Synthesis and characterization of cubic mesoporous bridged polysilsesquioxane for removing organic pollutants from water

Chemosphere 103 (2014) 188–196

Contents lists available at ScienceDirect

Chemosphere journal homepage: www.elsevier.com/locate/chemosphere

Synthesis and characterization of cubic mesoporous bridged polysilsesquioxane for removing organic pollutants from water Derong Lin a,b, Qing Zhao b, Lijiang Hu a, Baoshan Xing b,⇑ a b

Chemistry Department, Harbin Institute of Technology, Harbin 150001, China Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 3D cubic mesoporous BPS were

successfully synthesized and characterized.  Hydrophobic interaction dominantly controlled organic pollutants’ sorption on BPS.  Rigid aromatic BPS showed a higher sorption capacity than soft aliphatic BPS.  3D cubic mesoporous BPS show high sorption capacity and easy regeneration.

a r t i c l e

i n f o

Article history: Received 4 June 2013 Received in revised form 19 November 2013 Accepted 26 November 2013 Available online 26 December 2013 Keywords: Cubic mesoporous bridged polysilsesquioxane Organic pollutants Adsorption Removal efficiency

a b s t r a c t Hexane, octane, phenyl, and biphenyl-bridged bis(triethoxysilyl) precursors were used in synthesizing cubic mesoporous bridged polysilsesquioxane (BPS) copolymers. Structural characterization was carried out by FTIR, small angle XRD, Brunauer–Emmett–Teller-N2 sorption, 1H NMR, and TEM. We successfully synthesized both ‘‘rigid’’ and ‘‘soft’’ 3D cubic mesoporous BPS with high surface area and pore volume, as attested by the comprehensive characterization data. Adsorption of pyrene, phenanthrene, nitrobenzene, and 2,4-dichlorophenol on BPS was greatly affected by adsorbate properties, i.e., Kow, solvation properties and molecular size. Hydrophobic interaction dominantly controlled organic pollutants’ sorption on BPS. Other interactions, e.g., p–p and H-bond interactions, also have effects on sorption as indicated by Kow normalized sorption isotherms. Rigid aromatic BPS (phenyl and biphenyl) showed a higher sorption capacity than soft aliphatic BPS (hexane and octane). A conceptual model was proposed to further explain the phenomena. This study suggests a promising application of cubic mesoporous BPS in wastewater treatment. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Pollution from organic compounds has become one of the most serious global environmental issues today. Organic pollutants once released into the aquatic ecosystem can cause various environ-

⇑ Corresponding author. Tel.: +1 413 545 5212; fax: +1 413 545 3958. E-mail address: [email protected] (B.S. Xing). 0045-6535/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2013.11.062

mental problems, which include clogging sewage treatment plants, adversely affecting on the aquatic biota, increasing biochemical oxygen (Wang et al., 2010a). Therefore, how to deal with wastewater that contains organic pollutants is gaining more and more attention. Current technologies for wastewater treatment include ozonolysis (Beltran et al., 1992), photolysis (Yu et al., 2012), photocatalytic decomposition (Ide et al., 2012), adsorption (Kato et al., 2008; Merle et al., 2010), membrane filtration (Yang et al., 2012)

D. Lin et al. / Chemosphere 103 (2014) 188–196

and biological treatment (Nurisepehr et al., 2012). Among those technologies, adsorption by sorbents such as carbon nanotubes (Yang et al., 2006a,b; Wang et al., 2008), rice husk ash (Jang et al., 2009), activated carbon (Kato et al., 2008) and hydrophobic zeolite (Merle et al., 2010) is regarded as one of the most commonly used methods. Although many studies have been conducted on sorption, there are still some shortcomings needed to be addressed, e.g., low sorption efficiency, high cost, selective sorption, and difficult regeneration (Bercic et al., 1996; Matatov-Meytal and Sheintuch, 2000). More novel adsorbents are thus required to solve these problems. Surfactant templated hybrid mesoporous sieves with molecular-scale mixing of inorganic and organic species inside pore walls have been synthesized and studied by three research groups (Asefa et al., 1999; Inagaki et al., 1999; Melde et al., 1999; Yoshina-Ishii et al., 1999). Such materials are made by combining surfactants with (R0 O)3Si–R–Si(OR0 )3 precursors that are used in the fabrication of bridged polysilsesquioxanes (BPS) (Small et al., 1993; Loy and Shea, 1995). Bridging groups that have appeared in hybrid cubic and hexagonal structures of BPS include methane (Inagaki et al., 1999; Melde et al., 1999), ethane (Asefa et al., 1999; Melde et al., 1999), hexane (Whitnall et al., 2005), amido (Lin et al., 2011), octane (Tan et al., 2007), phenyl (Cho et al., 2009) and biphenyl (Kapoor et al., 2002). Because of mesoporous structure, BPS is suggested to be a potential sorbent. In addition, different organic bridging groups can create favorable surface-organic pollutants interactions, which can further enhance adsorption capacity of BPS. Burleigh et al. (2002) studied phenols’ sorption on aryleneand ethylene-BPS, and concluded that BPS was a candidate sorbent for the removal of phenols due to its high adsorption capacities, fast adsorption kinetics and easy regeneration. However, their study is limited in two dimensional structural BPS. BPS also has cubic structure. Cubic structure with interpenetrating networks of pores exhibits higher activity than two dimensional hexagonal structures (Peng et al., 2012). Hence we hypothesize that cubic structure BPS adsorbs more organic pollutions from wastewater than hexagonal structural one. We describe herein the synthesis and characterization of cubic mesoporous structure BPS and its application as a sorbent for the removal of organic pollutants. Four different bridging groups, two soft and aliphatic (hexane and octane) and two rigid and aromatic (phenyl and biphenyl) were used which will inevitably increase the hydrophobicity of BPS to different extents. Phenanthrene, pyrene, nitrobenzene and 2,4-dichlorophenol were selected to represent organic pollutants. The specific objectives of this study therefore were: (a) synthesize and characterize cubic mesoporous structure BPS with different bridging groups, (b) compare sorption ability of different bridged BPS and (c) study the influence of adsorbate properties on adsorption.

2. Experimental 2.1. Materials 1,8-Bis(triethoxysilyl) octane (BESO), 1,4-Bis(triethoxysilyl) phenyl (BESP), 4,40 -Bis(triethoxysilyl) biphenyl (BESB), polyethylene glycol monocetyl ether, n = 23 (Brij 56), unlabeled pyrene (99+%), phenanthrene (98+%), sodium azide, nitrobenzene (99+%) and 4-Chlorophenol (4-CP, 99+%) were purchased from Sigma– Aldrich Chemical Co. 2,4-Dichlorophenol (DCP, 99.5+%) was purchased from Shanghai Reagent Co. Fuming hydrochloric acid (37%), calcium chloride and anhydrous ethanol (100%) were obtained from Fisher Scientific. Deionized water (DW) was prepared with an Ionpure Plus 150 Service Deionization ion-exchange purification system. All reagents were used as received without further

189

purification. Selected properties of pyrene, phenanthrene, nitrobenzene and DCP are shown in Table S1. 2.2. Synthesis of 1,6-Bis(triethoxysilyl) hexane (BESH) The modified version of the preparation experiment developed by Oviatt et al. (1993) was used. Briefly, 1,5-hexadiene (13.3 g, 0.162 mol), triethoxysilane (58.66 g, 0.357 mol), 100 mL benzene (99.8%, anhydrous) and 0.10 g chloroplatinic acid were added to a round-bottom three neck flask. The solution was kept in dark for 24 h. The reaction was stirred at room temperature under nitrogen for 3 d. Benzene was removed in vacuo. Distillation (95– 105 °C, 0.05 Torr) afforded a clear colorless liquid. The yield was 58%. 1H NMR (300 MHz, CDCl3) d 3.85 (q, 12 H), 1.40 (m, 4 H), 1.34 (m, 4 H), 1.24 (t, 18 H), 0.66 (m, 4 H). 2.3. Synthesis of biphenyl, phenyl, octane, and hexane BPS Brij 56 (4 g, 3.56 mmol) was dissolved in 107 mL of 1 M HClaq and 115 mL of deionized water to form a clear Brij 56-HCl-DW mixture solution, which was stirred in a 250 mL round bottom flask equipped with a magnetic stir bar (under nitrogen) for 6 h at 45–55 °C. BESB (8.52 g, 17.8 mmol), BESP (7.17 g, 17.8 mmol), BESO (7.81 g, 17.8 mmol), and BESH (7.3 g, 17.8 mmol) were added dropwise over 2 h to the polymer-containing solution with vigorous stirring, respectively. The four opaque solutions were further stirred for 24 h at 5 °C. The products were filtered, washed three times with deionized water (450 mL) and dried at 23 °C in vacuo. The yields were 57%, 53%, 50%, and 45%, respectively. The template containing one gram sample was extracted with 150 mL of ethanol for 6 h at 50–60 °C. The surfactant (3 g, 2.67 mmol) was removed and the yields were 39.3%, 35.7%, 32.5%, and 28.6%, respectively. 2.4. Characterization of biphenyl, phenyl, octane, and hexane BPS Fourier transform infrared spectroscopy (FTIR), small angle X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET)-N2 sorption, 1H nuclear magnetic resonance (NMR), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) techniques were used for characterization. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra of four BPS were determined by Thermo Nicolet Avatar 370 Fourier Transform Infrared Spectrometer. Powder X-ray diffraction (PXRD) patterns were recorded on a Philips X’pert0 instrument using monochromatic Cu Ka radiation (k = 1.5418 Å A). TEM images were obtained using a Hitachi H-8100 instrument operated at an accelerating voltage of 200 kV. The TEM has high brightness LaB6 electron source and large specimen-tilt (>30°) capabilities. It offers phase contrast resolution of better than 0.26 (point) and 0.14 nm (line). The sample was sonicated for 60 min in an adequate quantity of ethanol and the solution was dropped onto a porous carbon film on a copper grid and then dried before measurement. The SEM was performed using a LEO 1530 Gemini instrument equipped with a field emission cathode with a lateral resolution of 2 nm. The acceleration voltage was chosen between 0.5 and 3 kV. Nitrogen adsorption–desorption isotherm was measured on a Quantachrome Instrument Corporation Autosorb-1 analyzer. The samples were degassed at 200 °C using vacuum below 20 mmHg. The BET (Brunauer–Emmet–Teller) specific surface areas were calculated from the adsorption data in the relative pressure range from 0.05 to 0.235 (Table S2). The Barrett–Joyner–Halenda method was used for calculations of pore diameter and pore volume (Barrett et al., 1951) (Table S2). A new Bruker 300 MHz instrument was used for NMR measurements. 1H NMR spectra were collected on a Bruker 300 MHz spectrometer in CDCl3. Chemical shifts were reported in ppm and referenced with the internal tetramethylsilane

190

D. Lin et al. / Chemosphere 103 (2014) 188–196

(TMS) signal at 0.0 ppm before use. 13C NMR spectra were collected on a Bruker 100 MHz spectrometer in CDCl3. Chemical shifts in 13C spectra were reported in ppm and referenced at 77.0 ppm with the internal chloroform signal. 29Si {1H} CP/MAS spectra were recorded on a Bruker AVANCE III 400 WB spectrometer equipped with a 7 mm standard bore CP/MAS probehead whose X channel was tuned to 79.50 MHz for 29Si and the other channel was tuned to 400.18 MHz for broadband 1H decoupling. The dried and finely powdered samples were packed in the ZrO2 rotor closed with Kel-F cap. The spun rate was 5 kHz. A total of 1400 scans were recorded with 60 s recycle delay for each sample. 29Si CP/MAS chemical shifts were referenced to the resonances of 3-(trimethylsilyl)-1-propan-esulfonic acid sodium salt (DSS) standard (d = 0.0). Molecular weight of BPS was estimated using gel permeation chromatography (GPC). Detection was performed with a Shimadzu SPD-10A detector and Class-VP software was used for the acquisition and treatment of data. The column was Styragel HR 1 THF 7.8  300 mm (Waters). The chromatographic conditions were used as follows: elution in the isocratic mode with mobile phase tetrahydrofuran (THF), 0.4 mL min1 flow rate, and 20 lL injection volume. Chromatograms were obtained at 254 nm line. Commercial narrow molecular weight distribution polystyrene was used as a standard. Chromatograms obtained by repeated runs were almost identical. The molecular weights of biphenyl, phenyl, octane, and hexane BPS were 79 430, 63 090, 31 620 and 15 860 g mol1, respectively. 2.5. Adsorption experiments All adsorption isotherms were obtained using a batch equilibration technique at 25 ± 1 °C. Phenanthrene, pyrene, nitrobenzene and DCP in methanol was injected into a background solution containing 0.01 M CaCl2 and 200 mg L1 NaN3 (as a biocide) in deionized water, respectively. Aqueous solution of phenanthrene, pyrene, nitrobenzene and DCP was mixed with four BPS respectively in 8- or 40-mL screw cap vials. Ratios of BPS to water were 1.2 mg:40 mL for phenanthrene, 0.9 mg:40 mL for pyrene, 35 mg:8 mL for nitrobenzene and 72 mg:8 mL for DCP. Initial concentrations of compounds were controlled to obtain the equilibrium concentration ranges over 3 orders of magnitude. The volume fraction of methanol in each vial was less than 0.002 to avoid cosolvent effect. The vials were then placed on a shaker for 12 d (preliminary tests indicated that apparent equilibrium was reached before 11 d) and centrifuged at 3000g for 30 min. The supernatant was sampled for high-performance liquid chromatography (HPLC) and UV-spectrophotometer analysis. The final pH of the supernatant after the sorption experiment was measured, showing that the pH values during the sorption process were unchanged. Experimental uncertainties were evaluated in vials without BPS, which showed that total uncertainty was less than 3% of the initial concentrations. Therefore, sorbed solute by BPS was calculated directly by mass difference.

sorbate for BPS regeneration in order to compare with the data in the literature (Burleigh et al., 2002). 2.7. Analytical methods Pyrene was determined by HPLC with a flow rate of 1 mL min1 and a mobile phase of 85% acetonitrile/15% water at excitation/ emission wavelengths of 331 nm/392 nm. Phenanthrene was also determined by HPLC with a flow rate of 1 mL min1 and a mobile phase of 90% methanol/10% water at excitation/emission wavelengths of 250 nm/370 nm. HPLC analysis performed with a fluorescence detector and an intertsil reversed-phase column (25 cm  4.6 mm  5 lm; Inertsil ODS-P, Gl Sciences Inc.). The DCP, nitrobenzene and 4-CP concentrations were determined by a UV-spectrophotometer (Shimadzu, UV-2450) at 245, 268 and 279 nm, respectively. 2.8. Sorption model Three different models were used in this study. They are as follows: Dubinin–Ashtakhov (DA) model:

log qe ¼ log Q 0  ðe=EÞb where qe (mg g1) is equilibrium sorbed concentration. Q0 (mg g1) is the saturated sorbed capacity. E (kJ mol1), namely ‘‘correlating divisor’’, is the effective adsorption potential and b is the fitting parameter of DA model. Langmuir (LM) model:

qe ¼ Q 0 C e =ðK L þ C e Þ where qe is the amount of contaminant adsorbed/unit mass of adsorbent (mg g1). Ce is the equilibrium (mg L1). Q0 is the amount of contaminant adsorbed to form a complete monolayer (mg g1) and KL is the affinity coefficient (mg L1). Freundlich (FM) model:

Q e ¼ K f C ne where Qe and Ce again refer to amount adsorbed/unit mass of adsorbent and equilibrium concentration respectively. Kf [(mg/g)/(mg/L)n] is the Freundlich affinity coefficient, and n is the Freundlich exponential coefficient. All experiments were performed in triplicates. All model parameters with their standard errors were determined by a commercial software program (SPSS 18.0). Mean-weighted-square-errors (MWSE), equal to 1=m½ðqmeasured  qmodeled Þ2 =q2measured ], and correlation coefficients (r2) were used to evaluate the goodness of fit (m is the degree of freedom; qmeasured is the measured equilibrium sorbed concentration and qmodeled is the estimated equilibrium sorbed concentration by respective models). 3. Results and discussion

2.6. BPS regeneration

3.1. Adsorbent characterization

100 mg BPS was added to 10 mL background solution with different initial concentration of 4-CP. After sorption equilibrium, the vials were centrifuged at 3000g for 30 min and the supernatants were discarded. The sediments remained in tubes were then freeze-dried and mixed with 10 mL ethanol for 60 min. Samples were centrifuged again at 3000g for 30 min. The supernatants were collected and determined by UV/vis spectroscopy. BPS in vial was then dried and mixed with different initial concentrations of 4-CP again to repeat the sorption procedure. After centrifuge, the supernatant was collected and determined. We selected 4-CP as

Infrared spectra of biphenyl, phenyl, octane, and hexane BPS are shown in Fig. S1. The 1044 cm1 peak for biphenyl BPS, 1044 cm1 peak for phenyl BPS, 1070 cm1 peak for octane BPS and 1027 cm1 peak for hexane BPS were probably ascribed to vibration of Si–O deformation. The 925 cm1, 915 cm1, 899 cm1, 907 cm1 peak for biphenyl, phenyl, octane, and hexane BPS respectively was assigned to the bending vibration of Si–OH. As for octane and hexane BPS, the peaks at 2922 cm1, 2854–2855 cm1 proved the existence of methylene groups. While for biphenyl and phenyl BPS, the 870 cm1 peak suggested that phenyl group was successfully

191

D. Lin et al. / Chemosphere 103 (2014) 188–196

bridged to BPS. The Si–C vibration of the hexane BPS located at 1208 cm1 indicates that no (Si–C) bond split appears during the course of hydrolysis/condensation reactions. All these IR bonds are consistent with the proposed product. The results of 1H NMR for BPS were as follows: Biphenyl BPS 1H NMR (300 MHz, CDC13) d 7.81 (m, 4 H), 7.47 (m, 2 H), 7.2 (m, 2 H); phenyl BPS 1H NMR (300 MHz, CDC13) d 7.39 (m, 2 H), 7.17 (m, 2 H); octane BPS 1H NMR (300 MHz, CDC13) d 1.30 (m, 4 H), 1.28 (m, 8 H), 0.62 (m, 2 H), 0.58 (m, 2 H); hexane BPS 1H NMR (300 MHz, CDC13) d 1.30 (m, 4 H), 1.28 (m, 4 H), 0.62 (m, 2 H), 0.58 (m, 2 H). Results of 13 C and 29Si CP MAS NMR for BPS are listed below: Biphenyl BPS 13 C CP MAS NMR (100 MHz, CDC13) d = 139.5, 130.6, 127.5, 141.9; phenyl BPS 13C CP MAS NMR (100 MHz, CDC13) d = 141.9, 133.6, 131.5; octane BPS 13C CP MAS NMR (100 MHz, CDC13) d = 23.1, 23.9, 34.7, 29.3, 13.6, 15.9; hexane BPS 13C CP MAS NMR (100 MHz, CDC13) d = 23.1, 23.9, 34.7, 13.6, 15.9. Biphenyl BPS 29 Si CP MAS NMR d = 64.3 (T3); phenyl BPS 29Si CP MAS NMR d = 61.9 (T3); octane BPS 29Si CP MAS NMR d = 56.5 (T2), 63.9 (T3); hexane BPS 29Si CP MAS NMR d = 55 (T2), 64.7 (T3). Combining the FTIR and NMR results, we can safely concluded that biphenyl, phenyl, octane and hexane bridged BPS was successfully synthesized and obtained. The XRD patterns of four BPS are shown in Fig. 1 and Table S3. The biphenyl BPS had two small angle XRD diffraction peaks in the 2h region of 2–3°, which can be indexed to the [110] and [210] reflections (Fig. 1a). This assignment is consistent with 3D cubic Pm3n symmetry of a typical SBA-1 mesoporous silica material (Kapoor et al., 2002). The unit cell parameter of the cubic lattice is 5.90 nm from a = d110. The phenyl BPS also had two small angle XRD diffraction peaks in the 2h region of 1.0–3°, which can be indexed to the [110] and [211] reflections (Fig. 1b). This assignment is consistent with 3D cubic Pm3n symmetry of a typical SBA-1 mesoporous silica material (Pan et al., 2009). The unit cell parameter of the cubic lattice is 8.385 nm. In the small angle XRD spectra of octane BPS, there are several peaks at about 2h = 1.10°, 1.50°, 1.83°, 2.10°, 2.40°, 2.60°, and 3.00° that correspond to d spacing of 5.89, 4.81, 4.17, 3.73, 3.40, and 2.95 nm (Fig. 1c). This is in accordance with a cubic Pm3n structure (the unit cell parameter of the

8000

d110 = 4.17 nm

(a)

biphenyl BPS

cubic lattice is 8.33 nm), similar to that reported for mesoporous silica material (Kim and Ryoo, 1999). Fig. 1d shows the small angle XRD curve of the hexane BPS indicated by one major narrow peak at the scattering vector (q) around 0.13 and three minor peaks indexed as [111], [200] and [211] according to the simply cubic Pm3n symmetry. In the small angle XRD spectra there are also several peaks at about 2h = 1.85°, 2.25°, 2.59°, and 3.18° that correspond to d spacing of 4.83, 3.93, 3.41, and 2.79 nm, This is in line with a cubic Pm3n structure (the unit cell parameter of the cubic lattice is 6.83 nm), similar to SBA-1 reported earlier by Kim and Ryoo (1999) and Huo et al. (1994). We also used TEM to further testify their cubic Pm3n structures. A set of the hexane BPS TEM images projected along the [110], [111], and [211] directions are displayed in Fig. S2. Hexane BPS TEM images exhibit typical [110], [111], and [211] projection planes of the cubic Pm3n mesophase, clearly indicting a highly ordered 3-D cubic mesostructure of the hexane BPS (Weinberger et al., 2010). The spherical clusters/particles from an interconnected 3D cubic network of octane BPS are displayed in Fig. 2. It can be seen that the external surface of the particle was composed of [100], [110], and [111] planes. It showed well-ordered 3-D cubic arrays of mesopores and confirmed that the octane BPS had a highly ordered 3D cubic (Pm3n) mesostructure (Tan et al., 2007). The formation of well-defined 3D cubic (Pm3n) mesostructures of phenyl BPS and bisphenyl BPS was confirmed in Fig. S3. In addition, SEM images of four BPS are also provided to display their 3D cubic structures (Fig. 3). Based on the above characterization, we can conclude that biphenyl, phenyl, octane, and hexane group were successfully bridged to BPS. All BPS showed a 3D cubic (Pm3n) mesostructure. 3.2. Adsorption Sorption isotherms of pyrene, phenanthrene, nitrobenzene and DCP on four BPS are shown in Fig. S4. All isotherms were nonlinear. Hence nonlinear models (Langmuir, Freundlich, and Dubinin– Ashtakhov (DA) model) were applied to fit the experimental data (Table S5). For all isotherms, Freundlich parameters ‘‘n’’ were smaller than 1, indicating the sorption sites were unevenly distributed

8000

6000

6000

4000

4000

Intensity

2000

phenyl BPS

d211 = 3.42 nm

2000

d210 = 2.64 nm

0

0 1

8000

d110 = 5.93 nm

(b)

2

3

4

d110 = 5.89 nm

(c)

5

1

6

octane BPS

d111 = 4.81 nm

8000

4

5

6

hexane BPS

(d)

d110 = 4.83 nm

6000

d111 = 3.93 nm

6000 4000

4000

3

2

d200 = 3.41 nm

d200 =4.17 nm 2000

2000

d210 = 3.73 nm d211 = 3.40 nm

d211 = 2.79 nm

0

0

d220 = 2.95 nm 1

2

3

4

5

6

1

2 theta/

2

3

4

5

0

Fig. 1. XRD patterns: biphenyl BPS (a), phenyl BPS (b), octane BPS (c), and hexane BPS (d).

6

192

D. Lin et al. / Chemosphere 103 (2014) 188–196

Fig. 2. TEM images of cubic octane BPS with orientations: (a) polymer of the octane BPS; (b) [100] plane; (c) [110] plane; and (d) [111].

Fig. 3. SEM images of cubic hexylene BPS (a), octylene BPS (b), phenyl BPS (c) and biphenyl BPS (d).

193

D. Lin et al. / Chemosphere 103 (2014) 188–196

on the adsorbents (Zhao et al., 2010). DA model had a better fit than LM and FM as indicated by the fitting adjusted square of correlation coefficient (r2adj ). Since DA model has been recognized as one of the most powerful available models for both gas and aqueous adsorption on energetically heterogeneous surfaces, the better fitting of DA model further suggested the unevenly energy distribution of sorption sites on adsorbents. Therefore, we use DA model parameters to correlate adsorption behavior with the properties of adsorbates and adsorbents.

and log Kow probably suggested that hydrophobic interaction between organic chemicals and BPS surface, primarily an entropic process involving the exclusion of water, is a main factor for sorption. In addition to hydrophobic effect, other interactions might also affect sorption by BPS. For biphenyl and phenyl BPS, aromatic ring may provide potential p–p interaction sites. The existence of Si– OH of four BPS might also provide sorption sites for H-bonds. Kow normalized sorption isotherms for phenanthrene, pyrene, nitrobenzene and DCP were obtained (Figs. S5 and 5). For phenanthrene and pyrene, because they have the same value of solvation parameters, i.e., p, a and b (Table S1), their Kow normalized sorption isotherms fell into a single curve (Fig. S5). While for nitrobenzene and DCP, higher sorption affinity of DCP was observed (Fig. 5). Nitrobenzene have a higher p value than DCP, hence it should have stronger p–p interactions than DCP. Thus the higher sorption affinity of DCP than nitrobenzene is probably attributed to the H-bond interaction. DCP has a higher a value (0.59), indicating it can act as a H-bond donor to form H-bond with BPS while nitrobenzene cannot (a = 0). For all BPS, the maximum volume sorption capacity of phenanthrene was higher than pyrene, while the maximum volume sorption of nitrobenzene was higher than DCP (Table 1). Molecular size is another adsorbate property that will affect sorption (Yang et al., 2006a; Wang et al., 2010b). Molecule cannot sorb on the sites with

3.3. Sorption mechanisms, removal efficiency and regeneration of BPS Linear relationships were observed between the DA model fitted E and b values (Table S4). With increasing b, the E also increased. The r2 of the biphenyl, phenyl, octane and hexane BPS were 0.891, 0.994, 0.991, and 0.975, respectively. E value is the adsorption energy for a given solute and represents all of the interaction forces responsible for adsorption (Yang and Xing, 2010). The linear intrinsic relationship suggests that the b of the DA model is also a parameter to represent the adsorption energy and the interaction forces responsible for adsorption, similar to the parameter E of the DA model (Yang et al., 2008). All DA fitted parameters, i.e., log Q0, E and b, were found to have linear relationships with log Kow (Fig. 4). Kow value is usually regarded as sorbates’ hydrophobicity. Hence, the linear relationship between DA model fitted parameters

2

2.5

20

log Q0

b

E

1.5

15

1

10

2 1.5 1

0.5

0

5

0

2

4

6

0

0.5

0

2

4

6

0

0

2

4

6

log Kow Fig. 4. Relationships between DA model fitted parameters and log Kow: } = biphenyl BPS, j = phenyl BPS, h = octane BPS,  = hexane BPS.

101

101

100

100

10-1

10-1 10-2

qe/Kow (mg/g)

10-2

Phenyl BPS

Biphenyl BPS 10-3 10-1

101

100

10-2

10-3 -1 10

101

101

100

100

10-1

10-1

10-2

10-2

100

10-1

100

101

10-2

Hexane BPS

Octane BPS 10-3 10-2

101

102

10-3 10-1

100

101

10-2

Equilibrium Concentrations, Ce (mg/L) Fig. 5. Kow normalized sorption isotherm of nitrobenzene and DCP on four BPS: } = nitrobenzene and j = DCP.

194

D. Lin et al. / Chemosphere 103 (2014) 188–196

Table 1 Maximum sorption capacities of phenanthrene, pyrene, nitrobenzene and DCP on various sorbents. Sorbent

Chemicals

Q0 (mg g1)a

Q0 (cm3 g1)b

References

0.0172 0.0198 0.0351 0.0423

Yang et al. (2006b) Chen and Yuan (2011) Yuan et al. (2010) Wang et al. (2009) This work This work This work This work

0.0301 0.0322 0.0445 0.0560

Chen and Yuan (2011) Yang et al. (2006b) This work This work This work This work

0.00854 0.00894 0.0129 0.0166

Huang and Chen (2010) Qin et al. (2007) Kato et al. (2008) This work This work This work This work

0.00589 0.00692 0.00631 0.00759

Wu and Yu (2006) Tutem et al. (1998) This work This work This work This work

Phenanthrene Fullerene Biochar (Soil p100) AC-4e MWNT100g Hexane BPS Octane BPS Phenyl BPS Biphenyl BPS

0.0678 9.55c 0.046 19.5c 21.9 25.1 44.7 53.7 Pyrene

Biochar (Soil p100) MWNT15g Hexane BPS Octane BPS Phenyl BPS Biphenyl BPS

0.0398 42.7 35.5 38.0 52.5 66.1 Nitrobenzene 2.54c 0.264d 15c 10.2 10.7 15.5 20

Ash-0 MCM-41f Ox OG-ACe Hexane BPS Octane BPS Phenyl Biphenyl DCP Bio-mass Bituminous shale Hexane BPS Octane BPS Phenyl BPS Biphenyl BPS a b c d e f g

4.07 9.2d 8.13 9.55 8.71 10.5

Q0, maximum sorption capacity. Q0, maximum volume sorption capacity. Q0 was calculated directly from the Freundlich model at their saturated equilibrium concentration. Q0 was obtained directly from the Langmuir model fitted isotherms. AC: activated carbon. MCM: mobile crystalline material. MWNT: multi-walled nanotube.

Fig. 6. Schematic diagram showing the adsorption mechanism of four BPS.

195

D. Lin et al. / Chemosphere 103 (2014) 188–196 Table 2 Percent adsorption, desorption and resorption (%) of 4-CP on BPS (solid liquid ratio was 100 mg:10 mL). Values are the means of triplicates. Sorbents

a b c d

C0a

1st Sorption

1st Desorption c

2nd Sorption

2nd Desorption

3rd Sorption

3rd Desorption

94.1 94.2 93.5 91.1 87.1

93.1 92.9 92.6 89.6 85.9

90.9 89.7 89.1 87.3 84.7

89.5 88.9 87.6 86.1 83.9

Hexylene BPS

0.1 0.25 0.5 0.75 1

98.2 97.8 97.1 94.9 89.9

97.7 97.2 96.3 94.1 89.1

Octylene BPS

0.1 0.25 0.5 0.75 1

98.5 98.1 97.4 95.2 90.3

98.1 97.6 96.5 94.5 89.3

95.1 94.9 93.7 91.7 87.4

94.2 93.5 92.7 89.6 85.7

92.1 91.3 90.1 87.6 85.0

91.2 90.5 88.9 86.9 84.3

Phenyl BPS

0.1 0.25 0.5 0.75 1

98.8 98.7 98.0 95.9 90.9

98.3 98.0 97.6 94.8 90.1

95.7 95.4 95.2 92.2 87.8

94.5 94.7 93.1 89.2 86.3

93.1 92.9 91.3 87.0 85.1

91.9 91.7 90.1 85.8 83.9

Biphenyl BPS

0.1 0.25 0.5 0.75 1

99.3 98.9 98.2 97.1 93.8

98.6 98.2 97.9 96.2 92.2

96.1 95.5 95.3 93.4 90.1

95.1 94.8 94.2 91.0 87.9

93.9 93.3 93.0 90.1 85.5

92.8 91.8 91.7 88.7 84.3

Arylene BPSb

0.1 0.25 0.5 0.75 1

98.6 98.3 97.5 95.3 90.0

>98 NDd ND ND ND

ND ND ND ND ND

ND ND ND ND ND

ND ND ND ND ND

ND ND ND ND ND

Initial 4-CP concentration (mmol L1). Data obtained from Burleigh et al. (2002). The percentage of the total initially added 4-CP mass. Not known.

pore size smaller than their size. Hence, the smaller the adsorbate is, the more sites it can access. Phenanthrene is smaller than pyrene (Table S1), thus resulting in a higher volume sorption capacity. So is DCP compared to nitrobenzene. Generally speaking, Q0 followed an order of biphenyl BPS > phenyl BPS > octane BPS > hexane BPS (Table 1), supported by the data from Burleigh et al. (2002). Burleigh et al. (2002) attributed the higher sorption capacity of aromatic BPS (arylene-bridged) than aliphatic ones to p–p interactions between the aromatic rings of the arylene-bridged sorbent and the phenolic compounds. However, the rigidity of bridging group may also have an effect on sorption. Aromatic bridging groups (biphenyl and phenyl) always result in fixed ‘‘rigid’’ pore structures, while aliphatic bridging groups (octane and hexane) which are more flexible and easily wrapped will form ‘‘soft’’ pore structures (Fig. 6). Compared to ‘‘soft’’ pore structures, ‘‘rigid’’ pore structures BPS have higher average pore diameters (5–5.8 nm). The higher pore diameters will result in more accessible sorption sites for chemical molecules and lead to more sorption. As all four BPS have similar surface areas (within 10% variation, Table S2), ‘‘rigid’’ pore structures BPS (biphenyl and phenyl) will certainly have higher sorption ability. A direct comparison of BPS with other adsorbents is made in Table 1. With regard to other sorbents i.e., multiwalled carbon nanotubes, fullerene, biochar, activated carbon, all cubic mesoporous structured BPS had relatively higher sorption ability for both nonpolar (phenanthrene and pyrene) and polar (nitrobenzene and DCP) chemicals, suggesting their potential application as adsorbents. In addition, we compared removal efficiency of 4-CP on 3D cubic BPS with that on 2D hexagonal BPS (Table 2). Our 3D cubic biphenyl and phenyl BPS showed higher removal efficiencies than 2D hexagonal arylene BPS with the initial 4-CP concentrations range from 0.1 mmol L1 to 1 mmol L1. Even ‘‘soft’’ pore 3D cubic structure BPS (hexane and octane) had similar removal efficiency with the ‘‘rigid’’ pore 2D hexagonal arylene BPS. These results confirmed our hypothesis.

Regeneration is always regarded as an important step for adsorbents. Easy regeneration allows for the repeated use of adsorbent, thus decreasing cost and solid waste generation (Burleigh et al., 2002). Because of high solubility in ethanol, 4-CP was readily desorbed from BPS. A high percentage of sorbed 4-CP was desorbed from BPS (Table 2). For example, at initial 4-CP concentration of 0.1 mmol L1, over 97% of the total initially added 4-CP mass was desorbed from BPS, i.e., almost all of the adsorbed 4-CP was desorbed from BPS. The percent sorption of 4-CP for the second and third sorption was still very high (Table 2), suggesting their potential application as adsorbents. 4. Conclusion High sorption capacity and easy regeneration make ‘‘rigid’’ mesoporous 3D cubic BPS a promising application as adsorbents. Our study suggests that the selection of bridging groups is very important for sorption of organic pollutants. Optimization method regarding the cost of BPS and the removal efficiency are required, which will help to promote the development and application of this mesoporous 3D cubic material. Acknowledgments This work was supported by USDA-AFRI Hatch program (MAS 00978). D.R. Lin thanks the China Scholarship Council to support his study in USA for two years. Appendix A. Supplementary material Infrared spectra of biphenyl, phenyl, octane, and hexane BPS (Fig. S1); TEM images of the cubes of hexane BPS after surfactant extraction (Fig. S2); TEM images for 3D cubic (Fm3n) phenyl and biphenyl BPS (Fig. S3); Sorption isotherms of pyrene, phenanthrene,

196

D. Lin et al. / Chemosphere 103 (2014) 188–196

nitrobenzene and DCP on four BPS fitted by DA model (Fig. S4); Kow normalized sorption isotherm of phenanthrene and pyrene on four BPS (Fig. S5); Selected properties of pyrene, phenanthrene, nitrobenzene and DCP (Table S1); Textural properties of biphenyl, phenyl, octane, and hexane BPS (Table S2); Crystal structure parameters of the BPS materials (Table S3); Regressions of DA model fitted parameter E with b in four different BPS (Table S4); Results of DA, LM and FM model fits to adsorption isotherms of pyrene, phenanthrene, nitrobenzene and DCP on BPS (Table S5). Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2013.11.062. References Asefa, T., MacLachlan, M.J., Coombs, N., Ozin, G.A., 1999. Periodic mesoporous organosilicas with organic groups inside the channel walls. Nature 402 (6764), 867–871. Barrett, E.P., Joyner, L.G., Halenda, P.P., 1951. The determination of pore volume and area distributions in porous substances.1. Computations from nitrogen isotherms. J. Am. Chem. Soc. 73 (1), 373–380. Beltran, F.J., Gomezserrano, V., Duran, A., 1992. Degradation kinetics of paranitrophenol ozonation in water. Water Res. 26 (1), 9–17. Bercic, G., Pintar, A., Levec, J., 1996. Desorption of phenol from activated carbon by hot water regeneration. Desorption isotherms. Ind. Eng. Chem. Res. 35 (12), 4619–4625. Burleigh, M.C., Markowitz, M.A., Spector, M.S., Gaber, B.P., 2002. Porous polysilsesquioxanes for the adsorption of phenols. Environ. Sci. Technol. 36 (11), 2515–2518. Chen, B.L., Yuan, M.X., 2011. Enhanced sorption of polycyclic aromatic hydrocarbons by soil amended with biochar. J. Soils Sediment 11 (1), 62–71. Cho, E.B., Kim, D., Gorka, J., Jaroniec, M., 2009. Three-dimensional cubic (Im3m) periodic mesoporous organosilicas with benzene- and thiophene-bridging groups. J. Mater. Chem. 19 (14), 2076–2081. Huang, W.H., Chen, B.L., 2010. Interaction mechanisms of organic contaminants with burned straw ash charcoal. J. Environ. Sci. – China 22 (10), 1586–1594. Huo, Q.S., Margolese, D.I., Ciesla, U., Demuth, D.G., Feng, P.Y., Gier, T.E., Sieger, P., Firouzi, A., Chmelka, B.F., Schuth, F., Stucky, G.D., 1994. Organization of organicmolecules with inorganic molecular-species into nanocomposite biphase arrays. Chem. Mater. 6 (8), 1176–1191. Ide, Y., Kagawa, N., Sadakane, M., Sano, T., 2012. Sunlight-induced effective heterogeneous photocatalytic decomposition of aqueous organic pollutants to CO2 assisted by a CO2 sorbent, amine-containing mesoporous silica. Chem. Commun. 48 (44), 5521–5523. Inagaki, S., Guan, S., Fukushima, Y., Ohsuna, T., Terasaki, O., 1999. Novel mesoporous materials with a uniform distribution of organic groups and inorganic oxide in their frameworks. J. Am. Chem. Soc. 121 (41), 9611–9614. Jang, H.T., Park, Y., Ko, Y.S., Lee, J.Y., Margandan, B., 2009. Highly siliceous MCM-48 from rice husk ash for CO2 adsorption. Int. J. Greenhouse Gas Contam. 3 (5), 545–549. Kapoor, M.P., Yang, Q.H., Inagaki, S., 2002. Self-assembly of biphenylene-bridged hybrid mesoporous solid with molecular-scale periodicity in the pore walls. J. Am. Chem. Soc. 124 (51), 15176–15177. Kato, Y., Machida, M., Tatsumoto, H., 2008. Inhibition of nitrobenzene adsorption by water cluster formation at acidic oxygen functional groups on activated carbon. J. Colloid Interface Sci. 322 (2), 394–398. Kim, M.J., Ryoo, R., 1999. Synthesis and pore size control of cubic mesoporous silica SBA-1. Chem. Mater. 11 (2), 487–491. Lin, D.R., Hu, L.J., You, H., Williams, R.J.J., 2011. Synthesis and characterization of a nanostructured photoluminescent silsesquioxane containing urea and dodecyl groups that can be patterned on carbon films. Eur. Polym. J. 47 (8), 1526–1533. Loy, D.A., Shea, K.J., 1995. Bridged polysilsesquioxanes – highly porous hybrid organic–inorganic materials. Chem. Rev. 95 (5), 1431–1442. Matatov-Meytal, Y., Sheintuch, M., 2000. Catalytic regeneration of chloroorganicssaturated activated carbon using hydrodechlorination. Ind. Eng. Chem. Res. 39 (1), 18–23. Melde, B.J., Holland, B.T., Blanford, C.F., Stein, A., 1999. Mesoporous sieves with unified hybrid inorganic/organic frameworks. Chem. Mater. 11 (11), 3302– 3308. Merle, T., Pic, J.S., Manero, M.H., Debellefontaine, H., 2010. Comparison of activated carbon and hydrophobic zeolite efficiencies in 2,4-dichlorophenol advanced ozonation. Ozone – Sci. Eng. 32 (6), 391–398.

Nurisepehr, M., Jorfi, S., Kalantary, R.R., Akbari, H., Soltani, R.D.C., Samaei, M., 2012. Sequencing treatment of landfill leachate using ammonia stripping, Fenton oxidation and biological treatment. Waste Manage. Res. 30 (9), 883–887. Oviatt, H.W., Shea, K.J., Small, J.H., 1993. Alkylene-bridged silsequioxane sol–gel synthesis and xerogel characterization – molecular requirements for porosity. Chem. Mater. 5 (7), 943–950. Pan, Y.C., Wu, H.Y., Kao, C.C., Kao, H.M., Shieh, Y.N., Fey, G.T.K., Chang, J.H., Tsai, H.H.G., 2009. Model system for solid-state NMR study on co-condensation behavior of silicon precursors in periodic mesoporous organosilicas. J. Phys. Chem. C 113 (42), 18251–18258. Peng, R., Zhao, D., Dimitrijevic, N.M., Rajh, T., Koodali, R.T., 2012. Room temperature synthesis of Ti-MCM-48 and Ti-MCM-41 mesoporous materials and their performance on photocatalytic splitting of water. J. Phys. Chem. C 116 (1), 1605–1613. Qin, Q.D., Ma, J., Liu, K., 2007. Adsorption of nitrobenzene from aqueous solution by MCM-41. J. Colloid Interface Sci. 315 (1), 80–86. Small, J.H., Shea, K.J., Loy, D.A., 1993. Arylene-bridged and alkylene-bridged polysilsesquioxanes. J. Non-Cryst. Solids 160 (3), 234–246. Tan, B., Vyas, S.M., Lehmer, H.J., Knutson, B.L., Rankin, S.E., 2007. Synthesis of inorganic and organic–inorganic hybrid hollow particles using a cationic surfactant with a partially fluorinated tail. Adv. Funct. Mater. 17 (14), 2500– 2508. Tutem, E., Apak, R., Unal, C.F., 1998. Adsorptive removal of chlorophenols from water by bituminous shale. Water Res. 32 (8), 2315–2324. Wang, X.L., Lu, J.L., Xing, B.S., 2008. Sorption of organic contaminants by carbon nanotubes: influence of adsorbed organic matter. Environ. Sci. Technol. 42 (9), 3207–3212. Wang, X.L., Tao, S., Xing, B.S., 2009. Sorption and competition of aromatic compounds and humic acid on multiwalled carbon nanotubes. Environ. Sci. Technol. 43 (16), 6214–6219. Wang, D., Silbaugh, T., Pfeffer, R., Lin, Y.S., 2010a. Removal of emulsified oil from water by inverse fluidization of hydrophobic aerogels. Powder Technol. 203 (2), 298–309. Wang, X.L., Liu, Y., Tao, S., Xing, B.S., 2010b. Relative importance of multiple mechanisms in sorption of organic compounds by multiwalled carbon nanotubes. Carbon 48 (13), 3721–3728. Weinberger, M., Puchegger, S., Froschl, T., Babonneau, F., Peterlik, H., Husing, N., 2010. Sol–gel processing of a glycolated cyclic organosilane and its pyrolysis to silicon oxycarbide monoliths with multiscale porosity and large surface areas. Chem. Mater. 22 (4), 1509–1520. Whitnall, W., Asefa, T., Ozin, G.A., 2005. Hybrid periodic mesoporous organosilicas. Adv. Funct. Mater. 15 (10), 1696–1702. Wu, J., Yu, H.Q., 2006. Biosorption of 2,4-dichlorophenol from aqueous solution by Phanerochaete chrysosporium biomass: isotherms, kinetics and thermodynamics. J. Hazard. Mater. 137 (1), 498–508. Yang, K., Xing, B.S., 2010. Adsorption of organic compounds by carbon nanomaterials in aqueous phase: polanyi theory and its application. Chem. Rev. 110 (10), 5989–6008. Yang, K., Zhu, L.Z., Xing, B.S., 2006a. Adsorption of polycyclic aromatic hydrocarbons by carbon nanomaterials. Environ. Sci. Technol. 40 (6), 1855–1861. Yang, K., Wang, X.L., Zhu, L.Z., Xing, B.S., 2006b. Competitive sorption of pyrene, phenanthrene, and naphthalene on multiwalled carbon nanotubes. Environ. Sci. Technol. 40 (18), 5804–5810. Yang, K., Wu, W.H., Jing, Q.F., Zhu, L.Z., 2008. Aqueous adsorption of aniline, phenol, and their substitutes by multi-walled carbon nanotubes. Environ. Sci. Technol. 42 (21), 7931–7936. Yang, Z., Juang, Y.C., Lee, D.J., Duan, Y.Y., 2012. Pore blockage of organic fouling layer with highly heterogeneous structure in membrane filtration: role of minor organic foulants. J. Membr. Sci. 411, 30–34. Yoshina-Ishii, C., Asefa, T., Coombs, N., MacLachlan, M.J., Ozin, G.A., 1999. Periodic mesoporous organosilicas, PMOs: fusion of organic and inorganic chemistry ‘inside’ the channel walls of hexagonal mesoporous silica. Chem. Commun. 24, 2539–2540. Yu, G.C., Xue, M., Zhang, Z.B., Li, J.Y., Han, C.Y., Huang, F.H., 2012. A water-soluble pillar 6 arene: synthesis, host-guest chemistry, and its application in dispersion of multiwalled carbon nanotubes in water. J. Am. Chem. Soc. 134 (32), 13248– 13251. Yuan, M.J., Tong, S.T., Zhao, S.Q., Jia, C.Q., 2010. Adsorption of polycyclic aromatic hydrocarbons from water using petroleum coke-derived porous carbon. J. Hazard. Mater. 181 (1–3), 1115–1120. Zhao, Q., Weise, L., Li, P.J., Yang, K., Zhang, Y.Q., Dong, D.B., Li, P., Li, X.J., 2010. Ageing behavior of phenanthrene and pyrene in soils: a study using sodium dodecylbenzenesulfonate extraction. J. Hazard. Mater. 183 (1–3), 881–887.