Research Leads to New Method for Rapid Detection of Uranium in Water

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.