Solid phase speciation of radiocaesium in bottom sediments

Abstract

One of the key issues to be resolved if we wish to forecast the behaviour of radiocaesium in freshwater bodies is the identification of the sorption pool, responsible for the radiocaesium sorption and the selectivity factors governing sorption in that pool. Radiocaesium may be adsorbed either in the micaceous Frayed Edge Sites (FES) characterised by a very high caesium selectivity, or the sites in the Regular Exchange Complex (REC), i.e. clays and humic acids, characterised by a relative low caesium selectivity. The partitioning of radiocaesium between FES and REC depends on the relative values of the respective sorption potentials, defined as the product of pool capacity and Cs to K (or NH 4 ) selectivity coefficients. A comprehensive characterisation study was carried out on some 75 sediments (riverine, lacustrine and estuarine), originating from various locations in Europe and covering a wide range in textural properties. Sediments were characterised in terms of cation exchange capacity (CEC), FES capacity, caesium ion selectivity in the FES (with respect to K and NH 4 ) and exchangeable K and NH 4 contents. It is shown that, on average, the FES represents some 4 (±2)% of the CEC and that the ratio of FES capacity to the amounts of exchangeable K and NH 4 in the REC is about unity. This means that the capacities of the two sorption pools are nearly equal in value. Both pools are however characterised by very large differences in radiocaesium ion selectivity. For the FES, the trace Cs/K selectivity coefficient is about 10 3 ; the trace Cs/NH 4 selectivity coefficient is about 2.10 2 . These coefficients are some two orders of magnitude lower in the REC: K c (Cs/K) and K c (Cs/NH 4 ) are about 5 in the clay fraction and about unity in the humic acid pool. Consequently, the radiocaesium sorption potential of the REC represents some 1 to 2% of that of the FES and it can be concluded that the radiocaesium sorption behaviour in sediments is ruled by the properties of the FES. Predictions of in situ short-term K d values can accordingly be based on FES characteristics of the sediment.