Solvent Extraction Research and Development, Japan, Vol. 24, No 2, 61 – 69 (2017)

A Comprehensive Extraction Study
Using a Mono-alkylated Diglycolamic Acid Extractant:Comparison Between a Secondary Amide Group and a Tertiary Amide Group
Tsuyoshi SUGITA1, Iori FUJIWARA1,2, Hiroyuki OKAMURA1, Tatsuya OSHIMA2,
Yoshinari BABA2, Hirochika NAGANAWA1 and Kojiro SHIMOJO1,*
1

Advance Science Research Center, Japan Atomic Energy Agency (JAEA), 2-4 Shirakata, Tokai-mura,

Ibaraki 319-1195, Japan; 2Department of Applied Chemistry, University of Miyazaki, 1-1 Gakuen
Kibanadai Nishi, Miyazaki 889-2192, Japan(Received December 28, 2016; Accepted March 3, 2017)
The extraction characteristics of N-dodecyldiglycolamic acid (C12DGAA), with a secondary amide group,for 56 metal ions have been investigated, and compared with those of N,N-dioctyldiglycolamic acid
(DODGAA) with a tertiary amide group. C12DGAA is capable of quantitative transfer for a variety of metal
ions through a proton-exchange reaction, and the extraction behavior as a function of the aqueous-phase pH
is similar for C12DGAA and DODGAA. Compared with DODGAA, C12DGAA has a poor extraction
performance and separation ability for rare-earth metal ions, except for Sc(III). However, C12DGAA tended
to provide better extraction for relatively small-sized metal ions than DODGAA. In addition, it was foundthat C12DGAA enables the selective removal of Hg(II) from aqueous solutions containing various divalent
metal ions (Hg(II), Pb(II), Cu(II), Cd(II), Zn(II), Mn(II), Co(II), and Ni(II)).

1. Introduction
Liquid–liquid extraction is an effective separation method for the purification of valuable metals[1-3], removal of toxic metals [4,5], and recovery of fission products [6-9]. The efficiency of metal
separation with liquid–liquid extraction depends on the extractant. To date, numerous extractants, whichhave superior extraction performance and separation ability, have been developed [10-14]. Most novel
extractants, however, are expensive for large-scale applications because of laborious and elaborate
synthetic processes, and are unsuitable for industrial extraction processes. Therefore, traditionalorganophosphorous

extractants

such

as

di(2-ethylhexyl)phosphoricacid(D2EHPA)and

2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (PC-88A) have been generally employed inindustrial processes.
Recently, we developed N,N-dioctyldiglycolamic acid (DODGAA, Figure1), which is a carboxylic
acid-type extractant with an amide group and an ether oxygen donor [15-21]. The extractant can besynthesized simply and readily in a single step, which helps to reduce its production cost. Although
DODGAA is a carboxylic acid-type extractant and has a simple molecular structure consisting only of C, H,
O, and N atoms, the compound provides an excellent extraction performance for a variety of metal ionscompared with commercial carboxylic acid-type extractants (e.g., Versatic 10). In fact, the extraction- 61 -

performance of DODGAA is comparable to that of organophosphorous extractants (e.g., D2EHPA and
PC-88A). This is attributed to the chelate effect created by the tridentate diglycolamic acid (DGAA)
framework.
We have been interested in the influence of amide groups on the extraction performance of
DGAA-type extractants. In a previous study, we synthesized N-dodecyldiglycolamic acid (C12DGAA,
Figure 1) with a secondary amide group (R–NH–C=O) as an analogous extractant to DODGAA with atertiary amide group (R–NR–C=O), and compared the extraction performance for rare-earth elements withboth extractants [22].
In the present study, we comprehensively investigated the extraction of 56 metal ions using
C12DGAA to collect basic data on the extraction performance and selectivity of C12DGAA. The obtained
extraction data for C12DGAA were compared with those for DODGAA to evaluate the effect of the amidegroup on the extraction performance and selectivity of DGAA-type extractants. In addition, selective
removal of toxic metal ions from a metal ion mixture was carried out using C12DGAA.

Figure 1. Molecular structures and abbreviations of extractants.
2. Experimental
2.1 ReagentsThe metal salts and metal ion standard solutions used in this study were: LiNO3, NaNO3, KNO3,
RbNO3, CsNO3, Mg(NO3)2·6H2O, Ca(NO3)2·4H2O, Sr(NO3)2, Ba(NO3)2, Al(NO3)3·9H2O, Cr(NO3)3·9H2O,
Mn(NO3)2·6H2O, Fe(NO3)3·9H2O, Co(NO3)2·6H2O, Ni(NO3)2·6H2O, Cu(NO3)2·3H2O, Zn(NO3)2·6H2O,
Ga(NO3)3·nH2O, AgNO3,

Cd(NO3)2·4H2O, In(NO3)3·3H2O, Hg(NO3)2·nH2O, TlNO3,

Pb(NO3)2,

Sc(NO3)3·4H2O, Y(NO3)3·6H2O, La(NO3)3·6H2O, Ce(NO3)3·6H2O, Pr(NO3)3·6H2O, Nd(NO3)3·6H2O,
Sm(NO3)3·6H2O, Eu(NO3)3·6H2O, Gd(NO3)3·5H2O, Tb(NO3)3·6H2O, Dy(NO3)3·5H2O, Ho(NO3)3·5H2O,
Er(NO3)3·5H2O, Tm(NO3)3·4H2O, Yb(NO3)3·3H2O, Lu(NO3)3·3H2O, HfCl4, RuCl3·nH2O, HAuCl4·4H2O,
Ti(SO4)2 in 1 M (1 M = 1 mol L−1) H2SO4 (1000 ppm), ZrO(NO3)2 in 1 M HNO3 (1000 ppm), NH4VO3 in
2-3% HNO3 (1000 ppm), NbF5 in 1 M HF (1000 ppm), TaF5 in 1 M HF (1000 ppm), Mo in 0.4 M HCl +0.2 M HNO3 (1000 ppm), Na2WO4 in H2O (1000 ppm), NH4ReO4 in H2O (1000 ppm), (NH4)2OsCl6 in 7%
HCl (1000 ppm), IrCl3 in 7% HCl (1000 ppm), RhCl3 in 0.1 M HCl (1000 ppm), PdCl2 in 1 M HCl (1000ppm), and H2PtCl6 in 1 M HCl (1000 ppm). The oxidation state of the Mo ion is unknown. All otherreagents were commercially available, of analytical grade, and used as received. Ultrapure water (18.2MΩ·cm), produced using a Direct-Q (Millipore), was used throughout this study.
2.2 Synthesis of DGAA-type extractants
C12DGAA and DODGAA were synthesized according to published procedures [15,16,18,22]. The
analogous extractant N-octyldiglycolamic acid (C8DGAA, Figure 1) was synthesized as follows: diglycolic- 62 -

anhydride (6.5 g, 53.2 mmol) was dispersed in dry CH2Cl2 (40 mL). Octylamine (7.0 g, 53.1 mmol)
dissolved in dry CH2Cl2 (10 mL) was slowly added dropwise to the solution in an ice-bath. The mixture
was stirred at room temperature for 22 h. The resulting clear solution was washed three times with 1 M HCl(3  60 mL) and water (4  100 mL). The organic layer was dried with anhydrous Na2SO4. After filtration,
the solvent was removed in vacuo to give the crude product, which was recrystallized from chloroform andn-hexane to obtain C8DGAA as a white powder (12.6 g, 96.8% yield). 1H NMR (400 MHz, CDCl3, TMS,
25°C): δ 0.88 (t, 3H, N–(CH2)7–CH3), 1.27, (d, 10H, N–CH2–CH2–(CH2)5–CH3), 1.54 (quintet, 2H,
N–CH2–CH2–C6H13), 3.31 (quartet, 2H, N–CH2–C7H15), 4.14 (s, 2H, N–CO–CH2–O), 4.21 (s, 2H,
CH2–COOH), 6.92 (s, 1H, NH). MS (MALDI-TOF) m/z: 246.1 [M + H]+. Anal. Calcd for C12H23NO4: C,
58.75; H, 9.45; N, 5.71. Found: C, 58.68; H, 9.50; N, 5.66.
The analogous extractant N-hexadecyldiglycolamic acid (C16DGAA, Figure 1) was synthesized asfollows: hexadecylamine (3.7 g, 15 mmol) was dissolved in dry THF (40 mL) by heating to 45°C.
Diglycolic anhydride (2.2 g, 18 mmol) dissolved in dry THF (15 mL) was slowly added dropwise to thesolution. The mixture was refluxed at 70°C for 1 day. After evaporation of the solvent in vacuo, theresulting crude product was dissolved in ethanol by heating and was re-precipitated by adding water. After
filtration, the compound was recrystallized from acetonitrile to obtain C16DGAA as a white powder (4.7 g,
87.6% yield). 1H NMR (400 MHz, CDCl3, TMS, 25°C): δ 0.88 (t, 3H, N–(CH2)15–CH3), 1.26 (d, 26H,
N–CH2–CH2–(CH2)13–CH3), 1.54 (quintet, 2H, N–CH2–CH2–C14H29), 3.31 (quartet, 2H, N–CH2–C15H31),
4.13 (s, 2H, N–CO–CH2–O), 4.21 (s, 2H, CH2–COOH), 6.76 (s, 1H, NH). MS (MALDI-TOF) m/z: 358.2[M + H]+. Anal. Calcd for C20H39NO4: C, 67.19; H, 10.99; N, 3.92. Found: C, 66.72; H, 11.02; N, 3.90.
2.3 Extraction procedure
Aqueous phases containing each metal ion were prepared with different pH values. The pH of theaqueous solutions was adjusted by the addition of either HNO3 or NaOH to a 10 mM or 100 mM
2-morpholinoethanesulfonic acid (MES) buffer. In the extraction tests with metal chlorides (Ru(III), Rh(III),
Pd(II), Hf(IV), Os(IV), Ir(III), Pt(IV), and Au(III)), the pH was adjusted by the addition of HCl to a 50 mM
sodium acetate buffer at a constant ionic strength of 0.1 M NaCl because the MES buffer can easily reduce
Au(III) to Au(0) [23-25]. In the extraction of K(I), Hg(II), and Ag(I), the contamination of aqueous
solutions by potassium ions and chloride ions from the pH electrode was prevented. For all aqueous phases,
stock solutions containing each metal salt were added to the aqueous solutions to give metal ion
concentrations of 0.01 mM. The organic phase was prepared by dissolving C12DGAA in isooctane
containing 5 vol% 1-octanol as a modifier. Equal volumes of the organic and aqueous solutions were mixed
and shaken mechanically at 25°C for 30 min to attain equilibrium. After separation of the two phases by
centrifugation, the metal ions in the organic phase were back-extracted into equal volumes of 1 M HNO3 or
1 M HCl. The concentrations of the metal ions in the aqueous phase and the receiving phase were
determined by inductively coupled plasma mass spectrometry (PerkinElmer NexION 300X) or inductively
coupled plasma atomic emission spectrometry (SII Nano Technology SPS 3500) to obtain the extractability(E = [Mn+]org/[Mn+]ini × 100). Mn+ denotes metal ions, and the subscripts org and ini denote the organic
phase and the initial condition, respectively. The equilibrium pH (pHeq) of the aqueous phases was alsomeasured. When the H+ concentration is over 0.1 M, the activity of H+ was calculated on the basis of the
Debye-Hückel theoretical equation. For comparison, extraction tests with DODGAA were also conducted- 63 -

according to the same procedure [19].
3. Results and Discussion
3.1 Extraction characteristics of C12DGAA for 56 metal ionsFigure 2. Extraction performance of C12DGAA for 56 metal ions as a function of pHeq in the aqueous
phase. Organic phase, 10 mM C12DGAA in isooctane containing 5 vol% 1-octanol; aqueous phase, 0.01
mM metal ions in corresponding buffer solutions. Extraction data on DODGAA were reconstructed on
the basis of the previous study [ref.19] and additional new data.
In a preliminary experiment, we examined the distribution of C16DGAA, C12DGAA, and C8DGAA
between the organic and aqueous phases. Although alkane solvents such as isooctane or dodecane are
acceptable in industrial extraction processes, the DGAA-type extractants with a linear alkyl chain werepoorly soluble in isooctane. In contrast, the extractants were readily soluble in 1-octanol. Thus, isooctane
containing a small amount of 1-octanol (5 vol%) was employed as an extracting solvent in this study. As aresult, C16DGAA was not dissolved in isooctane containing 5 vol% 1-octanol at all although C16DGAA
consists of the same number of carbon atoms as in DODGAA. C12DGAA and C8DGAA were soluble in theorganic solution, but C8DGAA slightly distributed from the organic phase into the aqueous phase,decreasing the pH values in the aqueous phases. Therefore, C16DGAA and C8DGAA are unsuitable forextraction of metal ions, and C12DGAA was selected as the extractant.
Figure 2 shows the extraction behavior of 56 metal ions using C12DGAA as a function of pHeq in the- 64 -

aqueous phase, together with that using DODGAA. For the alkali metal ions (group-1 elements), theextractability of the metal ions was negligible in both the C12DGAA and DODGAA systems. It is likely thatmonovalent alkali metal cations would form a 1: 1 complex with DGAA-type extractants to neutralize their
monovalent cationic charge. Therefore, the hydration waters of the metal ions are not sufficiently removed
and less hydrophobic metal complexes are formed.
For the alkaline-earth metal ions (group-2 elements), the extractability of the metal ions increased
with increasing pH. The metal ions were quantitatively extracted at around pHeq 5.2. The extraction
performance of C12DGAA was higher for Mg(II) and lower for Ca(II), Sr(II), and Ba(II) than that of
DODGAA. The selectivity of C12DGAA for the alkaline-earth metal ions was in the following order: Ca(II)> Sr(II) = Ba(II) > Mg(II), and C12DGAA exhibited a low separation ability for the alkaline-earth metal
ions compared with DODGAA (Ca(II) > Sr(II) > Ba(II) > Mg(II)). In addition, slope analysis showed thatthe slopes of the logarithmic distribution ratio versus pHeq plots were approximately 2 for the alkaline-earth
metal ions, except for Mg(II). The results indicate that two protons from the C12DGAA molecules werereleased to form neutral complexes with divalent alkaline-earth metal ions through a proton-exchange
reaction. The slope for Mg(II) was 0.98, which is not equivalent to the valence of Mg(II). It is likely thathydroxylated Mg(OH)+ is involved in the extraction [26].
For rare-earth (RE) metal ions (group-3 elements), C12DGAA enabled quantitative transfer of light
RE(III) at pHeq ≥ 3.5 and middle and heavy RE(III) at pHeq ≥ 2.8 [22]. Although C12DGAA performs better
than typical carboxylic acid-type extractants, the metal transfer with C12DGAA occurred in higher pH
ranges than that with DODGAA. The results indicate that C12DGAA has a lower extraction performancefor RE(III) cations than DODGAA. Both DGAA-type extractants exhibited selectivity for the heavier
RE(III) because the higher charge densities of the heavier RE(III) probably facilitated electrostaticinteractions with the anionic extractants. In particular, C12DGAA showed the highest selectivity for Sc(III)among 16 RE(III) cations, contrary to the low selectivity of DODGAA for Sc(III). However, C12DGAA had
a much lower mutual separation ability of individual RE(III) cations from each other than DODGAA. In
contrast, the extraction performance of DGAA-type extractants was much higher for RE(III) cations
compared with the other metal ions tested in this study, thus the extractants are useful for the separation of
RE(III) cations from foreign metal ions. In addition, slope analysis and structural characterization by X-ray
diffraction revealed that three C12DGAA molecules coordinated to the RE(III) cation in a tridentate fashion
through three oxygen atoms from the amide group, three oxygen atoms from the ether group, and threeoxygen atoms from the carboxy group, forming a 1:3 complex, RE(C12DGAA)3 [22].
For the group-4, 5 elements, Mo, W(VI), and Re(VII), the maximum extractabilities of Ti(IV) andZr(IV) with C12DGAA were approximately 40 and 55%. Hf(IV) was quantitatively extracted. In contrast,
the group-4 elements were not completely back-extracted from the organic phase with a 1 M HNO3
solution as the receiving phase. This is possibly attributed to aggregation of the unstable metal complexes
at the liquid−liquid interface. The extraction behavior of V(V) with C12DGAA was almost identical to thatwith DODGAA, and around 10% of V(V) was transferred. The stripping of V(V) was also successfully
achieved using a 1 M HNO3 solution. The partitioning of Nb(V), Mo, Ta(V), W(VI), and Re(VII) was
negligibly small.
For Cr(III), Mn(II), Fe(III), Co(II), Ni(II), and Cu(II), quantitative extraction was achieved in mildly- 65 -

acidic solutions and C12DGAA showed the following selectivity for the metal ions: Fe(III) >> Cu(II) >Cr(III) > Mn(II) = Co(II) = Ni(II). The extraction efficiency of Cr(III) was greatly enhanced by using
C12DGAA compared with DODGAA. In addition, a slightly greater extraction performance of C12DGAA
for Mn(II), Fe(III), Co(II), and Ni(II) was observed. The slopes of the logarithmic distribution ratio versus
pHeq plots for each metal ion were consistent with the valence of the corresponding metal ions, except for
Cr(III). The slope for Cr(III) was 1.6, likely because hydrolysis of Cr(III) partially occurred in the pH rangeof the extraction [26].
For noble metal ions (platinum group elements, Ag(I), and Au(III)), C12DGAA showed highextraction performance for Ru(III) and Pd(II), as in the case of DODGAA. However, complete
back-extraction of Ru(III) and Pd(II) was difficult using a 1 M HNO3 solution although back-extraction
was possible in the DODGAA system [19]. The other platinum group elements (Rh(III), Os(IV), Ir(III),
Pt(IV)) were not extracted at all. A similar extraction behavior was observed for Ag(I) when using
C12DGAA and DODGAA, and around 10 % of Ag(I) was extracted into the organic phase. In contrast,
Au(III) exhibited a different extraction behavior between C12DGAA and DODGAA, and C12DGAA did notshow any extractability for Au(III).
For the group-12 elements and Pb(II), C12DGAA quantitatively extracted all of the metal ions withthe following selectivity: Hg(II) >> Pb(II) >> Cd(II) > Zn(II). Compared with DODGAA, C12DGAA
showed a slightly higher extraction performance for Hg(II) and somewhat lower extraction performance for
Pb(II). The extraction behavior of Cd(II) and Zn(II) was similar between C12DGAA and DODGAA. The
slopes of the logarithmic distribution ratio versus pHeq plots for each metal ion were 0.9 for Hg(II), 2.1 for
Pb(II), 2.2 for Cd(II), and 1.9 for Zn(II). The slope values are consistent with the valence of thecorresponding metal ions, expect for Hg(II). To demonstrate the usefulness of C12DGAA in removing toxic

Figure 3. Extraction removal of the toxic metal Hg(II) from various divalent metal ions with (a)C12DGAA and (b) DODGAA. Organic phase, 10 mM C12DGAA or DODGAA in isooctane containing 5
vol% 1-octanol; aqueous phase, 0.01 mM metal ions in MES buffer solutions. Each aqueous phase
contained 8 divalent metal ions.

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metal ions, we investigated here the selective separation of Hg(II) from aqueous solutions containing
various divalent metal ions (Hg(II), Pb(II), Cu(II), Cd(II), Zn(II), Mn(II), Co(II), and Ni(II)). As shown in
Figure 3, C12DGAA provided the highest selectivity for Hg(II) among 8 divalent metal ions, which permits
the selective removal of Hg(II) from the metal mixture. In contrast, DODGAA can remove Hg(II) andPb(II) from the other metal ions, but mutual separation between Hg(II) and Pb(II) is less efficient than with
C12DGAA.
For the group-13 elements, quantitative partitioning was observed for the metal ions, expect for Tl(I),and C12DGAA had the following selectivity for the group-13 elements: In(III) > Ga(III) > Al(III) >> Tl(I).
Compared with DODGAA, the extraction performance of C12DGAA was slightly higher for Al(III) andGa(III), and was a little lower for In(III). Neither C12DGAA nor DODGAA were very effective for Tl(I)
extraction. The slopes of the logarithmic distribution ratio versus pHeq plots for Al(III), Ga(III), and In(III)were 3.4, 3.2, and 3.0, respectively.
3.2 Classification of metal ions based on extraction efficiency
Based on the extraction data described above, the metal ions are divided into the following two
groups, depending on whether C12DGAA provides a higher or lower extraction performance for the metal
ions than DODGAA:● C12DGAA > DODGAA
Mg(II), Al(III), Sc(III), Ti(IV), Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Ga(III), Pd(II), Hf(IV), Hg(II)● C12DGAA < DODGAACa(II), Sr(II), Y(III), Zr(IV), Ru(III), In(III), Ba(II), Au(III), Pb(II), Lanthanides
The DGAA framework with a tertiary amide group (DODGAA) has a relatively rigid molecular geometry
because of the partial double-bond character of the C–N bond of the amide group, which leads to a stronger
basicity of the amide oxygen atom [27]. In contrast, the DGAA framework with a secondary amide group(C12DGAA) makes the basicity of the amide oxygen atom weaker. Therefore, it is obvious that DODGAA
offers high coordination ability for metal ions compared with C12DGAA. In fact, the disadvantage of
C12DGAA was clearly observed for lanthanide transfer. However, C12DGAA tended to provide better
extraction for relatively small-sized metal ions compared to DODGAA, with a few exceptions. Although
the reason for the difference in extraction behavior between C12DGAA and DODGAA is unclear at thisstage, we postulate that the steric hinderance of the alkyl chain of the extractants is involved in theextraction of small-sized metal ions. In summary, the 56 metal ions are classified in the following sevengroups, according to the degree of extraction performance of C12DGAA for metal ions:(1) Quantitative extraction at pHeq > 2.8
Hf(IV), Sc(III), Lu(III), Yb(III), Tm(III), Er(III), Ho(III), Dy(III), Tb(III), Fe(III), Gd(III), Eu(III),
Y(III), Sm(III)(2) Quantitative extraction at pHeq > 3.5
Hg(II), In(III), Nd(III), Pr(III), Ce(III), La(III)(3) Quantitative extraction at pHeq > 4.0
Ga(III), Pb(II), Al(III)(4) Quantitative extraction at pHeq > 5.0
Cu(II), Cr(III), Cd(II), Zn(II)- 67 -

(5) Quantitative extraction at pHeq > 5.5
Ca(II), Mn(II), Co(II), Ni(II), Pd(II), Sr(II), Ba(II), Mg(II), Ru(III)(6) Extractability < 60%
Zr(IV), Ti(IV), V(V), Ag(I)(7) Unextractable metal ions
Li(I), Na(I), K(I), Rb(I), Cs(I), Nb(V), Mo, Ta(V), W(VI), Re(VII), Rh(III), Os(IV), Ir(III), Pt(IV),
Au(III), Tl(I)The metal ions in each category are arranged in order of high extractability.
4. Conclusion
In the present study, liquid–liquid extraction of 56 metal ions using C12DGAA with a secondary
amide group was comprehensively investigated, and compared with the results obtained using DODGAA
with a tertiary amide group. The extraction performance of C12DGAA for RE(III) cations was lower
compared to DODGAA. However, C12DGAA provided a slightly higher extraction performance forsmall-sized metal ions than DODGAA. In addition, C12DGAA showed the highest selectivity for Hg(II)among various divalent metal ions, thereby separating selectively Hg(II) from the metal mixture, whereas
DODGAA had difficulty in separating Hg(II) from Pb(II). The present findings suggest that the amidegroup of DGAA-type extractants has a significant influence on the extraction characteristics of theextractants. We might be able to control the extraction performance and separation ability of DGAA-type
extractants by modulating the chemical property of the amide group. Further studies on development ofnovel extractants by modification of the DGAA framework with appropriate functional groups are currently
under way.
Acknowledgement
This work was financially supported by JSPS KAKENHI Grant Number 25410161 (to K.S.). We
thank the Center for Instrumental Analysis at Ibaraki University for the elemental analysis, NMR, and
MALDI-TOF/MS measurements.
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