Alpha spectrometry isotopic ratios indications in the Paleozoic sedimentary rock of El Gor area, Southwestern Sinai, Egypt: insights on uranium mobility age.

The study area is located between longitude 33° 18' 00" - 33° 21' 00" E and latitude 28° 59' - 29° 01' N and covers approximately 700 km2. Uranium and thorium isotopes were determined by alpha spectrometry. The activity concentrations of 238U, 234U and 235U were ranged between 245.5 ± 8.3-465.2 ± 15.2 Bq.kg-1 with an average 345.5 ± 10.4-452.5 ± 9.3 Bq.kg-1 and 890.5 ± 21.3 Bq.kg-1 with an average 632.3 ± 14.9-11.40 ± 0.5 Bq.kg-1 and 21.50 ± 1.4 Bq.kg-1, respectively. The activity concentration of 232Th, 230Th and 228Th were ranged between 153.1 ± 0.3-318.1 ± 2.9 Bq.kg-1, 149.5 ± 0.7-280.8 ± 2.2 Bq.kg-1 and 36.9 ± 0.1-60.5 ± 0.9 Bq.kg-1. The 230Th/232Th activity ratios in all samples were lower than 20, indicating that these samples have been contaminated by detrital 230Th. Th/U ratio varied between 1.3 and 2.1 with an average 1.8; all values were lower than 3.5, indicated enrichment of uranium. 234U/238U activity ratios that higher than unity indicates that an isotope of uranium has migrated within the rock. From the isotopes of uranium and thorium and their activity ratios, the isochron age for the collected samples was about 58.96 ka.

Uranium isotopic ratio of black shale and its role in detection of oxic-anoxic conditions of uranium depositions.

The black shale is considered one of the most important rock units in the lower part of Um Bogma Formation, where it contains the uranium, heavy metals and rare earth elements mineralization. The black shale samples were analyzed radiochemically by using alpha spectrometry technique. Most of uranium in the studied samples is authigenic and the Th/U ratio confirms the deposition of uranium in reducing environment. The activity ratios of the studied black shale samples were characterized by 234U/238U > 1 and 230Th/234U < 1, which showed relatively recent precipitation of uranium from water in reducing conditions. 234U/235U and 238U/235U activity ratio was relatively deviated from equilibrium due to the changes in the oxidation-reduction conditions. The disequilibrium of 228Th/232Th can be due to the co-precipitation of 228Ra and the migration of 228Th from the black shale into the percolating water. So, the water was percolated through the paleochannels and caves instead of the rocks causing uranium mobilization and the fractionation of uranium, forming the oxidation-reduction interface in the periods from <6 × 104 to >3 × 105 year.

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.

Rapid collection of iron hydroxide for determination of Th isotopes in seawater.

This work introduces a novel method of recovery of iron hydroxide using a DIAION CR-20 chelating resin column to determine Th isotopes in seawater with a sector field (SF) inductively coupled plasma mass spectrometer (ICP-MS). Thorium isotopes in seawater were co-precipitated with iron hydroxide, and this precipitate was sent to chelating resin column. Ferric ions in the iron hydroxide were bonded to functional groups of the chelating resin directly, resulting in a pH increase of the effluent by release of hydroxide ion from the iron hydroxide. The co-precipitated thorium isotopes were quantitatively collected within the column, which indicated that thorium was retained on the iron hydroxide remaining on the chelating column. The chelating column quantitatively collected (232)Th with iron hydroxide in seawater at flow rates of 20-25 mL min(-1). Based on this flow rate, a 5 L sample was processed within 3-4 h. The >20 h aging of iron hydroxide tends to reduce the recovery of (232)Th. The rapid collection method was successfully applied to the determination of (230)Th and (232)Th in open-ocean seawater samples.