Geochemical reactions and dynamics during titration of a contaminated groundwater with high uranium, aluminum, and calcium

This study investigated possible geochemical reactions during titration of a contaminated groundwater with a low pH but high concentrations of aluminum, calcium, magnesium, manganese, and trace contaminant metals/radionuclides such as uranium, technetium, nickel, and cobalt. Both Na-carbonate and hydroxide were used as titrants, and a geochemical equilibrium reaction path model was employed to predict aqueous species and mineral precipitation during titration. Although the model appeared to be adequate to describe the concentration profiles of some metal cations, solution pH, and mineral precipitates, it failed to describe the concentrations of U during titration and its precipitation. Most U (as uranyl, UO ₂²⁺) as well as Tc (as pertechnetate, TcO ₄^-) were found to be sorbed and coprecipitated with amorphous Al and Fe oxyhydroxides at pH below ∼5.5, but slow desorption or dissolution of U and Tc occurred at higher pH values when Na ₂CO ₃ was used as the titrant. In general, the precipitation of major cationic species followed the order of Fe(OH) ₃ and/or FeCo _0.1(OH) _3.2, Al ₄(OH) ₁₀SO ₄, MnCO ₃, CaCO ₃, conversion of Al ₄(OH) ₁₀SO ₄ to Al(OH) _3,am, Mn(OH) ₂, Mg(OH) ₂, MgCO ₃, and Ca(OH) ₂. The formation of mixed or double hydroxide phases of Ni and Co with Al and Fe oxyhydroxides was thought to be responsible for the removal of Ni and Co in solution. Results of this study indicate that, although the hydrolysis and precipitation of a single cation are known, complex reactions such as sorption/desorption, coprecipitation of mixed mineral phases, and their dissolution could occur simultaneously. These processes as well as the kinetic constraints must be considered in the design of the remediation strategies and modeling to better predict the activities of various metal species and solid precipitates during pre- and post-groundwater treatment practices.