Fig. 2. Concentrations of undefined species other than Al₁₃ or Al_sym including aggregated or precipitated Al that cannot be detected by NMR (Al_NON) (open triangle) and precipitated Al (open circle) as a function of the humic acid carboxyl group versus Al ratios (COOH/Al) after 0.007 and 570 d of aging [10 mmol L^–1 hydroxyaluminum (HyA) solution].