Study of ¹³⁷Cs e ⁶⁰Co sorption in sediments from Saco de Piraquara de Fora, Angra dos Reis and its application for environmental monitoring

Both the suspended solid particles and the bed sediments sorb radionuclides, released in water systems. Sorption is usually represented mathematically by the distribution coefficient that is based in equilibrium between phases. Here the adsorption/desorption kinetics of ⁶⁰Co and ¹³⁷Cs in sea water were simulated by batch equilibrium experiments with sediments in two points (PT - 01, PT - 02) from Saco de Piraquara de Fora inlet (SPF). For both radionuclides, partition coefficient values (Kd) for the sampling point PT - 02 (509 and 385 L/kg for ⁶⁰Co and ¹³⁷Cs, respectively) were higher than the values determined for PT - 01 one (426 and 182 L/kg for ¹³⁷Cs and ⁶⁰Co, respectively). The higher values of Kd of PT-02 reflects the higher CEC (71,4 cmolc.dm³) and content of mud < 63 μm (87%) when compared to PT-01 (CEC of 39,5 cmol_c.dm³) and mud (55%). In comparison with the values reported in the literature, the found Kd values are low, which may be related to the predominance of kaolinite, which is a clay of low sorption capacity. The Kd values with an increase in temperature of 23 deg C to 27 deg C were similar (⁶⁰Co in PT-02 and ¹³⁷Cs in both sediment) or 27 ° C values were higher (⁶⁰Co the PT-01). With increasing temperature to 31 °C Kd values for the two radionuclides showed a decrease. However, increasing temperature increases the desorption of the two radionuclides for both sediments The sorption process is spontaneous and favorable for both sediments and the model of sorption can be fitted by both Freundlich and Langmuir sorption isotherms. The maximum amount of ⁶⁰Co that can be sorbed on sediments were 1,64 10^-5 moles/g (4,12 10¹⁰ Bq/g) and 2,79 10^-5 moles/g (7,03 10¹⁰ Bq/g) to PT-01 and PT-02, respectively, and of ¹³⁷Cs 1,99 10^-6 moles/g (9,70 10⁸ Bq/g) and 6,60 10^-6 mol/g (2,87 10⁹ Bq/g ) to PT-01 and PT-02. Two areas in SPF can potentially accumulate ¹³⁷Cs: sediments located near the effluent discharge and the area located north of this site. An area in the interface of SPF and the bay of Ribeira would potentially the best accumulated of ⁶⁰Co. (author)