Elution solutions
Mineral acids
In the adsorption-based separation process, the use of mineral acids for
desorption of adsorbed ions and regeneration of the adsorbent is
commonplace. However, in the case of REEs chromatographic separation
using strongly acidic cation-exchange resin, the use of mineral acids
such as HCl, H2SO4,
HNO3, and HBr as eluents results in very low separation
among REEs (Korkisch et al., 1967; Nelson, 1965). However, these acids
can be used to separate REEs from other monovalent and divalent ions
using strongly acidic cation resin (Fritz & Garralda, 1963; Page et
al., 2019; Strelow, 1960; Strelow et al., 1965). Between various strong
mineral acids, the distribution coefficients (Kd) for
different REE ions with strongly acidic cation-exchange resin are higher
for HCl (Strelow, 1960) than H2SO4 and
HNO3 (Strelow et al., 1965) (i.e., in 0.5 M HCl,
Kd = 2480, 2460, 1460 in 0.5 N H2SO4;
Kd = 1860, 1800, 1380 and in 0.5 N HNO3;
Kd = 1870, 1840, 1020 for La(III), Ce(III), and Y(III),
respectively).
In the case of weakly acidic cation-exchange resins, strong mineral
acids can be used for desorption. At very low pH, the excess of hydrogen
ions results in the desorption of REE ions from the resin surface
through ion-exchange. Thus, strong mineral acids are used for the
regeneration of resin in many cases. For example, HCl was used with
various carboxyl based weakly acidic cation-exchanger D152 (0.5 M HCl)
(Xiong et al., 2008), D155 (0.25 M HCl+0.5 M NaCl) (Xiong, 2008), D113
(4 M HCl) (Xiong et al., 2009), and D151 (0.5 M HCl) (Yao, 2010). In the
case of strongly basic anion-exchangers, the elution of the adsorbed
anionic REE-ligand complex is carried out with HNO3,
H2SO4, NH4SCN, and
CH3COOH solutions (Faris & Warton, 1962; Hubicki &
Olszak, 1998, 2002; Koodynska & Hubicki, 2012).
Similarly, chelating resins can be regenerated using mineral acids. In a
study with multiple chelating resins, Page et al. (2017) achieved high
elution (>97%) for iminodiacetic (IDA) resins with 0.1-1 M
HCl, the highest being 99.9% with 1 M HCl. For sulfonic/phosphonic (SP)
and aminophosphonic (AP) resins, a higher strength of HCl (2-4 M)
achieved 58-80% elution. In another study, 2 M HCl and 2 M
HNO3 were used for desorption of La(III), Ce(III), and
Nd(III) ions from chelating resin Dowex M 4195; however, the highest
desorption was only 43.9% for Ce(III) achieved with 2 M
HNO3 (Kołodyńska et al., 2019). Other examples of
mineral acid as eluent for chelating resins are 2 M HCl for
iminodiacetic acid resin (IDAAR) (Xiong et al., 2006); 1 M HCl, 6 M HCl,
1 M HNO3, and 6 M HNO3 for Tulsion CH-90
resin (Dutta et al., 2008).
Mineral acid, HNO3, was used in the desorption step for
REE adsorbed onto alumina and hematite (Kosmulski, 1997b; Marmier et
al., 1997). The elution of REE from amorphous ZrP with 1 M mineral acids
decreased in the order of H2SO4> HNO3 > HCl >
H3PO4, with the highest being 85.1% and
the lowest of 73.2% ( Xu et al., 2018). In the case of inorganic TiP,
0.5 M HNO3+0.5 M H3PO4eluted 98.3% Sc(III) (Wenzhong Zhang, Koivula, et al., 2017). Other
inorganic nanomaterials have been desorbed using 0.001 M HCl (Zhao et
al., 2019), 0.3 M HCl (Marwani et al., 2018), 0.2 M HNO3(Ghobadi et al., 2017, 2018), and 0.05-0.8 M HNO3 (Tu &
Johnston, 2018). 0.2 M HNO3 desorbed >80%
REE from
Co0.2Mn0.8Fe2O4and
Co0.8Mn0.2Fe2O4(Ghobadi et al., 2017) and >90% REE from
MnFe2O4 and MnFe2O4-GO
(Ghobadi et al., 2018). With CuFe2O4nanoparticles, 100% REE desorption was obtained with 0.05-0.8 M
HNO3 (Tu & Johnston, 2018).
Mineral acid HCl is very frequently used as an eluent in
solvent-impregnated resins. The examples of the use of different
concentrations of HCl as eluent for SIRs include the use of 0.15 M HCl
(Lee et al., 2010a, 2010b), 0.2 M HCl (Park et al., 2005), 0.5 M HCl
(Yin et al., 2020), 1 M HCl (Kumar et al., 2011), and 2 M HCl (Aardaneh
et al., 2008). Kumar et al. (2011) used HCl, HNO3,
H2SO4, and
(NH4)2CO3, and reported
99.7%, 99.1%, 100%, and 74.6% elution at 1 M concentration. HCl with
strength >0.1 M achieved almost complete desorption of
La(III) from P227-impregnated XAD-7HP (Yang et al., 2020). Nitric acid
solutions are also effective for elution from different SIRs (Helaly et
al., 2012; Louis & Duyckaerts, 1984, 1985). İnan et al. (2018) used
0.01 M HNO3 to desorb REEs from Cyanex impregnated
Amberlite XAD-7. 0.1 M HNO3 was used as a stripping
agent for SIL obtained through ([A336] [CA-100]) impregnation
into XAD-7 (Sun et al., 2009). Another SIL loaded with Sc(III) was
stripped using 1 M HCl and 1 M HNO3 with efficiencies
>90% and 100% with 1 M
H2SO4 (Avdibegović et al., 2017).
Similarly, for surface-functionalized adsorbents and ion-imprinted
polymers, mineral acids are mostly used for the desorption of REEs. The
strength of the acid solution used for desorption has varied from 0.1 M
HCl to 2.0 M HCl (e.g., 0.1 M HCl (Ahmed, Lee, et al., 2019; Kostenko et
al., 2019; Y.-R. Lee et al., 2018; Ryu et al., 2021), 0.25 M HCl (Hua et
al., 2019), 0.3 M HCl (Lou et al., 2019), 0.5 M HCl (Chen et al., 2014;
Ravi, Lee, et al., 2018), 1.0 M HCl (Babu et al., 2018; Gasser et al.,
2017; Moussa et al., 2017), 2 M HCl (Li et al., 2019; Qin et al., 2022;
Yusoff et al., 2017), and 1.0-2.0 M HCl (Ogata et al., 2015a)). Other
mineral acids such as HNO3 with the strength of
0.01-6.55 M (Ashour et al., 2017; Shuangyou Bao et al., 2022; Bertelsen
et al., 2019; Callura et al., 2018; Chen et al., 2014; Noack et al.,
2014; Rahman et al., 2020; Ravi, Zhang, et al., 2018; Zhao et al., 2021)
and H2SO4 (e.g., 0.1 M - 1 M
H2SO4 (Ogata et al.,
2015a), 0.1 M H2SO4(Gaete et al., 2021)) have been used as well. Mineral acids have usually
provided desorption in the order of 90-100% for functionalized
adsorbent and ion-imprinted polymers. The mechanism of desorption with
mineral acid is the ion-exchange reaction between protons and the
adsorbed ions (Ogata et al., 2015a).
Organic acids and complexing
ligands
Along with mineral acids, various organic acids and complexing ligand
solutions have been used for the desorption of REEs from different
adsorbents. In the case of strongly acidic cation-exchange resins, the
organic acid and complexing ligands play a vital part as eluent since
they can increase the chromatographic separation between REEs in the
elution step due to their difference in affinity for different REEs
(Boyd, 1978; B. Chen et al., 2017; Spedding et al., 1947, 1954; Strelow
& Victor, 1990). The examples of organic acids used as eluents in REEs
separation include acetic, malonic, phthalic, and other carboxyl acids,
hydroxy acids, phosphonic, and amino phosphonic acids (Koodynska &
Hubicki, 2012).
However, the best results for REEs desorption and separation from strong
acidic cation-exchange resins were obtained using complexing ligand as
eluent, primarily eluents based on polyamino-polycarboxylic acid such as
ethylenediaminetetraacetic acid (EDTA) (Smith & Hoffman, 1956; Spedding
et al., 1954a; Spedding & Powell, 1954); Diethylenetriaminepentaacetic
acid (DTPA) (Chuveleva et al., 1974, 1995; Hale & Hammer, 1972;
Kharitonov et al., 2009b; Kogan & Ratner, 1971), nitrilotriacetic
(Kharitonov et al., 2009a; Martynenko et al., 1968, 1972; Wheelwright,
1969), hydroxy-ethylethylenediaminetriacetic (Hagiwara, 1969; James et
al., 1961; Strelow & Victor, 1990), cyclohexane-1,2-diaminetetraacetic
(Gschneidner & Eyring, 1982), and iminodiacetic acids (Martynenko et
al., 1972).
The advantage of using polyamino-polycarboxylic acid as eluent is that
they form strong complexes with REE, reducing the consumption of ligand
and resulting in concentrated elutes. Also, the stability constant of
different REEs with these complexes differs more than their ion-exchange
constants, which results in a higher degree of separation ( Chen et al.,
2017; Ehrlich & Lisichkin, 2017; Koodynska & Hubicki, 2012). The most
widely used polyamino polycarboxylic is ethylenediaminetetraacetic acid
(EDTA). EDTA is readily available, inexpensive, and easy to regenerate.
Additionally, due to the multidentate nature of EDTA, the REEs form 1:1
complexes with EDTA, which reduces metal zone tailing.
EDTA has been used as a stripping agent for SLEs and functionalized
adsorbents (Gupta & Sengupta, 2017; Li et al., 2018; Liu et al., 2009;
Zhao et al., 2016). EDTA and oxalic acid were used for desorption with
amide functionalized multiwalled carbon nanotubes (Gupta & Sengupta,
2017). Other organic compounds used for desorption of REEs include
acetonitrile (Abdel-Magied et al., 2019; Jiang et al., 2016; Zheng et
al., 2015), methanol (Abdel-Magied et al., 2019), and ammonium oxalate
(Florek et al., 2014, 2015; Giret et al., 2018; Hu et al., 2017, 2019;
Juère et al., 2016; Perreault et al., 2017).