Competition between monovalent and divalent anions for calcined and uncalcined hydrotalcite: Anion exchange and adsorption sites

Hydrotalcite compounds are well-known anion exchangers. Their anionic exchange capacity (A.E.C.) increases with thermal treatment together with their specific surface area and porosity. In the presence of mono- and divalent anions such as Cl-, SO42- and CrO42-, calcined hydrotalcite behaves not only as an anion exchanger but also as an oxide adsorbent, In the case of divalent anions, the adsorbed quantity is larger than the A.E.C. It corresponds to 1.2 and 3 adsorbed molecules per nm(2) in the case of SO42- and CrO42- respectively. This latter value suggests that CrO42- precipitates at high equilibrium concentration. The affinity of each anion in mixed solutions (Cl- and SO42-) also depends on their solubility. In this case, the greater affinity of CrO42- than SO42- in NaCl electrolyte for calcined hydrotalcite is governed by its lower solubility.