Pertechnetate (TcO _4 -) reduction by reactive ferrous iron forms in naturally anoxic, redox transition zone sediments from the Hanford Site, USA

摘要

Technetium is an important environmental contaminant introduced by the processing and disposal of irradiated nuclear fuel and atmospheric nuclear tests. Under oxic conditions technetium is soluble and exists as pertechnatate anion (TcO _4 ~-), while under anoxic conditions Tc is usually insoluble and exists as precipitated Tc(IV). Here we investigated abiotic Tc(VII) reduction in mineralogically heterogeneous, Fe(II)-containing sediments. The sediments were collected from a 55m borehole that sampled a semi-confined aquifer at the Hanford Site, USA that contained a dramatic redox transition zone. One oxic facies (18.0-18.3m) and five anoxic facies (18.3-18.6m, 30.8-31.1m, 39.0-39.3m, 47.2-47.5m and 51.5-51.8m) were selected for this study. Chemical extractions, X-ray diffraction, electron microscopy, and M?ssbauer spectroscopy were applied to characterize the Fe(II) mineral suite that included Fe(II)-phyllosilicates, pyrite, magnetite and siderite. The Fe(II) mineral phase distribution differed between the sediments. Sediment suspensions were adjusted to the same 0.5M HCl extractable Fe(II) concentration (0.6mM) for Tc(VII) reduction experiments. Total aqueous Fe was below the Fe _(aq) detection limit (<2μM). Technetium(VII) reduction occurred in all anoxic sediments at depths greater than 18.3m and reaction time differed significantly between the sediments (8-219d). M?ssbauer analysis of the Tc-reacted, 30.8-31.1m sediment revealed changes in the concentrations of solid-phase Fe(II) and Fe(III). A decrease in the spectral areas of siderite and Fe(II)-containing phyllosilicates illustrated that these phases were oxidized following reaction with Tc(VII). XAS analysis demonstrated that Tc associated with sediments was in the Tc(IV) valence state and immobilized as clusters of a TcO _2·nH _2O-like phase. The speciation of redox product Tc(IV) was not affected by reduction rate or Fe(II) mineralogy.