Challenges for reliable analysis of uranium in natural waters using laser-induced fluorimetry/LED-fluorimetry in the presence of fluoride and diverse humic substances in hot arid regions and future advances – review

The presence of fluoride and diverse humic substances in arid regions results in changes in the content of uranium, major cations and anions if there is a time interval between sample collection and analysis, and this disrupts the reliable analysis of uranium in natural waters using laser/LED-fluorimetry, and this is discussed in this review. Complete and unequivocal preservation of samples, whether domestic waste water or natural water, is a practical impossibility. The physico-chemical and biological changes continue inevitably after sample collection due to changes in dynamic equilibrium. Thus, the use of a mobile geochemical laboratory for on the spot/quick analysis of water samples, preferably on the same day, is required. Laser-induced fluorimetry/pulsed LED-fluorimetry is a well documented, highly sensitive, and versatile technique for the determination of uranium in water samples at μg L⁻¹ levels. This is made more challenging due to the wide variety of types of water samples, which differ in total dissolved salts found, and these include saline water, diverse humic substances and fluoride content, especially in hot arid regions as well as due to the practical impossibility of preserving natural water samples. Therefore, it is the time interval between sample collection and analysis that is the most critical factor for the reliable analysis of uranium in hot arid regions. A high level of total dissolved solids (TDS) in water samples results in greater variation in the major cations and anions, and uranium content with respect to the time interval between sample collection and analysis. Moreover, there is no availability of certified reference materials for such matrices to cross-check/ensure the reliability of the results.