Contribution of metal ion precipitation to the adsorption of Co2+ ions from aqueous solutions onto sepiolite has been analyzed as a function of pH. Abstraction and precipitation isotherms are constructed to isolate the precipitation of cobalt from the real adsorption. The contribution of all cobalt species against pH is calculated from the available solubility products or acid constants. It is found that at pH 8.2, which is the onset of cobalt hydroxide precipitation, the distribution of adsorbed cobalt species is as follows: 92% Co2+, 7% CoOH+ and 1% Co(OH)(2). The experimental values are in accord with the calculated uptake of cobalt species onto sepiolite. Adsorption of cobalt ions onto sepiolite before precipitation of cobalt is governed by ion exchange between the released Mg2+ ions from sepiolite matrix and those adsorbed Co2+ ions; this behavior differs from typical oxide (titanium dioxide) and silicate (quartz) minerals. However, adsorption of cobalt onto the same materials including sepiolite follows the same trend after the region of cobalt precipitation despite distinct differences in their charge profiles. (C) 2007 Elsevier B.V. All rights reserved.