Complexation of Ln(III) Ions by Gluconate: Joint Investigation Applying TRLFS, CE-ICP-MS, NMR, and DF Calculations

The potential of gluconate, a common cement additive, to mobilize lanthanides (used as analogues of actinides) from cement is investigated. For this purpose, complex formation of trivalent lanthanides, Ln(III), (Ln: La, Sm, Eu, Gd, Lu) with gluconate (GLU) was studied applying time-resolved laser-induced luminescence spectroscopy (TRLFS) in combination with parallel factor analysis (PARAFAC), capillary electrophoresis-inductively coupled plasma mass spectrometry (CE-ICP-MS), nuclear magnetic resonance (NMR) spectroscopy, and density functional (DF) calculations. Up to circumneutral conditions, binary complexes form with Ln(III):GLU stoichiometric ratios of 1:1-1:4 depending only on the Ln:GLU ratio, regardless of the concentration regime (micromolar to millimolar). Coordination facilitates via the carboxyl group (C1) and the adjacent hydroxyl group (at C2) forming a five-membered ring chelation motif, with a probable participation of the C3 hydroxyl group. Beyond circumneutral pH, with the exact onset depending on the specific lanthanide, a fundamental change in speciation takes place. Speciation then becomes more complex upon coexistence and interconversion of several (isomeric) complexes concluded to involve one or more deprotonated GLU hydroxyl groups not necessarily participating in coordination.