Complexation of Uranium(VI) with the Main Components of Mine-Wood Degradation. Part 1. Complexation with Protocatehuic Acid, Vanillic Acid and Vanillin

To consider the process of mine wood degradation and its concequences on the contaminant migration via aquatic path in risk assessments, e.g. for uranium mine restoration in the southern region of Saxony (Germany), the complexation of U(VI) with 3,4-dihydroxybenzoic acid (proto-catechuic acid), 4-hydroxy-3-methoxy-cinnamic acid (ferulic acid), 4-hydroxy-3-methoxybenzoic acid (vanillic acid) and 4-hydroxy-3-methoxybenzaldehyde (vanillin) was studied by potentiometric pH titration in carbonate-free solutions of 0.1 M NaClO₄. These four complexants are reasonably stable monomeric intermediates in the microbial or hydrothermally driven spruce-wood lignin degradation and therefore play a role in natural water chemistry.

The evaluation of the pH titration curves was carried out with the classical theory of stepwise complex formation, analyzing Bjerrum’s formation function [1] and by non-linear fitting of the pH titration curve with Sayce’s multi-equilibria program "SCOGS" [2], considering hydrolysis and mixed-ligand complexation.

In the case of protocatechuic acid, a series of very stable (1:1) to (1:3) complexes are formed (log ß: 14.8/25.9/33.2). The constants calculated by Bjerrum’s formation function are in full agreement with the values of the SCOGS curve fitting, i.e. U(VI) hydrolysis is surpressed in the presence of this ligand. The complexes with ferulic acid and vanillic acid have a much lower stability: log ß₁₁ values are to 6.88 and 7.16, respectively. In the case of ferulic acid, the assumption of the acidic species [UO₂H(fer)]⁺ with log ß₁₁₁ =11.9 improves the pH curve fit. Vanillin as complexant leads to pH curves that can be best fitted with the species [UO₂(van)]⁺ and [UO₂(van)₂(OH)₂]^2- with log ß values of 4.47 and -3.95, respectively.

Using these data speciation calculations for some calcite-rich mine waters show that the strong U(VI)-carbonato complexes predominate. Under certain conditions only protocatechuic acid complexes can be formed.