Elsevier

Chemical Engineering Journal

Volume 345, 1 August 2018, Pages 252-259
Chemical Engineering Journal

Insights into adsorption mechanism for fluoride on cactus-like amorphous alumina oxide microspheres

https://doi.org/10.1016/j.cej.2018.03.174Get rights and content

Highlights

  • Cactus-like amorphous alumina oxide microspheres were fabricated by a facile method.

  • The CA-AlOx exhibited excellent fluoride removal performance.

  • Actual groundwater can reach drinking standard after treated with the CA-AlOx.

  • The removal mechanism was mainly chemical reaction of coordinatively unsaturated Al.

  • Ion exchange, electrostatic adsorption and pore filling also played important roles.

Abstract

Cactus-like amorphous alumina microspheres (CA-AlOx) were fabricated by a facile solvothermal method without templates. The as-prepared CA-AlOx material exhibited the amorphous structure which is beneficial to the large specific surface area, developed pore structure, abundant functional groups and high reactivity of coordinatively unsaturated Al species. The maximum adsorption capacity of CA-AlOx was calculated to be 129.4 mg/g for fluoride, which was higher than that of many adsorbents reported. Moreover, the CA-AlOx exhibited fast adsorption rate, high interference resistance and good reusable ability. Adsorption mechanism and relationship between amorphous structure and adsorption performance for fluoride on the CA-AlOx were investigated systematically by various characterization techniques. The adsorption mechanism of fluoride on the CA-AlOx was mainly through the chemical coordination of unsaturated Al-species AlO4 and AlO5 by fluoride ions to form the AlF3 and [AlF6]3− species by XPS and NMR analysis. Also, ion exchange, electrostatic adsorption and pore filling were also involved in the adsorption process. These results suggested that the CA-AlOx has great application potential for deep purification of fluoride-contaminated drinking water.

Graphical abstract

Coctus-like amorphous alumina oxide microspheres (CA-AlOx) were fabricated by a facile method, exhibiting an excellent fluoride removal performance. Adsorption mechanism and relationship between amorphous structure and adsorption performance for fluoride on the CA-AlOx were investigated systematically by various characterization techniques.

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Introduction

Fluoride pollution in drinking water poses serious risks to the eco-environment and human health [1], [2]. Therefore, the high efficient removal of fluoride from drinking water has become an issue of great concern [3], [4]. Among various methods, adsorption is one of the most extensively adopted approaches due to its high efficiency, easy operation and cost-effective [5], [6], [7]. Recently, many researchers investigated the removal of fluoride on various adsorbents such as carbon-based materials [8], [9], [10], aluminum-based materials [11], [12], [13], [14], [15], [16], hydrotalcite [17], [18], [19] and mixed metal oxides [20], [21], [22]. Aluminum-based materials have attracted considerable attention in water treatment owing to their high efficiency, low-cost and environmentally benign nature. Zhao et al. [12] synthesized Fe3O4@Al(OH)3 magnetic nanoparticles for fluoride removal and found that its adsorption capacity was 88.48 mg/g. Wei et al. [13] reported Al(OH)CO3 nanospheres with good adsorption property for fluoride (59 mg/g). However, relatively low adsorption capacities limited theirs application. Therefore, it is desirable to design and synthesize adsorbents with high removal efficiency for fluoride.

Understanding of the adsorption mechanism is important for rational design and fabrication of high-efficiency adsorbents. In general, the adsorption mechanisms of fluoride mainly include electrostatic attraction and ion exchange [23]. Chai et al. [11] proposed that anion exchange of sulfate by fluoride and formation of inner-sphere fluoride complex were the mainly adsorption mechanisms of fluoride removal by the sulfate-doped Fe3O4/Al2O3 nanoparticles. However, whether other functional groups are also involved in the adsorption process and how fluoride ions are bound to the adsorbents has not been elucidated, especially the role of the other groups remains unclear.

Herein, cactus-like amorphous alumina microspheres (CA-AlOx) were prepared by a facile method without template, surfactant or toxic materials. The as-prepared CA-AlOx exhibited excellent adsorption performance for fluoride removal. The objectives of this study are to design adsorbent with high removal efficiency for fluoride, and then reveal how amorphous structure affects their adsorption performance for fluoride, finally investigate adsorption mechanism of fluoride. The ultimate goals are to establish relationship between material structure and adsorption performance, and thus providing theoretical basis for development of high efficiency defluoridation materials.

Section snippets

Materials synthesis

All chemical reagents, including sodium fluoride (NaF), aluminum nitrate (Al(NO3)3·9H2O), acetic acid (HAc) and isopropanol (IPA), were purchased from Sinopharm Chemical Regent Beijing Co., Ltd. All of the reagents are of analytical grade and used as received without further purification. Ultrapure water was used throughout the synthesis and treatment processes.

The synthesis of amorphous alumina was simply achieved by a solvothermal reaction followed by calcination in air. In a typical

Characterization of materials

The morphology of the CA-AlOx was observed by SEM. As shown in Fig. 1a, the CA-AlOx was composed of well-developed cactus-like microspheres with a diameter of about 40 µm, which was assembled from large continuous nanosheets with relatively small thickness. Therefore, a developed porous structure could be formed due to many pile holes, slit pores and other apertures produced during the formation process of cactus-like microspheres. Crystal structure of the as-prepared material was characterized

Conclusions

CA-AlOx was fabricated by a facile solvothermal method. The Sips model fitting results suggested that the fluoride might be adsorbed in a mixed form of monolayer and multilayer coverage with an energetic nonuniform distribution on the adsorbent surface. The maximum adsorption capacity of CA-AlOx for fluoride can reach 129.4 mg/g, which was higher than that of many adsorbents reported. Moreover, the CA-AlOx exhibited fast adsorption rate, high interference resistance and good reusable ability,

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21407152, 21590811, 21777166) and the China Postdoctoral Science Foundation (2017M611001).

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