Introduction
In the ongoing assessment of chloride equilibrium concentrations in bentonite, we here take a closer look at the study by Muurinen et al. (2007), in the following referred to as Mu07. We thus assess the 11 points indicated here
Mu07 actually report 9 more data points, but these originate from Muurinen et al. (2004) (which we have already assessed). This is not fully acknowledged in Mu07, but below I try to sort out the status of all data presented. We refer to Muurinen et al. (2004) as Mu04.
In similarity to Mu04, Mu07 is an equilibrium study (i.e. not a diffusion study) performed on purified “MX-80” bentonite. One of the main objectives in Mu07 is to investigate possible influence of sample preparation on the chloride equilibrium concentrations. The samples in Mu07 cover a large density range (0.6 — 1.5 g/cm3), but were all equilibrated with a single type of solution: 0.1 M NaCl.
Originally conducted and already reported tests
Mu07 state that the study is a continuation of the investigations presented in Mu04 and present data on five different sets of samples, prepared and equilibrated using different methods (labeled A — E). What is not explicitly stated — but what is obvious if comparing tables 1 and 2 in Mu04 with table 1 in Mu07 — is that sample sets D and E are the same as previously reported in Mu04.
I find this quite remarkable, since two of these samples were dismissed as “not reliable” in Mu04 (In my assessment, I dismissed all tests in Mu04); here the same results — which show an increase in equilibrium chloride content with density — are not only re-reported, but modeled! The authors don’t even seem aware that they have previously discarded the samples, writing: “Surprisingly, it seems that the concentrations in the sample types D and E start to increase at the highest densities“. Furthermore, one of the (previously reported) data points of sample set E, which have a clay concentration larger than the corresponding concentration of the equilibrating solution, is not included in Mu07. Needless to say, excluding data points without motivation, or including previously discarded data is not good scientific practice.
As the sample overview table in Mu07 also has some misprints,1 I here present a (hopefully) correct version that also indicates original publication (for the indicated sample-IDs, see the assessment of Mu04).
| Type | Density (g/cm3) | Time d.w. (days) | Time 0.1 M (days) | Clay conc. (mM) | origin/remark |
| A | 0.625 | 18+35 | 217 | 37 | Mu07 |
| A | 0.812 | 18+35 | 217 | 29 | Mu07 |
| A | 1.200 | 18+35 | 217 | 20 | Mu07 |
| B | 0.670 | 222 | 107 | 35 | Mu07 |
| B | 0.937 | 222 | 107 | 23 | Mu07 |
| B | 1.389 | 222 | 107 | 15 | Mu07 |
| C | 0.622 | 30 | 36 | 48 | Mu07 |
| C | 0.731 | 30 | 36 | 30 | Mu07 |
| C | 1.113 | 30 | 36 | 17 | Mu07 |
| C | 1.382 | 30 | 36 | 14 | Mu07 |
| C | 1.517 | 30 | 36 | 12 | Mu07 |
| D | 0.754 | 0 | 40 | 65 | Mu04 (S2-02) |
| D | 0.855 | 0 | 40 | 39 | Mu04 (S2-21) |
| D | 1.273 | 0 | 36 | 22 | Mu04 (S2-04) |
| D | 1.636 | 0 | 36 | 24 | Mu04 (S2-17; deemed “not reliable” in Mu04) |
| D | 1.764 | 0 | 85 | 48 | Mu04 (S2-18; deemed “not reliable” in Mu04) |
| E | 0.750 | 0 | 12(?) | 109 | Mu04 (not included!) |
| E | 0.875 | 0 | 12 | 61 | Mu04 |
| E | 1.225 | 0 | 12 | 25 | Mu04 |
| E | 1.516 | 0 | 12 | 12 | Mu04 |
| E | 1.543 | 0 | 12 | 14 | Mu04 |
In the following, the focus is solely on samples sets A — C (as mentioned, the others have already been assessed).
Material
The material appear to be the same as used in Mu04. I therefore refer to the assessment of that study for a detailed discussion. In brief, the material is purified “MX-80” bentonite, with a montmorillonite content above 90% and about 90% sodium as exchangeable cation.
Samples
Samples in the three different sample sets A — C were prepared in different ways. For set A, the clay was initially dispersed in deionized water at quite low density. After an equilibration time of 18 days (which included ultrasound treatment), the dispersion was slowly squeezed to achieve the intended densities. This squeezing phase lasted 35 days, after which the samples were contacted with 0.1 M NaCl and equilibrated for 217 days.
Samples in set B were prepared in the same type of sample holder as those in set A, but the bentonite powder was directly compacted to the desired density, and the samples were water saturated by contact with deionized water for 222 days (!). Thereafter, the samples were contacted with 0.1 M NaCl and equilibrated for 107 days.
The external solution was not circulated in the preparation of samples in sets A and B. In contrast, samples in set C were prepared in cells with external circulation. The bentonite powder was directly compacted to the desired density, and the samples were water saturated by contact with (circulating) deionized water for 30 days. The samples were then equilibrated with (circulating) 0.1 M NaCl solution for 36 days.
Even if the preparation protocols are described quite detailed in Mu07, we are not given any information on sample geometry. We are not even told if the samples have the same geometry! (Given that they were prepared in different types of equipment, different geometries may certainly be the case.) Without knowledge on e.g. the characteristic diffusion lengths, it is impossible to assess e.g. whether the adopted equilibration times are adequate. Reasonably, the size of the samples are on the cm scale, and since the equilibration times are very long, we can guess that they have had time to equilibrate. This is in contrast to the samples in Mu04, which we have reason to suspect have not been completely equilibrated, as discussed in the assessment of that study. (Note that these samples are included in Mu07, as sample sets D and E.)
Mu07 does not provide any information on how sample density was measured. Since we neither know the dimensions of the samples it is therefore impossible to estimate any uncertainty of the reported densities.
Chloride equilibrium concentrations
The following plot summarizes the reported chloride equilibrium concentrations and corresponding densities in sample sets A — C.
Although the data show some significant scatter (e.g. for the two lowest densities in sample set C), the main impression is that the three different ways of preparing and equilibrating samples result in quite similar values for the chloride equilibrium concentrations. Thus, even if we know little about the samples, this coherence in the results indicates that they have been properly equilibrated.
Possible interface excess salt
As we have discussed in several previous blog posts, when performing equilibrium tests it is important to handle the possibility that the samples have an increased salt content in the interface regions. In the assessment of Mu04, my guess was that the samples had not been handled specifically to deal with this possible measuring artifact, and I neither see any reason to believe that this issue has been addressed in sample sets A — C (we can, however, rule out that too much salt entered these samples during saturation, since deionized water was used in this phase).
The possible influence of interface excess depends, apart from general sample treatment, on e.g. sample thickness and the concentration of the equilibrating external solution. As noted above, we have no information on sample thickness, but the external concentration is in this regard quite low (we showed in an earlier post that the problem of interface excess salt becomes more severe for thin samples and low external concentrations). Therefore, we can certainly not exclude the possibility that the reported equilibrium concentrations are systematically overestimated due to possible influence of an interface excess, especially for the denser samples (see here for details on this).
An argument against that interface excess has significantly influenced the results is the similar result for the three different sample sets. Of course, this depends on how similar (or dissimilar) the samples in the different sets are, of which we have no information. Under any circumstance, it is very clear that Mu07 provides too little information to fully rely on the reported values.
Summary and verdict
From one perspective, Mu07 is a very straightforward study: samples of purified bentonite (almost pure Na-montmorillonite) at various density have been equilibrated with a single type of external solution (0.1 M NaCl). The results also look reasonably coherent. However, the paper contains way too little information on e.g. sample geometry and how density and concentration were measured to fully rely on the results. In particular, we cannot rule out a systematic overestimation due to influence of interface excess salt. Furthermore, the main reason to believe that equilibrium was achieved, is the similarity between the different test sets.
My decision, however, is to keep these result to use e.g. for possible qualitative process understanding (specifically, chloride exclusion). But I will certainly keep in mind the quite extensive lack of information associated with this data.
- Chloride content: UNKNOWN
- Extracting anion equilibrium concentrations from through-diffusion tests
- Assessment of chloride equilibrium concentrations: Muurinen et al. (1988)
- Assessment of chloride equilibrium concentrations: Molera et al. (2003)
- How salt equilibrium concentrations may be overestimated
- Assessment of chloride equilibrium concentrations: Muurinen et al. (2004)
- Assessment of chloride equilibrium concentrations: Van Loon et al. (2007)
Footnotes
[1] The presented diagram in Mu07 (fig. 5) don’t suffer from these typos.