the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Dec 2024
The clay mineralogy rather that the clay content determines radiocaesium adsorption in soils
Abstract. The transfer of radiocaesium (137Cs) from soil to crops is the main long-term radiation risk after nuclear accidents. The prevailing concept is that 137Cs sorption in soil, and hence its bioavailability, is controlled by soil clay content (0–2 µm). This study tested this assumption using 24 soils collected worldwide. The Radiocaesium Interception Potential (RIP), i.e., 137Cs adsorption, was measured for the bulk soils and for their clay and silt fractions. The RIP varied by factor 438 among soils and was unrelated to its clay content (p >0.05). The RIP in the clay fractions was lowest for young volcanic soils with allophane and mica, and for highly weathered tropical soils with kaolinite. In contrast, about two order of magnitude higher RIP values were found in intermediate-weathered temperate soils dominated by illite. Soil RIP was, hence, related to soil illite content (R2 = 0.50; p <0.001). Significant fraction of soil RIP originated from clay minerals embedded in the silt fraction. The sum of RIP in clay and silt fractions overestimated the soil RIP by, on average, factor of 2, indicating that isolation of clay opens selective 137Cs sorption sites inaccessible in intact soils. Soil mineralogy, not just clay content, governs soil RIP. The validity of existing 137Cs bioavailability models require recalibration for its use on a global scale.
- Preprint
(3239 KB) - Metadata XML
- BibTeX
- EndNote
Status: final response (author comments only)
-
RC1: 'Comment on egusphere-2024-3585', Atsushi Nakao, 01 Jan 2025
I enjoy reading this manuscript. It is well-written and includes scientific novelty. I fully agree with your central concept that “137Cs sorption is not controlled by the clay content but rather by the mineralogy, which is influenced by parent material and weathering states” (line 299-300 in Conclusion) . Figure 2 provides us a nice view of this idea. Although a more sampling set would be more increased data reliability, I can imagine the time and labor work required for fractionation and measurement of RIP and the other related physicochemical properties.
However, the reason why illite is more dominated at the intermediate weathering stage requires a more careful discussion. In my opinion, many of the soils currently distributed around the world (especially in Europe and North America) that are intermediate in development are happen to be rich in illite, but there may be many that are not. For example, soils developed mainly from loess deposition (e.g. Chernozem) are rich in illite, whereas those from mafic or ultramafic materials (e.g. serpentinite rock and glassy volcanic ash) have no illite even being developed intermediately. These differences are controlled by original mineralogy of the parent materials. I doubt if illite will be formed in the Miyakonojo soil (Figure C3) developed from glassy volcanic ash, even times fly to the long future.
Four possible processes are considered behind the “illite accumulation”: 1) neoformation during pedogenesis, 2) selective concentration as the other clay minerals are dissolved, 3) downsizing of coarser (sandy or silty) illite to clay fraction through weathering, 4) translocation of exogenic illite from other environments. Which do you think of the major possible process to cause the dominance of illite at intermediate weathering stage?
1) Neoformation of illite or other mica phases is not probable through chemical weathering reaction in pedogenesis. Isomorphic substitution of 4-fold Al3+ in Si-O tetrahedron, which is required to provide a series of permanent negative charges in the interlayer site, cannot dominantly occur at normal pressure and temperature in soil environment(Mackenzie et al., 1987; Marsh et al., 2024). Most of the 2:1 phyllosilicates with a considerable tetrahedrally isomorphic substitution are, therefore, considered to be crystallized at higher temperature and pressure than soil environments (e.g. magmatic melt and pressure solution during diagenesis).
2) Selective concentrations of illite are also not probable because dissolution of allophane or other poorly crystalline alminosilicates often associated with neoformation of the other (more resistant to weathering) clay minerals as smectite or kaolinite (e.g. Figure C2 showing XRD for soil clays in the Philippines).
3) If parent material contains illite, downsizing through pedogenesis is the most probable process to increase illite content in clay fraction.
4) If soil sampling sites are located at lower elevation close to illite-rich bedrock area or at the region where aeolian dusts are frequently deposited, translocation of exogenic illite could be a major process of the “illite accumulation”.I believe a more careful discussion of the above-mentioned issues with adding more citations will increase the value of your study.
Additionally, careful definition of “illite” is recommended. If my understanding is correct, illite content was determined by XRD peak intensity of 1.0 nm d-spacing. Although it is not bad, this approach cannot discriminate muscovite or biotite from the “illite”. RIP values are largely different between muscovite, biotite, and illite, and are more variable depending on their weathering stage (Kitayama et al., 2020). Especially, muscovite shows very low RIP. The significant but not very high R2 value in Figure 3 may partly due to the inclusion of muscovite in some soils (e.g. Mount Elgon, Kenya). I understand that it is technically difficult to separate illite from muscovite, so it is good to point out the possibility that some white mica is mixed in.
Result showing in Figure 5 is excellent. Most of the plots are close to 1:1 line, which is probably due to the fractionation without chemical decomposition of SOM. If you add the result with SOM decomposition (just a comment, no need to add in this study), you can show the inhibition effect of SOM coverage on illite for 137Cs adsorption. Although it is an exceptional case, unexpectedly high RIPclay + RIPsilt for Pagsanjan (Philippines) soil is curious. Opening up FES not accessible in bulk soil may be one reason, but it looks too high increments. Further inspection of mineralogical composition for this soil might be interesting.
Kitayama, R., Yanai, J., Nakao, A., 2020. Ability of micaceous minerals to adsorb and desorb caesium ions: Effects of mineral type and degree of weathering. Eur. J. Soil Sci. 71, 641–653.
Mackenzie, K.J.D., Brown, I.W.M., Cardile, C.M., Meinhold, R.H., 1987. The thermal reactions of muscovite studied by high-resolution solid-state 29-Si and 27-AI NMR. J. Mater. Sci. 22, 2645–2654.
Marsh, A.T.M., Brown, A.P., Freeman, H.M., Walkley, B., Pendlowski, H., Bernal, S.A., 2024. Determining aluminium co-ordination of kaolinitic clays before and after calcination with electron energy loss spectroscopy. Appl. Clay Sci. 255, 107402.Citation: https://doi.org/10.5194/egusphere-2024-3585-RC1 -
RC2: 'Comment on egusphere-2024-3585', Anonymous Referee #2, 07 Jan 2025
The fate of radiocaesium in soils is probably one of the most documented subjects of study compared to the behavior of other radioelements. It is pleasant to note that despite these numerous studies, this knowledge can still be improved.
The novelty of the current work is clearly based on three aspects:
- the collection of soil, taken from different continents, which made it possible to cover a wide range of soils characterized by a variable stage of weathering (which was quantified by an index),
- the mineralogical characterization of the clay fraction, rarely carried out in other studies,
- the consideration of the silt fraction and its role in the selective adsorption of radiocaesium (to my knowledge, never estimated before).
These advances represent an obvious added value of this work and effectively complement the ancient study of Vandebroek et al. (JER 2012) which had notably analyzed the correlation between soil texture (and other parameters) and the RIP of soils on a global scale (not cited in this work).
The paper is well structured and written with a rigorous presentation of methods and treatment of results, and an attractive and convincing interpretation. I have just a few questions and have suggested minor changes that could lead to interesting clarifications.
Title : For clarification, I would add "... on a global scale."
L134 : The formulation is confusing: « finer texture »; do you mean a higher clay content, or something else ? What is the relationship with 32 g 100 g-1 which seems to refer to the silt+sand fraction ?
L141 : We have to regret the few number of young soils (N=3) which could have been higher to consolidate the conclusions of the study.
L196-199 : « … in mica. … in pure illite. … in kaolinite » - Of which size ?
L215, Figure 2 : We suppose that reference value for illite and kaolinite are also for a clay size fraction ? Not obvious like it is mentioned in the legend.
L276 : To avoid confusion between Cs and heavy metals, I would write: « … for other trace elements like x or y ... »
L280 : Aging or ageing ?
L296 : « hence, proxies for this will be needed for implementing better models » - That formulation seems a little elusive without more explicit recommandations or suggestions. Why not explore the combination of soil type (as tested by Vandebroek et al;) normalized by geological, climate and topography index for example ? According to your experience, a proposal would be appropriate to guide future research.
L299 : I would precise : « This study reveals that, on a global scale, ... »
Citation: https://doi.org/10.5194/egusphere-2024-3585-RC2
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 95 | 33 | 9 | 137 | 1 | 1 |
- HTML: 95
- PDF: 33
- XML: 9
- Total: 137
- BibTeX: 1
- EndNote: 1
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1