Modelling and optimization of a new complexing retardant-enhanced polymer inclusion membrane system for highly selective separation of Zn2+ and Cu2+

Polymer inclusion membranes (PIMs) have much application potential for metal ion separation. However, its separation selectivity highly depends on the extractant, and new efficient extractants are difficult to develop. This work proposes another approach, i.e., adding a complexing retardant to enhance the selectivity. The impurity ion (e.g., Cu 2+ ) was retarded in the feed solution by the complexing retardant (e.g., citric acid), while the target ion (e.g., Zn 2+ ) was transported across PIM. To reveal the enhancement mechanism, a mass transfer mathematical model was established. Contrast experiments proved that the complexing retardant can significantly enhance the overall separation coefficient β overall,exp . The β overall,exp of a conventional PIM system was 61.80, while β overall,exp increased to 853.45 after the retardant was added. The effects of retardant concentration, initial pH of feed solution, membrane thickness, extractant dosage, and plasticizer dosage on the retardant-enhanced PIM system were investigated. Model calculation results agreed well with the experimental data. The maximal β overall,exp 17525.68 was obtained at the optimal separation conditions of [CA] f,0  = 0.02 M, pH f ,0  = 4.5, [H 2 SO 4 ] s ,0  = 0.5 M, and the optimal composition of the PIM casting solution, i.e., PVC : P507: DBP: THF  = 0.125 g: 0.188 mL: 0.032 mL: 10 mL. Further studies indicated that the retardant mechanism was reflected in the difference increase of interfacial partition coefficients P Zn 2 + , f $P_{{\mathit{Zn}}^{2+},f}$ and P Cu 2 + , f $P_{{\mathit{Cu}}^{2+},f}$ . Finally, the optimization strategies of the separation effects for the retardant-enhanced PIM system are discussed in depth based on the simulated calculation.