When aluminum-rich acid mine drainage mixes with near-neutral surface waters, fluffy aluminum oxyhydroxide flocs precipitate and move downstream as suspended solids, transporting adsorbed pollutants. Scientists previously thought that such flocs formed by aggregation of monomeric Al(OH)₄^– complexes into a Al₆(OH)₁₂(H₂O)₁₂⁶⁺ multimer, even though such a multimer has never been found in nature. Now, researchers are challenging that belief with ²⁷Al nuclear magnetic resonance (NMR) data, which suggest instead that the flocs are formed from aggregation of the more toxic aqueous polyoxocation, AlO₄Al₁₂(OH)₂₄(H₂O)₁₂⁷⁺ (Al₁₃).

The finding is important because Al₁₃ is phytotoxic and is likely to be responsible for the declining fish populations in rivers polluted by acid mine drainage. The compound also has a high affinity for heavy metals cations, such as Pb²⁺, Cu²⁺, and Zn²⁺, and is therefore likely to transport metals over long distances.

Gerhard Furrer of the Institute of Terrestrial Ecology in Zürich, William Casey of the University of California–Davis, and colleagues examined flocs from nine streams polluted from acid mine drainage in Germany and California. Using ²⁷Al magic-angle spinning NMR spectroscopy, they observed the presence of distinct Al(O)₄ centers in the flocs. To confirm that the source of Al(O)₄ is Al₁₃, they performed experiments with solutions of GaO₄Al₁₂(OH)₂₄(H₂O)₁₂⁷⁺ (GaAl₁₂), in which a Ga(O)₄ is substituted for Al(O)₄. As expected, they saw no signal for Al(O)₄ when GaAl₁₂ was aggregated, whereas they did see a signal for Al(O)₄ when Al₁₃ was aggregated. (Science 2002, 297, 2245–2247)