Lanthanum molybdenum oxide as a new platform for highly selective adsorption and fast separation of organic dyes

Meng Sun, Yuan-Yuan Ma, Huaqiao Tan*, Jian Yan, Hong-Ying Zang*, Hong-Fei Shi, Yong-Hui Wang and Yang-Guang Li*
Key Laboratory of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun, 130024, P. R. China. E-mail: tanhq870@nenu.edu.cn; zanghy100@nenu.edu.cn; liyg658@nenu.edu.cn

Received 1st June 2016 , Accepted 8th September 2016

First published on 8th September 2016


Exploration of new adsorbents for quick and highly selective adsorption and efficient separation of dyes in waste water is in urgent demand due to environmental problems. Herein, a new lanthanum molybdenum oxide adsorbent, La2Mo2O9, is prepared by the calcination of a new crystalline compound [La(CH3COO)(H2O)]MoO4 (La–Mo). After the precursor (La–Mo) is calcined at 600 °C, the as-prepared La2Mo2O9 (abbr. La–Mo600) exhibits fast and highly selective adsorption for multi-sulfonic dyes. Furthermore, the dyes adsorbed on La–Mo600 can be easily eluted by a dilute alkaline solution. Thus, it can be used as a new type of adsorbent to separate dye mixtures. The La–Mo600 adsorbent possesses excellent stability in the pH range of 1–13 and can be easily re-used seven times with a negligible adsorption decline. This study suggests that the adsorbent, La2Mo2O9, may represent a promising new generation of dye adsorbents that can realize quick adsorption, high selectivity and efficient separation of organic dye mixtures.


Introduction

Organic dyes are important chemical products in various fields in industry, such as textile, plastics, paper, leather tanning, rubber, food processing and printing.1–7 To date, over 100[thin space (1/6-em)]000 commercial dyes are known, and the annual production is more than a couple of million tons.8 Many dyes are toxic to human beings and even mutagenic, carcinogenic, or teratogenic for other living organisms.9–11 About 10% of dye effluents produced in industry are directly discharged into aqueous ecosystems, due to their good solubility in water,12 which has led to environmental pollution. It is therefore an imperative task to eliminate dyes from the sewage before discharge. To date, many methods have been explored to remove dyes from different effluents, such as coagulation, membrane filtration, photocatalysis, electrochemical degradation, aerobic and anaerobic microbial degradation, and adsorption.13–17

Among them, adsorption is one of the most efficient approaches because of its easy operation, versatility and low cost.18,19 Conventional adsorbents include activated carbon,20–23 biosorbents,24–27 clay materials,28,29 zeolites30,31 and metal–organic frameworks (MOFs).32–35 In this research field, chemists have mainly focused on developing adsorbents with low cost and high specific surface area.36–39 Low-cost carbon-based adsorbents are the main focus, since they can effectively adsorb a variety of dyes; there are, however, some disadvantages as well, such as low recovery and relatively poor selectivity. MOFs and their derivatives have been developed as new adsorbents in recent years because of their potentially selective adsorption properties;40,41 however, their relatively long adsorption equilibrium time and low separation efficiency have hindered their practical application. To date, the exploration of new types of adsorbents with superior adsorption properties, remarkable adsorption selectivity and easy renewability is still a challenging project.

Lanthanide (Ln)-based adsorbents as one type of environmentally friendly material have shown high adsorption efficiency for anionic pollutants.42–50 In recent years, a number of Ln-based adsorbents, such as lanthanide oxide,42 lanthanide hydroxide, hydroxyl–transition metal–lanthanide43,44 and Ln3+-modified bentonite, vesuvianite, chelex resin, and mesoporous zeolite materials,45 have been developed to remove nitrates, organic sulfur compounds,46 fluorides47,48 and phosphates49,50 from waste water. However, the Ln-based adsorbents used for dye removal and separation have been seldom researched.51 Based on the fact that the Ln(III) metal center possesses high charge, high coordination number and strong affinity for oxoanions, we attempted to explore new Ln(III)-based adsorbents with fast adsorption properties and high selectivity, to separate and remove of dyes from aqueous solution.

The introduction of another metal center into Ln(III)-based adsorbents may be a feasible way to develop new Ln(III)-based adsorbents for dye separation and removal. For this purpose, lanthanum molybdenum (Ln–Mo) oxide is a reasonable choice, since the combination with a molybdenum center may enhance the thermal and pH stability of the compound.52,53 Usually, Ln–Mo oxides are prepared from a mixture of Ln(III) salts and molybdate by hydrolyzation.54,55 However, such Ln–Mo oxides are not stable in low pH aqueous solution, and cannot be used to adsorb dyes in acidic solution. Based on the above consideration, we explored a new way to prepare thermally and pH stable Ln–Mo oxide compounds. Herein, a new hybrid compound [La(CH3COO)(H2O)]MoO4 (abbr. La–Mo) is synthesized. The La–Mo compound is calcined in air to remove water and acetic acid molecules, and the final La–Mo oxide with the formula La2Mo2O9 (abbr. La–MoT, T = calcination temperature) is obtained. The La–Mo oxide possesses high stability in a wide pH range of 1–13. Dye adsorption experiments reveal that La–Mo600 exhibits a fast adsorption and high selectivity for multi-sulfonic dyes. By controlling the pH of the mixed dye aqueous solution, La–Mo600 can separate bi-component dye mixtures freely. After the adsorption experiments, the adsorbed dyes on the La–Mo600 adsorbents can be alternately eluted by the dilute alkaline solution. More importantly, the adsorption activity of La–Mo600 exhibits negligible change after seven adsorption–desorption cycles.

Results and discussion

Synthesis and crystal structure of [La(CH3COO)(H2O)]MoO4 (La–Mo)

The crystalline compound [La(CH3COO)(H2O)]MoO4 (La–Mo) was hydrothermally synthesized by mixing (NH4)6Mo7O24·4H2O, La(NO3)3, Mo powder and acetic acid in an aqueous solution at pH 1–3, at a temperature of 180 °C for 1 day. Single crystal X-ray diffraction analysis reveals that compound La–Mo crystallizes in a triclinic P[1 with combining macron] space group. In the crystal structure of La–Mo, each La center exhibits a nine-coordinate mode with four O atoms derived from four {MoO4} (Om) units, four O atoms derived from three acetic molecules (Oa) and one H2O molecule (Ow). The bond lengths of La–O are in the range of 2.436(9)–2.715(9) Å. Each Mo center adopts a tetrahedral coordination geometry with four oxygen atoms, and the bond lengths of Mo–O are in the range of 1.748(7)–1.782(8) Å. Each acetate ligand acts as both a bidentate chelate and a bridging ligand, coordinating with three La centers (Fig. 1). Based on the above connection mode, the La centers are connected by the acetate ligands into a 1-D chain (Fig. 1 and 2); moreover, these chains are further linked by {MoO4} units to form a 3-D framework (Fig. 2).
image file: c6ra14179j-f1.tif
Fig. 1 The lanthanum centers are connected by acetic acid molecules into a 1D La–Ac chain.

image file: c6ra14179j-f2.tif
Fig. 2 (a) Ball-and-stick representation of La–Ac chains; (b) the La–Ac chains are connected by {MoO4} tetrahedron units to form a 3D structure.

Preparation and characterization of La2Mo2O9 (La–MoT)

In the crystal structure of La–Mo, each La center is coordinated with three acetate ligands and one H2O molecule. In order to improve the adsorption capability of La centers, it is better to remove these ligands, so as to release more of the coordination sites of the Ln atoms. The TG curve of compound La–Mo displays two weight loss steps in the temperature range of 30–800 °C (Fig. S1). The first weight loss is 5.15% in the temperature range 30–350 °C, corresponding to the loss of coordinated water molecules (cal. 4.74%). The second weight loss (11.92%) in the temperature range of 350–600 °C can be assigned to the decomposition of acetate ligands. After 600 °C, the weight does not change further. According to this result, crystalline samples of La–Mo are then calcined at 600, 700, 800, and 900 °C, respectively. The as-prepared La–Mo oxide samples are labeled as La–MoT (T = calcination temperature).

The IR spectra of compound La–Mo before and after calcination are shown in Fig. S2. The characteristic peaks at 3386, 2946, 1647, 1468, and 1405 cm−1 are attributed to H2O and acetate ligands in compound La–Mo. However, these peaks disappear or are dramatically weakened after calcination, indicating the removal of water and acetic acid molecules. Powder X-ray diffraction was used to determine the product after calcination of compound La–Mo. As shown in Fig. 3a, XRD patterns reveal that the calcined samples at different temperatures, are all ascribed to La2Mo2O9 (JCPDS Card No. 28-0509). Usually, the strong diffraction peaks mean that La–MoT products are highly crystalline. In this case, with the increase of calcination temperature, the intensities of the main XRD peaks are gradually increased, suggesting that the crystallization and particle size of La–MoT products are increased. Such results can be further demonstrated by SEM images. After calcination at 600 °C, the average diameter of La–Mo600 nanocrystals is ca. 270 nm (Fig. 3b). With the increase of calcination temperature however, the particle sizes of La–MoT products gradually increase (Fig. S3). The average diameter of La–Mo700 and La–Mo800 nanocrystals are ca. 300 nm and ca. 320 nm, respectively (Fig. S3b and c). When the temperature rises to 900 °C, the small nanocrystals melt into the bulk material (Fig. S3d). The N2 adsorption measurements also show the changes in the specific surface area of these calcined samples. The BET of La–Mo600 and La–Mo900 are 18.515 m2 g−1 and 13.426 m2 g−1, respectively (Fig. S4 and S5), exhibiting the decrease in specific surface area.


image file: c6ra14179j-f3.tif
Fig. 3 (a) The XRD patterns of the calcined samples La–MoT at different temperatures; (b) the SEM image of La–Mo600; (c) the TEM images of La–Mo600; (d) the HRTEM of La–Mo600, the inset in (d) is a corresponding FFT pattern.

Fig. 3c and d shows the TEM images of the calcined sample La–Mo600. The well-resolved diffraction lattice feature indicates that the La–Mo600 is highly crystalline. The lattice plane distances are ca. 0.32 nm, which can be assigned to the lattice plane [2 1 0] of La2Mo2O9 (JCPDS Card No. 28-0509). The EELS mapping of the TEM micrographs are shown in Fig. 4a–d. It reveals that La, Mo, and O are uniformly distributed. X-ray photoelectron spectroscopy (XPS) and the energy dispersive X-ray (EDX) spectra are used to further verify the composition of the sample. As shown in Fig. 4e, the binding energies at 837 eV, 529 eV and 231 eV can be attributed to the XPS signals of La 3d, Mo 3d and O 1 s, respectively. In the EDX spectra, the signals of La, Mo and O elements are clearly observed. The atomic ratio of La, Mo and O is about 2[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]9 (Fig. 4f), in accordance with the compositions of La–Mo600.


image file: c6ra14179j-f4.tif
Fig. 4 (a)–(d) HADDF-EELS images of La–Mo600; (e) X-ray photoelectron spectra (XPS) of La–Mo600; (f) the energy dispersive X-ray (EDX) spectra of La–Mo600.

Organic dye adsorption and separation

Eight dyes with different compositions and structures were selected to investigate the adsorption properties of La–MoT. UV-vis absorption spectroscopy was used to monitor the content of the dyes in solution. The calcination temperature is an important factor for adsorbent activation. Thus, the adsorption properties of La–Mo oxide adsorbents at different calcination temperatures were initially checked with an anionic dye, Evans Blue (EB), as the model. Simultaneously, the adsorption performance of the precursor compound La–Mo was also measured as the contrast. As shown in Fig. 5, La–Mo600 shows the best adsorption property for EB. After calcination, the adsorption activity of La–Mo600 is obviously better than that of the precursor compound La–Mo. It is presumed that the loss of acetate and aqua ligands in the crystalline compound La–Mo may release more adsorption-active metal sites, which dramatically improve the adsorption properties of La–Mo oxide adsorbent materials. Furthermore, the adsorption activities of La–Mo700, La–Mo800 and La–Mo900 were gradually reduced with the increase of the calcination temperature (Fig. 5), and the crystallization and particle size of the La–MoT adsorbents were increased with the gradually increasing calcination temperature (Fig. 3a and S3). It is well known that the high crystallinity always leads to low defects in materials, and large particle size results in relatively low specific surface area. Both defects and the specific surface area are significant factors for the adsorption properties of materials. Thus, it is easy to understand that the adsorption properties of La–Mo adsorbents at high temperatures (>600 °C) gradually decrease, due to the promotion of crystallization and the reduction of specific surface area. In the following dye adsorption experiments, the optimized La–Mo600 was investigated as the representative example.
image file: c6ra14179j-f5.tif
Fig. 5 The adsorption properties of the precursor compound La–Mo, La–Mo600, La–Mo700, La–Mo800 and La–Mo900 for Evans Blue.

The aqueous stability in a wide pH range is another important factor for the adsorbent materials. Thus, the stability of the La–Mo600 adsorbent in the pH range of 1–13 is investigated. As shown in Fig. S6, there are negligible changes in the XRD patterns of La–Mo600 treated with different pH solutions, indicating that La–Mo600 possesses good stability in a wide pH range of 1–13. Then, the adsorption properties of La–Mo600 for a series of organic dyes in different pH solutions were investigated. As shown in Fig. 6a, when the pH of the solution is about 7.0, La–Mo600 exhibits fast and highly efficient adsorption for multi-sulfonic dyes, such as Evans Blue (EB), Methyl Blue (MB) and Congo Red (CR). However, La–Mo600 displays very poor adsorption properties for mono-sulfonic dyes (Methyl Orange (MO) and Orange II (OII)), and other organic dyes, such as the amino dyes Rhodamine (RhB), Methylene Blue (MB-1) and the carboxyl dyes Alizarin yellow GG (GG) (Fig. 6a and b). It was also observed that the number of sulfonic groups in organic dye molecules significantly affects the adsorption performance of La–Mo600. In comparison to the curves of C/C0 vs. T of EB, CR and MO (Fig. 6a), the adsorption efficiency of La–Mo600 follows the order of EB > CR > MO, since EB, CR and MO molecules possess four, two and one negative sulfonic groups, respectively. It was observed that the MO molecule with just one sulfonic group was almost not adsorbed by the La–Mo600 adsorbent.


image file: c6ra14179j-f6.tif
Fig. 6 The molecular structures of the dyes. (a) Concentration changes of the dyes vs. time in a pH = 7 solution with La–Mo600; (b) the histogram of the percentage removal of the fifteen dyes in a pH = 7 solution at about 15 min; (c) concentration changes of the dyes vs. time in a pH = 13 solution, with La–Mo600; (d) the histogram of percentage removal of the fifteen dyes in a pH = 13 solution at about 15 min.

When the pH of the above solution is adjusted to 13, most of dyes cannot be adsorbed by La–Mo600, except that GG exhibits a slight adsorption (Fig. 6c and d). This means the dilute alkaline (0.1 mol L−1 NaOH) solution might be used to elute the adsorbed multi-sulfonic dyes in neutral or acidic aqueous solution. Thus, the adsorbent La–Mo600 can be easily recycled. In addition, the equilibrium adsorption capacity (Qe) of various multi-sulfonic dyes are Qe (EB) = 23 mg g−1, Qe (MB) = 10.82 mg g−1, Qe (CR) = 20.6 mg g−1, respectively. The ionic strength of the solution is another factor that can affect the adsorption of dye molecules, especially for practical application. Therefore, the dye adsorption experiments of La–Mo600 in different concentrations of Na2SO4 solution have been investigated using EB as the example. As shown in Fig. S7, with the increase of concentration of Na2SO4 in solution, the adsorption efficiency of La–Mo600 decreases. In 0.1 mol L−1 and 0.5 mol L−1 of pH = 7 Na2SO4 solution, 40 mL of 20 ppm of EB can be almost entirely adsorbed in 10 min and 30 min, respectively; i.e., the presence of SO42− in solution shows an important influence on the adsorption of EB, which might be caused by the competitive adsorption of SO42− and dye molecules.

In order to explore the adsorption mechanism of the La–Mo oxide adsorbents, the adsorption active sites of La–Mo600 adsorbent have been investigated. As shown in Fig. 7, MoO3 exhibits almost no adsorption for EB with multi-sulfonic groups. However, La2O3, La–Mo precursor and La–Mo600 (Fig. S8) show obvious adsorption for EB. Such results indicate that the La center might be the adsorption-active site for the dyes with multi-sulfonic groups in a pH = 7 solution. It is well known that the Ln metal centers lack electrons in their 4f orbitals, so they have a strong affinity for electron-rich groups, e.g., the sulfonic group. In this case, La centers on the La–Mo600 surface may adsorb the EB molecules with multi-sulfonic groups. With the decreasing number of sulfonic groups in the organic dye molecules, the adsorption properties of La–Mo600 with respect to the dyes gradually decrease.


image file: c6ra14179j-f7.tif
Fig. 7 The adsorption properties of MoO3, La2O3, La–Mo precursor and La–Mo600 for EB in pH = 7 aqueous solution.

Based on the above adsorption features, La–Mo600 can be used as an efficient adsorbent for separating multi-component dyes. EB/GG and RhB/EB in solution at pH = 7 were selected as the models to investigate the separation capability of La–Mo600 for bi-component dyes (Fig. S9). As shown inset in Fig. 8a and b, once La–Mo600 adsorbent is immersed into the green solution mixed with EB and GG dyes, the blue EB with multi-sulfonic groups is quickly adsorbed by La–Mo600, and the solution changes to yellow, which is the typical color of GG dye. When La–Mo600 is added to the violet solution mixed with RhB and EB dyes, the blue EB with multi-sulfonic groups is also immediately adsorbed by La–Mo600, and the solution changes to pink, which is the typical color of RhB dyes. As shown in Fig. 8, the characteristic peaks of EB decrease dramatically, and EB can be completely adsorbed in 5 min, while the characteristic peaks of GG and/or RhB show negligible decrease. After adsorption, the EB dyes on La–Mo600 can be quickly released by virtue of the elution of dilute alkaline (0.1 mol L−1 NaOH) solution, as shown in video 1; simultaneously, the La–Mo600 adsorbent is recycled.


image file: c6ra14179j-f8.tif
Fig. 8 (a) Temporal evolution of UV-vis absorption spectra of the bi-component dye mixture (EB and GG) in aqueous solution at pH = 7, with La–Mo600 adsorbent; (b) temporal evolution of UV-vis absorption spectra of bi-component dye mixture (RhB and EB) in aqueous solution at pH = 7, with La–Mo600 adsorbent.

In order to investigate the stability and the adsorption efficiency changes of La–Mo600, the recycling experiments were performed with the use of EB dye. According to the XRD and SEM results (Fig. S10 and S11), the crystal phase and morphology of La–Mo600 adsorbent exhibited no obvious changes. Furthermore, the adsorption properties of La–Mo600 for EB had few changes after seven cycles (Fig. S12). All these results reveal that La–Mo600 is stable during the dye adsorption.

Conclusions

In summary, a new kind of La–Mo oxide adsorbent La2Mo2O9 (La–Mo600) has been prepared, which shows a fast adsorption property and high selectivity for the organic dyes with multi-sulfonic groups. It is worth mentioning that most dye adsorbents usually lack selectivity. In this case, the La–Mo600 adsorbent exhibits high selectivity for the multi-sulfonic dyes. Furthermore, the adsorbed dyes can be easily recycled by the elution of dilute alkaline solution. Moreover, the recycling experiments indicate that La–Mo600 is stable and can keep the adsorption activity for seven cycles. This work suggests that La–Mo oxide may be one type of new promising adsorbent that can be utilized in dye adsorption with high adsorption efficiency, good selectivity and easy reusability. Further research in this field may concentrate on smaller nanocrystalline materials of La2Mo2O9, so as to further improve the specific surface area and adsorption capability of such an adsorbent. The work is ongoing in our research team.

Experimental

Materials and methods

All chemicals and organic solvents used for the syntheses were of reagent grade, and were used without further purification. Elemental analyses (C, H and N) were carried out on a Perkin-Elmer 2400 CHN elemental analyzer. FT-IR spectra were performed on a Mattson Alpha-Centauri spectrometer with KBr pellets in the range of 4000–400 cm−1. TG analyses were performed on a Pyris Diamond TG instrument in flowing N2, with a heating rate of 10 °C min−1. The powder X-ray diffraction (XRD) patterns were obtained with a Rigaku D/max-IIB X-ray diffractometer at a scanning rate of 1° per minute with 2θ ranging from 5° to 50°, using Cu Kα radiation (λ = 1.5418 Å). The transmission electron microscopy (TEM) studies were carried out using a JEOL-2100F transmission electron microscope. A SU8000 SEM-FEG microscope coupled with an energy-dispersive X-ray (EDX) spectrometer was used to characterize the sample morphology. XPS studies were carried out on an ESCALABMKII spectrometer with an Al-Kα (1486.6 eV) achromatic X-ray source.

Synthesis of [La(CH3COO)(H2O)]MoO4 (La–Mo)

A mixture of (NH4)6Mo7O24·4H2O (0.336 g, 0.272 mmol), La(NO3)3 (1.95 g, 4.52 mmol) and molybdenum powder (0.046 g, 0.476 mmol) were added to distilled water (10 mL). The solution was adjusted to pH 1.0–3.0 with acetic acid, and stirred for 0.5 h, the mixture was then sealed in a Teflon-lined autoclave and kept at 180 °C for 24 h. After slowly cooling to room temperature, the colorless block crystals were isolated. The crystalline products were filtered, washed with distilled water, and kept in a vacuum desiccator (yield: 70% based on (NH4)6Mo7O24·4H2O). Elemental analysis calcd (%) for C2H5O7LaMo: C 6.38, H 1.33, La 36.95, Mo 25.52; found (%): C 6.47, H 1.22, La 37.03, Mo 25.71.

Preparation of La2Mo2O9 (La–MoT, T = calcination temperature)

The oxide, La2Mo2O9, was prepared from the precursor compound, La–Mo. The crystals of the La–Mo precursor were ground in the agate mortar and placed in an alumina crucible. Then, the samples were heated at 600–900 °C for 3 h in air. After cooling to room temperature, the final La–MoT oxide products were prepared.

X-ray crystallography

The crystallographic data of the precursor compound La–Mo was collected at 298 K on the Rigaku R-axis Rapid IP diffractometer using graphite monochromatic Mo Kα radiation (λ = 0.71073 Å) and IP technique. A multi-scan absorption correction was applied. The crystal data of La–Mo was solved by the direct method and refined by a full-matrix least-squares method on F2 using the SHELXTL-97 crystallographic software package.56–58 All non-H atoms were refined anisotropically. The detailed crystal data and structure refinement for La–Mo are listed in Table S1. Selected bond lengths and angles of La–Mo are listed in Table S2. Crystallographic data for this paper has been deposited at the Cambridge Crystallographic Data Center: CCDC 1447976 for La–Mo.

Dye adsorption

The solid La–Mo600 adsorbent (0.16 g) was dispersed in solution (40 mL) with the initial dye concentration of 20 ppm. The mixture was stirred at a constant temperature of 25 °C. At different time intervals, about 3 mL of suspension were extracted and centrifuged for subsequent analysis. Dye concentration was determined by a Xinmao ultraviolet-visible (UV-vis) spectrophotometer (UV-7504 PC).

For the equilibrium experiment, solid La–Mo600 adsorbent (20 mg) was dispersed in dye solution (10 mL) with the initial concentration in the range of 20–40 ppm. The mixture was stirred continuously for 24 h at ambient temperature until the equilibrium was reached. The dye concentration in the clear supernatant was determined by UV spectrophotometer. The adsorption capacity Qe (mg g−1) was calculated by eqn (1):

 
Qe = (C0Ce)V/m (1)
where C0 (mg L−1) is the initial concentration, Ce (mg L−1) is the equilibrium concentration after adsorption. V (L) is the solution volume and m (g) is the mass of the adsorbent.

Separation of bi-component dyes mixture

La–Mo600 adsorbent (0.16 g) was added to 40 mL bi-component dye mixture (EB and GG) at pH = 7, with the dye concentration of 20 ppm. The mixture was stirred at a constant temperature of 25 °C. At different time intervals, about 3 mL of suspension was extracted and centrifuged for subsequent analysis. Dye concentration was determined by a Xinmao ultraviolet-visible (UV-vis) spectrophotometer (UV-7504 PC). EB was completely adsorbed in 5 min. EB dye on the adsorbent was eluted by dilute alkaline solution (0.1 mol L−1 NaOH solution). In order to dynamically show the separation of the bi-component dye mixture, the video 1 was shot as an example. La–Mo600 adsorbents (1.0 g) were tiled in the sand funnel and then 50 mL of the bi-component dye mixture (EB and GG) in pH = 7 solution was poured into the funnel.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (grant no. 21301166, 21271039, 21201032, 21471028 and 21401131), Program for New Century Excellent Talents in University (grant no. NCET120813), Fundamental Research Funds for the Central Universities (grant no. 11SSXT140), and Science and Technology Department of Jilin Province (no. 20130522127JH).

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Footnote

Electronic supplementary information (ESI) available: Summary of crystal data, selected bond length (Å) and angles (°), the crystal structures of La–Mo; TG, IR, SEM, XRD, BET dyes adsorption properties and recycle experiment of La–Mo600. CCDC 1447976. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra14179j

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