Technical Report NTB 04-02

Experimental and Modelling Investigations on Na-Illite: Acid-Base Behaviour and the Sorption of Strontium, Nickel, Europium and Uranyl

In an extensive study the physico-chemical, protolysis and sorption characteristics of Sr(II), Ni(II), Eu(III) and U(VI) have been measured on illite and modelled over a wide range of pH, sorbate and NaClO4 concentrations.

Samples of Illite du Puy, collected in the region of Le Puy-en-Velay, France, were carefully conditioned to the Na-form and physico-chemically characterised. Potentiometric titrations on suspensions of the Na-illite were carried out using a batch back titration technique in 0.01, 0.1 and 0.5 M NaClO4 background electrolytes from pH~2 to ~12 in an inert atmosphere glove box. The supernatant solutions from each titration experiment in each series were analysed for K, Mg, Ca, Sr, Si, Al, Fe and Mn.

Sorption edges (solid/liquid distribution ratios versus pH at trace sorbate concentrations and constant ionic strength) were determined for Sr, Ni, Eu and U on Na-illite as a function of NaClO4 concentration under anoxic conditions (CO2 ≤ 2 ppm, O2 ≤ 2 ppm.). Sorption isotherms for the same set of radionuclides under similar conditions were measured for Na-illite suspensions in 0.1 M NaClO4 at fixed pH values.

The titration data were modelled in terms of the protolysis of two amphoteric edge sites (=SW1OH and =SW2OH) without an electrostatic term. The protonation/deprotonation constants and site capacities obtained from the titration measurements were then fixed. The sorption edge and isotherm data were modelled with strong (=SSOH) and weak (=SW1OH) surface complexation sites, assumed to have the same protolysis constants, again without electrostatic terms. Uptake by cation exchange was included in all of the calculations. This sorption model, the 2 site protolysis non electrostatic surface complexation and cation exchange model, had been developed previously for montmorillonite and was successful in describing the sorption characteristics of Sr, Ni, Eu and U on Na-illite over a wide range of conditions. Cation exchange capacity, strong and weak site capacities and protolysis constants for Na-illite are given, together with surface complexation constants and selectivity coefficients for Sr, Ni, Eu and U.

At 0.01 M NaClO4 and pH < 8 the sorption of Sr, Ni, Eu and U was dominated by a cation exchange mechanism. The strong dependency of sorption on pH observed under these conditions arose from the competitive effects of Ca and Al on the uptake of the sorbate. Selectivity coefficients for Ca and Al with respect to Na were deduced from these measurements.