A comprehensive extraction study using a mono-alkylated diglycolamic acidextractant: Comparison between a secondary amide group and a tertiary amidegroup
T. Sugita1), I. Fujiwara1,2), H. Okamura1), T. Oshima2) , Y. Baba2) , H. Naganawa1) and K. Shimojo1)
1) : Interfacial Gr., JAEA 2): Miyazaki Univ.

Liquid-liquid extraction is an effective separation method forthe purification of valuable metals, removal of toxic metals, andrecovery of fission products. The efficiency of metal separation
with liquid-liquid extraction depends on the extractant. To date,numerous extractants, which have superior extraction andseparation ability, have been developed. Most novel extractants,
however, are expensive for large-scale applications because oflaborious and elaborate synthetic processes, and are unsuitablefor industrial extraction processes.
Recently, we developed N,N-dioctyldiglycolamic acid
(DODGAA, Fig. 1), which is a carboxylic acid-type extractant
with an amide group and an ether oxygen donor [1, 2]. The
extractant can be synthesized simply and readily in a single step,which helps to reduce its production cost. Although DODGAA isa 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 ability for a variety of metal ionscompared with commercial carboxylic acid-type extractants.
In this study, we comprehensively investigated the extractionof 56 metal ions using N-dodecyldiglycolamic acid (C12DGAA,Fig. 1). The obtained extraction data for C12DGAA werecompared with those for DODGAA to evaluate the effect of theamide group on the extractability and selectivity of DGAA-typeextractants. In addition, selective removal of toxic metal ionsfrom a metal ion mixture was carried out using C12DGAA.
In a preliminary experiment, we examined the distribution of
C16DGAA, C12DGAA, and C8DGAA (Fig. 1) between theorganic 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
were poorly soluble in isooctane. In contrast, the extractants werereadily 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 a result, C16DGAA was not dissolved in
isooctane containing 5 vol% 1-octanol. C12DGAA and C8DGAA
were soluble in the organic solution, but C8DGAA slightly
distributed into the aqueous phase. Therefore, C12DGAA wasselected as the extractant.
To evaluate the effect of amide group in DGAA-type extractant
on the extraction ability, extraction behavior for various metal
ions were compered between C12DGAA and DODGAA. Basedon the extraction data, the metal ions are divided into thefollowing two groups, depending on whether C12DGAA provides
a higher or lower extraction ability 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
In addition, we investigated here the selective separation ofHg(II) from aqueous solutions containing various divalent metalions (Hg(II), Pb(II), Cu(II), Cd(II), Zn(II), Mn(II), Co(II), andNi(II)) for the purpose of demonstrate the usefulness ofC12DGAA in removing toxic metal ions. As shown in Fig. 2,
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) and Pd(II) from the other metal ions, but mutual separation
between Hg(II) and Pb(II) is less efficient than with C12DGAA.
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 amidegroup, which leads to a stronger basicity of the amide oxygen
atom. In contrast, the DGAA framework with a secondary amidegroup (C12DGAA) makes the basicity of the amide oxygen atom
weaker. Therefore, it is obvious that DODGAA offers highcoordination ability for metal ions compared with C12DGAA.
However, C12DGAA tended to provide better extraction for
relatively small-sized metal ions compared to DODGAA, with afew exceptions. Although the reason for this difference is unclear
at this stage, we postulate that the steric hinderance of the alkyl
chain of the extractants is involved in the extraction of smallsized metal ions.

Fig. 1 Molecular structures and abbreviations of extractants.

Fig. 2 Extraction removal of the toxic metal Hg(II) fromvarious divalent metal ions with (a) C12DGAA and (b)DODGAA.
References[1] H. Naganawa et al., Solvent Extr. Res. Dev., Jpn. 14, 151(2007).
[2] K. Shimojo et al., Anal. Sci. 30, 513 (2014).