Tailored Design of Differently Modified Mesoporous Materials To

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Tailored design of differently modified mesoporous materials to deeply understand the adsorption mechanism for PAHs Pei Yuan, Xiaoling Li, Wangyang Wang, Haiyan Liu, Yan Yan, Haifeng Yang, Yuanyuan Yue, and Xiaojun Bao Langmuir, Just Accepted Manuscript • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on November 29, 2018

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Tailored design of differently modified mesoporous materials to deeply understand the adsorption mechanism for PAHs Pei Yuana, Xiaoling Lia, Wangyang Wangb, Haiyan Liub, Yan Yanc, Haifeng Yangc, Yuanyuan Yuea*, Xiaojun Baod* a

National Engineering Research Center of Chemical Fertilizer Catalyst, School of Chemical Engineering, Fuzhou University, Fuzhou 350002, China

b

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China c

d

Chinese Academy of Inspection and Quarantine, Beijing 100176, China

State Key Laboratory of Photocatalysis on Energy & Environment, Fuzhou University, Fuzhou 350116, China *Emails: [email protected]; [email protected]

1

ACS Paragon Plus Environment

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ABSTRACT: A series of SBA-15 with different modifications have been successfully prepared and applied as adsorbents to remove polycyclic aromatic hydrocarbons (PAHs) from aqueous solutions. The morphology and structural properties of the chemically modified materials are all similar to pure SBA-15, and thus the difference of PAHs adsorption capacity can be directly attributed to the different functional groups, which is favorable to deeply explore the adsorption mechanism. Adsorption kinetics and isotherms experiments for naphthalene (Nap), anthracene (Ant) and pyrene (Pyr) were carried out and the results reveal that the adsorption processes follow a pseudo-second-order rate equation and the equilibrium can be achieved within 120 min for Nap and Ant while only 90 min for Pyr, indicating the more hydrophobic molecules, the easier and faster adsorption can be obtained. All of the adsorption isotherms fit well with Freundlich model, suggesting the unevenly distributed active sites on adsorbents. The phenyl-functionalized materials possess the highest adsorption capacity, implying that the π-π interaction is the most primary interaction and plays the predominant role in PAHs adsorption, superior to the acidic and hydrophobic interaction. Our research sheds light on the design and synthesis of advanced and highly-efficient adsorbents to remove PAHs from aqueous solutions.

KEYWORDS: SBA-15, polycyclic aromatic hydrocarbons, modification, adsorption, π-π

interaction

2


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INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) are widespread in atmosphere, soil, ocean and other ecosystem and have been recognized as the most toxic and highly persistent organic pollutants due to their high toxicity and adverse impacts to the environment and human health.1,2 Therefore, it is essential and urgently needed to effectively remove PAHs from the environment, which has become a hot issue all around the world. Researchers have made many efforts to develop a variety of methods to eliminate PAHs via chemical, physical, thermal or biological progresses, e.g. adsorption,3 sedimentation,4-7 oxidation,8-9 biodegradation10-13 and photocatalysis.14-16 Among these, adsorption is proved to be a promising method for PAHs removal because it is low-cost, easy to operate and recycle, and also environmental friendly. In recent years, a number of efficient adsorbents have been developed and utilized to remove PAHs from contaminated water and soil, such as biomass materials,17-20 activated carbons,21-25 graphene,15,

26-28

metal-organic frameworks (MOFs)29-32 and modified

mesoporous materials.33-38 Accordingly, several adsorption mechanisms, involving hydrophobic interaction, π-π interaction and acidic interaction, were proposed to illustrate the adsorption processes and explain the adsorption behaviour of different absorbents. Hu et al.39 compared the adsorption performance of hydrophobic and hydrophilic MCM-41 molecular sieves for phenanthrene, and the results showed that the template-containing hydrophobic MCM-41 exhibited a higher adsorption capacity due to the hydrophobic interactions with phenanthrene. Wang et al.40 studied the 3


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adsorption properties of graphene oxide (GO), chemically reduced graphene and annealing reduced graphene and it was indicated that the reduced GO, in which the oxygen-containing functional groups were removed and a sp2-hybridized structure was restored, had a better adsorption performance for naphthalene and its derivative owing to the strengthened π-π interactions between the organic pollutants and graphene nanosheets. Araujo et al.41 described the removal of naphthalene, anthracene and pyrene in isooctane by MCM-41 and Al-MCM-41, the adsorption performance was greatly enhanced as increasing the Al/Si molar ratios, resulting from the electrostatic interactions between the π-electrons of PAHs and the acidic centers of the Al-MCM-41. Researchers have tried to modify the adsorbents based on the proposed adsorption mechanism to improve the adsorption ability, however, what is the most principal interaction between PAHs and adsorbents and which mechanism plays a predominant role on PAHs adsorption are still ambiguous. Till now it is intriguing and essential, but still a challenge to identify the effect of PAHs-absorbent interactions on the adsorption efficiency in the same system, excluding other possible influencing factors (e.g. morphology, structure and texture properties). In this study, we have designed a series of chemically modified mesoporous materials with similar morphology, structure and texture properties to compare their adsorption ability of PAHs in the same system. Five modified materials, including aluminum

doped

SBA-15

by

two

different

methods

(O-Al-SBA-15

and

P-Al-SBA-15), hydrophobically treated SBA-15 (CH3-SBA-15), phenyl-groups grafted SBA-15 (Ph-SBA-15) and phenyl-groups modified both in the framework and 4


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on the surface of SBA-15 (BT-SBA-15), have been carefully prepared and used as adsorbents to remove the naphthalene (Nap), anthracene (Ant) and pyrene (Pyr) from the aqueous solutions. The adsorption kinetics and isotherms of PAHs on the obtained absorbents are systematically investigated and the influence of different interactions between PAHs and absorbents on the adsorption performance is clarified.

EXPERIMENTAL SECTION Materials. Nonionic triblock copolymer poly (ethylene glycol)-block-poly (propylene glycol)-block-poly (ethylene glycol) (P123, Mw = 5800), aluminium isopropoxide (Al(O-i-Pr)3) and aluminium nitrate (Al(NO3)3) were purchased from Aldrich.

Tetraethyl

orthosilicate

(TEOS),

phenyltrimethoxysilane

(PTMS),

1,4-bis(triethoxysilyl)benzene (BTEB) and trimethyl chlorosilane (TMCS) were obtained from Macklin (Shanghai, China). Nap, Ant and Pyr were obtained from Aladdin. All the chemicals were of analytical grade and used without any further purification. Synthesis of SBA-15 and chemically modified SBA-15. Ordered mesoporous SBA-15 was synthesized using nonionic triblock copolymer P123 as a template and TEOS as silica source under acidic condition following the method reported by Zhao et al.42 In a typical synthesis, P123 (3.0 g) was completely dissolved in 2M HCl solution (80 mL) by continuously stirring at 38 ºC, then TEOS (6.24 g) was added into the above solution. The resultant mixture was stirred at 38 ºC for 8 min and kept static for 24 h and then crystallized at 100 ºC for another 24 h in a Teflon autoclave 5


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crystallization. Eventually, the white solids were filtered, washed with deionized water, dried at room temperature, and then calcined at 550 ºC for 5 h to remove the templates. Two Al-SBA-15 materials with different acidic properties were obtained by a one-step synthesis and post-grafting method, respectively. The direct synthesis of Al-SBA-15 was conducted as follows: firstly, P123 (3.0 g) was dissolved in HCl solution (2M, 80 mL) for 6 h with stirring, followed by the addition of TEOS (6.24 g), then the gel mixture was continuously stirred at 38 ºC for 24 h and the suspension was filtered out and washed with deionized water. After that, the wet solids were dispersed in 60 mL aluminum precursor solution containing 0.375g of Al(NO3)3 with the adjusted solution pH value of 1.4. Then the mixture was crystallized in a Teflon autoclave at 100 ºC for 24 h. Finally, the product was separated by filtration, washed with deionized water, dried in air and calcined at 550 ºC for 5 h. The obtained white powder was named as O-Al-SBA-15. For the post-grafting method, SBA-15 (1.8 g) was dispersed into 100 mL of aluminum precursor ethanol solution containing 0.204 g of Al(O-i-Pr)3 with stirring at 60 ºC for 24 h. The product was then filtered, washed, dried at room temperature and calcined at 550 ºC for 5 h and the product was named as P-Al-SBA-15. To modify the surface of SBA-15 with hydrophobic groups, 1.8 g of SBA-15 was added to a solution of TMCS (10 mL) in toluene (150 mL) and then the suspension was refluxed at 80 ºC for 10 h. After that, the resulting solid was recovered by filtration, washed with ethanol for several times, and dried at room temperature. 6


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The surface functionalization of SBA-15 with PTMS was carried out by the post-grafting method as follows: SBA-15 (1.8 g) was first dispersed in 100 mL toluene, and then PTMS (0.72 mL) was added. The mixture was heated up to 80 ºC and refluxed for 10 h, and the product was collected by filtration, washed with ethanol for several times, and dried at room temperature. BT-SBA-15 with the phenyl groups both in the framework and on the surface was prepared as follows: P123 (3.0 g) was dissolved in HCl solution (2M, 80 mL), followed by adding a mixture of BETB (2.52 mL), PTMS (0.72 mL) and TEOS (4.8 mL) at 40 ºC with stirring for 24 h and the mixture was crystallized at 100 ºC for another 24 h. The product was separated by filtration, washed with ethanol, and dried in air. The template was removed by means of solvent extraction at 70 ºC with a solution of HCl (5 mL) in ethanol (100 mL) for three times (8 h each time). Characterizations. Small-angle X-ray diffraction (XRD) patterns were performed on a Philips X’Pert diffractometer (Netherlands) equipped with a Cu K𝛼 radiation (a wavelength of 1.5406 Å). Measurements were achieved with 2θ values in the range of 0.5-5º, a step size of 0.01º with 4 s per step. The unit cell parameters a0 was determined

from

the

d100

reflection

for

SBA-15-type

materials.

N2

adsorption-desorption measurements of the samples were performed on a Micromeritics ASAP 2002 instrument (USA) at -196 ºC. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method and the pore size distribution was evaluated by the Barrett-Joyner-Halenda (BJH) method.43 The samples to be measured were firstly degassed in the preparation station at 180 ºC and 7


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a vacuum of 10-5 Torr for 15 h, and then switched to the analysis station for sorption experiment. Scanning electron microscopy (SEM) images were acquired using an FEI Quanta 200F microscope (USA) with the operating voltage of 15 kV. Transmission electron microscopy (TEM) was carried out on a Philips Tecnai G2 F20 instrument (Netherlands) with an accelerating voltage of 200 kV. UV-vis diffuse reflection spectroscopy (UV-vis DRS) experiment was conducted on a Hitachi U-4100 UV-vis spectrophotometer (Japan) attached with the integrating sphere diffuse reflectance. The powder sample was put into a transparent quartz cell and measured ranging from 200 to 400 nm. The hydrophilic/hydrophobic properties of pure SBA-15 and modified SBA-15 were measured on a JC2000DM contact angle measurement instrument (China). Fourier transform infrared spectroscopy (FTIR) spectra was performed on a Thermo Nicolet Nexus 470 FTIR instrument (USA) with wave numbers ranging from 4000 to 400 cm-1. The acidic properties of the two different Al-SBA-15 were characterized by pyridine-FTIR on a Thermo Nicolet Nexus 470 FTIR instrument with a resolution of 1 cm-1. The samples were first dehydrated under vacuum for 5 h, then adsorbed pyridine vapor at room temperature. IR spectra recorded the signals when degassing and evacuating at 200 ºC and 350 ºC, respectively. The quantities of L or B acid sites (Q) were calculated according to Eq. 1:

IA( B or L active sites )  Q

Awafer Wwafer

C

(1)

Where IA(B or L active sites) (cm-1) indicates the integrated absorbance band for L or B 8


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acid sites, Awafer (cm2) is the self-supporting wafer’s cross-sectional area, Wwafer (g) is the wafer weight, and C (cm/μmol) means the extinction coefficient. The extinction coefficients corresponding to the wavenumbers of ~1540 and ~1450 cm-1 are 1.13 and 1.28 cm/μmol, respectively.44-45 Adsorption studies. The standard solutions were prepared by dissolving 50 mg Nap in 50 ml of methanol, the concentrations of the stock standard solutions were 1000 mg/L. In order to investigate the adsorption kinetics of Nap onto the SBA-15 and modified SBA-15 adsorbents, the standard solution was diluted using deionized water to 50 mg/L, and 10 mg of adsorbents were added into 50 mL of Nap aqueous solution in a 250 mL glass flask with stirring at 25±1 ºC. After adsorption for a certain time (5-150 min), the mixture was separated by a membrane filter and the residual Nap concentration was determined by a UV/vis detector. The Nap uptakes at any time, qt (mg/g), was calculated using Eq. 2: qt 

V (C0  Ct ) m

(2)

Where C0 (mg/L) and Ct (mg/L) are the Nap concentration at initial and time t, V (L) is the volume of the solution and m (g) is the mass of adsorbent. The adsorption kinetics for Ant and Pyr were investigated following the same procedures as Nap. Pseudo-first-order and pseudo-second-order models were applied and expressed as (Eqs. 3 and 4).

log(qe  qt )  log qe 

K1t 2.303

t 1 t = + 2 qt K 2 qe qe 9


(3)

(4)

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Where qe (mg/g) and qt (mg/g) are the adsorption amounts of PAHs on adsorbents at equilibrium and at time t (min), respectively; K1 (min-1) and K2 (g/(mg·min)) are represented as the rate constants of the first-order and pseudo-second-order model, respectively. Adsorption isotherm experiments were carried out by mixing 10 mg of adsorbent with 50 ml of the PAHs solution with different initial concentrations (5 ~ 150 mg/L). The mixture was stirred at 25 ºC for 2 h to reach equilibrium, and the adsorption capacity were expressed in terms of the amount of PAHs absorbed at equilibrium per gram of adsorbent (qe, mg/g) and calculated as Eq. 5: qe 

V (C 0  Ce ) m

(5)

The Langmuir isotherm model is expressed by Eq. 6: K L qmCe 1+K L Ce

(6)

Ce C 1 = + e qe K L qm qm

(7)

qe =

The linear form is given as Eq. 7:

Where Ce (mg/L) is the PAHs concentration at equilibrium, qe (mg/g) is the adsorption amount of PAHs at equilibrium, qm (mg/g) is the maximum adsorption capacity per unit mass of adsorbent, and KL is the Langmuir constant related to the energy of adsorption. The Freundlich isotherm model is expressed by Eq. 8: qe  K F Ce1 n

The logarithmic form is described as Eq. 9: 10


(8)

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1 ln qe  ln K F  ln Ce n

(9)

Where KF is the Freundlich constant related to the adsorption capacity, and 1/n indicates the affinity between the adsorbent and adsorbate, which can be determined from a linear plot of lnqe against lnCe.

RESULTS AND DISCUSSION The morphology and structural characterization of mesoporous materials. Figure 1A shows the small-angle XRD patterns of the six adsorbents and they all exhibit three well-resolved peaks, assigned to (100), (110) and (200) diffractions associated with a two-dimensional hexagonal symmetry (P6mm),46 indicating the highly ordered mesostructures of SBA-15 and functionalized SBA-15 materials. The d100 values and unit cell parameters (a0) of the samples can be calculated as given in Table 1. It is clearly seen that the chemical modification processes have almost no influence on the structure and orderness of materials and the a0 values are all around 10.5 nm.

Figure 1. Small-angle XRD patterns (A), N2 adsorption-desorption isotherms (B) and pore size distribution curves (C) of SBA-15 (a), O-Al-SBA-15 (b), P-Al-SBA-15 (c), CH3-SBA-15 (d), Ph-SBA-15 (e), and BT-SBA-15 (f). 11


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Figure 1B and C display the N2 adsorption-desorption isotherms and the pore size distribution curves calculated from adsorption branch for SBA-15 and modified SBA-15 and the textual properties are summarized in Table 1. All of the six materials present type IV isotherms with type H1 hysteresis loops according to the IUPAC classification, demonstrating that these materials have one-dimensional straight pore channels.47 The modified materials synthesized by a post-grafting method, such as P-Al-SBA-15, CH3-SBA-15 and Ph-SBA-15, have slightly decreased SBET, Vp and dp compared with SBA-15, suggesting the successful graft of heteroatoms or functional groups to the wall surface of SBA-15. O-Al-SBA-15 material (the incorporation of Al atoms via the direct synthesis) has the highest SBET (889 m2/g) and Vp (1.12 cm3/g) and the largest pore diameter (7.7 nm), which might be caused by the embedding of Al atoms in the silica framework. BT-SBA-15 prepared by the co-condensation of TEOS and two organic silica precursors (BTEB and PTMS) shows the lowest SBET (616 m2/g) and Vp (0.80 cm3/g), approximately 23.5% and 18.4% lower than the values of SBA-15, probably due to the decrease of micropores and the surface coverage by phenyl groups. The pore size distribution curves indicate the uniformly distributed mesopores (Figure 1C) and the pore sizes of the prepared materials excluding O-Al-SBA-15 are measured to be similar to each other (Table 1).

12


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Table 1. Textural and structural characteristics of SBA-15 and modified SBA-15. Sample

d100(nm)a

a0(nm)b

SBET(m2/g)c

Vp(cm3/g)d

dp(nm)e

SBA-15

9.0

10.4

805

0.98

6.9

O-Al-SBA-15

9.3

10.7

889

1.12

7.7

P-Al-SBA-15

9.0

10.4

664

0.85

6.8

CH3-SBA-15

9.1

10.6

739

0.93

6.8

Ph-SBA-15

9.1

10.6

758

0.93

6.8

BT-SBA-15

9.0

10.7

616

0.80

6.9

Note: a d-spacing calculated from the (100) diffraction peak, d100 = λ/2sinθ; b the unit cell parameter calculated according to the p6mm symmetry, a0 = 2d100/√3; c BET surface area; d total pore volume; e pore diameter obtained from adsorption branch using BJH model.

SEM images in Figure 2 show that all the samples have short-rod morphologies with uniform diameters and lengths, indicating the maintenance of the morphology and particle sizes after the different chemical modifications. TEM images were also applied to investigate the internal structure of the adsorbents and as shown in Figure 3, the straight one-dimensional channels viewed along the (1 1 0) directions can be clearly seen, suggesting the highly ordered hexagonal mesostructures. All the XRD, N2 sorption, SEM and TEM results indicate that the different synthesis processes and chemical modifications have little effect on the morphology and structural properties which is favorable for us to explore and understand the effect of the distinct chemical properties of adsorbents on PAHs adsorption without regarding to the morphology, structure and textual properties of adsorbents.

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Figure 2. SEM images of SBA-15 (A), CH3-SBA-15 (B), Ph-SBA-15 (C), P-Al-SBA-15 (D), O-Al-SBA-15 (E), BT-SBA-15 (F).

Figure 3. TEM images of SBA-15(A), CH3-SBA-15 (B), Ph-SBA-15 (C), P-Al-SBA-15 (D), O-Al-SBA-15 (E), BT-SBA-15 (F).

The surface chemical properties of modified mesoporous materials. Figure 4 is the pyridine-adsorption FTIR spectra of O-Al-SBA-15 and P-Al-SBA-15 taken after evacuations at 200 and 350 ºC in the wavenumber range of 1600-1400 cm-1 to 14


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determine the acidic types, strength and amounts. There are several peaks can be seen in the spectra corresponding to the C-C stretching vibrations of pyridine. According to the literature,48 the strong band at 1493 cm-1 is due to pyridine adsorbed onto both Brönsted (B) and Lewis (L) acid sites, while the peak at 1548 cm-1 is ascribed to the protonation of pyridine molecules by B acid sites, and the above two peaks are similar in O-Al-SBA-15 and P-Al-SBA-15. It is assumed that Al atoms incorporation into SBA-15 can generate bridging hydroxyl groups (Si-OH-Al) on Al-SBA-15 resulting in B acid sites. The peaks at 1448, 1578 and 1597 cm-1 in O-Al-SBA-15 are attributed to pyridine molecules adsorbed onto L acid sites (Figure 4A), while only two peaks at 1456 and 1578 cm-1 are observed as L acid sites in P-Al-SBA-15 (Figure 4B). It was reported that the direct synthesis can incorporate Al atoms effectively into the silica framework and remarkably increase L acid sites.49 All the bands still exist after degassing at 350 ºC, indicating the existence of medium/strong acidity in O-Al-SBA-15 and P-Al-SBA-15. Based on the adsorption peaks, the amounts of B and L acid sites for these two samples can be quantitatively estimated and the acid strength distributions as well as the acid quantities are listed in Table 2. It is indicated that after degassing at 200 ºC, O-Al-SBA-15 possesses more (B+L) acid sites than P-Al-SBA-15 but the latter possesses more B acid sites. However, both the B and L acid sites are calculated to be more in O-Al-SBA-15 when the degassing temperature is increased to 350 ºC, indicating the much stronger B acid sites and more L acid sites in Al-modified SBA-15 prepared via the direct synthesis.

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Figure 4. Pyridine-FTIR spectra of O-Al-SBA-15 (A) and P-Al-SBA-15 (B).

Table 2. Amounts of B and L acid sites of O-Al-SBA-15 and P-Al-SBA-15 determined by pyridine-FTIR. Amounts of acid sites (μmol/g) Sample O-Al-SBA-15 P-Al-SBA-15

200 ºC L 199.1 105.4

B 22.1 35.7

350 ºC L+B 221.2 141.1

L 173.3 89.4

B 10.4 5.3

L+B 183.7 94.7

In order to further confirm the successful modification of organic groups on SBA-15, FTIR, UV-vis DRS and contact angle detection were carried out as displayed in Figure 5. Comparing the FTIR spectrum of CH3-SBA-15 with that of SBA-15 (Figure 5A), two new adsorption peaks are observed in CH3-SBA-15. One is the weak adsorption peak around 2900 cm-1 correponding to the C-H stretching vibration of -CH3 group50 and the other is the adsorption peak at 830 cm-1 attributed to the presence of Si-C(H) bonds,51 confirming the anchoring of -CH3 groups to the silica surface. UV-vis spectra of BT-SBA-15 and Ph-SBA-15 in the wavelength range of 200-400 nm (Figure 5B) present two peaks centered at (224 and 274 nm) and (214 and 264 nm), respectively, which are ascribed to the characteristic π-π* electron 16


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transitions of benzene ring.52 Such two peaks shift to higher wavelengths than ultraviolet adsorbances of free benzene (at 203 and 254 nm) due to the restricted mobility. It should be noted that the adsorption peaks shift towards much longer wavelengths for BT-SBA-15 compared with Ph-SBA-15 because of the substitution in p-position of benzene ring by siloxy, implying the embedding of phenyl groups in the silica framework.53 Figure 5 also shows the shapes of a water droplet on the surface of pellet-type SBA-15 and modified SBA-15 to evaluate their hydrophilic/hydrophobic properties. Generally, the higher contact angle indicates the stronger hydrophobicity.54 It is clearly seen that the contact angle for water on SBA-15 without any modification is nearly 0° implying a super hydrophilicity of pure SBA-15, while after post-grafting with -CH3 groups, it gives the highest contact angle of 35.4° indicating CH3-SBA-15 is the most hydrophobic material among all the modified materials. The hydrophobic property follows the order: CH3-SBA-15 > BT-SBA-15 > Ph-SBA-15 > SBA-15, and the enhancement in hydrophobicity is further verified the successful modification on SBA-15. It should be noted that the contact angles of the modified materials are all lower than 90°, meaning that they have a certain wetting property which is beneficial for the adsorbents to sufficiently contact with contaminated water to adsorb PAHs from it.

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Figure 5. FTIR spectra (A) of CH3-SBA-15(a) and SBA-15(b), UV-vis spectra (B) of BT-SBA-15 (a) and Ph-SBA-15 (b), and the contact angle of droplet formed on different adsorbents.

Adsorption kinetics. Adsorption kinetics is vital for understanding the adsorption mechanism and process efficiency. The variation of adsorption capacity with time is depicted in Figure 6 in order to probe the adsorption kinetics of Nap, Ant and Pyr. The adsorption initial concentration of PAHs is set to be 50 mg/L and after 120 min, the adsorption process tends to reach the equilibrium state (Figure 6A and B), but for Pyr with more benzene rings and higher molecular weight, shorter time is needed for completing the adsorption (Figure 6C). Two classical adsorption kinetics models,55 pseudo-first-order and pseudo-second-order, are used to evaluate the adsorption rate of each PAH onto SBA-15 with different modifications and verify the mechanisms involved. Linear regression was applied to determine the best-fitting kinetic rate equation and the correlation coefficients R2 were calculated and used to evaluate how well the 18


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experimental data is fitted to the kinetic model. Figure 7 shows the plots of log (qe-qt) vs. t and t/qt vs. t and the corresponding parameters and R2 for the kinetic models are given in Table 3. It is clear that the linear correlation coefficients for pseudo-second-order model are 0.999 for all the studies, and the calculated qe values are in good accordance with the experimental values (qe

(exp.)),

suggesting the

adsorption process of Nap, Ant and Pyr on pure and modified SBA-15 can be well represented by the pseudo-second-order kinetic model associated with the chemical sorption.56 Based on K2 and qe values (Table 3), the adsorption rates for Nap decrease in the order of BT-SBA-15 > Ph-SBA-15 > O-Al-SBA-15 > P-Al-SBA-15 > CH3-SBA-15 > SBA-15, and the equilibrium adsorption capacities for Nap show the trend of BT-SBA-15 > Ph-SBA-15 > O-Al-SBA-15 > CH3-SBA-15 > P-Al-SBA-15 > SBA-15. BT-SBA-15 shows the highest adsorption rate and equilibrium adsorption capacity although it has the lowest pore diameter and pore volume (Table 1), that is because BT-SBA-15 with the phenyl groups both in the silica framework and on the surface not only has the hydrophobic property but also provides a relatively strong π-π interaction with Nap. Ph-SBA-15 with the phenyl groups on the surface exhibits higher adsorption rate and capacity than SBA-15 with acidic modification (O-Al-SBA-15 and P-Al-SBA-15) and hydrophobic modification (CH3-SBA-15), implying π-π interaction is more important for the interaction between adsorbents and PAHs. Al-modified materials show higher K2 for Nap adsorption than methyl-modified SBA-15, probably owing to the relatively low hydrophobicity of 19


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Nap molecules. O-Al-SBA-15 with more L acid sites and strong B acid sites performs a higher adsorption capacity than P-Al-SBA-15, revealing that the interaction between π-electrons of the aromatic structure in PAHs and acid sites (especially L acid sites), plays a vital role in PAHs adsorption.48

Figure 6. Adsorption kinetics of Nap (A), Ant (B) and Pyr(C) onto pure and modified SBA-15. 20


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Figure 7. Pseudo first-order kinetics for adsorption of Nap (A1), Ant (B1) and Pyr (C1) and pseudo second-order kinetics for adsorption of Nap (A2), Ant (B2) and Pyr (C2) onto pure and modified SBA-15.

For the adsorption of Ant and Pyr, the adsorption rate and equilibrium adsorption capacity order are similar to those of Nap. BT-SBA-15 still demonstrates the best adsorption capacity, followed by Ph-SBA-15, further certifying π-π interaction is the most significant effect. However, CH3-SBA-15 exhibits higher K2 and qe values for 21


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Ant and Pyr than Al-SBA-15, and such enhancement might be attributed to the intensification of hydrophobic effect. Additionally, the adsorption capacity is increased with the increase of benzene-ring numbers for a certain absorbent, Nap < Ant < Pyr. This is because (1) the increase of hydrophobic property of PAHs could lead to a stronger hydrophobic effect between the absorbents and PAHs; and (2) more benzene-ring numbers provides more π-electrons, resulting in the enhanced interaction of π-electrons in PAHs with the phenyl groups or acid sites on the surface of the adsorbents.

Table 3. Pseudo first-order and Pseudo second-order kinetics model parameters and correlation coefficients for adsorption of PAHs on modified SBA-15. Adsorbate

Adsorbent

qe(exp)

Nap

SBA-15 O-Al-SBA-15 P-Al-SBA-15 CH3-SBA-15 Ph-SBA-15 BT-SBA-15

Pseudo-first-order

Pseudo-second-order

qe

K1

R2

192.2 229.1 206.9 214.4 232.2 241.0

92.34 75.31 96.82 86.26 80.28 58.25

0.0318 0.0338 0.0463 0.0305 0.0326 0.0365

0.942 0.955 0.960 0.984 0.956 0.987

195.69 231.48 209.21 217.39 234.74 242.72

0.858 1.634 1.430 1.214 1.731 2.098

0.999 0.999 0.999 0.999 0.999 0.999

Ant


196.7 233.9 222.9 237.8 241.2 242.9

116.82 85.91 90.34 87.76 94.55 89.55

0.0346 0.0340 0.0276 0.0287 0.0296 0.0302

0.963 0.871 0.964 0.909 0.908 0.934

201.61 236.97 226.24 240.96 244.50 245.70

0.928 1.537 1.479 1.719 1.810 2.173

0.999 0.999 0.999 0.999 0.999 0.999

Pyr


205.6 229.1 226.5 239.4 242.1 243.0

91.24 112.30 111.39 202.48 114.15 113.93

0.0530 0.0566 0.0571 0.0459 0.0451 0.0473

0.941 0.976 0.971 0.960 0.969 0.965

202.02 238.73 229.30 241.10 244.81 247.50

1.067 1.671 1.558 1.816 1.999 2.422

0.999 0.999 0.999 0.999 0.999 0.999

22


qe

K2 (10-3)

R2

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Adsorption isotherms. The adsorption isotherms for Nap, Ant and Pyr on each prepared sample are investigated to identify the relationship between the amounts of solute absorbed at equilibrium per weight of adsorbent (qe) and the PAHs concentration after reaching the equilibrium (Ce). The experiments were implemented in a batch reaction using various initial concentrations of PAHs aqueous solution with a constant adsorbent weight. Two most commonly used isotherm models57 (Langmuir and Freundlich) were applied to fit the experimental data. Langmuir model proposes that adsorption takes place on a homogeneous surface by a monolayer sorption and there is no interaction among adsorbed molecules with the assumption of uniform surface adsorption energy. Freundlich model is an empirical formula and widely used for describing the multilayer and heterogeneous adsorption with non-uniform active sites. The linear fitting of adsorption isotherms data with Langmuir and Freundlich models are shown in Figure 8 and the fitting parameters are listed in Table 4. The higher correlation coefficients can be obtained when fitting with Freundlich model, deducing that the heterogeneous active sites on the surface of modified SBA-15 play an important role in PAHs adsorption.58-59 It can be seen that all the 1/n values calculated from the Freundlich model are less than 1, indicating a preferential adsorption60 and imlying a strong interaction between Nap, Ant and Pyr and the adsorbents. In addition, KF values for the adsorption of Nap, Ant or Pyr on different adsorbents have the same trend of BT-SBA-15 > Ph-SBA-15 > CH3-SBA-15 > O-Al-SBA-15 > P-Al-SBA-15 > SBA-15, which is in accordance with the 23


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experimental data.

Figure 8. Langmuir adsorption isotherm for adsorption of Nap (A1), Ant (B1) and Pyr (C1) and Freundlich adsorption isotherm for adsorption of Nap (A2), Ant (B2) and Pyr (C2) onto modified SBA-15.

24


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Table 4. Langmuir and Freundlich isotherm model parameters and correlation coefficients for adsorption of PAHs on modified SBA-15. Adsorbate

Adsorbent

qs a

Langmuir

Freundlich

qm

KL

R2

KF

1/n

R2

Nap


196.79 254.26 214.94 222.63 281.27 332.64

254.45 284.90 317.46 326.80 343.64 346.02

0.279 0.848 0.242 0.316 0.705 1.267

0.980 0.973 0.753 0.906 0.992 0.997

5.213 5.870 5.307 6.219 6.789 7.145

0.627 0.581 0.621 0.595 0.497 0.485

0.998 0.985 0.998 0.987 0.996 0.995

Ant


212.26 263.85 222.52 291.77 326.39 363.01

213.68 313.48 229.89 343.64 357.14 321.54

0.633 0.482 0.654 0.534 0.605 1.797

0.903 0.905 0.916 0.872 0.883 0.959

5.414 6.044 5.590 6.60 7.345 8.298

0.616 0.586 0.610 0.491 0.480 0.458

0.996 0.997 0.996 0.996 0.999 0.998

Pyr


229.62 291.26 244.76 345.27 374.26 389.28

214.59 249.38 258.40 349.65 409.84 416.67

0.462 0.723 0.495 0.715 0.748 1.171

0.954 0.979 0.986 0.926 0.920 0.926

5.669 6.404 5.573 7.121 10.011 11.365

0.611 0.514 0.602 0.483 0.470 0.412

0.998 0.994 0.998 0.998 0.998 0.998

a q is the saturation adsorption obtained from isotherm experiment, mg/g. s

Adsorption mechanisms. Several interactions including π-π interaction,61-65 hydrophobic effect39, 66 and electrostatic interaction48, 67 between aromatic compounds and adsorbents have been reported to promote the adsorption of PAHs. The adsorption performance can be greatly influenced by the surface properties of the absorbents, but to the best of our knowledge, there is no report to inspect and compare these widely-accepted adsorption interactions in the same system. Compared with unmodified SBA-15, the enhancement of adsorption capacity for PAHs varies with different functionalization on materials. O-Al-SBA-15 and P-Al-SBA-15 exhibit a 25


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higher adsorption efficiency than SBA-15, and the improvement could be attributed to the presence of a homogeneous distribution of acid sites which can perform the electrostatic interaction with the π-electron system of PAHs. O-Al-SBA-15 with more L acid sites always exhibits better adsorption capacity compared to P-Al-SBA-15 no matter what type of PAHs, indicating L acid sites might play a vital role in PAHs adsorption compared with B acid sites. CH3-SBA-15 with the strongest hydrophobicity among the six prepared materials shows a slightly improved adsorption capacities for Nap with a relatively weak hydrophobicity and largely improved activity for Ant and Pyr with a strong hydrophobicity, attributed to the intensification of hydrophobic effect. The phenyl groups modified materials, both Ph-SBA-15 and BT-SBA-15, display much better adsorption performance for Nap, Ant and Pyr than SBA-15 with alumina modifications via two different synthesis routes and hydrophobic modifications, suggesting π-π force might be the most important interaction between adsorbents and the studied PAHs. Especially for the co-condensed material BT-SBA-15, it exhibits the highest adsorption capacity due to its more phenyl groups both in the framework and on the surface and higher hydrophobicity. The importance of acidic and hydrophobic effect depends on the adsorbates. For PAHs with a low hydrophobicity, the electrostatic interaction might be more helpful than hydrophobic effect, but for PAHs with a high hydrophobicity, the enhancement from hydrophilic effect could be more remarkable than that from electrostatic interaction. Overall, considering the structural and hydrophobic nature of the studied PAHs, π-π interaction plays a dominant role in PAHs adsorption, next is 26


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Langmuir

hydrophobic effect and then is the electrostatic interaction. With regard to different PAHs, the adsorption capacity for a certain absorbent is increased as increasing the benzene rings: Nap < Ant < Pyr, owing to the increase of π-electrons and hydrophobic property of PAHs.

SUMMARY AND CONCLUSIONS A series of modified SBA-15 with similar morphology and structural properties, including BT-SBA-15, Ph-SBA-15, CH3-SBA-15, O-Al-SBA-15 and P-Al-SBA-15, have been successfully synthesized, which allows us to evaluate the adsorption capacity for different PAHs in the same system. Kinetic studies reveal that all the sorption processes can achieve equilibrium within 120 min for Nap, Ant and 90 min for Pyr, following with the pseudo-second-order model. The investigation of adsorption isotherms illustrates that Freundlich model provides the best description of PAHs adsorption on different adsorbents. BT-SBA-15 shows the highest adsorption capacities for Nap, Ant and Pyr (332.6.0, 363.0 and 389.3 mg/g, respectively), indicating that the π-π interaction is the primary interaction between PAHs and adsorbents. The presence of Lewis acid sites can improve the PAHs absorption in a certain degree, but the enhancement is limited especially for the PAHs containing more benzene rings in the molecules. Our result is of great importance for deeply understanding the adsorption mechanism which may provide a foundation on the tailored design and controllable synthesis of facile and efficient adsorbents for the enrichment and removal of aromatic compounds from aqueous solution. 27


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ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (Grant 21576290, 21776048), Fujian Province Natural Science Funds for Distinguished Young Scholar (2018J06002), the research fund for public welfare project (201410015). REFERENCES (1) Abdel-Shafy, H. I.; Mansour, M. S. A review on polycyclic aromatic hydrocarbons: Source, environmental impact, effect on human health and remediation. Egypt. J. Petrol. 2016, 25, 107-123. (2) Kim, K.-H.; Jahan, S. A.; Kabir, E.; Brown, R. J. A review of airborne polycyclic aromatic hydrocarbons (PAHs) and their human health effects. Environ. Int. 2013, 60, 71-80. (3) Li, Z.; Liu, Y.; Yang, X.; Xing, Y.; Yang, Q.; Yang, R. T. Adsorption thermodynamics and desorption properties of gaseous polycyclic aromatic hydrocarbons on mesoporous adsorbents. Adsorption 2017, 23, 361-371. (4) Yan, Z.; Zhang, H.; Wu, H.; Yang, M.; Wang, S. Occurrence and removal of polycyclic aromatic hydrocarbons in real textile dyeing wastewater treatment process. Desalin. Water. Treat. 2016, 57, 22564-22572. (5) Li, R.; Feng, C.; Wang, D.; Li, B.; Shen, Z. Multiphase redistribution differences of polycyclic aromatic hydrocarbons (PAHs) between two successive sediment suspensions. Front. Env. Sci. Eng. 2016, 10, 381-389. (6) Li, P.-h.; Wang, Y.; Li, Y.-h.; Wai, K.-m.; Li, H.-l.; Tong, L. Gas-particle partitioning and precipitation scavenging of polycyclic aromatic hydrocarbons (PAHs) in the free troposphere in southern China. Atmos. Environ. 2016, 128, 165-174. (7) Ghosal, D.; Ghosh, S.; Dutta, T. K.; Ahn, Y. Current state of knowledge in microbial degradation of polycyclic aromatic hydrocarbons (PAHs): A Review. Front. Microbiol. 2016, 7, 1639. 28


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339x174mm (150 x 150 DPI)


Tailored Design of Differently Modified Mesoporous Materials To

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