New Heterocyclic Dithioether Ligands for Highly Selective Separation

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New Heterocyclic Dithioether Ligands for Highly Selective Separation and Recovery of Pd(II) from Acidic Leach Liquors of Spent Automobile Catalyst Kannan Senthil, Yoshihiko Kondo, Uichi Akiba, Kenshu Fujiwara, and Fumio Hamada Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03874 • Publication Date (Web): 08 Jan 2017 Downloaded from http://pubs.acs.org on January 8, 2017

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Industrial & Engineering Chemistry Research

Table of Contents (TOC) Graphic Selective extraction of Pd(II) with BMDTE and BMTDTE from the ACR solution containing six metal ions (i.e., Pt(IV), Pd(II), Rh(III), La(III), Al(III) and Ce(II)).

ACS Paragon Plus Environment

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New Heterocyclic Dithioether Ligands for Highly Selective Separation and Recovery of Pd(II) from Acidic Leach Liquors of Spent Automobile Catalyst Kannan Senthil,a Uichi Akiba,b Kenshu Fujiwara,b Fumio Hamada,b Yoshihiko Kondob* a

Center for Regional Revitalization in Research and Education, Akita University, 1-1 Tegatagakuen-cho Akita 010-8502, Japan

b

Graduate School of Engineering Science, Akita University, 1-1 Tegatagakuen-cho Akita 010-8502, Japan

Abstract In this paper, the solvent extraction behaviour of Pd(II) from a hydrochloric acid solution with heterocyclic dithioether ligands, namely, 1,2-bis((5-methyl-1,3,4-thiadiazol-2-yl)thio)ethane (BMDTE)

and

1,2-bis((5-(methylthio)-1,3,4-thiadiazol-2-yl)thio)ethane

(BMTDTE)

were

investigated, for the first time, as potential extractants by using chloroform as diluent. Various experimental parameters such as effects of the concentration of hydrochloric acid, extractant concentration, hydrogen ion, chloride ion, and diluents on the extraction of Pd(II) has also been investigated in detail. By comparing the extraction time, BMDTE exhibits very rapid and


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quantitative extraction of Pd(II) even with 5 min than BMTDTE (30 min). The loading experiments indicated that the maximum loading capacity of BMDTE and BMTDTE for Pd(II) was determined to be 124 ppm and 118 ppm, respectively. The stoichiometry of the extracted species during extraction was confirmed by 1H NMR and Job's method analysis. Both of the two extractants exhibits high extraction percentage (>99%) and selectivity towards Pd(II) from an automotive catalyst residue (ACR) solution comprising platinum group (PGMs) metal ions (i.e., Pd(II), Pt(IV), Rh(II), La(III), Al(III) and Ce(III)). After Pd(II) stripping from loaded organic phase, the water washed stripped organic phase can be reused for further extracting Pd(II). The numbers of theoretical stages needed for the effective recovery and stripping of Pd(II) has been determined by McCabe Thiele’s plot. Studies of the reusability of the both extractants have been found to be no significant loss in activity even after five cycles of successive extraction and stripping experiments. Based on the extraction protocol, these two extractants may be promising candidates for high selectivity extraction of Pd(II) from ACR solution. Keywords: Heterocyclic dithioether ligands; Solvent extraction; Palladium(II);Recovery; automotive catalyst leach liquors; Recycling. 1. Introduction In recent years, there has been a growing interest in noble metals due to their demands, mainly natural resources of platinum group metals (PGMs), namely, palladium (Pd), platinum (Pt), rhodium (Rh), ruthenium (Ru), etc. because of their wide range of industrial applications such as automobile exhaust emission control catalysts, petroleum and electronic industries, in chemical, and pharmaceutical.1,2 Palladium is slowly replacing other PGMs as a lower-price material and hence can be used in different industries such as metallurgy, catalysts in hydrogenation and dehydrogenation of organic processes, fuel cell, dentistry, photography,


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coinage, in Jewellery making, and auto catalysts, etc.3-6 Nowadays palladium has also been used in hydrogen purification, value added components in metal alloys, and various electrical equipment. These partially correspond to the high demand causes the high price of palladium in recent years.6,

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Demand for PGMs, particularly palladium has been expected that the annual

gross demand in autocatalysts will increase by as much as 8.6 million ounces between 2014 and 2017.8 The natural occurrences of palladium have found rather scarce within the earth's crust is only 0.0063 ppm, this places it as one of the least metals on earth and the available resources are geographically exist in limited locations in South Africa, North America, Russia, and Zimbabwe. 9-16

Due to those facts, there is broad acceptance to recycling is compulsory from appropriate waste spills /or secondary sources such as spent automotive catalysts for economic, environmental and preservation of these critical metals from their natural resources.17,19 Literature survey indicates that extraction and recovery process of PGMs are extremely complicated because of their constituent chemical similarities and it is substituted with other metals species. Over the past few years, various methods have been pointed out and utilized for the recovery of individual Pd(II) from their various chloride-based leaching solutions such as biological methods,20 electro- deposition,21 ion exchange,22 liquid‐liquid extraction,23 membrane separation and adsorption,24 and cloud point extraction solid-phase extraction.25,

26

However,

some factors that restrict wider applicability of these techniques include initial cost of instruments, limited budgets, consumable and costly maintenance. Among the different methods for separation, liquid-liquid extraction has been widely used for the separation and recovery of PGMs from mineral ores, 27 spent catalysts in hydrometallurgical processes,28, 29 and reprocessing of nuclear fuels.30 Besides, it offers several advantages due to its high extraction efficiency with


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a selectivity of metals, high purity, and complete removal of metals is possible by the multiple extraction stages.31 According to Pearson's Hard Soft [Lewis] Acid Base (HSAB) principle, palladium is a soft metal, and it can be effective co-ordination towards with soft ligands containing 'S' and 'N' as donor atoms. Recently, it was established that many variety of ligands had been synthesized and extensively investigated for their extraction efficiency and selectivity behavior towards Pd(II), namely dibutyl sulfoxide (DBSO),32 dihexyl and dioctyl sulfides (DHS and DOS),33,34 alkylated monoamides,35 8-hydroxyquinoline,36 thiocarbamoyl-functionalized thiacalix[n]arenes,37 N,N′dimethyl-N,N′-dicyclohexyl ethylhexyl)

thiodiglycolamide

thiodiglycolamide

(DMDCHTDGA),38

(T(2EH)TDGA),12

N,N,N′,N′-tetra

(2-

N,N,N’,N’-tetraoctyl-thiodiglycolamide

(TOTDGA),39 etc. However, the use of those ligands suffers from several disadvantages such as pH sensitivity, poor solubility, slow extraction time, co-extraction of other metals, and instability in acidic medium. Consideration of these facts provoked us to search for new ligands having soft donor atoms 'S' with outstanding extraction rate as well as high extraction selectivity. In this study, we report our systematic studies on the use of the alkyl (-CH3) and thioalkyl (-SCH3) groups as electron donors substituents on the heterocyclic dithioether ligands BMDTE and BMTDTE for the liquid‐liquid extraction of Pd(II) from hydrochloric acid medium have been carried out. For this purpose, the influence of various experimental condition on the extraction of Pd(II) has been investigated in more detail. Furthermore, the mechanism of the extracted species has been determined by using spectroscopies and Job’s continuous variation method. From a practical point of view, selective recovery of Pd(II) in the presence of Pt(IV), Rh(II), La(III), Al(III) and Ce(III) from acidic leach liquors of ACR have also been reported. In addition, stripping of extracted Pd(II) from the organic phase and its reusability cycles were


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performed. 2. Experimental section 2.1 Reagents 5-Methyl-1,3,4-thiadiazole-2-thiol

and

2-Mercapto-5-methylthio-1,3,4-thiadiazole

were

purchased from Wako Pure Chemical, Japan and used as received. PdCl2 (Kanto Chemical, Japan), PtCl4 (Acros Organics, Japan), RhCl3·3H2O, AlCl3, LaCl3·7H2O, and CeCl3·7H2O (Nacalai Tesque, Japan) were commercially available and were used without additional purification. All other chemicals and reagents used in this study (potassium carbonate, 1, 2dibromoethane, ethanol, acetone, chloroform, diethyl ether, etc.) were of analytical grade and used without further purification. Standard aqueous solutions (1000 ppm) of Pd(II), Pt(IV), Rh(III), La(III), Al(III) and Ce(III) were purchased from Wako Pure Chemicals and were used by appropriate dilution of standard stock solutions using Millipore water. 2.2 Instruments and analysis The 1H NMR spectra of the ligands and their complexes were obtained in CDCl 3 with an Bruker DPX 300 MHz NMR Spectrometer employing TMS as an internal reference at 300 K. The elemental analysis of carbon, hydrogen, and nitrogen (CHN analysis) was carried out on a Model CE-440M CHN/S elemental analyzer (Systems Engineering, Japan). The mass spectroscopy analysis was carried out using a Matrix-assisted laser desorption ionization time-offlight mass spectroscopy (MALDI-TOF MS, Bruker auto flex speed-AK1). The UV-Vis absorption spectra of ligands and their complexes were measured using UV-2501 PC Shimadzu. The Pd(II) content in aqueous phases, before and after extraction was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES, SII Nano Technology Inc.).


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2.3. Material synthesis The syntheses of BMDTE and BMTDTE was carried out according to the procedure reported earlier with some modifications,40 and the structure is shown in Scheme 1. However, the synthesis steps are describing here, which makes it easier for reader understanding. In a 150 mL round-bottomed flask, a mixture of 5-Methyl-1, 3, 4-thiadiazole-2-thiol or 2-Mercapto-5methylthio-1, 3, 4- thiadiazole (20 mmol) and potassium carbonate (20 mmol) in 75 mL of acetone was heated for 30 min with constant stirring. After heated, the reaction mixture was cooled to room temperature and 1, 2-dibromoethane (10 mmol) was added to the mixture slowly. After the addition was complete, the reaction mixture was then refluxed for an additional 4 h and concentrated in vacuo. The residue was dissolved in 100 mL of chloroform and extracted with 2.0 N hydrochloric acid and then washed with water. The combined chloroform extracts were evaporated to dryness to yield a white crystalline product. BMDTE: Yield: 2.3 g (79.18%). Anal. Calcd for C8H10N4S4 (%): C, 33.08; H, 3.47; N, 19.26. Found: C, 33.20; H, 3.63; N, 19.23. MALDT-TOF MS: Calcd: 290.45, Found: 291.42 [M]+. BMTDTE: Yield: 2.9 g (81.78%). Anal. Calcd for C8H10N4S6 (%): C, 27.10; H, 2.84; N, 15.80. Found: C, 26.98; H, 2.93; N, 15.64. MALDT-TOF MS: Calcd: 354.58, Found: 355.39 [M]+. The corresponding mass spectra of BMDTE and BMTDTE can be found in Figure S1 and S2 (Supplementary Information).


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Scheme 1.Synthesis of BMDTE and BMTDTE. 2.4 Extractions studies The extractions were conducted with a certain amount of BMDTE or BMTDTE dissolved in chloroform was employed as organic phase and the solution containing desired concentration of Pd(II) or mixed metal ions of Pd(II), Pt(IV), Rh(III), La(III), Al(III) and Ce(III) in hydrochloric acid was employed as aqueous phase. An equal volume (5 mL each) of the two phases were mixed in a glass tube (50 mL) and horizontally shaken at 300 strokes per min at an appropriate time. The mixture was then separated by a centrifuge at 2000 rpm for 3 min. All the extractions experiments were carried out at room temperature. Each experiment was performed 2 times and the percentage relative standard deviation (RSD%) for the results did not exceed 5%. The metal ions concentration in the aqueous phase was measured by ICP-AES. The concentration of metal ions in the organic phase was determined from the difference between the concentration in the aqueous phase before and after extraction. The extraction and stripping percentage of metal ions are defined as Eqs. (1) and (2).


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weight of a metal ions extracted into organic phase x100 initial weight of a metal ions in the aqueous phase before extraction

(1)

weight of a metal ions stripped into aqueous x100 initial weight of a metal ions in the organic phase before stripping

(2)

Extraction percentages (% E ) =

Stripping percentage (% S ) =

3. Results and discussion 3.1 Effect of hydrochloric acid concentration The acid concentration of aqueous phase plays key parameters in the metal ions extraction process since high acid concentration is involved in the hydrometallurgical industry. To evaluate the influence of aqueous HCl in the concentration range from 0.01 to 1.0 M HCl on the extraction of 1.0 mM Pd(II) was investigated using 1.5 mM of BMDTE and BMTDTE as extractants in chloroform as the organic phase. The extraction results are displayed in Figure 1. It can be seen that the alkyl (-CH3) substituents in BMDTE showed better %E value, which was greater than 99% between 0.01 M and 0.4 M HCl and 97-89% in 0.5-1.0 M HCl, while for thioalkyl (-SCH3) substituent in BMTDTE shows higher than 99% in 0.01-0.1 M HCl and then decreased to 40% with increasing concentration from 0.2 to 1.0 M HCl. The significant difference in extraction behavior between BMTDTE and BMTDE can be explained on the basis of donor substituents such as -CH3 or -SCH3 into the heterocyclic part. It is notable that the introduction of the donor -CH3 compared to -SCH3 would allow these dithioether groups to act as additional donor sites causes the extraction rate of Pd(II) and increase the stability of Pd(II) complexes. Therefore, the structural effect of the coordination site can significantly affects the extraction behavior of the extractant. From these observations, it is concluded that the extraction efficiency of both extractants is dependent on the HCl concentration. Hence, 0.1 M HCl


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concentration was recommended for subsequent experiments, in order to achieve the quantitative extraction of Pd(II). -Figure 13.2 Effect of contact time Contact time is an essential parameter because an extractant can be of practical use only if it has a lesser contact time. Hence, the effect of phase contact time on the extraction of Pd(II) has been studied by shaking organic phase containing 1.5 mM of extractants (BMDTE and BMTDTE) in chloroform with fixed concentration of 1.0 mM Pd(II) in 0.1 M HCl as the aqueous phase. The contact time between both phases were examined in the 0.5 to 35 min time range. The results obtained are depicted in Figure 2, indicating that the rate of extraction of Pd(II) was quite rapid with the contact time of 5 min for BMDTE, while extractant BMTDTE shows a maximum extraction yield of Pd(II) after 30 min. At this time, the extraction values of extractants BMDTE and BMTDTE was 99.9% and 99.8%, respectively. The contact time is compared with some other extractants reported in the literature (Table S1 (Supporting Information)).41-46 Therefore, a contact time of 45 min has been used subsequently in order to ensure complete extraction equilibrations. -Figure 23.3 Influence of extractant concentration on extraction of palladium(II) Since the extractant concentration in liquid‐liquid extraction has a significant impact on the metal extraction. Also, low extractant concentration is a general requirement for any commercial extraction process. To determine the effect of extractant concentration on Pd(II) extraction, the experiment was carried out at fixed 1.0 mM Pd(II) in 0.1 M HCl and extractants BMDTE and


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BMTDTE concentration was varied between 0.1 and 3.0 mM in chloroform. The experimental results (Figure 3) revealed that the Pd(II) extraction (%E) increases with an increase of extractant concentration and reaches a maximum at the extractant concentration of 1.0 mM for both extractants. Further increases (beyond 1.0 mM) the extractant concentration has no changes in the extraction percentage of Pd(II) due to excess of extractant. Maximum extraction values have been found to be 99.8% and 99.6% at 1.0 mM of extractants BMDTE and BMTDTE, respectively. Hence, it can be suggested that the stoichiometry of the species may be in the organic phase is 1:1 metal:extractant. Thus, in the subsequent experiments, 1.5 mM of extractants (BMDTE and BMTDTE) was used in order to completely extract of Pd(II) from the aqueous phase. However, there was no adverse effect if one can use excess of extractant. -Figure 33.4 Effect of H+ and Cl- concentration on palladium(II) extraction To discern the role of H+ ion concentration on the extraction of Pd(II), the experiments was studied by varying H+ ion concentration from 0.01 M to 1.0 M using HCl with a fixed chloride concentration (1.0 M) in the aqueous phase using LiCl. The corresponding experimental data are plotted in Figure 4. With 1.5 mM both of the two extractants in chloroform, with increase of H+ ion concentration the Pd(II) extraction efficiency is almost keep constant under the studied experimental conditions. The above observation indicates that the H+ ion had no influence in the Pd(II) extraction from hydrochloric acid. This is because the both of the two extractants are neutral and contain no hydrogen ions.47,

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The influence of Cl- ion concentration on the

extraction of Pd(II) was studied with organic phase containing 1.5 mM of extractant (BMDTE and BMTDTE) and 1.0 mM Pd(II). The Cl- ion concentrations in the aqueous phase was varied between 0.05 and 1.0 M using LiCl at constant [H+] of 0.1 M. The results are demonstrated in


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Figure 5. It can be seen that the %E of Pd(II) from 0.05 to 0.1 M Cl is above 99% and then the further increase in Cl- concentration from 0.2 to 1.0 M causes decrease in extraction efficiency of extractants BMDTE and BMTDTE. This indicated the presence of high Cl- ions leads to formation of non-extractable moieties of Pd(IV) between Cl- and the Pd(II) in the aqueous phase. Based on this study, it can be concluded that the %E of Pd(II) of both extractants is depends on the chloride concentration of the aqueous phase. -Figure 4-Figure 53.5. Effect of the nature of diluents on the extraction It is well known that the diluent plays a predominant role in choosing the best diluent in the solvent extraction process.48 With that objective in mind, it was decided to check if the possibility of other diluents could replace chloroform some solvents with different low-density and high-density solvents with varying dielectric constant of 17 different aromatic, aliphatic diluents and alkanes diluents. In this case, the organic phases were of 1.5 mM extractants BMDTE and BMTDTE in each diluent, whereas the aqueous phase contained 1.0 mM Pd(II) in 0.1 M HCl. The results were given in Table 1. It reveals that the %E of Pd(II) for both extractants are lower in non-chlorinated diluents compared to chlorinated diluents. High extraction of >99% of Pd(II) for both extractants BMDTE and BMTDTE was effectively achieved in chloroform, 1,2 Dichloromethane, 1,1,2,2-Tetrachloroethane, Nitrobenzene, 1,2 Dichloroethane and 1, 2 Dichlorobenzene with no third phase formation. This indicates that solubility of the formed complex is higher than compared to the other studied solvents. The extraction of Pd(II) was found to be incomplete and formation of the third phase with such diluents as Benzene, Ethylbenzene, Toluene, p-Xylene and 1-Octanol. Also, the use of a


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commercial diluents (kerosene and Shellsol D70®) and alkane diluents were not able to quantitative extraction of Pd(II) and also greater contact time (more than 4 h) was required for complete extraction.The results clearly demonstrate that there was no direct relationship between %E and the density or dielectric constant of the diluents. On the basis of availability, chloroform was selected as diluents for the succeeding experiments. -Table 13.6 Loading experiments High loading capacities of the extractant are the most important consideration for the solvent extraction process. Hence, the loading test was investigated by varying the concentration of Pd(II) in aqueous phase between 10 to 180 ppm in 0.1 M HCl with a fixed concentration of 1.5 mM extractants BMDTE or BMTDTE in chloroform. The results obtained were shown in Figure 6. It was observed that the loading capacity increases with increasing Pd(II) concentration of 140 ppm in the aqueous phase and further increases the Pd(II) concentration of 140 to 180 ppm, there was the extraction efficiency becomes constant. BMDTE showed slightly more loading capacity than the BMTDTE. The loading capacity for Pd(II) was found to be 124 ppm and 118 ppm for BMDTE and BMTDTE, respectively. Hence, this result implies that one mole of Pd(II) is coordinate to one mole of both extractants BMDTE and BMTDTE, separately. -Figure 63.7 Absorption spectra The UV-Vis absorbance spectra of both extractants and their Pd(II) complexes in chloroform were measured in the wavelength range between 200 and 600 nm. From this spectrum (Figure 7), the maximum absorbance wavelength was found to be at 268 nm for BMDTE and 292 nm for


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BMTDTE, while the yellow colored complex (Pd(II)-BMDTE or Pd(II)-BMTDTE) was also shows absorption in this region. Hence, neat BMDTE or BMTDTE/chloroform has been used as reagent blank during the absorbance measurement of Pd(II)-BMDTE or Pd(II)-BMTDTE complex. Therefore, further spectral measurements were measured at 268 nm for Pd(II)-BMDTE and 292 nm for Pd(II)-BMTDTE. -Figure 73.8 Nature of the extracted species The stoichiometry of the extracted species formed during the extraction of Pd(II) with BMDTE or BMTDTE in chloroform was determined using Job’s continuous variation method.49 For this purpose, concentration of both extractants and Pd(II) was maintained at 1.0 mM. Both extractants and Pd(II) solutions were contacted in different ratios and keeping the total volume 12 mL. The absorbance of each organic phase were measured and plotted against the mole fraction the respective ligand to Pd(II). Results from the absorption spectra (Figure 8), the maximum absorbance value at the mole fraction was found to be 0.50 for both extractants BMDTE and BMTDTE. These values indicating the ratio of metal to ligand is 1:1 stoichiometry, which is good agreement with the results obtained by molar ratio and loading capacity experiments. -Figure 83.9 NMR spectral studies The 1H-NMR spectra of the free ligands (BMDTE and BMTDTE) and their Pd(II) complexes are shown in Figure 9. In the spectra of the free ligands BMDTE and BMTDTE show a singlet around 3.754 ppm and 3.750 is due to the ethane protons (a and c). In the spectrum of the Pd(II) complexes, the signal of ethane protons is shifted upfield by 0.073 ppm for BMDTE and 0.061


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ppm for BMTDTE from its position. The chemical shifts of alkyl (-CH3) and thioalkyl (-SCH3) protons attached to heteroatoms in the free ligands were also moved upfield in their complexes: b-b’, 2.783-2.701 ppm; d-d’, 2.821-2.666 ppm, respectively. Thus, the chemical shift of these data confirm that the coordination of the ligands BMDTE and BMTDTE to the Pd(II) through dithioether. Based on the results obtained from Job’s plot and NMR spectra above, the Pd(II) extraction mechanism can be represented as follows

(3)

(4) -Figure 93.10 Acid Durability To evaluate the acid durability of both extractants, the experiments were carried out with equal volumes (50 mL) of 5.0 mM extractants (BMDTE or BMTDTE) in chloroform was contacted with a mixed solution of 2.0 M HCl and 1.0 M HNO3. The mixture was stirred for 2 days at room temperature. After which, the organic phase was separated, washed with water, dried over MgSO4 and then evaporated in vacuo at 85 oC for 4 h. The obtained material was subjected to FT-IR studies. The spectrum of acid treated extractant showed that similar features as that of FTIR spectrum of pure extractant (Figure 10a and b), which indicates good acid durability of both extractants structure in acidic medium. Hence, extractants BMDTE and BMTDTE can be a more suitable candidate for the recovery of Pd(II) through multiple regeneration/reuse cycles in acidic medium.


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- Figure 10a and b-

4. Applications The obtained results for the extraction of Pd(II) by the two heterocyclic dithioether ligands studied, encouraged us to explore the possibility of selective recovery of Pd(II) from simulated mixed metal ions and real ACR solution. 4.1 Selectivity extraction of palladium(II) from other metals The separation possibility of extractants (BMDTE and BMTDTE) was investigated through a competitive extraction of a mixture of Pd(II), Pt(IV), Rh(II), La(III), Al(III) and Ce(III) with 20.0 mM BMDTE and BMTDTE, separately. These metal ions have been chosen in the similarity of automotive catalyst residue. The mixed metal ions solution was prepared by dissolving 100 ppm of each metal chloride salt in 0.1 M HCl. The obtained data of this study are plotted in Figure 11. It was observed that the both extractants exhibited higher affinity towards divalent Pd(II) and tetravalent Pt(IV) ions than the other metal ions in the aqueous phase. Maximum selectivity of 99.7% of Pd(II) was extracted by BMDTE, and that of BMTDTE was found to be 99.3%. However, extractants BMDTE and BMTDTE has low extraction percentages for other metal ions: 32.6% and 27.1% for Pt(IV), 9.1% and 7.9% for Rh(III), 5.2% and 3.9% for La(III), 4.9 and 3.8% for Al(III) and Ce(III) was 3.4% and 2.3%. This fact obvious that the possibility separation and recovery of Pd(II) from mixed-metal ions solutions. -Figure 114.2 Palladium(II) extraction from automotive catalysts residue (ACR) solution To evaluate the practical applicability of the prepared extractants BMDTE and BMTDTE for the separation and recovery of Pd(II) from ACR solution containing six metals- Pd(II), Pt(IV),


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Rh(II), La(III), Al(III), and Ce(III). The ACR solution was prepared by dissolving an automotive catalyst residue in the acid mixture (12 M HCl +1.0 vol% H2O2)50 and diluted to 5 times their original volume with distilled water, and the pH of the ACR solution was found to be 0.30 (~ 0.5 M HCl). For this purpose, 20.0 mM extractant was used to extract PGMs metals. Before extraction, the metal ions concentrations in the ACR solution are presented in Table 2. Figure 12 shows that the both extractants has excellent selectivity towards Pd(II) were extracted up to 99.9% efficiency and small amount of 17.3% (31.6 ppm) and 10.6% (19.4 ppm) for Pt(IV), 7.9% (1.4 ppm) and 2.7% (0.48 ppm) for Rh(III), 2.8% (0.82 ppm) and 3.4% (1.0 ppm) for La(III), and Al (III) and Ce(III) was < 1.0%. The simplicity, separation and recovery of Pd(II) shown by the both BMDTE and BMTDTE demonstrate its potential application for the selective separation of Pd(II) from ACR. -Table 2-Figure 12-

4.3 Effect of phase ratio (O/A) and construction of McCabe–Thiele diagram The fact that the O/A ratio (organic to aqueous) is an extremely important for a solvent extraction process. Therefore, in the present investigation, an attempt was made to the effect of the phase ratio (O/A) for the complete extraction of Pd(II) by contacting the ACR solution and 20.0 mM of examined extractants BMDTE and BMTDTE in chloroform, separately. Both the bases were mixed at different volume ratios from 1/4 to 4:1 and a contact time of 45 min were maintained. The results are shown in Table 3. It can be seen from Table 3 that the extraction efficiency of Pd(II) for the extractant BMDTE has increased from 44.7 to 99.30% with the increase of O/A ratio from 1/4 to 4:1 and for extractants, BMTDTE was found to be 42.5 to


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99.6%. Thus, the result of O/A ratio study concluded that O/A ratio of 1:1 is sufficient for complete selective recovery of Pd(II) from ACR solution for the two investigated extractants (BMDTE and BMTDTE). It is one of the important characterizations to investigate the number of theoretical stages required under certain conditions for the extraction of Pd(II), since it is directly related to the cost of measurement. Based on the data obtained from phase ratio variation study for extraction of Pd(II) with extractants BMDTE and BMTDTE, corresponding McCabe-Thiele plot was constructed as shown in Figure 13a and b. A vertical line is then plotted starting from the concentration of Pd(II) in the ACR solution on the x-axis. The operating line was then inserted with a slope equal to the phase ratio (A/O = 1).51-52 The results clearly suggest that for both the extractants, two theoretical extraction stages were needed for complete separation of Pd(II) and to leave a very tiny amount of Pd(II) in ACR solution using an O/A phase ratio of 1/1. -Table 3-Figure 13a and b-

4.4 Scrubbing of co-extracted impurities from loaded organic phase The purpose of scrubbing is to achieve high metal purity and it is necessary to remove coextracted impurities from loaded organic phase before the stripping step. Since small amounts of Pt(IV), Rh(II), La(III) with traces of Al(III), Ce(III) were co-extracted along with Pd(II) loaded organic phase (examined extractants BMDTE and BMTDTE). Therefore, scrubbing of coextracted impurities was carried out with 5 M HCl solutions at an O: A ratio of 1 and shaking time of 5 min at room temperature. This removed more than 98 % of the co-extracted impurities (Rh(II), Al(III), La(III), and Ce(III)), accompanied by a maximum of 67.8% (21.5 ppm) and


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61.1% (11.8 ppm) for Pt(IV) and almost the whole of Pd(II) remains in the organic phase. This may be due to the interaction between Pt(IV)/Pd(II), and extractants are very strong. The scrubbed loaded organic phases with the examined extractants BMDTE and BMTDTE containing Pd(II) and Pt(IV) was used for further stripping studies. 4.5 Palladium (II) stripping It is an important parameter to back extract the extracted metals from the loaded organic phase. The back extraction of Pd(II) in the scrubbed organic phases of the both extractants was investigated at room temperature with 8 stripping agents of different concentrations. In this experiment, the mixture of the two phases (5 mL of each) was shaken for 5 min and then allowed to obtain a clear phase separation. The results collected are reported in Table 4. It was observed among the studied strippants, ammonia solution (v/v, 5%) and 1.0 M HCl + 0.1 M thiourea are found to almost completely strip the Pd(II) from the loaded organic phase into the aqueous phase with a yield exceeding 99%. The other strippants were found to be incomplete in the stripping of Pd(II). Here, we used ammonia solution (v/v, 5%) as strippants. Therefore, the probable stripping mechanism of ammonia for both extractants can be given as follows: Stripping mechanism:

(5)

(6) -Table 4-


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4.6 Stripping isotherm and McCabe–Thiele diagram In order to study the effect of O/A ratios on the stripping of Pd(II) from the scrubbed loaded organic phases with the two extractants BMDTE and BMTDTE using a strip solution of Ammonia solution (v/v, 5%) (according to the results obtained from Table 4), a series stripping experiments were carried at different O/A ratios from 1:4 to 4:1 for 5 min. The results are shown in Figure 14a and b. The slope analysis implies that two theoretical stripping stages are sufficient to strip most of the extracted Pd(II) from the loaded organic phase containing extractants BMDTE and BMTDTE at an O/A phase ratio of 1/1. For both the extractants, the co-extracted Pt(IV) were less than 5% at an O/A phase ratio of 1/1. - Figure 14a and b 4.7 Reusability of the extractant The regenerated organic phase was first washed with distilled water and then used for extraction of Pd(II) from ACR solution (2nd cycle) and the organic phase was treated with ammonia solution (v/v, 5%) to back extraction of Pd(II) (Phase ratio). The procedure of extraction-scrubbing-stripping was repeated five times. The extraction-stripping cycles for Pd(II) by BMDTE and BMTDTE are given in Figure S3 (Supporting Information). The results demonstrate that extraction efficiency of Pd(II) from the regenerated organic phase of both extractants was not noticeable change even after five cycles of extraction and the extraction efficiencies was found that above 99%. Therefore, the both extractant can be a used for recovery of Pd(II) from acidic leach automotive catalyst residue solution. - Figure S3 (Supporting Information)-


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5. Conclusion In the research, two heterocyclic dithioether ligands BMDTE and BMTDTE have been synthesized, and their solvent extraction performances of Pd(II) in hydrochloric acid solution was fully investigated for the first time. The results clarified that the extraction of Pd(II) was dependent on the concentration of hydrochloric acid, extractant concentration, chloride ion and hydrogen ion. Compared to BMTDTE, BMDTE can extract more than 99% Pd(II) from 0.01 to 0.4 M HCl solutions in a short time of 5 min. Of the studied diluents, it is found that chloroform, 1,2 Dichloromethane, 1,1,2,2-Tetrachloroethane and Nitrobenzene gave the better phase separation and extraction. The alkyl (-CH3) substituent in BMDTE was found to be faster phase separation, and no third phase (layer/emulsion) was formed during the loading capacity studies compared to that for the thioalkyl (-SCH3) substituent in BMTDTE. The results of Job’s plot analysis of both ligands demonstrated the 1:1 (metal: ligand) stoichiometric ratio of the complexes. It has been shown that both extractants BMDTE and BMTDTE exhibit a high extractability and selectivity of more than 99% of Pd(II) from the leach liquor solution of the automotive catalysts residue. Among the studied strippants, almost complete back extraction of Pd(II) from loaded organic phase can be achieved in single contact using ammonia solution (v/v, 5%) and acidified thiourea solution. Complete extraction and stripping of Pd(II) was obtained in two theoretical stages. Additionally, both of the recycled extractants have demonstrated the same extractive abilities as the fresh ligand. The findings of the various studies demonstrated that the synthesized low molecular weight of both extractants (BMDTE and BMTDTE) could be a potential candidate for Pd(II) recovery from ACR in an acidic medium in the future.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Figure S1 MALDT-TOF MS spectra of BMDTE; Figure S2 MALDT-TOF MS spectra of BMTDTE; Figure S1. Extraction and stripping cycle of (a) BMDTE and (b) BMTDTE; Table S1 Pd(II) extraction properties of some other extractants reported in the literature.

Author information Corresponding author Yoshihiko Kondo, b E-mail Id: [email protected]. Notes The authors declare no competing financial interest. Acknowledgments The research work was financially supported by “Tohoku Innovative Materials Technology Initiatives for Reconstruction (TIMT)” project, “High-Efficiency Rare Elements Extraction Technology Area” under the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

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Figure Captions

Figure 1. Extraction behavior of Pd(II)) from different molarities of HCl by 1.5 mM BMDTE and 1.5 mM BMTDTE in chloroform. Aqueous phase, 1.0 mM Pd(II).


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Figure 2. Variation of %E of Pd(II) as a function of contact time. Conditions: organic phase = 1.5 mM BMDTE or BMTDTE in chloroform, Aqueous phase = 1.0 mM Pd(II) in 0.1 M HCl. Contact time varied from 0.5 to 35 min. Figure 3. Dependence of %E of Pd(II) on the BMDTE or BMTDTE concentration. Conditions: extractants BMDTE or BMTDTE concentration varied between 0.1 and 3.0 mM in chloroform, Aqueous phase = 1.0 mM Pd(II) in 0.1 M HCl. Contact time = 45 min. Figure 4. Effect of H+ concentration on the extraction of Pd(II) by 1.5 mM BMDTE and 1.5 mM BMTDTE in chloroform. Pd(II)=1.0 mM, [H+] ions varied from 0.01 M to 1.0 M using HCl, [Cl-] = 1.0 M. Figure 5. Effect of Cl- concentration on the extraction of Pd(II) by 1.5 mM BMDTE and 1.5 mM BMTDTE in chloroform. Pd(II) = 1.0 mM, [Cl-] ions varied from 0.05 M to 1.0 M using LiCl, [H+] = 0.1 M. Figure 6. Loading capacity test for Pd(II) with BMDTE and BMTDTE. Conditions: organic phase = 1.0 mM BMDTE or BMTDTE in chloroform, Aqueous phase = 10 to 180 ppm Pd(II) in 0.1 M HCl. Figure 7. Uv-Vis absorption spectrum of BMDTE or BMTDTE and their Pd(II) complexes in chloroform. Conditions: [BMDTE or BMTDTE] = 0.1 mM, [Pd(II)-BMDTE or Pd(II)BMTDTE] = 0.1 mM . Figure 8. Job’s plot. Conditions: organic phase = 1.0 mM BMDTE or BMTDTE in chloroform, Aqueous phase =1.0 mM Pd(II) in 0.1 M HCl. Figure 9. 1H-NMR spectra of the free ligands (BMDTE and BMTDTE) and their Pd(II) complexes.


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Figure10 FT-IR spectra of (a) BMDTE and acid treated BMDTE, (b) BMTDTE and acid treated BMTDTE. Figure 11. Selective extraction of Pd(II) with BMDTE and BMTDTE from simulated mixedmetal solutions containing six metal ions (i.e., Pd(II), Pt(IV), Rh(II), La(III), Al(III) and Ce(III)). Conditions: organic phase = 20.0 mM BMDTE or BMTDTE in chloroform. [Metal] = 100 ppm in 0.1 M HCl. Figure 12. Selective extraction of Pd(II) with BMDTE and BMTDTE from the ACR solution containing six metal ions (i.e., Pt(IV), Pd(II), Rh(III), La(III), Al(III) and Ce(II). Conditions: organic phase = 20.0 mM BMDTE or BMTDTE in chloroform. Figure 13. McCabe–Thiele plot for palladium(II) extraction. (a) BMDTE (b) BMTDTE. Organic phase: 20.0 mM BMDTE or BMTDTE in chloroform, Aqueous phase: 102.22 ppm in 0.1 M HCl (O/A varied from 1:4 to 4:1), Contact time = 45 min. Figure 14. McCabe–Thiele plot for stripping from organic solution. (a) BMDTE (Pd(II) = 101.05 ppm) (b) BMTDTE (Pd(II) = 101.52 ppm), O/A varied from 4:1 to 1:4, Contact time = 5 min.


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Figure 1. Extraction behavior of Pd(II)) from different molarities of HCl by 1.5 mM BMDTE and 1.5 mM BMTDTE in chloroform. Aqueous phase, 1.0 mM Pd(II).


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Figure 2. Variation of %E of Pd(II) as a function of contact time. Conditions: organic phase = 1.5 mM BMDTE or BMTDTE in chloroform, Aqueous phase = 1.0 mM Pd(II) in 0.1 M HCl. Contact time varied from 0.5 to 35 min.


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Figure 3. Dependence of %E of Pd(II) on the BMDTE or BMTDTE concentration. Conditions: extractants BMDTE or BMTDTE concentration varied between 0.1 and 3.0 mM in chloroform, Aqueous phase = 1.0 mM Pd(II) in 0.1 M HCl. Contact time = 45 min.


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Figure 4. Effect of H+ concentration on the extraction of Pd(II) by 1.5 mM BMDTE and 1.5 mM BMTDTE in chloroform. Pd(II)=1.0 mM, [H+] ions varied from 0.01 M to 1.0 M using HCl, [Cl-] = 1.0 M.


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Figure 5. Effect of Cl- concentration on the extraction of Pd(II) by 1.5 mM BMDTE and 1.5 mM BMTDTE in chloroform. Pd(II) = 1.0 mM, [Cl-] ions varied from 0.05 M to 1.0 M using LiCl, [H+] = 0.1 M.


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Figure 6. Loading capacity test for Pd(II) with BMDTE and BMTDTE. Conditions: organic phase = 1.5 mM BMDTE or BMTDTE in chloroform, Aqueous phase = 10 to 180 ppm Pd(II) in 0.1 M HCl.


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Figure 7. Uv-Vis absorption spectrum of BMDTE or BMTDTE and their Pd(II) complexes in chloroform. Conditions: [BMDTE or BMTDTE] = 0.1 mM, [Pd(II)-BMDTE or Pd(II)BMTDTE] = 0.1 mM .


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Figure 8. Job’s plot. Conditions: organic phase = 1.0 mM BMDTE or BMTDTE in chloroform, Aqueous phase =1.0 mM Pd(II) in 0.1 M HCl.


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Figure 9. 1H-NMR spectra of the free ligands (BMDTE and BMTDTE) and their Pd(II) complexes.


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Figure 10. FT-IR spectra of (a) BMDTE and acid treated BMDTE, (b) BMTDTE and acid treated BMTDTE.


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Figure 11. Selective extraction of Pd(II) with BMDTE and BMTDTE from simulated mixedmetal solutions containing six metal ions (i.e., Pd(II), Pt(IV), Rh(II), La(III), Al(III) and Ce(III)). Conditions: organic phase = 20.0 mM BMDTE or BMTDTE in chloroform. [Metal ions] = 100 ppm in 0.1 M HCl.


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Figure 12. Selective extraction of Pd(II) with BMDTE and BMTDTE from the ACR solution containing six metal ions (i.e., Pd(II), Pt(IV), Rh(II), La(III), Al(III), and Ce(III)). Conditions: organic phase = 20.0 mM BMDTE or BMTDTE in chloroform.


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Figure 13. McCabe–Thiele plot for Pd(II) extraction. (a) BMDTE (b) BMTDTE. Organic phase: 20.0 mM BMDTE or BMTDTE in chloroform, Aqueous phase: 102.22 ppm Pd(II) in 0.1 M HCl (O/A varied from 1:4 to 4:1), Contact time = 45 min.


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Figure 14. McCabe–Thiele plot for stripping from organic solution. (a) BMDTE (Pd(II) = 101.05 ppm) (b) BMTDTE (Pd(II) = 101.52 ppm), O/A varied from 4:1 to 1:4, Contact time = 5 min.


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Table Captions

Table1 Effect of various diluents on the extraction of Pd(II)*. Table 2 Individual concentration of each metal ions in the ACR solution before extraction. Table 3 Effect of O/A ratio on Pd(II) extraction by 20.0 mM of examined extractants BMDTE and BMTDTE in chloroform (aqueous feed = ACR solution, contact time = 45 min). Table 4. Effect of various stripping reagents on the stripping of Pd(II).


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Table.1 Effect of various diluents on the extraction of Pd(II)*

S. No. Name of the diluent

Boiling point (°C)

Density (g/cm3)

Dielectric constant

%E Pd(II)

1

Chloroform

61.0

1.489

4.81

BMDTE 99.8

Remarks BMTDTE CS 99.6

Remarks CS

2

1,2 Dichloromethane

40.0

1.326

8.93

99.6

CS

99.4

CS

3

1,1,2,2Tetrachloroethane

146.5

1.590

8.42

99.6

CS

99.4

CS

4

Nitrobenzene

210.0

1.204

34.80

99.5

CS

99.6

CS

5

Benzene

80.1

0.879

2.28

98.9

TP

95.3

TP

6

Ethylbenzene

136.0

0.867

2.41

93.6

TP

87.4

TP

7

Tolune

110.0

0.867

2.38

94.4

TP

84.3

TP

8

p-Xylene

138.4

0.861

2.20

91.1

TP

86.7

TP

9

Benzonitrile

191.0

1.000

26.00

90.7

NCS

93.8

TP

10

1-Octanol

195.0

0.824

10.34

89.4

NCS

94.3

TP

11

Kerosene

150–300

0.810

1.80

46.3

CS

64.4

CS

12

Shellsol D70®

193–245

0.790

2.10

71.2

CS

66.8

CS

13

1,2 Dichloroethane

83.5

1.250

10.36

99.5

CS

99.6

CS

14

n-Decane

214.0

0.740

2.00

73.4

CS

45.5

CS

15

n-Hexane

68.0

0.655

1.88

10.2

CS

8.4

CS

16

n-Heptane

98.4

0.684

1.92

12.1

CS

15.3

CS

17

1, 2 Dichlorobenzene

180.3

1.30

9.93

99.6

CS

99.6

CS

*Conditions: [BMDTE or BMTDTE] = 1.5 Mm, [Pd(II)] = 1.0 mM in 0.1 M HCl. CS, clear separation; TP, third phase formation; NCS, no clear separation.


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Table 2 Individual concentration of each metal ions in the ACR solution before extraction S. No.

Metal ions in the ACR solution Metal ions

before extraction (ppm)

1

Pd(II)

102.22

2

Pt(IV)

182.72

3

Rh(III)

19.74

4

Al(III)

1894.72

5

La(III)

30.97

6

Ce(III)

1630

Table 3 Effect of O/A ratio on Pd(II) extraction by 20.0 mM of examined extractants BMDTE and BMTDTE in chloroform (aqueous feed = ACR solution, contact time = 45 min).

BMDTE

Pd(II) conc. (ppm) O/A ratio

Aqueous

Organic

BMTDTE

Ext. coeff.

%E

Pd(II) conc. (ppm)

(D)

Pd(II)

Aqueous Organic coeff. (D) Pd(II)

Ext.

%E

1

4

56.4

182.9

3.2

44.7

58.7

173.9

2.9

42.5

1

3

38.2

191.8

5.0

62.6

41.4

182.4

4.4

59.5

1

2

11.3

181.8

15.9

88.9

15.4

173.5

11.2

84.9

1

1

0.23

101.9

428.6

99.8

0.25

101.9

399.1

99.8

2

1

0.54

50.8

93.4

99.5

0.62

50.7

81.6

99.4

3

1

0.28

33.9

118.9

99.7

0.46

33.9

73.5

99.5

4

1

0.31

25.4

79.8

99.7

0.27

25.4

92.6

99.6


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Table 4. Effect of various stripping reagents on the stripping of Pd(II).

S. No.

%S Pd(II) Stripping reagent

BMDTE

BMTDTE

1

DI water (pH =7)

0.51

3.62

2

Ammonia solution (v/v, 5%)

99.8

99.6

3

Ammonia solution (v/v, 2.5%)

63.0

69.3

4

1.0 M HCl + 0.1 M thiourea

99.7

99.4

5

0.1 M thiourea

38.3

47.1

6

Acetic acid (2.0 M)

8.4

14.2

7

1.0 M HNO3

25.3

21.4

8

3 M H2SO4

17.3

24.6


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