Precious Metal Extraction with Thiol and Dithioether Derivatives of a Trident Molecule

Transcription

1 olvent Extraction Research and Development, Japan, Vol. 24, No 2, (27) Precious Metal Extraction with Thiol and Dithioether Derivatives of a Trident Molecule Keisuke OHTO*, Hiroaki FURUGOU, Takuyou YOHINAGA, hintaro MORIADA, Hidetaka KAWAKITA and Katsutoshi INOUE Department of Chemistry and Applied Chemistry, Faculty of cience and Engineering, aga University, -Honjo, aga , Japan (Received December 3, 26; Accepted February 23, 27) Thiol and dithioether derivatives of tripodal extraction reagents have been newly prepared and employed to investigate the extraction behavior of precious metals. Both sulfur-containing compounds did not exhibit a ph dependency for precious metal extraction. The thiol type derivative exhibited a high extraction ability for gold(iii), silver and palladium(ii), and a : 3 (extractant : metal) stoichiometry for gold(iii) and silver. The results showed that the thiol derivative exhibited little structural effect due to the strong functionality of thiol group. The dithioether derivative also extracted gold(iii), silver and palladium(ii), however the silver extraction is caused by the structural effect of the tripodal framework, while gold(iii) and palladium(ii) extraction is probably attributed to a partial structural effect. The tripodal derivative possesses a high extraction ability compared with the corresponding monopodal compound. Tripodal and monopodal derivatives showed : and : 2 (extractant : metal) stoichiometry for silver. This means that the tripodal derivative exhibited the structural effect as a size effect, converging effect of the multi functionality, complementary effect. The coordination site of the tripodal derivative was confirmed by H-NMR spectra before and after the silver loading. tripping of the loaded silver from the tripodal derivative was also investigated. Finally, the stepwise separation of silver and palladium(ii) was carried out and roughly achieved by using different eluents.. Introduction Precious metals, consisting of 8 elements, have been employed as jewelry and advanced materials. The classification of precious metals as a valuable metal resource is significant, but access to them is not easy due to the limited number and location of the mines. Recycle of such metals from an urban mine has been of great interest. However, precious metals have various and complicated valencies and species, and the chemical properties of some metal species are very similar. Mutual separation of precious metals is therefore difficult []. Although many conventional reagents have been tested, more effective ones are still required [2-4]. Various extraction reagents based on calixarenes [5,6] and tripodal molecules [4] have been prepared for the separation of metal ions. The so-called trident molecules are tripodal alkyl trimethylol derivatives [7]. The compounds do not have any cyclic structure, but possess three-dimensionally arranged coordination sites and their framework provides high uptake ability and selectivity for target metal ions caused by the structural effect [8-2] and their multifunctionality [3,4]

2 In our previous work, a thioester type of trident molecule was prepared to investigate silver extraction [5]. Precious metals are classified as being soft metals [6] and exhibit a strong affinity with soft atoms such as sulfur. In the present study, thiol and diether derivatives of trident molecules have been prepared for the investigation of precious metal extraction. For silver extraction with the diether derivative, a structural effect was observed supported by stoichiometric analyses and H-NMR spectroscopy. tepwise separation of silver and palladium(ii) was also investigated. The chemical structures of the extraction reagents prepared in the previous and present work are shown in Figure. Figure. Chemical structures of the compounds employed in the present work. 2. Experimental 2. Reagents,,-Tris(thioacetylmethyl)-9-decene ( 8 None{3}CH 2 C(O)CH 3 ) was prepared in a similar manner to that described previously [5]. The extraction reagents employed in the present work were prepared from 8 None{3}CH 2 C(O)CH 3. The synthetic scheme of the reagents is shown in Figure 2. The corresponding monopodal diether molecule was similarly prepared from -octanethiol. -Octanethiol was not employed as the corresponding monopodal thiol reagent due to its volatile and toxic nature. Other chemicals were of analytical grade and employed without further purification. Figure 2. ynthetic scheme for the present extraction reagents.,,-tris(mercaptolmethyl)-9-decene ( 8 None{3}CH 2 H) [7,8] Under nitrogen stream,,,-tris(thioacetylmethyl)-9-decene (4.74 g,.7 mmol), dry THF 2 cm 3, and sodium borohydride (4. g, 5 mmol) were mixed in an ice bath and refluxed for 42 h. After complete reaction confirmed by TLC, M (M = mol dm -3 ) hydrochloric acid (2 cm 3 ) was added to the ice bath to deactivate the excess amount of sodium borohydride. THF was removed in vacuo, then

3 chloroform (2 cm 3 ) was added three times to extract the desired compound. The combined organic solution was washed twice each with M hydrochloric acid ( cm 3 ) and distilled water ( cm 3 ). The organic solution was dried over anhydrous magnesium sulfate, filtered, and the solvent was removed in vacuo. The residue was purified by column chromatography using silica gel with hexane to give a yellow liquid. Yield.75 g (53.7%), TLC (io 2, hexane : chloroform = 3 : v/v) R f =.55, FT-IR (neat) ν C-H 2927 cm -, ν -H 256 cm -, ν C=C 639 cm -,ν C- 43 cm - ; H-NMR (3 MHz, CDCl 3, TM, 298 K) δ.7 (3H, t, H),.3 (2H, m, CH 2 (CH 2 ) 6 C), 2.5 (2H, q, CH 2 (CH 2 ) 6 ), 2.58 (6H, d, CH 2 H), 4.95 (2H, m, CH 2 CH), 5.82 (H, m, CH 2 CH) ; E.A. Found : C, ; H, 9.4%, Calcd. For C 3 H 26 3 : C, 56.6, H, 9.4.,,-Tris(2-methylthio)ethylthiomethyl)-9-decene ( 8 None{3}CH 2 CH 2 CH 2 Me) [9] Under nitrogen stream,,,-tris(mercaptolmethyl)-9-decene (.5 g,.539 mmol) and 2-chloroethyl methyl sulfide (.56 g, 4.85 mmol, 9eq), plus dry DMF 5 cm 3 were mixed, then sodium t-butoxide (.3 g, 3.23 mmol, 6eq) was added to the mixture in the ice bath. After shaking for 4 h in an ice bath, the mixture was further shaken for 24 h at room temperature. After complete reaction was confirmed by TLC, M hydrochloric acid (2 cm 3 ) was added to the ice bath and the mixture was shaken for 24 h to deactivate the excess amount of sodium t-butoxide. Chloroform (2 cm 3 ) was added three times to extract the desired compound. The combined organic solution was washed twice each with M hydrochloric acid ( cm 3 ) and distilled water ( cm 3 ). The organic solution was dried over anhydrous magnesium sulfate, filtered, and the solvent was removed in vacuo to give a brown liquid. Yield.4 g (5.9%), TLC (io 2, hexane : chloroform = 2 : 3 v/v) R f =.6, FT-IR (neat) ν C-H 298 cm -, ν-h 256 cm - (disappeared), νc=c 639 cm -,ν C- 427 cm - ; H-NMR (3 MHz, CDCl 3, TM, 298 K) δ.29 (2H, m, CH 2 (CH 2 ) 6 C), 2.3 (2H, q, CH 2 (CH 2 ) 6 ), 2.4 (9H, s, CH 3 ), 2.67 (6H, s, CCH 2 ), 2.74 (2H, m, CH 2 CH 2 ), 4.95 (2H, m, CH 2 CH), 5.82 (H, m, CH 2 CH) ; E.A. Found : C, ; H, 8.79%, Calcd. For C 22 H 44 6 : C, 52.74, H, (2-Methylthio)ethylthiomethyl)-octane (Hex{}CH 2 CH 2 CH 2 Me) [9] Under nitrogen stream, -octanethiol (.5 g,.3 mmol) and 2-chloroethyl methyl sulfide (2.6 g, 23.5 mmol, 2.3eq), plus dry DMF cm 3 were mixed, then sodium t-butoxide (.8 g, 8.7 mmol,.8eq) was added to the mixture in an ice bath. The further procedure was carried out in a similar manner described for 8 None{3}CH 2 CH 2 CH 2 Me to give a brown liquid. Yield 2.26 g (7.6%), TLC (io 2, hexane : chloroform = : v/v) R f =.55, FT-IR (neat) ν C-H 292 cm -,ν C- 464 cm - ; H-NMR (3 MHz, CDCl 3, TM, 298 K) δ.88 (3H, t, CH 3 C),.27 (H, m, CH 3 (CH 2 ) 5 ),.59 (2H, m, (CH 2 ) 5 CH 2 ), 2.4 (3H, s, CH 3 ), 2.54 (6H, s, (CH 2 ) 5 CH 2 CH 2 ), 2.72 (4H, m, CH 2 CH 2 ). 2.2 Extraction of metal ions Extraction was carried out by the conventional batch method. A typical procedure is described below; The organic phase was prepared by dissolving the extraction reagent in chloroform to 5 mm for trident molecules and to 5 mm for the monopodal molecule. The aqueous phase was prepared by dissolving each metal salt in. M nitric acid and. M HEPE solution, and by mixing them to adjust to

4 the desired ph values. Equal volumes of both phases were mixed and shaken at 38 rpm for 24 h at 33 K. It took 4 h and 8 h to reach equilibrium for silver extraction with,,-tris(acetylsulfanylmethyl)-9-decene [5] and,,-tris(ethylthiomethyl)-9-decene [22]. The extraction mechanism for both extraction reagents was known as solvation exhibiting a very slow extraction rate. haking for 24 h seems to be sufficient. After phase separation, the ph value and metal concentration in the aqueous solution were measured using a ph meter (Orion, φ72a), and an atomic absorption spectrophotometer (abbreviated as AA, himadzu, AA-665) and an inductively coupled plasma atomic emission spectrophotometer (abbreviated as ICP-AE, himadzu, ICP-8), respectively. The sulfur concentration in the aqueous phase after extraction was measured by ICP-AE to check the elution of the extraction reagent. When using the continuous variation (Job s) method, the total concentration of metal and the extraction reagent was adjusted to mm. For the loading test, the extractant concentration was adjusted to mm. For stripping of the loaded metal, an organic solution containing a trident molecule was contacted with the above-mentioned aqueous solution to quantitatively extract silver. The silver-loaded organic solutions were contacted with an equal volume of various eluents for 24 h. The silver concentrations of the strip aqueous solution were measured by AA, and the stripping percentage was calculated by mass balance before and after extraction together with after stripping. The peak shift of the extraction reagent on extraction of silver was also monitored by H-NMR spectroscopy (Jeol, JNM-GX3). Experimental conditions were nearly the same as those described above except for the use of deuterium solvents. After the attainment of equilibrium, the H-NMR peaks of the extraction reagent molecule in the organic phase were recorded. tepwise separation of silver and palladium with 8 None{3}CH 2 CH 2 CH 2 Me was also carried out. 3. Results and Discussion 3. Extraction of precious metals with thiol type of trident molecule The thiol type of the trident molecule, 8 None{3}CH 2 H, possesses three thiol groups, although it is well-known that, in general, the pk a values of alkylthiols are around.7. The elution of 8 None{3}CH 2 H, was preliminarily checked before metal extraction. The elution percentages of 8 None{3}CH 2 H at ph.2 and 5.5 were.7 and.%. The degree of elution was negligible. The effect of the initial ph on the percentage extraction of various metals with 8 None{3}CH 2 H is shown in Figure 3. The fact that the ph values were little changed after the extraction suggests that the metal extraction was not driven by an ion-exchange mechanism, but by coordination with the sulfur atoms. Although the extraction of copper and iron seemed to be by an ion-exchange mechanism, the ph dependent behavior may be caused by complexation with hydroxide ions at high ph. ilver and palladium (II) were quantitatively extracted. The extraction percentages of gold(iii), copper(ii) and iron(iii) were remarkably high, while platinum(iv), rhodium(iii) and lanthanum(iii) were hardly extracted at all at ph to 5. Among the soft metals, the easily coordinated ones were extracted. For determination of the stoichiometry of the extraction reaction, the continuous variation method for gold and silver with 8 None{3}CH 2 H was investigated. The Job s plots for gold and silver with - 8 -

5 8 None{3}CH 2 H are shown in Figure 4(a) and (b), respectively. The maximum ratios of 8 None{3}CH 2 H to the total concentrations of metal and 8 None{3}CH 2 H was.28 for gold and.25 for silver. These results indicate that the gold ion forms : 2 : 3 (extraction reagent : metal) complexes with 8 None{3}CH 2 H, while silver ion forms a : 3 complex. For further confirmation, a loading test was also carried out. The results are shown in Figure 5. The ratio of the initial concentration of 8 None{3}CH 2 H to the loaded %Extraction La(III) Fe(III) Cu(II) Pt(IV) Rh(III) Pd(II) Metal Au(III) Ag Figure 3. Effect of ph on the percentage extraction of various metals with 8 None{3}CH 2 H. [ 8 None{3}CH 2 H] org = 5 mm, [metal] =. mm in. M nitric acid -. M HEPE solution. ph gold became close to 2.4 with increasing gold concentration, while that for the loaded silver became close to 2.7 with increasing silver concentration. This result revealed that the stoichiometries of gold and silver complexes with 8 None{3}CH 2 H were : 2 : 3 (extraction reagent : metal) and : 3, respectively. The results of the loading test were completely consistent with those of the continuous variation method. The ligand provides three thiol groups with high affinity to soft gold and silver. One ion loaded on each group is reasonable. It means that 8 None{3}CH 2 H cannot involve its structural effect due to the strong effect of thiol group. [Au(III)]org / mm.6.4 (a) Au(III).2 : 2 and : [ 8 None{3}CH2H]org [ 8 None{3}CH2H]org+[Au] [ 8 None{3}CH2H]org [ 8 None{3}CH2H]org+[Ag] Figure 4. Job s plots of 8 None{3}CH 2 H for (a) Au(III) and (b) Ag. [ 8 None{3}CH 2 H] org + [metal] = mm,. M HNO 3. [Ag]org / mm : 3 (b) Ag - 8 -

6 tripping of the silver loaded on 8 None{3}CH 2 H was also carried out using various eluents. The results are shown in Figure 6. Hydrochloric acid of more than 3 M and a mixture of hydrochloric acid and thiourea stripped the loaded silver well, however sulfur-containing neutral eluents such as sodium thiocyanate solution hardly stripped silver at all, but M thiourea solution exhibited quantitative stripping. [Au]org [ 8 None{3}CH2H]i,org 5 4 : 3 (Ag) : 2 and : 3 (Au) [Metal]i / mmol dm [ 8 None{3}CH2H]i,org [Ag]org Figure 5. Loadings test for Au(III) and Ag on 8 None{3}CH 2 H. [ 8 None{3}CH 2 H] org = mm. 8 %tripping Water satd. NaCl satd. NaClO4 M HCl 3 M HCl 6 M HCl M HCl+ M TU M TU satd. NaCN Figure 6. Percentage stripping of silver loaded on 8 None{3}CH 2 H with various stripping reagents. At any rate, it was found that the thiol group is too strong to allow observation of any structural effects for the ligand from the results of the continuous variation method, the loading and stripping tests Extraction of precious metals with the dithioether type of trident molecule The effects of the initial ph on the percentage extraction of various metals with 8 None{3}CH 2 CH 2 CH 2 Me and Hex{}CH 2 CH 2 CH 2 Me are shown in Figure 7(a) and (b), respectively. Although the extraction of copper and iron seemed to occur through an ion-exchange mechanism as with 8 None{3}CH 2 H, the ph dependent behavior may be caused by complexation with hydroxyl ions at high ph, because dithioether compounds are neutral and metal extraction was not driven by ion-exchange. These results supported that the extraction of copper and iron is not driven by an ion-exchange mechanism, but by coordination

7 (a) 8 %Extraction La(III) Fe(III) Cu(II) Pt(IV) Metals Rh(III) Pd(II) Au(III) Ag 5 3 ph (b) 8 %Extraction La(III) Fe(III) Cu(II) Pt(IV) Metals Rh(III) Pd(II) Au(III) Ag 5 3 ph Figure 7. Effect of ph on the percentage extraction of various metals with (a) 8 None{3}CH 2 CH 2 CH 2 Me and (b) Hex{}CH 2 CH 2 CH 2 Me. [metal] =. mm in. M nitric acid -. M HEPE solution, [ 8 None{3}CH 2 CH 2 CH 2 Me] org = 5 mm in chloroform, [Hex{}CH 2 CH 2 CH 2 Me] org = 5 mm in chloroform. ilver and palladium (II) were quantitatively extracted with 8 None{3}CH 2 CH 2 CH 2 Me at ph to 5. The extraction of gold(iii), copper(ii), iron(iii), and platinum(iv), rhodium(iii) were also observed, while lanthanum(iii) was very poorly extracted. Among the soft metals, the easily coordinated ones were extracted. Compared with Hex{}CH 2 CH 2 CH 2 Me, the percentage extraction of metals except iron and gold with 8 None{3}CH 2 CH 2 CH 2 Me were higher probably due to the structural effect. The stoichiometries of gold, silver and palladium with 8 None{3}CH 2 CH 2 CH 2 Me were also determined by the continuous variation method. The Job s plots of gold, silver and palladium with 8 None{3}CH 2 CH 2 CH 2 Me are shown in Figure 8(a) - (c), respectively. Maximum ratios of 8 None{3}CH 2 CH 2 CH 2 Me to total concentrations of metal and 8 None{3}CH 2 CH 2 CH 2 Me was.2 for gold,.5 for silver and.38 for palladium. The results indicate that gold, silver and palladium form :

8 4, :, and probably 2 : 3 (extraction reagent : metal) complexes with 8 None{3}CH 2 CH 2 CH 2 Me. The ligand provides six sulfur atoms with a high affinity to soft metals. [Au(III)]org / mm (a)au(iii) : [ 8 None{3}CH2C2H4Me]org [ 8 None{3}CH2C2H4Me]org+[Au] [Pd(II)]org / mm.4 (c) Pd(II) : [ 8 None{3}CH2C2H4Me]org [ 8 None{3}CH2C2H4Me]org+[Pd] For confirmation of the obtained stoichiometries, loading tests were also carried out. The results are shown in Figure 9. The ratio of the initial concentration of 8 None{3}CH 2 CH 2 CH 2 Me to the loaded gold, silver and palladium became close to 2.,., and.5 with increasing metal concentration, respectively. These results for silver and palladium were consistent with the results obtained by the continuous variation method for the metals and consequently support the stoichiometries, while that for gold was not consistent with the continuous variation method data, the reason for which is not clear. The stoichiometries of gold and [Ag]org / mm (b) Ag. : [ 8 None{3}CH2C2H4Me]org [ 8 None{3}CH2C2H4Me]org+[Ag] [Metal]org [ 8 None{3}CH2C2H4Me]i,org Figure 8. Job s plots of 8 None{3}CH 2 CH 2 CH 2 Me for (a) Au(III), (b) Ag and (c) Pd(II). [ 8 None{3}CH 2 CH 2 CH 2 Me] org + [metal] = mm,. M HNO : (Ag) : 2 (Au) 2 : 3 (Pd) [Metal]i / mmol dm -3 Figure 9. Loadings tests of Au(III), Ag and Pd(II) on 8 None{3}CH 2 CH 2 CH 2 Me. [ 8 None{3}CH 2 CH 2 CH 2 Me] org = mm

9 palladium with 8 None{3}CH 2 CH 2 CH 2 Me indicate that both ions were not surrounded by the many sulfur atoms of the three sets of dithioether groups inside a single trident molecule due to the strong affinity or discrepant coordination geometry, whereas silver with a : stoichiometry, is probably surrounded by sulfur atoms inside a single trident molecule. The stoichiometries of gold and silver with 8 None{3}CH 2 CH 2 CH 2 Me at a lower level of 8 None{3}CH 2 CH 2 CH 2 Me concentration were not fixed as shown above, because 8 None{3}CH 2 CH 2 CH 2 Me provides excess sulfur atoms for complexation with gold and silver, and one of them has a sufficiently strong affinity for these ions. ilver stoichiometry was further checked by slope analysis. The effects of the extraction reagent concentration on the silver distribution ratio with 8 None{3}CH 2 CH 2 CH 2 Me together with Hex{}CH 2 CH 2 CH 2 Me are shown in Figure (a) and (b), respectively. The plots for 8 None{3}CH 2 CH 2 CH 2 Me lie on a straight line with a slope of, while those for Hex{}CH 2 CH 2 CH 2 Me lie on a straight line with a slope of 2. The results indicate that the trident and monopodal molecules form : and 2 : (extraction reagent : metal) complexes with silver, respectively. The fact that two molecules of Hex{}CH 2 CH 2 CH 2 Me are required for silver uptake suggests that silver can be extracted outside the pseud-cavity of 8 None{3}CH 2 CH 2 CH 2 Me two of whose dithioether groups were used. In order to elucidate the coordination site of silver in 8 None{3}CH 2 CH 2 CH 2 Me, the peaks of the H-NMR spectra for 8 None{3}CH 2 CH 2 CH 2 Me before and after silver loading, together with those for Hex{}CH 2 CH 2 CH 2 Me were checked. A peak shift is mainly caused by the decreased electron density with the silver loading on a sulfur atom. (Protons close to the benzene ring can be shifted by an electronic shielding effect [2,2].) The relationship between the peak positions of 8 None{3}CH 2 CH 2 CH 2 Me and Hex{}CH 2 CH 2 CH 2 Me in the H-NMR spectra and the silver loading percentage with showing the observed protons is shown in Figure (a) and (b), respectively. All of the observed protons for Hex{}CH 2 CH 2 CH 2 Me and methyl and ethylene protons for 8 None{3}CH 2 CH 2 CH 2 Me were shifted to a low magnetic field after silver loading, while the rest of the methylene protons next to the branched carbon atom.4.2 (a).5 (b) log D slope = [ 8 None{3}CH2C2H4Me]e,org / mm.5 slope = [Hex{}CH2C2H4Me]e,org / mm Figure. Effect of extraction reagent concentration on the silver distribution ratio with (a) 8 None{3}CH 2 CH 2 CH 2 Me and (b) Hex{}CH 2 CH 2 CH 2 Me. [Ag + ] =. mm in. M nitric acid. log D

10 3 2.8 (a) (b) / ppm / ppm %Loading %Loading CH 2 CH(CH 2 ) 7 C CH 3 (CH 2 ) 6 Figure. Relationship between the peak positions of (a) 8 None{3}CH 2 CH 2 CH 2 Me and (b) Hex{}CH 2 CH 2 CH 2 Me in the H-NMR spectra and percentage silver loading. : -CH 3, : -CH 2 CH 2 -, : C-CH 2 -, [ 8 None{3}CH 2 CH 2 CH 2 Me] = 5 mm in CDCl 3, [Hex{}CH 2 CH 2 CH 2 Me] = 5 mm in CDCl 3, [Ag + ] = mm in. M nitric acid. for 8 None{3}CH 2 CH 2 CH 2 Me were hardly shifted. For Hex{}CH 2 CH 2 CH 2 Me, even the end methylene protons of the octyl group whose electron density was not drastically changed after silver loading due to the electron-donating property of the long alkyl chain were slightly shifted. These results mean that the coordination site of 8 None{3}CH 2 CH 2 CH 2 Me consists of only three sulfur atoms next to methyl groups and not those close to the branched carbon atom, while that of Hex{}CH 2 CH 2 CH 2 Me uses both sulfur atoms. The peak shift differences between the free ligand and the fully loading ligand for the trident molecule are higher than those for the monopodal compound. This means that the peak shift is caused not only by the decreased electron density but also the electronic shielding effect with the structural change after silver loading. Based on these results, the silver complexes are proposed in Figure 2. CH 3 (CH 2 ) 6 CH 2CH(CH 2 ) 7 C Ag + Ag + (CH 2 ) 6 CH 3 Figure 2. Proposed silver complexes of 8 None{3}CH 2 CH 2 CH 2 Me and Hex{}CH 2 CH 2 CH 2 Me

11 tripping of the silver loaded on 8 None{3}CH 2 CH 2 CH 2 Me was also carried out using various eluents. The results are shown in Figure 3. Compared with 8 None{3}CH 2 H, all eluents except water and a saturated sodium perchlorate solution were effective for stripping. ulfur-containing eluents are not preferable for stripping, even though they can completely strip the silver. The fact that saturated sodium chloride solution exhibited a high stripping percentage is interesting. %tripping Water satd. NaCl satd. NaClO4 M HCl 3 M HCl 6 M HCl M HCl+ M TU M TU satd. NaCN Figure 3. Percentage stripping of silver loaded on 8 None{3}CH 2 CH 2 CH 2 Me with various stripping reagents. From a practical point of view, the stepwise recovery of silver and palladium was carried out by changing the eluents after metal loading. The flow sheet is shown in Figure 4. It was suggested that 8 None{3}CH 2 CH 2 CH 2 Me for stepwise recovery of both metals can be effective by the optimization of the treating conditions. Organic phase 5 cm 3 of 5 mm 8 None{3}CH 2 CH 2 CH 2 Me Extraction % Ag 99.6%, Pd 8.% Aqueous phase 5 cm 3 of. mm Ag + + Pd 2+ in. M HNO 3 solution 2 h haking 4 cm 3 Organic phase 36 cm atd. NaCl soln. tripping % Ag 74.9%, Pd % 4 h haking tripping % Ag 6.9%, Pd 6.4% 2 cm 3 Organic phase 5 h haking 6 cm 3 M TU + M HCl Figure 4. Percentage stripping of silver ion loaded on 8 None{3}CH 2 CH 2 CH 2 Me with various stripping reagents

12 4. Conclusion Thiol and dithioether type tripodal extraction reagents have been prepared and employed to investigate their extraction behavior for precious metals. Although both derivatives exhibited a high extraction ability for gold(iii), silver and palladium(ii), the thiol derivative exhibited little structural effect due to the strong thiol coordination ability, while the dithioether derivative exhibited a structural effect for silver. The coordination site of the tripodal derivative was confirmed by H-NMR spectra before and after silver loading. The stepwise recovery of silver and palladium(ii) was also investigated by using the dithioether derivative and different stripping reagents. References ) J. hibata, A. Okuda, higen to ozai, 8, -8 (22). (in Japanese) 2) M. Iwakuma, T. Oshima, Y. Baba, olvent Extr. Res. Dev., Jpn., 5, 2-35 (28). 3) H. Narita, M. Tanaka, J. MMIJ, 27, 75-8 (2). (in Japanese) 4) Y. Ueda, K. Ohto, Bunseki, 492, (25). (in Japanese) 5) K. Ohto, olvent Extr. Res. Dev., Jpn., 7, -8 (2). 6) K. Ohto, Ion Exch. olv. Extr., 2, 8-27 (24). 7) R. Yamaguma, A. Yamashita, H. Kawakita, T. Miyajima, C. Takemura, K. Ohto, N. Iwachido, ep. ci. Technol., 47, (22). 8) K. Ohto, H. Nakagawa, H. Furutsuka, T. hinohara, T. Nakamura, T. Oshima, K. Inoue, olvent Extr. Res. Dev., Jpn.,, 2-34 (24). 9) C. Yamamoto, H. eto, K. Ohto, H. Kawakita, H. Harada, Anal. ci., 27, (2). ) Y. Ueda,. Morisada, H. Kawakita, K. Ohto, olvent Extr. Res. Dev., Jpn., 2, (23). ) Y. Ueda,. Morisada, H. Kawakita, K. Ohto, olvent Extr. Res. Dev., Jpn., 2, 9-9 (24). 2) Y. Ueda,. Morisada, H. Kawakita, K. Ohto, ep. ci. Technol., 5, (26). 3) K. Ohto, A. Yamashita, Y. Ueda, R. Yamaguma,. Morisada, H. Kawakita, olvent Extr. Res. Dev., Jpn., 2, 73-8 (24). 4) K. Ohto, Y. Hashimoto, Y. Ueda, A. Yamashita,. Morisada, H. Kawakita, olvent Extr. Res. Dev., Jpn., 23, 8-86 (26). 5) H. Furugou, K. Ohto, H. Kawakita, H. Harada, K. Inoue, Ars eparatoria Acta, 5, (27). 6) R. G. Pearson, J. Am. Chem. oc., 85, (963). 7). W. Chaikin, W. G. Brown, J. Am. Chem. oc., 7, (949). 8) T. uzuki, Y. Nagano, A. Kouketsu, A. Matsumura,. Maruyama, M. Kurotani, H. Nakagawa, N. Miyata, J. Med. Chem., 48, 9-32 (25). 9) J. Buter, R. M. Kellog, J. Org. Chem., 46, (98). 2) K. Ohto, E. Murakami, T. hinohara, K. hiratsuchi, K. Inoue, M. Iwasaki, Anal. Chim. Acta, 34, (997). 2) K. Ohto, H. Yamaga, E. Murakami, K. Inoue, Talanta, 44, 23-3 (997). 22) K. Ohto, T. Yoshinaga, H. Furugou, H. Kawakita, H. Harada, K. Inoue, unpublished data