6th Response Submittal to the EPA Enclosure 1 

1-23-6 EPA Follow-up Comment #1 

There seems to be a disconnect between the previous (CRA-2004 PABC) FMT modeling 

results and the experimental investigation of Nd(III) solubilities. Borkowski et al. (2008) 

assumed that the stable Nd(III) phase in the carbonate system would be NdOHCO3(s). In 

the CRA-2004 PABC modeling results, the stable +III phase was predicted to be 

Am(OH)3(s). I have looked at a few of the CRA-2009 PABC FMT output files that you 

sent to me, and it appears that the stable +III phase predicted by the FMT model 

continues to Am(OH)3(s). In Figure 4-8 of Borkowski et al (GWB brine with carbonates), 

it appears that they were able to get fairly good reproducibility of results from 

undersaturation (dissolution of NdOHCO3) and oversaturation (precipitation) 

experiments. There appears to have been limited to no characterization of the postexperiment solid phases. Although precisely what was done to characterize the solids 

after the experiments was unclear to me after reading the test plan and Borkowski et al. 

2008 report, Marion Borkowski stated in our conference call this week that there were 

insufficient amounts of solids to characterize after the oversaturation (precipitation) 

experiments. As a result, the identity of the solid phases at the end of the experiments is 

not certain. The data in Figure 4-8 and the interpretation by Borkowski et al 2008 seem 

to indicate that the stable phase was NdOHCO3. Consequently, FMT calculations predict 

that Am(III) solubility for WIPP PA is controlled by Am(OH)3(s) but the experiments 

seem to indicate the solubility-controlling phase will be AmOHCO3. This raises the 

question of whether the FMT modeling is accurately predicting the concentrations of 

Am(III) (and by analogy Pu(III)) in the WIPP brines for PA. 

EPA Reference 

Borkowski, M., J.-F. Lucchini, M.K. Richmann, and D.T. Reed. 2008. “Actinide (III) 

Solubility in WIPP Brine: Data Summary and Recommendations.” LCO-ACP-08, 

LANL\ACRSP Report. Carlsbad , NM: Los Alamos National Laboratory. 

DOE Response

The DOE believes that the current Nd(III) experimental investigations agree with WIPP 

FMT modeling results. The results summarized in the Nd(III) report, in fact, support the 

modeling position that one phase is controlling the solubility of neodymium in these 

brine experiments (Borkowski et al., 2008). These data are part of an on-going 

experimental program that has not been completed. Based on the experimental results to 

date, the DOE does not believe the solubility controlling phase used in the FMT 

modeling should be changed (see Section 4.4.2 of Borkowski et al., 2008). 

The perception of a disconnect likely exists because there is not a full recognition of the 

differences between the experimental goals of a long-term solubility study (such as 

Borkowski et al., 2008) and the short term studies that are reported in the literature. In 

1 March 30, 2010 
6th Response Submittal to the EPA Enclosure 1 

short-term studies, the goal is to design the experiment to measure the solubility of a 

specific and well defined phase that is carefully prepared and added to the experiment. 

Here, equilibrium is rapidly established and a case is made that the solubility measured is 

directly linked to the characterized phase added to the experiment. 

In long term studies, the expectation is that the solubility controlling phase will be 

established as a response to the brine chemistry (pH, ionic strength, anion/cation 

concentrations, and Eh). In this experimental approach, given enough time, the solubility 

controlling phase will approach the same phase or group of phases and the measured 

solubility will be somewhat independent of the initial phase present. Results reported for 

the WIPP-relevant Nd(III) undersaturation experiments (Borkowski et al., 2008) are 

shown in Table 1. 

Table 1. Solubility of neodymium in the GWB undersaturation experiments 

Initial Nd Phase Brine and pCH+ Measured Solubility (M) 

Nd(OH)3 GWB / 8.6 6.9 x 10-7 (avg. of two) 

1

NdOHCO3 GWB / 8.4 3.8 x 10-7 (avg. of two) 

2

NdOHCO3 GWB / 8.4 5.8 x 10-7

3

FMT model prediction Reacted GWB / 9.7 2.25 x 10-7 

1

data taken from experiments with a total carbonate value of 0.1 mM 

2

data taken from experiments with a total carbonate value of 1 mM 

3

FMT calculations based on Brush et al., 2009 (in the absence of organics) 

The agreement in the neodymium solubilities measured for the two solids in GWB brine 

are a factor of 1.7 to 3.1 higher but are at a pCH+ that is almost a pH unit lower than the 

FMT calculations. The CRA-2009 PABC calculations (Brush et al., 2009) specifically 

indicate that Am(OH)3 is the solubility controlling phase at carbon dioxide fugacities 

established by the brucite-hydromagnesite buffer (WIPP reference case) but carbonate 

phases can be formed at the higher fugacities established by the brucite-nesquehonite 

buffer. Overall, the modeling and experimental results are well within the “order of 

magnitude” agreement typical of solubility studies in complex groundwater systems and 

support the existence of a single solubility-controlling phase or group of phases. The 

DOE believes this agreement is consistent with the Nd(OH)3 phase in the Nd experiments 

performed at WIPP-relevant carbonate levels (see Table 1). In Borkowski et al. 2008, 

there were no firm conclusions about the nature of the solubility-controlling solid phase 

present in these experiments other than to note that the solubility results presented are 

consistent with the current WIPP PA position that it is the Am(OH)3 phase (and by 

analogy the Nd(OH)3 phase in the neodymium experiments performed). 

2 March 30, 2010 
6th Response Submittal to the EPA Enclosure 1 

DOE References 

Borkowski, M., J.-F. Lucchini, M.K. Richmann, and D.T. Reed. 2008. “Actinide (III) 

Solubility in WIPP Brine: Data Summary and Recommendations.” LCO-ACP-08, 

LANL\ACRSP Report. Carlsbad , NM: Los Alamos National Laboratory. 

Brush, L.H., Y.-L. Xiong, and J.J. Long. 2009. “Results of the Calculations of Actinide 

Solubilities for the CRA-2009 PABC.” Analysis Report, October 7, 2009. Carlsbad, 

NM: Sandia National Laboratories. ERMS 552201 

3 March 30, 2010 
6th Response Submittal to the EPA Enclosure 1 

1-23-6 EPA Follow-up Comment #2

The observed differences in the concentrations at similar pcH in GWB and ERDA-6 brine 

are attributed to borate complexation. GWB has approximately 2.5 times the borate 

concentration of ERDA-6. The neodymium concentrations measured at pcH 8.7 in 

ERDA-6 brine (no carbonate) is approximately 5 x 10-8 M (Figure 4-10). In GWB brine 

at pcH 8.7 (Figure 4-9) is approximately 5 x10-7 M. What is the neodymium-borate 

speciation that would explain a ten-fold increase in neodymium concentrations with a 

2.5-fold increase in borate concentration? If it is a one-to-one complex such as 

NdB(OH)4

++, then one would expect that increasing the borate concentration by a factor 

of 2.5 would increase the Nd(III) concentration by approximately the same factor. 

EPA Reference

Borkowski, M., J.-F. Lucchini, M.K. Richmann, and D.T. Reed. 2008. “Actinide (III) 

Solubility in WIPP Brine: Data Summary and Recommendations.” LCO-ACP-08, 

LANL\ACRSP Report. Carlsbad , NM: Los Alamos National Laboratory. 

DOE Response 

The DOE agrees with the hypothetical argument presented by the EPA on the potential 

effects of borate complexation on the solubility of the III actinides but does not agree that 

the differences noted in the GWB and ERDA-6 neodymium solubility experiments 

should be attributed to borate complexation alone (Borkowski et al., 2008). The results 

summarized in this report are part of a long-term experimental program and the full 

effects of borate complexation will not be understood until research to establish 

appropriate Pitzer parameters for this species and the subsequent modeling of the full 

brine system is performed. There are, however, significant differences in the neodymium 

solubilities measured in these brine systems and differences are also predicted by current 

WIPP FMT modeling of the III actinides without organics present for the GWB (2.25 x 

10-7 M) and ERDA-6 (8.6 x 10-8 M) reacted brine systems (Brush et al., 2009). These are 

best explained by the many differences that exist between these two brines, (e.g., ionic 

strength, pH, sulfate/borate/chloride concentrations, and cation concentrations) and 

should not be attributed solely to differences in the concentration of borate. There are 

statements in the Borkowski et al. 2008 report that speculate on the relative importance of 

borate – but this should be viewed as preliminary in nature and not yet the accepted 

position of the WIPP project. 

DOE References 

Borkowski, M., J.-F. Lucchini, M.K. Richmann, and D.T. Reed. 2008. “Actinide (III) 

Solubility in WIPP Brine: Data Summary and Recommendations.” LCO-ACP-08, 

LANL\ACRSP Report. Carlsbad , NM: Los Alamos National Laboratory. 

4 March 30, 2010 
6th Response Submittal to the EPA Enclosure 1 

Brush, L.H., Y.-L. Xiong, and J.J. Long. 2009. “Results of the Calculations of Actinide 

Solubilities for the CRA-2009 PABC.” Analysis Report, October 7, 2009. Carlsbad, 

NM: Sandia National Laboratories. ERMS 552201 

5 March 30, 2010 
6th Response Submittal to the EPA Enclosure 1 

1-23-6 EPA Follow-up Comment #3 

The "shoulder" in the Nd concentration in ERDA-6 brine as a function of increasing pcH 

was attributed by Borkowski et al. 2008 to borate complexation. However, the reason for 

the decreasing Nd complexation as a function of pcH above about 9.75 was not clear. 

There is no reason that I know of for borate complexation to cease at higher pcH, and if 

hydrolysis species such as Nd(OH)3(aq) became more important at high pcH, these 

concentrations would be present in addition to the Nd-borate species concentrations. It is 

unclear whether a different solid might be controlling the Nd(III) solubilities in the highpcH region. An understanding of the solubility in this pcH region is important because 

in ERDA-6 the maximum concentration is observed at approximately the repository pcH 

predicted by FMT calculations (9.6-9.7). [Note: Clarification on this issue was requested 

in a phone call with LANL staff, but it was not resolved.] 

DOE Response 

The DOE believes that the concentration of complexes in solution, once formed, can 

decrease. The concentration of any neodymium (or III actinide) complex in solution is 

defined by the formation constant and the concentration of each contributing species. 

This is illustrated for a mononuclear complex of neodymium in the following: 

( ) n

n

K nXNd XNd + − − ←+ ⎯→ 3 3

[complex] = [ ( ) ] [ ] [ ] n n

n XNdkXNd − −+ = 3 3

In this context, the solution concentration of the complex will decrease when the 

concentration of the complexant is decreased or the concentration of the cation is 

decreased. For a fixed concentration of the complexant X-

, the complex will decrease 

when the concentration of the cation decreases (e.g., when its solubility decreases). For 

the Nd3+ solubility experiments reported, it remains the position of the DOE that the 

Nd3+(aq) concentration decreases with increasing pH (above pCH+ = 9) due to the 

formation of hydroxide phases which correspondingly decrease the concentration of 

borate complexes in solution (as well as other complexes). An overall decrease in the 

solubility of the III actinides with increasing pH (above 9.75) is supported by the WIPPspecific and literature data (see Section 3) reported in Borkowski et al. (2008). 

6 March 30, 2010 
6th Response Submittal to the EPA Enclosure 1 

DOE Reference 

Borkowski, M., J.-F. Lucchini, M.K. Richmann, and D.T. Reed. 2008. “Actinide (III) 

Solubility in WIPP Brine: Data Summary and Recommendations.” LCO-ACP-08, 

LANL\ACRSP Report. Carlsbad , NM: Los Alamos National Laboratory. 

7 March 30, 2010 
6th Response Submittal to the EPA Enclosure 1 

1-23-6 EPA Follow-up Comment #4

If the trend in Nd concentrations in GWB (Figure 4-8) is simply extrapolated up to pcH 

9.4 (the pcH predicted by FMT modeling for GWB brine in the repository), the Nd 

concentration could be as high as 4 x 10-5 M, which is much higher than the 

concentrations predicted for the CRA-2004 PABC (GWB no ligands) of 2 x 10-7 M. In 

our discussion, LANL staff said that the solubility at the pcH of 9.4 would be best 

represented by the solubility of Nd(III) in ERDA-6, because as pcH is increased, Mg 

precipitates and the composition of GWB approaches that of ERDA-6. However, if one 

compares the predicted Mg concentration of GWB at pcH 9.4 based on FMT modeling 

for the CRA-2004 PABC to the composition of ERDA-6, the Mg concentration in the 

higher-pcH GWB is still 0.55 M, which is 29 times the Mg concentration in ERDA-6. 

Even if one accepts that the solubility in ERDA-6 is a perfect way to estimate the 

concentration of Nd in pcH 9.4 GWB (1 x 10-6 M or higher), then the concentration is 

still, at a minimum, 5 times what was predicted by FMT (2 x 10-7 M). There is a lot of 

uncertainty in such a prediction, because the higher-pcH GWB is not the same 

composition as ERDA-6 and the results with carbonate in ERDA-6 (Figure 4-11) were 

only approached from undersaturation in this pcH region. Consequently, the 

concentration of Nd(III) in pcH 9.4 GWB could be higher than 10-6 M. 

EPA Reference 

Borkowski, M., J.-F. Lucchini, M.K. Richmann, and D.T. Reed. 2008. “Actinide (III) 

Solubility in WIPP Brine: Data Summary and Recommendations.” LCO-ACP-08, 

LANL\ACRSP Report. Carlsbad , NM: Los Alamos National Laboratory. 

DOE Response 

The DOE continues to maintain that the extrapolation of the Nd3+ solubility data in GWB 

brine (as described in the EPA comment above) does not properly reflect what is 

understood about the high pH chemistry of the III actinides in these brines. It is also 

important to note that the chemistry of these brine systems is complex and depends on 

many factors (see DOE answers to 1-23-6 follow-up comments #2 and #3) that are not 

properly captured by linear extrapolations. 

At high pH, hydroxide complexation and precipitation overwhelms the carbonate and 

borate complexes formed to lower the overall solubility of the III actinides (see Section 3 

in Borkowski et al., 2008). In the Nd3+ experiments this was best captured by the ERDA6 data which covered the current WIPP repository pCH+. Although no solubility data was 

collected for Nd3+ beyond the GWB “cloud point”, this was however done for the U(VI) 

experiments performed in carbonate-free GWB brine (Lucchini et al., 2009). Although 

U(VI) is not an analog for the III actinides, qualitatively similar solubility trends should 

8 March 30, 2010 
6th Response Submittal to the EPA Enclosure 1 

be present in the GWB brine system. These data are shown in Figure 1 along with the 

corresponding neodymium data and illustrate the expected dependence of the III actinide 

solubility on pH. 

6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

 Nd oversaturated

 U(VI) oversaturation

 Nd undersaturated

Metal Concentration, M

pCH+

GWB

Figure 1. Plot of the uranium(VI) over-saturation data (upper curve) and 

neodymium over and undersaturation data (lower curve) in GWB brine. The pCH+

~ 9.2 data point for uranium already shows a decrease in solubility due to hydrolysis 

and is beyond the cloud point of GWB brine meaning that some magnesium 

precipitation has occurred. 

DOE Reference 

Borkowski, M., J.-F. Lucchini, M.K. Richmann, and D.T. Reed. 2008. “Actinide (III) 

Solubility in WIPP Brine: Data Summary and Recommendations.” LCO-ACP-08, 

LANL\ACRSP Report. Carlsbad , NM: Los Alamos National Laboratory. 

Lucchini, J.-F., H. Khaing, M. Borkowski, M.K. Richmann, and D.T. Reed. 2009. 

“Actinide (VI) Solubility in Carbonate-free WIPP Brine: Data Summary and 

Recommendations.” LCO-ACP-10, LANL\ACRSP Report. Carlsbad NM: Los Alamos 

National Laboratory. 

9 March 30, 2010 
6th Response Submittal to the EPA Enclosure 1 

1-23-6 EPA Follow-up Comment #5

When discussing possible effects of increased +III actinide solubilities on PA during our 

call, a LANL staff member said that he thought the increased ligands concentrations 

predicted for the CRA-2009 PABC FMT calculations might have increased the +III 

actinide solubilities to the extent that americium was inventory limited. Although he did 

not think Pu would be inventory-limited, he suggested that high EDTA concentrations 

might result in sufficient EDTA complexation that the effects of borate would be 

relatively unimportant. I have only looked at what I believe are the ERDA-6 results for 

the CRA-2009 PABC calculations. This output file 

(FMT_PABC09_E6_HMAG_ORGS_013.OUT) is for ERDA-6 brine, with organics, and 

was run on July 8 2009. The total concentration of Am(III) was predicted to be 1.51 x10-6

M at pcH 9.7. This concentration does not dwarf the concentration observed in Figure 4-

11 (about 1 x 10-6 M), so it does not appear that EDTA complexation of the +III 

actinides will be so high that the potential effects of borate complexation described by the 

results of Borkowski et al. 2008 can be neglected. 

EPA Reference 

Borkowski, M., J.-F. Lucchini, M.K. Richmann, and D.T. Reed. 2008. “Actinide (III) 

Solubility in WIPP Brine: Data Summary and Recommendations.” LCO-ACP-08, 

LANL\ACRSP Report. Carlsbad , NM: Los Alamos National Laboratory. 

DOE Response 

The initial results of the ongoing neodymium experiments remain inconclusive about the 

overall effect of borate complexation on the III actinides under WIPP-relevant conditions. 

The most important reason for this, as discussed in 1-23-6 EPA follow-up comment # 2, 

is that the issue of borate complexation has not been fully addressed by the WIPP project 

and continues to be the subject of ongoing research. The full quantification of the true 

impact of borate complexation cannot be established until all the needed Pitzer data and 

species information are obtained by the project and calculations can be done that reflect 

all the key solution parameters at WIPP-relevant ionic strengths and conditions. 

It is also important to note, as stated in 1-23-6 EPA follow-up comment # 2, that there are 

potentially other factors besides borate that account for the small differences between the 

modeling and experimental results (e.g., pH, brine component concentrations) and it 

remains the DOE position that a factor of 10 or less agreement between modeling and 

experimental results is excellent agreement for complex groundwater systems such as the 

WIPP-relevant brines used in Borkowski et al. (2008). In this context, differences of 

factors of 2 or 3 are not considered significant. The DOE’s position is that the FMT 

model calculations for An(III) were confirmed by the data in Borkowski et al. (2008). 

10 March 30, 2010 
6th Response Submittal to the EPA Enclosure 1 

Addressing the influence of organic ligands (i.e. EDTA), comparisons of the FMT 

calculations with and without organics show an increase in americium solubility from 

2.25 x 10-7 M to 1.51 x 10-6 M, about a factor of seven (Brush et al., 2009). The final 

concentration of the actinides is a function of several uncertainties and is calculated as a 

function of time (Garner, 2010). For example, after including the effects of the 

uncertainties, this leads to a range of total potential mobilized americium in the Castile 

brine used in PA of 3 x 10-8 M to 2 x 10-3 M (Garner, 2010). The DOE believes that this 

range is sufficient to account for any possible borate effects or other uncertainties. 

DOE References 

Borkowski, M., J.-F. Lucchini, M.K. Richmann, and D.T. Reed. 2008. “Actinide (III) 

Solubility in WIPP Brine: Data Summary and Recommendations.” LCO-ACP-08, 

LANL\ACRSP Report. Carlsbad , NM: Los Alamos National Laboratory. 

Brush, L.H., Y.-L. Xiong, and J.J. Long. 2009. “Results of the Calculations of Actinide 

Solubilities for the CRA-2009 PABC.” Analysis Report, October 7, 2009. Carlsbad, 

NM: Sandia National Laboratories. ERMS 552201. 

Garner, J. 2010. “Analysis Package for PANEL: CRA-2009 Performance Assessment 

Baseline Calculation.” Carlsbad, NM: Sandia National Laboratories. ERMS 553032. 

U. S. Department of Energy (DOE) 2009. CRA-2009 Appendix SOTERM. Carlsbad 

NM: Carlsbad Area Field Office. 

Xiong, Y.L., L.H. Brush, A.E. Ismail, and J.J. Long. 2009. “Uncertainty Analysis of 

Actinide Solubilities for the WIPP CRA-2009 PABC.” Analysis report, December 1, 

2009. Carlsbad, NM: Sandia National Laboratories. ERMS 552500. 

11 March 30, 2010 
6th Response Submittal to the EPA Enclosure 1 

1-23-6 EPA Follow-up Comment #6 

Figure 4-9 presents Nd(III) solubility data in GWB as a function of pcH in the absence of 

carbonate. At pcH values of approximately 7.3 and 8.0, the concentrations measured in 

experiments approached from undersaturation are higher by approximately an order of 

magnitude than the concentrations measured in experiments approached from oversaturation. Does DOE have an explanation for this? 

EPA Reference 

Borkowski, M., J.-F. Lucchini, M.K. Richmann, and D.T. Reed. 2008. “Actinide (III) 

Solubility in WIPP Brine: Data Summary and Recommendations.” LCO-ACP-08, 

LANL\ACRSP Report. Carlsbad , NM: Los Alamos National Laboratory. 

DOE Response 

The EPA has correctly noted that there is often disequilibrium in experiments that start 

from undersaturation. This is commonly observed in many solubility studies in complex 

media that start with a specific solid. The explanation for this is that it takes time for the 

solubility-controlling phases to form and define the observed solubility. In this context, 

the initially-observed solution concentrations are defined by the relative rates of the 

dissolution of the “unstable” phase added to the experiment and the precipitation/ 

formation of the solubility controlling phases – rather than simply the solubility of the 

solubility-controlling phases. 

In time, the undersaturation experiments and oversaturation experiments are expected to 

approach the same value because, at least in the time frame of the experiments, 

equilibrium has been reached. This is what is generally observed in the III actinide 

experiments reported (Borkowski et al., 2008). The figure noted in the EPA response is 

shown in Figure 1 (Figure 4-9, Borkowski et al., 2008) and corresponds to the 

neodymium concentrations measured after approximately 9.2 months. The same data 

taken at 13.0 months (~ 4 months later), which was after the CRA-2009 data cut-off, are 

shown in Figure 2. As can be seen, although some discrepancy remains, the over- and 

under saturation results are reaching better agreement (a factor of ~3 rather than ~10). 

DOE Reference 

Borkowski, M., J.-F. Lucchini, M.K. Richmann, and D.T. Reed. 2008. “Actinide (III) 

Solubility in WIPP Brine: Data Summary and Recommendations.” LCO-ACP-08, 

LANL\ACRSP Report. Carlsbad , NM: Los Alamos National Laboratory. 

12 March 30, 2010 
6th Response Submittal to the EPA Enclosure 1 

6.0 6.5 7.0 7.5 8.0 8.5 9.0

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

Nd Concentration, M

pCH+

 - oversaturation

 - undersaturation

 - carbonate trend

GWB

over- and under-saturation

carbonate free 

Figure 1. Solubility of neodymium in GWB brine at 9.2 months (this is Figure 4-9 in 

Borkowski et al., 2008). 

6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

- oversaturation

- undersauration

Nd Concentration, M

pCH+

GWB

carbonate free

Figure 2. Neodymium solubility in GWB brine at 13 months. This compares to the 

data presented in Figure 1 and shows increased agreement between the 

undersaturation and oversaturation data. 

13 March 30, 2010 
6th Response Submittal to the EPA Enclosure 1 

1-23-6 EPA Follow-up Comment #7 

DOE's response to 1-23-6c dated November 25, 2009 statement that "For the III 

actinides, the complexation of borate effectively substitutes for the calculated effect of 

carbonate presently in the WIPP model." Is inconsistent with the modeling results from 

the CRA-2004 PABC. These results, both with and without ligands present in the brine, 

showed that aqueous americium-carbonate complexes constituted negligible proportions 

of the total americium in solution in all four solution compositions (GWB and ERDA-6 

with and without ligands). In addition, because there is an assumed excess of Am(OH)3(s) 

in the WIPP repository, "competition" between borate and carbonate for americium in 

the system will not occur. Americium-borate aqueous complexes, if formed, will form in 

addition to americium-carbonate complexes. However, under WIPP conditions, current 

FMT modeling does not predict significant carbonate complexation of the +III actinides. 

DOE Response 

The DOE agrees with the EPA that carbonate complexation does not significantly impact 

the solubility of the III actinides in the WIPP. The effects of borate complexation have 

not yet been fully evaluated (see responses to 1-23-6 EPA follow-up comments #2, #3 

and #5). The intention of the 1-23-6c DOE response was to make a general point that 

borate and carbonate complexation behave in a similar way in the brine systems 

investigated. The reasons for this are 1) they have a similar pKa: 9.0 for borate vs. 9.6 

for carbonate; and 2) the effective complexation strength (Kf [concentration]) value is 

very similar (Keff = ~320 for carbonate, 370 for borate in GWB and 150 for borate in 

ERDA-6). Therefore, even though the formation constant for borate is smaller than 

carbonate, both anions have similar complexation strengths. 

14 March 30, 2010 
6th Response Submittal to the EPA Enclosure 1 

15 March 30, 2010 

1-23-6 EPA Follow-up Comment #9 

Another question is related to the carbonate concentrations in the experiments that they 

conducted. I understand that they spiked an initial carbonate concentration into the 

solutions and used "gas-tight polypropylene bottles" for the experiments. Did they 

measure carbonate concentrations in solution at the end of any of the experiments to 

confirm that there was no loss of CO2? It is my understanding that all of the experiments 

were performed in a CO2-free glove box, so gain of CO2 should not have occurred. Do 

they assume that the amounts of NdOHCO3 precipitated in their experiments were so 

small that it did not affect the total carbonate in the solutions? 

DOE Response 

Carbonate levels were measured in the experiments reported. After three years, which is 

well beyond the duration of the experiments reported (Borkowski et al., 2008), there is 

satisfactory agreement between the measured carbonate concentrations and the initial 

concentrations added to the brine (typically 80% or greater). It was not stated in 

Borkowski et al., 2008 that carbonate phases were formed in the experiments reported. 

The DOE position continues to be that the solubility controlling phase is the hydroxide 

phase since there is good agreement between FMT model predictions and the 

experimental results (good agreement in this context is less than an order of magnitude). 

DOE Reference 

Borkowski, M., J.-F. Lucchini, M.K. Richmann, and D.T. Reed. 2008. “Actinide (III) 

Solubility in WIPP Brine: Data Summary and Recommendations.” LCO-ACP-08, 

LANL\ACRSP Report. Carlsbad , NM: Los Alamos National Laboratory.