Precious metal extraction by N,N,N′,N′-tetraoctyl-thiodiglycolamide and its comparison with N,N,N′,N′-tetraoctyl-diglycolamide and methylimino-N,N′-dioctylacetamide
Introduction
Studies using diglycolamide compounds and related compounds have been widely reported, particularly related to the partitioning of high-level radioactive waste (Sasaki et al., 2001, Sasaki and Tachimori, 2002, Zhu et al., 2004, Yaita et al., 2004, Modolo et al., 2008, Magnusson et al., 2009, Matloka et al., 2005, Hirata et al., 2003, Ansari et al., 2005, Gujar et al., 2012). The most widely studied compound is N,N,N′,N′-tetraoctyl-diglycolamide (TODGA), which was initially reported by Sasaki et al.1 TODGA has very high distribution ratios for actinide (An) and lanthanide (Ln) from nitric acid to n-dodecane. TODGA was modified to have a high loading capacity for Nd extraction (N,N,N′,N′-tetradodecyl-diglycolamide, TDdDGA) (Sasaki et al., 2005, Sasaki et al., 2007a), relatively low D(An,Ln) values due to easy stripping (N,N,N′,N′-tetra-2-ethylhexyl-diglycolamide, TEHDGA) (Sharma et al., 2008, Sharma et al., 2010), and high solubility in water to use it as a masking agent (N,N,N′,N′-tetraethyl-diglycolamide, TEDGA, water-miscible) (Sasaki et al., 2006, Sasaki et al., 2007b, Vanel et al., 2012). These compounds are also available for use in the partitioning of HLW. For this purpose, methylimino-N,N′-dioctylacetamide (MIDOA) was first synthesized in 2009 (Sasaki et al., 2007c). This compound has a nitrogen donor atom instead of the oxygen atom in TODGA. MIDOA also exhibits high D values for oxonium anions, Mo, Tc, and Re, from nitric acid to n-dodecane (Sasaki et al., 2007c, Sasaki et al., 2009, Sasaki et al., 2013). Therefore, the central frame, O, or (NCH3)(CH2CONR2)2 has a very strong affinity for various metals. These compounds are known to be tridentate ligands, and two amidic oxygen and one ether oxygen (nitrogen) work as donor atoms. These extractants have the following advantages: a neutral donor that extracts metals in acidic solution; high solubility into n-dodecane; non-polar solvent; and no precipitation without the neutralization of the valence of metal ions.
With regard to these, an analogous compound including a sulfur donor, N,N,N′,N′-tetraalkyl-thiodiglycolamide (TDGA) (Ruhela et al., 2010, Ito et al., 2013, Sasaki et al., 2016, Mowafy et al., 2015, Sasaki et al., 2015), was introduced by Sasaki et al. (Sasaki and Tachimori, 2002). However, metal extraction using TDGA was incomplete. TDGA was examined for the extraction of Ln, An, and platinum metal groups from nitric acid and/or hydrochloric acid and recently for the extraction of Ag from HNO3, HClO4, and H2SO4 into n-dodecane (Ruhela et al., 2010, Ito et al., 2013, Sasaki et al., 2016). In this study, the extraction of other soft acid metals in addition to Ag by TDGA was investigated. Using these data, the D values of TODGA and MIDOA were compared with those of TDGA, allowing the differences in extractability among S, N, and O donors to be discussed more clearly. The extractants used herein, including 2,2′-thiodiacetic acid (commercially available for IR measurement), are shown in Fig. 1. In addition to examining their extractability, spectrochemical studies were employed to reveal the chemical forms of metal-TDGA complexes. Spectrochemical studies using 1H NMR and IR in relation to solvent extraction have been reported previously (Martin et al., 1986, Takeuchi et al., 1994, Paiva and Lemaire, 1996, Tarabanko et al., 2007, Fan et al., 2009, Yamaguma et al., 2012, Kandil et al., 2012, Sasaki et al., 2007d, Saeki et al., 2012). These studies can be assigned the spectrum as existence of hydrogen atoms and absorbance by vibrational energy for the functional groups; therefore, peak shifts before and after metal extractions might be considered as an evidence of the chemical bond between metals and the corresponding functional groups of the extractant. In addition, molecular simulation by chemical calculation was performed to help elucidate the mechanism of solvent extraction. All extractants, metal complexes, metals, and anions were modeled using the density functional theory (DFT) in the Gaussian03 program, and their heats of formation were obtained. To elucidate the relationship between metal complex stability and metal extraction by TDGA, the heats of formation of the metal complexes were calculated and compared with the apparent extraction constants.
Section snippets
Reagents
The extractants TDGA, TODGA, and MIDOA were purchased from Wako Pure Chemical Industries, Ltd. The synthetic method for these extractants is reported elsewhere (Siddall and Good, 1967, Sasaki and Choppin, 1996). Extractants with > 98% purity were used for solvent extraction. Standard solutions (1000 ppm) of metals (Wako Pure Chemical Industries, Ltd.) were used for solvent extraction. The radioactive elements Tc(VII), U(VI), and Pu(IV) used in this study were obtained from Sceti Company LTD.
Extraction of various metals by TDGA
We investigated the extraction of various metals by TDGA. Twenty-eight selected metals were examined for extraction in 3 M HNO3 and 0.2 M TDGA/n-dodecane. The chosen metal ions belong to the hard, soft, and oxonium anions. The results, shown in Table 1, can be divided into three groups: hardly extracted metals (D < 0.1), extractable metals (D = 0.1–10), and well-extractable metals (D > 10). Pd(II), Hg(II), Au(III), Ag(I), and Pu(IV) were found to be well-extractable metals. Some papers suggest that
Summary
The results of the present study are summarized as follows: (1) TDGA can effectively extract Ag(I), Pd(II), Au(III), and Hg(II). (2) Various mineral acids can be applied to extract these precious metals using TDGA. (3) The extraction results obtained using TDGA, TODGA, and MIDOA confirmed the metal complexation ability of their respective donor atoms: S, N, and O. (4) IR and 1H NMR measurements revealed that etheric sulfur and amidic nitrogen donors in TDGA complex with metals. (5) The ΔHf
Acknowledgments
The authors deeply thank the staff of Wako Pure Chemical Industries, Ltd., TCI chemicals, and Yamamoto Precious Metal Co., Ltd. and Dr. S. Suzuki of the Japan Atomic Energy Agency for NMR measurements.
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- 1
The University of Kitakyushu, Hibikino 1-1, Kitakyushu 808-0135, Japan.