Uranium sequestration in fracture filling materials from fractured granite aquifers.

The migration of the uranium (U) in high-level radioactive waste that is held in deep geological repositories via fractures in deep granite aquifers is a serious safety concern, thus, this study investigates the effect of fracture filling materials designed to mitigate these concerns. Geochemical analysis was conducted on granite rock core and groundwater samples collected from boreholes located in granite areas. Sequential extraction tests on fracture filling material (FFM) samples were also conducted. The rock core samples were classified as two-mica granite that had uranium (U) content ranging from 1900 to 22,100 µg/kg with an arithmetic mean of 8500 µg/kg. The total U concentration in the FFM samples was found to range from 790 to 80,781 µg/kg. The U in the FFM samples was mainly associated with a carbonate phase that made up from 29.9 to 100% of the total U in the FFM. The U fraction of carbonate phase was closely correlated with the Ca fraction. U associated with crystalline inorganic FFM constituents (e.g, clay minerals and metal oxyhydroxides) was also found in FFM samples in fractions ranging from 21.1 to 70.1%. U in FFM is mainly incorporated via Ca-carbonate, which might have not been formed in modern groundwater, but the time and temperature during formation are unknown. In addition, the Fe, Si, Al, Ca, K, and U levels were found to be well correlated with each other, suggesting that U can also become geochemically associated with crystalline clay minerals or Fe-oxyhydroxides.

Chlorite alteration in aqueous solutions and uranium removal by altered chlorite.

Chlorite alteration and the U removal capacity of altered chlorite were investigated. Batch kinetic dissolution tests using clinochlore CCa-2 were conducted for 60days in aqueous solutions of various pHs and ionic strengths. Batch sorption tests using these altered chlorite samples were conducted for 48h with natural groundwater containing 3.06×10-6M U. Chlorite dissolution was influenced more by pHo than by the ionic strength of the solution. TEM analysis revealed Fe(oxy)hydroxide aggregates in the solid residue from the batch dissolution test with 0.1M NaClO4 solution at pHo=10. The U removal capacity of the reacted chlorite samples at pHo=6-10 was higher than that of the reacted chlorite samples at pHo=3. The degree of dissolution of chlorite samples reacted at pHo=3-8 was inversely proportional to the U removal capacity, but that of chlorite samples reacted at pHo=10 was proportional to the U removal capacity. The positive correlation between the U removal capacity and degree of chlorite dissolution at pHo=10 might be due to the formation of Fe-containing secondary minerals and changes in the reactive sites.

Effect of Cr(VI) concentration on gas and particle production during iron oxidation in aqueous solutions containing Cl- ions.

Zero-valent iron (ZVI) is commonly used as a medium in permeable reactive barriers (PRBs) because of its high reducing ability. The generation of H2 gas in PRBs, however, can decrease the permeability of PRBs and reduce the contact area between the PRB and contaminated groundwater. This study investigated the effect of the initial Cr(VI) concentration ([Cr(VI)init]) in aqueous solutions containing Cl- ions on the generation of H2 gas. ZVI chips were reacted in reactors with 0.5-M NaCl solutions with [Cr(VI)init] ranging between 51 and 303 mg/L. The initial pH was set at 3. The oxidation of ZVI chips by Cr(VI) in aqueous solutions containing Cl- ions produced H2 gas and particles (Fe(III)-Cr(III)(oxy)hydroxides). The Cr(VI) removal from aqueous solutions increased as the [Cr(VI)init] increased, as did H2 gas generation. The positive effect of [Cr(VI)init] on H2 gas generation might be due to an increase in the redox potential gradient as [Cr(VI)init] increases. This increased gradient would enhance H+ ion penetration through the passive film (Fe(III)-Cr(III)(oxy)hydroxides), which formed on the ZVI surface, by diffusion from the solution to pits beneath the passive film.

Adsorption characteristics of U(VI) on Fe(III)Cr(III) (oxy)hydroxides synthesized at different temperatures.

In this study, the adsorption behavior of U(VI) on (oxy)hydroxides synthesized at different temperatures (25 and 75 °C) was investigated. Four (oxy)hydroxides were synthesized by drying slurries of Fe(III) and Fe(III)Cr(III) (oxy)hydroxide in a vacuum desiccator (25 °C) or in an oven (75 °C). Batch adsorption tests were conducted using the (oxy)hydroxides thus synthesized and groundwater containing uranium ions. In general, the U(VI) removal fraction significantly increased with increasing pH from 3 to 5, remained constant with increasing pH from 5 to 9, and decreased at pH greater than 9, regardless of the type of (oxy)hydroxides and solid-to-liquid ratio. The effect of pH on the U(VI) removal fraction was more significant at a low solid-to-liquid ratio. The oven-dried Fe(III) (oxy)hydroxide exhibited a U(VI) removal fraction lower than that of the vacuum-dried one, whereas the oven-dried Fe(III)Cr(III) (oxy)hydroxide exhibited a U(VI) removal fraction higher than that exhibited by the vacuum-dried one. X-ray photoelectron spectroscopy (XPS) analysis results indicated that the difference in the U(VI) removal fraction is attributed to the dissolution and precipitation of the Fe(III) (oxy)hydroxide during oven drying and dehydration of the Fe(III)Cr(III) (oxy)hydroxide during oven drying.