Uranium precipitation in a permeable reactive barrier by progressive irreversible dissolution of zerovalent iron

A permeable reactive barrier (PRB) containing zerovalent iron-[Fe(0)] was i nstalled at a former uranium milling site in Monticello, UT. A large-scale column experiment was conducted at the site to test the feasibility of Fe(0 ) to treat U prior to installing the PRB. Effluents from the field column e xperiment had pH values near 7.34, moderate decreases in C(IV) and Ca conce ntrations, and an elevated Fe concentration (27.1 mg/L). In contrast, groun dwater exiting the PRE had a pH value of 9.82, decreases in C(IV) and Ca co ncentrations, and a low concentration of Fe (0.17 mg/L). A geochemical mode was used to explain the chemical changes that occurred in both the field c olumn experiment and the PRB. The model simulated the systems by the progre ssive irreversible dissolution of Fe(0). Modeling results indicated that a longer residence time in the PRB compared with the shorter residence time, in the column contributed to the disparate effluent qualities. Prior to mod eling, a controlled laboratory column experiment was conducted to help eval uate the,dominant chemical mechanisms by which Fe(0) removes U-from aqueous solutions. Results of the laboratory column experiment indicated that only a small amount of U could be adsorbed to-ferric minerals, and, therefore, this mechanism was not considered in the model.