Hexavalent uranium is a prominent radioactive contaminant in both sediments and aquifers around nuclear activity sites, and poses a potential health and environmental risk to the biosphere. Understanding on the detailed interaction at the solid-liquid interface between uranium and granite is crucial for the safety assessment of high level radioactive waste geological repository in granitic terrain, as well as prediction of uranium’s geochemical fate in the environment. However, the interaction between uranium and granite is complicated by the ubiquitous ligands in natural media such as phosphate, humic substances et al, thus the detailed insight into U-granite interface is critical in governing the subsurface mobility of uranium in disposal environment. Full understand on complicated interaction mechanism between uranium and ligands at the granite-water interface requires full identification of surface species with the aid of sensitive spectroscopic techniques.
In this work, the adsorption of uranium on granite in both absence and presence of phosphate was investigated by a combination of batch measurements and spectroscopic techniques, including cryogenic time-resolved laser induced fluorescence spectroscopy (TRLFS) and extended x-ray absorption fine structure (EXAFS). Results showed that phosphate is beneficial for uranium immobilization, the spectroscopic confirmation revealed that multiple surface species including inner-sphere complexes and surface precipitates were formed on granite surface with their abundances varying as a function of acidity. The EPMA results showed that uranium mainly located on mica mineral, thus the detailed interaction between uranium and mica was further investigated. The results showed that uranium sorption on phlogopite mica was strongly dependent on pH while minimally affected by the ionic strength, multiple inner-sphere surface species (including ≡SOUO2+, ≡SO(UO2)2(OH)2CO3- and ≡SOUO2(CO3)x1-2x) were formed with their abundance varying as a function of pH, and a portion of uranium precipitated as uranyl oxyhydroxides at pH > 9. The presence of HA made significant difference on uranium sorption behavior as well as surface species. The finding in this work is helpful for understanding on the geochemical fate of uranium in granitic environment as well as setting a reliable reference for surface complex models.