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Effective Isotopic Fractionation Factors for Solute Removal by Reactive Sediments: A Laboratory Microcosm and Slurry Study

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Laboratory results demonstrate the muting of observed isotopic fractionation when a diffusive transport barrier exists between a measured reservoir and reaction sites in sediments. Wetlands remove many dissolved pollutants from surface waters by various mechanisms. Stable isotope ratio measurements may provide a means of detecting and possibly quantifying certain removal processes, such as reduction of SeO42−, Cr(VI), NO3−, and HClO4−, that fractionate isotopes. However, the magnitude of the isotopic fractionation for a given reaction depends on the setting in which it occurs. We explore the case where isotope ratio shifts in surface waters are used to detect or quantify reactions occurring in pore waters of underlying sediments. A series of SeO42− reduction experiments reveals that the effective isotopic fractionation, observed in the water column as a result of SeO42− diffusion into underlying, Se-reducing sediments, is weaker than the intrinsic fractionation induced by the same reduction reactions in well-mixed systems in which reaction sites are not separated from measured SeO42−. An intact sediment core yielded an effective ε (≈ δreact − δinstantaneous prod) of 0.20‰, whereas the intrinsic ε was 0.61‰. These results are consistent with previously published reactive transport models. Isotopic studies of sediment-hosted reactions in wetlands and other surface water systems should use the smaller effective fractionation values, which can be estimated using the models.

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