TY - JOUR

T1 - High manganese redox variability and manganate predominance in temperate soil profiles as determined by X-ray absorption spectroscopy

AU - Zahoransky, Teresa

AU - Kaiser, Klaus

AU - Mikutta, Christian

N1 - Funding Information: We are indebted to Leopold Sauheitl, Ulrike Pieper, Rebekka Stünkel, and Anne Herwig who contributed to the determination of basic soil parameters. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515s. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. We are also thankful to PETRA III at DESY for providing us with beamtime. Assistance at synchrotron facilities by Ritimukta Sarangi (SSRL), Qing Ma (APS), and Wolfgang Caliebe, Akhil Tayal, and Vadim Murzin (all DESY) is gratefully acknowledged. This work was financially supported by the DFG (project no. 326242261).

PY - 2022/12/1

Y1 - 2022/12/1

N2 - 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%, x¯ = 54%), followed by Mn3+ (≤80%, x¯ = 32%) and Mn4+ (≤55%, x¯ = 14%). Mineral soil horizons contained significantly less Mn2+ (≤56%, x¯ = 23%), about equal quantities of Mn3+ (≤68%, x¯ = 31%), and were enriched in Mn4+ (≤89%, 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%, 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%, 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%, x¯ = 11%), silicate-bound Mn (≤30%, x¯ = 8%), Mn(III) oxyhydroxides (≤37%, x¯ = 7%), Mn oxides without tunnel- or layer structure (≤18%, x¯ = 5%), and physisorbed Mn (≤14%, 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. 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.

AB - 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%, x¯ = 54%), followed by Mn3+ (≤80%, x¯ = 32%) and Mn4+ (≤55%, x¯ = 14%). Mineral soil horizons contained significantly less Mn2+ (≤56%, x¯ = 23%), about equal quantities of Mn3+ (≤68%, x¯ = 31%), and were enriched in Mn4+ (≤89%, 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%, 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%, 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%, x¯ = 11%), silicate-bound Mn (≤30%, x¯ = 8%), Mn(III) oxyhydroxides (≤37%, x¯ = 7%), Mn oxides without tunnel- or layer structure (≤18%, x¯ = 5%), and physisorbed Mn (≤14%, 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. 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.

KW - Extended X-ray absorption fine structure spectroscopy

KW - Manganese

KW - Soils

KW - Speciation

KW - X-ray absorption near edge structure spectroscopy

UR - http://www.scopus.com/inward/record.url?scp=85140926911&partnerID=8YFLogxK

U2 - 10.1016/j.gca.2022.10.016

DO - 10.1016/j.gca.2022.10.016

M3 - Article

AN - SCOPUS:85140926911

VL - 338

SP - 229

EP - 249

JO - Geochimica et cosmochimica acta

JF - Geochimica et cosmochimica acta

SN - 0016-7037

ER -