Impact of fracture filling materials on selenium sorption in granite

Abstract

Selenium (Se) sorption behaviors have been widely studied for evaluating radionuclide reactive transport in fractured granite. However, with disparate mineral compositions, the impact of fracture filling materials (FFMs) is often ignored in characterizing radionuclide sorption and migration. This study aims to investigate the contribution of FFMs to the sorption of radionuclide by conducting flow-through experiments with granite, FFMs, and their mixtures (GFMs) separately. The relative contribution of major minerals in these materials is further identified by column experiments with pure minerals. Taking the granite core samples collected from a potentialrepositorysite as an example, the Se(IV) sorption and migration is mainly influenced by biotite and albite in granite samples, and is controlled by smectite and illite in FFMs. The sorption distribution coefficient (Kd) estimated from the individual mineral experiments decreased in the following order: smectite > illite > biotite > K-feldspar > albite > quartz > calcite. When all minerals were mixed, the sorption ability increased with the mass fraction of FFMs. The component additivity model (CAM) was further tested by predicting Kd in different mineral mixtures. The calculated Kd values were lower than those obtained from experimental fitting results. The largest discrepancy occurred when the mass fractions of the granite and FFMs were equal. This implies that CAM has certain limitations in upscaling Kd directly from individual minerals to mineral mixtures, especially when the mass fraction of FFMs is larger than 0.2. The results from this study provide valuable insights for understanding the sorption behaviors of radionuclides in fractured granite.