Ionic strength and pH dependent multi-site sorption of Cs onto a micaceous aquifer sediment

Caesium-137 (t1/2 = 30 years) is a common contaminant at nuclear legacy sites. Often the mobility of 137Cs in the environment is governed by its sorption to charged sites within the sediment. To this end it is important to understand the sorption behaviour of caesium across a wide range of environmental conditions. This work investigates the effect of varying solution composition (pH and competing ions) on the sorption of caesium to micaceous aquifer sediment across a large concentration range (1.0 × 10−11 – 1.0 × 10−1 mol L−1 Cs+). Experimental results show that Cs+ exhibits three distinct sorption behaviours at three different concentration ranges. At very low concentrations < 1.0 × 10−6 mol L−1 Cs+ sorption was unaffected by competition with Na+ or H+ but significantly reduced in high ionic strength K+ solution. Secondly between 1 × 10−6 and 1.0 × 10−3 mol L−1 Cs+ is strongly sorbed in a neutral pH, low ionic strength background but sorption is significantly reduced in solutions with either a high concentration of Na+ or K+ ions or low pH. At high concentrations > 1.0 × 10−3 mol L−1 Cs+ sorption is reduced in all systems due to saturation of the sediment’s sorption capacity. A multi-site cation exchange model was used to interpret the sorption behaviour. From this it was determined that at low concentrations Cs+ sorbs to the illite frayed edge sites only in competition with K+ ions. However, once the frayed edge sites are saturated the Cs+ sorbs to the Type II and Planar sites in competition with K+, Na+ and H+ ions. Therefore sorption of Cs+ at concentrations > 1.0 × 10−6 mol L−1 is significantly reduced in both high ionic strength and low pH solutions. This is a significant result with regard to predicting the migration of 137Cs+ in acidic or high ionic strength groundwaters.