X-ray absorption spectroscopy study of Mn reference compounds for the identification and quantification of Mn species in soils

Chemical reactivity, mobility, and bioavailability of manganese (Mn) in the environment depend crucially on its speciation. Despite the broad application of X-ray absorption spectroscopy (XAS) to environmental samples, studies covering the identification and quantification of Mn species in soils are surprisingly scarce. In this study, we analyzed 32 organic and inorganic Mn reference compounds by Mn K-edge (6,539 eV) XAS to assess the potential of XAS to differentiate various Mn coordination environments in soils. X-ray absorption near-edge structure (XANES) spectra of reference compounds were evaluated for the oxidation state of Mn using linear combination fit analysis. Results of this analysis were validated by redox titrations. The average local coordination environment (<5 Å) of Mn was analyzed by shell-fitting of extended X-ray absorption fine structure (EXAFS) spectra. Based on spectroscopic data and statistical data analysis, Mn reference compounds were grouped into physically and/or chemically meaningful clusters with diagnostic spectral features. Our results show that XANES spectroscopy can accurately determine the average oxidation state of Mn within 0.12 valence units. EXAFS spectroscopy is capable of differentiating at least three major Mn species groups, which include Mn(III/IV) manganates, Mn(III) oxyhydroxides and organic Mn(III) compounds. Using this information, we elucidated the oxidation state and local coordination environment of Mn in Cambisols, Luvisols, and a Stagnosol (L, O, A, B, and C horizons). Our talk will highlight the capabilities and limitations of XAS in analyzing Mn speciation of bulk soils and provide a guide for scientists exploring the biogeochemical Mn cycle in soil environments.