Indones. J. Chem., 2023, 23 (6), 1735 - 1741

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Short Communication:
Extraction of Au(III), Pt(IV), and Pd(II) from Aqueous Media with Deep Eutectic Solvent
Dissolved in n-Heptane as Extractant
Pius Dore Ola1* and Michiaki Matsumoto2
1

Department of Chemistry, Faculty of Science and Engineering, Nusa Cendana University,
Jl. Adisucipto No. 10, Penfui, Kupang 85001, Indonesia

2

Department of Chemical Engineering and Materials Science, Doshisha University, Kyotanabe, Kyoto 610-0321, Japan* Corresponding author:
email: pius_ola@staf.undana.ac.id
Received: January 2, 2023
Accepted: September 5, 2023
DOI: 10.22146/ijc.80862

Abstract: Due to the scarcity, studying precious metals extraction such as gold,
platinum, and palladium is very critical. Solvent extraction is one of the establishedmethods due to its simplicity and quickness but the low operational cost. In this study, adeep eutectic solvent (DES) consisting of decanoic acid and lidocaine in the molar ratioof 2:1 was applied as an extractant to extract Au(III), Pt(IV), and Pd(II) from aqueous
media. Au(III) was solvated and extracted with DES in the form of AuCl3. Pd(II) was
extracted by the cation exchange reaction between Pd2+ and H+. Complete extraction ofAu(III) 98.2% and Pd(II) 100% can be achieved using 500 and 300 g/L DES, respectively.
This finding opens new opportunities for improving Au(III) and Pd(II) extraction using
an environmental friendly and inexpensive extractant.
Keywords: Au(III); deep eutectic solvent; Pd(II); Pt(IV); solvent extraction■ INTRODUCTION
Valuable metals like gold (Au), platinum (Pt), andpalladium (Pd) become strategic elements due to thescarcity of resources and uneven distribution of metal
production. They are extensively used not only forjewellery but also for the production of various advanced
materials like fuel cells and electronic devices, as well as
for medical use [1]. Therefore, the resources' scarcity andextensive applications encourage researchers to study theseparation process of precious metals intensively.
Liquid-liquid extraction is a very establishedmethod due to its simplicity and quickness but the low
operational cost [2-4]. In most cases of metal extraction,hydrophobic extractants such as chelating reagents andacidic organophosphorus compounds were needed topartition metal ions into the organic solvent phase [5].
Recently, ionic liquids have been successfully applied as
extractants in this technique for separating precious
metals [6-10]. However, the complexity of their synthesis,
particularly in the purification step, causes ionic liquids tobe high cost and will limit their practical use for larger-

Pius Dore Ola and Michiaki Matsumoto

scale applications [11]. A comparable solvent called deep
eutectic solvent (DES) was developed to address theionic liquid's restrictions.
DES is identified by a notable reduction in its
melting point upon combining two or three affordable
and safe compounds, which function as a hydrogenbond donor (HBD) and a hydrogen bond acceptor(HBA), respectively [12]. The melting point of DES isnotably decreased compared to its components,
primarily because of a large, asymmetrical ion with low
lattice energy [13-14]. Because mixing HBD and HBA toform DES is performed at a moderate temperature anddoes not need the purification step, DES synthesis iseasier than that of ionic liquid. In addition, because most
HBD is a natural product; DES is environmentallybenign, safe, and sustainable [15]. An extensively
explored application of DES involves its utilization as an
extractant of metal ions within the solvent extraction
technique. In our previous study, DES, which iscomposed of decanoic acid and lidocaine dissolved in nheptane was successfully employed as an extractant to1736

Indones. J. Chem., 2023, 23 (6), 1735 - 1741

extract transition metal ions, including Fe(III), Mn(II),Co(II), Ni(II), and Cu(II) [16-17]. n-Heptane was
employed as a diluent agent for DES because water wastransferred to the DES phase without dilution, generating
a considerable change in the volume of both phases frombefore to after equilibration [18]. In this study, we
examined the extraction of valuable metals such as
Au(III), Pt(IV), and Pd(II) from aqueous solutions using
a DES consisting of lidocaine and decanoic acid dissolved
in n-heptane as the extracting agent, and the discussion
was focused on the extraction mechanism.
■

EXPERIMENTAL SECTION

Materials

The Wako Pure Chemicals company provided
decanoic acid, lidocaine, and sodium acetate trihydrate.
Fig. 1 shows the molecular structure of the DES
components. We bought HAuCl4, HPtCl5, PdCl2 from
Nacalai Tesque, Inc. in Kyoto. Without additional
purification, all GR-grade compounds were utilized.
Instrumentation

This work used a thermostat water bath in the
shaking process. The metal concentration in the aqueous
phase was determined using inductively coupled plasma
atomic emission spectroscopy (ICP-AES, ICPS-8100,
Shimadzu, Kyoto, Japan). A pH meter (F-52, Horiba,
Kyoto, Japan) was used to measure the pH value of theaqueous solution.
Procedure
DES preparation

DES was prepared by mixing decanoic acid as HBD
and lidocaine as HBA following the procedure according

to the previous paper [19]. HBD and HBA were weighedin a beaker at a predefined molar ratio(lidocaine:decanoic acid = 1:2), and covered with parafilm to avoid water contamination. The mixture wasstirred at a temperature of 323 K until a transparent andconsistent liquid was witnessed. Subsequently, the DESs
were left at room temperature overnight to ensure theprevention of any recrystallization.
The procedure of solvent extraction

Due to the DES structure, hypothetically, the pH of
the metal ion solution highly influences the metal ion
extraction. This study investigated the impact of the pHof the initially prepared metal ion solution on
extractability. The data can also be used to predict theextraction mechanism. The detailed procedure forextracting three kinds of metal ions is as follows: therequired concentration of the organic phase wasprepared by dissolving an exact amount of DES diluted
in a certain volume of n-heptane. The aqueous phase wasprepared by mixing 0.1 mol/L of nitric acid, 0.1 mol/L ofacetic acid, and 0.1 mol/L of sodium acetate containing
10 mmol/L of metal ion, which made this media bufferedby acetate at the pH of higher than 1. A thermostat water
bath set at 303 K was used to mix and shake 2 mL of eachphase at 120 rpm. ICP-AES spectroscopy was used todetermine the metal content in the aqueous solution
after 3 h of shaking to reach equilibrium. A pH meter
was used to measure the aqueous solution's pH values.
Percentage extraction (%E) and distribution ratio (D)were calculated using Eq. (1) and (2):%E D[M]aq,int  [M]aq,eq[M]aq,int

 100[M]aq.int  [M]aq.eq[M]aq.eq(1)(2)

where [M] denotes the metal concentration, aq is theaqueous phases, int and eq are the initial andequilibrium states.
■
Fig 1. DES components' molecular structures: (a)decanoic acid, (b) lidocaine (2-(diethylamino)-N-(2,6dimethylphenyl)acetamide

Pius Dore Ola and Michiaki Matsumoto

RESULTS AND DISCUSSION

In our previous study, the maximum extractability
to transition metals was shown by DES, which consistedof decanoic acid as HBD and lidocaine as HBA in a 2:1

Indones. J. Chem., 2023, 23 (6), 1735 - 1741

molar ratio compared to the combinations of other DES
constituents, dodecanoic acid and DL-menthol [17].
Therefore, we employed this type of DES as an extractant
to extract precious metals in this work.
Extraction of Precious Metals

Firstly, we evaluated the effects of DES concentrationand initial pH on %E of Au(III), Pt(IV), and Pd(II), whichis shown in Figs. 2 and 3. From Fig. 2, metal extractabilities
were in the following order Pd(II) > Au(III) >> Pt(IV).
Pt(IV) was scarcely extracted with DES. Fig. 3 shows that

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Au(III) percentage extraction reached 95%, far higher
than the percentage extraction of Pt(IV) (lower than 9%)
at initial pH = 1 and the percentage extraction increased
in decreasing the initial pH for Au and Pt. Structurally,
decanoic acid can donate a proton, and lidocaine is a
proton acceptor, so the pH of metal ion solution highly
affects the percentage extraction of metal ions.
Considering the dependency of percentage extraction on
the initial pH of Au(III) solution, experiments wereattempted at higher hydrochloric acid concentrations(≥0.5 mol/L) to dissolve Au(III). In this case, the %E

Fig 2. Effect of DES concentration on the percentage extraction of Au(III) (initial pH 1), Pt(IV) (initial pH 1), andPd(II) (initial pH 4.4)

Fig 3. Effect of the initial pH of the metal ion solution on the percentage extraction (DES conc. = 300 g/L for Au(III)and Pt(IV), 50 g/L for Pd(II))

Pius Dore Ola and Michiaki Matsumoto

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Indones. J. Chem., 2023, 23 (6), 1735 - 1741

reached 97%, but the solution tended to emulsify. On theother hand, the %E increased with the initial pH for
Pd(II). This suggests that Au(III) and Pt(IV) have distinct
extraction mechanisms than Pd(II). We also used asolution of lidocaine and decanoic acid as the sole
extractant to extract Au(III) to help explain the extraction
mechanism, and the results are given in Fig. 4. From Fig.
4, it can be thought that the extraction of Au(III) by DES
shown in Fig. 3 was contributed by lidocaine as HBA. Fig.
4 also shows that %E of Au(III) with lidocaine solutions
as an extractant reached 85%, approach to the Au(III)percentage extraction using DES as an extractant.
However, the mixture of lidocaine solution and Au(III)solution was emulsified; and then, DES was still superior
to lidocaine.
Extraction Mechanisms

The %E in Figs. 2 and 3 were converted to D value
and the relations between D and pH at equilibrium or
DES concentration to clarify the extraction mechanismof Au(III) and Pd(II) were shown in Figs. 5 and 6,
respectively, with a logarithmic scale.
From Fig. 5, the slopes between log D of Au(III)and Pd(II), and pH at equilibrium were −3.8 and 0.37,
respectively. From Figs. 3 and 5, the pH values changed
significantly between initial and equilibrium states andFig 4. The extractability of Au(III) using decanoic acid solution (100 g/L) and lidocaine solution (50 g/L) with nheptane as solvent

Fig 5. Relation between D of Au(III) and Pd(II) and equilibrium pH in the extraction with DES

Pius Dore Ola and Michiaki Matsumoto

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Indones. J. Chem., 2023, 23 (6), 1735 - 1741

Fig 6. Relation between D of Au(III) and Pd(II) and DES concentration in the extraction with DES
pH values at equilibrium were regulated within a narrow
range from 4.8 to 5.8. This may be caused by the strong
buffering effect of lidocaine solution [19]. Because pH
dependency in a narrow pH range possesses lower
reliability, what we found in Fig. 5 is a negative correlation
between D and pH for Au(III) and a positive correlation
between D and pH for Pd(II). Generally, a positive
correlation between D and pH indicates the extraction
reaction contains the ion exchange between the metal
cation and proton. In the Au(III) extraction, HAuCl4 is
prone to hydrolyze in the aqueous solution to form

AuCl3OH−.
Fig. 6 clearly shows that the slopes between D and
DES concentration were 1 for Au(III) and 2 for Pd(II).
Based on these results, Au(III) and Pd(II) were extractedwith DES ((RCOOH)2·Lid) according to the following
reactions:
AuCl 3OH (aq)  (RCOOH)2  Lid(org)  H(aq) AuCl 3 (RCOOH)2 (Lid)2(org)  H2O(aq)Pd2 (aq)  2((RCOOH)2 Lid)(orq) Pd(RCOO)2 (RCOOH)2 (Lid)2(org)  2H(aq)Table 1. Comparison of Au(III) and Pd(II) extraction in this work with those in the previous work
Metal
Au(III)

Pd(II)

Extractant
500 g/L DES(Lidocaine + decanoic acid)
Cyphos IL 109
2-Octylaminopyridine
MBIMTT
PKFAD[C16mim]+Cl−
300 g/L DES (Lidocaine + decanoic acid)[P66614]+Br−[P66614]+Cl−
N-Hexadecylpiperidinium chloride
N-Hexadecylpyridinium chloride

Diluent
n-Heptane

Aq. solution
Acetate media

D
55.7

%E
98.2

Ref.
This work

XyleneXyleneChloroform
No diluent
Chloroform
n-Heptane
TolueneXyleneToluene
Chloroform

Chloride media
Malonate media
Chloride media*
Chloride media
Chloride media
Acetate media
Chloride media
Chloride media
Chloride media
Chloride media

45.7
∞**
-

99.4
99.5
99.6
99.0
99.6
100.0
100.0
98.0
±100.0
98.8

[20]
[21]
[22]
[23]
[24]
This work
[25]
[26]
[27]
[28]

MBIMTT: (4-methoxybenzylideneimino)-5-methyl-4H-1,2,4-triazole-3-thiol*Chloride media in the presence of other metal ions
PKFAD: palm kernel fatty acid distillate∞**: The equilibrium aqueous metal concentration was not detectable through ICP

Pius Dore Ola and Michiaki Matsumoto

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Indones. J. Chem., 2023, 23 (6), 1735 - 1741

We also compared the extraction efficiency and/or
distribution ratio with the other extractants, as shown in
Table 1. Table 1 shows that the extraction efficiency
and/or distribution ratio with DES composed oflidocaine-decanoic acid as extractant to extract Au(III)and Pd(II) was not inferior to the other extractants.
However, DES is superior from an economic andecological point of view because lidocaine and decanoic
acid can be found in natural products. Then, DES ischeaper and more environmentally friendly.
■

CONCLUSION

This study demonstrates that extracting valuable
metals using a deep eutectic solvent (DES) composed oflidocaine and decanoic acid is notably influenced by the
pH of the initial metal ion solution. Gold ion was solvatedand extracted with DES in the form of AuCl3. The
extractability of platinum ion was very low (<9%).
Palladium ion underwent extraction through a cationexchange reaction involving Pd2+ and H+. Au(III) andPd(II) were fully extracted using 500 and 300 g/L of DES,
respectively.
■

ACKNOWLEDGMENTS

The author would like to thank the Department ofChemistry, Faculty of Science and Engineering, Nusa
Cendana University, for supporting the publication ofthis article.
■

AUTHOR CONTRIBUTIONS

Pius Dore Ola formulated the idea, performed theexperiments and data analysis, wrote the original draft,
and revised the manuscript. Michiaki Matsumoto
formulated the idea, secured funding, supervised the
project, and revised the manuscript.
■

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