Complexing capacity of low-level radioactive waste leachates for 60Co and 109Cd using an ion-exchange technique Original Research Article

Low-level radioactive wastes (LLRW) intended for disposal at Chalk River laboratories (CRL) are composed primarily (>90% by volume) of organic material (paper, used clothing, packaging material, mop heads, etc.). Upon contact with water, microbial degradation of this material will produce dissolved organic matter (DOM) which has the potential to change the speciation and mobility of radionuclides in soils. The determination of the aqueous speciation of nuclides will provide a tool that will enable a better prediction of the behaviour of nuclides and their potential to migrate in soils. This work is one aspect of the characterisation of the DOM produced by the microbial degradation of LLRW. An ion-exchange technique was set up to demonstrate the applicability of the method to determine the complexing capacity of the DOM in a difficult matrix. Leachates from a LLRW degradation experiment were used with two radioisotopes (109Cd and 60Co). The method allows the determination of the free-ion content of a metal in a solution, which also leads to the determination of the complexed fraction. The leachate matrix consisted of ∼0.1 M ionic strength of inorganic salts, with 4177 mg C/l of DOM. The complexing capacity of the DOM in the leachates was 50 μg/l for Co, and 0.54 μg/l for Cd. This represents 0.0028% and 0.000015% of the available DOM sites to Co and Cd, respectively. The conditional stability constant of the DOM with Cd was slightly higher (log β = 1.98) than that of acetic acid, whereas it was ambiguous for Co (log β = 2.02 or 2.54). DOM fouling did not constitute a problem, and the method could be used for other radionuclides and metals.