Soil Manganese: Speciation, Transformation, and Reactivity
Final Report Abstract
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 evaluated Mn K-edge (6,539 eV) X-ray absorption spectroscopy (XAS) to identify and quantify Mn species in surface environments by means of linear combination fit (LCF), fingerprint, and shell-fit analyses of bulk Mn XAS spectra. Therefore, we first investigated a suite of 32 natural and synthetic Mn reference compounds, including Mn oxide, oxyhydroxide, carbonate, phosphate, and silicate minerals, as well as organic and adsorbed Mn species, by Mn K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy. The ability of XAS to infer the average oxidation state (AOS) of Mn in environmental samples was assessed by comparing XANES-derived AOS with the AOS obtained from redox titrations of Mn reference compounds. All reference compounds were studied for their local (<5 Å) Mn coordination environment using EXAFS shell-fit analysis. Statistical analyses were employed to clarify how well and to what extent individual Mn species (groups) can be distinguished by XAS based on spectral uniqueness. Based on these results, we investigated 47 soil horizons (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 XAS, and related the obtained speciation information to major soil properties. Amounts of Mn2+, Mn3+, and Mn4+, as well as the average oxidation state of Mn, were evaluated by LCF analysis of XANES spectra. Additionally, we used 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: (1) manganates, (2) organically complexed Mn, (3) Mn(III) oxyhydroxides, (4) silicate-bound Mn, (5) Mn oxides without tunnel- or layer structure, and (6) physisorbed Mn. In litter horizons (Oi), 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 and 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) and/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 confirm the release of Mn from primary minerals already at early stages of soil formation, and low concentrations of physisorbed Mn demonstrate 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 soil and biogeochemical element cycles, as these minerals play an important role in metal binding, plant nutrition, and redox-related processes.
