Adsorption and intercalation of organic pollutants and heavy metal ions into MgAl-LDHs nanosheets with high capacity

Abstract

Hexagonal MgAl layered double hydroxide (MgAl-LDH) materials comprised of nanosheets and microsheets with different precipitants were synthesized via a facile hydrothermal route. XRD, FESEM, TEM and BET were employed to characterize the samples. Structural characterization revealed that MgAl-LDHs nanosheets and microsheets are 100 nm and 2 μm in width, respectively. Furthermore, MgAl-LDHs nanosheets have a higher specific surface area (65.94 m² g⁻¹) than that of microsheets (15.75 m² g⁻¹). Methyl orange and Cr(VI) anion and Ni(II) cation adsorption on the as-synthesized MgAl-LDHs nanosheets and microsheets were systematically assessed by measuring the residual concentration during the adsorption process. The MgAl-LDHs nanosheets showed better adsorption performance than the MgAl-LDHs microsheets for methyl orange and Cr(VI) anions and Ni(II) cations. The adsorption performance versus time for the adsorption of methyl orange by MgAl-LDHs nanosheets has an excellent adsorption quantity (229.82 mg L⁻¹) with high adsorption rate. The adsorption kinetics and adsorption isotherms of Cr(VI) anions and Ni(II) cations of MgAl-LDHs nanosheets can be described by the pseudo-second order kinetic and Langmuir isotherm with saturated adsorption of 63.8 and 92.3 mg g⁻¹, respectively. Combined with results from XRD, FTIR, EDS and XPS experiments, the adsorption mechanisms of MgAl-LDHs nanosheets including precipitation, surface complexation, isomorphic substitution and ion exchange in the interlayer space of MgAl-LDHs nanosheets are discussed in detail. Finally, based on the quick and efficient removal of heavy metal ions by MgAl-LDHs nanosheets, a filtering-type water purification device was constructed.

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1. Introduction

The domestic and industrial effluents contain a large amount of organic pollutants and heavy metal ions have become a global concern due to their high toxicities and carcinogenic and mutagenic activities.^1–4 With the rapid development of industrialization and urbanization, wastewater is mainly discharged from sewage, metallurgy, electroplating, paper printing and textile dyeing processes. Efficient ways to remove organic pollutants and heavy metal ions from wastewater before it is discharged to the receiving water, particularly drinking-water, are needed to prevent adverse effects on human health.

Various methods have been developed for the removal of organic pollutants and heavy metal ions in effluent, including the use of membrane filtration, chemical oxidation, photocatalysis, adsorption, and ionic exchange.^5–7 Among these methods, the use of adsorption has attracted particular attention due to its low cost, high efficiency, ease of operation, and regeneration. Unfortunately, traditional adsorbing materials, including activated carbon,⁸ zeolite⁹ and clay,¹⁰ possess disadvantages of low adsorption capacity, high required amounts, and unsatisfactory recyclable performances. Some novel adsorbents have not been widely applied due to their toxicity.² Thus, there is a need for new adsorbent materials with nontoxic, low cost and high adsorption capacity for pollutants.^11,12

LDHs are a class of two dimensional (2D) sheets, with the generic formula:¹³ [M_1−x²⁺, M_x³⁺(OH)₂]^x+A_x/2ⁿ⁻·mH₂O, where M²⁺ and M³⁺ represent divalent and trivalent cations, Aⁿ⁻ is the interlayer anion or organic anion, and x is normally between 0.2–0.33. LDHs have potential applications in adsorbent processes, electrocatalyst, and corrosion, as well as fire retardant additives.^14–17 In addition, an example is the naturally occurring layered magnesium aluminium hydroxy carbonate,¹⁸ that suggests MgAl-LDHs are more stable than ZnAl-LDHs and NiAl-LDHs, consistent with the results of Wang et al.¹⁹ The remarkable structural features enable MgAl-LDHs to exhibit a powerful ability to capture pollutants in aqueous solution. Moreover, the large specific surface of MgAl-LDHs with accessible diffusion pathways of mesoporous networks promote excellent adsorption capacity.²⁰ Due to the striking features mentioned above, MgAl-LDHs can be widely applied for water treatment.

The domestic and industrial wastewater is a complex of organic pollutants and heavy metal ions.²¹ Although most adsorbents can adsorb organic pollutants or heavy metal ions, separately, the comprehensive adsorption of organic pollutants and heavy metal ions has rarely been reported. Additionally, batch experiments are typically used in adsorption studies and results from these studies may differ from performance in a comprehensive practical situation. To overcome these shortcomings, a filtering device must be built for a specific filtering system. In particular, the treatment of drinking water requires adsorbents which are nontoxic. Nevertheless, the utilization of such a device has rarely been reported. Hence, further studies are needed to synthesize an efficient and nontoxic adsorbent and construct a filtering device based on that adsorbent.

The objective of this study was to evaluate the possible use of MgAl-LDHs to remove both organic pollutants and heavy metal ions from aqueous solution. First, MgAl-LDHs nanosheets and microsheets were prepared through a hydrothermal method. Methyl orange (MO), Cr(VI) and Ni(II) were selected to imitate organic pollutants, heavy metal anions and cations in wastewater. Next, based on XRD, FTIR, EDS and XPS experiments, we proposed a synergistic mechanism of MgAl-LDHs nanosheets. Finally, a facile filtering adsorption device was constructed. The adsorption mechanism and absorption method can provide guidance for the investigation of new efficient adsorbents.

2. Experimental

2.1. Materials

Aluminium nitrate (Al(NO₃)₃·9H₂O), magnesium nitrate (Mg(NO₃)₂·6H₂O), nickel nitrate (Ni(NO₃)₂·6H₂O), potassium bichromate (K₂Cr₂O₇), nitric acid (HNO₃), ammonium hydroxide (NH₃·H₂O), EDTA, dimethylglyoxime, triammonium citrate, methyl orange, urea, ethanol and iodine solution were purchased from Sinopharm Group Co. LTD.

2.2. Methods

MgAl-LDHs nanosheets and microsheets were synthesized using NO₃⁻ based divalent and trivalent salts as precursor materials. For MgAl-LDHs nanosheets, 0.015 mol Mg(NO₃)₂·6H₂O and 0.005 mol Al(NO₃)₃·9H₂O were dissolved in boiling deionized water to remove CO₂. Then, ammonia was added dropwise to the solution at a temperature of 338 K under N₂ atmosphere. The mixed solution was controlled at a pH value of 10.0. Similarly to the above method, MgAl-LDHs microsheets were synthesized by mixing equal amounts of Mg(NO₃)₂·6H₂O and Al(NO₃)₃·9H₂O with 0.04 mol urea. After vigorous stirring for 30 min, both types of suspensions were transferred into 100 mL Teflon-lined autoclaves and maintained at 120 °C for 24 h. The precipitate was then collected by filtration, washed with deionized water and ethanol several times, and finally dried at 80 °C for 12 h.

2.3. Organic pollutants adsorption experiment

Due to its relatively stable chemical state, MO is difficult to degrade under natural conditions, presenting much greater harm to water resources. In this paper, MO selected as the representative of anionic dyes to probe the adsorption performance. To evaluate organic pollutant adsorption, 5 mg MgAl-LDHs nanosheets were added into 25 mL of MO solution at concentrations ranging from 20 to 50 mg L⁻¹ (the linear relationship (R² = 0.999) as illustrated in Fig. S1(a)†) under magnetic stirring in 50 mL centrifuge tubes and agitated in a temperature controlled shaker at 298 K. For comparison, 5 mg MgAl-LDHs microsheets were added into a 25 mL 20 mg L⁻¹ MO solution under the same conditions. At predetermined time intervals, samples were removed from the solution by centrifugation at 10 000 rpm for 5 min. The residual MO concentrations were detected by measuring the absorbance at 465 nm; this value was selected based on previous analysis of the UV-vis spectrum exhibited by a MO aqueous solution and measured using a UV-3101 PC spectrophotometer.

The equilibrium adsorption capacity q_e (mg g⁻¹) for LDHs was evaluated by the mass balance equation:²²

q_e = (c₀ − c_e)V/m (1)

where c₀ (mg L⁻¹) is the initial concentration of pollutants, c_e (mg L⁻¹) is the concentration of residual pollutants solution after maximum adsorption, V (mL) stands for the volume of the solution, and m (mg) is the mass of the adsorbent.

2.4. Heavy metal ion adsorption experiment

Aqueous solutions of K₂Cr₂O₇ and Ni(NO₃)₂·6H₂O were used to provide Cr(VI) anions and Ni(II) cations, respectively. The residual concentrations during the adsorption process were measured directly by the colorimetric method for Cr(VI) anions at a wavelength of 353 nm (ref. 2) and dimethylglyoxime was used as the chromogenic reagent for Ni(II) cations at a wavelength of 530 nm.

In the adsorption kinetics studies, 40 mg MgAl-LDHs nanosheets were added to 25 mL 150 mg L⁻¹ aqueous solution of Cr(VI) anions. 50 mg MgAl-LDHs nanosheets were mixed with 25 mL 140 mg L⁻¹ Ni(II) cations solution. At different time intervals, the mixtures were rapidly separated from the solid adsorbent by centrifugation at 10 000 rpm for 10 min. MgAl-LDHs microsheets were used as adsorption references under the same experimental conditions. In the adsorption isotherm studies, 25 mL of heavy metal ions solution (Cr(VI) anions and Ni(II) cations) at initial concentrations from 100 mg L⁻¹ to 200 mg L⁻¹ were mixed with the same amount of MgAl-LDHs nanosheets adsorbent for 2 h under magnetic stirring at a rotating speed of 550 rpm and then centrifuged to remove particles for analysis by UV-Vis spectrophotometer. Fig. S1(b) and S2† are shown the linear relationship (R² = 0.999) between the initial concentration and solution absorbance.

2.5. Column adsorption experiments

To evaluate filtering adsorption, 1 g of MgAl-LDHs nanosheets or microsheets were pumped into glass column of inner diameter 1 cm and heights 25 cm with different volumes of a solution at a Cr(VI) anions initial concentration of 200 mg L⁻¹. The Cr(VI) anions solution was passed through the column in up-flow direction at a flow rate of 2 mL min⁻¹ using a digital peristaltic pump. Subsequently, 200 mL of deionized water was allowed to percolate through and wash the inside of the column. Then, 0.1 M NaOH solution was used as an eluent at a flow rate of 2 mL min⁻¹. Finally the column was washed again with 200 mL of deionized water at the same flow rate. Effluent solutions were successively collected using a fraction collector during adsorption and desorption to determine the phosphate ion concentrations. This cycle was repeated five times to investigate the reusability of the adsorbent.

2.6. Characterization

Phase identification of the sintered LDHs powders was performed using an X-ray diffractometer (XRD; D/MAX2500PC model, Rigaku Co., Japan) with Cu Kα radiation at room temperature over a 2θ range of 5° to 70°. The morphology of the LDHs was observed by field emission scanning electron microscopy (FESEM) (Nanosem 450, FEI, Japan) with an energy dispersive spectrometer (EDS) and ​high-resolution transmission electron microscope (HR-TEM, JEOL JEM-2100). The valence analysis was conducted by X-ray photoelectron spectroscopy (XPS). The specific surface area of the LDHs was calculated using the Brunauer–Emmett–Teller (BET) N₂ adsorption method using a Quantachrome Quadrasorb surface analyser. The FTIR spectrum was determined using a FTIR Spectrophotometer (NEXUS670, Thermo Nicolet Corporation, U.S.) over a range of 4000–400 cm⁻¹ at room temperature. Particle size distribution were analyzed by Mastersizer-2000.

3. Results and discussion

3.1. Structure and morphology of MgAl-LDHs

To determine the crystal structure and possible phase changes during preparation, XRD spectra were collected for pristine LDHs nanosheets and microsheets prepared with various precipitants, as shown in Fig. 1. The LDHs peaks of (003) and (006) appeare extremely strong, indicating that the crystal grows along a certain axis, and (012), (015), (110) peaks exhibit a hexagonal lattice with rhombohedral 3R symmetry because of lattice constants a = b, c = 3d₀₀₃.²³ In spectrum (a), the typical peaks of MgAl-LDHs (JCPDS 54-1030) at (003), (006), (012), (009), (018), (110) and (113) plane were apparent.¹¹ According to the interlayer separation of characteristic (003) planes d₀₀₃ = 7.56 Å, LDHs microsheets are characteristic basal reflections of LDHs-CO₃²⁻ materials. LDHs microsheets with intense and sharp reflections indicate that the LDHs microsheets have high crystallinity. However, some impure peaks can be indexed to MgCO₃. In spectrum (b), the three symmetric and intense peaks correspond to the (003), (006) and (012) harmonic reflections and the basal spacing d₀₀₃ = 8.90 Å, consistent with LDHs-NO₃⁻ materials, in agreement with previously reported values.¹² The (003) peak significantly shifts to small angle, possibly due to a decrease of attractive interaction within the brucite-like layer when CO₃²⁻ is substituted for NO₃⁻, resulting in the increase of (003) plane distance.^24,25 The intensities of (00l) peaks become weaker because of the small size or the poor crystallinity of the MgAl-LDHs nanosheets.

Fig. 1 X-ray diffraction patterns of MgAl-LDHs (a) microsheets and (b) nanosheets (★ peak of MgCO₃ (JCPDS 08-0479)).

The morphology and particle size of the as-prepared MgAl-LDHs were characterized by FESEM. Fig. 2(a) and (b) shows the typical SEM images of MgAl-LDHs microsheets, which are 2 μm in width. These images show a mass of chips, indicating the material falling apart due to their unstable structure. For comparison, MgAl-LDHs nanosheets were prepared using ammonia as precipitant reagents. The images of these materials show that the MgAl-LDHs nanosheets are 100 nm in width and the shape retains its integrity, as shown in Fig. 2(c) and (d). Based on the size of the samples, MgAl-LDHs nanosheets are expected to expose more sites for active adsorption than MgAl-LDHs microsheets, and thus this material could be expected to demonstrate superior adsorption activity. It is worth noting that the evident particles (Fig. 2(d)) on the surface of MgAl-LDHs may result from metal spraying (Pt).

Fig. 2 FE-SEM images of MgAl-LDHs (a, b) microsheets and (c, d) nanosheets.

The crystal structure, morphology, and microstructure of the obtained MgAl-LDHs nanosheets are analyzed via TEM and HRTEM analyses. The TEM images of MgAl-LDHs nanosheets show that the typical hexagonal crystal are about 100 nm in width (Fig. 3(a)), which agrees well with the above SEM result. From the side view of MgAl-LDHs nanosheets, the MgAl-LDHs nanosheets dominates the stratified structure, constituted by layer upon layer stacking. The pristine MgAl-LDHs nanosheets display clearly resolved and well-defined lattice fringes even on the surface of the nanocrystal, which are shown in the HRTEM image (Fig. 3(b)), in which the porous structure are clearly visible. The interplanar spacing of 0.26 nm corresponds to the (012) crystal planes of MgAl-LDHs nanosheets. Both nanosheets and microsheets were synthesized without detection of any templates possessing hexagonal symmetry, which indicates that the MgAl-LDHs may be single crystals.²⁶

Fig. 3 (a) TEM and (b) HRTEM images of MgAl-LDHs nanosheets.

3.2. Specific surface area and porosity and particle size distribution

Nitrogen adsorption–desorption isotherm measurements were recorded to examine the structural characteristics of MgAl-LDHs nanosheets and microsheets. The surface and pore size distributions of LDHs nanosheets and microsheets are depicted in Fig. 4. The BET surface areas of the MgAl-LDHs as determined by the BET data were 65.94 m² g⁻¹ for the nanosheets and 15.75 m² g⁻¹ for the microsheets. The smaller nanostructure volume and the unique structure of the interlayer result in a specific surface area of nanosheets that is greater than that of TMCS absorbent materials²⁷ or calcined graphene/MgAl-LDH,²³ and thus presents higher adsorption capacity. Both of the isotherms display two hysteresis loops, which appear as type-IV isotherms with hysteresis loops at a higher pressure between 0.6 and 1.0. This is possibly associated with capillary condensation in the uniform mesopores. As shown in the insets, the MgAl-LDHs nanosheets are composed of mesopores of around 8.87 nm and the microsheets are mainly constituted by mesopores of around 4.87 nm. Particle size distribution of MgAl-LDHs nanosheets and microsheets are shown in Fig. S4.† The particle sizes of MgAl-LDHs nanosheets and microsheets (Fig. S3(a)†) are about 200 ± 100 nm and 2 ± 1 μm, respectively. The latter values may be due to the agglomeration of nanosheets. Besides, the particle sizes of MgAl-LDHs nanosheets is smaller than those of microsheets (5 ± 1 μm) (Fig. S3(b)†). The larger nanoscale mesopores and smaller particle sizes increase the specific surface area of MgAl-LDHs nanosheets and enhance the transport of organic pollutants and heavy metal ions deep inside the materials.

Fig. 4 Nitrogen adsorption–desorption isotherms and pore size distributions (insets) of MgAl-LDHs nanosheets and microsheets.

3.3. Comparisons of adsorption performance of MgAl-LDHs nanosheets and microsheets

The adsorption performance of MO, Cr(VI) anions, and Ni(II) cations for MgAl-LDHs nanosheets and microsheets, as shown in Fig. S4.† The results give clear evidence that MgAl-LDHs have excellent adsorption capacity for organic pollutants and heavy metal ions. The MgAl-LDHs nanosheets exhibit much higher adsorption and the adsorption amount at equilibrium can reach 99.9, 88.3 and 69.9 mg g⁻¹ for the adsorption of MO, Cr(VI) anions and Ni(II) cations, respectively, values that are much higher than those of the microsheets (29.1, 37.0 and 8.9 mg g⁻¹). Furthermore, the adsorption rate of MO, Cr(VI) anions, Ni(II) cations for MgAl-LDHs nanosheets was still superior to microsheets. The excellent adsorption capacity of organic pollutants and heavy metal ions can be attributed to the larger specific surface area and the smaller in width of the MgAl-LDHs nanosheets.

The fast adsorption rate and enhanced adsorption capacity of MgAl-LDHs nanosheets can be ascribed to the following factors. First, the larger interlayer spacing of MgAl-LDHs nanosheets offer facilities for the adsorption of organic pollutants and heavy metal ions into the interlayer of LDHs based on the XRD results (Fig. 1). Second, the MgAl-LDHs nanosheets with mesopores have a larger specific surface area that provides adequate adsorptive sites for organic pollutants and heavy metal ions. In contrast, the MgAl-LDHs microsheets have some relatively obvious shortcomings such as smaller interlayer spacing and specific surface area, resulting in unsatisfactory adsorption performance. Thus, the MgAl-LDHs nanosheets were selected for further study of the adsorption properties and investigation of the adsorption mechanism for organic pollutants and heavy metal ions.

3.4. Adsorption of organic pollutants by MgAl-LDHs nanosheets

A series of systematic experiments were performed to examine MO adsorption by the MgAl-LDHs nanosheets in aqueous solution. The adsorption performance versus time for the adsorption of MO by MgAl-LDHs nanosheets at different initial MO concentrations is shown in Fig. 5. The adsorption amount of MO increased rapidly within the first 5 min for all concentrations and then reached equilibrium after 20 min. The adsorption amount at equilibrium was approximately 99.9, 148.9, 196.7 and 229.8 mg g⁻¹, corresponding to initial concentrations of 20, 30, 40 and 50 mg L⁻¹, respectively. The high adsorption rate suggests this type of adsorbent might work well for water purification. The adsorption amount increased as the initial concentration of MO increased from 20 mg L⁻¹ to 50 mg L⁻¹, which is attributed to an increase in the driving force of concentration gradient to overcome the mass transfer resistance of the MO between the aqueous phases and the solid phases with the increase of the initial concentration. Overall, MgAl-LDHs nanosheets exhibit efficient removal of organic pollutants that is better than other sorbents, as listed in Table 1.

Fig. 5 Effect of contact time on MO adsorption using MgAl-LDHs nanosheets for initial MO concentrations of 20, 30, 40 and 50 mg L⁻¹.

Table 1 Comparison of adsorption capacities of MO for various adsorbents

Adsorbents	qmax (mg g^−1)	References
MgAl-LDHs nanosheets	229.8	This paper
Hypercrosslinked polymeric	71.0	28
MoS2 nanosheets	60.0	29
Activated carbon	11.8	8
Porous g-C3N4	2.5	30
KGM/GO	51.6	31

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3.5. Adsorption of heavy metal ions by MgAl-LDHs nanosheets

Due to their integrated specific nanosheets structures, the as-obtained LDHs nanosheets show promise for application in the uptake of heavy metal ions for water purification. Because Ni(II) cations and Cr(VI) anions are two typical toxic heavy metal ions in water resources, they were selected to illustrate the adsorption capability of nanosheets. The adsorption kinetic and the adsorption capacity of adsorbents are commonly used to investigate adsorption performance.

The adsorption kinetic studies were performed using LDHs nanosheets in Cr(VI) anions and Ni(II) cations solutions, as shown in Fig. 6(a). The adsorption amount was more than 80% within the first 10 min, which is attributed to the increased driving force coming from the adequate free adsorptive sites and a high metal ions concentration gradient. The adsorption amount can reach approximately 95% after 30 min, considered the adsorption–desorption equilibrium as there was no significant change in Cr(VI) anions and Ni(II) cations concentration with further increase of time. The rate of adsorbate uptake on LDHs and adsorption equilibrium time was well described by the pseudo first-order and pseudo second-order adsorption model,² as shown in Table S1.†

Fig. 6 (a) Adsorption kinetics and (b) adsorption isotherms of Cr(VI) anions and Ni(II) cations for MgAl-LDHs nanosheets at 298 K.

The pseudo first-order kinetic model assumes that the change rate of solute adsorption with time monotonely decreases with the concentration of solution, which is generally suitable to describe the preliminary stage of an adsorption process. The pseudo second-order kinetic model states that the adsorption before reaching the adsorption equilibrium is chemisorption and predicts the behavior over the whole range of adsorption. The adsorption of Cr(VI) anions and Ni(II) cations for the MgAl-LDHs nanosheets was consistent with both the pseudo first-order and pseudo second-order equations. The correlation coefficient (listed in Table S1†) for the pseudo second-order model (R² = 1.000) was appreciably larger than that of the pseudo first-order model (R² < 0.985). It is worth noting that the best fit pseudosecond-order parameters (q_e) can vary with equilibrium adsorption capacity.

To eliminate the effect of required different equilibrium times of Cr(VI) anions and Ni(II) cations at different concentrations, the adsorption experiments were conducted for 180 min to achieve complete adsorption equilibrium and obtain a more accurate value of q_max. As illustrated in Fig. 6(b), the adsorption capacity was investigated under different initial Cr(VI) anions and Ni(II) cations concentrations from 100 to 200 mg L⁻¹, and the maximum adsorption capacity (q_max) of Cr(VI) anions and Ni(II) cations was 90.2 and 63.8 mg g⁻¹, respectively. To further elucidate the adsorption performance, the Langmuir and Freundlich adsorption models¹⁹ were used to described the results.

The outstanding adsorption capacity is reflected by the linear fitting of the Langmuir and Freundlich isotherms, as shown in Table S2.† According to the values of R² of the two isotherms, the experimental data fit the Langmuir equation better than the Freundlich equation. This suggests that the homogeneous adsorption of Cr(VI) anions and Ni(II) cations results in monolayer coverage on the surface of LDHs nanosheets. From the Langmuir equation fitting data, the maximum absorption capacity was determined to be 92.3 and 63.8 mg g⁻¹ for Cr(VI) anions and Ni(II) cations, respectively, corresponding well to the data in Fig. 6(b). Overall, the efficient adsorption capacity of LDHs nanosheets is higher than that of other hydrotalcite materials.^32,33

3.6. Adsorption mechanisms

To investigate the adsorption mechanisms of MgAl-LDHs nanosheets, the structures of samples before and after adsorption were analyzed by XRD measurements, as shown in Fig. 7(a). For the XRD pattern of MgAl-LDHs nanosheets containing Ni(II) cations, a displacement of the (003) peak from 10.0 to 10.9° was observed. The interlayer distance thus decreased from 0.89 to 0.82 nm. This means that NO₃⁻ anions within the interlayer space of MgAl-LDHs are partly replaced by carbonate ones, provided by CO₂ in water from atmosphere. The XRD pattern of MgAl-LDHs nanosheets containing MO molecules and Cr(VI) anions shows that the peaks of (003) evidently shift to small angle and became weaker compared to the patterns of MgAl-LDHs nanosheets containing Ni(II) cations, due to the exchange of NO₃⁻ anions with MO molecules and Cr(VI) anions. MO molecules and Cr(VI) anions are intercalated into the interlayer space of the MgAl-LDHs nanosheets, causing an increase of the interlayer spacing distance. These results agree well with those previously reported.^12,14,34

Fig. 7 (a) X-ray diffraction patterns and (b) infrared spectra of MgAl-LDHs nanosheets before and after MO, Cr(VI) anions and Ni(II) cations adsorption.

The structures of the as prepared samples before and after adsorption were further analyzed by FTIR spectroscopy, as illustrated in Fig. 7(b). The absorption band near 3460 cm⁻¹ in the spectrum of the MgAl-LDHs nanosheets can be assigned to the stretching vibration of the hydroxyl groups of LDHs layers. The peak at 1630 cm⁻¹ can be ascribed to the H₂O bending vibration. A sharp band at 1382 cm⁻¹ may be caused by contamination by NO₃⁻ in the synthesis of MgAl-LDHs nanosheets. The peak at 1122 cm⁻¹ are assigned to aromatic S=O groups that link the dye molecules and the MgAl-LDH layer, indicating that MO pollutants are adsorbed onto the external surface of MgAl-LDHs nanosheets.³⁵ The characteristic band of chromate is due to mode ν_d(Cr–O), recorded at 882 cm⁻¹ for the MgAl-LDHs nanosheets after adsorption of Cr(VI) anions. This shows that the interlayer NO₃⁻ anions exchanged with Cr(VI) anions in solution.

To evaluate whether Cr(VI) anions and Ni(II) cations are adsorbed onto MgAl-LDHs nanosheets, XPS was performed, as shown in Fig. 8. Fig. 8(a) displays the survey spectrum of original MgAl-LDHs nanosheets in which the peaks of Mg 1s and Al 2p are clearly observed and no contaminants other than C are observed. The MgAl-LDHs nanosheets after Cr(VI) anions (red curve in Fig. 8(a)) and Ni(II) cations (blue curve in Fig. 8(a)) are analyzed. The XPS peak located at 579.4 eV corresponding to Cr 2p was found in the spectrum of MgAl-LDHs nanosheets containing Cr(VI) anions, indicating the adsorption of Cr(VI) anions. The XPS peak located at 855.6 eV assigned to the Ni 2p was also be found in the spectrum of MgAl-LDHs nanosheets containing Ni(II) cations, which confirms that Ni(II) cations are successfully absorbed by the MgAl-LDHs nanosheets. High-resolution spectra of elemental Cr 2p and Ni 2p (Fig. 8(c) and (d)) further confirm the adsorption performance of heavy metal ions. Spectra obtained from Cr 2p_3/2 and Cr 2p_1/2 show symmetrical features at binding energies of 578.9 eV and 587.5 eV, as depicted in Fig. 8(c), which can be unambiguously assigned to emission from the 2p levels of Cr(VI). The peaks centered at 854.2 eV and 872.9 eV can be assigned to Ni 2p_3/2 and Ni 2p_1/2 in Fig. 8(d), demonstrating the presence of Ni(II). The above results further confirm that Cr(VI) anions and Ni(II) cations are adsorbed onto the MgAl-LDHs nanosheets without valence variation of Cr and Ni. The details of the adsorption mechanism can be explained through the change of XPS peak of elemental N 1s before and after Cr(VI) anions and Ni(II) cations adsorption, as shown in Fig. 8(b). The element of N disappears after the adsorption of Cr(VI) anions due to the exchange of NO₃⁻ anions with MO molecules and Cr(VI) anions. And then Cr(VI) anions are successfully intercalated into the interlayer space of the MgAl-LDHs nanosheets, consistent with the XRD results (Fig. 6(a)).

Fig. 8 X-ray photoelectron spectroscopy of MgAl-LDHs nanosheets before and after Cr(VI) anions and Ni(II) cations adsorption (a) fully scanned spectra, high resolution spectra of elemental (b) N 1s, (c) Cr 2p and (d) Ni 2p.

EDS mapping was used to confirm the adsorption behavior of MgAl-LDHs nanosheets, as shown in Fig. 9. The pollutants were well-distributed in the MgAl-LDHs nanosheets with pollutants. The elements of Mg, Al and O were chosen to represent MgAl-LDHs, and the element of S, Cr and Ni indicated MO, Cr(VI) anions, and Ni(II) cations, respectively. Obviously, the elements of S, Cr and Ni were uniformly distributed on the surface of the MgAl-LDHs nanosheets, which confirms the successful absorption performance of MgAl-LDHs nanosheets. The nanosheets structures provide uniform adsorption sites for the adsorption of pollutants.

Fig. 9 EDS mapping of MgAl-LDHs nanosheets after the adsorption of (a) MO, (b) Cr(VI) anions, and (c) Ni(II) cations.

On the basis of the above results, the absorption mechanism of pollutants for MgAl-LDHs nanosheets is illustrated in Scheme 1. The MgAl-LDHs has variable charges due to the adsorption of OH⁻ from solution when the solution pH value after adsorption is about 9.0. The deprotonation of surface hydroxyls of MgAl-LDH can be described as follows:³⁶

Sur-OH + OH⁻ → Sur-O⁻ + H₂O (2)

where Sur denotes the surface of MgAl-LDH.

Scheme 1 Schematic diagram of adsorption of pollutants for the MgAl-LDHs nanosheets.

There may be homologous mechanisms for MO and Cr(VI) anions adsorption onto MgAl-LDHs nanosheets. The ion exchange plays an essential role in adsorption, due to the presence of NO₃⁻ in the interlayer of the MgAl-LDHs nanosheets after exchange with MO and Cr(VI) anions. Additionally, the residual Sur-OH₂⁺ of MgAl-LDH forms outer-sphere surface complexes with MO and Cr(VI) anions.¹⁴ The FT-IR results show that there is no significant change in the spectrum, indicating that the adsorption happened through a kind of weak interaction and not strong enough to take big effect on the compound. The phenomenon is consistent with the fact that hydrogen bonding is a kind of weak forces.³⁷ So the –SO₃ group is the binding bridge between dye molecules and the layer of MgAl-LDHs by hydrogen bonding.³⁸ The Ni(II) classical metal cations adsorption onto MgAl-LDHs nanosheets may be attributed to (i) formation of outer-sphere complexes with Ni(II) cations by electrostatic binding, consistent with the results of FITR, (ii) isomorphic substitution because the atomic radium of Ni(II) to Mg(II) in MgAl-LDHs is much closer,³⁶ or (iii) formation of metal hydroxides (Ni(OH)₂) at high pH.

3.7. The filtering adsorption for MgAl-LDHs nanosheets and microsheets

The above results have indicated that the MgAl-LDHs nanosheets showed better adsorption performance than microsheets for heavy metal ions. The quick and efficient adsorption inspires us to adopt filtering adsorption for water purification. In addition, MgAl-LDHs nanosheets meet the requirements as a nontoxic adsorbent in dealing with drinking water. It is the key to water pollution treatment that an efficacious way to keep the pollutants to avoid re-contamination is expected. For these above reasons, a filtering adsorption device was constructed to verify the advantages of MgAl-LDHs nanosheets, as shown in Fig. 10(a). The device includes three parts: (i) a container holds simulated polluted water, (ii) the digital peristaltic pump system is applied to pump polluted water through the filtering device; (iii) the column is filled with MgAl-LDHs. Fig. 10(b) displays the breakthrough curves for adsorption of Cr(VI) for MgAl-LDHs nanosheets and microsheets. It can be seen that the breakpoint time onto MgAl-LDHs nanosheets and microsheets were 35 and 8 h respectively. The results indicated that the treatment capacity of the nanosheets was much better than that of microsheets due to the quick and efficient heavy metal ions removal ability of nanosheets. The cyclic utilization of the adsorbent for adsorption of Cr(VI) was carried out after it was regenerated (Fig. S5†). It is found that the MgAl-LDHs nanosheets exhibits a very stable performance on adsorption of Cr(VI). The adsorption rate is only reduced by approximately 20% after five cycles. This device containing MgAl-LDHs nanosheets can be used to eliminate pollutants in aqueous solution by filtering, which is potentially practical for water treatment.

Fig. 10 (a) Filtering device for pollutant removal (b) breakthrough curves of MgAl-LDHs microsheets and nanosheets.

4. Conclusions

In summary, MgAl-LDHs microsheets and nanosheets were synthesized via a simple hydrothermal method. The as-prepared mesoporous MgAl-LDHs nanosheets present small nanosheets in width (∼100 nm), a high specific surface area (65.94 m² g⁻¹), a large pore size (∼8.87 nm), and a small particle size (∼200 ± 100 nm). As a result, the MgAl-LDHs nanosheets showed better adsorption performance than the MgAl-LDHs microsheets for MO and Cr(VI) anions and Ni(II) cations. The excellent adsorption of MO by MgAl-LDHs nanosheets was 229.8 mg g⁻¹. The adsorption kinetics and the adsorption isotherms of Cr(VI) anions and Ni(II) cations onto the MgAl-LDHs nanosheets can be described by the pseudo second-order kinetics and the Langmuir isotherm, with saturated adsorption of 63.8 and 92.3 mg g⁻¹, respectively. The MgAl-LDHs nanosheets showed enhanced adsorption performance for organic pollutants and heavy metal ions. First, MgAl-LDHs nanosheets were found to efficiently adsorb MO from aqueous solution by ion exchange, hydrogen bonding adsorption and outer-sphere surface complexes. Second, the adsorption mechanism indicated that adsorption of Cr(VI) anions for MgAl-LDHs nanosheets arises from the synergistic effect of outer-sphere surface complexes on the adsorbent surface and ion exchange in the interlayer space. Third, the adsorption of Ni(II) cations for MgAl-LDHs nanosheets can be divided into three categories: outer-sphere complexes, isomorphic substitution and metal hydroxides. To take advantage of the quick and efficient heavy metal ions removal ability of MgAl-LDHs nanosheets, a filtering-type water purification device was constructed that presents an attractive option to improve the overall economics of water treatment systems.

Acknowledgements

The authors are thankful for fundings from the National High Technology Research and Development Program of China (863 Program, No. 2015AA034404), National Natural Science Foundation of China (No. 51502160 and 51272141), Taishan Scholars Project of Shandong Province (No. TS20110828), Natural Science Foundation of Shandong Province (No. ZR2015EQ001) and SDUST Research Fund (No. 2015JQJH101).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18783h

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