Uranium is a naturally occurring radioactive chemical element that has mainly found use in nuclear reactors and in weapons manufacture. As a natural element, it is found in mineral deposits and in ground water, but as a result of economic activities, such as: mining, enrichment, nuclear waste disposal and industrial agriculture, elevated levels of uranium can be present in surface, ground or sub-surface water. Its presence in drinking water can pose a threat to human health even at low concentrations. Specifically, uranium dioxide – chemically referred to as uranyl, is the most common bioavailable form of uranium in water. This compound usually has a unique visible green emission that has been known for over 150 years. Unfortunately, it is quite difficult to detect uranium in water, especially in the field. In particular, at low concentrations in an aqueous environment, the green fluorescence of the uranyl can be quenched and attenuated making it undetectable. Therefore, it is imperative that a novel field measurement technique be devised that will facilitate rapid detection of uranium in water.
Recently, Virginia Commonwealth University scientists: Professor Gary C. Tepper and Brandon M. Dodd (PhD student in Professor Tepper’s lab) used nanoporous silica gel to collect, concentrate and enhance the fluorescence intensity and increase the fluorescence lifetime of uranyl in water samples. They anticipated that their endeavour would facilitate and enhance detection of uranyl in water in the field. Their work is currently published in the research journals, Materials and Design, Journal of Radioanalytical and Nuclear Chemistry and Journal of Fluorescence.
In brief, the research method employed commenced with sample preparation where the researchers acquired silica gel and aqueous uranyl nitrate in pure deionized water for use in their experiments. Next, they conducted a full spectral measurement where, in particular, baseline fluorescence emission measurements were performed. Finally, they engaged in fluorescence lifetime measurements where the QuantaMaster-3 Spectrofluorometer was used.
The authors observed that for uranyl adsorbed within pores larger than 4 nm, the lifetime was relatively independent of pore size, whereas below 4 nm, the lifetime increased with decreasing pore size. In addition, a blue shift in the emission spectra was observed at the smallest pore size (2.2 nm) and is believed to have been caused by quantum confinement. The researchers also found out that the lifetime was longer at a neutral pH than in an acidic pH, an observation they attributed to the formation of a uranyl hydroxyl complex at higher pH values.
In summary, the study presented a novel technique for detecting uranium in water, even at very low concentrations. Their work provided insights into the effect pore size has on the fluorescent spectrum; such as the blue shift and the additional peaks observed. Altogether, the results by Virginia Commonwealth University researchers have potential to be used in the development of more sensitive methods to collect, accumulate, and detect uranium from water samples.
Brandon M. Dodd, Gary C. Tepper. Uranyl fluorescence lifetime in nanoporous silica gel: The influence of pore size, pH, and water. Materials and Design, volume 151 (2018) page 48–52.Go To Materials and Design
Brandon Dodd, Gary Tepper, The Effect of Cations on Uranyl Transport and Fluorescence in Mesoporous Silica Gel, Journal of Fluorescence, 30 September 2018, ISSN 1573-4994.Go To Journal of Fluorescence
Brandon Dodd, Michael Cartwright, Braden Goddard, Gary Tepper, Investigation of uranium(VI) sorption in mesoporous silica gel using gamma spectroscopy, Journal of Radioanalytical and Nuclear Chemistry, Volume 318, 17 August 2018, Pages 1077-1083, ISSN 1588-2780.Go To Journal of Radioanalytical and Nuclear Chemistry
Brandon Dodd, Gary Tepper, Uranyl Adsorption kinetics within silica gel: dependence on flow velocity and concentration, SPIE Conference Paper
Brandon Dodd, Hooman Tafreshi, Gary Tepper, Flow-enhanced kinetics of uranyl (UO2) transport into nano-porous silica gel, Materials & Design, Volume 106, 15 September 2016, Pages 330-335, ISSN 0264-1275.Go To Materials & Design