Reduction of pertechnetate in solution by heterogeneous electron transfer from Fe(II)-containing geological material

We have studied the surface-mediated reduction of pertechnetate (TcO4-) in solution by Fe(II)-bearing fracture filling material from a natural fracture in granite, hornblende, and magnetite. The disappearance of technetium from solution was found to follow pseudo first order kinetics, the rate constant being dependent on the specific surface area and Fe(II) content of the solid. Comparison of the rate constants obtained in the experiments with fracture filling material containing chlorite as Fe(II)-bearing mineral, hornblende, and magnetite indicates a strong influence of the binding manner of Fe(II) in the solid phase. Magnetite, with a low band gap (0.1 eV) between the valence and conduction bands was found to be the most efficient reductant, and based on the ionic strength and pH dependence of the rate of TcO4- reduction, it is concluded that sorption of TcO4- on the magnetite surface by a ligand-exchange mechanism is the rate-determining reaction step: > SOH + TcO4- reversible arrow > SOTcO3 + OH-. Oxidative desorption of sorbed/precipitated TcO2(s) into air-saturated groundwater was found to be very slow, most probably due to competing reactions between oxygen and the surface of the Fe(II)-bearing solid.