The dissolution rates of gibbsite in the presence of chloride, nitrate, silica, sulfate, and citrate in open and closed systems at 20°C

The dissolution of well crystallized gibbsite far at from equilibrium was studied in batch and mixed flow through reactors. The dissolution experiments were carried out between pH 2 and 6 in the presence of 10 mmol L-1 citrate, at pH 2 and 3 in the presence of 10 mmol L-1 chloride, nitrate, and sulfate, and at pH 2 and 3 in the presence of 1.5 mmol L-1 silica at 20 degrees C. The dissolution rate of gibbsite, R-A1 (Mol m(-2) s(-1)), increases in the order of chloride approximate to nitrate < silica < sulfate approximate to citrate. In presence of silica, sulphate, and citrate dissolution is catalysed by the formation of aluminium complexes at the gibbsite surface (pH 2 and 3). From pH 2 to 3 no effect of R-A1 on hydrogen activity is predicted as singly coordinated surface sites at the edges of the platy gibbsite crystals, [ AlOH2+0.5] approximate to [ AlOH], are almost saturated with protons. However at pH > 3 dissolution is slowed by a decrease of [equivalent to AlOH2+0.5]. Gibbsite dissolution rates measured in closed and open systems were identical within the experimental and analytical uncertainty. This observation indicates that gibbsite dissolution is a surface controlled process. If dissolution of gibbsite occurs close to equilibrium R-A1 values may be predicted by an approximately linear function of Delta G(r). Copyright (c) 2005 Elsevier Ltd.