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XAS study of iron speciation in soils and waters from a boreal catchment
Umeå University, Faculty of Science and Technology, Department of Chemistry.
Umeå University, Faculty of Science and Technology, Department of Chemistry.
Umeå University, Faculty of Science and Technology, Department of Chemistry. Centre for Environmental and Climate Research & Department of Biology, Lund University, Lund.
2014 (English)In: Chemical Geology, ISSN 0009-2541, E-ISSN 1872-6836, Vol. 364, 93-102 p.Article in journal (Refereed) Published
Abstract [en]

Iron (Fe) is a key element, strongly influencing the biogeochemistry of soils, sediments and waters, but the knowledge about the variety of Fe species present in these systems is still limited. In this work we have used X-ray absorption spectroscopy (XAS) to study the speciation of Fe in soils and waters from a boreal catchment in northern Sweden. The aim was to better understand the controls of Fe speciation across different, but adjacent landscape elements including soil, soil solution, groundwater and stream water draining catchments with contrasting land characteristics. Our results showed that all samples contained mixtures of Fe(II) and Fe(III). The soils consisted of Fe phyllosilicates, Fe (hydr)oxides and Fe complexed by natural organic matter (NOM). All aqueous samples contained Fe(II)– and Fe(III)–NOM complexes, often in combination with Fe(III) (hydr)oxides that were associated with NOM. The variation in contribution from Fe–NOM and Fe (hydr)oxides was controlled by pH and total concentrations of NOM. The XAS spectra suggested formation of mononuclear Fe–NOM complexes consisting of chelate ring structures, but it could not be determined whether they originated solely from Fe(III)– or from a mixture of Fe(II)/Fe(III)–NOM complexes. Our collective results showed that the Fe speciation was highly variable across the different landscape elements and streams. This variation was manifested both in the distribution between mononuclear Fe–NOM complexes and Fe (hydr)oxides associated with NOM and between Fe(II) and Fe(III). These results highlight the complexity of Fe speciation in natural environmental systems and thus the challenges in interpreting Fe reactivity.

Place, publisher, year, edition, pages
Elsevier, 2014. Vol. 364, 93-102 p.
Keyword [en]
Fe speciation, XAS, Stream water, soil solutions, Ground water, Krycklan catchment
National Category
Chemical Sciences Geochemistry Geophysics
URN: urn:nbn:se:umu:diva-85430DOI: 10.1016/j.chemgeo.2013.11.023OAI: diva2:693456
NIH (National Institute of Health)
Available from: 2014-02-04 Created: 2014-02-04 Last updated: 2014-03-05Bibliographically approved
In thesis
1. Interactions between Fe and organic matter and their impact on As(V) and P(V)
Open this publication in new window or tab >>Interactions between Fe and organic matter and their impact on As(V) and P(V)
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Iron (Fe) speciation is important for many biogeochemical processes. The high abundance and limited solubility of Fe(III) are responsible for the widespread occurrence of Fe(III) minerals in the environment. Co-precipitation and adsorption onto mineral surfaces limits the free concentrations of compounds such as arsenate (As(V)), Fe(III) and, phosphate (P(V)). Mineral dissolution, on the other hand, might lead to elevated concentrations of these compounds. Fe speciation is strongly affected by natural organic matter (NOM), which suppresses hydrolysis of Fe(III) via complexation. It limits the formation of Fe(III) minerals and Fe(III) co-precipitation. This thesis is focused on interactions between Fe(III) and NOM as well as their impact on other elements (i.e. As(V) and P(V)). X-ray absorption spectroscopy (XAS) was used to obtain molecular scale information on Fe and As speciation. This was complemented with infrared spectroscopy, as well as traditional wet-chemical analysis, such as pH and total concentration determinations. Natural stream waters, soil solutions, ground water and soil samples from the Krycklan Catchment, in northern Sweden, were analyzed together with model compounds with different types of NOM. A protocol based on ion exchange resins was developed to concentrate Fe from dilute natural waters prior to XAS measurements. Iron speciation varied between the stream waters and was strongly affected by the surrounding landscape. Stream waters originating from forested or mixed sites contained both Fe(II, III)-NOM complexes and precipitated Fe(III) (hydr)oxides. The distribution between these two pools was influenced by pH, total concentrations and, properties of NOM. In contrast, stream waters from wetland sites and soil solutions from a forested site only contained organically complexed Fe. Furthermore, the soil solutions contained a significant fraction Fe(II)-NOM complexes. The soil samples were dominated by organically complexed Fe and a biotite-like phase. Two pools of Fe were also identified in the ternary systems with As(V) or P(V) mixed with Fe(III) and NOM: all Fe(III) was complexed with NOM at low total concentrations of Fe(III), As(V) and/or P(V). Hence, Fe(III) complexation by NOM reduced Fe(III)-As(V)/P(V) interactions at low Fe(III) concentrations, which led to higher bioavailability. Exceeding the Fe(III)-NOM complex equilibrium resulted in the occurrence of Fe(III)-As(V)/P(V) (co-)-precipitates.

Place, publisher, year, edition, pages
Umeå: Umeå Universitet, 2014. 68 p.
XAS, Fe(III), As(V), P(V), natural organic matter, Fe(II, III)-NOM complexes, Krycklan catchment
National Category
Chemical Sciences Geochemistry
Research subject
Earth Sciences with Specialization Environmental Analysis
urn:nbn:se:umu:diva-86703 (URN)978-91-7601-020-4 (ISBN)
Public defence
2014-03-28, Lilla hörsalen, KB3A9, KBC-huset, Umeå universitet, Umeå, 13:00 (English)
Swedish Research Council, 621-2012-3890
Available from: 2014-03-07 Created: 2014-03-05 Last updated: 2014-04-02Bibliographically approved

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