Ferragels: A New Family of Materials for Remediation of Aqueous Metal ion Solutions
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Fe2+ + 2eFe 3รท + 3e-
E0 = .447 V E0 =.037 V
(1) (2)
The strong thermodynamic force driving elemental iron to iron oxides provides a means of reducing halogenated organics to relatively innocuous hydrocarbons. 8' 9 Two possible mechanisms for this reaction are reductive hydrolysis and dehydrochlorination. 9-" Considerable work is still ongoing regarding the mechanisms and kinetics of the reactions between zero-valent iron and 12 15 chlorinated hydrocarbons.9, Some work has also been done on the use of zero valent iron to remediate waters containing dissolved metals. 16,17 Toxic, environmentally-mobile metal ions, such as CrO42-, can be reduced to stable, insoluble oxides by using the reducing power of zero valent iron. The contaminant metals are immobilized and separated from the aqueous stream by precipitation as the oxide, or in some cases, as the elemental metal. Candidates for remediation by zero valent iron include Cr(VI), Pb(II), Hg(II), and Tc(VII). The reductive half-reactions are:
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Mat. Res. Soc. Symp. Proc. Vol. 556 0 1999 Materials Research Society
Cr(VI) + 3e --+ Cr(III) Pb(II) + 2e -- Pb0 Hg(Hl) + 2e -- Hg0 Tc(VII) + 3e -- Tc(IV)
E0 = E0 = E0 = E0 =
.111 V -.126 V .851 V .782 V
(3) (4) (5) (6)
In the case of technetium, which is present in aqueous solution primarily as the pertechnetate ion (TcO4), the reductive process terminates in the insoluble dioxide. Technetium is a byproduct of nuclear fission and is a pernicious waste problem due to its 2.13 x 105 year halflife.' 8 As the pertechnetate anion, it is both difficult to remove from aqueous solution, and environmentally mobile. Typically, the technetium from military wastes such as that stored at Hanford, WA is vitrified, although other locations may use grout as a final wasteform. Pertechnetate presents additional problems to these wasteforms because it is both mobile in wet environments and volatile at typical vitrification temperatures.19' 20 The insoluble dioxide, TcO 2, has a much lower vapor pressure, and is thus much more easily and safely vitrified. Current separation methods for the removal of Tc from aqueous wastes are focused on the use of anion exchange resins.2 1,' 22 These resins are highly effective at removing Tc(VII) from solutions such as are found at Hanford. However, organic resins foam profusely during the vitrification process, creating an unacceptable, porous glass. Consequently, the separated Tc must be eluted from the exchange resins, often creating a new waste of a greater volume than the original. Also, exchange resins do not usually reduce the valence state of the technetium. Thus, the problem of Tc(VII) volatility still remains. In contrast, the reduction of pertechnetate onto an inorganic support such as ZrO 2 could proceed directly to the vitrification process without further development steps. The inorganic metal oxides do not cause foaming, and the technetium would enter the vitrification process in a nonvolatile 4+ valence state. One important factor in the use of zero-valent iron is the surface are
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