Cobalt adsorption/desorption on sediments from Piraquara de Fora bay, Angra dos Reis, Brazil

Among the radionuclides released with the liquid effluents from PWR power plants, cobalt isotopes (⁶⁰Co and ⁵⁸Co) play an important role for risk and environmental impact assessments. The laboratory experiments on radionuclide adsorption-desorption, followed by the use of adsorption isotherms are useful tools for prediction of transport, distribution, accumulation, and fate of a contaminant into a specific medium. Adsorption and desorption experiments were carried out in batches, using two sediment samples collected in Piraquara de Fora Bay, near the discharge channel of the liquid effluents from CNAAA. Aiming the assessment of the radioisotope adsorption mechanisms, sediment samples were shaken with solutions containing cobalt chloride which concentrations ranged between 10^-10 and 10^-3 M. In order to estimate the efficiency of adsorption, 100 Bq of ⁶⁰Co were used as radioactive tracer. The experimental data fitted well with Freundlich and Langmuir isotherms. Results point out a favorable and high cobalt adsorption on the sediments, although smaller than the ones reported in literature studies. Such smaller trend of adsorption coefficients may be caused by the low exchange capacity of the prevailing clay, kaolinite. The maximum adsorption capacities were found to be 10 and 17 moles/g for sediments in PT-01 and PT-02, respectively. Values lower than 8% of the adsorbed cobalt underwent desorption and this amount decreased to 4%, with time, which show the high retention capacity of these sediments. (author)