Fig. 3. Concentrations of (a, d) aluminum tridecamer (Al₁₃), (b, e) undefined species other than Al₁₃ or Al_SYM, including aggregated or precipitated Al that cannot be detected by NMR; (Al_NON), and (c, f) Al monomer and dimer (Al_SYM) as a function of humic acid carboxyl group versus Al ratios (COOH/Al) after 0.007 (a, b, c) and 570 d (d, e, f) of aging. Total Al concentrations are 5 (closed triangle) and 10 mmol L^–1 (closed circle).