Event

Electrochemistry and speciation of cerium in aqueous and organic solutions

Mark R. Antonio

Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL

Since the 1920s, volumes of literature have accumulated about the extraction and purification of cerium from the rare-earth-elements, which are pivotal to the national energy economy. Yet, even today, ambiguities about cerium valence and speciation frequently arise from unexpected trivalent/tetravalent redox activity in solution and solid-state chemistry.[1] The electroactive nature of Ce puts it at the intersection of two ostensibly disparate fields: separation science, on the one hand, and electroanalysis, on the other. We have observed that the physical manipulations and chemical reactions in the liquid-liquid extraction of tetravalent Ce lead to organic solutions with peculiar behaviors and unusual structures.[2] This illustrates the point that—despite nearly nine decades of research and a deep practical knowledge about Ce extraction technology vis-à-vis nuclear fuel reprocessing—there is a comparatively shallow understanding of the fundamental chemistry in aqueous and organic media. For example, in a normal paraffinic hydrocarbon (e.g., n-dodecane) with tri-n-butyl phosphate—the workhorse extractant of separation science and engineering[3]—Ce(IV) distribution isotherms are still incomplete, mainly because of the strong oxidizing power of Ce(IV), which makes it difficult to determine water and acid concentrations in the organic phases. Moreover, the formation of peculiarly-dense organic phases (see Figure) confounds expectations. This presentation will focus upon the characterization of Ce(IV) in aqueous and organic solutions of relevance to chemical separations, including the phenomenon of third phase formation in a PUREX-like process.[4] Widely-held assumptions and long-standing assertions about this ages-old system chemistry will be revisited with new insights obtained using modern techniques.

[1] R. J. Ellis, M. R. Antonio, ChemPlusChem 2012, 77, 41. [2] M. R. Antonio, R. J. Ellis, S. L. Estes, M. K. Bera, Phys. Chem. Chem. Phys. 2017, 19, 21304. [3] Science and Technology of Tributyl Phosphate, Vols. I–IV, W. W. Schulz, J. D. Navratil, et al., Eds., CRC Press: Boca Raton, 1984. [4] The PUREX acronym was coined by W. B. Lanham and T. C. Runion for Plutonium Uranium Reduction EXtraction, Report ORNL-479, 7 October 1949, see http://www.osti.gov/scitech/servlets/purl/4165457

Inquires please contact Rosa M. Vargas rvargas@sas.upenn.edu