Disappearance of aluminum tridecamer from hydroxyaluminum solution in the presence of humic acid

We investigated the influences of humic acid on the removal of Al tridecamer (Al₁₃) from a hydroxyaluminum (HyA) solution at various humic acid/Al ratios. The Al species contained in the solution were analyzed by using a liquid-state ²⁷Al-NMR and an atomic absorption spectrometer and fractionated into three Al species: (i) Al₁₃, (ii) Al monomer and dimer (Al_SYM), and (iii) other undefined species including aggregated/precipitated Al (Al_NON). By the addition of humic acid to the HyA solution, the concentration of Al₁₃ was rapidly decreased within 0.007 d (10 min). The decrease in Al₁₃ and the increase in Al_NON were more pronounced at a higher humic acid/Al ratio. When the molar ratio of humic acid carboxylic groups to Al exceeded 0.8, Al₁₃ was undetected from solution within 0.007 d. The formation of Al ₁₃-humic acid complexes and the aggregation/precipitation of those complexes were a predominant mechanism in removing aqueous Al₁₃ at the early stage of the reaction. Approximately 10 mol of carboxylic groups in humic acid were required to remove 1 mol of Al₁₃ from the HyA solution. Aqueous Al₁₃ had greater preference in precipitating with humic acid than Al_SYM. After 5 to 570 d of aging, the concentration of Al₁₃ and Al_NON also decreased and increased, respectively, both in the presence and absence of humic acid. In conclusion, aqueous Al₁₃ would not exist in soil solution under a high humic acid condition.