Uranium photo-precipitation coupled with fulvic acid oxidation under anoxic and oxic conditions

Uranium (U) is of vital importance for the nuclear power industry. Its mining, processing and disposal impose significant risks to the environment. Precipitation and fate of uranium in water bodies are of interest for environmental protection. In this work, we propose that fulvic acid (FA), a natural organic matter, can photo-precipitate ∼ 93.2% U under anoxic conditions within 3 h and ∼ 88.3% under oxic conditions within 48 h, respectively. Based on HRTEM, SAED and XPS analyses of the anoxic precipitate, U₂O₅ and UO₂(OH)₂ nanocrystals precipitate with FA and its decomposition intermediates. XPS analysis of C1s of the anoxic precipitate further reveals that FA is partly oxidized and has lost its conjugated benzoic ring and aldehyde groups, forming more carboxylic groups. On the other hand, XRD and XPS analyses on the oxic precipitate indicates the formation of U^IV(C₂O₄)₂·2H₂O and U^VIO₃·0.75CO₂·0.68H₂O, demonstrating much deeper oxidation of FA under oxic conditions. In contrast to numerous previous studies focusing on heterogeneous photocatalytic reduction of U using solid catalyst, the uranium removal technique here spared the cost of catalyst synthesis, the necessity of sacrificer for uranium photo-reduction and U retrieval from the catalyst's surface. The results also shine a light on new pathways for uranium fates in uranium-bearing water bodies such as rivers and lakes.