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Uranium from natural and anthropogenic sources is continuously and increasingly released into the environment, enters the food chain, and eventually accumulates into the bio-systems including humans posing both chemical and radiological hazards. Therefore, exposure of human to uranium becomes more and more a subject of interest in many scientific disciplines such as environmental medicine, toxicology, and radiation protection. Ingestion of uranium via food and drinking water is one of the most important pathways of exposure. The ongoing discussion in Germany and the European Union (EU) on the maximum uranium levels in drinking water reflects the increasing awareness of uranium as a public health problem. Knowledge about uranium chemical binding forms (speciation) can provide detailed information on uranium behavior (mobility, bioavailability, and toxicity) in both geo-systems and bio-systems. In the current work, uranium speciation in natural samples (water, bio-fluids) was elucidated by both spectroscopic means and theoretical modeling using the most recent thermodynamic stability constants for aqueous uranium complexes. For this purpose, environmentally relevant waters (mineral water, groundwater, acid drainage waters) and natural human body fluids (saliva, sweat, urine) have been investigated. As a technical basis, the cryogenic time-resolved laser-induced fluorescence spectroscopy (cryo-TRLFS) was successfully used with a remarkably good capability for direct determination of uranium speciation in such complex matrices of high organic content and low uranium concentrations without any prior treatment. The findings of the present work add new data regarding the uranium(VI) speciation in both natural waters and bio-fluids which offers a reliable basis and a better understanding as a basic mechanism of redistribution of uranium after exposure, and thus for more insight on its biokinetics. The suitability of the cryo-TRLFS as a non-invasive technique for direct determination of uranium speciation in such complex matrices with possibility for further improvement was demonstrated.
