Dissolution of nano-size Mg-Al-Cl hydrotalcite in aqueous media

摘要

Aqueous dissolution of a synthetic chloride-containing Mg~(2+)-Al~(3+) layered double hydroxide was studied at 298 K as a function of pH. Mild acidification (final pH 9 -5) results in incongruent dissolution of Mg~(2+), exclusively. Formation of an amorphous A1(OH)_3 passive layer prevents further Mg~(2+) leaching, thus dissolved Mg~(2+) amounts for only 20% of the solid. At lower pH values, massive dissolution readily takes place in a congruent fashion. Congruent dissolution profiles are accounted for by a contracting volume kinetic law. The pH dependence of the observed rate constant shows that the kinetic order on proton concentration is 0.41, indicating that dissolution is surface controlled. The implications of these findings are discussed.