Comparative study of carboxylic acid adsorption on calcite: l-malic acid, d-malic acid and succinic acid

Succinic acid and l-malic acid are known to affect calcite dissolution/crystallization due to their tendency to adsorb on calcite's surface. Taking into account the structural similarity among succinic acid, l-malic acid, and d-malic acid, a comparative study of their adsorption behavior on calcite was investigated for clarifying their adsorption mechanism. Kinetic results showed that l-malic acid adsorption reached equilibrium within 12 h, while succinic acid required 48 h. Results from the pH effect study showed that the strength of surface complex of succinic acid on calcite was weaker, even compared to its aqueous complex with Ca2+ ion. Alternately, l-malic acid exhibited a much stronger surface complex strength, even compared to its aqueous complex with Ca2+ ion. The adsorption isotherm results indicated that the saturated adsorption amount of l-malic acid was twice that of succinic acid, suggesting the surface complex of succinic acid may occupy a larger space on calcite surface compared to that of l-malic acid. Based on these results and those from previous studies, we proposed that succinic acid adsorption is through its two carboxyl groups binding to two adjacent surface sites (> Ca+), and not only one. Also, l-malic acid adsorption occurs through one carboxyl group and one hydroxyl, which forms a claw on calcite surface. The similar adsorption behavior between l-malic acid and d-malic acid reinforces this proposition for l-malic acid adsorption mechanism. Our study will serve as good reference information to provide better understanding into the adsorption of other carboxylic acids on calcite.