Spatial Distribution and Speciation of Arsenic in Peat Studied with Microfocused X-ray Fluorescence Spectrometry and X-ray Absorption Spectroscopy

Arsenic binding by sulfhydryl groups of natural organic matter (NOM) was recently identified as an important As sequestration pathway in the naturally As-enriched minerotrophic peatland Gala di Lago, Switzerland. Here, we explore the microscale distribution, elemental correlations, and chemical speciation of As in the Go la di Lago peat. Thin sections of undisturbed peat samples from 0-37 cm and 200-249 cm depth were analyzed by synchrotron microfocused X-ray fluorescence (mu-XRF) spectrometry and X-ray absorption spectroscopy (mu-XAS). Additionally, peat samples were studied by bulk As, Fe, and S K-edge XAS. Micro-XRF analyses showed that As in the near-surface peat was mainly concentrated in 10-50 mu m sized hotspots, identified by mu-XAS as realgar (alpha-As4S4). In the deep peat layer samples, however, As was more diffusely distributed and mostly associated with particulate NOM of varying decomposition stages. The NOM-associated As was present as trivalent As bound by sulfhydryl groups. Arsenopyrite (FeAsS) and arsenian pyrite (FeAsxS2-x) of <25 mu m size, which have escaped detection by bulk As and Fe K-edge XAS, were found as minor As species in the peat. Bulk S K-edge XAS revealed that the deep peat layers were significantly enriched in reduced organic S species. Our findings suggest an authigenic formation of realgar and arsenopyrite in strongly reducing microenvironments of the peat and indicate that As(III)-NOM complexes are formed by the passive sorption of As(III) to NOM. This reaction appears to be favored by a combination of abundant reduced organic S and comparatively low As solution concentrations preventing the formation of secondary As-bearing sulfides.