Co-extraction of Iron and Sulfate by Bis(2,4,4-trimethylpentyl)phosphinic Acid, CYANEX®272

ABSTRACT

The mode of action of iron(III) uptake from sulfate solutions by the commercial extractant CYANEX®272, bis(2,4,4-trimethylpentyl)phosphinic acid, has been studied using conventional solvent extraction methods, together with ³¹P{¹ H} NMR spectroscopy, electrospray ionisation mass spectrometry, and DFT calculations that have not previously been used to analyse the content of the ISOPAR M water-immiscible phase. The maximum Fe-loading recorded was 177% of the theoretical maximum, based on the formation of an Fe(III) complex [Fe(L)₃] with a 1:3 molar ratio of Fe to phosphinate (L). Inductively coupled plasma optical emission spectroscopy (ICP-OES) indicates that sulfate or hydrogen sulfate ions are co-extracted in a 1:2 molar ratio with iron across the whole of the Fe-loading range, which helps to account for the loadings being greater than 100% of the theoretical value. ³¹P{¹ H} NMR spectroscopy indicates that the limiting factor in Fe-loading is the availability of uncomplexed CYANEX®272 (sulfate is present in large excess). In contrast to the behaviour of Co(II) and Zn(II) extraction by CYANEX®272, there is no evidence for the formation of polymeric Fe(III) complexes and highly viscous solutions at high metal loadings. The identity of the extracted species is likely to be a polynuclear Fe(III) sulfate complex.

ABSTRACT

KEYWORDS:

• CYANEX®272
• iron(III) extraction
• solvent extraction

Research Highlights

• Study of Fe(III) uptake from sulfate solutions by CYANEX®272 using solvent extraction, ³¹P{¹H} NMR spectroscopy, ESI-MS, and DFT calculations, to understand the nature of iron complexes formed.

• Some, but not all, of the findings are consistent with previous literature reports, which we attribute to a change in extractant mechanism with Fe(III) concentration.

• Findings indicate that at ca. 0.08 M Fe concentration sulfate or hydrogen sulfate ions are co-extracted in a 2:1 (Fe:S) molar ratio with Fe(III) across the whole of the Fe-loading range.

• Limiting factor in Fe-loading is the availability of uncomplexed CYANEX®272.

• In contrast to Co(II) and Zn(II) extraction by CYANEX®272, there is no evidence for the formation of polymeric Fe(III) complexes and highly viscous solutions at high metal loadings. Extracted species is most likely a polynuclear Fe(III) sulfate complex.

Acknowledgments

The authors gratefully acknowledge Solvay for funding this research and for providing samples of CYANEX®272, CYANEX®272Y, and ISOPAR M. We thank the School of Chemistry at the University of Edinburgh for funding and for access to analytical services and the Edinburgh Computer and Data Facility for access to high-performance computing facilities. We are also very grateful to Dr Logan Mackay for assistance with the operation of mass spectrometry facilities.

Additional information

Funding

This work was supported by the Cytec Industries Inc [R83378].