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Technical Report NTB 99-04

Experimental Studies of Cs, Sr, Ni and Eu Sorption on Na-illite and the Modelling of Cs Sorption

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A natural illite (illite du Puy) was purified and converted to the homo-ionic Na-form. The conditioned Na-illite was characterised in terms of its mineralogy, chemical inventory and physico-chemical properties. The structural formula was determined from energy dispersive spectroscopic analyses (SEM/TEM-EDS) and bulk chemistry measurements. A cation exchange capacity of 127 meq kg-1 was determined by the 22Na isotope dilution method at neutral pH. The Na-CEC was also measured as a function of pH. The stability of Na-illite as a function of pH in the range ~3 to ~6 was investigated. At low pH values partial dissolution of the illite occurs releasing the structural elements Al, Si, Mg, and K into solution. The presence of Ca and Sr in solution was interpreted as being due to desorption from cation exchange sites. All of these elements are also present at neutral pH but at considerably lower levels. Such effects cannot be avoided and must be considered in the interpretation of the sorption measurements.

The main focus of the experimental work presented here is on the sorption behaviour of Cs, Sr, Ni and Eu on conditioned Na-illite as a function of NaClO4 background electrolyte concentration (0.1 and 0.01 M), nuclide concentration and pH in the range ~3 to ~11. Sorption edge data (Rd versus pH) and sorption isotherms (quantity of nuclide sorbed versus equilibrium nuclide concentration) are presented for these four elements. Prior to beginning these experiments, sorption kinetics were measured. The broad based pool of sorption measurements generated from this work will provide the source data sets for subsequent modelling.

So far only the Cs sorption measurements have been modelled. A two site cation exchange model was developed to describe the sorption of Cs over the whole range of experimental conditions. The two site types were termed “frayed edge sites” (FES, high affinity/low capacity) and “type II sites” (low affinity/high capacity). Selectivity coefficient values for NaCs Kc {FES}, NaK Kc {FES}, NaH Kc {FES} and NaCs Kc {II} are given for illite together with the corresponding site capacities.

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