Liquid–liquid extraction of palladium(II) from chloride media by N,N′-dimethyl-N,N′-dicyclohexylthiodiglycolamide

Separation and Purification Technology xxx (2015) xxx–xxx

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Liquid–liquid extraction of palladium(II) from chloride media by N,N0 -dimethyl-N,N0 -dicyclohexylthiodiglycolamide Osvaldo Ortet a,b, Ana Paula Paiva a,⇑ a b

Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal Departamento de Ciência e Tecnologia, Universidade de Cabo Verde, 379C Praia, Santiago Island, Cape Verde

a r t i c l e

i n f o

Article history: Received 29 May 2015 Received in revised form 6 October 2015 Accepted 9 October 2015 Available online xxxx Keywords: Palladium(II) Liquid–liquid extraction Thiodiglycolamide derivative Hydrochloric acid

a b s t r a c t Previous research showed that N,N0 -dimethyl-N,N0 -dicyclohexylthiodiglycolamide (DMDCHTDGA) in 1,2dichloroethane is able to co-extract platinum(IV) and palladium(II) from concentrated hydrochloric acid solutions. Following a detailed study about Pt(IV), the present work focuses on the thorough investigation of Pd(II) extraction by DMDCHTDGA in toluene. The removal of Pd(II) is rather efficient from 0.5 M to 5.5 M HCl, and progressively decreases until 7.5 M HCl. Pd(II) stripping is better achieved by an acidic thiourea solution, but ammonia aqueous phases can alternatively be used for some cases. Pd(II) extraction kinetics is relatively favored (5–15 min). The robustness of DMDCHTDGA to extract Pd(II) has been confirmed through five successive reutilization experiments. The maximum loading capacity for Pd(II) reaches a 2.5 DMDCHTDGA:Pd(II) molar ratio. Pd(II) can selectively be recovered by DMDCHTDGA from 4.0 M or 6.0 M HCl complex mixtures containing equivalent concentrations of Pt(IV) and Rh(III), and fivefold Fe(III) and Al(III) concentrations, although with a slight Pt(IV) contamination. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction The platinum-group metals (PGMs) occupy a relevant place in the forecast market balance for critical raw materials, as they are considered to have small deficit supplies in 2015 and 2020 in a recent European Union report [1]. The collapse of PGMs natural resources may be considered a threat to the preservation of the everyday life in the western world, since PGMs have been gradually required for use in several technological applications. The irreplaceable catalytic properties of PGMs are currently explored in the automobile catalysts, as a means to reduce the harmful emissions of the motor engines, and also in the electronic and chemical industry [2]. Accordingly, the PGMs recycling from anthropogenic sources becomes essential, aiming to fulfill the worldwide requirements of these critical metals [3]. A few processes, all of them relying on chloride-based leaching media, have been developed for the direct hydrometallurgical leaching of PGMs from spent catalysts [4,5], the sequential steps often applied to further isolate and concentrate PGMs being ion exchange [6,7] and liquid–liquid extraction (solvent extraction, SX) [8]. The general compositions of these leaches may be more complex than those coming from the primary raw materials, and ⇑ Corresponding author. E-mail addresses: [email protected] (O. Ortet), [email protected] (A.P. Paiva).

even the most frequent contaminants are different for many cases. This situation has been justifying the crescent commitment of the researcher’s community to further adapt already well known SX processes [9–11] or to investigate the viability of new molecules as PGMs extractants [11,12]. Amide derivatives are one of the main families of organic compounds that has been extensively considered for PGMs extraction, particularly for Pd(II), Pt(IV) and Rh(III) [13–16]. Focusing on Pd(II) recovery, sulfide containing monoamides [17] and a dithiodiglycolamide [18] have recently been proposed, as well as tertiary thioamides [19] and particular thiodiglycolamide derivatives [20–23]. A cautious assessment of the Pd(II) extraction capabilities shown by these latter compounds points out to remarkable findings in terms of efficiency and selectivity for Pd(II) recovery, therefore justifying additional research. The SX of Pt(IV) from hydrochloric acid media by N,N0 dimethyl-N,N0 -dicyclohexylthiodiglycolamide (DMDCHTDGA) – Fig. 1 – in 1,2-dichloroethane (1,2-DCE) has already been cautiously investigated [23]. Furthermore, the quantitative co-extraction of Pt(IV) and Pd(II) from 8 M HCl was also described, the separation of the two metal ions from the loaded organic phase being accomplished by selective stripping [23]. It is well known that the use of chlorinated diluents is not conceivable in practical applications. However, for preliminary fundamental investigation, it is better to opt for a diluent not

http://dx.doi.org/10.1016/j.seppur.2015.10.023 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.

Please cite this article in press as: O. Ortet, A.P. Paiva, Liquid–liquid extraction of palladium(II) from chloride media by N,N0 -dimethyl-N,N0 -dicyclohexylthiodiglycolamide, Separ. Purif. Technol. (2015), http://dx.doi.org/10.1016/j.seppur.2015.10.023

O. Ortet, A.P. Paiva / Separation and Purification Technology xxx (2015) xxx–xxx

O H3C

Fig. 1. Structure (DMDCHTDGA).

N

of

O S

N

CH3

N,N0 -dimethyl-N,N0 -dicyclohexylthiodiglycolamide

presenting difficulties to solubilize the extractant. As previously reported [23], DMDCHTDGA is not soluble enough in aliphatic commercial diluents of the kerosene type, but it is soluble in toluene, xylene and in other aromatic diluents. In addition to the ease of Pd(II) extraction and stripping and favorable extraction kinetics, the reutilization patterns and selectivity performance shown by DMDCHTDGA in toluene are rather promising if Pd(II) recovery from complex aqueous matrices is aimed. These encouraging results justify future attempts to investigate if more environmentally-friendly diluents can alternatively be used.

2. Experimental The synthesis and characterization of DMDCHTDGA have already been described elsewhere [23]. Feed aqueous phases containing about 9.4  104 M Pd(II) were prepared from an atomic absorption spectroscopy standard (1003 ± 4 mg L1 Pd(II) in 5% HCl, Fluka), in the required volumes of hydrochloric acid (37%, PA, Fisher Chemicals). These aqueous solutions were involved in the general extraction experiments. An aqueous phase with 2.8  103 M Pd(II) in 4.5 M HCl was also arranged for obtaining the equilibrium extraction isotherm. For the selectivity experiments, 4.0 M and 6.0 M HCl solutions containing 100 mg L1 Pd(II) and Pt(IV) (atomic absorption spectroscopy standard, 1000 ± 4 mg L1 Pt(IV) in 5% HCl, Fluka) were prepared, as well as similar ones with the additional presence of 100 mg L1 Rh(III) (atomic absorption spectroscopy standard, 1001 ± 6 mg L1 Rh(III) in 5% HCl, Fluka). Four- and five-metal ion solutions were further arranged, containing the three PGMs with 500 mg L1 Al (III) (prepared from aluminum(III) chloride, >98%, Merck) and/or 500 mg L1 Fe(III) (prepared from iron(III) chloride hexahydrate, 99%, Riedel-de Haën). Organic phases with 0.02 M DMDCHTDGA were settled in toluene (Sigma–Aldrich, P99.3%). For the selectivity experiments, a 0.03 M DMDCHTDGA concentration was alternatively considered. A 0.1 M thiourea solution (99%, Sigma–Aldrich) in 1.0 M HCl, and concentrated (25%, Panreac) and diluted ammonia aqueous phases were used as Pd(II) stripping media. Extraction and stripping experiments were generally carried out by stirring equal volumes of aqueous and organic phases (A/O = 1) at room temperature and adopting a rotation speed between 900 and 1000 rpm. Except for the experiments developed for the kinetics investigation, the agitation period considered for all the remaining extraction assays varied between 15 and 30 min. Stripping experiments were always performed for 30 min. After separation of the two phases, the aqueous and organic extracts were always filtered to minimize mutual entrainments. Fresh portions of an aqueous solution containing 2.8  103 M Pd (II), in 4.5 M HCl, were successively contacted with the same fraction of 0.02 M DMDCHTDGA in toluene, using A/O = 1 ratios, to construct the equilibrium extraction isotherm and achieve the maximum Pd(II) loading saturation path. In reutilization, the same volume of 0.02 M DMDCHTDGA in toluene suffered five sequential

extraction and stripping contacts, with A/O = 1. In the selectivity experiments, the tests included an additional scrubbing step with distilled water. The determination of Pd(II) contents in single ion aqueous phases, before and after extraction, was carried out by flame atomic absorption spectrophotometry (AAS, novAA 350, Analytik Jena). For the analysis of the multi-metallic aqueous solutions, inductively coupled plasma – atomic emission spectrometry (ICP-AES, Horiba Jobin–Yvon, Ultima) was alternatively used. Metal ion concentrations in organic phases were calculated by mass balance. Two replicates were generally considered for all the experiments, and the analysis of the aqueous solutions before and after extraction was systematically performed in triplicate. The uncertainties determined for the extraction and stripping results presented in this article do not exceed ±5%.

3. Results and discussion 3.1. Effect of hydrochloric acid concentration on palladium(II) extraction Equal volumes of aqueous solutions containing about 9.4  104 M Pd(II) in 0.5 M to 7.5 M HCl, and organic phases with 0.02 M DMDCHTDGA in toluene, were put in contact for 30 min. The extraction results obtained are displayed in Fig. 2, in which a plot showing the dependence of log D Pd(II) on HCl concentrations can be observed. Pd(II) extraction is rather efficient and more or less constant between 0.5 M and 5.5 M HCl, and reduces when Pd(II) extraction is performed from 6.5 M and particularly from 7.5 M HCl. These results are different from those achieved for Pd(II) extraction by 0.05 M DMDCHTDGA in 1,2-DCE [23], since Pd(II) was quantitatively extracted regardless the HCl content until 8.0 M HCl. Narita and co-workers studied other derivatives of the same family to extract Pd(II), for instance N,N0 -dimethyl-N,N0 -diphenyl thiodiglycolamide (DMDPHTDGA) in chloroform [20], and N,N,N0 ,N0 -tetraoctylthiodiglycolamide (TODTDGA), in 80 vol% n-dodecane–20 vol% 2-ethylhexanol [21], and also found that Pd (II) was totally extracted from HCl solutions until 8.0 M HCl. More recently, another thiodiglycolamide, e.g., N,N0 -dimethyl-N, 0 N -didecylthiodiglycolamide (MDTDGA), dissolved in 80 vol% ndodecane–20 vol% 2-ethylhexanol, has been investigated for Pd (II) extraction as well, and a completely different result was found, as D values of about 1–2 were achieved for 1.0–2.0 M HCl, of about 5 for 4.0 M HCl, 60 for 5.0 M and >100 for 6.0 M and 7.0 M HCl [22]. Hence, it can be concluded that factors such as the length and/or combination of substituent groups, and diluents as well, play

2.5 2

log D Pd (II)

2

1.5 1 0.5 0 0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

[HCl] / M Fig. 2. Variation of the log D of Pd(II) with the HCl concentration (9.4  104 M Pd (II) in 0.5–7.5 M HCl; 0.02 M DMDCHTDGA in toluene; A/O = 1, room temperature, 900–1000 rpm, 30 min). Standard deviations: ±5%.

Please cite this article in press as: O. Ortet, A.P. Paiva, Liquid–liquid extraction of palladium(II) from chloride media by N,N0 -dimethyl-N,N0 -dicyclohexylthiodiglycolamide, Separ. Purif. Technol. (2015), http://dx.doi.org/10.1016/j.seppur.2015.10.023

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O. Ortet, A.P. Paiva / Separation and Purification Technology xxx (2015) xxx–xxx

determinant roles on Pd(II) extraction, resulting in completely different profiles shown by thiodiglycolamides when HCl concentration in the aqueous chloride media vary. Comparing with the results previously obtained with DMDCHTDGA in 1,2-DCE, it seems that toluene hampers the affinity of this particular derivative toward Pd(II) extraction from more concentrated HCl solutions. Previous studies regarding the investigation of diluent effects in similar liquid–liquid SX systems [24,25] support the assumption that distinct solute–diluent interactions between the surface active extractant ion pair species and each of the diluents, 1,2-DCE or toluene, are likely to occur; accordingly, the higher dipolar moment and polarizable characteristics of 1,2-DCE should facilitate the extraction of metal species as ion pairs to its bulk. These interfacial issues are therefore proposed to be the key-factors justifying the poorer Pd(II) and Pt (IV) [23] extractive performance shown by DMDCHTDGA in toluene, when compared with the situations where 1,2-DCE is present, for more concentrated HCl solutions.

3.2. Palladium(II) stripping Earlier investigation performed with DMDCHTDGA in 1,2-DCE for Pd(II) extraction has already shown that 0.1 M thiourea in 1.0 M HCl is an efficient stripping agent for the metal ion [23], irrespective of the HCl content in the feed aqueous phases. Hence, in order to confirm such a behavior when 1,2-DCE is replaced by toluene, two Pd(II) loaded organic phases obtained from the previous experiments suffered subsequent contacts with the thiourea solution, the remaining ones being stripped with different concentrated ammonia aqueous phases (A/O = 1, 30 min contacts). This latter stripping agent was chosen to check if the successful results obtained with TODTDGA [21] could also be attained with DMDCHTDGA. The overall data achieved are summarized in Table 1. Table 1 shows that 0.1 M thiourea in 1.0 M HCl continues to be a quite efficient stripping medium for Pd(II) removal from the DMDCHTDGA organic phases. Aqueous ammonia is also able to extract Pd(II) from the organic solutions, but always worse than thiourea, as Pd(II) stripping was never complete (D values varying between 0.6 and 2.7). The influence of the ammonia concentration in Pd(II) release does not appear to be a key factor, at least within the concentration range studied. On the contrary, the HCl content in the initial feed aqueous solutions seems to be a determinant parameter affecting Pd(II) stripping, particularly when the two values attributed to the 13.0 M NH3 solution are taken into account. This fact is in agreement with the findings of Narita and co-workers, who reported that Pd(II) release from TODTDGA is dependent on the extractant and HCl concentrations of the forward extraction step, and on Pd(II) concentration in the loaded organic phases as well [21].

3.3. Kinetics of palladium(II) extraction This set of experiments was carried out with 1.5 M and 4.5 M HCl solutions, maintaining the Pd(II) concentration at 9.4  104 M, and using 0.02 M DMDCHTDGA in toluene as organic phase. The time of contact for the two phases varied between 2 and 60 min, and all the other parameters were kept constant. It was found that the extraction kinetics for these systems is rather favorable (see Fig. 3): a 5 min contact for the 1.5 M HCl solution is sufficient to reach the maximum Pd(II) extraction, whereas the same situation is clearly achieved for the 15 min contact when the 4.5 M HCl aqueous phase is involved (probably even for a shorter period). Longer time contacts seem to cause a slight decrease in the Pd(II) distribution ratios. These equilibrium shaking times are in agreement with the ones previously reported for other thiodiglycolamide derivatives, e.g., less than 5 min for DMDPHTDGA [20], 10 min for MDTDGA [22] and 15 min for TODTDGA [21]. Accordingly, a contact time of 15 min was adopted for all the subsequent experiments with DMDCHTDGA, as the thermodynamic equilibrium for these extraction systems has undoubtedly been reached.

3.4. Equilibrium isotherm and reutilization of DMDCHTDGA In order to acquire knowledge about the capacity shown by DMDCHTDGA for Pd(II) extraction, successive contacts of a given volume of 0.02 M DMDCHTDGA in toluene with fresh portions of a 4.5 M HCl solution containing 2.8  103 M Pd(II) have been carried out (using A/O = 1 ratios). The correspondent equilibrium extraction isotherm of DMDCHTDGA toward Pd(II) is presented in Fig. 4. It can be observed that the solvent exhibits a high ability to accumulate Pd(II). Saturation plateaus from equilibrium isotherms provide an estimation of the stoichiometry of extractant:metal ion species and, for this case, a maximum 0.008 M Pd(II) loading was achieved, corresponding to a molar DMDCHTDGA:Pd(II) ratio of 2.5. Narita and collaborators determined a TODTDGA:Pd(II) ratio of about 1.5 from the saturation plateau of an equilibrium isotherm for Pd(II) extraction from 3.0 M HCl [21]. In order to confirm the robustness of DMDCHTDGA to recover Pd(II) when subject to consecutive extraction–stripping experiments, a given portion of 0.02 M DMDCHTDGA in toluene has been contacted with fresh aqueous solution containing 9.4  104 M Pd (II) in 4.5 M HCl (A/O = 1), subsequently stripped by 0.1 M thiourea in 1.0 M HCl, and this sequential procedure has been repeated four times. The results obtained are presented in Fig. 5, in which a

3

a

[HCl] feed (M)

Stripping agents (M)

D Pd(II)

0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5

13.0 M NH3 1.0 M NH3 2.0 M NH3 1.0 M NH3 0.1 M TU in 1.0 M HCl 2.0 M NH3 13.0 M NH3 0.1 M TU in 1.0 M HCl

0.6 1.0 1.3 1.6 >99.0 1.5 2.7 >99.0

Initial [Pd(II)] 1.1  103 M.

in

organic

phases

varying

between

8.4  104 M

a

log D Pd (II)

2.6 Table 1 Pd(II) distribution ratios (D) obtained after stripping from loaded 0.02 M DMDCHTDGA toluene phases (TU: thiourea).

2.2 1.8 1.4 1 0.6

0

15

30

45

60

Time of contact / min 1.5 M HCl

and

4.5 M HCl

Fig. 3. Variation of the log D of Pd(II) with the contact time, for 1.5 M and 4.5 M HCl solutions (9.4  104 M Pd(II); 0.02 M DMDCHTDGA in toluene; A/O = 1, room temperature, 900–1000 rpm). Standard deviations: ±5%.

Please cite this article in press as: O. Ortet, A.P. Paiva, Liquid–liquid extraction of palladium(II) from chloride media by N,N0 -dimethyl-N,N0 -dicyclohexylthiodiglycolamide, Separ. Purif. Technol. (2015), http://dx.doi.org/10.1016/j.seppur.2015.10.023

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O. Ortet, A.P. Paiva / Separation and Purification Technology xxx (2015) xxx–xxx

0.01

[Pd(II)] org. / M

0.008 0.006 0.004 0.002 0 0

0.002

0.004

0.006

0.008

0.01

0.012

[Pd(II)] aq. / M Fig. 4. Equilibrium extraction isotherm for Pd(II) by DMDCHTDGA (2.8  103 M Pd (II) in 4.5 M HCl; 0.02 M DMDCHTDGA in toluene; A/O = 1, room temperature, 900– 1000 rpm, 15 min). Standard deviations: ±5%.

graphic exhibiting the dependence of log D Pd(II) on the five consecutive extraction experiments is shown. It can be perceived that DMDCHTDGA nearly maintains its performance toward Pd(II) extraction; a rather slight decrease in the Pd(II) distribution ratio has been found for the last three stages, but the variation path falls within the experimental error. The thiourea solution was able to quantitatively strip Pd(II) from the loaded DMDCHTDGA organic phase in each cycle. 3.5. Selectivity for palladium(II) extraction The selectivity shown for Pd(II) by DMDCHTDGA is a determinant key point that requires evaluation. Focusing the research on the use of HCl media with a more practical interest, 4.0 M and 6.0 M HCl solutions with two – Pd(II) and Pt(IV) – and three PGMs – Pd(II), Pt(IV) and Rh(III) – were considered, all with initial 100 mg L1 concentrations. Additional experiments were subsequently carried out involving four metals in the feed aqueous solutions, namely the three PGMs with either 500 mg L1 Al(III) or Fe(III). Finally, five-metals aqueous media were prepared, containing the three PGMs, Al(III) and Fe(III), all with similar concentrations as depicted above. The overall results obtained for the DMDCHTDGA extractive behavior are illustrated in Table 2, expressed by the separation factors (SF) of Pd(II) in relation to each of the other metal ions (calculated as SF = DPd/Dmetal). Thus, the higher the SF, the more selective for Pd(II) recovery DMDCHTDGA is. 1.4

log D Pd (II)

1.2 1.0 0.8 0.6 0.4 0.2 0.0 1

2

3

4

5

Number of extraction stages Fig. 5. Variation of the log D of Pd(II) along consecutive extraction stages, included in five extraction–stripping cycles (9.4  104 M Pd(II) in 4.5 M HCl; 0.02 M DMDCHTDGA in toluene; 0.1 M thiourea in 1.0 M HCl; A/O = 1, room temperature, 900–1000 rpm, 15 min for extraction, 30 min for stripping). Standard deviations: ±5%.

Some differences can be perceived for the extraction results found for the two HCl media tested, particularly concerning the data involving Pt(IV) and Fe(III). For the binary aqueous media – solution 1 – it can be observed that the SFPd/Pt are good for both HCl media, although better for 4.0 M HCl. Previous data obtained for individual Pt(IV) extraction from 4.0 M and 6.0 M HCl solutions by DMDCHTDGA in toluene pointed out to D values of about 3.4 [23], whereas D Pt(IV) values found now are 0.4 and 1.1, respectively. It should be pointed out, however, that 0.05 M DMDCHTDGA has been employed for the previous experiments, almost twofold of the concentration used in this investigation (0.03 M). Furthermore, the extraction patterns obtained for individual metal ion solutions are not always totally reproducible for situations where diverse metal ions are involved. A similar tendency as for solution 1 has been verified for the solution with the three PGMs (solution 2): the SFPd/Pt are a bit lower than before, and again superior for the 4.0 M HCl solution. Rh(III) is practically not extracted. Pd(II) was efficiently stripped by 0.1 M thiourea in 1.0 M HCl from all the loaded organic phases – 82–90 mg L1 – with a maximum Pt(IV) contamination of 6 mg L1. Solutions 3 and 4 in Table 2 refer to the SF values obtained for Pd(II) separation when four metal ions coexist in the feed aqueous media, the three PGMs and either Al(III) or Fe(III), respectively. When Al(III) is present, the SFPd/Pt are comparable to the ones previously achieved; Rh(III) is less removed than before, and Al(III) is extracted only traceably (SF values higher than 7000). Therefore, Al (III) does not interfere with the behavior of DMDCHTDGA toward Pd(II) extraction. Inversely, Fe(III) is extensively extracted by DMDCHTDGA, particularly from 6.0 M HCl (SFPd/Fe = 1). Curiously, Pt(IV) is much more extracted than before from 4.0 M HCl, but a similar SFPd/Pt value as those previously obtained has been found for the 6.0 M HCl solution. Furthermore, the Pd(II) distribution ratio remains high (D = 26). The results depicted in Table 2 for the extraction of the five metals aqueous phase – solution 5 – is generally in agreement with the results found before, e.g., Rh(III) and Al(III) are practically not extracted, whereas the highest SFPd/Pt and the lowest SFPd/Fe were found for the 6.0 M HCl solution. The D value for Pd(II) was of about 41. The maintenance of high D values for Pd(II) in the presence of Fe (III) is encouraging, since water can efficiently scrub Fe(III) from the loaded organic media. Accordingly, 89–98% of Fe(III) has been transferred to water, accompanied by a maximum of 7 mg L1 Pd (II), and that procedure left the organic phases cleaner for a more selective Pd(II) stripping by the usual thiourea solution. Thus, Pd (II) was efficiently stripped by 0.1 M thiourea in 1.0 M HCl from all the loaded organic phases – 83–92 mg L1 – with a maximum Pt(IV) contamination of 4 mg L1, and with slight traces of Fe(III) that did not surpass 1 mg L1. Fe(III) extraction by DMDCHTDGA and its efficient stripping by contact with water is not surprising, since several other thioamide, mono-, and malonamide derivatives previously investigated [19,26,27] showed a similar performance.  Accordingly, species of the general type [nLH+FeCl 4 (n  1)Cl ], with L being DMDCHTDGA and n varying between 1 and 3, are proposed as being responsible for Fe(III) extraction. In conclusion, the selectivity patterns shown by DMDCHTDGA for Pd(II) are rather promising; amongst the metal ions tested, only Fe(III) is extensively extracted together with Pd(II), but easily removed through an intermediate scrubbing step with water. The slight contaminations with Pt(IV) may be difficult to avoid; nevertheless, the lack of affinity observed for Al(III) configures DMDCHTDGA as a potential good extractant to be applied on Pd (II) recovery from leaching solutions coming from the treatment of anthropogenic sources such as palladium spent catalysts. Additionally, toluene is clearly more adequate as a diluent for the

Please cite this article in press as: O. Ortet, A.P. Paiva, Liquid–liquid extraction of palladium(II) from chloride media by N,N0 -dimethyl-N,N0 -dicyclohexylthiodiglycolamide, Separ. Purif. Technol. (2015), http://dx.doi.org/10.1016/j.seppur.2015.10.023

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O. Ortet, A.P. Paiva / Separation and Purification Technology xxx (2015) xxx–xxx

Table 2 Selectivity of DMDCHTDGA for extraction of two and three PGMs solutions (solutions 1 and 2, respectively), and three PGMs with Al(III) or Fe(III) (solutions 3 and 4, respectively) and three PGMs with Al(III) and Fe(III) (solution 5) evaluated through the correspondent separation factor values (SF). Initial aqueous phases: [Pd(II)] = [Pt(IV)] = [Rh(III)] = 100 mg L1; [Fe(III)] = [Al(III)] = 500 mg L1; organic phase: 0.03 M DMDCHTDGA in toluene. Standard deviations: ±5%. [HCl] (M)

4 6

Pd(II) separation factors (SF) Sol. 1

Sol. 2

Pt(IV)

Pt(IV)

Rh(III)

Sol. 3 Pt(IV)

Rh(III)

Al(III)

Sol. 4 Pt(IV)

Rh(III)

Fe(III)

Pt(IV)

Rh(III)

Al(III)

Fe(III)

354 170

289 133

1997 1997

314 154

2178 3188

7295 >10,000

20 161

996 >10,000

30 1

32 193

1648 3452

2263 >10,000

38 1

selective recovery of Pd(II) from mixed metal ion solutions than 1,2-DCE [23]. In the context of the research described in this article, the knowledge about the reactions involved in Pd(II) extraction by DMDCHTDGA is obviously important. Consequently, the interpretation of spectroscopic records (UV–visible, NMR, FTIR, Raman), together with distribution results and apparent molar volume data (these latter to rule out the possibility of aggregates) is in progress. The overall information collected up to now support the assumption that Pd(II) extracted species may be a mixing of innersphere complexes and outer-sphere ion-pairs, whose proportion seems to directly depend on the HCl concentration of the aqueous phase.

4. Conclusions N,N0 -dimethyl-N,N0 -dicyclohexylthiodiglycolamide (DMDCHTDGA), that previously showed an appreciable affinity to recover Pd(II) from concentrated HCl media, is investigated in more detail in this work, envisaging the evaluation of its practical usefulness as a SX reagent for complex chloride solutions. DMDCHTDGA in toluene is rather efficient for extracting Pd(II) until 5.5 M HCl, with a progressive reduction afterwards. The extraction kinetics is rather favorable (15 min maximum) and the equilibrium extraction isotherm points out to high loading Pd(II) contents, as an extractant:Pd(II) ratio of about 2.5 at saturation is reached. When subject to five extraction–stripping cycles, with an acidified thiourea solution as stripping medium, DMDCHTDGA exhibits comparable extraction and stripping Pd(II) distribution ratios for all the cycles. The selectivity profiles attained can be considered promising, since the Pd(II) distribution ratios are not critically affected by the coextraction of Fe(III), this latter metal being easily removed from the loaded organic phases by a water scrubbing step. A total separation of Pt(IV) from Pd(II) is not achieved, but Al(III), one of the main contaminants in real leaching solutions produced in the hydrometallurgical treatment of secondary raw materials, is not extracted. Further research with DMDCHTDGA is in progress, particularly regarding the interpretation of equilibrium, spectroscopic and apparent molar volume data information, to better understand the reactions responsible for Pd(II) extraction.

Acknowledgements The financial support for the work reported in this article has been kindly provided by Portuguese national funds through ‘‘FCT – Fundação para a Ciência e a Tecnologia” (Lisbon, Portugal) under the projects with references UID/MULTI/00612/2013 and PTDC/ QUI-QUI/109970/2009, including the PhD grant offered to Osvaldo Ortet (SFRH/BD/78289/2011).

Sol. 5

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Please cite this article in press as: O. Ortet, A.P. Paiva, Liquid–liquid extraction of palladium(II) from chloride media by N,N0 -dimethyl-N,N0 -dicyclohexylthiodiglycolamide, Separ. Purif. Technol. (2015), http://dx.doi.org/10.1016/j.seppur.2015.10.023

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Please cite this article in press as: O. Ortet, A.P. Paiva, Liquid–liquid extraction of palladium(II) from chloride media by N,N0 -dimethyl-N,N0 -dicyclohexylthiodiglycolamide, Separ. Purif. Technol. (2015), http://dx.doi.org/10.1016/j.seppur.2015.10.023