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Schuth, Stephan, Hurrass, Julia, Muenker, Carsten and Mansfeldt, Tim (2015). Redox-dependent fractionation of iron isotopes in suspensions of a groundwater-influenced soil. Chem. Geol., 392. S. 74 - 87. AMSTERDAM: ELSEVIER SCIENCE BV. ISSN 1872-6836

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Abstract

Redox conditions control the release of iron (Fe) into soil pore waters. A fluctuating groundwater table in soils results in significant changes in redox conditions with both time and depth. The effects of short-term differences in redox conditions on the stable isotope inventory of dissolved Fe in such soils have not yet been studied. Bulk Fe isotope compositions of a Gleysol yielded delta Fe-57 values from + 0.3 parts per thousand (humic topsoil, Ah horizon) to -0.2 parts per thousand (Fe-enriched subsoil, CrBg horizon). In microcosm experiments, soil suspensions of the Ah and CrBg horizons were subjected to controlled redox conditions ranging from high redox potential (E-H) (>430 mV, pH 5.1 to 5.6), moderate E-H (similar to 330 mV, pH 4.9 to 5.9), to low E-H (<= 170 mV, pH 5.2 to 6.7). Membrane-filtered (0.45 mu m) solutions taken from the suspensions were analysed for their Fe concentrations (Fe2+, and total Fe: Fe-tot) and isotopic compositions. The microcosm experiments demonstrated that the ferrihydrite-and organic-rich Ah horizon is a highly dynamic and rapidly responding reservoir with respect to Fe mobilization and isotopic fractionation at low E-H. Iron concentrations and isotope ratios of the solutions from the Ah horizon varied depending on E-H with negative delta Fe-57 values (-0.4 parts per thousand) and Fe-tot (-1.6 mg L-1) at moderate E-H, and even lower delta Fe-57 values (-1.1 parts per thousand) but high Fe-tot (similar to 7.8 mg L-1) at low E-H. At high E-H, delta Fe-57 values slowly increased from + 0.3 parts per thousand to + 1.0 parts per thousand and Fetot decreased to similar to 0.2 mg L-1 within six weeks. The goethite-rich CrBg horizon constitutes a stable redox-insensitive pool with very low amounts of mobilized Fe and a small degree of isotopic fractionation, even after exposure to low E-H over several weeks. In a natural open system, removal of Fe from the dynamic Ah horizon will result in progressively higher soil delta Fe-57 values due to preferential release of Fe-54. Vertical movement of a low-delta Fe-57 solution from the topsoil may result with time in the formation of a subsoil with delta Fe-57 values that are lower than the topsoil after repeated low and high E-H cycles. At high E-H, Fe mobility in the Ah horizon is much lowered, but release of Fe with high delta Fe-57 values is in agreement with earlier studies for the formation of Fe pools (Fe oxides, colloids, organic complexes) with high delta Fe-57 signatures. (C) 2014 Elsevier B.V. All rights reserved.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Schuth, Stephan UNSPECIFIED UNSPECIFIED UNSPECIFIED Hurrass, Julia UNSPECIFIED UNSPECIFIED UNSPECIFIED Muenker, Carsten UNSPECIFIED UNSPECIFIED UNSPECIFIED Mansfeldt, Tim UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-414497
DOI:	10.1016/j.chemgeo.2014.11.007
Journal or Publication Title:	Chem. Geol.
Volume:	392
Page Range:	S. 74 - 87
Date:	2015
Publisher:	ELSEVIER SCIENCE BV
Place of Publication:	AMSTERDAM
ISSN:	1872-6836
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language SURFACE STRUCTURAL MODEL; AQUEOUS FERROUS IRON; ORGANIC-MATTER; ATOM EXCHANGE; FE; DISSOLUTION; MASS; FE(III); MICROORGANISMS; NANOPARTICLES Multiple languages Geochemistry & Geophysics Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/41449