Cobalt-60 behaviour in aquatic systems: Experimental determination of sorption-desorption processes of 60Co on the Meuse river sediments

When introduced into aquatic systems, cobalt-60 binds to suspended particles and sediments. The distribution coefficient is an empirical value that indicates the ability of a pollutant (e.g., radionuclides) to adsorb and desorb sediments. The adsorption process largely depends on both the type of pollutant and the mineralogy of the sediment; as a result, the time lapsed before equilibrium between the liquid and solid phases is attained is also variable in the case of cobalt. The reverse process (desorption) is very slow. A good knowledge of both processes helps to understand how cobalt-60 behaves in an aquatic system after an accidental spill or disaster. Here, the adsorption and desorption behaviour of cobalt-60 in freshwater environments was simulated experimentally by using nine different sediments taken along the Meuse river and the Albert canal in the province of Liège. These locations are of interest due to the presence of the nuclear installations in Tihange (Belgium) and Chooz (France). The experimental set-up consisted of a mixture of water, sediment, and cobalt-60 from which aliquots were sampled at different time intervals over a period of 48 hours. This allowed us to keep track of the evolution of the distribution coefficient and determine when equilibrium between phases was reached. The study found minor variations in Kd values at the equilibrium point, with values that range between 249,5 ml/g being the lowest value and 322,4 ml/g being the highest value across the complete set of Meuse sediment samples. The results show that location and flow rate are influencing factors. The experiments carried out on sediments collected at Maizeret and Vezin displayed higher values for the Kd while the Albert canal sediments showed stronger cobalt-60 bonds, possibly due to differences in sediment characteristics. An one-way ANOVA test showed that there are no significant differences between the obtained Kd values. Moreover results of the desorption experiments performed on the Albert canal sediments showed stronger cobalt-60 bonds. This behaviour can be largely explained by the differences in sediment characteristics (e.g., mineralogy of the sediment) and competition with other pollutants present in the water column. It was observed that the change in cobalt-60 concentration with time followed an exponential curve for all sediments. The desorption experiment also revealed a correlation between Kd values and cobalt-60 desorption percentages. It is also worthy to mention that the percentage of desorption from the sediment taken at Frappo was observed to be higher despite having a lower initial activity per gram of sediment. A one-way ANOVA test confirms that there are significant differences between the percentages obtained. Our findings suggest that other factors like pH and solid-phase chemistry could significantly influence both the adsorption and desorption kinetics of cobalt-60, highlighting the need for further research.