Size and shape dependences of the adsorption kinetics of malachite green on nano-MgO: a theoretical and experimental study.

Compared with bulk materials, there is a considerable difference in the adsorption kinetics of nanoparticles, which mainly depend on particle size and shape. Herein, by introducing the shape factor, we have derived the relations between the kinetic parameters of adsorption and the particle size of nanoparticles of different shapes. Then, the influence of the regularities and mechanisms of particle size and shape on the kinetic parameters of adsorption were discussed. In the experiment, spherical nano-MgO and cubic nano-MgO of different particle sizes were synthesized via a sol-gel method, the kinetic parameters of the adsorption of malachite green on nano-MgO were determined, and the influence of the regularities of particle size and shape on the adsorption kinetic parameters were obtained. The experimental results show that shape and particle size may significantly influence the kinetic parameters of adsorption. For the adsorption of nano-MgO with the same equivalent particle diameter, compared to spherical shapes, the rate constant of adsorption k for cubic shapes is larger, while the apparent activation energy E-a and the pre-exponential factor A are smaller. For the adsorption of spherical or cubic nano-MgO, k increases with decreasing particle size, while E-a and A decrease, and there exist good linear relationships between ln k, E-a, ln A and the reciprocal of particle size. The experimental results are well in agreement with the theoretical relations. Furthermore, k is influenced by the shape factor, E-a by the shape factor and the specific surface enthalpy, and A by the shape factor and the temperature coefficient of surface tension. The kinetic theory of adsorption can quantitatively describe the influence of the regularities and mechanisms of particle size and shape on the adsorption kinetics of nanoparticles and provide significant guidance for the research and the application of nano-adsorptions in the related fields.