Spectroscopic and Theoretical Insights into Surprisingly Effective Sm(III) Extraction from Alkaline Aqueous Media by o‑Phenylenediamine-Derived Sulfonamides

• Published in: Inorganic chemistry 2020-05, Vol.59 (10), p.6884-6894
• Main Authors: Govor, Evgen V, Morozov, Alexander N, Rains, April A, Mebel, Alexander M, Kavallieratos, Konstantinos
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.0c00309

Alkaline high-level waste (HLW) generated as a result of years of nuclear weapons production has complicated composition and requires comprehensive treatment methods, which would allow concentrating its most radiotoxic components in a small volume for geological disposal. We have investigated six alkyl-substituted o-phenylenediamine-derived sulfonamides for extraction and consecutive stripping of Sm­(III) from alkaline aqueous media. Up to 81% of Sm­(III) recovery at pH 13.0–13.5 was achieved by disulfonamide (dsa) or dsa/Et3N in CH2Cl2, measured after contact with organic phases and subsequent stripping with 0.1 M HNO3. The use of Et3N dramatically enhances Sm­(III) extraction at lower pH ranges (10.5–11.5) but decreases extraction at pH 13.0–13.5, while control experiments with Et3N and no dsa showed no extraction. Analysis of the extraction equilibria gave a 1:1 sulfonamide–Sm­(III) complexation ratio, with the extracted species also presumed to contain coordinated H2O or OH–, as also shown by DFT calculations. Titration experiments of sulfonamides with Sm­(III) in CH3CN were consistent with a 1:1 complexation ratio of dsa-6 to Sm­(III) with a K 11 = 6.6 × 107 M–1 derived from nonlinear regression analysis of the 1:1 binding isotherm. Theoretical DFT calculations determined the structures of possible species formed during extraction and the thermodynamics of extraction processes based on several initial [Sm­(OH) y (NO3) z (H2O) x ]3–y−z species and 1:1 Sm­(III)/dsa-3 2– complexes formed in the organic phase, in which dsa complexes to Sm­(III) in its bis-deprotonated form (denoted below as dsa-3 2– ). Organization of close ion pairs of type {Na­[Sm­(dsa-3 2– )­(OH)2]·2H2O} was shown to be thermodynamically favorable for extraction from alkaline aqueous media with pH = 13.0–13.5. Theoretical calculations also demonstrated thermodynamically favorable coordination to Am­(III).