Kinetics of gibbsite dissolution under low ionic strength conditions

Experiments measuring synthetic gibbsite dissolution rates were carried out using both a stirred-flow-through reactor and a column reactor at 25 degre es C, and pH range of 2.5-4.1. All experiments were conducted under far fro m equilibrium conditions (Delta G < -1.1 kcal/mole). The experiments were p erformed with perchloric acid under relatively low (and variable) ionic str ength conditions. An excellent agreement was found between the: results of the well-mixed how -through experiments and those of the (nonmixed) column experiments. Tills agreement shows that the gibbsite dissolution rate is independent of the st irring rate and therefore supports the conclusion of Bloom and Erich (1987) that gibbsite dissolution reaction is surface controlled and not diffusion controlled. The Brunauer-Emmett-Teller (BET) surface area of the gibbsite increased dur ing the flow-through experiments, while in the column experiments no signif icant change in surface area was observed. The agreement between the dissol ution rates of the mixed flow-through experiments that were normalized to t he final surface area, with these of the column experiments, supports the a ssumption that the changes in surface area occurred early in the experiment , before the first steady state was approached. The significant differences in the BET surface area between the column experiments and the flow-throug h experiments, and the excellent agreement between the rates obtained by bo th methods, enable us to justify the substitution of the BET surface area f or the reactive surface area. The dissolution rate of gibbsite varied as a function of the perchloric aci d concentration. At pH > 3.5 the dissolution rate increased as a function o f acid concentration, while at pH < 3.5 it decreased with acid concentratio n. We interpret the gibbsite dissolution rate as a result of a combined eff ect of proton catalysis and perchlorate inhibition. Following the theoretic al study of Ganor and Lasaga (1998) we propose specific reaction mechanisms for the gibbsite dissolution in the presence of perchloric acid. The mathe matical predictions of two of these reaction mechanisms;adequately describe the experimental data. Copyright (C) 1999 Elsevier Science Ltd.