The mechanisms of uranium removal from water by iron oxyhydroxides, nanocrystalline titanium dioxide, and elemental iron

Abstract

The removal mechanisms and efficiencies of uranium from water by iron oxyhydroxides, nanocrystalline titanium dioxide, and elemental iron were investigated in artificial and natural systems. Batch tests, zeta potential measurements, and Fourier Transform Infrared (FTIR) spectroscopic studies were utilized to characterize the nature of hexavalent uranium [U(VI)] adsorption on ferrihydrite and nanocrystalline titanium dioxide. Adsorption isotherms demonstrated that carbonate had a negative effect on U(VI) adsorption on ferrihydrite at pH > 6. Zeta potential measurements indicated that U(VI) was adsorbed on ferrihydrite as a cationic species (SO-UO2+) in the absence of carbonate and as anionic U(VI) complexes in the presence of carbonate at neutral pH. FTIR spectroscopic measurement of adsorbed U(VI) on ferrihydrite suggested that it was retained as uranyl carbonate complexes in the presence of carbonate. An increase in carbonate concentration caused a shift in the antisymmetric stretching vibration of the uranyl (UO2 2+) U-O bond toward lower wavenumbers, which indicated an increasing carbonate effect in the adsorbed uranyl carbonate complexes. Adsorption isotherms indicated that carbonate also had negative effect on U(VI) adsorption on titanium dioxide at pH > 6.0. Zeta potential and FTIR studies indicated that U(VI) was adsorbed on titanium dioxide as a cationic species in the presence and absence of carbonate. An increase in carbonate concentration caused a shift in the chemical equilibrium in favor of aqueous uranyl carbonate complexes. The adsorbed U(VI) species were successfully incorporated into a surface complexation model to describe the adsorption of U(VI) by ferrihydrite and titanium dioxide from artificial solutions and contaminated water. Batch tests indicated that U(VI) was reduced to tetravalent uranium [U(IV)] by elemental iron under anaerobic conditions. Evidence suggested that U(VI) reduction by elemental iron was catalyzed by the iron surface. Even though the reaction was thermodynamically feasible, U(VI) was not reduced by ferrous iron in a homogenous solution under anaerobic conditions at pH > 7.0. In the presence of dissolved oxygen, uranium was removed from water by ferrous and elemental iron through U(VI) adsorption onto iron hydroxides.

A better description of the adsorbed uranium surface species was achieved through the use of batch tests, Zeta potential and FTIR spectroscopic experiments. These results were used to predict the mobility of uranium in natural environments and the removal efficiencies of uranium by iron oxyhydroxides, nanocrystalline titanium dioxide, and elemental iron. The knowledge gained was successfully used to remediate depleted uranium contaminated water at a Department of Defense site.