1. Introduction
Rare Earth Elements (REEs) are the collection of 17 special elements which are usually divided into heavy rare earth elements (HREE) and light rare earth elements (LREE). REEs could be widely used in many fields based on their excellent properties. For instance, the chemical, electrical, magnetic, and optical characteristics make them suitable for fields, including the nuclear industry, metallurgy, and luminescence [
1]. Moreover, REEs can be combined with other element or compounds and then can be used as alloys, catalysts, ceramics, etc. Therefore, these strategic usages of REEs have been attracting much attention in recent years. However, due to the limited availability, the shortage of REEs resources and the necessity of a stable supply of these metals have been concerned over the past years. Therefore, the development of a recovery method for these trace metals is important from the viewpoint of environmental protection [
2,
3]. Metallic ions in aqueous solution can interact in various ways with a solid surface, including precipitation, adsorption, and ion exchange. Among these methods, adsorption has been regarded as an efficient way to remove metallic ion in aqueous solution [
4,
5,
6]. For ecofriendly purposes, biological materials as an adsorbent for water purification have used by a number of researchers. The biological adsorbent has the advantages of recyclable, low cost, easy operation, and little possibility of secondary pollution.
From above-mentioned, the laboratory model experiments for biosorption of REEs using seaweed biomasses [
7] and shell biomass [
8] have been carried out in our previous work. It was found that seaweed and shell biomass show excellent sorption capacity for lanthanides even under the presence of common cations (refer to
Figure S1). In addition, surface modification of activated carbon has been recognized as an attractive approach to improve and/or enhance the removal of heavy metals [
9] or lanthanides [
10].
On the other hand, layered double hydroxides (LDH) are a kind of lamellar ionic compound containing positively charged brucite-like layers and exchangeable anions in the interlayer. Their abundance in nature, low cost, and high specific surface area make them suitable for adsorption of metal ions. The general formula of an LDH is
, where cationic M
II and M
III are divalent and trivalent metals, and occupy the center of octahedral in the brucite-like layer. A
n− is the interlayer anion of charge
n that leads to the electric neutrality of the LDH, and the coefficient
x is the molar ratio of M
II/(M
II + M
III). The structure of LDH suggests that these materials can be intercalated easily with other anions. Various types of anions can be intercalated in its interlayer space. It has been extensively reported by both inorganic [
11,
12] and organic [
13,
14] chemistry researchers. The ability of LDH to intercalate anions makes them useful as catalysts, anion exchangers, and adsorbents. We note that several physicochemical models were proposed to rationalize the anion-exchange constants [
15]. Some formalisms were derived by relating the activity of an anion to the anion-exchange constant [
16], while geometric features of atoms, such as charge radii of the anions were taken into account to investigate the anion-exchange ability of halide anions on LDH [
17].
Moreover, LDHs modified with hybrids have also been studied as potential adsorbents of heavy metals from aqueous solution [
18,
19,
20]. Chelation is a type of bonding of ions or molecules to metallic ions. It involves the formation of coordinate bonds between a single central atom and ligand within multiple separate binding sites. Therefore, it can be expected that chelating agents modified LDH have higher adsorption capacity and better selectivity compared with other adsorbents for metallic ions. In our previous work [
21], we synthesized and characterized LDH intercalated with the chelating agent ethylenediaminetetraacetic acid (EDTA) or
N,
N′-1,2-ethanediylbis-1-aspartic acid (EDDS) and studied the uptake of Cd(II), Cu(II), and Pb(II) by these hybrid compounds. It turned out that the LDHs synthesized in that work were very effective for removing heavy metal ions from aqueous solutions. In fact, in our another previous work [
22], to theoretically clarify the adsorption mechanism and adsorption structures of LDH, we have performed quantum chemistry calculations of reactants, locally stable states, transition states, and products among phosphorous anion, water and hydrotalcite in a variety of pH ranges.
As mentioned above, a number of works have been devoted to researches on the adsorption phenomena of lanthanide [
4,
5,
6,
23,
24,
25,
26]. These works were mainly focused on the potential application of materials for removal of lanthanides from wastewater. On the other hand, the objectives of our present work are to experimentally investigate the efficiency of LDHs for adsorbing REEs from aqueous solution, as well as to unveil the adsorption mechanism and adsorption structures of lanthanide adsorbed onto LDH using quantum chemistry calculations. We carried out the uptake experiments for the adsorption of REEs (lanthanum, europium) from aqueous solutions by LDH modified with EDTA. In addition, we performed quantum chemistry calculations, including locally stable states of LDH-EDTA and optimized structures of the chemical systems. Particular emphasis is put on the elucidation of the effect of EDTA on structural stability. In the calculations, we used molecular cluster models for hydrotalcite lamellae, whose chemical structure is described by Mg
12Al
7(OH)
369+ [
22,
27]. Finally, to get a deeper insight into the uptake mechanism, we calculated the desorption rates of the adsorbates.
2. Materials and Methods
Chemical reagents, including Zn(NO3)2·6H2O, Al(NO3)3·9H2O, NaOH, NaNO3 and C10H14N2Na2O8 (Na2H2EDTA·2H2O) were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan). REEs nitrate salts Eu(NO3)3·6H2O (99%, 446.07) and La(NO3)3·6H2O (97%, 433.01) were purchased from Kishida Chemical Co., Inc.(Osaka, Japan). All reagents used were of analytical grade. The La(III) or Eu(III) ion stock solutions were prepared and suitably diluted with ultrapure water. The ultrapure water (>18.2 MΩ) which was treated by an ultrapure water system (Suite A Dublin, California, USA, Advantec aquarius: RFU 424TA) was employed throughout the work.
2.1. Synthesis of the Adsorbent
In this section, we present the synthesis processes of L1 and L2. The synthesis of LDHs intercalated with EDTA includes two steps: The preparation of the precursor NO3-LDH (L1), and the ion-exchange reaction between NO3-LDH and Ethylenediaminetetraacetic acid disodium salt.
2.1.1. Synthesis of Precursor L1
L1 was prepared by dropping addition of 100 mL aqueous solution of 0.02 mol Zn(NO3)2·6H2O and 0.01 mol Al(NO3)3·9H2O to 100 mL NaOH/NaNO3 solution (pH 10, molar ratio 1:1). Then, the solutions were agitated at 70 °C for 8 h by maintaining the pH, separated by centrifugation, and washed until they became neutral.
2.1.2. Synthesis of L2
L2 was synthesized as follows. Under an N2 atmosphere, 0.015 mol of EDTA was added to the 150 mL of the suspended solution of L1. Then, the mixing solutions were agitated at 70 °C for 8 h under a certain pH degree, separated by centrifugation, washed until they became neutral, and then dried at 60 °C overnight.
The above synthesis processes were conducted with N
2 purging to avoid CO
2 uptake from the atmosphere. The preparation and purification of the adsorbent, and the characterization of the samples were reported in detail in our previous paper [
21].
2.2. Characterization of the Adsorbent
Infrared spectra were obtained using the KBr disc method, with wavenumbers from 400 to 4000 cm−1 on an FT-IR (JASCO, Tokyo, Japan: FTIR-4200). The XRD pattern of LDHs samples was carried out on a RINT2500HR-PC (RIGAKU Corporation, Tokyo, Japan) using Cu Kα radiation in the scanning range of 2–80°. A Nova Nano SEM450 scanning electron microscope (FEI Co., Ltd., New York, NY, USA) was performed to examine the surface morphology and element distribution of L1 and L2 after the adsorption of lanthanide. The zeta-potentials of LDHs samples were measured by electrophoretic light scattering method (Otsuka, Tokyo, Japan: ELSZ-2000ZS).
2.3. Adsorption Experiments Using L1 and L2 as Adsorbents
Twenty milligrams of L1 or L2 were put into contact with 30 mL of an aqueous solution containing La(III) or Eu(III) ion with known initial concentration. Batch adsorption experiments were conducted in the pH range of 4–6 (optimum pH), contact time from 10 min to 8 h, temperature 25 °C, and adsorbent dosage 20 mg. The pH of each solution was adjusted using 0.1 mol·L
−1 NH
4OH and 0.1 mol·L
−1 HNO
3. Following the adsorption experiment, the suspension was filtered (0.45 μm, Mixed Cellulose Ester 47 mm, Advantec MFS, Inc., Dublin, CA, USA). In order to discuss the target ions before and after the adsorption, we have dissolved the sample and measured the concentration of Ln ions by an Inductively Coupled Plasma-Mass Spectrometry (Agilent HP4500, Santa Clara, CA, USA). The operating conditions of Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) are shown in
Table 1.
2.4. Desorption and Regeneration Experiments
After the adsorption experiment, the exhausted adsorbent was washed and dried overnight. Then it was shaken in a 100 mL flask which contained 30 mL HCl or HNO3 solution with different concentrations. After the system reached equilibrium, the suspension was filtered, and finally, Eu(III) and La(III) content in the filtrate was determined. Subsequently, the adsorbent was neutralized by ultrapure water for several times and reconditioned for adsorption in succeeding cycles. The second adsorption experiments were carried out by using regeneration adsorbent under the optimum condition. The adsorption-desorption processes were repeated five times in this work.
2.5. Quantum Chemistry Calculation
All of the present quantum chemistry calculations were performed using the GAUSSIAN09 program package [
28]. The molecular locally stable state structures were optimized by density functional theory (DFT) at the theoretical level of B3LYP. The basis sets were SDD (Stuttgart-Dresden) for Europium atoms and 6-311G (d, p) for other atoms. To take into account the effect of a water environment on our target systems, polarizable continuum medium (PCM method) with an appropriate dielectric constant for the solvent (water) was used [
29]. To model the adsorbent, molecular cluster models for hydrotalcite lamellae whose chemical structure is described by Mg
12Al
7(OH)
369+ were used [
27,
30]. The interlayer distance was kept 9.3 Å to imitate the experimentally observed one [
31].
2.6. Calculation of Adsorption and Desorption Rates
The following equation was derived in Reference [
32]:
Here,
is the metal ion concentration at a given time
t in the aqueous solution in mmol L
−1 and
is its initial concentration in mmol L
−1. In Equation (1), we defined
In Equations (2) and (3), B, D, and M/(AV) are the constants determined by adsorption constant kad, desorption constant kd, and the volume of the solution in liters V. By fitting the experimental data on the metal ion concentration, , versus time, t, by Equation (1), we obtain the constants, α, β, and D because and t are determined experimentally. Using the constants, α, β, and D, obtained in this way, we can get B, D, and M/(AV) from Equations (2) and (3),
In Equation (2), we have
where
K is the adsorption equilibrium constant of the solute,
From Equation (4), we immediately find
Because we have obtained
M/(
AV) above and
K is known from the experiment, we can obtain
from Equation (6). Then, from Equation (5),
is expressed as
Above, and are the required adsorption and desorption rates, respectively.
To fit the aqueous concentration of adsorbates versus time by Equation (1) and to obtain
and
, we have used a genetic algorithm [
33]. The remarkable difference between our work and Ref. [
32] is that
was determined by iterating Equation (1) until the best fit was obtained to the experimental results in the latter case, while the genetic algorithm makes it unnecessary to do such computationally demanding iterations in order to obtain
and
in the former case.
4. Conclusions
In conclusion, we performed XRD, SEM-EDS, and FT-IR experiments to characterize layered double hydroxides (LDHs) modified with EDTA and to identify the prepared LDH-NO3 (L1) and LDH-EDTA (L2). In addition, based on these experimental results, the adsorption capabilities and mechanisms of REEs ions on L1 and L2 were experimentally accessed. It was shown that both La(III) and Eu(III) could rapidly be adsorbed, and that the adsorption capacity of L2 was larger than that of L1 no matter whether the concept of adsorption capacity or that of the PC was used. In addition, comparing with other adsorbents, the PC of both L2-Eu and L2-La are relatively larger. It indicates that LDH intercalated by EDTA has superior adsorption performance. The complexation of REEs ions with EDTA (i.e., L2) brought about a lower zeta potential, which indicates that the former plays the role more dominant than the latter and that the latter is just a secondary result stemming from the former. In accord with this, quantum chemistry calculations showed that the complexation actually leads to the negatively charged species being adsorbed on LDHs, which promotes the adsorption of the complex compared with REEs without complexation. They also showed that the species in the presence of EDTA is more stable than that in its absence and that the intercalation of EDTA does indeed improve the stability of the adsorbed species. Finally, from our numerical results of adsorption and desorption rates, it was suggested that the recovery efficiency is large if the rate of desorption is large. Both the quantum chemistry calculation results and the adsorption and desorption rate results are consistent with the experimental ones. The LDH-EDTA synthesized in this work have a high affinity for removing REEs ions.