Strontium dissolution effect on the adsorption experiment with rhyolitic pumice tuff

Determination of adsorption coefficient of strontium (Sr), one of the major elements of low and intermediate level radioactive wastes, is an important step to assess the performance of a waste repository. The adsorption coefficient of strontium on rhyolitic type pumice tuff, already selected as a potential host rock for radioactive waste repositories in Japan, was investigated with batch adsorption experiments. Some parts of the pumice tuff were oxidized and this redox effect was compared with fresh tuff. The distribution coefficient yielded negative values ranging from –0.3 to –9.6 mL/g at low initial concentration of 10–6 and 10–7 mol/dm3, because a higher strontium concentration over the initial one was found in the aqueous phase at equilibrium. Batch dissolution experiment revealed that pumice tuff releases considerable amount of strontium, 8.27 × 10–7 and 4.76 × 10–6 mol/dm3, respectively for fresh and oxidized tuff. Incorporating the dissolved strontium concentration in equilibrium, major changes in distribution coefficient values were found for the oxidized tuff. Considering the dissolution effect is thus necessary, especially for redox zone solid, to properly evaluate the distribution coefficient of easily soluble nuclides when a lower initial concentration is used. To incorporate the dissolution effect, a slight modification of the existing equation was made to determine the distribution coefficient.