One-step synthesis of aluminum magnesium oxide nanocomposites for simultaneous removal of arsenic and lead ions in water

Abstract

Aluminum magnesium oxide nanocomposites were prepared via a one-step microwave assisted solvothermal method, and they showed high adsorption capacities for the removal of both of As(V) and Pb(II) ions in water.

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Heavy metal ions, such as As(V), Pb(II) and Cd(II), are highly toxic pollutants, which can have serious side effects and toxicities on human health when their concentrations are higher than the permissible limits.^1–3 Therefore, their efficient removal from water is an actively pursued goal. Various methods, including chemical coagulation, ion exchange, membrane process, electrochemical method, and adsorption, have been developed for the removal of these heavy metal ions.^3–15 Among these methods, the adsorption technique is possibly the most extensively adopted because of its low cost and simplicity. On the other hand, traditional absorbents, such as activated carbon, activated alumina, clay and zeolite, show limited adsorption abilities for heavy metal ions. Nanomaterials, including metal oxides with hierarchical nanostructures and layered double hydroxides, have shown excellent adsorption capacities for the removal of heavy metal ions.^14–20 The adsorption capacity of nanomaterials can be attributed to high surface area, facile mass transportation and abundance active adsorption sites. Song et al. developed many different kinds of nanomaterials, such as flowerlike α-Fe₂O₃, CeO₂ hollow nanospheres, ordered mesoporous γ-Al₂O₃, urchin zinc silicate, which showed higher adsorption capacities than commercial adsorbents.^21–28 Recently, they reported a novel protocol for synthesizing flowerlike magnesium oxides and aluminum basic carbonate microporous nanospheres for the removal of Pb(II) and As(V) with maximum adsorption capacities of 1980 mg g⁻¹ and 170 mg g⁻¹, respectively.^29,30 Despite these outstanding achievements, most of the results can only remove either negatively charged heavy metal anions or positively charged heavy metal cations alone.

Wang et al. and Lou et al. fabricated chrysanthemum-like α-FeOOH³¹ and urchin-like α-FeOOH hollow nanospheres,³² respectively, and found their good adsorption properties for both of As(V) and Pb(II). Yang et al. prepared a hierarchical porous magnetic nanomaterial with high adsorption capacities for the removal of Pb(II), As(V) and Cr(VI) ions from aqueous solutions.³³ On the other hand, the adsorption capacities of these adsorbents are still relatively low. In addition, expensive chemicals and complicated synthesis processes are normally used, which limit their practical applications. Therefore, there is urgent demand for the development of a low-cost and facile method to fabricate nanomaterials with high adsorption capacities for the removal of various cationic and anionic heavy metal ions.

Herein, we present a facile one-step synthesis of aluminum magnesium oxide nanocomposites via a microwave assisted solvothermal method, which is a simple, template-free and low-cost route. In addition, the morphologies and structures of the aluminum magnesium oxide nanocomposites can be easily tuned by changing the molar ratio of Al³⁺ and Mg²⁺ addition. These nanocomposites had a large surface area and showed excellent adsorption properties for both As(V) and Pb(II) with maximum adsorption capacities of 133 mg g⁻¹ and 423 mg g⁻¹, respectively.

In a typical synthesis, a certain amount of Al(NO₃)₃·9H₂O, Mg(NO₃)₂·6H₂O (total metal molar is 10 mmol) and 20 mmol of urea were dissolved in 100 mL of anhydrous ethanol under sonication to form a clear solution, and about 40 mL of the solution was then poured into a Teflon-lined autoclave for microwave heating. The oven was heated to 150 °C in 2 min by microwave irradiation, and then kept at that temperature for an additional 30 min. The precipitates were collected by centrifugation after cooling to room temperature, and then washed with water and ethanol. Finally, the samples were dried at 80 °C for 5 h. The detail experiment procedures are shown in the ESI.† The samples obtained with different molar ratios of Al³⁺ and Mg²⁺ were defined as Al_xMg_y nanocomposites, x is the molar of Al(NO₃)₃·9H₂O and y is the molar of Mg(NO₃)₂·6H₂O.

Unlike previously reported multistep methods for the synthesis of nanocomposites,³³ a one-step microwave assisted solvothermal method without an organic template or solvent was used to prepare hierarchical aluminum magnesium oxide nanocomposites in this work. Microwave heating has major advantages in cost- and time-efficiency, in which the reaction time for the solvothermal process can be within 30 min. In addition, microwave heating leads to uniform heating of the whole synthesis mixture, resulting in homogeneous nanocomposites in terms of shape and size. Overall, this method is facile, low-cost and environment-friend.

Fig. 1a shows a typical SEM image of Al₃Mg₇ nanocomposites samples, where the surface of the nanocomposites was rough and many sheets were deposited on the surface. TEM (Fig. 1b) clearly shows that Al₃Mg₇ nanocomposites are composed of “graphene-like” sheets wrapped with nanospheres. The corresponding X-ray diffraction (XRD) pattern for the Al₃Mg₇ nanocomposites is shown in Fig. 1c. All diffraction peaks can be indexed to Mg₂(OH)_3.24(NO₃)_0.76(H₂O)_0.24 (JCPDS 47-0436), while no obvious diffraction peaks of aluminum chemicals were observed, suggesting that the aluminum chemicals may existed as an amorphous form. The energy dispersive spectrum (EDS) (Fig. S1, ESI†) indicated that the sample consisted of carbon, oxygen, aluminum and magnesium.

Fig. 1 (a) SEM image, (b) TEM image and (c) XRD pattern of typical sample of Al₃Mg₇ nanocomposites.

To investigate the formation process and the structures of the Al₃Mg₇ nanocomposites, samples prepared at different reaction times were collected and investigated by SEM, XRD and EDS. Solid and amorphous spheres with mean diameters of ca. 200 nm were obtained in 5 min (Fig. S2†). More interestingly, neither magnesium nor nitrogen was detected in the EDS spectrum, indicating there was no magnesium containing chemicals formed during this reaction time. In addition, the XRD pattern also showed no diffraction peak. When the reaction time was prolonged to 30 min, “graphene-like” sheets wrapped with nanospheres were observed, and magnesium chemicals were also formed, as discussed above. The diameter of the inner nanospheres was almost the same as those obtained in 5 min, suggesting that magnesium chemicals were formed after 5 min and wrapped the surface of the inner nanospheres. During 5 min, the synthesis procedures in this communication were similar to the previous paper reported by Song et al., where the samples were defined as Al(OH)CO₃ nanospheres.³⁰ Therefore, we can ascribe the solid and amorphous inner nanospheres formed during 5 min as Al(OH)CO₃. After 5 min, “graphene-like” sheets of Mg₂(OH)_3.24(NO₃)_0.76(H₂O)_0.24 grew and wrapped the Al(OH)CO₃ nanospheres to form the final aluminum magnesium oxide nanocomposites. The possible reason for forming these structures was the different nucleation speed of Al(OH)CO₃ and Mg₂(OH)_3.24(NO₃)_0.76(H₂O)_0.24.

In addition, the morphologies and structures of aluminum magnesium oxide nanocomposites can be easily tuned by changing the molar ratio of Al³⁺ and Mg²⁺. As shown in Fig. 2, aluminum magnesium oxide nanocomposites with different sizes of “graphene-like” sheets and inner nanospheres were obtained. With increasing Mg²⁺ concentration or decreasing Al³⁺ concentration, the size of the “graphene-like” sheets became larger and the inner Al(OH)CO₃ nanospheres became smaller.

Fig. 2 TEM images of aluminum magnesium oxide nanocomposites with different molar ratio of aluminum and magnesium (scale bars: 200 nm).

Nitrogen adsorption–desorption isotherms of aluminum magnesium oxide nanocomposites with different molar ratios of aluminum and magnesium are shown in Fig. 3. The Brunauer–Emmett–Teller (BET) surface area of Al₈Mg₂, Al₇Mg₃, Al₅Mg₅, Al₃Mg₇, and Al₂Mg₈ nanocomposites were 270, 262, 220, 191 and 126 m² g⁻¹, respectively. The gradual decrease in specific surface area with decreasing aluminum content was due to the inner Al(OH)CO₃ nanospheres being microporous structures with a higher specific surface area. These aluminum magnesium oxide nanocomposites with different chemical structures and surface areas have different adsorption properties for heavy metal ions.

Fig. 3 Nitrogen adsorption–desorption isotherms of aluminum magnesium oxide nanocomposites with different molar ratio of aluminum and magnesium.

Owing to the high specific surface area of the aluminum magnesium oxide nanocomposites, they were expected to show an advantage in heavy metal ions adsorption. As(V) and Pb(II) are two typical toxic heavy metal ions in water resources, and their efficient removal is of great importance. Fig. S3† shows the adsorption rates of As(V) and Pb(II) solutions with an initial concentration of 50 mg L⁻¹ on the Al₈Mg₂ spheres at room temperature. The adsorption processes are quite fast as equilibrium is reached in only 30 min. Fig. 4a and b show the adsorption isotherms of aluminum magnesium oxide nanocomposites with different molar ratios of aluminum and magnesium for As(V) and Pb(II), respectively. The adsorption data was fitted to the Langmuir model as follows:

q_e = q_mbC_e/(1 + bC_e)

where C_e is the equilibrium concentration of heavy metal ions (mg L⁻¹), q_e is the amount of heavy metal ions adsorbed per unit weight of the adsorbent at equilibrium (mg g⁻¹), q_m (mg g⁻¹) is the maximum adsorption capacity, and b is the equilibrium constant related to the adsorption energy.

Fig. 4 Adsorption isotherms of (a) As(V), (b) Pb(II) and (c) maximum adsorption capacities on aluminum magnesium oxide nanocomposites with different molar ratio of aluminum and magnesium.

This showed that the experimental data agreed well with the Langmuir model, indicating the mono-surface complexion processes. The maximum adsorption capacities of aluminum magnesium oxide nanocomposites can be calculated according to the fitting curves, as shown in Fig. 4c. The maximum adsorption capacities for As(V) and Pb(II) were 133 mg g⁻¹ on Al₈Mg₂ and 423 mg g⁻¹ on Al₂Mg₈, respectively. These values are significantly higher than those reported recently, as shown in Table 1. In addition, these adsorbents can be regenerated with NaOH (0.1 M) for re-usability. Taking Al₈Mg₂ spheres for example, a recycling test shows that their capacities can be maintained at 124 mg g⁻¹ and 200 mg g⁻¹ for As(V) and Pb(II) after regeneration.

Table 1 Maximum adsorption capacities of different adsorbents for As(V) and Pb(II)

Adsorbents	Maximum adsorption capacity (mg g^−1)
	As(V)	Pb(II)
Al8Mg2 (this study)	133	215
Al2Mg8 (this study)	38.7	423
Chrysanthemum-like α-FeOOH^31	66.2	103
Urchin-like α-FeOOH hollow nanospheres^32	58	80
Porous magnetic Fe2O3@AlO(OH)^33	74.9	84.1

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The difference in the adsorption capacities for Pb(II) and As(V) on the aluminum magnesium oxide nanocomposites with different molar ratios of Al³⁺ and Mg²⁺ can be ascribed to the following reasons. According to the literature, the active adsorption sites for As(V) and Pb(II) are aluminum and magnesium, respectively.^29,30 Therefore, when the amount of aluminum in the composite is high, the adsorption capacity for As(V) is also high, while the adsorption capacity for Pb(II) is low. On the other hand, low adsorption capacity for As(V) and high adsorption capacity for Pb(II) can be found when the composites contain low aluminum and high magnesium contents. Moreover, aluminum magnesium oxide nanocomposites with different chemical structures showed different surface areas, which also affected the adsorption properties for heavy metal ions.

In conclusion, aluminum magnesium oxide nanocomposites were prepared using a one-step microwave assisted solvothermal method. This is a simple, template-free and low-cost route. These nanocomposites had a large surface area and could adsorb both heavy metal anions and cations. The maximum adsorption capacities for As(V) and Pb(II) were 133 mg g⁻¹ and 423 mg g⁻¹, respectively.

Acknowledgements

We gratefully thank the National Basic Research Program from Ministry of Science and Technology (no. 2011CB933700), National Natural Science Foundation of China (no. 51378014, 51338008, 51338010).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: EDS spectra of aluminum magnesium oxide nanocomposites obtained at 5 min and 30 min. See DOI: 10.1039/c4ra13146k

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