Monovalent ion exchange kinetics of hydrated calcium-alumino layered double hydroxides

This study reveals ion exchange kinetics in hydrated calcium-alumino layered double hydroxides of the alumino-ferrite monosubstituent (AFm) subgroup. By careful analysis of solution phase (ion) concentrations and the solid phases, the rate of exchange of NO₃^- for Cl^- ions from the interlayer positions is studied across a range of temperature, solution compositions, and solution conditions (i.e., static and convectively mixed). Ion exchange kinetics conform to an exponential first-order reaction that follows an Arrhenius formalism. The activation energy of ion exchange is 38.2 ± 4.6 kJ/mol for exchange occurring in the thermodynamically preferred (i.e., NO₃^- for Cl^- ion exchange) direction and it is 1.8 times greater for the inverse less-preferred pathway. For ion exchange occurring in the favored direction NO₃-AFm converts to Cl-AFm; whereas in the disfavored (less-favored) direction, the compositional change occurs in two steps with the formation of a Cl-NO₃-AFm solid solution as an intermediate step; before Cl^- species are exhausted and NO₃-AFm forms as the product. By comparison of ion exchange rates in static and mixed solutions, transport through the Nernst diffusion layer (NDL) is identified as the rate-limiting step in ion exchange kinetics. (Graph Presented).