Superb fluoride and arsenic removal performance of highly ordered mesoporous aluminas
Highlights
► Highly ordered mesoporous aluminas and calcium-doped aluminas are produced. ► Ca-doped mesoporous alumina has a maximum defluoridation capacity of 450 mg/g and excellent selectivity towards fluoride ion. ► 1 g of highly ordered mesoporous aluminas can treat 4 kg of fluoride contaminated water (5 ppm). ► 1 g of highly ordered mesoporous aluminas can treat 200 kg of arsenic contaminated water (100 ppb).
Introduction
Long-term ingestion of fluoride contaminated water can cause dental and skeletal fluorosis [1], [2] while intake of arsenic contaminated water can lead to arsenicosis, various types of cancers and blackfoot disease [3], [4]. The World Health Organization (WHO) has set the guideline values of 1.5 mg L−1 for fluoride [2] and 0.01 mg L−1 for arsenic [5] in drinking water. However, the concentrations of fluoride and/or arsenic in groundwater in China, India, Bangladesh area, Central Africa, USA and South America exceed these values, threatening the health of millions of people [6].
Many defluoridation and arsenic removal methods have been developed [3], [7]. These methods include chemical precipitation [8], [9], [10], [11], [12], [13], ion exchange [14], [15], membrane process [16], [17], [18], [19], adsorption technique [1], [2], [20], [21], Donnan dialysis [22], [23] and electrodialysis [24]. Each technology may be suitable for certain conditions, such as the population of resident, the electricity supply and funds. Among these methods, the adsorption technique is perhaps the most extensively adopted one because of its simplicity and low cost. While other technologies require complicated setups, the simplest adsorption unit may require only a filter, making it ideal for small scale water treatment units for households or small villages. A wide range of adsorption materials have been reported for fluoride or arsenic removal, such as activated alumina [25], [26], [27], activated carbon [28], hydroxyapatite [20], [29], alum [30], lime [31], red mud [32], bone char [33], rare earth oxides [34], hydrous ferric oxide [35], [36], Mg/Al layered double hydroxide [37], [38] and zeolite [39]. However, most of these adsorbents have low defluoridation or arsenic removal capacities in treating complicated groundwater. It is still a challenge to develop better adsorbents with superior adsorption capacities for fluoride or arsenic.
Porous materials are known for their high adsorption capacities [40], [41]. The large surface areas, narrow pore-size distributions and highly uniformly channels are favorable features for removing fluoride ions and arsenic species. Due to its large-scale production, high affinity and selectivity towards fluoride and arsenic, commercial activated alumina is perhaps the most frequently used material for removal of fluoride and arsenic from drinking water. The defluoridation capacity of activated alumina is only about 7.6 mg/g [25] and its arsenic removal capacity is 9.0 mg/g [26]. Mesoporous alumina is a new generation of porous alumina. Lee et al. found that the adsorption capacity of mesoporous aluminas for fluoride was 14.3 mg/g [2]. Kim et al. [41] used a similar method to prepare mesoporous γ-alumina, whose maximum uptake capacity for As (V) was 121 mg/g. However, though the maximum adsorption capacities of many adsorbents for arsenic were reported to be high, many of them showed quite low removal capacities at low arsenic concentrations, e.g. 500 ppb and the arsenic concentrations of most arsenic contaminated underground water supply are usually lower than 500 ppb.
In this study, we synthesized highly ordered mesoporous aluminas and calcium-doped aluminas through a reproducible and high-yield method and investigated their adsorption characteristics for fluoride and arsenic. These mesoporous alumina materials exhibited excellent fluoride and arsenic removal capacities. The best sample, Ca-doped mesoporous alumina showed a maximum fluoride removal capacity of 450 mg/g and excellent selectivity towards fluoride ion. In addition, these highly ordered mesoporous aluminas exhibited superior arsenic removal ability at low initial arsenic concentrations (i.e. 100 ppb). 1 g of these materials can treat 200 kg of arsenic contaminated water, reducing the concentration of arsenate from 100 ppb to 1 ppb, resulting in an impressive arsenic adsorption capacity of 20 mg/g for practical application.
Section snippets
Chemicals and materials
Pluronic P123 (Mav = 5800) and sodium arsenite (NaAsO2) were purchased from Sigma–Aldrich. Aluminum isopropoxide (98+ wt%), calcium nitrate tetrahydrate and sodium hydrogen arsenate heptahydrate were purchased from Alfa Aesar. Anhydrous ethanol and nitric acid were purchase from Beijing Chemical Reagents. Sodium fluoride (ACS grade >99%) was purchased from Acros Organic. All chemicals were used as received. Deionized water used in all of the experiments was prepared using Milli-Q water by Milli-Q
Characterization of adsorbents
As shown in Fig. 1, TEM images confirm the presence of highly uniform and hexagonally ordered mesopores with p6mm symmetry on all samples [43]. The cylindrical pores along [1 1 0] direction and hexagonal pore openings along [0 0 1] direction from the meso-Al-400 sample are shown in Fig. 1a and b, respectively. Fig. 1c shows that the Ca-doped mesoporous alumina (meso-Al-10Ca-400) also possesses similar ordered pore networks and highly uniform channels. The TEM image of sample meso-Al-900 (Fig. 1d)
Conclusions
We produced highly ordered mesoporous aluminas and Ca-doped aluminas through a reproducible and high yield sol–gel method. Fluoride adsorption tests demonstrated that they had excellent defluoridation capacities and selectivity towards fluoride in the presence of competing anions. The highest fluoride removal capacity reached 450 mg/g. These highly ordered mesoporous aluminas also exhibited superb arsenic removal capacities and adsorption kinetics. 1 g of sample meso-Al-400 can treat 200 L of
Acknowledgements
We thank the financial supports from the National Natural Science Foundation of China (NSFC 50725207, 20821003), National Basic Research Program of China (2009CB930400, 2011CB933700) and the Chinese Academy of Sciences.
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