Effect of sulfate on the release rate of Al³⁺ from gibbsite in low- temperature acidic waters

Solubility experiments were performed at 5 °C by reacting 1 g of powdered gibbsite with 45 g of aqueous H₂SO₄-NaCl and HCl-NaCl (0.01 m H⁺) solutions at 0.1 m ionic strength. The kinetics of dissolution were monitored under extremely undersaturated conditions and appeared to favor a zero-order rate-determining process. The dissolution rate of gibbsite in the sulfate solution was approximately 10 times faster than in the HCI solution, which is consistent with trends reported in the literature for studies performed at higher temperatures. The enhanced dissolution kinetics of gibbsite (and by analogy, other aluminum-containing minerals) in the presence of sulfate may contribute to the elevated Al concentrations observed in natural low- temperature waters associated with acid rain and acid mine drainage.