the last two sampling points.
Figure 6.11 – Normalised mass losses of selected elements from PCT experiments on CSXC-NaAlO2 and CSXC-BORAX at 90°C with a SA/V of ~2000 m-1 in 18.2 MΩ water.
The NLi values obtained from CSXC-BORAX follow four different trends over the course of this PCT experiment, which are most clearly observed in Figure 6.12. NLCs follows the same trend as was noted was noted in CSXC-NaAlO2. The second noted trend is where a large initial increase of NLi occurs, followed by a gradual decrease. This is the case for NLSi, NLAl and NLBa. The third trend noted is that the NLi first reaches a steady value until the final sampling point prior to a decrease in the NLi, as is the case for NLCa, NLSr, NLNa and NLS. The final trend is only observed for NLB which attains a steady value of NLB after 3 days and shows no deviation from this outside of the expected error.
Figure 6.12 – Graphs comparing the normalised elemental mass loss of key elements from PCT type experiments on CSXC-NaAlO2 and CSXC-BORAX at 90°C with a SA/V of ~2000 m-1 in 18.2 MΩ water.
Although the descriptions provided above appear to show the behaviour of the two wasteforms is very different, when looking at the generic behaviour of these glasses with
respect to the release of major elements several similarities are evident. After 28 days of testing both CSXC-NaAlO2 and CSXC-BORAX both show the following trends in the values of NLi,the values of which are provided in Table 6.5; NLSi ≤ NLCs < NLBa < NLS < NLCa < NLSr. These NLi values are all comparable within at least the same order of magnitude. This suggests that the dissolution behaviour concerning the majority of elements in the wasteforms is similar.
Although all the NLi values for these elements are within the same order of magnitude, it should be noted that the NLi of the above mentioned elements is higher in CSXC-BORAX after 28 days than was determined for CSXC-NaAlO2. The NLi of Sr and Ca from CSXC-BORAX were up to three times greater than was observed for CSXC-NaAlO2.
However, the NLi of Na and Al differ more significantly between the two wasteforms. NLNa
and NLAl are both an order of magnitude greater in CSXC-NaAlO2 than was observed from CSXC-Borax. A comparison of the behaviour of NLB is impossible due to the absence of B from the composition of CSXC-NaAlO2.
MCC-1 Experiments
Figure 6.13 displays data obtained from MCC-1 experiments on CSXC-NaAlO2 and CSXC-BORAX. This figure provides plots of NLi against time for each element of both CSXC-NaAlO2 and CSXC-BORAX, which highlight the relationship between individual NLi
values. Figure 6.14 provides plots of NLi against time for each element of interest in both CSXC-NaAlO2 and CSXC-BORAX which allows comparison of the dissolution behaviour between the two wasteforms. The error from these experiments was determined from the RSD on the concentrations of multiple identically prepared samples, as measured by ICP-MS.
This error was large by comparison to that previously observed for MCC-1 type experiments (± 5%) and was set to ± 25 % of the measured value. The origin of this larger error is discussed later in this section. The pH data is shown for the experiment in Figure 6.14 showing a gradual increase during testing to a peak of approximately pH 8.5 after 28 days. Figure 6.15 provides the same comparison as Figure 6.14 with the addition of data from PCT experiments described earlier in this section. This allows comparison of NLi under different experimental methodologies. Table 6.5 displays both the NLi and NRi values from PCT and MCC-1 experiments from the 28 days sampling.
In the case of CSXC-NaAlO2 Figure 6.13 shows that the comparison of individual NLi values is unlikely to prove conclusive. No pronounced differentiation based on behaviour can be made from Figure 6.13. Figure 6.14 shows that three different trends exist within the plots of NLi
for CSXC-NaAlO2. NLSi, NLBa and NLCs all increase steadily over the course of the testing. NLS,
NLCa, NLNa and NLAl all increase in NLi over the course of testing, however the rate at which these changes occurs varies between sampling points, accelerating towards the end of the experiment. Finally NLSr at first shows a steady increase but after 28 days drops significantly.
The values of NLi obtained for CSXC-BORAX follow two different trends as can be seen more clearly in Figure 6.14. NLSr increases gradually at first with a large increases after 21 days.
NLSi, NLAl, NLNa, NLCs, NLBa, NLCa and NLB increase over time steadily over the course of the experiment.
Figure 6.13 – Normalised elemental mass losses of selected elements from MCC-1 type experiments on CSXC-NaAlO2 and CSXC-BORAX at 90°C with a SA/V of ~10 m-1 in 18.2 MΩ water.
Due to the large experimental error no comparison is made here of the significance of variations in NLi within each experiment. It can however be surmised from Figure 6.14 that both CSXC-NaAlO2 and CSXC-BORAX produce similar profiles to each other for all but three of the key elements. NLSr shows a decrease in CsXC-NaAlO2 after 28 days testing, whereas it shows an increase in the case of CSXC-BORAX. As in the PCT experiments described above, the most apparent variation is observed for NLAl and NLNa. Here the release in the CSXC-NaAlO2 sample is over an order of magnitude greater than that of the CSXC-BORAX. As in the
PCT experiments a comparison of NLB is not possible due to the composition of CSXC-NaAlO2
containing no boron.
Figure 6.14 – Graphs comparing the normalised elemental mass loss of key elements from MCC-1 type experiments on CSXC-NaAlO2 and CSXC-BORAX at 90°C with a SA/V of ~10 m-1 in 18.2 MΩ water.
Comparison of PCT and MCC-1 Experiments Solution Releases
Figure 6.15 provides a comparison of the results obtained from PCT and MCC-1 experiments on CSXC-NaAlO2 and CSXC-BORAX. In all cases the NLi from each composition is between one and two orders of magnitude higher when tested by MCC-1 experiments than observed in PCT experiments.
MCC-1 experiments consistently show a greater slope in the NLi vs. time than is observed from PCT experiments throughout the course of testing. This shows the NRi is always greater in the MCC-1 experiments.
Plotting the combined results on a log scale helps to illustrate where consistencies in the behaviour of elemental release behaviour from each composition exist in both experiments.
Three behaviours can be noted with respect to the consistency of behaviour. The first is that NLi values from each composition are broadly similar when examined with comparable experimental methodologies. This is true of NLSi, NLCs, and NLS.
The second behaviour observed was evident when consistent and significant differences existed between each composition tested under both experimental methodologies. This is the case for NLNa, NLAl and NLSr. NLNa and NLAl are over an order of magnitude greater for CSXC-NaAlO2 than for CSXC-BORAX in both experiments. NLSr shows the CSXC-BORAX consistently releases Sr in greater quantities in both experiments than is seen for CSXC-NaAlO2.
The third behaviour is shown when no correlation of behaviour between composition and experimental methodology are observed. This is the case with NLCa and NLBa. NLCa shows roughly equivalent behaviour of the compositions during MCC-1 experiments, but a greater NLCa from CSXC-BORAX during PCT experiments. NLBa shows higher release from CSXC-BORAX during MCC-1 experiments but roughly similar behaviour of each composition during PCT experiments.
Figure 6.15 - Graphs comparing the normalised elemental mass loss of key elements from MCC-1 and PCT type experiments on CSXC-NaAlO2 and CSXC-BORAX at 90 °C in 18.2 MΩ water.
Table 6.5 – Values of normalised elemental mass loss and normalised elemental loss rates from both 28 days testing of PCT and MCC-1 type experiments of CSXC-NaAlO2 and CSXC-BORAX.
Both experiments were performed in 18.2 MΩ H2Oat 90°C with a SA/V of 2000 m-1 and 10 m-1 respectively. Errors estimated at ± 10% of stated values for PCT and ±25 % for MCC-1.
Element
NLi (g m-2) NRi (g m-2 day-1)
PCT MCC-1 PCT MCC-1
CSXC-NaAlO2 CSXC-BORAX CSXC-NaAlO2 CSXC-BORAX CSXC-NaAlO2 CSXC-BORAX CSXC-NaAlO2 CSXC-BORAX Si 1.18 x 10-2 1.90 x 10-2 3.09 x 100 3.86 x 100 4.21 x 10-4 6.80 x 10-4 1.10 x 10-1 1.38 x 10-1 Al 4.96 x 10-2 8.41 x 10-3 6.85 x 100 3.30 x 100 1.77 x 10-3 3.00 x 10-4 2.45 x 10-1 1.18 x 10-1 Na 3.02 x 10-1 5.93 x 10-2 1.11 x 10-1 3.73 x 100 1.08 x 10-2 2.12 x 10-3 3.96 x 10-1 1.33 x 10-1 Cs 1.38 x 10-2 1.90 x 10-2 1.94 x 100 2.44 x 100 4.91 x 10-4 6.79 x 10-4 6.93 x 10-2 8.71 x 10-2 Sr 1.61 x 10-1 4.78 x 10-1 3.91 x 100 2.26 x 101 5.75 x 10-3 1.71 x 10-2 1.40 x 10-1 8.05 x 10-1 Ba 3.04 x 10-2 3.70 x 10-1 8.09 x 100 1.10 x 101 1.08 x 10-3 1.32 x 10-3 2.89 x 10-1 3.94 x 10-1 Ca 1.00 x 10-1 2.12 x 10-1 2.02 x 101 1.26 x 101 3.57 x 10-3 7.56 x 10-3 7.21 x 10-1 4.49 x 10-1 S 4.15 x 10-2 5.23 x 10-2 1.20 x 100 1.95 x 100 1.48 x 10-3 1.87 x 10-3 4.29 x 10-2 6.95 x 10-2
B N/A 9.15 x 10-2 N/A 1.20 x 101 N/A 3.27 x 10-3 N/A 4.28 x 10-1