Ionic Speciation and Coordination Mechanisms of Vanadium, Iron, and Aluminum in the Oxalic Acid Leachate of Shale

The oxalic acid leachate of vanadium-bearing shale (OALS) is a complex system in which the ion states and coordination mechanisms of the primary metallic elements—vanadium, iron, and aluminum—are not fully understood. This study investigated the ionic speciation and coordination mechanisms of vanadium, iron, and aluminum in OALS. The results indicate that vanadium predominantly existed as VO(C2O4)22− anions, iron as Fe(C2O4)2− and Fe(C2O4)33− anions, and aluminum as Al(C2O4)2− and Al(C2O4)33− anions. The coordination reaction processes and equations of various oxalate complexes were examined. Regardless of whether the molar ratio was 1:1 or 1:2, the iron–oxalate complex exhibited the lowest reaction Gibbs free energy (ΔG), with values of −5343.69 and −1470.72 kJ/mol, respectively. The aluminum–oxalate complex followed, with ΔG values of −5169.23 and −1318.87 kJ/mol, respectively. The vanadium–oxalate complex displayed the highest reaction ΔG, at −2760.65 and −714.12 kJ/mol, respectively. Therefore, the coordination mechanism of vanadium, iron, and aluminum with oxalate ions in OALS is such that iron coordinated with oxalate first, followed by aluminum, and finally vanadium. The research results have important guiding significance for the purification, enrichment, and coordination mechanisms of complex solutions.