Fig. 1. Changes in the concentrations of (a) aluminum tridecamer (Al₁₃), (b) undefined species other than Al₁₃ or Al_SYM including aggregated or precipitated Al that cannot be detected by NMR (Al_NON), and (c) Al monomer and dimer (Al_SYM) and (d) in pH with aging time (10 mmol L^–1 hydroxyaluminum [HyA] solution). Humic acid carboxy group versus Al ratios (COOH/Al) are 0 (open square), 0.07 (open triangle), 0.15 (open inverted triangle), and 0.38 (open circle).