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  • Title: Isotopically exchangeable concentrations of elements having multiple oxidation states: the case of Fe(II)/Fe(III) isotope self-exchange in coastal lowland acid sulfate soils.
    Author: Collins RN, Waite TD.
    Journal: Environ Sci Technol; 2009 Jul 15; 43(14):5365-70. PubMed ID: 19708367.
    Abstract:
    Isotope exchange techniques have been used to probe isotopically exchangeable concentrations of Fe (E value) that are in dynamic equilibrium between the aqueous- and solid-phase of coastal lowland acid sulfate soils. Isotope self-exchange between Fe(II) and Fe(III) was rapid and complete in <1 min (p < 0.05) indicating that this reaction was initially occurring solely in the aqueous-phase and the surface of the soil solid-phase. It is further demonstrated that accurate and valid measurements of isotopically exchangeable concentrations of Fe do not require corrections for Fe speciation. This also holds for any element existing in two or more oxidation states which are completely isotopically self-exchangeable in soils. As isotope self-exchange between Fe(II) and Fe(III) is rapid, the distribution coefficient (Kd) and E value determined via this methodology are, therefore, truly representative of Fe regardless of the relative importance of Fe(II) or Fe(III) to the isotopically exchangeable pool of Fe. In the 21 soil samples examined, isotopically exchangeable concentrations of Fe varied from 90 mg/kg to values as high as 3610 mg/kg in acidic, saturated samples collected below the groundwater table from the transition soil horizon. The combination of low Evalues and extremely high Kd values in the upper oxidized layers of these soils indicate that these soil horizons are a relatively insignificant source of transportable (labile) Fe. As such, given our knowledge on the general rates of microbial Fe(III) reduction in, and the hydraulic properties of, the coastal lowland acid sulfate soils of this region, only those soils adjacent to agricultural drains are likely to contribute to the load of Fe entering surrounding aquatic systems.
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