Enhancement mechanism of an improved liquid membrane using selective permeation retardant for heavy metal ions separation

Liquid membrane (LM) separation is a kinetic process that can be enhanced by increasing the difference between the permeation rates of the components. The addition of H 3 Cit/Na 3 Cit as a selective permeation retardant can enlarge the difference between the permeation rates of Cu(II) and Cd(II) ions when P507 was used as the extractant. This retardant is analogous to a masking agent employed in liquid–liquid extraction (LLE). However, the retardant mainly affects the LM’s kinetics, while the masking agent impacts the LLE's equilibrium. It was found that the separation coefficient of the H 3 Cit/Na 3 Cit LM system could be much greater than that of the H 3 Cit/Na 3 Cit LLE system owing to the LM's kinetic effect. Following the addition of H 3 Cit/Na 3 Cit, the Cu(II) ion permeation rate was selectively retarded, and the LM selectivity of Cd(II)/Cu(II) was significantly enhanced in comparison with a control LM system. To reveal this enhancement mechanism, a new LM kinetic model was established in which the interfacial chemical reactions and mass transfer resistance in the middle connection zone of a H-type device were considered. The model's predictions agreed well with the experimental data. According to the model' quantitative analysis, the retardant affected the apparent reaction rate constants of Cd(II) and Cu(II) ions differently, and this is key to the enhancement mechanism. Aside from concentration, the LM model also predicted the variations in separation coefficient, purity, and yield with time. An optimal LM extraction time zone was defined to obtain high yield and high purity simultaneously. Finally, the enhanced LM was further verified and applied on a preliminary basis in a supported liquid membrane with strip dispersion, which is a LM technology with potential for industrial application.