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Adsorption Performance of Hydrophobic/Hydrophilic Silica Aerogel for Low Concentration Organic Pollutant in Aqueous Solution

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Adsorption Performance of Hydrophobic/Hydrophilic Silica Aerogel for Low Concentration Organic Pollutant in Aqueous Solution Nanotechnol Rev 2019; 8:266–274 Research Article Zhigang Yi*, Qiong Tang, Tao Jiang, and Ying Cheng Adsorption performance of hydrophobic/hydrophilic silica aerogel for low concentration organic pollutant in aqueous solution https://doi.org/10.1515/ntrev-2019-0025 ganic pollutants with the feature of bioaccumulation and Received Apr 14, 2019; accepted May 14, 2019 difficulty biochemical degradation, can present high risk to ecosystem and human health even at low concentra- Abstract: Hydrophobic silica aerogels (SiO2(AG)) was pre- tion [1, 2]. The prevention and removal organic contamina- pared via sol-gel and solvent exchange method under tions in aquatic environment has attracted great attention. ambient pressure, which could be transformed to hy- ∘ Conventional methods like adsorption [3, 4], biodegrada- drophilic after heated under 500 C. Heat treatment can- tion [5], advanced oxidation processes [6–9], etc. have not change its structure. SiO2(AG) samples were the been in-depth investigated and widely used. Compared micro-porous structure formed by numerous fine parti- with other ways, adsorption process is the most commonly cles and had high specific surface area, pore size and technology to remove organic pollutants from water due pore volume. The absorption performance of hydropho- to its simplicity, high efficiency [3, 4]. In general, the ad- bic/hydrophilic SiO2(AG) on nitrobenzene, phenol and sorption property of adsorbents is determined by the mor- methylene blue (MB) showed that hydrophobic SiO2(AG) phology and structure of porous materials, such as spe- exhibited strong adsorption capacity on slightly soluble or- cific surface areas, pore volumes, pore distributions and ganic compounds, while hydrophilic SiO2(AG) was much special pore surface chemistries [10]. Thus, the porous ma- more effective on adsorbing soluble compounds, which terials with special size and properties, such as graphene, could be analyzed by the hydrophobic and hydrophilic in- carbon aerogels, activated carbon, polymers, porous silica, teraction theory between the adsorbent and adsorbate. Hy- and metal-organic frameworks are actively investigated as drophobic/hydrophilic SiO2(AG) adsorption performance advanced sorbents [11–16]. Among them, activated carbon for MB is superior to that for phenol, which could be ex- is a widely used adsorbent due to its availability and high plained via the electrostatic interaction theory. adsorption capacity [17]. However, carbons display disad- Keywords: Silica aerogels; Adsorbent; Organic pollutant vantages such as limited modification flexibility and low selectivity, which limit their applications [18]. As an unique material with wide application [19, 20], 1 Introduction silica aerogel (SiO2(AG)) is a three-dimensional and multi- scaled porous nano-material formed by numerous fine par- ticles and networks. Traditional preparation of SiO (AG) In the last decades, with the rapid development of indus- 2 involve two different ways: the process of supercritical dry- try, large amounts organic compounds were widely used ing and ambient pressure drying (APD) technique. The in different field, such as pesticides, dyes, synthetic rub- method of supercritical drying can avoid capillary stress ber, plastics. The large-scale use of chemical raw materi- and associated drying shrinkage. However, this process als has led to a series of environmental pollution. Many or- is energy intensive and can be dangerous, which leads to high costs and results in limited practical applications [21, 22]. APD technique could overcome the disadvantages of *Corresponding Author: Zhigang Yi: Department of Chemistry, supercritical aerogel process, and made the manufacture Leshan Normal University, Leshan, 614004, China; and application of SiO2(AG) in large scale possible [23–25]. Email: [email protected]; Tel: +86-8332270785; The SiO2(AG) has been increasingly researched as an ad- Fax: +86-8332270785 sorbent owing to its high porosity (up to 99%), high spe- Qiong Tang, Tao Jiang, Ying Cheng: Department of Chemistry, Leshan Normal University, Leshan, 614004, China cific surface area, low density, and ease of surface modifi- Open Access. © 2019 Z. Yi et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 License Adsorption performance of hydrophobic/hydrophilic silica aerogel Ë 267 cation, etc. [26–29]. Hrubesh et al. [28] prepared hydropho- (C6H5NO2), phenol (C6H5OH) and methylene blue (MB) bic SiO2(AG) via the supercritical drying method, and pro- were analytical grade and commercially purchased from ceeded the adsorption experiments of decomposing differ- Chron Chemicals Co., Ltd., Chengdu. All reagents were ent organic compounds in water. The results showed that used without any further purification. Deionized water the adsorption capacity of the hydrophobic SiO2(AG) was made from laboratory, was used in all experiments. 30 times as that of the carbon for the soluble organic com- pounds, and up to 130 times for the insoluble organic com- pounds. Reynolds et al. [29] carried out adsorption exper- 2.2 Preparation of SiO2(AG) iments about hydrophobic SiO2(AG) to the oil slick in wa- ter, and the results indicated that the capacity of the oil Based on the sol-gel method, hydrophobic SiO2(AG) was absorption into the hydrophobic SiO2(AG) was up to 273 synthesized by controlling the solvent exchanging pro- times as its own volume. Perdigoto et al. [30] got SiO2(AG) cedure under ambient pressure. A certain proportion from methyltrimethoxysilane (MTMS) as a precursor by su- of TEOS, EtOH, and deionized water (mole ratios of percritical drying, and adsorption capacities of SiO2(AG) TEOS/EtOH/water=1:3:6) were put into a 200 mL beaker, were shown to uptake more than 50 mg/g of benzene at dropping 0.1 mol/L HCl in order to control the pH of the 50 mg/L of benzene solution. Wang et al. [31] prepared hy- solution in the rage of 2.0-3.0. After stirring for 60 min, drophobic SiO2(AG) and tested its adsorption for organic 0.1 mol/L NH3·H2O solution was added to the silica sol compounds from aqueous phase. Adsorption equilibrium to adjust the pH in the rage of 5.0-6.0, until wet gels were of SiO2(AG) was reached in under 20 min and the adsorp- gotten. The obtained wet gels were collected and aged for tion capacities were 223 mg/g for toluene and 87 mg/g for 24 h at room temperature, followed a soaking process in benzene. EtOH(50%) for 24 h, then soaked in a mixed solvent with Previous researches suggested that the adsorption ca- volume ratio of EtOH/TMCS/hexane=1:0.8:1 for 48 h. After pacity of adsorbent was primarily decided by the pore filtrating, the wet gels were washed 3 times by hexane and ∘ structure and surface area, the surface chemical proper- dried 60 C for 24 h, to obtain the hydrophobic SiO2(AG). ties were also important, furthermore, the hydrophobicity, The hydrophilic SiO2(AG) was gotten by above processes ∘ polarity, electron accepting or donating property and the and calcination at 500 C for 3h [10]. structure of the pollutants could also affect the adsorption affinity [32]. However, the results discussed in many stud- ies on adsorption performance of SiO2(AG) has focused on 2.3 Characterization hydrophobic media. Investigation on other factors of ad- sorption property and adsorption mechanisms were still The microstructure of fabricated materials was investi- fragmentary. gated by SEM (Inspect F,FEI,Holland) and TEM (JEM-2100F, In this work, in order to give a fundamental knowl- JEOL, Japan). X-ray diffraction (XRD DX-2500) was used to edge for potential application of SiO2(AG) on pollutants re- analyzed the crystal structure, the X-ray target made of moval. Hydrophobic/hydrophilic SiO2(AG) were prepared Cu Kα-ray generator (40 kV, 40 mA, λ = 0.15406 nm).The via the APD route, and to compare different adsorption pore structure and specific surface area (SSA) of the materi- performance for nitrobenzene, phenol and methylene blue als were respectively determined by the automatic surface (MB) with different physicochemical properties, which area and porosity analyzer (3H-2000PS4, Beishide instru- were common pollutants in industrial wastewater. The ment Technology Co., Ltd., Beijing), the test conditions of adsorption isotherm were evaluated, and the adsorption automatic surface area and porosity analyzer were: nitro- mechanisms were also discussed. gen as adsorbate, degassing mode of heating-vacuum, de- ∘ gassing temperature of 150 C, degassing time of 180 min, saturated steam pressure of 1.0434 bar, ambient temper- ∘ 2 Experimental ature of 14.0 C. The surface functional groups of materi- als were characterized by the FTIR spectra of the mate- rials by using the Fourier transform infrared spectrome- 2.1 Materials ter (Nicolet 5700 Spectrophotometer, FTIR), the samples were performed by pressing the power mixed with some Tetraethyl orthosilicate (TEOS), anhydrous ethanol (EtOH), of KBr into disks, and the scanning wave number range hexane (C6H14), trimethylchlorosilane (TMCS), hydrochlo- was set to 4000~ 400 cm−1. The hydrophobic properties of ric acid (HCl), ammonia water (NH3·H2O), nitrobenzene the materials were obtained by measuring contact angle of 268 Ë Z. Yi et al. water droplets and materials using a contact angle meter 2.4.2 Freundlich isothermal adsorption model (DSA100, KRUSS, Germany). Freundlich isothermal adsorption model is an empirical equation, which describes the adsorption equilibrium in 2.4 Research of adsorption performance the solid-liquid adsorption process, that adsorption heat decreased logarithmically with the increases of adsorp- The adsorption experiments were conducted in three dif- tion amount under isothermal conditions. the model takes ferent organic solution (nitrobenzene, phenol and methy- uneven surface into account, expressed by equation 4 lene blue (MB) solution, respectively). 250 ml conical flask or 5 [34]: containing 100 ml organic solution and 2 g/l adsorbent 1 n q = KF Ce (4) (hydrophobic/hydrophilic SiO2(AG)) were shaken at 220 ∘ r/min. Temperature was kept at 25 C.
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