Manganese speciation in soil studied by Mn K-edge X-ray absorption spectroscopy

Manganese (Mn) is one of the most redox-sensitive elements on Earth, participating in a plethora of environmental processes. Its reactivity and mobility largely rely on its specific chemical form, i.e., Mn2+, Mn3+, or Mn4+. In soils, Mn is recognized as a major player controlling oxidative transformation of organic and inorganic constituents. Various Mn minerals are found in soils, but among all, Mn oxides and hydroxides are commonly referred to as key species, whose precipitation and dissolution primarily control the sequestration of (heavy) metal pollutants and nutrients. Despite their ecological relevance, their typically low concentration and poor crystallinity in soils render their analytical accessibility challenging. Thus, studies identifying and quantifying effectively occurring chemical forms of Mn in soils are remarkably rare. To overcome this lack of knowledge, this work provides the first Mn K-edge (6,539 eV) X-ray absorption spectroscopy (XAS) library of soil Mn species and presents the first quantitative species inventory of bulk soils. The first study compiles a database of 32 well characterized (in)organic Mn compounds potentially occurring in soils. Their Mn average oxidation state (AOS) was inferred from Mn K-edge X-ray absorption near edge structure (XANES) and their local (<5 Å) Mn coordination environment form extended X-ray absorption fine structure (EXAFS) spectroscopy. Principal component and cluster analyses of k2-weighted EXAFS spectra of Mn compounds implied that at least five primary Mn species groups can be identified and quantified by EXAFS linear combination fit analysis of environmental samples. The results highlight the potential of Mn K-edge EXAFS spectroscopy to assess bulk Mn speciation in soils and establish the first extensive framework for the analysis and interpretation of Mn XAS spectra of natural samples. The second study explores Mn speciation of 47 soil samples (45.1-2,280 mg/kg Mn) of nine Central European soils by XAS and relates the obtained information to major soil properties. In litter horizons, Mn was mainly present in the form of organically complexed and ‘physisorbed’ Mn, but also minor amounts of manganates, Mn(III) oxyhydroxides, and silicate-bound Mn occurred. In all mineral soil horizons, manganates clearly dominated, but we also highlight the occurrence of feitknechtite (β-MnOOH), groutite (α-MnOOH), and hausmannite (Mn3O4) in acidic soils. The low occurrence of primary silicate-bound and exchangeable Mn phases confirms the early release of Mn from primary silicate minerals and the rapid conversion into manganates, respectively. These results have far-reaching implications for the functioning of soil and biogeochemical element cycles, as manganates play a fundamental role in metal binding, plant nutrition, and redox-related processes in the critical zone

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

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 Mn$^{2+}$, Mn$^{3+}$, and Mn$^{4+}$ 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 Mn$^{2+}$ being most abundant (≤100%, $\bar{x}$  = 54%), followed by Mn$^{3+}$ (≤80%,  $\bar{x}$= 32%) and Mn$^{4+}$ (≤55%, $\bar{x}$ = 14%). Mineral soil horizons contained significantly less Mn$^{2+}$ (≤56%, $\bar{x}$  = 23%), about equal quantities of Mn$^{3+}$ (≤68%,  $\bar{x}$= 31%), and were enriched in Mn$^{4+}$ (≤89%,  $\bar{x}$= 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%, $\bar{x}$ = 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%,  $\bar{x}$= 67%). Correlation analysis showed that manganates dissolve completely during dithionite-citrate and acid ammonium oxalate extractions, and suggested that Mn$^{4+}$-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%,  = 11%), silicate-bound Mn (≤30%,  $\bar{x}$= 8%), Mn(III) oxyhydroxides (≤37%,  = 7%), Mn oxides without tunnel- or layer structure (≤18%,  $\bar{x}$= 5%), and physisorbed Mn (≤14%,  $\bar{x}$<1%). The detection of Mn(III) oxyhydroxides such as feitknechtite ($β$-MnOOH) or groutite ($α$-MnOOH) as well as the spinel hausmannite (Mn$_3$O$_4$) 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

X-ray absorption spectroscopy study of Mn reference compounds for Mn speciation in terrestrial surface environments

X-ray absorption spectroscopy (XAS) offers great potential 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. However, these approaches are complicated by the lack of a comprehensive and accessible spectrum library. Additionally, molecular-level information on Mn coordination in some potentially important Mn species occurring in soils and sediments is missing. Therefore, we 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 was assessed by comparing XANES-derived AOS with the AOS obtained from redox titrations. 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. Our results show that LCF analysis of normalized XANES spectra can reliably quantify the Mn AOS within ~0.1 v.u. in the range +2 to +4. These spectra are diagnostic for most Mn species investigated, but unsuitable to identify and quantify members of the manganate and Mn(III)-oxyhydroxide groups. First-derivative XANES fingerprinting allows the unique identification of pyrolusite, ramsdellite, and potentially lithiophorite within the manganate group. However, XANES spectra of individual Mn compounds can vary significantly depending on chemical composition and/or crystallinity, which limits the accuracy of XANES-based speciation analyses. In contrast, EXAFS spectra provide a much better discriminatory power to identify and quantify Mn species. Principal component and cluster analyses of k$^2$-weighted EXAFS spectra of Mn reference compounds implied that EXAFS LCF analysis of environmental samples can identify and quantify at least the following primary Mn species groups: (1) Phyllo- and tectomanganates with large tunnel sizes (2 × 2 and larger; hollandite sensu stricto, romanèchite, todorokite); (2) tectomanganates with small tunnel sizes (2 × 2 and smaller; cryptomelane, pyrolusite, ramsdellite); (3) Mn(III)-dominated species (nesosilicates, oxyhydroxides, organic compounds, spinels); (4) Mn(II) species (carbonate, phosphate, and phyllosilicate minerals, adsorbed and organic species); and (5) manganosite. All Mn compounds, except for members of the manganate group (excluding pyrolusite) and adsorbed Mn(II) species, exhibit unique EXAFS spectra that would allow their identification and quantification in mixtures. Therefore, our results highlight the potential of Mn K-edge EXAFS spectroscopy to assess bulk Mn speciation in soils and sediments. A complete XAS-based speciation analysis of bulk Mn in environmental samples should preferably include the determination of Mn valences following the “Combo” method of Manceau et al. (2012), EXAFS LCF analyses based on principal component and target transformation results, as well as EXAFS shell-fit analyses for the validation of LCF results. For this purpose, all 32 XAS reference spectra are provided in the Online Materials1 for further use by the scientific community