High manganese redox variability and manganate predominance in temperate soil profiles as determined by X-ray absorption spectroscopy

Journal Article · Thu Oct 13 04:00:00 EDT 2022 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/j.gca.2022.10.016· OSTI ID:2423449

Zahoransky, Teresa ^[1]; Kaiser, Klaus ^[2]; Mikutta, Christian ^[1]

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1. University Hannover (Germany)
2. Martin Luther University Halle-Wittenberg (Germany)

Manganese speciation is a key to understanding the fate of contaminants, nutrients, and organic matter in soils. To date, quantification of Mn species in bulk soils has been performed mainly by sequential extraction methods and rarely supported by spectroscopic analysis. In order to obtain quantitative information on the Mn species inventory of soils, we investigated 46 soil horizons (<2-mm fraction, 45.1–2,280 mg/kg Mn) of nine typical Central European soils (Cambisols, Chernozems, Luvisols, Podzol, Stagnosol) by chemical Mn analyses and Mn K-edge X-ray absorption spectroscopy, and related speciation results to major soil properties. Amounts of Mn²⁺, Mn³⁺, and Mn⁴⁺ and the average oxidation state of Mn were evaluated by linear combination fitting (LCF) of X-ray absorption near edge structure (XANES) spectra. Additionally, we used extended X-ray absorption fine structure (EXAFS) spectroscopy to identify and quantify major Mn species. For this, EXAFS spectra of 20 organic and mineral soil samples from five soils (Cambisols, Chernozem, Luvisol) were analyzed by LCF and shell fitting. XANES analyses revealed a high Mn redox variability in organic surface layers, with Mn²⁺ being most abundant (≤100%, $$\overline{x}$$ = 54%), followed by Mn³⁺ (≤80%, $$\overline{x}$$ = 32%) and Mn⁴⁺ (≤55%, $$\overline{x}$$ = 14%). Mineral soil horizons contained significantly less Mn²⁺ (≤56%, $$\overline{x}$$ = 23%), about equal quantities of Mn³⁺ (≤68%, $$\overline{x}$$ = 31%), and were enriched in Mn⁴⁺ (≤89%, $$\overline{x}$$ = 46%). EXAFS analyses implied the presence of six major Mn species groups: manganates, organically complexed Mn, Mn(III) oxyhydroxides, silicate-bound Mn, Mn oxides without tunnel- or layer structure, and physisorbed Mn. In litter horizons, Mn was mainly present in organic complexes (58–91%, $$\overline{x}$$ = 78%) and as physisorbed Mn (≤15%), but individual horizons also comprised manganates, Mn(III) oxyhydroxides, and silicate-bound Mn. Manganates, likely mixtures of phyllomanganates with hexagonal layer symmetry and tectomanganates, dominated in all mineral soil horizons (37–94%, $$\overline{x}$$ = 67%). Correlation analysis showed that manganates dissolve completely during dithionite-citrate and acid ammonium oxalate extractions, and suggested that Mn⁴⁺-rich manganates preferentially form under less acidic soil conditions, partly by oxidation of organically complexed Mn(II), and that they are enriched in the soil clay fraction. Mineral soil horizons also contained minor quantities of organically complexed Mn (≤39%, $$\overline{x}$$ = 11%), silicate-bound Mn (≤30%, $$\overline{x}$$ = 8%), Mn(III) oxyhydroxides (≤37%, $$\overline{x}$$ = 7%), Mn oxides without tunnel- or layer structure (≤18%, $$\overline{x}$$ = 5%), and physisorbed Mn (≤14%, $$\overline{x}$$ <1%). The detection of Mn(III) oxyhydroxides such as feitknechtite (β-MnOOH) or groutite (α-MnOOH) as well as the spinel hausmannite (Mn₃O₄) in acidic soils is remarkable, since their formation is normally linked to neutral or alkaline pH conditions. Minor contributions of silicate-bound Mn indicate the release of Mn from primary minerals already at early stages of soil formation, and low concentrations of physisorbed Mn suggest that exchangeable Mn is rapidly converted into manganates in oxic soils. In conclusion, the predominance of manganates in mineral soils has far-reaching implications for the functioning of soils and biogeochemical element cycles, as these minerals play an important role in metal binding, plant nutrition, and redox-related processes.

• Research Organization: Argonne National Laboratory (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF)
• Grant/Contract Number: AC02-06CH11357; AC02-76SF00515
• OSTI ID: 2423449
• Journal Information: Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: C Vol. 338; ISSN 0016-7037
• Publisher: Elsevier; The Geochemical Society; The Meteoritical Society
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
58 GEOSCIENCES
Extended X-ray absorption fine structure spectroscopy
Geochemistry
Geophysics
Manganese
Soils
Speciation
X-ray absorption near edge structure spectroscopy