Effect of Isosaccharinic Acid on the Mobility of Nickel in the Near- and Far-Field Repository Conditions

Abstract

This study investigates the effect of isosaccharinic acid (ISA), a product of cellulose degradation, on the transport behavior of radioactive nickel (Ni) in relation to radioactive waste disposal facilities. Sorption and solubility experiments were conducted under far-field (groundwater, GW) and near-field (porewater, PW) conditions. However, diffusion experiments were performed only under far-field conditions. The results indicate a complex interaction between Ni and ISA. Under far-field conditions, Ni sorption decreases as the concentration of ISA increases, probably because ISA forms a soluble complex with Ni and prevents its sorption on granite surface. Conversely, Ni sorption increases under near-field conditions at high pH (>12) when Ni and ISA coexist. This may be because of an increase in its adsorption on cement surface as a Ca–ISA–Ni ternary system. Furthermore, Ni and ISA may be slightly adsorbed or coprecipitated on CaCO3. ISA increases the solubility of Ni by approximately 3.5- and 60-fold under far- and near-field conditions, respectively. ISA exerts a greater impact under near-field conditions probably because of the improved deprotonation rate of ISA in alkaline environment. The diffusion experiment revealed that ISA as a ligand slightly enhances the diffusion of Ni through porous media by hampering its sorption on negatively charged granite surfaces regardless of the overall minimal difference. Collectively, these results underscore the crucial role of ISA in influencing the transport behavior of Ni under near-field and far-field conditions. Based on the experiment, in the disposal facility where ISAs are saturated, Ni radionuclides might adsorb onto cement, posing no issues; however, in far-field environments, Ni's binding with ISAs could enhance its mobility. Consequently, the influence of ISA should be carefully considered when conducting safety assessments and developing management strategies for radioactive waste disposal facilities.