Column experiments and the full dissolution rate law of gibbsite

The dissolution of a gibbsitic bauxite in natural systems was simulated in the laboratory, at 25°C, using a column reactor, with input pHs ranging from 3.2 to 4.5 and fluid velocities ranging from 61 to 1085 m/y. As a result, the dissolution of gibbsite was measured under a wide range of saturation state conditions from equilibrium or near-equilibrium conditions to very far from equilibrium conditions. Far-from-equilibrium dissolution rates were measured under steady-state conditions. At slower flow rates, the variation of the rates with deviation from equilibrium was also extracted. The slowest flow rates yielded the equilibrium solubility of gibbsite. The solubility (K_sp) of natural gibbsite and the column output solution saturation states (expressed as the Gibbs Free Energy of reaction, ΔG_r) were determined with respect to the overall reaction: Al(OH)₃ + 3H⁺ = Al³⁺ + 3H₂O Despite the impurities present in the natural sample material, the calculated log K_sp, 7.83 ± 0.12, is in excellent agreement with published K_sp values for pure gibbsite. Far-from-equilibrium, ΔG_r < -0.7 kcal/mol, the dissolution rates attain a constant value of -7.4 × 10^-13 moles/m²/sec at input pH values of 3.5. A reaction order of 0.33 with respect to a_H+ was found. The variation of the rates with deviation from equilibrium was found to be very similar to the results of Nagy and Lasaga (1992), even though our study used natural gibbsite, a column device, and different pH and temperature. Therefore, the comparison of the results of this study and Nagy and Lasaga (1992) validates the use of a general rate law and suggests that the same kind of ΔG_r functionality is valid over a broad range of pH and temperature conditions.