Enhanced Nitrobenzene Adsorption in Aqueous Solution by Surface

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Enhanced nitrobenzene adsorption in aqueous solution by surface-silanized fly-ash-derived SBA-15 Ge Li, Baodong Wang, Wayne Qiang Xu, Yonglong Li, Yifan Han, and Qi Sun ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.7b00127 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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ACS Earth and Space Chemistry

Enhanced nitrobenzene adsorption in aqueous solution by surface-silanized fly-ash-derived SBA-15

Ge Lia,b, Baodong Wanga,*, Wayne Qiang Xua, Yonglong Lia, Yifan Hanb, Qi Suna,*

a.

National Institute of Clean-and-Low-Carbon Energy, Beijing 102211, China

b.

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

Abstract: In this study, a two-step alkaline hydrothermal method was used to prepare SBA-15 mesoporous molecular sieves from fly ash obtained from a thermal power plant in Inner Mongolia (China). CH3O-SBA-15 was prepared using trimethoxychlorosilane as a modifier and toluene as the solvent. The silylated SBA-15 was characterized using X-ray diffraction, infrared spectroscopy, N2 adsorption–desorption, and scanning and transmission electron microscopies. The adsorption of nitrobenzene (NB), which is a persistent organic hydrophobic pollutant, by silylated SBA-15, and improvements in its hydrophobicity were studied. The effects of adsorbent dosage, time, temperature, and pH on the NB adsorption rate were investigated. The results show that CH3O-SBA-15 has the same two-dimensional hexagonal pore structure as SBA-15, but its specific surface area, average pore size, and the pore volume were lower. The NB removal rate gradually increased with increasing adsorbent dosage and time. The NB removal rate gradually decreased with increasing temperature and pH; an alkaline environment is not conducive to NB removal. The NB removal rate achieved with CH3O-SBA-15 was higher than that with SBA-15. At 25 °C, pH 5, and 5.5 g/L of adsorbent in an 8 µmol/L NB solution, SBA-15 and CH3O-SBA-15 both rapidly adsorbed NB. After 60 min, the adsorption rate became constant. The maximum values were 77.5% and 89.06% with CH3O-SBA-15 and SBA-15, respectively. Langmuir and Freundlich adsorption isotherms were obtained for NB solutions of various initial concentrations. The results show that NB adsorption on both SBA-15 and CH3O-SBA-15 was 1

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basically consistent with the Langmuir equation, and the adsorption was physical the adsorption process is mainly physical adsorption, accompanied by chemical adsorption. The adsorption capacity of silylated SBA-15 reached 10.42 µmol/g. A pseudo-second-order kinetic model described NB adsorption on CH3O-SBA-15 well. The coefficient of determination was over

0.99. The thermodynamic data show that NB adsorption on CH3O-SBA-15 was a spontaneous, exothermic process, accompanied by an decrease in entropy. The developed fly-ash-derived SBA-15 molecular sieves provide an effective technique for the treatment of industrial organic wastewater, and a new method for efficient use of fly ash. Key words: fly ash; mesoporous molecular sieve; silanization; adsorption; nitrobenzene

Corresponding author: Baodong Wang

E-mail：[email protected].

1. Introduction

Fly ash is a solid waste produced during coal combustion. Its properties are similar to those of volcanic ash, and it consists mainly of compounds such as SiO2, Al2O3, Fe2O3, CaO, and MgO. China has rich coal reserves, therefore the power industry is still based on coal-fired thermal processes. Statistics show that each ton of coal burnt produces 20–30 kg of slag and 250–300 kg of fly ash [1]. Fly-ash production reached 620 million tons in 2015, and it is estimated that by 2020 the total amount of fly ash accumulated will reach more than 30 million tons [2]. The amounts of these industrial solid wastes are large, and they are a serious hazard to the environment and human health, because of land occupation, air pollution, water pollution, soil pollution, and radioactive contamination. The efficient use of fly ash in various fields is being widely studied, and some progress has been made. In developed countries, more than 60% of fly ash is used. However, China’s current utilization rate is only 40%; applied research lags behind that in developed countries, and large-scale industrial applications are rare. Research in China on fly-ash applications has focused on use of large quantities, e.g., in concrete, bridge construction, dam 2


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construction, high-rise building floors, and nuclear power plant containment. Fly ash is also used for processes such as mining area backfill and soil improvement [2-3]. However, in terms of the three-way principle of resource use, existing methods only exploit the physical and chemical characteristics of fly ash, and separation of high-value elements such as Al and Si contained in fly ash is largely ignored. This is a waste of resources. The rapid development of modern industry has resulted in the widespread use of nitroaromatic hydrocarbons. Nitrobenzene (NB) is a commonly used in dyes, pharmaceuticals, and pesticide intermediates. Degradation of NB is difficult, and it causes long-term pollution of water. NB is teratogenic, mutagenic, and has reproductive toxicity [4]. Common NB wastewater treatment methods are divided into biodegradation, and chemical and physical adsorption. Because NB is stable and difficult to degrade, physical adsorption is the preferred method for NB removal. Adsorption methods are simple, efficient, fast, and a range of suitable adsorbents are available [5-13]. Activated carbon is a commonly used adsorbent and has a strong adsorption capacity. It has a well-developed pore structure, large specific surface area, it can be used across a wide concentration range, and it does not cause secondary pollution. However, activated carbon is expensive, the operating and regeneration costs are high, and it is not suitable for large-scale industrial application [13]. The development of new, highly efficient, low-cost, and high-yield adsorbents is therefore attracting increasing attention. Mesoporous molecular sieves are highly selective and efficient adsorbents, are easy to use, and are important in the effective use of resources. Mesoporous molecular sieves can effectively adsorb organic matter such as phenols, chlorophenols, and dyes [4]. The mesoporous materials exhibited high surface area with high pore volumes and exhibited high adsorption capacity as recent articles reported[14-23]. In general, molecular sieves are hydrophobic and oleophobic, and their adsorption capacities are low. Research has therefore focused on modifying mesoporous molecular sieves with organic functional groups such as –NH2, –SH, and –CH3 [24-33]. Qin et al. [24,25] reported that MCM-41 molecular sieves quickly adsorb NB, and adsorption equilibrium is achieved in 1 min. Qin et al. [16] enhanced the hydrothermal stability of MCM-41 and increased its adsorption rate by methylation and studied the adsorption of NB by CH3-MCM-41. The results showed that adsorption depends on the electronic properties of the substituent. When the substituent is an 3



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electron-withdrawing group, adsorption is good, and vice versa. This is because siloxane groups on the CH3-MCM-41 surface have excess electrons, which are affected by electron acceptors/donors, and this affects the NB adsorption efficiency. Tang et al. [27] prepared CH3-SBA-15, NH2-SBA-15, and CH3/NH2-SBA-15, and studied their use in bilirubin adsorption. The adsorption efficiency of CH3-SBA-15 was better than those of the other two. Bilirubin is hydrophobic, therefore electrostatic interactions are the main driving force in adsorption. The addition of NaCl increases the ionic strength and decreases the adsorption capacity for bilirubin; this further demonstrates that electrostatic interactions are the driving force. In addition, they proposed that the hydrophobic action of methyl can also promote adsorption. The hydrothermal stability of SBA-15 molecular sieves is better than that of MCM-41 molecular sieves, and SBA-15 has a larger number of channels [29-33]. However, little research has been done on the adsorption of NB by SBA-15. The methods currently used for preparing molecular sieves from fly ash include hydrothermal synthesis, microwave-assisted synthesis, the alkali-melting hydrothermal method, and alkali-soluble hydrothermal method. ZSM-5, MCM-41, SBA-15, and other molecular sieves have been prepared from fly ash [34-39]. In this study, we successfully synthesized fly-ash-derived SBA-15 molecular sieves and silanized fly-ash-derived SBA-15 mesoporous molecular sieves to improve its hydrophobicity, and used the modified SBA-15 for NB adsorption from wastewater. The modification and functionalization of SBA-15 mesoporous molecular sieves were investigated to improve the adsorption selectivity and adsorption capacity, and to reduce costs. The results of this study will help in the development of low-cost and efficient molecular sieve adsorbents and effective techniques for treating industrial organic wastewater. The developed adsorbents also provide new methods for the comprehensive use of fly ash.

2. 2.1.

Experimental Raw materials and chemicals The raw material was high-alumina fly ash (FA-01) from a thermal power plant in Zhungeer, 4


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Inner Mongolia, China. The chemical composition of the fly ash was SiO2 53.17%, Al2O3 36.86%, Fe2O3 2.58%, TiO2 2.48%, and CaO 2.27%; the total combined content of SiO2 and Al2O3 was above 90%. The powder X-ray diffraction (XRD) pattern in Fig. 1 shows that the predominant phase was an amorphous aluminosilicate glass phase with a high internal chemical energy. The main crystalline phases were mullite, quartz, hematite, and rutile.

Fig. 1. Powder X-ray diffraction pattern of high-alumina fly ash

Based on the material balance principle, the results of chemical composition analysis and main phase analysis of high alumina fly ash (FA-01) were calculated by software LINPRO. The main phase of FA-01 consisted of 15.9% mullite, 3.7% quartz, 67.1% glass, 0.9% rutile, and 4.7% hematite. HCl (analytical grade), NaOH (analytical grade), Pluronic P123, toluene (analytical grade), trimethoxychlorosilane (analytical grade), and NB (chromatographic purity) were all provided by the National Pharmaceutical Group Chemical Reagent Co., Ltd.

2.2.

Preparation of CH3O-SBA-15 from fly ash The preparation of fly-ash-derived SBA-15 mesoporous molecular sieves has been described

in the literature [33]. The surface of a mesoporous silica material contains three different structures: isolated (– SiOH) and twin (=SiOH) silicon-hydroxyl groups, and hydrogen-bonded hydroxyl groups. The 5



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first two have high chemical reactivities, but the last one does not. However, heating transforms the hydrogen-bonded hydroxyl groups into highly active free silicon-hydroxyl groups. The chemical modification of mesoporous silica materials is based on the use of covalent bonds to link organic functional groups with silicon-hydroxyl groups on the surface of the mesoporous silica [40,41]. In this study, Si–O–Si(CH3O)3 was formed by grafting of a halogenated silane with a silanol group, to eliminate the acidity of the silyl group and increase the hydrophobicity (lipophilicity). Fig. 2 shows the reaction between the molecular sieves and trimethoxychlorosilane.

Fig. 2. SBA-15 silanization

First, SBA-15 molecular sieves were dried in an oven at 110° C for 12 h. A mixture of the dried SBA-15 molecular sieves and toluene was heated at 110 °C in an oil bath for 2 h. Trimethoxychlorosilane was added to give a SBA-15:trimethoxychlorosilane mass ratio of 1:2.5, and the mixture was refluxed at 125 °C in an oil bath for 7 h. After the reaction, the mixture was filtered, and the solid was washed with toluene and dried at 95 °C to give CH3O-SBA-15. The technology route of preparation and silanization of SBA-15 molecular sieves are shown as Fig. 3.

Fig. 3. Preparation and silanization of SBA-15 molecular sieves

2.3. Nitrobenzene adsorption experiments A certain amount of SBA-15 or CH3O-SBA-15 powder was added to a NB aqueous solution (25 mL). The pH was adjusted with 1 M HCl or NaOH and the mixture was shaken at constant temperature. At specified time intervals, appropriate amounts of liquid were filtered through a 0.22 µm needle filter and analyzed using high-performance liquid chromatography. A standard curve was used to determine the NB concentration from the peak area. The effects of adsorbent dosage, 6


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adsorption time, adsorption temperature, and pH on NB adsorption were investigated. The NB removal rate and adsorption capacity were calculated using equations (1) and (2)[40]:

%Removal = =

(

)×

× 100

(1) (2)

where C0 (mg/L) is the initial concentration of NB, Ce (mg/L) is the concentration of NB after adsorption for a period of time; qe (mg/g) is the adsorption capacity; V (L) is the volume of NB solution; and m (g) is the mass of adsorbent used.

2.4. Characterization methods XRD was performed using a D8 Advance X-ray diffractometer, with Cu Kα radiation, at 40 kV and 30 mA, and a scanning velocity of 5°/min, over the 2θ range from 5° to 90°. Small-angle XRD was performed using a Brukerg (Germany) diffractometer with Cu Kα radiation at 40 kV and 200 mA. Scans were performed at a scanning velocity of 0.05°/min, over the 2θ range from 0.5° to 5°. N2 adsorption at 77 K was performed using a Micromeritics ASAP-2020 apparatus. The Brunauer–Emmett–Teller (BET) equation was used to calculate the surface area from the linear part of the resulting plot. The pore size distributions were calculated using the Barrett–Joyner– Halenda method. Functional groups were identified using Fourier-transform infrared (FT-IR) spectroscopy (Bomen, MB-154S, Canada). Transmission electron microscopy (TEM; JEM ARM200F, JEOL, Japan) and scanning electron microscopy (SEM; Hitachi S4700, Japan) were used to examine the morphology of the synthesized SBA-15 molecular sieves. The NB concentration was determined using a HPLC system (Agilent 1260 Infinity II Prime, USA).

3. Results and discussion 3.1. Preparation of CH3O-SBA-15 The small-angle powder XRD patterns of SBA-15 and CH3O-SBA-15 were used to investigate the changes in the SBA-15 molecular sieves caused by silanization. The patterns are shown in Fig. 4.

7



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Fig. 4. Small-angle powder X-ray diffraction patterns of SBA-15 before and after modification

The patterns in Fig. 4 show three strong diffraction peaks from pure SBA-15 at 2θ values between 0.6° and 2°; these correspond to the (100), (110), and (200) crystal planes, and are typical of a hexagonal pore structure. After silanization, the (100) plane peak shifted to a lower angle, and the peak intensity weakened; the diffraction peaks from (110) and (200) gradually disappeared, indicating that the Si–O content of SBA-15 decreased, the number of defects increased, and the channels were filled with a large number of silane groups. The silane groups disrupted the orderly mesoporous channels. The results show that SBA-15 silanization was successful. The surface functional groups of the molecular sieves were identified using IR spectroscopy, to determine whether silane groups were grafted onto SBA-15. The IR spectra of SBA-15 and CH3O-SBA-15 are shown in Fig. 5. In the SBA-15 spectrum, the absorption peaks at 3445 and 1635 cm−1 correspond to the stretching and bending vibrations of O–H in water. The absorption peak at 466 cm−1 corresponds to the bending vibration of Si–O–Si bonds in the molecular sieves. The absorption peaks at 802 and 1095 cm−1 correspond to the symmetric and antisymmetric stretching vibrations of Si–O–Si bonds. The CH3O-SBA-15 spectrum shows absorption peaks at 2962, 2860, 1437, and 1402 cm−1, which are characteristic silane group absorption peaks. The peak at 2962 cm−1 corresponds to the asymmetric stretching vibration of C–H and the peak at 2860 cm−1 corresponds to the symmetric stretching vibration of C–H. The peaks at 1437 and 1402 cm−1 correspond to the asymmetric and symmetric in-plane bending vibrations, respectively, of C– 8


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H [6]. The presence of these peaks indicates successful grafting of silane onto SBA-15.

Fig. 5. Infrared spectra of (a) SBA-15 molecular sieves and (b) CH3O-SBA-15

a

b

c

d

Fig. 6. Scanning electron microscopy images of (a&b) SBA-15 mesoporous molecular sieve samples and (c&d) CH3O-SBA-15 9



a

b

c

d

Fig. 7. Transmission electron microscopy images of

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(a&b) SBA-15 mesoporous molecular sieve samples and

(c&d) CH3O-SBA-15

The morphologies and pore structures of the SBA-15 and CH3O-SBA-15 molecular sieves were examined using SEM and TEM; the images are shown in Figs. 6 and 7. Fig. 6 shows that the pure SBA-15 and CH3O-SBA-15 molecular sieves both consist of spherical particles. Modification did not change the morphology, but slightly increased the particle size. Fig. 7a and 7b are hexagonal and stripe images of the SBA-15 sample in the (100) direction. The SBA-15 sample has a typical highly ordered two-dimensional hexagonal phase structure. The mesopore size is about 5–7 nm and the wall thickness is about 3 nm. Fig. 7c and 7d show that modification did not change the pore channels; this is consistent with the XRD results. After silanization (Fig. 7c and 7d), CH3O-SBA-15 still has a regular hexagonal pore structure. CH3O-SBA-15 has a pore size of about 3–5 nm and a pore wall thickness of about 2 nm. The CH3O-SBA-15 pores are smaller than those in the pure SBA-15, and the pore wall is thicker, indicating that organic functional groups were grafted onto the pore wall, which increased the thickness.

10


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Fig. 8. N2 adsorption–desorption isotherms (a) and pore size distributions (b) of CH3O-SBA-15 and SBA-15

Fig. 8 shows the adsorption–desorption isotherms of the SBA-15 and CH3O-SBA-15 molecular sieves. The isothermal nitrogen adsorption curves of both samples are type Ⅳ, with H1-type hysteresis. A large hysteresis loop appeared with increasing relative pressure, indicating that the isotherms are Langmuir type IV, i.e., typical mesoporous adsorption curves. The more regular pore structure and larger pore size and larger specific surface area provide the necessary conditions for adsorption[14-18]. Compared with SBA-15, the nitrogen adsorption capacity of the CH3O-SBA-15 molecular sieves was significantly lower and the pressure at which mutation occurred shifted to a slightly lower pressure, indicating that the pore size of the sample was smaller. The pore structure parameters of the SBA-15 and CH3O-SBA-15 molecular sieves are shown in Table 1. The data show that the specific surface area, pore volume, and pore size of CH3O-SBA-15 were smaller than those of the original SBA-15 sample, which is similar to the previous studies[19-28]. Table 1 Pore structure parameters for SBA-15 before and after modification Sample

BET surface area/

d100/nm

a0/nm

2

Pore volume

Pore size D /nm

3

（m /g）

（cm /g）

Wall thickness/nm

SBA-15

793.6

8.32

10.74

0.76

6.11

2.21

CH3O-SBA-15

637.8

10.24

11.29

0.55

4.93

5.31

Notes: d100:crystal face spacing; a0:unit cell parameter, a0 = (2/√3)d100; pore size calculated from adsorption branch; wall

thickness calculated as (a0 − D).

11



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3.2. Nitrobenzene adsorption on molecular sieves

The NB concentrations were determined using HPLC. The mobile phase was water:acetonitrile = 3:7 (volume ratio), and the flow rates were 0.3 and 0.7 mL/min, respectively. The UV wavelength was 262 nm. A series of NB solutions of various concentrations were prepared, and the peak areas of 20 µL samples were measured. A standard curve for NB was constructed. The standard curve equation is y = 72568x + 7969.9. The fitting coefficient R2 is 0.9955.

3.2.1. Effect of adsorbent amount

Fig. 9. Effect of adsorbent amount on nitrobenzene adsorption efficiency

The effect of the amount of adsorbent used on the NB adsorption efficiencies of SBA-15 and CH3O-SBA-15 were investigated. The adsorbents were added to an 8 µmol/L (20 mL) NB solution at 25 °C. The concentrations of SBA-15 and CH3O-SBA-15 were all 0.5, 1.0, 2.5, 4.0, 5.5, 7.0, 8.5, and 10.0 g/L respectively. The adsorption efficiency was evaluated after adsorption for 1 h. The NB removal rates of the adsorbents varied with the amount of adsorbent, as shown in Fig. 9. The NB removal rate increased with increasing amount of adsorbent, for both SBA-15 and CH3O-SBA-15. This is because a larger amount of adsorbent provides a larger surface area and 12


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more adsorption sites, therefore the removal rate increases [31]. For both SBA-15 and CH3O-SBA-15, when the amount of adsorbent was 5.5 g/L, the adsorption rate increased slowly, but continued increasing. This suggests that as long as there is sufficient adsorbent, NB will be removed from the water. This similar phenomenon also appears in our reference [13,14]. Continue to increase the amount of adsorbent, nitrobenzene removal rate changed little, it may be that the adsorption has reached equilibrium at this time (5.5g/L), is close to the maximum adsorption capacity due to excessive adsorbent makes the saturation of surface-active sites. Our previous results show that, adsorption capacity of SBA-15 and CH3O-SBA-15 was 5.4mg/g and 6.3mg/g, respectively. Therefore, treating with high NB concentration, dilution of the solution would be required. If continue to increase the amount of adsorbent, not only resulted in the waste of molecular sieve, but also increase the cost of wastewater treatment. On the basis of the adsorption capacity and cost, the optimum amount of SBA-15 was 5.5 g/L and the NB removal rate was 77.5%. The NB removal rate with CH3O-SBA-15 was significantly higher than that with SBA-15, indicating that silanization improved the rate of NB adsorption by SBA-15. Further increases in the CH3O-SBA-15 dosage did not increase the NB removal rate; the adsorption rate was 89.6%, and the optimum CH3O-SBA-15 concentration was 5.5 g/L.

3.2.2. Effect of adsorption time

Fig. 10. Effect of time on nitrobenzene adsorption efficiency

The NB removal rates at various times were investigated. The adsorption temperature was 13



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25 °C, the SBA-15 and CH3O-SBA-15 concentrations were 5.5 g/L, and the NB concentration was 8 µmol/L. The removal rate was evaluated after adsorption for 1, 2, 3, 5, 10, 20, 30, 40, 50, 60, and 90 min. The results are shown in Fig. 10. Adsorption of NB on SBA-15 and CH3O-SBA-15 was rapid in both cases. This is because their excellent pore structures promote diffusion and mass transfer of NB. In addition, the adsorbent and NB are both hydrophobic, therefore adsorption equilibrium is rapidly reached [29]. It is clear from the figure that there were three adsorption stages, the first from 0 to 10 min, the second from 10 to 50 min, and the third from 50 to 90 min. This indicated that adsorption onto the external surfaces occurred in the first 5 min. This stage was the fastest, and it depended on the bulk diffusion of NB. The adsorption rates at 10 min for SBA-15 and CH3O-SBA-15 were 68.8% and 85.1%, respectively. Once the external surface is saturated, NB started to diffuse within the pores to reach internal sites and that is why adsorption rate get slower. The last stage represents reaching equilibrium. All of this indicated that diffusion within the pores is not the sole step and this can be proved by applying the intraparticle diffusion model. When SBA-15 was used as the adsorbent, the nitrogen removal rate was 85.52% at 60 min. The adsorption rate then decreased slightly with time, indicating that a small amount of desorption occurred. For CH3O-SBA-15, the adsorption rate did not change after 60 min, indicating that desorption of NB had not yet occurred. This proves that the adsorption capacity of the silanized molecular sieves was higher than that of the unmodified molecular sieves.

3.2.3. Effect of pH

14


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Fig. 11. Effect of pH on nitrobenzene adsorption efficiency

The pH of the 8 µmol/L NB solution was 5; the pH was adjusted with 1 mol/L HCl and 1 mol/L NaOH to explore the effect of pH on the NB adsorption efficiency. The effect of pH on the nitrogen adsorption efficiencies of the two adsorbents were investigated at pH 3, 5, 7, 9, and 11. The adsorbent dosage was 5.5 g/L, the concentration of the NB solution was 8 µmol/L, and the adsorption time was 60 min. The results are shown in Fig. 11. There are six mechanisms for adsorption on solid surfaces, namely electrostatic interactions, ion exchange, ion–dipole interactions, ion coordination on a metal surface, hydrogen bonding, and hydrophobicity [30]. NB is a non-ionic compound, and mainly non-ionic in water, therefore the involvement of the first four mechanisms is extremely low. Furthermore, because it is a non-ionic compound, the effect of pH on the adsorption process may be determined by the adsorbent surface [31]. The zero-charge point pHzpc of SBA-15 is about 4, and if hydrogen bonding is the primary mechanism of adsorption, the optimum adsorption pH should coincide with the pHzpc of SBA-15. Fig. 12 shows that the optimum pH was not 4 but 5. This indicates that hydrophobic interactions are also involved [29]. NB and SBA-15 are both hydrophobic, and silanization increases their hydrophobicities, therefore the removal rate with CH3O-SBA-15 molecular sieves is higher. However, the stability of SBA-15 prepared in an acidic environment is affected by an alkaline environment, which leads to changes in the hydrophobicity and affects NB adsorption [31]. The adsorption efficiency therefore decreased with increasing pH at pH greater than 7.

3.2.4. Effect of temperature

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Fig. 12. Effect of temperature on nitrobenzene adsorption efficiency

The effect of temperature on the NB adsorption efficiency was investigated. Adsorption was performed at 5, 25, 35, and 45 °C; the amount of adsorbent was 5.5 g/L, the concentration of the NB solution was 8 µmol/L, the pH was 5, and the adsorption time was 60 min. The results are shown in Fig. 12. Between 25 and 45 °C, the higher the temperature was, the lower the adsorption efficiency was, for SBA-15 and CH3O-SBA-15. This shows that low temperatures are favorable for adsorption. This may be because the solubility of NB in water increases with increasing temperature, which is not conducive to adsorption or the nitrobenzene adsorption was an exothermic process[13]. At 25 °C, the removal rates of NB achieved using SBA-15 and CH3O-SBA-15 were 89.6% and 77.05%, respectively.

3.2.5. Adsorption isotherms The Langmuir and Freundlich models are commonly used to interpret the adsorption isotherms of solid–liquid adsorption processes. The Langmuir model is suitable for monolayer adsorption with a limited number of adsorption sites on the surface [12]. It is widely used because of its simple form, and high fitting with adsorption results. The Langmuir adsorption model equation is

1 1 1 = + qe qm Ce (qmb)

(3)

The Freundlich model is suitable for non-uniform surfaces [30]. It assumes that adsorption occurs on a non-uniform surface, and that the adsorption capacity is related to the equilibrium concentration, as shown in equation (4):

lg qe = (1/ n) lg Ce + lg K F

(4)

where qe is the equilibrium adsorption capacity (µmol/g), Ce is the equilibrium concentration of NB (µmol/L), qm is the static saturated adsorption capacity (µmol/g), b is the adsorption intensity (L/mg), KF is the adsorption coefficient, and 1/n is the adsorption index. Table 2 shows the corresponding adsorption isotherm parameters. The results show that both models are suitable for NB adsorption, but the Langmuir model is better. The values of 1/n for the Freundlich model are less than 1, indicating that the SBA-15 and CH3O-SBA-15 surfaces have 16


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good affinities for NB; an n value less than 1 indicates chemical adsorption, and n greater than 1 indicates physical adsorption. The n values therefore show that NB is mainly removed by physical adsorption on the CH3O-SBA-15 molecular sieves.

Table 2 Adsorption isotherm parameters Langmuir parameter

Temperature /℃

Freundlich parameter 2

b/(L/µmol)

qm/(µmol /g)

R

KF/(µmol/g)

1/n

R2

SBA-15

0.1768

6.3857

0.9942

1.9101

0.6472

0.9839

CH3O-SBA-15

0.1917

10.4167

0.9993

2.0311

0.7086

0.9848

3.2.6. Adsorption kinetics

Fig. 11 shows that NB adsorption varied with time. Adsorption equilibrium was reached at about 60 min, and adsorption increased sharply in the first 10 min. After 10 min, the adsorption rate gradually decreased and equilibrium was eventually reached. Lagergren's first-order adsorption rate equation (equation 5) is commonly used for simulating solid–liquid adsorption systems. ln( − ) = −

(5)

!

where k1 is the quasi-first-order adsorption kinetic rate constant (min−1), qt is the adsorption amount at time t (mg/g), and qe is the equilibrium adsorption capacity (mg/g). McKay’s quasi-second-order kinetic equation (equation 5) is also commonly used to describe adsorption behavior. "#

=$

! % % "&

!

+ " ( (6) &

where k2 is the second-order adsorption rate constant [g/(mg min]. The data in Fig. 11 were fitted to the linear form of these two kinetic equations. The kinetic parameters obtained from the slopes and intercepts of the straight lines are shown in Table 3. Table 3 Kinetic parameters for nitrobenzene adsorption on CH3O-SBA-15 Adsorbents

qe( mg/g)

Lagergren -1

k1(min )

qe·c(mg /g)

McKay 2

R

k2( g

qe·c(mg /g)

R2

17



Page 18 of 27

/( mg·min)) CH3O-SBA-15

1.282

0.0321

0.3578

0.9874

0.75

0.64

0.9981

molecular sieves

The data in Table 3 show that the coefficient of determination for the fitting curve of the quasi-second-order kinetic equation is greater than 0.99 and the fitting degree is very good. The theoretical equilibrium adsorption capacity calculated using the equation is similar to the actual adsorption capacity. The adsorption of NB on CH3O-SBA-15 therefore conforms to the quasi-second-order kinetic equation.

3.2.7. Adsorption thermodynamics The adsorption enthalpy change ∆H is obtained using the Clausius–Clapeyron equation: ∆/

[ln(* )]" = −,- + 01

(7)

where Ce is the equilibrium solution concentration at the absolute temperature at which the adsorption capacity is q (mg/L), R is the ideal gas constant, T is the absolute temperature (K), and K0 is a constant. In practice, the enthalpy change of the adsorption reaction is often calculated using the equilibrium coefficient KD (the ratio of the amount of adsorbate in the solid phase to that in the liquid phase at equilibrium) instead of the equilibrium constant. The free energy can be deduced using the Gibbs equation: 8

∆2 = −34 5-

67 7

(8)

where ∆G is the adsorption free energy change of the unit adsorbent, X is the mole fraction of the solute in the solution, q is the adsorption isothermal equation, and the appropriate adsorption isotherm equation can be used. When the Freundlich adsorption isothermal equation is used to replace q in the formula, it is deduced that the adsorption free energy of the unit adsorbate is independent of q: ∆G = −nRT

(9)

where n is the Freundlich constant, and is closely related to the adsorption force between the 18


Page 19 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


adsorbent and the adsorbate, according to Powell et al. [41]. The adsorption entropy can be calculated using the Gibbs–Helmholtz equation: ∆9 =

∆/ ∆: 1

(10)

The Freundlich equation was used to calculate the equilibrium concentrations Ce of the solution at the same adsorption capacity but different temperatures, and the values were plotted with lnCe as the ordinate and 1/T as the abscissa (Fig. 13). The values of the adsorption enthalpy ∆H and ∆S are obtained from the slope and intercept of the straight line (Table 4).

Fig. 13. Relationship between lnCe and 1/T Table 4 Thermodynamic parameters for nitrobenzene adsorption on CH3O-SBA-15

qe/(mg/g)

ΔH

ΔG(kJ/mol)

ΔS(kJ/mol)

(kJ/mol)

288.5K

298.5K

308.5K

288.5K

298.5K

308.5K

1.0

-18.09

-4.869

-5.038

-5.206

0.045

0.043

0.041

1.5

-29.56

-3.214

-4.289

-5.017

0.091

0.084

0.079

The data in Table 4 show that △G is less than 0 in all cases, indicating that NB adsorption on CH3O-SBA-15 is a spontaneous process, and the degree of spontaneity increases with increasing temperature. The adsorption enthalpy △H is negative, indicating that NB adsorption on CH3O-SBA-15 is an exothermic reaction; the value is less than 40 kJ/mol. Usually the adsorption heat of physical adsorption is small, similar to the condensation heat. The adsorption heat of chemical adsorption is approximately equal to the chemical reaction heat, generally more than 40 kJ/mol [42]. Adsorption on CH3O-SBA-15 19



Page 20 of 27

molecular sieves is therefore probably physical adsorption. This result is consistent with the adsorption isotherm calculations. The adsorption entropy reflects the degree of order of matter in the system; The negative value of ∆S suggested decreased randomness at the solid/solution interface occurred in the internal structure of the adsorption of nitrobenzene onto CH3O-SBA-15.

3.2.8. Elution

Regeneration after successful elution is an important for recycling and reusing the adsorbent to improve cost-effectiveness[32-45]. As shown in Fig. 14, the fly-ash-derived SBA-15 was efficiently eluted using petroleum ether and subsequently regenerated for the next cycle. However, the adsorption efficiency was slightly decreased after seven cycles. The data clarified that fly-ashderived SBA-15 exhibited regeneration ability for NB for potential use on a large scale. Fly-ashderived SBA-15 has excellent adsorption capacity and efficient regeneration capacity and has potential uses in dyes, spices, explosives and other organic synthesis industry.

Fig.14. Elution and simultaneous regeneration using petroleum ether for the multiple reuses in a series of adsorption-elution-regeneration operations.

4. Conclusions In this study, we synthesized fly-ash-derived SBA-15 mesoporous molecular sieves. CH3O-SBA-15 mesoporous molecular sieves were prepared using the SBA-15 mesoporous molecular sieves as a support and trimethoxychlorosilane as a modifier. The synthesized SBA-15 20


Page 21 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and CH3O-SBA-15 molecular sieves were characterized using N2 adsorption–desorption analysis, powder XRD, FT-IR spectroscopy, SEM, and TEM. The adsorption of NB in wastewater with the SBA-15 and CH3O-SBA-15 molecular sieves as the adsorbent was studied. The efficiency of NB adsorption on CH3O-SBA-15 was significantly greater than that on SBA-15. The results show that the optimum adsorption conditions were temperature 25 °C, pH 5, adsorbent amount 5.5 g/L, and NB solution concentration 8 µmol/L. The adsorption efficiency did not change after 60 min, and the maximum values for SBA-15 and CH3O-SBA-15 were 77.5% and 89.6%, respectively. Langmuir and Freundlich adsorption isotherms were obtained for the initial NB solution concentration. The results show that the adsorption behaviors of NB on SBA-15 and CH3O-SBA-15 were basically consistent with the Langmuir equation, and the adsorption was physical adsorption. The adsorption capacity of CH3O-SBA-15 reached 10.42 µmol/g.. The adsorption of NB on CH3O-SBA-15 conformed to a quasi-second-order kinetic equation. The coefficient of determination of the quasi-second-order kinetic equation was more than 0.99, and the fitting degree was good. Investigation of the adsorption thermodynamics showed that the reaction was spontaneous and endothermic, and the entropy increased. High temperatures were conducive to the adsorption reaction.

Acknowledgements The authors gratefully acknowledge support by the National High Technology Research and Development Program (“863”program) of China (2012AA06A115) and the China Postdoctoral Science Foundation (2017M610723). We thank Helen McPherson, PhD, from Liwen Bianji, Edanz Editing China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript.

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TOC

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Enhanced Nitrobenzene Adsorption in Aqueous Solution by Surface

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