English

• 1. INTRODUCTION

  Until the end of the nineteenth century, all the pigments were extracted from natural substances, like flowers and roots. However, with the development of modern synthetic chemistry and chemical industry, natural dyes have been replaced by synthetic ones because they can be manufactured on a large scale, which is one the most important factors in water pollution. The artificial dyes are so stable that it is difficult to degrade in nature, which may significantly impact the health of human beings^{[1, 2]}. MG is an organic dye widely used in cotton, silk, wool, leather and paper industries, and also a drug used to treat parasites, fungal and bacterial infections^{[3, 4]}. Despite its extensive usage, MG dye has caused several health hazards as carcinogenesis, mutagenesis, teratogenesis and respiratory toxicity agent^{[5, 6]}. Therefore, the removal of dyes from effluents is very important. Up to now, several treatment methods including adsorption^{[2-4, 7]}, photodegradation^[8] and biological treatment^[9] have been developed to remove dyes from effluents. Among them, adsorption technology grows rapidly because of its relatively low cost and effect in dye removal.

  Up to now, various adsorbents have been reported to eliminate MG from effluents, such as porous carbon^[10], zeolites^[11], mesoporous materials^{[12, 13]}, some metal-organic frameworks (MOFs)^[14] and so on. However, the results of these studies showed limited adsorption efficiencies which restrict their applications in large scale effluents treatment. Recently, coordination complexes consisting of metal ions or clusters and organic ligands have been used as alternative precursors for the synthesis of nanostructure adsorbents^[15–18], and some of these materials exhibit good adsorption properties of organic dyes from aqueous solution. However, most of them are focused on transition metal complexes, and the design and control over lanthanide-transition bimetal complexes are relatively less.

  In this study, Nd₂Cu₂O_4+δ samples were prepared by a coordination-complex method (CCM) taking [NdCu(3,4-pdc)₂(OAc)(H₂O)₅]·6.5H₂O (1) as a precursor to yield particles with large special surface areas when compared to those prepared by simple solution method (SSM). The as-prepared Nd₂Cu₂O_4+δ particles were characterized by using XRD, SEM, HRTEM, XPS, and BET methods. The adsorption properties are studied in detail, showing Nd₂Cu₂O_4+δ samples exhibit selective adsorption towards MG with significant Q_m (maximum adsorption capacity) values reaching up to 1.55 g/g at room temperature.

  2. MATERIALS AND METHODS

  2.1 Synthesis of Nd₂Cu₂O_4+δ

  The [NdCu(3,4-pdc)₂(OAc)(H₂O)₅]·6.5H₂O (1) was prepared according to the method published in the literature^{[19, 20]} as the CCM precursor. A mixture of Cu(OAc)₂·4H₂O (0.5 mmol, 127 mg), Nd(NO₃)₃·6H₂O (0.5 mmol, 219 mg), 3,4-pdc (1 mmol, 167 mg) and triethylamine (0.2 mL) was dissolved in a mixture of water-methanol at the volume ratio of 1:1. The solution was stirred for 3 hours then filtered off and allowed to stand undisturbed. After about 5 days, blue block crystals suitable for X-ray analysis were obtained by filtration, washing with ethyl ether and drying in air. The obtained single-crystal precursor was then calcined at different temperature for 1 hour under N₂ atmosphere, followed by 0.5 h in air to yield Nd₂Cu₂O_4+δ.

  The SSM samples were synthesized according to the method reported in the literature^{[19, 20]}. Under constant stirring, Cu(OAc)₂·4H₂O (7.9 mmol, 2 g) and Nd(NO₃)₃·6H₂O (7.9 mmol, 3.46 g) were dissolved in 50 mL distilled water at the stoichiometric proportions. After 1 h, the solution was heated to a gel and then calcined at 800 ℃ for 1 h to yield Nd₂Cu₂O_4+δ.

  2.2 Characterization

  Single-crystal X-ray diffraction data of the coordination precursor [NdCu(3,4-pdc)₂(OAc)(H₂O)₅]·6.5H₂O (1) were collected at 100 K by a Rigaku Saturn CCD diffractometer, equipped with graphite-monochromated Mo-Kα radia­tion with a radiation wavelength of 0.71073 Å using an ω-φ scan mode. The structure was solved by direct methods and refined with a full-matrix least-squares technique based on F² using the SHELXS-97 and SHELXL-97 programs^[21]. Anisotropic thermal parameters were assigned to all non-hydrogen atoms. Crystallographic data and structural refinement parameters are listed in Table S1 (the Supporting Information). The selected bond lengths and bond angles for 1 are given in Table S2.

  XRD patterns were examined on a Rigaku D/Max-RB X-ray diffractometer with CuKα irradiation with a scan speed (2θ) of 0.05 °/s from 10 to 90°. Powder morphologies were performed by SEM (Zeiss Supra 55) and HRTEM (FEI Tecnai F30). The specific surface areas of the as-prepared samples were characterized by N₂ adsorption/desorption experiments at 77 K with a Builder SSA-4300 instrument. The XPS measurements were measured on a PHI 5000 C ESCA System with Mg K source operating at 14.0 kV and 25 mA.

  2.3 Adsorption experiments

  Adsorption properties were carried out to evaluate the adsorption capacity of Nd₂Cu₂O_4+δ towards MG. Batch adsorption experiments were carried out in conical flasks containing 1000 mL of 0.4 g/L MG aqueous/ethanol mixture solution at 1:1 volume ratio and various adsorbent doses (0.03~0.07 g) at different temperature (298, 313, and 338 K) for 7 days. Once the equilibrium was established, 5 mL sample solution was extracted by a micropipette and the adsorbent particles were removed by centrifugation. The concentration of residual MG in the sample was determined by UV-Vis spectrophotometer (T9) at maximum absorbance of 618 nm. The efficiencies of the adsorbent in sequestration of MG dye, expressed in terms of adsorption capacity at the equilibrium concentration, q_e (mg/g), were calculated according to Eq. (1):

  $q_e=\frac{\left(C_0-C_e\right) \times V}{m}$

  (1)

  where C₀ (mg/L) and C_e (mg/L) are the initial and equilibrium concentrations of MG in the solution, respectively, V (L) is the initial solution volume, and m (g) is the used dry adsorbent mass.

  3. RESULTS AND DISCUSSION

  3.1 Characterization

  The X-ray diffraction analysis reveals that [NdCu(3,4-pdc)₂(OAc)(H₂O)₅]·6.5H₂O (1) crystallizes in the monoclinic system with space group P2₁/n. The asymmetric unit in 1 (Fig. 1) consists of one crystallographically independent Nd³⁺, one Cu²⁺, two 3,4-pdc ligand anions, one acetate ion and five coordinated water molecules. Nd³⁺ is coordinated by two carboxylic oxygen atoms from two 3,4-pdc ligands, three oxygen atoms from two acetate ions and four water molecules, and the coordination geometry for the nine-coordinated Nd³⁺ can be described as a distorted tricapped trigonal prism. Two N atoms and two carboxylic oxygen atoms located at diagonal positions respectively and one water molecule lies at the axial position, finishing the square pyramidal coordination sphere of Cu²⁺. The bond lengths of Nd–O (3,4-pdc) are 2.372(4) and 2.471(4) Å, and the bond lengths of Nd–O (acetate) are 2.637(4), 2.471(4) and 2.542(5) Å. The bond lengths of Cu–O (carboxylic) are 1.982(4) and 1.962(3) Å, and those of Cu–N are 2.016(4) and 2.005(4) Å. The Cu–O (water) is 2.304(4) Å, which is longer than other Cu–O bonds, due to the Jahn-Teller effect. Compared to the similar complexes reported in Ref. [19], the axial Cu–O bond length of complex 1 is longer, indicating that the coordination ability of the water molecule is weaker. Both of 3,4-pdc ligands adopt a tri-dentate mode to coordinate with one Nd³⁺ ion and two Cu²⁺ ions (Fig. 1b). Cu²⁺ ions are bridged by 3,4-pdc ligands, giving rise to a (4,4)-connected 2D lattice plane with a regular rhombus of about 9 × 9 Ų size (defined by the distances between Cu²⁺ ions), as shown in Fig. 1c. Two Nd³⁺ ions are bridged by acetate ligands, adopting a μ₂-η₂: η₁ bridging mode, forming a dinuclear Nd(Ⅲ) unit (Fig. 1d). The 2D lattice planes are further connected by the dinuclear Nd(Ⅲ) units, giving rise to a 3D sandwich-like framework (Fig. 1e). It should be noted that the structure has a large porosity along the a and b directions. A PLATON program analysis suggests that there is approximately 38% of the crystal volume accessible to the solvents after taking off the lattice water molecules.

  Figure 1

  

  Figure 1.  (a) Molecular structure of 1, (b) Coordination mode of 3,4-pdc ligand, (c) 2D lattice plane constructed by Cu²⁺ and 3,4-pdc, (d) Connection mode of the 2D lattice plane and the dinuclear Nd(Ⅲ) unit, (e) 3D sandwich-like framework viewed along the a direction

  DownLoad: Full-Size Img PowerPoint

  XRD was used to investigate phase structures of the as-prepared Nd₂Cu₂O_4+δ particles, and Fig. 2 shows the XRD patterns of the sample prepared at different temperature, indicating that Nd₂Cu₂O_4+δ was formed above 800 ℃. All the observed diffraction peaks can be indexed to the standard data of Nd₂CuO₄ with PDF card no 80-1644^[22-25]. When below 800 ℃, the CuO and Nd₂O₃ crystallites were initially formed due to their low activation energies and relatively high energies of formation^[26]. No obvious impurity phase was detected above 800 ℃. The XRD of SSM shows that the samples of SSM contain more Nd₂O₃ impurities under the same conditions (Fig. S1). Compared to SSM, the sample of CCM was purer, which could be conjectured by the fact that the metal ions were evenly distributed in the coordination precursor. However, the stoichiometric ratio of the coordination precursor is 1:1, which is different with the stoichiometric ratio of Nd₂CuO₄, indicating that Cu²⁺ ions were doped into the host lattice of Nd³⁺.

  Figure 2

  

  Figure 2.  XRD patterns of Nd₂Cu₂O_4+δ prepared through CCM in comparison with PDF # 80-1644

  DownLoad: Full-Size Img PowerPoint

  SEM and TEM are performed to observe the morphology of Nd₂Cu₂O_4+δ particles. Fig. 3 shows the results of SEM and TEM investigations for samples prepared at 900 ℃. Figs. 3a and 3b show the results of SEM investigations for Nd₂Cu₂O_4+δ prepared by CCM, which can be described as a leaf-like nanosheet with a width of dozens of microns and a thickness of 50~80 nm. Most leaves are stacked together, but the layer structure is still clearly observed. It is suggested that the coordination precursor may be lamellar division firstly during the decomposition process and the organic matter volatilized at high temperature^[26]. From the high magnification picture, we can see droplet particles on the lamellae, which are characterized by SEM elemental analysis (Figs. 3c and 3d), indicating that the distribution of elements in the droplet is also Nd: Cu = 1:1 phase^[23]. These results are consistent with the Nd/Cu stoichiometric ratio and show that the proportion of elements in the configuration of Nd₂Cu₂O_4+δ is indeed 1:1 and distributes evenly.

  Figure 3

  

  Figure 3.  SEM images of CCM (a, b) and elemental mapping (c, d) and HRTEM SAED patterns (e), and SEM of SSM (f)

  DownLoad: Full-Size Img PowerPoint

  The clear lattice spacing detected by HRTEM

  (Fig. 3e) suggested that Nd₂Cu₂O_4+δ particles were highly crystalline in nature with single crystalline structure^[27]. The interplanar crystal spacing is 0.374 nm, indicating that the side of Nd₂Cu₂O_4+δ term is (101) crystal face^[28]. Fig. 3f shows the morphology of SEM investigations for Nd₂Cu₂O_4+δ prepared by SSM, which can be described as an inhomogeneous massive particle of 0.5~4 um. Compared with the samples prepared by CCM, the SSM one has a smaller surface area and is not suitable for the adsorbent.

  To further characterize the porosity of Nd₂Cu₂O_4+δ prepared by CCM, nitrogen adsorption-desorption experiments were performed at 77 K. The nitrogen adsorption-desorption isotherms and pore size distribution are shown in Fig. 4. As evidenced in Fig. 4, the Nd₂Cu₂O_4+δ of CCM showed type Ⅲ isotherm profiles according to IUPAC classification, indicating weak adsorbent-adsorbate interaction^{[29, 30]}. Although the leaf-like nanosheets prepared by CCM do not have channels deep into the matrix, the distance between slices is relatively close. Therefore, the pore distribution statistics show that there are aperture distributions in the range of 10~100 nm, which are consistent with the SEM observations. The calculated BET surface area is 17.9 m²/g.

  Figure 4

  

  Figure 4.  Nitrogen adsorption-desorption isotherms and corresponding pore size distributions of Nd₂Cu₂O_4+δ of CCM

  DownLoad: Full-Size Img PowerPoint

  The surface chemical composition and elemental states of Nd₂CuO₄ adsorbents are investigated by XPS. Fig. 5a presents the XPS survey spectrum of Nd₂CuO₄ of CCM, showing that the sample contains Nd, Cu and O. The high-resolution XPS spectra of Nd, Cu and O were attentively deconvoluted by considering spin-orbital coupling. The high-resolution XPS spectra of Nd 3d are shown in Fig. 5b. The peaks of 3d_3/2 and 3d_5/2 observed respectively at 1004.8 and 982.0 eV suggest the presence of Nd ion with a formal charge of +3^[30]. Meanwhile, one little shoulder peak, observed at 1000.1 eV, might be attributed to the satellite peak of Nd ion with +3~δ₁ charge, which possibly implies that a part of Nd ions occupies the O4-coordinated sites (Fig. 1). As shown in Fig. 5c, the Cu 2p XPS of CCM showed core level of Cu 2p spectral region with two spin-orbital doublets. The main peaks presented Cu 2p_1/2 at 953.8 eV and Cu 2p_3/2 at 934 eV with an energy difference of about 20 eV, which could be attributed to Cu ion in CuO₄ group with a formal charge of +2^[31]. The second doublet with binding energies at 962.9 and 943.2 eV could be ascribed to the emission from Cu 2p_1/2 and Cu 2p_3/2 core levels of Cu atoms with more positive charges, denoted as +2 + δ₂, suggesting that an inequivalent Cu ion locates at an 8-coordinated position^{[31, 32]}. Fig. 5d shows two different valences of O at 528.9 and 531.3 eV (more positive)^{[33, 34]}, respectively, indicating that there are two kinds of non-equivalent O atoms: one (528.9 eV) is related to 4-coordinated O species^[35] and the other (531.3 eV) to 6-coordinated O species. The XPS profiles of Nd₂CuO₄ by SSM are almost the same (Fig. S2), which implies that the Cu and Nd ions cloud occupy the 4- and 6-coordinated sites in the A₂BO₄ structure, respectively, and have a mixed valence state. This further demonstrates that the A₂BO₄-type compounds with perovskite structure have good solubility to metal ions^[31].

  Figure 5

  

  Figure 5.  XPS spectra of Nd₂Cu₂O_4+δ of CCM: survey scan (a) and high-resolution spectra of Nd (b), Cu (c) and O (d)

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  3.2 Maximum adsorption capacity of Nd₂Cu₂O_4+δ for MG

  The adsorption capabilities toward MG of the as-prepared Nd₂Cu₂O_4+δ were evaluated at 298, 318 and 338 K. In order to investigate the adsorption capacity of Nd₂Cu₂O_4+δ, a series of absorbent doses of CCM samples (0.03, 0.04, 0.05, 0.06 and 0.07 g) were employed to study the equilibrium isotherm, and the concentration of residual MG in the sample was determined by UV-Vis spectrophotometer (T9) at maximum absorbance, 618 nm, as shown in Fig. 6.

  Figure 6

  

  Figure 6.  Varieties MG equilibria concentrations in the maximum adsorption experiment of CCM samples at 298 (a), 318 (b), and 338 K (c) and the corresponding fitting lines according to Eqs. (2), (3) and (4) are depicted in d, e, and f, respectively

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  The adsorption isotherms play an important role in determining the adsorption ability of MG over Nd₂Cu₂O_4+δ. The adsorption isotherms were investigated by Langmuir and Freundlich isotherm models, which could be expressed as follows:

  $\frac{1}{q_e}=\frac{1}{K^\theta Q_m} \times \frac{1}{C_e}+\frac{1}{Q_m}$

  (2)

  $\ln q_e=\ln K_F+\frac{1}{n} \ln C_e$

  (3)

  The correlation between measuring equilibrium concentrations of MG, C_e, and their corresponding equilibrium adsorption capacity (q_e) can induce a linear plot of 1/C_e versus 1/q_e for Langmuir model (Eq. 2) or a linear plot of lnC_e versus lnq_e for Freundlich model, Eq. (3)^[31]. The best fitting yields the maximum adsorption amount (Q_m) and K^θ (Langmuir constants) or K_F (Freundlich constants) and n (intensity factor). Furthermore, the values of related parameters and R² of CCM samples are provided in Table 1. Fig. 6d suggests that the adsorption experimental data of dye over Nd₂Cu₂O_4+δ are well described using Langmuir isotherm model with R² being closer to 1. The Q_m of Nd₂Cu₂O_4+δ estimated by Langmuir model reached 1.55 g/g at 298 K with the equilibrium constant (K^θ) equal to 6395 L/mol. To the best of our knowledge, the maximum adsorption amount of Nd₂Cu₂O_4+δ towards MG is relatively larger, which is comparable to the reported value in the literature^{[35, 36]}. As temperature increased, the maximum adsorption value of Nd₂Cu₂O_4+δ decreased to 1.25 g/g at 338 K, accompanying the equilibrium constant falling to 2254 L/mol. The thermodynamic parameters can be obtained from the fitting of lnK^θ versus T⁻¹ according to Eq. (4), as shown in Fig. 6f.

  $\ln K^\theta=\frac{-\Delta_r G_m{ }^\theta}{R T}=-\frac{\Delta_r H_m^\theta}{R} \times \frac{1}{T}+\frac{\Delta_r S_m{ }^\theta}{R}$

  (4)

  Table 1

  

  Table 1.  Partial Fitting Results Obtained from the Maximum Adsorption Capacity Experiment of CCM

  DownLoad: CSV

  T/K		Langmuir				Freundlich
  		Qm (g/g)	KL (L/mol)	R2		KF (L/mol)	1/n	R2
  298		1.55	6395	0.9968		2099	0.339	0.8494
  318		1.35	4323	0.9951		1282	0.363	0.7756
  338		1.25	2254	0.9994		1357	0.563	0.7860

  The standard Gibbs free energy change (Δ_rG_m^θ), standard enthalpy (Δ_rH_m^θ) change and standard entropy change (Δ_rS_m^θ) for adsorption of 1 mol MG are calculated to be –21.80, –21.68 and 0.51 J/mol·K, respectively. The negative value of Δ_rG_m^θ indicates that the adsorption reaction is spontaneous. The negative value of Δ_rH_m^θ further confirms the decrease of equilibrium constant with increasing the temperature. The positive value of Δ_rS_m^θ may imply that the surface of the adsorbent is covered initially by water molecules and the adsorbed MG molecule occupies a larger area on the surface.

  4. CONCLUSION

  Nd₂Cu₂O_4+δ adsorbents were successfully synthesized via CCMs taking [NdCu(3,4-pdc)₂(OAc)(H₂O)₅]·6.5H₂O as the coordination precursor. Compared to the particles prepared by SSM, the sample of CCM was purer and showed leaf-like morphology composed of nanosheets with an average thickness of 50~80 nm and a BET surface area up to 17.9 m²/g. XRD, SEM mapping, and XPS measurements showed that Nd₂Cu₂O_4+δ is A₂BO₄ -type compound with Cu²⁺ ions occupying the A site of the host lattice. Adsorption capabilities were investigated, and Nd₂Cu₂O_4+δ samples exhibit selective adsorption towards malachite green (MG) with significant Q_m (maximum adsorption capacity) values reaching up 1.55 g/g at room temperature. The fitting of the isotherms at different temperature estimated the following thermodynamic parameters: Δ_rG_m^θ –21.8 kJ/mol, Δ_rH_m^θ –21.68 kJ/mol, and Δ_rS_m^θ 0.51 J/mol·K, indicating the adsorption reaction is a spontaneous process. Therefore, the Nd₂Cu₂O_4+δ may be utilized as an ideal candidate for applications in environmental and separation science.

  
  
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  Figure 1  (a) Molecular structure of 1, (b) Coordination mode of 3,4-pdc ligand, (c) 2D lattice plane constructed by Cu²⁺ and 3,4-pdc, (d) Connection mode of the 2D lattice plane and the dinuclear Nd(Ⅲ) unit, (e) 3D sandwich-like framework viewed along the a direction

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  Figure 2  XRD patterns of Nd₂Cu₂O_4+δ prepared through CCM in comparison with PDF # 80-1644

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  Figure 3  SEM images of CCM (a, b) and elemental mapping (c, d) and HRTEM SAED patterns (e), and SEM of SSM (f)

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  Figure 4  Nitrogen adsorption-desorption isotherms and corresponding pore size distributions of Nd₂Cu₂O_4+δ of CCM

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  Figure 5  XPS spectra of Nd₂Cu₂O_4+δ of CCM: survey scan (a) and high-resolution spectra of Nd (b), Cu (c) and O (d)

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  Figure 6  Varieties MG equilibria concentrations in the maximum adsorption experiment of CCM samples at 298 (a), 318 (b), and 338 K (c) and the corresponding fitting lines according to Eqs. (2), (3) and (4) are depicted in d, e, and f, respectively

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  Table 1.  Partial Fitting Results Obtained from the Maximum Adsorption Capacity Experiment of CCM

  T/K		Langmuir				Freundlich
  		Qm (g/g)	KL (L/mol)	R2		KF (L/mol)	1/n	R2
  298		1.55	6395	0.9968		2099	0.339	0.8494
  318		1.35	4323	0.9951		1282	0.363	0.7756
  338		1.25	2254	0.9994		1357	0.563	0.7860

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