Role of aliphatic and phenolic hydroxyl groups in uranyl complexation by humic substances

Relativistic density functional calculations of uranyl complexes with alcohols were carried out to study how phenolic and aliphatic hydroxyl groups of humic substances may contribute to uranyl complexation by humic substances. According to recent experimental work, blocking of phenolic OH groups decreases the loading capacity, but has no effect on the key geometric parameters of uranyl humate complexes. This can be understood on the basis of our calculations which showed uranium–oxygen distances to be very similar for complexes with rather different types of O-donor ligands, with average U–Oeq ∼ 237 pm for fivefold coordinated uranyl (VI) complexes, both for O− and OH functional groups. Uranyl complexation by alcohol moieties seems unlikely at environmental conditions as a high pH is required for the deprotonation of these groups; we confirm an alternative complexation mechanism that overcomes the ligand deprotonation problem. Similarities in structures and energetic suggest that complexes of both aliphatic and phenolic alcoholates may well contribute in comparable fashion to the complexation of uranyl by humic acids.

Graphical abstractDFT calculations on uranyl complexes with aliphatic and phenolic alcohols as models of OH and O– functional groups of humic substances yield similar local structure parameters, in agreement with experiment. Further structural similarity with uranyl complexes of carboxylic groups corroborates experiments on humic acids with blocked phenolic OH groups where uranyl complexes showed no change in geometry. Thermodynamic modeling suggests uranyl alcoholates to form already at weakly acidic conditions.Figure optionsDownload full-size imageDownload as PowerPoint slide