Citric and malic acids influence uranium(VI) uptake into Brassica napus in hydroponic culture by affecting solubility and speciation

The migration of uranium (U) in soil and its uptake into plants is known to be affected by many factors, one of which is the presence of organic acids, e.g. as root exudates of plants, in soil. To date, the influence of the organic acids on mobilization and uptake is known but very little has been elucidated about the mechanisms involved. In this study, using hydroponic cultivations of Brassica napus and combining the analytical methods time-resolved laser-induced fluorescence spectroscopy and transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy, we explored the influence of two organic acids on the U speciation in hydroponic culture medium and the bioassociation of U to B. napus plants. Both citric acid and malic acid significantly increased the solubility of U in the hydroponic solution by forming U(VI) citrate and U(VI) malate complexes compared to control samples without the addition of the organic acids, in which a significant amount of U precipitated. By using this multi-method approach, for the first time, we could demonstrate the correlation between certain spectroscopically observed U species in solution and varying degrees of bioassociation to the plant as well as differences in U translocation patterns in B. napus between citric and malic acids, providing more insights into the interaction of U with plants.