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

Sorption of CrO(4)(2-) and HAsO(4)(2-) 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 CrO(4)(2-) equals the ion-exchange capacity of the solid, whereas sorption of HAsO(4)(2-) saturates at a higher value. Chloride ions inhibit the uptake of both anions, the amount of sorbed CrO(4)(2-) declining rapidly to zero. The uptake of HAsO(4)(2-), 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≈pQ(a3). CrO(4)(2-) and HAsO(4)(2-) have notable, albeit different, effects on the electrophoretic behavior of hydrotalcite; the positive particle charge is screened almost completely by CrO(4)(2-), whereas sorption of HAsO(4)(2-) 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.