Reversibility of radiocesium

Kinetics and Reversibility of Radiocesium Sorption on Illite and Sediments Containing Illite... 

The relevance of the kinetics and reversibility of radiocesium sorption is immediately obvious from observations following the introduction of the anthropogenic radioisotopes of cesium in the environment, e.g. by nuclear weapons tests and nuclear... 

In order to study the kinetics and reversibility of radiocesium sorption on natural sediments, the experimental setup described in (7 ) was applied to sediments sampled from the lakes Ketelmeer (4) and Hollands Diep (16) in the Netherlands. The top 2 (Ketelmeer) and 4 cm (Hollands Diep) from sediment cores were dialysed for 2 weeks... 

The kinetics and reversibility of radiocesium sorption on illite and natural sediments have been reviewed and interpreted in terms of a mechanistic framework. This framework is based on the premise that radiocesium is almost exclusively and highly-selectively bound to the frayed particle edges of illitic clay minerals. It is shown that in-situ Ko of radiocesium in sediments are consistent with this ion-exchange process on illite. 

It is important to realise that the above models should not be used to predict Cs-sorption reversibility over time frames of more than a few months. Observations that radiocesium, which has been in contact with contaminated sediments for many years, can still be mobilised by enhanced ammonium concentrations (3,4) give evidence for a backwards reaction. This reaction, which is interpreted as a remobilization of radiocesium from fixed edge-interlayer sites on illite (i.e. box Z in Figure 5), is apparently too slow to be observed in laboratory experiments of many weeks. Therefore, extractions of radiocesium from historically contaminated sediments have been used, and are described below, to obtain a first estimate of this slow remobilization rate. 

Slow (reverse) migration of radiocesium from clay-mineral interlayers into solution. In the kinetic models discussed above, a reverse process of radiocesium remobilization from fixed edge-interlayer sites was not considered. The equilibration times of up to 4-weeks were too short for the reverse process to become apparent. Nevertheless, the fact that radiocesium in sediments is still exchangeable to a certain extent after more than 20 years of contact with sediments (3), indicates that such a reverse process must exist. 

Table II shows the average fraction of exchangeable- Cs in sediments after 3 sequential 24-hr Nlij-extractions and the additional fraction mobilized after the fourth, long-term (400-842 days) extraction. Assuming (1) that all (rapidly) exchangeable radiocesium had been removed by the three prior extractions, and (2) a first order remobilization process, we can roughly calculate the reverse rate constant that describes the slow remobilization of Cs from the sediments. Table II indicates that the half-life of this reaction, which is interpreted as slow release from the edge-...

Reverse (remobilization) rates. Smith Comans (16) have developed a transport model that includes radiocesium sorption kinetics to simulate radiocesium in each of three phases in the sediment profiles of Ketelmeer and Hollands Diep aqueous, exchangeably-bound and slowly reversible ( fixed ). The model gave evidence for a reverse reaction from less-exchangeable to exchangeable sites with a half-life of order 10 years, which is close to the independent estimates by the long-term extractions in Table II. 

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