Surface complexation modeling of nickel sorption under varying geochemical conditions at the Wolsung repository site

Low-and-intermediate-level (LILW) radioactive wastes generated from NPPs will be disposed in the Wolsong repository site in Gyeongju, which has been built as underground concrete silo type and operated since 2015. For a long-term safety assessment of the repository, reactive transport models utilize the distribution coefficient (constant-K_d) modeling approach to describe the mobility of radionuclides in the subsurface environments. However, Kd values are very sensitive to the varying environmental conditions, specially by different pH conditions due to the reaction between groundwater (pH: ⁓8) and cement (pH: ⁓12). Wolsong repository utilizes lots of cementitious materials by engineered barrier system, backfill, and solidified materials. After closure of repository, these cementitious materials will interact with groundwater, then will buffer the pH to hyperalkaline conditions (pH 10.5-13.1). In contrast to the constant-Kd approach, surface complexation models (SCM) can describe changes in nickel's sorption onto minerals as environmental conditions vary. A lot of literatures on SCM indicated that the SCM approach has been successfully applied to predict sorption of contaminants (metal and radionuclide ions) on soils and sediments. Therefore, in this study, nickel sorption onto minerals was studied in batch experiments as a function of pHs and different initial nickel concentrations.