Behavior of Cs and Co in marine sediments of Enseada de Piraquara de Fora, Angra dos Reis, RJ, Brazil

The release of ¹³⁷Cs and ⁶⁰Co in the liquid effluents from the Angra dos Reis power plants could radiologically impact the Saco de Piraquara de Fora (SPF) inlet. The mobility and trend of the radionuclides in the water-sediment compartment were investigated by studies of thermodynamic and sorption-desorption. The values of distribution coefficients (Kd) were around 500 L. kg^-1 for ⁶⁰Co and 400 L. kg^-1 for ¹³⁷Cs that are lower than the literature values. The predominance of kaolinite in the clay fraction may have played a major role in the low observed Kd values. For both radionuclides, the sorption was higher for the sediment with higher cation exchange capacity, and the equilibrium conditions were reached only after the third day. Despite the relatively low Kd values and low sorption rates, both can considerably be accumulated in sediments, and the sorption reactions are partially irreversible. ⁶⁰Co and ¹³⁷Cs accumulate in orders of magnitude of 1010 and 109 Bq g^-1 sediment, respectively, which shows the ability of the sediments to remove and retain much of the ¹³⁷Cs and ⁶⁰Co released by the power plants. Considering the range of temperature of the SPF water, both sorption and desorption of the two metals increase with temperature. However, in general, the retention rates of the radionuclides in sediments remain constant. The sorption sites are heterogeneous since the sorption data can be adjusted by the Langmuir and Freündlich isotherms. The sorption of Co and Cs are endothermic processes, which are spontaneous at low temperature. (author)