Optimization of synergistic extraction of calcium ion via an emulsion liquid membrane using dicyclohexano-18-crown-6 and D2EHPA carriers: A design of experiment study

Abstract

Calcium ions play an important role in various fields, with their stable isotopes being of particular importance in medical research and in the production of superheavy metals. Emulsion liquid membranes (ELMs) offer a promising alternative to conventional methods for metal ion extraction due to their remarkable mass transfer rates. However, the effectiveness of crown ethers as carriers in ELM processes has been hindered by their limited ability to extract calcium ions. The aim of this study is to optimize the extraction of calcium ions using an ELM containing dicyclohexano-18-crown-6 (DC18C6) as a carrier and di-(2-ethylhexyl) phosphoric acid (D2EHPA) as a synergistic carrier. To achieve this objective, a two-level fractional factorial design (FFD) was initially implemented to screen the key factors. Subsequently, a central composite design (CCD) was employed to optimize the four identified key factors. The CCD method yielded a quartic regression model for calcium extraction efficiency. The optimum values were: DC18C6 concentration at 0.035 mol/L, feed phase pH of 4, a feed/emulsion volume ratio of 2.3, and a W/O/W stirring time of 23.4 min. Under these optimal conditions, DC18C6 exhibited negligible extractability, while D2EHPA achieved an extraction efficiency of 62.2%. Combining both carriers, however, significantly increased the extraction efficiency to 97.3%, indicating a synergistic effect that enhanced the extraction of Ca(II) ions. Furthermore, the selectivity of the ELM towards Ca(II) ions was examined in binary solutions of Ca(II)/Li(I), Ca(II)/Na(I), and Ca(II)/Mg(II). The ELM demonstrated selectivity for Ca(II) over Li(I), Na(I), and Mg(II) with separation factors of 4.4, 2.0, and 8.1, respectively. It is noteworthy that the combination of carriers positively influenced calcium selectivity exclusively in the Ca(II)/Mg(II) solution. Conversely, no noticeable effect on selectivity was observed in the Ca(II)/Li(I) solution, while the combination had an adverse effect on calcium selectivity in the Ca(II)/Na(I) solution.