Determination of the cation exchange capacity of bentonite exposed to hyperalkaline fluid

The hyper alkaline fluid that could be generated by cementitious degradation in a repository, can affect important properties of backfilling materials, host-rocks and soils. It is well known that mineralogical characteristics of materials can change with the exposition to high pH conditions, such as found in a disposal installation. These modifications are likely to cause deterioration in physical and chemical properties of these materials, e.g., hydraulic conductivity, radionuclides sorption (Kd), diffusivity and cation exchange capacity (CEC). This paper presents the preliminary results for bentonite exposed to alkaline solutions in order to establish the time for mineralogical modifications to occur. The property of bentonite used as an indicator of the modification was CEC. A commercial bentonite was treated with NaOH 1 mol/L at 30 deg C, for different periods of time that varied from 4 hours to 28 days. After that, the CEC was determined by the conventional methylene-blue adsorption titration method. Although the methylene-blue titration has given fast results, it has not demonstrated confidence, due to the semi-quantitative nature of this technique. Generally, the values of CEC decreased from 68 meq/100g to 42 meq/100g with the time of contact showing that hyper alkaline conditions provide significant changes in the CEC. Additionally, a second group of samples was modified with moderate alkaline solution and had the CEC determined by Centrifuge Method, reported in Soil Survey Laboratory Methods Manual, number 42. The second group presented an increase in the CEC in few hours of alteration, which can be explained by the formation of zeolites when an alkaline solution containing K and Ca is used. (author)