Effect of Solution Composition on the Rate and Mechanism of Gibbsite Dissolution in Acid Solutions

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Abstract
The rate of dissolution of gibbsite was measured as a function of H+ concentration and anion concentration. The anions examined were nitrate, sulfate, and phosphate and the pH range used was 1.5 to 4.0. Rates of gibbsite dissolution were low and comparable to reported dissolution rates for feldspars. In the presence of nitrate and sulfate the dissolution reaction was first‐order with respect to H+ below pH 2.5. In phosphate there was no dependence of the rate on H+. Above pH 3.5 in nitrate the reaction order with respect to H+ approached zero. The order of the reaction with respect to anion concentration was about 0.6, 0.4, and 0.9 for nitrate, sulfate, and phosphate, respectively. The rate dependence of gibbsite dissolution on H+ and anion concentration suggested that the mechanism of dissolution, as well as the rate, is a function of solution composition. In solutions containing ions not specifically adsorbed (e.g., NO‐3 and SO2‐4), proton attack of the surface is the rate‐determining step. In solutions containing phosphate, anion attack predominated, and the rate of the reaction is not dependent on pH.

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