Fe3+ and amino functioned mesoporous silica: Preparation, structural analysis and arsenic adsorption
Highlights
► Materials were prepared by grafting amino and then coordinating Fe3+ on silica gel. ► The materials had mesoporous structure, large surface area, abundant amino and Fe3+. ► The materials had high removal ability and adsorption speed for As(V) and As(III). ► Langmuir and Freundlich model were used to fit the adsorption isotherms. ► The effect of chloride and sulfate for As(V) and As(III) removal was investigated.
Introduction
High concentration of arsenic in drinking water can cause many diseases, such as lung, liver, kidney, bladder cancer and keratosis [1], [2], [3]. A maximum arsenic content of 10 ppb in the drinking water has been established by WHO and adopted by most countries [1], [4], [5]. However, due to the limits of economic and technical conditions, millions of people suffer from drinking water with arsenic content far above the standard of 10 ppb in Bengal basin of Bangladesh and India, Chianan plain of Taiwan, western USA, Huhhot basin of Inner Mongolia (China), Red River basin of Vietnam and so on [6], [7]. Therefore, it is important to develop efficient technologies for the removal of arsenic in the water.
Compared with other methods for arsenic removal, such as precipitation–coagulation, membrane separation and ion exchange, more and more attention was attracted on adsorption in recent years, for it is cheap and facile [8], [9]. Active carbon, alumina, zeolite, magnesia and iron are the adsorbents most widely used to treat the water contaminated by arsenic [9], [10], [11], [12], [13], [14]. Among these materials, iron was more popular for its high affinity for both arsenate(V) and arsenite(III), the two arsenic forms mainly existing in nature water [15], [12], [16], [17], [18], [19]. The latter form is usually hard to be removed by other adsorbents. Recently, nano-iron particles are used as candidate of adsorbents, since they have larger surface area and higher ability for arsenic removal [20], [21]. However, the nanoparticles are difficult to separate from water and easy to block the filtrate [22]. In order to solve these problems, alumina, iron and its oxides were loaded on materials with high surface area, such as activated carbon, zeolite, and mesoporous silica. These methods showed promising results in the adsorption of arsenic in water [1], [15], [22], [23], [24], [25]. Feng et al. put forward a novel synthetic route by grafting functionalized monolayers on ordered mesoporous supports with silylation, then some researchers used this approach to bring useful groups, mainly amino, on mesoporous silica for the removal of arsenic in water [26], [27], [28].
Fe3+ in hydrated reactive form, mainly nanoparticles of ferrihydrite, is an excellent adsorbent, and has been widely used in water treatment plants for arsenic and other contaminants removal [29], [30]. However, it usually takes long time for ferrihydrite to precipitate and easy to block the filter when filtration was used. So it is not suitable for water treating in remote area. On the contrary, to incorporate Fe3+ into supporting materials with high surface area is more preferable. Silica gel is a good selection for the incorporation of Fe3+. As SiO2 aerogel attracted more and more attention in recent year, the preparation of silica gel with large block and high surface area has been developed [31]. However, Fe3+ can be easily dissolved from SiO2 without specific interaction. To joint SiO2 and Fe3+ together, the strategy of grafting amino on the SiO2 first, and then coordinating Fe3+ with amino was used here [28], [29]. To the best of our knowledge, the load of functional groups on mesoporous silica gel prepared by the sol–gel technologies for arsenic removal has rarely been reported.
In this work, mesoporous silica gel with high network skeleton strength and large block was prepared by using sol–gel method with two-step acid–base catalysis. This sample was expected to have larger surface area and higher hydroxyl density, as no calcination was used to decompose the template. Amino was introduced on silica gel, and then Fe3+ was coordinated with amino to remove the arsenic in the water.
N2 adsorption isotherm, Fourier transformed infrared spectroscopy, X-ray photoelectron spectroscopy and 29Si NMR spectra analytic techniques were used to characterize the materials. The effect of pH, and the kinetic and thermodynamic properties of adsorption were studied to investigate the removal ability of adsorbents for arsenic. The influence of coexistent anion was also investigated.
Section snippets
Chemicals
Tetraethyl orthosilicate (TEOS), ethanol, oxalic acid, and ammonia were used as the silica source, solvent, acid catalyst, and base catalyst, respectively. All of them were bought from Sinopharm Chemical Reagent Co. Ltd. Aminopropyltriethoxysilane (H2N(CH2)3Si(OC2H5)3) and N-[3-(trimethoxysilyl)propyl]ethylenediamine ((CH3O)3Si(CH2)3NHCH2CH2NH2) were purchased from Aladdin reagent as monoamine and diamine source, respectively. FeCl3·6H2O was also purchase from Sinopharm Chemical Reagent Co.
FTIR result
To investigate the change of functional group during synthesis process, the infrared spectra of pure SiO2, 1NSiO2, and Fe1N were tested and the results are given in Fig. 1. The peaks at 2984, 2940, 2902, 1478, 1450, and 1396 cm−1 are assigned to characteristic banding of CH, while the peaks at 1086 and 800 cm−1 are assigned to the characteristic banding of SiOSi [27], [33], [34]. The peak at 1630 cm−1 is probably attributed to the bending of water residual on the surface. Compared with SiO2, the
Conclusions
In this work, two adsorbents for arsenic removal were prepared by grafting monoamine and diamine on the silica gels obtained using sol–gel method with two-step acid–base catalysis, and then coordinating amino with Fe3+. The adsorbents have mesoporous structure, large surface area and pore volume, abundant amino and Fe3+, thus have high adsorption capacity for both arsenate and arsenite. The Fe1N adsorbent has the highest adsorption capacity of 132 mg/g at pH 4 and a high adsorption capacity of 95
Acknowledgment
This work was supported by the National Basic Research Program of China (973 Program, No. 2011CB933700) of Ministry of Science and Technology of China.
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