Complexation at the edges of hydrotalcite: The cases of arsenate and chromate

Sorption of CrO42- and HAsO42- by hydrotalcite, in its chloride form, was studied as a function of anion concentration. In both cases, the shape of the isotherms is langmuirian. The maximum uptake of CrO42- equals the ion-exchange capacity of the solid, whereas sorption of HAsO42- saturates at a higher value. Chloride ions inhibit the uptake of both anions, the amount of sorbed CrO42- declining rapidly to zero. The uptake of HAsO42-, however, attains a constant value at high chloride concentrations. The excess of arsenate uptake follows, at constant pH, a langmuirian dependence with equilibrium concentration and decreases with increasing pH, depicting a marked change in slope at pH ≈ pQa3. CrO42- and HAsO42- have notable, albeit different, effects on the electrophoretic behavior of hydrotalcite; the positive particle charge is screened almost completely by CrO42-, whereas sorption of HAsO42- produces charge reversal. These results reflect the formation of inner-sphere arsenate surface complexes at the edges of hydrotalcite particles. The underlying rationale is discussed in terms of the crystal structure of hydrotalcite surfaces.

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► Hydrotalcite edges contribute to anion uptake.
► Arsenate forms inner-sphere edge complexes.
► Chromate has a much lesser affinity for edge adsorption sites.
► Stability follows the predictions of the surface complexation concept.