Hi Folks,
This excerpt is from a recent paper, Adsorption of phosphate by halloysite (7 Å) nanotubes (HNTs)Nia Gray-Wannell (a1) (a2), Peter J. Holliman (a2), H. Christopher Greenwell (a3), Evelyne Delbos (a1) ... DOI: https://doi.org/10.1180/clm.2020.24Published online by Cambridge University Press: 06 August 2020
The excerpt below says a few interesting things, but also highlights the difference of halloysite from site to site around the world:
This study has investigated the phosphate adsorption capacities of
two tubular halloysites and shown that surface area greatly influences the adsorption capacity. Because surface area is correlated
with halloysite morphology (cylindrical vs polygonal), cylindrical
varieties will generally show greater adsorption than polygonal
prismatic halloysites, presumably due to a greater number of
edge sites for reaction. However, there is some tentative evidence
for an additional, more direct effect of morphology in that the
polygonal forms may have more edge sites per unit surface.
Additionally, a further difference between the two morphologies
in terms of adsorption behaviour was observed in the fitting of
the data to Langmuir isotherms, where the prismatic sample
obtained a good fit whilst the cylindrical sample did not. This
raises the possibility that the two do not adsorb phosphate at
the same adsorption sites and that the lumen may have a great
influence on adsorption in the cylindrical forms.
The adsorption of phosphate onto kaolin-group clay minerals
shows a significant dependency on pH. The maximum adsorption
for both halloysites and kaolinite occurs at pH ∼6, which is
slightly greater than their PZC values and similar to the pH
that would exist in many aquatic and soil systems. This suggests
that the halloysites may be able to act as effective phosphate
traps or sinks in natural systems. Comparison of the normalized
adsorption capacities of halloysites with other clay minerals
such as montmorillonite, illite and kaolinite suggests that halloysites have a greater adsorption capacity with respect to phosphate.
This contrasts with the adsorption of phosphate by Fe-oxides,
which can be seen to be greater than that of clay minerals by a
factor of at least 10.
The relationships between adsorption, morphology and surface area imply that pH-dependent sites must exist on the external
surfaces of the HNTs and that the model of the halloysite tubular
structure, as is often assumed in technological applications with a
uniform external siloxane surface, is too simplistic. Future work
will be aimed at more direct ways of examining the surface of
HNTs and documenting the true structure of the external surface,
which is fundamental to understanding the attempts at and
results of HNT functionalization in emerging technological applications and the role of halloysite in adsorption processes in soils.
As one can surmise, phosphate adsorption by halloysite looks potentially superior with high aspect ration cylindrical as opposed to prismatic, but the jury is still out and more research needs to be done, not least with the a broader range of halloysite samples!
Generally, in my understanding, the 7 Angstrom material comes from Applied Minerals in Utah. The aspect ratio appears inferior to ADN resource, in my layman's opinion, and I would like to stress that I may well be wrong and if so am certainly prepared to admit that and set the record straight.
Imerys in NZ is I believe generally prismatic halloysite.
Other companies are mentioned here solely to give context and highlight the importance of the resource used in the research as I think it is these small differences that carry a significant impact with regard to the final suitability of different halloysite reserves in the world. Please note, that if this turns into a thread that starts wilfully denigrating other companies, I will cease to participate, but I think that pointing out what I believe are important factual differences is ok. It is just that it is a really slippery slope when mentioning other companies and their resources, and unless the moral bar is kept high, one quickly arrives at the bottom rung on the ladder! Personally, I do not want to be part of a low level discussion that revolves around slagging off other companies, and fortunately many others on this thread and board are of a similar mindset.
That being said, it would appear that the high aspect ratio of the tubular ADN halloysite may be well suited to phosphate adsorption compared to other clays. However, the statement that Iron oxides are 10 times more effective than any clay minerals would suggest that there are better products out there, if I have understood the paper correctly.
Most of all, I think the real important take away is that one halloysite is not necessarily like another, and that different halloysite structure will often change the suitability of the resource for one process or another, often quite radically. And that is really important, as we try to move beyond the broad brush myth that halloysite is the best product in the world for everything, and more towards the specific commercial applications that may have legs.
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