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

Abstract

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.