Decontamination of TCE- and U-Rich Waters by Granular Iron: Role of Sorbed Fe(II)

Abstract
Uranium (UO22+) and chlorinated aliphatics [tetrachloroethane (PCE) and trichloroethane (TCE)] can be reduced and thus immobilized or degraded (i.e., dehalogenated), respectively, by the same abiotic mechanism. In this mechanism the reduction reaction is coupled to the oxidation of Fe(II) sorbed on iron corrosion products such as hematite. This is indicated by the equilibrium Eh values measured during uranium immobilization and PCE degradation reactions of zerovalent iron. These values fit closely with those measured in the Fe(II)-αFe2O3-H2O system (in the absence of U or PCE), not those of the Fe(o)/Fe(II) or H2(g)/H2O couples. Because iron (II) is very unstable in environments that are not strictly anaerobic, Fe(o) serves as a source of Fe(II). The reduction kinetic rate, analyzed in detail for the reduction of U(VI), is found to be a function of the concentration of OH−, Fe2+ and reactive surface sites, and is given in terms of sorbed species concentrations by {d[U(VI)]ads}/dt={−k′[≡FeOFeOH0][U(VI)]ads...