Geochemical and U-Th isotopic insights on uranium enrichment in reservoir sediments.

Uranium (U) geochemistry and its isotopic compositions of reservoir sediments in U mine area were poorly understood. Herein, U and Th isotopic compositions were employed to investigate source apportionment and geochemical behavior of U in 41 reservoir sediments from a U mining area, Guangdong, China. The remarkably high contents of both total U (207.3–1117.7 mg/kg) and acid-leachable U (90.3–638.5 mg/kg) in the sediments exhibit a severe U contamination and mobilization-release risk. The U/Th activity ratios (ARs) indicate that all sediments have been contaminated apparently by U as a result of discharge of U containing wastewater, especially uranium mill tailings (UMT) leachate, while the variations of U/Th ARs are dominated by U geochemical behaviors (mainly redox process and adsorption). The U isotopic compositions (δ238U) showed a large variance through the sediment profile, varying from − 0.62 to − 0.04‰. The relation between δ238U and acid-leachable U fraction demonstrates that the U isotopic fractionation in sediments can be controlled by bedrock weathering (natural activity), UMT leachate (anthropogenic activity) and subsequent biogeochemical processes. The findings suggest that U-Th isotopes are a powerful tool to better understand U geochemical processes and enrichment mechanism in sediments that were affected by combined sources and driving forces. [Display omitted] • Extremely U contamination and re-migration risk were observed in reservoir sediments. • U sources and geochemical behaviors were studied using U-Th isotopes. • U enrichment mechanisms (reduction/adsorption) were distinguished by isotopic perspective. • U6+ to U4+ is the dominant mechanism leading to the U enrichment and isotopic fractionation. [ABSTRACT FROM AUTHOR]