Adsorption performance of hydrophobic/hydrophilic silica aerogel for low concentration organic pollutant in aqueous solution

Abstract Hydrophobic silica aerogels (SiO2(AG)) was prepared via sol-gel and solvent exchange method under ambient pressure, which could be transformed to hydrophilic after heated under 500∘C. Heat treatment cannot change its structure. SiO2(AG) samples were the micro-porous structure formed by numerous fine particles and had high specific surface area, pore size and pore volume. The absorption performance of hydrophobic/hydrophilic SiO2(AG) on nitrobenzene, phenol and methylene blue (MB) showed that hydrophobic SiO2(AG) exhibited strong adsorption capacity on slightly soluble organic compounds, while hydrophilic SiO2(AG) was much more effective on adsorbing soluble compounds, which could be analyzed by the hydrophobic and hydrophilic interaction theory between the adsorbent and adsorbate.Hydrophobic/hydrophilic SiO2(AG) adsorption performance for MB is superior to that for phenol, which could be explained via the electrostatic interaction theory.


Introduction
In the last decades, with the rapid development of industry, large amounts organic compounds were widely used in different field, such as pesticides, dyes, synthetic rubber, plastics. The large-scale use of chemical raw materials has led to a series of environmental pollution. Many or-ganic pollutants with the feature of bioaccumulation and difficulty biochemical degradation, can present high risk to ecosystem and human health even at low concentration [1,2]. The prevention and removal organic contaminations in aquatic environment has attracted great attention. Conventional methods like adsorption [3,4], biodegradation [5], advanced oxidation processes [6][7][8][9], etc. have been in-depth investigated and widely used. Compared with other ways, adsorption process is the most commonly technology to remove organic pollutants from water due to its simplicity, high efficiency [3,4]. In general, the adsorption property of adsorbents is determined by the morphology and structure of porous materials, such as specific surface areas, pore volumes, pore distributions and special pore surface chemistries [10]. Thus, the porous materials with special size and properties, such as graphene, carbon aerogels, activated carbon, polymers, porous silica, and metal-organic frameworks are actively investigated as advanced sorbents [11][12][13][14][15][16]. Among them, activated carbon is a widely used adsorbent due to its availability and high adsorption capacity [17]. However, carbons display disadvantages such as limited modification flexibility and low selectivity, which limit their applications [18].
As an unique material with wide application [19,20], silica aerogel (SiO 2 (AG)) is a three-dimensional and multiscaled porous nano-material formed by numerous fine particles and networks. Traditional preparation of SiO 2 (AG) involve two different ways: the process of supercritical drying and ambient pressure drying (APD) technique. The method of supercritical drying can avoid capillary stress and associated drying shrinkage. However, this process 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 supercritical aerogel process, and made the manufacture and application of SiO 2 (AG) in large scale possible [23][24][25]. The SiO 2 (AG) has been increasingly researched as an adsorbent owing to its high porosity (up to 99%), high specific surface area, low density, and ease of surface modifi-cation, etc. [26][27][28][29]. Hrubesh et al. [28] prepared hydrophobic SiO 2 (AG) via the supercritical drying method, and proceeded the adsorption experiments of decomposing different organic compounds in water. The results showed that the adsorption capacity of the hydrophobic SiO 2 (AG) was 30 times as that of the carbon for the soluble organic compounds, and up to 130 times for the insoluble organic compounds. Reynolds et al. [29] carried out adsorption experiments about hydrophobic SiO 2 (AG) to the oil slick in water, and the results indicated that the capacity of the oil absorption into the hydrophobic SiO 2 (AG) was up to 273 times as its own volume. Perdigoto et al. [30] got SiO 2 (AG) from methyltrimethoxysilane (MTMS) as a precursor by supercritical drying, and adsorption capacities of SiO 2 (AG) were shown to uptake more than 50 mg/g of benzene at 50 mg/L of benzene solution. Wang et al. [31] prepared hydrophobic SiO 2 (AG) and tested its adsorption for organic compounds from aqueous phase. Adsorption equilibrium of SiO 2 (AG) was reached in under 20 min and the adsorption capacities were 223 mg/g for toluene and 87 mg/g for benzene.
Previous researches suggested that the adsorption capacity of adsorbent was primarily decided by the pore structure and surface area, the surface chemical properties were also important, furthermore, the hydrophobicity, polarity, electron accepting or donating property and the structure of the pollutants could also affect the adsorption affinity [32]. However, the results discussed in many studies on adsorption performance of SiO 2 (AG) has focused on hydrophobic media. Investigation on other factors of adsorption property and adsorption mechanisms were still fragmentary.
In this work, in order to give a fundamental knowledge for potential application of SiO 2 (AG) on pollutants removal. Hydrophobic/hydrophilic SiO 2 (AG) were prepared via the APD route, and to compare different adsorption performance for nitrobenzene, phenol and methylene blue (MB) with different physicochemical properties, which were common pollutants in industrial wastewater. The adsorption isotherm were evaluated, and the adsorption mechanisms were also discussed.

Preparation of SiO 2 (AG)
Based on the sol-gel method, hydrophobic SiO 2 (AG) was synthesized by controlling the solvent exchanging procedure under ambient pressure. A certain proportion of TEOS, EtOH, and deionized water (mole ratios of TEOS/EtOH/water=1:3:6) were put into a 200 mL beaker, dropping 0.1 mol/L HCl in order to control the pH of the solution in the rage of 2.0-3.0. After stirring for 60 min, 0.1 mol/L NH 3 ·H 2 O solution was added to the silica sol to adjust the pH in the rage of 5.0-6.0, until wet gels were gotten. The obtained wet gels were collected and aged for 24 h at room temperature, followed a soaking process in EtOH(50%) for 24 h, then soaked in a mixed solvent with volume ratio of EtOH/TMCS/hexane=1:0.8:1 for 48 h. After filtrating, the wet gels were washed 3 times by hexane and dried 60 ∘ C for 24 h, to obtain the hydrophobic SiO 2 (AG). The hydrophilic SiO 2 (AG) was gotten by above processes and calcination at 500 ∘ C for 3h [10].

Characterization
The microstructure of fabricated materials was investigated by SEM (Inspect F,FEI,Holland) and TEM (JEM-2100F, JEOL, Japan). X-ray diffraction (XRD DX-2500) was used to analyzed the crystal structure, the X-ray target made of Cu Kα-ray generator (40 kV, 40 mA, λ = 0.15406 nm).The pore structure and specific surface area (SSA) of the materials were respectively determined by the automatic surface area and porosity analyzer (3H-2000PS4, Beishide instrument Technology Co., Ltd., Beijing), the test conditions of automatic surface area and porosity analyzer were: nitrogen as adsorbate, degassing mode of heating-vacuum, degassing temperature of 150 ∘ C, degassing time of 180 min, saturated steam pressure of 1.0434 bar, ambient temperature of 14.0 ∘ C. The surface functional groups of materials were characterized by the FTIR spectra of the materials by using the Fourier transform infrared spectrometer (Nicolet 5700 Spectrophotometer, FTIR), the samples were performed by pressing the power mixed with some of KBr into disks, and the scanning wave number range was set to 4000~400 cm −1 . The hydrophobic properties of the materials were obtained by measuring contact angle of water droplets and materials using a contact angle meter (DSA100, KRUSS, Germany).

Research of adsorption performance
The adsorption experiments were conducted in three different organic solution (nitrobenzene, phenol and methylene blue (MB) solution, respectively). 250 ml conical flask containing 100 ml organic solution and 2 g/l adsorbent (hydrophobic/hydrophilic SiO 2 (AG)) were shaken at 220 r/min. Temperature was kept at 25 ∘ C. Adsorption capacity of SiO 2 (AG) was reflected by measuring the change of concentration of organic compounds, which were determined by UV-vis spectrophotometer (UV-2550, Shimadzu, Japan).
The adsorption amount of organic compounds on adsorbent is calculated as: Where q is the adsorption amount of organic compounds on adsorbent, mg/g; C 0 is the initial concentration of organic compounds in solution, mg/l; Ce is the equilibrium adsorption concentration of organic compounds in solution after adsorption equilibrium, mg/l; v is the volume of solution, l; w is the dosage of adsorbent, g. Langmuir isotherm adsorption model and Freundlich isotherm adsorption model were usually to fit the adsorption behavior of the organic compounds in dilute solution [33].

Langmuir isothermal adsorption model
Langmuir isothermal adsorption model used to describe the adsorption behavior in the solid-liquid system, expressed by equation 2 or 3 [34]: Where Ce is the equilibrium concentration in solution, mg/l; q is the adsorption amount in the adsorbent, mg/g; qm is the saturated adsorption amount in the adsorbent, mg/g; k L is the Langmuir adsorption constant, l/mg, reflects the affinity between the adsorbate and the adsorption sites.

Freundlich isothermal adsorption model
Freundlich isothermal adsorption model is an empirical equation, which describes the adsorption equilibrium in the solid-liquid adsorption process, that adsorption heat decreased logarithmically with the increases of adsorption amount under isothermal conditions. the model takes uneven surface into account, expressed by equation 4 or 5 [34]: ln q = ln K F + 1 n ln Ce Where Ce is the equilibrium concentration in solution, mg/l; q is the adsorption amount on the adsorbent, mg/g; K F is the Freundlich adsorption constant, which is a measure of the adsorption capacity, the greater of its value, then the bigger of the adsorption amount; n is a constant and usually greater than 1, and its inverse is a measure of the adsorption intensity.

Morphology of SiO 2 (AG)
Morphologies of hydrophobic/hydrophilic SiO 2 (AG) samples were investigated by SEM and TEM. As shown in Figure 1(a), the hydrophobic SiO 2 (AG) samples prepared under ambient pressure presented a spongy-liked shape, which were made of numerous fine particles and formed a loose and porous structure, and uniform particle size distribution. Hydrophilic SiO 2 (AG) was gained via hydrophobic SiO 2 (AG) calcined 3h under 500 ∘ C, Figure 1(b) indicated its surface appearance had no significant difference from hydrophobic SiO 2 (AG).

Specific surface area and pore structure of SiO 2 (AG)
The textural properties of SiO 2 (AG) were analyzed by using Nitrogen adsorption-desorption method. As can be seen in Figure 2, the N 2 adsorption-desorption isotherms of hydrophobic/hydrophilic SiO 2 (AG) samples were a typical type IV adsorption isotherms characteristic with an adsorption hysteresis, indicated that the nano-porous structure exists on hydrophobic/hydrophilic SiO 2 (AG), and the hole shape was the narrow tubular pores of the ends open  and mouth width [10]; The SSA was determined by using BET (Brunauer-Emmett-Teller) method. Pore size distribution and total pore volume of the materials were evaluated from the adsorption branch of nitrogen isotherms by using the BJH (Barrett-Joyner-Halenda) method. The insets of Figure 2 displayed the BJH differential integral hole volume and pore size distribution curve of SiO 2 (AG).
The experimental results on the SSA, pore size and total pore volume of the SiO 2 (AG) powders have been compiled in Table 1. The results showed that hydrophobic/hydrophilic SiO 2 (AG) samples had high specific surface area, pore size and pore volume. The conclusions were consistent with the characterization results by TEM. The reason for the decrease in pore size and volume of hydrophilic SiO 2 (AG) might be the shrinkage and collapse during the calcination process.

The FT-IR analysis of SiO 2 (AG)
The FT-IR spectrum of hydrophobic/hydrophilic SiO 2 (AG) were shown in Figure 3. The adsorption peaks at around 3490 cm −1 of hydrophobic SiO 2 (AG) was significantly weaker than hydrophilic SiO 2 (AG), it was due to that the -OH group on the surface of SiO 2 (AG) was replaced by the organic group in the process of modification. As shown in FT-IR spectrum of hydrophobic SiO 2 (AG), the characteristic peaks at 2968 cm −1 corresponding to the C-H of -CH 3 antisymmetric stretching vibration, 1387 cm −1 and 928 cm −1 appeared the symmetric vibration and in-plane vibration of -CH 3 group respectively, which indicated -CH 3 to existed on the surface of hydrophobic SiO 2 (AG). In addition, the peak at 758 cm −1 corresponding to the symmetric stretching vibration of Si-CH 3 , suggested that Si-CH 3 group was contained on the surface of hydrophobic SiO 2 (AG). Analyzing by FT-IR, the hydrophobic -CH 3 connected with branched chain of SiO 2 (AG) that prepared by solvent exchange method, which let it has hydrophobic properties. The FT-IR of hydrophilic SiO 2 (AG) showed the peaks of -OH group existed on the surface of hydrophilic SiO 2 (AG), which let it has hydrophilic performance, it was due to that -CH 3 group on the surface of hydrophobic SiO 2 (AG) has been translated into -OH group after calcination at 500 ∘ C [10].

Contact angle of SiO 2 (AG)
The hydrophobicity of SiO 2 (AG) was measured by testing the contact angle of SiO 2 (AG), the results were displayed in Figure 4. The prepared hydrophobic SiO 2 (AG) had a good hydrophobicity, 500 ∘ C heat treatment promoted the conversion from hydrophobicity to hydrophilicity. The conclusions were consistent with the characterization results by FT-IR.

Adsorption curves of different organic compounds
The adsorption experiments were carried out in 100 ml nitrobenzene solution(12 mg/l), phenol solution(10 mg/l) and MB solution(10 mg/l) respectively. The dosage of hydrophobic/hydrophilic SiO 2 (AG) was 2.0 g/l. As shown in the adsorption curves of hydrophobic SiO 2 (AG) on different adsorbate, Figure 5(a), the hydrophobic SiO 2 (AG) exhibited the best adsorption capacity of by removing 51.8% nitrobenzene within 1 h when the system reached adsorption equilibrium, on the other hand, it exhibited poor adsorption capacity on phenol and MB by adsorbing only 9.9% and 17.6%, respectively, and did not get adsorption equilibrium even after 10 h. Figure 5(b) displayed adsorption property of hydrophilic SiO 2 (AG), the removal ratio of phenol and MB by adsorption of hydrophilic SiO 2 (AG) was 57.8% and 64.3% respectively, and reached adsorption equilibrium within 0.5 h. On the contrary, it showed poor adsorption performance on nitrobenzene by adsorbing only 17.8% and got adsorption equilibrium in 1.5 h.

Adsorption isotherms
Isothermal adsorption experiment were conducted in nitrobenzene solution (12,24,36,48,60 mg/l), phenol solution (10,20,30,40,50 mg/l) and methylene blue (MB) solution (10,20,30,40,50 mg/l), respectively. The liquid adsorption isotherms of hydrophobic/hydrophilic SiO 2 (AG) on organic solution, and fitted curves by Langmuir and Freundlich isotherm models were shown in Figure 6(a)-6(c) and Figure 6(d)-6(f), respectively. The results indicated that the equilibrium adsorption amount of hydrophobic/hydrophilic SiO 2 (AG) increased with the increase of equilibrium concentration of organic compounds. In the range of the concentration in this experiment, the equilibrium adsorption amount of hydrophobic SiO 2 (AG) for nitrobenzene was 6.32 mg/g, while for phenol and MB was 1.19 mg/g and 2.63 mg/g, respectively; but the equilibrium adsorption amount of hydrophilic SiO 2 (AG) for nitrobenzene is only 2.38 mg/g, obviously lower than the equilib-  showed that Langmuir and Freundlich isotherm models could fit well the adsorption isotherms of SiO 2 (AG) for nitrobenzene, phenol and methylene blue. Related studies [34] suggested that Freundlich constant K F was a constant related to the adsorption capacity, the greater K F would course greater adsorption; when the value of con- stant 1/n was in the range from 0.1 to 0.5, the adsorbate was easily absorbed, and the value was smaller, the adsorption properties were better; with the increases of 1/n, while the equilibrium concentration of the adsorbate was higher, the absorption capacity would play more sufficient. Compared with the relevant parameters from Table 2, adsorption performance of hydrophobic SiO 2 (AG) for nitrobenzene was much better than that for phenol and MB; while the hydrophilic SiO 2 (AG) expressed a better adsorption property for phenol and MB; Both the hydrophobic and hydrophilic SiO 2 (AG), its adsorption properties for MB were superior to that for phenol.

Adsorption mechanism research
In the organic compounds adsorption process in solution, the adsorbent surface area, pore size distribution, pore structure and other characteristics of material plays a decisive role to its adsorption properties [35]. In addition, adsorbents exhibit differences of adsorption performance to different adsorbates, which could be analyzed by the hydrophobic and hydrophilic interaction theory between the adsorbent and adsorbate, molecular polarity theory, electrostatic interaction theory, the electron donor-acceptor interaction theory, π-π dispersion theory [36,37]. According to the hydrophobic and hydrophilic interaction theory in water phase, the hydrophobicity of SiO 2 (AG) plays a leading role in the organic compounds adsorption process. Nitrobenzene is an insoluble hydrophobic organic compounds, MB and phenol are hydrophilic organic compounds and freely soluble in water, and the hydrophilic SiO 2 (AG) is infiltrated easily by water molecules which occupy a part of pores. So, the hydrophobic SiO 2 (AG) shows better adsorption to nitrobenzene than other one; hydrophilic SiO 2 (AG) has better adsorption to MB and phenol. Hydrophobic and hydrophilic SiO 2 (AG) adsorption performance for MB is superior to that for phenol, basing on the electrostatic interaction theory, SiO 2 isoelectric point is very low [38], less than the pH of the phenol and MB solution, SiO 2 (AG) in phenol and MB solution will show electro-negativity, MB is a typical cationic dye with a positive charge, while phenol is electrically neutral. So there exist an electrostatic attraction between MB and SiO 2 (AG), and the adsorption performance of SiO 2 (AG) for MB prove better than phenol.

Conclusion
Via sol-gel and solvent exchange method, hydrophobic SiO 2 (AG) was prepared under ambient pressure, and it could be transformed to hydrophilic after 500 ∘ C heat treatment. The results of material characterization showed that the synthesized hydrophobic/hydrophilic SiO 2 (AG) were the micro-porous structure formed by numerous fine particles and had high specific surface area, pore size and pore volume.
The absorption experiments of hydrophobic/hydrophilic SiO 2 (AG) for nitrobenzene, phenol and MB showed that hydrophobic/hydrophilic SiO 2 (AG) was effective in removing organic compounds from aqueous solution. Hydrophobic SiO 2 (AG) showed much stronger adsorption effectiveness on slightly soluble organic com-pounds, while hydrophilic SiO 2 (AG) was much more effective on adsorbing soluble compounds, which could be analyzed by the hydrophobic and hydrophilic interaction theory between the adsorbent and adsorbate. Adsorption performance of hydrophobic/hydrophilic SiO 2 (AG) for MB is superior to that for phenol, which could be explained via the electrostatic interaction theory.
Due to the high adsorption efficiency on a wide range of organic pollutant in water, easy modification of the properties, and the relative low manufacture cost, hydrophobic/hydrophilic SiO 2 (AG) has great application potential in the wastewater treatment field.
Data accessibility: We have conducted our experiment systematically and reported their experimental procedure clearly in the experimental section and provided all necessary data in results and discussion section in the main manuscript.
Author Contributions: Zhigang Yi designed, supervised experiments and led the drafting of manuscript. Qiong Tang and Tao Jiang assisted in the analysis and testing work during the experiments. Ying Cheng contributed to characterization of property of material.