Uranium bearing dissolved organic matter in the porewaters of uranium contaminated lake sediments
Abstract Uranium (U) mobility in the environment strongly depends on its oxidation state and the presence of complexing agents such as inorganic carbon, phosphates, and dissolved organic matter (DOM). Despite the importance of DOM in U mobility, the exact mechanism is still poorly understood. Therefore, the aim of our investigation was to characterise sediment porewater DOM in two lakes in Ontario, Canada (Bow and Bentley Lakes) that were historically contaminated with U and propose possible composition of UO 2 -bearing DOM. Depth profiles of U concentrations in porewaters and total sediment digests reveal U levels of up to 1.3 mg L −1 in porewater and up to 0.8 mg −1 g in sediment. Depth profiles of U did not correlate with Fe, Mn, SO 4 2− , or Eh profiles. Therefore, porewater DOM was analysed and taken into consideration as the primary source of U mobility. Porewater DOM in each sediment section (1 cm sections, 20 cm core length) was analysed by high-resolution electrospray ionisation mass spectrometry. PCA analyses of porewater DOM mass spectra showed grouping and clear separation of DOM in sediment sections with elevated U concentrations in comparison to sections with background U concentrations. Several criteria were set to characterise UO 2 -bearing DOM and more than 70 different molecules were found. The vast majority of these UO 2 -DOM compounds fell in the category of carboxyl-containing aliphatic molecules (H/C between 0.85 and 1.2 and O/C ≤ 0.4) and had a mean value of m/z about 720. Highlights U concentrations up to 1.3 mg L −1 in pore waters and 0.8 mg −1 g in sediment are reported for two historically contaminated lakes in Ontario. High resolution mass spectrometry was performed on pore waters. More than 7000 different molecules were evaluated to find UO 2 -bearing dissolved organic matter. Over 70 different UO 2 -bearing molecules had their molecular formula assigned. UO 2 -bearing molecules are plotted in a Van Krevelen diagram according to O/C and H/C ratios. The majority of UO 2 -bearing molecules fell in the category of carboxyl-containing aliphatic molecules with H/C between 0.85 and 1.2 and O/C ≤ 0.4. Size distribution of UO 2 -bearing molecules is presented with a mean size value of m/z of about 720. Graphical abstract [DISPLAY OMISSION]
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