The Effect of pH and Time on the Extractability and Speciation of Uranium(VI) Sorbed to SiO₂

The effect of pH and contact time on uranium extractability from quartz surfaces was investigated using either acidic or carbonate (CARB) extraction solutions, time-delayed spikes of different U isotopes (²³⁸U and ²³³U), and liquid helium temperature time-resolved laser-induced fluorescence spectroscopy (TRLFS). Quartz powders were reacted with ²³⁸U­(VI) bearing solutions equilibrated with atmospheric CO₂ at pH 6, 7, and 8. After 42 days, the suspensions were spiked with ²³³U­(VI) and reacted for an additional 7 days. Sorbed U was then extracted with either dilute nitric acid or CARB. For the CARB, but not the acid, extraction there was a systematic decrease in extraction efficiency for both isotopes from pH 6 to 8, which was mimicked by less desorption of ²³⁸U, after the ²³³U spike, from pH 6 to 8. The efficiency of ²³³U extraction was consistently greater than that of ²³⁸U, indicating a strong temporal component to the strength of U association with the surface that was accentuated with increasing pH. TRLFS revealed a strong correlation between CARB extraction efficiency and sorbed U speciation as a function of pH and time. Collectively, the observations show that aging and pH are critical factors in determining the form and strength of uranium-silica interactions