Stability and structural aspects of complexes forming between aluminum(III) and D-heptagluconate in acidic to strongly alkaline media: An unexpected diversity

In the current contribution, the complex formation equilibria between Al3+ and α-D–heptagluconate (Hpgl−) ions, which is of relevance in, e.g., the Bayer process for obtaining alumina from bauxitic ores, has been investigated. Potentiometric titrations and polarimetric measurements performed at 25 °C and 4 M ionic strength uncovered the formation of several complexes forming in the hyperalkaline (pH ≥ 13) pH range: Al(OH)4Hpgl2−, Al(OH)5Hpgl3−, Al(OH)5Hpgl42−, Al3(OH)13Hpgl62− and Al4(OH)15Hpgl63−. NMR spectroscopy measurements revealed that diverse complexation processes took place in the hyperalkaline region. In the 9 to 7 pH range, ligand–stabilized aluminum hydroxides were formed. In acidic medium, the binding sites of the ligand could be identified as C2H(OH), C3H(OH), C4H(OH) and C5H(OH). Freezing point depression (FPD) measurements indicated, that association occurred to a large extent. ESI–MS measurements confirmed the predominant formation of Al2(OH)6Hpgl− at pH = 7, and implied the coordination via the carboxylate and the adjacent OH functional groups of the ligand.