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Aluminum is a strongly hydrolysing metal and is relatively insoluble in the neutral pH range (6.0–8.0). In the presence of complexing ligands and under acidic (pH < 6) and alkaline (pH > 8) conditions, aluminum solubility is enhanced. At low pH values, dissolved aluminum is present mainly in the aquo form (Al3+). Hydrolysis occurs as pH rises, resulting in a series of less soluble hydroxide complexes (e.g., Al(OH)2+, Al(OH)2+).

Aluminum solubility is at a minimum near pH 6.5 at 20°C and then increases as the anion, Al(OH)4, begins to form at higher pH. Thus, at 20°C and pH < 5.7, aluminum is present primarily in the forms Al3+and Al(OH)2+. In the pH range 5.7 to 6.7, aluminum hydroxide species dominate, including Al(OH)2+and Al(OH)2+, and then Al(OH)3. Typically, at a pH of approximately 6.5, Al(OH)3predominates over all the other species. In this range, aluminum solubility is low, and availability to aquatic biota should also be low. At pH > 6.7, Al(OH)4becomes the dominant species.

Mononuclear aluminum hydrolytic products combine to form polynuclear species in solution. Aluminum begins to polymerize when the pH of an acidic solution increases to over 4.5. Polymerization gradually proceeds to larger structures, eventually leading to the formation of the Al13polycation.

The Joint Research Committee of the European Chemicals Bureau (ECBI/61/95 Add. 84) reported about the dissolution of AlCl3 carried out at Alcan Chemical’s Chalfont Park Laboratories. The experiment investigated the hydrolytic decomposition of aluminium chloride solutions over the pH range 6.0 – 8.5. The data of this study “indicate that aluminium chloride decomposes rapidly in aqueous environments, around neutral pH, leaving low concentrations of soluble aluminium species in the body of the liquid medium, above insoluble aluminium hydroxide precipitate. Comparison of these solubility data with published ecotoxity data indicate that aluminium chloride is unlikely to present a long term hazard to the environment.”