From Roger Watters
I disagree with Brenda Howard about how particles of caesium, or any other
cation for that matter, are trapped by clay minerals
(This Week, 21 November 1998, p 12).
And, indeed, whether or not clay minerals play a significant part at all.
In 1979 I was joint author of a paper arguing that anomalous quantities of
cations cannot be held by clay minerals for the simple reason that there are a
limited number of exchange sites, all of which are competed for fiercely by
macro-elements, such as potassium (as Howard rightly points out), as well as
magnesium, iron and the like.
Larger ions will displace smaller ones, highly charged ones will displace the
lesser charged. One cannot successively plate more ions onto a site—one
site, one ion, that is the rule, depending on relative charges.
I also suggested that coprecipitation with iron and manganese was the
mechanism by which one gets anomalous concentrations in stream sediments and
soils, not adsorption on or in clay minerals. Much of the subsequent 20 years of
research supports this conclusion.
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Darwin, Australia
