| Summary: | The variation in geographical distribution of selenium (Se) in environmental settings and the food chain can cause serious human health deficiencies, or poisoning and fatal death. Although Se toxicity is usually inferred as caused by local geology or human activities, the current food trade practices raise Se concerns to a global level, as the Se contained in agricultural products is often unknown. In most natural settings, the prime -. source of Se are shale rocks. Yet, Se weathering pathways and release mechanisms are poorly understood because the locus and the distribution of Se in shales are unknown. Therefore, this thesis assessed the geochemical environment surrounding Se in shales and identified the main inorganic and organic host phases and elucidated the Se speciation and the way this may affect the Se mobilization pathways. This was done by combining simple and complex geochemical, mineralogical and spectroscopic techniques -that were statistically validated to analyse shale samples from the UK, Colombia and China that were representative for typical (1 - 10 ug/g) and extreme (>1 %) Se concentrations. The first important result was the identification of pyrite and organic matter as the two main Se host phases. Additionally, the data showed that in typical shales (e.g. with <6% organic C and 1-2% reduced inorganic S) Se was preferentially associated to pyrite, while in low pyrite shales the association between Se and organic matter was favoured. Interestingly, the data also revealed that depending on the formation pathways pyrite morphology also differentially bound Se with euhedrals concentrating more Se than framboids. Finally, the spectroscopic data showed that Se was not substituting S in pyrite, instead Se was present as an independent FeSex species in close association with both euhedral and framboidal pyrite. Conversely, in the organic matrix, nanosized elemental Se and organo-Se species (Se=C, Se-Se and Se-C bonds) were the main Se carriers.
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