Introduction

In recent years, organic dye pollutants produced by various dyestuff manufactures, plastic, paper, textile, cosmetics, leather, pharmaceutical, food, and other industries are frequently found in groundwater, and this is becoming a serious environmental and health problem. Obviously, the removal of color synthetic organic dye stuff from waste effluents becomes environmentally important. It is rather difficult to treat these dyes due to their complex molecular structure and xenobiotic properties. Many methods, including adsorption (Arami et al. 2006), ion-exchange (Liu et al. 2007), photocatalytic degradation (Muruganandham and Swaminathan 2006), chemical oxidation (Dutta et al. 2001), ozone treatment (Selcuk 2005), membrane filtration (Buonomenna et al. 2009), precipitation (Lee et al. 2006), flocculation (Lee et al. 2006), coagulation (Lee et al. 2006), and biological treatment (Kornaros and Lyberatos 2006) have been investigated to remove dyes from aqueous systems. Among these chemical, physical, or biological treatment processes, adsorption is the most promising one for the removal of dyes, mainly because of its high effectiveness and low cost and simplicity. Various kinds of materials including activated carbons (Degs et al. 2001), zeolite (Faki et al. 2008), fly-ashes (Janos et al. 2003), wood (Mckay and Poots 1980), pith (Namasivayam and Kanchana 1992), clay (Ozcan and Ozcan 2004; Wang et al. 2004; Kacha et al. 1997), polymer (Ai et al. 2010; Kopinke et al. 2001), graphene (Sharma and Das 2013), porous metal–organic frameworks (Yang et al. 2011), and titanium peroxide (Zhao et al. 2014) could be used to adsorb dyes from waste water.

Layered double hydroxides (LDHs), well known as hydrotalcite-like compounds, can be expressed in general as [MII 1 − x MIII x (OH)2]x+An− x/n m·H2O, where, MII and MIII are the divalent and trivalent cations, respectively. An− is the charge-balancing interlayer gallery anion (Meyn et al. 1990; Woo et al. 2011; Chang et al. 2005) and easy to exchange with other anions. Due to its excellent stability, unique microstructure and chemical composition, exchangeable interlayer anions, compositional flexibilities, large surface areas, ease of preparation, and low cost, LDHs have been widely used as adsorbents or exchangers to remove various anionic species in waste water. For example, LDHs and its derivatives with different composition and morphology have been used to remove anionic and cationic dyes (Aguiar et al. 2013), indigo carmine dyes (EI Gaini et al. 2009), acid green (dos Santos et al. 2013), methyl orange (Ai et al. 2011), orange II (Abdelkader et al. 2011), and anionic relative dye (Ahmed and Gasser 2012).

The LDHs commonly used to remove dyes from industrial effluent are the form of powder, it must be recovered by solid–liquid separation subsequent to the purification process. The separation and regeneration of adsorbent is one of the key to influence its application. If an adsorbent is magnetic, it can be readily separated from complex multiphase systems by applying an external magnetic field. Recently, Chen et al. reported the synthesis of the colloidal Fe3O4-LDH nanohybrids via an electrostatic interaction between the Fe3O4 nanoparticle and LDH nanoparticle, and it demonstrated excellent performance for removal of organic dyes in water (Chen et al. 2011), but the synthesis processes of Fe3O4 nanoparticles and LDH nanoparticles are tedious. Pan et al. have synthesized Fe3O4@DFUR-LDH submicro particles and exhibited its application in controlled drug delivery and release (Pan et al. 2011). In this paper, we synthesized core/shell Fe3O4@MgAl-LDH microspheres by a simple template-induced growth process, and the Fe3O4@MgAl-LDH microspheres demonstrated excellent adsorption performance toward congo red (CR); furthermore, it can be separated and regenerated easily. To our knowledge, this unique Fe3O4@MgAl-LDH microsphere with excellent adsorption ability has not been reported yet.

Experimental

Materials

Congo red used for this study was purchased from Tianjin Damao Reagent Factory. The chemicals, FeCl3·6H2O, NaAc, ethylene glycol, ethanediamine, Na2CO3, NaOH, Al(NO3)3·9H2O, Mg(NO3)2·6H2O, polyethylene glycol 20,000 (PEG-20000), and H2O were all of analytical grade and obtained from Kelong Chemical Reagent Co. Ltd., (China). The desired pH was adjusted by adding NaOH and Na2CO3 (2:1).

Synthesis of Fe3O4 nanospheres

The Fe3O4 microspheres were synthesized by a hydrothermal process. At first, FeCl3.6H2O (1.35 g) was dissolved in ethylene glycol (40 mL), followed by the addition of NaAc (3.6 g) providing the weak base environment and PEG-20000 (1 g) which will be an active agent. The mixture was vigorously mechanical stirred for 30 min. Then, the solution was transferred into a teflon-lined stainless steel autoclave (100 mL capacity) for hydrothermal treatment at 200 °C for 18 h. After cooling to room temperature, the black precipitate was collected by a magnet and washed several times using ethanol and water in sequence. Finally, the sample was dried overnight at 60 °C.

Synthesis of MgAl-LDH and Fe3O4@MgAl-LDH microspheres

The magnetic Fe3O4@MgAl-LDH microspheres were prepared by a coprecipitation method. The synthesized Fe3O4 microspheres (0.5 g) were ultrasonically dispersed into 50-mL water/methanol mixed solution (methanol/water = 1:1) in a 500-mL round bottom flask to obtain a uniform suspension; then, the flask was put into 60 °C oil bath under vigorous stirring. Four grams NaOH and 5.3 g Na2CO3 were dissolved in 1 L aqueous methanol as solution A; 1.155 g Mg(NO3)2·6H2O and 0.565 g Al(NO3)3·9H2O were dissolved in aqueous methanol as solution B. The A was added to B and kept the pH at about 10. The mixed A and B (1:1) solution was added to the round bottom drop by drop. After being aged in solution for 24 h, the precipitate was washed with deionized water for several times; finally, the product was dried at 100 °C for 6 h.

Pure MgAl-LDH nanoflakes were synthesized by the same process in the absence of Fe3O4 microspheres.

Characterization methods

Morphology of Fe3O4, MgAl-LDH, and Fe3O4@MgAl-LDH composite were characterized using the scanning electron microscope (SEM, JEOLS-3400N, Japan). X-ray powder diffractometry (XRD) patterns of products were obtained from DX 1000 X-ray diffractometer (Philip, Netherland) with Cu Ka radiation (40 kV, 300 mA, λ = 0.154 nm) to confirm the structure of the material, the XRD data were collected in a scan range from 5 to 80°(2θ) with a step size of 0.03°. Nitrogen adsorption and desorption isotherm was measured using micromeritics tristar II3020 sorptometer. The specific surface area of the sample was derived using the multipoint Brunauer–Emmett–Teller (BET) method and the pore-size distribution was determined using the Barret–Joyner–Halenda (BJH) mathematical model. Fourier transform infrared spectroscopy (FTIR) spectra were recorded in the range of 4000–400 cm−1 on a FTIR spectrometer (Nicolet-6700, USA). UV–vis absorbance of products was measured by using UV1101 spectrophotometer (Techcomp).

Adsorption experiments

The adsorption performance of synthesized materials toward congo red was studied. Solutions containing the dye were prepared by dissolving a known quantity of the dye in DI water (1000 mg/L), followed by serial dilutions to reach the needed concentrations. All the adsorption experiments were conducted under stirring conditions throughout the test at room temperature (25 °C) in the dark. Twenty milligrams of as-prepared adsorbent was added to 50 mL of dye solutions (100 mg/L). At appropriate time intervals, the aliquots were withdrawn from the solution and the adsorbents were separated from the suspension via magnet. The concentration of residual CR in the supernatant solution was detected using a UV–vis spectrophotometer at the 500 nm. The sampling continued until the adsorption process reaches its equilibrium. Every adsorption experiment was repeated three times.

The removal efficiency of dye was given according to the following formula:

$$ \mathrm{removal}\;\left(\%\right)=\frac{\left({C}_0-{C}_t\right)}{C_0}\times 100\% $$

where C 0 (mg/L) is the initial concentration of adsorbent, C t (mg/L) is the concentration of adsorbate at time t (min).

The adsorption capacity q e (mg/g) was calculated by the following formula:

$$ qe=\frac{\left({C}_0-{C}_e\right) V}{m} $$

where C e (mg/L) is the concentration of the adsorbate at equilibrium, V (L) is the volume of adsorbate solution, and m (g) is the mass of adsorbent.

The recycling of the adsorbent was evaluated by repeating cycles of adsorption–separation–regeneration using the same Fe3O4@MgAl-LDH sample. After the adsorption, the adsorbent was separated by a magnet, then it was annealed in a tube furnace at 400 °C for 6 h. Appropriate new adsorbent was added into recycled sample to replenish the adsorbent lost (about 5%) in the adsorption, separation, regeneration process.

Results and discussion

Figure 1 shows the XRD patterns of pure and composite materials. The diffraction patterns shown by curve (a) and curve (b) can be well indexed to a cubic phase of Fe3O4 (JCPDS NO.74-0748) and MgAl-LDH (JCPDS NO.70-2151), respectively. The XRD pattern shown in Fig. 1c clearly indicates that the final product is a mixture of Fe3O4 and MgAl-LDH; all the diffraction peaks can be attributed to cubic Fe3O4 and MgAl-LDH. The peaks at 30.1°, 35.4°, 43.1°, 53.4°, 56.9°, and 62.5° come from the (220), (311), (400), (422), (511), and (440) of Fe3O4, and the signal at 11.6°, 23.3°, 34.9°, 38.1°, 44.4°, and 60.7° come from the (003), (006), (012), (015), (018), (110), and (113) of MgAl-LDH, and there is no other impurity.

Fig. 1
figure 1

XRD patterns of (a) pure Fe3O4, (b) pure MgAl-LDH, and (c) composite Fe3O4@MgAl-LDH nanomaterials

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The SEM images of Fe3O4, MgAl-LDH, and Fe3O4@MgAl-LDH are shown in Fig. 2. The pure Fe3O4 is sphere-like structure with an average diameter (the average diameter of all the particles in a SEM field of view was calculated) of 350 nm; many nanopores can be observed on the surface of Fe3O4 microspheres (Fig. 2a). Based on SEM image with higher magnification, it demonstrates that Fe3O4 porous microspheres are comprised of many smaller nanoparticles with a diameter about 20 nm (Fig. 2b). The pure MgAl-LDH is flake-like nanostructures with an average thickness of about 70 nm (labeled with black arrows in Fig. 2d. Figure 2e, f shows the SEM images of hybrid Fe3O4@MgAl-LDH product; it indicates that the hybrid nanostructures are hierarchical microspheres, which consist of inner Fe3O4 core and outer MgAl-LDH nanoflake shell. The average diameter of the Fe3O4@MgAl-LDH microspheres is about 430 nm, which is a little larger than that of pure Fe3O4 microspheres, and the size of inner Fe3O4 core has no obvious change; it suggests that the Fe3O4 microspheres act as a template to induce the deposition and growth of MgAl-LDH; finally, core/shell Fe3O4@MgAl-LDH microspheres are obtained.

Fig. 2
figure 2

SEM images with different magnification of pure Fe3O4 (a, b), pure MgAl-LDH (c, d), and composite Fe3O4@MgAl-LDH (e, f) nanomaterials

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Figure 3 shows the energy dispersive spectroscopy (EDS) spectra of Fe3O4 microspheres, MgAl-LDH nanoflakes, and Fe3O4@MgAl-LDH core/shell microspheres, and Table 1 lists the corresponding element analysis result. As we can see, the measured Fe:O molar ratio of pure Fe3O4 microspheres is about 0.78, which is agreement with the atom ratio in the Fe3O4. The measured Mg:Al molar ratio in pure MgAl-LDH and composite Fe3O4@MgAl-LDH are about 2.2 and 2.3, respectively, which are smaller than the value in the precursor solution (3.0). A similar observation was reported earlier due to the leaching of Mg2+ under the current synthesis conditions (Ai et al. 2011; EI Gaini et al. 2009; Abdelkader et al. 2011).

Fig. 3
figure 3

EDS spectra of Fe3O4 microspheres (a), MgAl-LDH nanoflakes (b), and Fe3O4@MgAl-LDH core/shell microspheres (c)

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Table 1 Element analysis of Fe3O4 microspheres, MgAl-LDH nanoflakes, and Fe3O4@MgAl-LDH core/shell microspheres
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The FTIR spectra of the pure Fe3O4, pure MgAl-LDH, and composite Fe3O4@MgAl-LDH are shown in Fig. 4. In curve (a), the absorption band observed around 572 cm−1 belongs to Fe-O stretching and torsional mode of Fe3O4 (Racuciu 2009). In curve (b), the strong and broad absorption band observed around 3473 cm−1 corresponds to the O-H stretching vibration of the layer surface and/or interlayer water molecules (Ai et al. 2011; EI Gaini et al. 2009; Abdelkader et al. 2011). The adsorption peaks in the range of 500–800 cm−1 are associated with M-O, O-M-O, and M-O-M lattice vibrations (M = Mg and Al) (Ai et al. 2011; EI Gaini et al. 2009; Abdelkader et al. 2011). The strong peak at 1371 cm−1 is due to the interlayer carbonate species (mode v3) in the MgAl-LDH (Ai et al. 2011; Abdelkader et al. 2011), and the band at 1636 cm−1 belongs to the hydroxyl deformation mode of the water molecules in the interlayer (Ai et al. 2011; Abdelkader et al. 2011). The FTIR spectrum of composite Fe3O4@MgAl-LDH mainly demonstrates the MgAl-LDH absorption (3419, 1630, 1358, 762 cm−1) companying a weaker Fe3O4 adsorption at around 570 cm−1(curve (c)); the weaker absorption of Fe3O4 can be attributed to the coating of LDH on the surface of Fe3O4. The strong peak at 1358.74 cm−1 comes from the interlayer carbonate species, which act as charge-balancing interlayer anion in MgAl-LDH.

Fig. 4
figure 4

FTIR spectra of Fe3O4 (a), MgAl-LDH (b), and Fe3O4@MgAl-LDH (c)

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The specific surface area and porosity of the as-prepared samples were determined by nitrogen adsorption measurements. Figure 4 displays the N2 adsorption–desorption isotherms and the corresponding pore-size distribution curve for pure Fe3O4, pure MgAl-LDH, and composite Fe3O4@MgAl-LDH microspheres. All the pure Fe3O4, MgAl-LDH, and composite Fe3O4@MgAl-LDH microspheres exhibit a typical IV isotherm with a narrow hysteresis loop according to IUPAC classification (Rouquerol et al. 1994). The measured specific surface area of Fe3O4, MgAl-LDH, and Fe3O4@MgAl-LDH microspheres is 13.46, 139.47, and 57.2 m2/g, respectively. Although the aggregation of magnetic Fe3O4 and Fe3O4@MgAl-LDH nanostructures induced smaller specific surface area than pure MgAl-LDH, the specific surface area of Fe3O4@MgAl-LDH increase about four times than that of pure Fe3O4 microspheres.

The adsorption performance of synthesized nanomaterials was studied. Figure 6 shows the UV–vis spectra of congo red solutions after different contact time in the presence of different adsorbent. The pure Fe3O4. microspheres have weak adsorption ability toward CR; it is able to slowly (within 40 min) adsorb 53.1% of congo red with an initial concentration of 100 mg/L; the adsorption capacity is about 220.56 mg/g (Fig. 6a, d). The adsorption capacity of Fe3O4 microspheres should come from its porous microstructures, but its small surface area lead to its lower adsorption performance. The MgAl-LDH nanoflakes show a much better adsorption ability than pure Fe3O4 microspheres; it can adsorb 86.3% of CR in 40 min; the adsorption capacity is about 345.72 mg/g (Fig. 6b, d). The MgAl-LDH nanoflakes have a larger specific surface and a plate-like structure, and the anionic CR can exchange with CO3 2− anions of MgAl-LDH (Shan et al. 2014), at the same time, due to the memory effect, the intercalation induced by microstructure reconstruction of LDHs will also improve the adsorption (Crepaldi et al. 2002), so it demonstrates good adsorption ability. Although the composite Fe3O4@MgAl-LDH microspheres have a smaller specific surface area than pure MgAl-LDH nanoflakes (Fig. 5), it demonstrates better adsorption ability than both pure Fe3O4 microspheres and pure MgAl-LDH nanoflakes (Fig. 6c, d). Of the CR, 99.8% can be removed by Fe3O4@MgAl-LDH microspheres in 30 min; its adsorption capacity can reach to 404.6 mg/g. The excellent adsorption performance can be attributed to its unique microstucture. The growth of MgAl-LDH nanoflakes on the surface of Fe3O4 microspheres improves the dispersity of MgAl-LDH nanoflakes which mainly provide adsorption site in the nanocomposites, and the hierarchical microstructures filled with micropores and tunnels which facilitate the reserve of adsorbed dye molecular. LDHs can uptake anions from a solution by three different machanisms: adsorption, intercalation by anion exchange, and intercalation by reconstruction of calcined precursor (Crepaldi et al. 2002). Due to the carbonate, which presents strong electrostatic interaction with the layers, is difficult to exchange, in this composite Fe3O4@MgAl-LDH microsphere system, dye anions are mainly uptaken by adsorption and intercalation induced by reconstruction of calcined LDHs. Due to the presence of magnetic Fe3O4 cores, the composite Fe3O4@MgAl-LDH adsorbent can be quickly separated by a magnet. Figure 7 shows the micrographs of dye solution before and after adsorption and separation. It is clear that the red CR solution become colorless after Fe3O4@MgAl-LDH treatment for 40 min, and the black adsorbent can be easily separated from the solution by a magnet.

Fig. 5
figure 5

N2 adsorption–desorption isotherms (at −196 °C) of Fe3O4, MgAl-LDH, and Fe3O4@MgAl-LDH nanomaterials

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Fig. 6
figure 6

the UV–vis spectra of congo red solutions after different contact time in the presence of pure Fe3O4 microspheres (a), pure MgAl-LDH nanoflakes (b), and Fe3O4@MgAl-LDH nanocomposite (c); and the removal efficiency of different adsorbent toward CR (d). (Initial concentration 100 mg/L, catalyst dosage 0.2 g/L, temperature 25 °C)

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Fig. 7
figure 7

Micrographs of dye solution before and after adsorption and separation

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The regeneration ability is an important consideration for the application of adsorbent. The commonly reported regeneration methods include chemical oxidation, solvent, and thermal regeneration (Boulinguiez and Cloirec 2010; Song et al. 2009; Wang et al. 2006; Tamon and Okazak 1997; Huling et al. 2007, 2005). Here, we applied thermal technology for regeneration of our used nanocomposite adsorbent due to the high efficiency and low cost of thermal treatment process. After heat treatment at 400 °C for 6 h, the dye could be removed from adsorbent, and the structure of adsorbent could restore to the original state before adsorption. Figure 8 shows the CR remove rate of Fe3O4@MgAl-LDH after different recycle runs. The CR remove rate can reach to 99.3% at the second run, and it can retain to about 78.1% after 5 cycle runs. The decrease of adsorption ability can be attributed to the partial destruction of MgAl-LDH microstructure and the remanent impurity comes from dye which take up the adsorption site after repeated adsorption and heat treatment.

Fig. 8
figure 8

CR remove rate of Fe3O4@MgAl-LDH after different recycle runs

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Conclusions

A novel hierarchical Fe3O4@MgAl-LDH composite nanomaterial with good adsorption performance has been successfully synthesized. The composite microspheres composed of inner Fe3O4 core and outer MgAl-LDH-nanoflake layer. Under a magnetic field, it could be easily separated from the solution. The Fe3O4@MgAl-LDH composite microspheres exhibit excellent adsorption performance toward congo red in the solution. It demonstrates a high adsorption capacity of 404.6 mg/g, and the saturated adsorption capacity of pure Fe3O4 and MgAl-LDH is only 220.56 and 345.72 mg/g, respectively. Furthermore, the composite microspheres exhibit fast adsorption rate; 99.8% CR could be removed in 30 min, which is much higher than that of pure Fe3O4 (53.1%) and MgAl-LDH (86.3%). The used nanocomposite adsorbent can be fast separated by the magnet and regenerated using thermal treatment. It was found that about 78.1% of CR removal rate can still be retained after five recycle runs. The Fe3O4@MgAl-LDH nanocomposites combined nanostructured and magnetic features should be a potential adsorbent with excellent performance.