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

• Published in: Journal of colloid and interface science 2013-03, Vol.393, p.314-318
• Main Authors: Jobbágy, Matías, Regazzoni, Alberto E.
• Format: Article
• Language: English
• Subjects: Adsorption; Aluminum Hydroxide - chemistry; Anions; Arsenate; Arsenates; Arsenates - chemistry; Chemistry; Chlorides; Chromate; Chromates - chemistry; Crystal structure; electrophoresis; Exact sciences and technology; General and physical chemistry; Hydrogen-Ion Concentration; Hydrotalcite; ion exchange; Isotherms; Layered double hydroxides; Magnesium Hydroxide - chemistry; Solid-liquid interface; Sorption; Surface complexation; Surface physical chemistry; Surface Properties; Uptakes
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/j.jcis.2012.10.069

[Display omitted] ► 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. 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.