Surface Complexation of Calcite by Carboxylates in Water

Small molecules that have two carboxylic functions can adsorb from water onto calcite. The adsorption site is a −Ca+site. The mechanism of adsorption is a complexation of the −Ca+site by the two carboxylates, similar to the solution complexation of Ca++ions. The complex has a ring structure where the two carboxylates are joined on one side by the −Ca+ion and on the other by thenCH2groups of the small molecule. Five-bond rings (n= 0) are the most stable, followed by six-bond rings (n= 1) and seven-bond rings (n= 2). Five-bond rings can also be formed with one carboxylate and one hydroxyl group (this is the case for α-hydroxycarboxylates) or with one enolate and one hydroxyl group (catechol). The sequence of binding strengths is enolate > carboxylate > hydroxyl; it matches the sequence of complexation efficiencies of these groups in solution and their characters as electron donors toward the metal cation.