Industrial & Engineering Chemistry Research, Vol.56, No.1, 63-74, 2017
Monovalent Ion Exchange Kinetics of Hydrated Calcium-Alumino Layered Double Hydroxides
This study reveals ion exchange kinetics in hydrated calcium-gut-lino 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.