The H+/M2+ exchange stoichiometry of calcium and zinc adsorption by ferrihydrite

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Abstract
Summary: The specific adsorption of Ca2+ and Zn2+ by ferrihydrite results in the net release of H+. The rate and H+/M2+ exchange stoichiometry of this reaction were monitored with a pH‐stat. A rapid reaction of less than 6 min was followed by a slower reaction which continued at a diminishing rate for at least 2 days. Adsorption of Ca2+ at pH 7.8 and Zn2+ at pH 5.4 resulted in the net release of 0.92 and 1.70 mol H+/mol M2+ adsorbed, respectively. For Zn2+ adsorption, this stoichiometry was shown to be independent of pH. These estimates agree well with independent estimates based on the pH dependence of adsorption. The difference between the Ca2+ and Zn2+ stoichiometries was related to the differing acidity of the –OH2 ligands attached to the adsorbed ions.