Bi-Functional Organophosphorus Extractants and Computational Modeling for Copper(II) and Zinc(II) Extraction

Abstract

In the liquid-liquid extraction technology, the extractant is well-known to play a key role for separating target metal ions. The selection of appropriate extractant often decides the success of the extraction process. To date, a lot of extractants have been developed and their extraction properties have been evaluated for a variety of metal ions. [1][2][3][4][5][6][7][8][9][10][11] In a previous study, we have developed novel bi-functional organophosphorus extractants in which two phosphonic acids were introduced as functional groups in the molecular structure for the separation of rare earth metals. 12 The result indicated that extraction performance of bi-functional extractants is very sensitive to the molecular structure of the spacer which connects the two functional moieties. In the present study, we applied this novel type of bi-functional extractants to the separation of Cu(II) and Zn(II). Since the structures of Cu(II) and Zn(II) complexes with a ligand are different from each other, an interesting extraction behavior is expected for the bi-functional extractants. We focused on the difference in the length of the spacer because the spacer length was considered to affect the complex formation of Cu(II) (square planar) and Zn(II) (tetrahedral). In this work, three bifunctional organophosphorus extractants which have a different spacer length were prepared, and their extraction performance for Cu(II) and Zn(II) was investigated compared to that of a monomeric extractant as an analog.Recently, computational chemistry attracted much attention for the design of novel extractants. [12][13][14][15][16] In the field of solvent extraction, interfacial properties of extractants at the oil-water interfaces were discussed on the extraction rate using MD method. 17 In our study, extraction behavior of the bi-functional extractants for Cu(II) and Zn(II) is discussed with computational modeling. Calculated results support the conclusion that the bi-functional organophosphorus extractants show a high selectivity towards Zn(II) over Cu(II) compared to that of mono-functional analog. We also demonstrate that the computational modeling is very helpful for predicting extraction behavior and designing new extractants for a target metal ion. ExperimentalReagents 1,4-Butanediol-O,O′-diphenyl phosphonic acid (abbreviated as BDPA), 1,8-octanediol-O,O′-diphenyl phosphonic acid (abbreviated as ODPA) and 1,12-dodecanediol-O,O′-diphenyl phosphonic acid (abbreviated as DDDPA) were synthesized by the procedures described in the following section. A mono-functional extractant, phenyl phosphonic acid mono-2-ethylhexyl ester (abbreviated as 2EHPA), was also synthesized as the analog to discuss the bi-functional effect. Figure 1 shows the molecular structures of the newly developed extractants applied in this study. Novel bi-functional extractants, which possess two phosphonic groups on both sides of the extractants, have been synthesized for the separation of Zn(II) and Cu(II). The separation properties and extractability of the novel ex...