Modeling cesium retention onto Na-, K- and Ca-smectite: Effects of ionic strength, exchange and competing cations on the determination of selectivity coefficients

Abstract

Cesium (137Cs) retention onto three homoionic smectites (Na-, K- and Ca-smectite), obtained from natural Spanish FEBEX bentonite, was studied. Special emphasis was given to the analysis of non-linear sorption behaviour and the dependence of selectivity on the ionic strength. A very large set of experimental sorption data was generated from sorption tests under a wide range of pHs (2–11), ionic strengths (10-3 to 1 M), and radionuclide concentrations (10-10 to 10-3 M). The aqueous phase, in contact with the clay, was analysed to quantify the effects of the presence of trace aqueous ions on Cs retention. For all the exchanged clays, Cs sorption was non-linear and a two-site exchange model approach was adopted to interpret and model sorption data. Highly selective sites for Cs sorption (Type 1 sites, T1), resembling those present in micaceous materials, with very low capacity but controlling uptake of Cs at low concentration, were observed. The logarithm value of selectivity coefficients determined for Cs+ in respect to Na+, K+ and Ca2+ in these sites is: Log(SEL(Cs,Na)(T1))=7.59; Log(SEL(Cs,K)(T1))=5.15; Log(SEL(Cs,Ca)(T1))=14.41 respectively. The exchange sites at the surface of smectite sheets (planar sites), with a capacity approximately equivalent to the cation exchange capacity (CEC) of the clay, constitute the second type of sorption sites (Type 2 sites, T2). The logarithm of the selectivity coefficients determined for Cs+ with respect to Na+, K+ and Ca2+ is: Log(SEL(Cs,Na)(T2))=1.68; Log(SEL(Cs,K)(T2))=1.16; Log(SEL(Cs,Ca)(T2))=3.02 , respectively. The analysis of the dependence of sorption values on the ionic strength clearly indicated that for a correct interpretation of data, competition effects of trace ions in solution must be always accounted for. Data obtained in this work and performed analyses are basic to explain the behaviour of raw FEBEX bentonite, and other smectite-based clay materials, under more complex experimental conditions.