Hydrophobic poly(alkoxysilane) organogels as sorbent material for oil spill cleanup

Marine Pollution Bulletin xxx (2015) xxx–xxx

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Hydrophobic poly(alkoxysilane) organogels as sorbent material for oil spill cleanup Gulsah Ozan Aydin, Hayal Bulbul Sonmez ⇑ Gebze Technical University, Department of Chemistry, P.O. Box 141, 41400 Gebze, Kocaeli, Turkey

a r t i c l e

i n f o

Article history: Received 25 March 2015 Revised 7 May 2015 Accepted 12 May 2015 Available online xxxx Keywords: Poly(alkoxysilane) organogel UNOXOL™ Alkyltriethoxysilane Oil absorbency Oil spill Reusability

a b s t r a c t In this study, reusable poly(alkoxysilane) organogels with high absorption capacities were synthesized by the condensation of a cyclo aliphatic glycol (UNOXOL™) and altering the chain length of the alkyltriethoxysilanes. The structural and thermal properties of cross-linked poly(alkoxysilane) polymers were determined by FTIR, solid-state 13C and 29Si CPMAS NMR and TGA. The oil absorbency of poly(alkoxysilane)s was determined through oil absorption tests, absorption and desorption kinetics. Results showed that the highest oil absorbency capacities were found to be 295% for hexane, 389% for euro diesel, 428% for crude oil, 652% for gasoline, 792% for benzene, 792% for toluene, 868% for tetrahydrofuran, and 1060% for dichloromethane for the poly(alkoxysilane) gels based on UNOXOL™ and dodecyltriethoxysilane. Owing to their hydrophobic structure, the poly(alkoxysilane) organogels can selectively absorb crude oil from water. The reusability of the absorbents was quantitatively investigated, demonstrating that absorbents can be used effectively at least nine times. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Oil is one of the most important energy resources and raw materials for many chemicals and synthetic polymers used in the modern industrial world (Annunciado et al., 2005; Rengasamy et al., 2011; Wei et al., 2003). During its production, transportation, storage and eventual usage, spills can often occur, resulting not only in energy loss but also significant environmental pollution. This is a major problem which requires the immediate attention of researchers, environmentalists and the petroleum industry. It is necessary to clean the affected body of water after an oil spill because of its catastrophic effect on the ecological environment (Li et al., 2014a; Lin et al., 2012). A variety of methods have been employed to clean oil from water surfaces; mechanical techniques (oil skimmers (Ventikos et al., 2004) and oil containment booms (Wong and Barin, 2003)), in situ combustion (Mullin and Champ, 2003), use of dispersants (Chapman et al., 2007), biodegradation (Atlas, 1995) and use of absorbent materials (Adebajo et al., 2003). The use of absorbent materials has come to prominence because it can be easily applied with an altogether superior efficiency and effectiveness (Chu et al., 2015; Deng et al., 2015; Gu et al., 2014; Li et al., 2014b; Pan et al., 2014; Sabir, 2015; Wang et al., 2014; Wu et al., 2014a).

⇑ Corresponding author. E-mail address: [email protected] (H. Bulbul Sonmez).

Absorbent materials can be classified into the three following major classes: Inorganic absorbents – zeolites (Mazˇeikiene et al., 2005; Sakthivel et al., 2013), silica aerogels (Gurav et al., 2010), graphite (Toyoda and Inagaki, 2003), active carbon, perlite (Bastani et al., 2006), vermiculites (Mysore et al., 2005) and absorbent clay (Carmody et al., 2007). Natural organic absorbents – Straw, corn corb, wood fiber, cotton fiber, cellulosic kapok fiber and kenaf (Adebajo et al., 2003). Synthetic organic absorbents – Polypropylene (Wei et al., 2003), polyurethane foam (Yang et al., 2005), polyesters (Tanaka et al., 2012), polycarbonates, styrene-alkyl acrylate polymers (Jang and Kim, 2000), hydrophobic aerogels (Hrubesh et al., 2001), cellulosic fibers (Payne et al., 2012), polyelectrolyte gels (Ono et al., 2012), cryogels based on rubber (Dogu and Okay, 2008), poly(orthocarbonate)s (Karadag et al., 2011; Sonmez and Wudl, 2005; Yati et al., 2013). Inorganic absorbents do not possess adequate buoyancy retention and their oil absorption capacity is generally low (Choi and Cloud, 1992). Natural sorbents have the advantage of economy and biodegradability but they also have poor buoyancy characteristics, relatively low oil absorption capacity and low hydrophobicity (Adebajo et al., 2003). Synthetic polymeric absorbents however, have favorable hydrophobic and oleophilic properties whilst their high absorption capacity has also attracted attention (Jin et al., 2012). It is generally expected that absorbents employed in the cleanup of oil from water surfaces boast sufficient hydrophobicity, high uptake capacity, high rate of uptake, buoyancy, retention over time, durability in aqueous media, reusability or biodegradability

http://dx.doi.org/10.1016/j.marpolbul.2015.05.033 0025-326X/Ó 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Ozan Aydin, G., Bulbul Sonmez, H. Hydrophobic poly(alkoxysilane) organogels as sorbent material for oil spill cleanup. Mar. Pollut. Bull. (2015), http://dx.doi.org/10.1016/j.marpolbul.2015.05.033

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G. Ozan Aydin, H. Bulbul Sonmez / Marine Pollution Bulletin xxx (2015) xxx–xxx

and recoverability of oil (Ceylan et al., 2009). Unfortunately, it is very difficult for a single material to yield all of these properties. Although studies are still ongoing, there is a pressing need to develop new materials with high and fast uptake abilities that are also ultimately renewable (Karan et al., 2011). One way to obtain good oil absorbents is to use organogels are hydrophobic crosslinked networks have strong ability to absorb oils. These polymeric organogels should be hydrophobic in nature with elastic network and intersticial spaces (Atta et al., 2007; Liu et al., 1996). Organoalkoxysilanes have been widely used as structural units to construct a variety of silica based organic–inorganic hybrid materials (Judeinstein and Sanchez, 1996; Shimojima and Kuroda, 2002; Siramanont et al., 2009). Materials modified with alkoxysilanes to enhance their hydrophobicity have been used as an oil absorbent (Tao et al., 2011; Wu et al., 2014b), whilst gels based on organoalkoxysilanes have also enjoyed some use (John and Reynolds, 2001). Organic–inorganic absorbents have been prepared from alkoxysilanes by sol–gel methods (Chirica and Remcho, 2000; Patrushev and Sidelnikov, 2013). Condensation of alkoxysilanes by bulk polymerization processes could be an alternative to the sol–gel equivalent. Previously, we synthesized crosslinked poly(orthosilicate)s based on tetraethyl orthosilicate (TEOS) with hydroxyl functional monomers (cyclohexanediols (Karadag et al., 2010), cyclohexanedimethanols (Sonmez et al., 2011) and linear aliphatic diols (Karadag and Sonmez, 2013) which have fast and high organic solvent absorption properties with oil-derived pollution, reusability and good thermal stability. Recently, we synthesized crosslinked poly(alkoxysilane)s as an absorbent for organic solvent and oil-derived fuel from n-octyltriethoxysilane crosslinker and linear aliphatic diols (Karadag and Bulbul Sonmez, 2013). We investigated why TEOS-derived alkyltriethoxysilanes (having 3-functional ethoxy groups) were used instead of TEOS (having 4-funtional ethoxy groups); ultimately, poly(alkoxysilane)s’ oil absorbency was increased due to the decreasing of the density of crosslink points. Additionally, the effects of increasing the linear chain lengths of the aliphatic diols on the absorbency of organic solvents were also investigated. In one of our previous studies, poly(alkoxysilane) absorbents synthesized from an aromatic glycol (1,3-benzenedimethanol), the different chain length of alkyltriethoxysilanes and the effect of the alkyltriethoxysilanes’ chain length on the organic solvent and oil-derived fuels’ absorption properties were examined systematically (Kizil et al., 2015). In the present study, we synthesized novel poly(alkoxysilane) organogels as an oil absorbent through the condensation of a cyclo aliphatic glycol (UNOXOL™) and altering the chain length of the alkyltriethoxysilanes (from ethyltriethoxysilane to hexadecyltriethoxysilane). We attempted to obtain a higher oil absorption capacity for poly(alkoxysilane) organogels synthesized using a cyclo aliphatic glycol (UNOXOL™) instead of an aromatic glycol (1,3-benzenedimethanol). By using UNOXOL™ instead of 1,3-benzenedimethanol, p–p interaction of chains of polymer eliminated thus, holes of polymer network increase. Ultimately, oil absorption of poly(alkoxysilane) organogels increased thus oil molecules diffuse to network, easily. 2. Experimental 2.1. Materials The mixture of 1,3–1,4 cyclohexanedimethanol (UNOXOL™) was obtained Dow Chemical Company, USA. Ethyltriethoxysilane

(ETES), hexyltriethoxysilane (HTES), octyltriethoxysilane (OTES), decyltriethoxysilane (DTES) were purchased from Alfa Aesar. Dodecyltriethoxysilane (DDTES) were received Aldrich. Butyltriethoxysilane (BTES), hexadecyltriethoxysilane (HDTES) and octadecyltriethoxysilane (ODTES) were supplied by Gelest. All monomers were used without further purification. Dichloromethane (DCM), tetrahydrofuran (THF), benzene, toluene and hexane were obtained from Aldrich, and all solvents were used as received. 95 octane gasoline and euro diesel were purchased from British Petroleum (BP) and crude oil was obtained TUPRAS (Turkish Petroleum Refineries Co.). Seawater and lake water were respectively obtained from Sea of Marmara and Sapanca Lake in Turkey. 2.2. Characterization Fourier transform infrared spectroscopy (FT-IR) spectra were recorded on a Perkin-Elmer Spectrum 100 FT-IR spectrophotometer with an attenuated total reflectance (ATR) objective. Solid state 13 C and 29Si cross polarization magic angel spinning nuclear magnetic resonance (CMPAS NMR) spectra were recorded Bruker Superconducting FT-NMR Spectrometer Avance TM 300 MHz WB. Thermogravimetric analysis (TGA) was performed under an argon atmosphere at 10 °C min1 using a Mettler Toledo model TGA/SDTA 851 (Mettler Toledo, Greifensee, Switzerland). Elemental analysis was obtained using a LECO, CHNS-932 analyzer. Innova 2000/platform shaker was used in order to increase polymer–solvent interaction. 2.3. Synthesis of poly(alkoxysilane) organogels A series of poly(alkoxysilane) organogels were synthesized by using UNOXOL™ monomer and alkoxysilane crosslinking agents with different alkyl chain lengths. A moderately high temperature was applied (160 °C) in an argon atmosphere using a Pyrex (75 mL) pressure vessel (Chemglass, Vineland, NJ) to produce a condensation reaction. In this study, eight kinds of alkoxysilane, from ethyltriethoxysilane to octadecyltriethoxysilane with different alkyl chain lengths were used to obtain the poly(alkoxysilane) organogels (Scheme 1). Synthesized organogels were named according to the carbon number of alkyltriethoxysilane crosslinking agents so they could be understood more easily. For the synthesis of Poly C2, Poly C4, Poly C6, Poly C8, Poly C10, Poly C12, Poly C16 and Poly C18 the following alkoxysilane crosslinking agents were used, respectively: ethyltriethoxysilane (C2), butyltriethoxysilane (C4), hexyltriethoxysilane (C6), octyltriethoxysilane (C8), decyltriethoxysilane (C10), dodecyltriethoxysilane (C12), hexadecyltriethoxysilane (C16), and octadecyltriethoxysilane (C18). 2.3.1. Synthesis of Poly C2 The reaction was carried out with ETES (0.52 mL, 2.40 mmol) and UNOXOL™ (0.52 g, 3.60 mmol) for 1 day in a Pyrex pressure vessel placed in an oil bath at 160 °C, until gelation occurred. The polymer was then washed with water, ethanol, acetone and dichloromethane and dried in a vacuum for 24 h: 0.41 g of a glassy, light yellow, cross-linked polymer was obtained. FTIR: 3596–3175, 2919, 2857, 1450, 1071, 932, 832 cm1. 13C CPMAS NMR: 68.0, 58.5, 40.8, 30.0, 19.0, 7.0 ppm. 29Si CPMAS NMR: 44.0, 52.0, 59.5 ppm. Elemental analysis [C26H47O6Si]n: Theoretical: C 64.56%, H 9.79%. Experimental: C 59.98%, H 9.41%. 2.3.2. Synthesis of Poly C4 BTES (0.64 mL, 2.56 mmol) and UNOXOL™ (0.55 g, 3.85 mmol) were allowed to react for 3 days in a Pyrex pressure vessel placed in an oil bath at 160 °C, until gelation occurred. The polymer was then washed with water, ethanol, acetone and dichloromethane and dried in a vacuum for 24 h to produce 0.49 g of a glassy,

Please cite this article in press as: Ozan Aydin, G., Bulbul Sonmez, H. Hydrophobic poly(alkoxysilane) organogels as sorbent material for oil spill cleanup. Mar. Pollut. Bull. (2015), http://dx.doi.org/10.1016/j.marpolbul.2015.05.033

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G. Ozan Aydin, H. Bulbul Sonmez / Marine Pollution Bulletin xxx (2015) xxx–xxx

Poly C2 Poly C18

Poly C4 O O O Si

O O Si

O

O O Si O

OH

O O Si O

O Si O O

HO

Poly C16

Poly C6 HO

OH

O Si O O O Si O O O

Si O O

Poly C12

Poly C8 Poly C10

Scheme 1. Polymerization reactions of UNOXOL™ with different alkyl chain lengths alkoxysilane crosslinking agents.

colorless, cross-linked polymer. FTIR: 3500–3177, 2917, 2862, 1452, 1075, 957, 832 cm1. 13C CPMAS NMR: 64.0, 54.0, 36.0, 25.0, 22.0, 20.0, 14.0, 9.4, 5.7 ppm. 29Si CPMAS NMR: 61.0, 68.0 ppm. Elemental analysis [C28H51O6Si]n: Theoretical: C 65.71%, H 10.04%. Experimental: C 62.36%, H 10.41%. 2.3.3. Synthesis of Poly C6 The reaction of HTES (1.10 mL g, 3.79 mmol) and (0.82 g 5.69 mmol) UNOXOL™ at 160 °C was reacted for 5 days in a Pyrex pressure vessel until gelation occurred. After the polymer was washed with water, ethanol, acetone and dichloromethane, it was dried in a vacuum for 24 h to yield 0.74 g of a white, glassy polymer. FTIR: 3590–3200, 2922, 2861, 1449, 1071, 959, 826 cm1. 13 C CPMAS NMR: 69.0, 59.0, 41.0, 33.0, 30.0, 24.0, 12.0 ppm. 29Si CPMAS NMR: 45, 71 ppm. Elemental analysis [C30H55O6Si]n: Theoretical: C 66.74%, H 10.27%. Experimental: C 64.61%, H 10.17%. 2.3.4. Synthesis of Poly C8 The reaction was carried out with OTES (1.57 mL, 4.99 mmol) and UNOXOL™ (1.08 g, 7.49 mmol) for 5 days in a Pyrex pressure vessel placed in an oil bath at 160 °C, until gelation occurred. The synthesized polymer was washed with water, ethanol, acetone and dichloromethane and then dried in a vacuum for 24 h. 0.99 g of a glassy, white polymer was obtained. FTIR: 3569–3202, 2919, 2858, 1468, 1076, 952, 835 cm1. 13C CPMAS NMR: 68.0, 40.3, 29.0, 11.0 ppm. 29Si CPMAS NMR: 60.0, 67.0 ppm. Elemental analysis [C32H59O6Si]n: Theoretical: C 67.68%, H 10.47%. Experimental: C 65.24%, H 10.65%. 2.3.5. Synthesis of Poly C10 DTES (1.42 mL, 4.10 mmol) and UNOXOL™ (0.89 g, 6.15 mmol) were allowed to react for 5 days in a Pyrex pressure vessel placed in an oil bath at 160 °C, until gelation occurred. The polymer was then washed with water, ethanol, acetone and dichloromethane and dried in a vacuum for 24 h to produce 0.91 g of a glassy, white, cross-linked polymer. FTIR: 3604–3217, 2916, 2854, 1452, 1077,

948, 828 cm1. 13C CPMAS NMR: 68.0, 40.0, 30.0, 10.0 ppm. 29Si CPMAS NMR: 60, 67 ppm. Elemental analysis [C32H59O6Si]n: Theoretical: C 68.52%, H 10.66%. Experimental: C 68.39%, H 11.05%. 2.3.6. Synthesis of Poly C12 The reaction was carried out with DDTES (1.26 mL, 3.66 mmol) and UNOXOL™ (0.79 g, 5.49 mmol) for 4 days in a Pyrex pressure vessel placed in an oil bath at 160 °C, until gelation occurred. The polymer was then washed with water, ethanol, acetone and dichloromethane and dried in a vacuum for 24 h: 0.56 g of a glassy, white, cross-linked polymer was obtained. FTIR: 3522–3200, 2919, 1449, 1071, 955, 835 cm1. 13C CPMAS NMR: 68.0, 66.0, 57.0, 41.0, 33.0, 30.0 23.0, 18.0, 14.0, 10.0 ppm. 29Si CPMAS NMR: 60.0, 67.0 ppm. Elemental analysis [C36H67O6Si]n: Theoretical: C 69.29%, H 10.82%. Experimental: C 67.83%, H 11.34%. 2.3.7. Synthesis of Poly C16 HDTES (1.00 mL, 2.37 mmol) and UNOXOL™ (0.51 g, 3.55 mmol) were allowed to react for 2 h in a Pyrex pressure vessel placed in an oil bath at 160 °C, until gelation occurred. The synthesized polymer was washed with water, ethanol, acetone and dichloromethane, and then dried in a vacuum for 24 h. 0.56 g of a glassy white polymer was obtained. FTIR: 3599–3179, 2925, 2852, 1456, 1076, 959, 833 cm1. 13C CPMAS NMR: 64.0, 36.0, 26.0, 19.0, 6.0 ppm. 29Si CPMAS NMR: 60.0, 67.0 ppm. Elemental analysis [C40H75O6Si]n: Theoretical: C 70.64%, H 11.12%. Experimental: C 69.41%, H 11.55%. 2.3.8. Synthesis of Poly C18 The reaction was carried out with ODTES (1.15 mL, 2.40 mmol) and UNOXOL™ (0.52 g, 3.60 mmol) for 1.5 days in a Pyrex pressure vessel placed in an oil bath at 160 °C, until gelation occurred. The polymer was then washed with water, ethanol, acetone and dichloromethane and dried in a vacuum for 24 h. 0.70 g of a glassy, light yellow, cross-linked polymer was obtained. FTIR: 3563–3229, 2918, 2855, 1471, 1073, 955, 831 cm1. 13C CPMAS NMR: 64.0,

Please cite this article in press as: Ozan Aydin, G., Bulbul Sonmez, H. Hydrophobic poly(alkoxysilane) organogels as sorbent material for oil spill cleanup. Mar. Pollut. Bull. (2015), http://dx.doi.org/10.1016/j.marpolbul.2015.05.033

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53.0, 36.0, 28.0, 26.0, 19.0 ppm. 29Si CPMAS NMR: 60.0, 67.0 ppm. Elemental analysis [C42H79O6Si]n: Theoretical: C 71.23%, H 11.24%. Experimental: C 70.34%, H 11.39%. 2.4. Extraction of soluble fraction During the gelation process, some polymer chains that are not attached to the infinite network are subsequently named soluble fraction (SF). The probable SF can be extracted from gel fraction. For this purpose, a weighed quantity of polymer was put in a solvent and the SF was extracted three times at room temperature over a 24 h period by using DCM. After its extraction, the swelled polymer was dried in a vacuum at room temperature for 24 h. The SF was estimated according to the following Eq. (1).

ðW 0  WÞ SF ð%Þ ¼  100 W0

ð1Þ

where W0 and W are the weights of the polymers before and after the extraction, respectively (Atta and Arndt, 2003). 2.5. Determination of oil absorption capacity of poly(alkoxysilane) organogels Oil absorbency was determined by a weighing method: The swelling behavior of poly(alkoxysilane) organogels was determined with the oils; dicloromethane, tetrahydrofuran, hexane, benzene and toluene. Bags prepared from filter paper were used to determine the extent to which the organogels swelled. First, the empty bags were immersed in the oil and blotted quickly, then weighed and recorded. A dried gel sample of known weight was placed in the bag. The bag filled with organogel was immersed in the oil and allowed to equilibrate for 24 h at room temperature. Excess oil was removed and the surfaces dried gently by blotting – they were then weighed in a stoppered weighing bottle at room temperature. The oil-uptake percentages were calculated with the following Eq. (2):

Oil absorbency ð%Þ ¼

Ws  Wb  Wd  100 Wd

ð2Þ

where Wb is the weight of the empty bag after the oil treatment, Wd is the weight of dry polymer, Ws is the weight of the bag including swollen gel was measured (Zhou et al., 2000). The absorption capacities were also calculated for the gels in different oils, such as gasoline, diesel and crude oil as explained previously. Crude oil’s absorption capacity was determined by directly immersing it in oil. Then, it was studied for both natural seawater and lake water alike and also simulated seawater (3.5% w/w NaCl solution). For this experiment; 20 mL crude oil and 30 mL water were added to a bottle, forming an oil and water mixture suspension. The swelling measurements were then conducted as explained above. 2.6. Absorption kinetics of poly(alkoxysilane) organogels Absorption kinetics measurements were conducted using the procedure described above, using DCM. After taking away the bags from the solvent at various time intervals, they were blotted quickly to get rid of DCM attached to the surface and then weighed. To understand the sorption process, the experimental data was further analyzed using kinetic models (Dutta et al., 2013; Schott, 1992). For first order kinetics, the swelling rate is expressed as shown in the following equation:

dQ ¼ kðQ max Q Þ dt

ð3Þ

where Q is the weight of the swollen gel at time t, Qmax is the swollen gel at equilibrium. Integration of Eq. (1) gives:

ln



Q max Q max Q



¼ k1 t

ð4Þ

Plotting ln (Qmax/QmaxQ) versus t gives a straight line, swelling process follows first order kinetics. The second order rate for the swelling process is shown in the following equation:

dQ ¼ k2 ðQ max Q Þ2 dt

ð5Þ

Integrating;

t 1 1 ¼ þ t Q k2 Q max2 Q max

ð6Þ

If the plot of t/Q versus time is a straight line, the swelling tends to follow second order kinetics. 2.7. Desorption kinetics of poly(alkoxysilane) organogels After the swollen polymer reached maximum absorbent capacity, the DCM retention of the polymer in air was determined by weighing the swollen polymer as a function of time. 3. Results and discussions 3.1. Synthesis of poly(alkoxysilane) organogels In this study, poly(alkoxysilane) organogels with hydrophobic characteristics were synthesized by using UNOXOL™ monomer and alkoxysilane crosslinking agents with different alkyl chain lengths for use as an oil absorbent. To examine the effect of using different alkoxysilane monomers on absorbent properties, starting from ethyltriethoxysilane to octadecyltriethoxysilane which have even number of carbon atoms were used and eight polymers were synthesized (Scheme 1). A condensation reaction was carried out between –OH groups of UNOXOL™ and –OCH2CH3 groups of alkoxysilane crosslinking agents at 160 °C in an argon atmosphere. The reaction occurred in a solvent-free medium in one step without using a catalyst by bulk polymerization. 3.2. Characterization of poly(alkoxysilane) orgonogels The resulting organogels were characterized by Fourier transform infrared spectroscopy (FTIR) and solid-state cross-polarization magic angle spinning (CPMAS) 13C and 29Si NMR, and the thermal properties were analyzed by thermal gravimetric analysis (TGA). A schematic representation of the proposed structure of poly(alkoxysilane) organogels was shown in Scheme 2. The appearance of a broad absorption band between around 3600 cm1 and 3175 cm1 indicates stretching vibrations of the hydroxyl group originating from the UNOXOL™ monomers at the end of the chain of organogels. At around 2920 cm1 and 2850 cm1 stretching vibrations of the aliphatic C–H bonds were observed. The strong stretching vibrations detected at approximately 1070–1080 cm1, 960–950 cm1, 840–820 cm1 indicated the presence of Si–O–C, Si–OH and Si–O–CH, respectively (Fig. 1). The chemical structure evolution of poly(alkoxysilane)s was also confirmed through solid state CPMAS 13C and 29Si NMR analysis, as shown in Fig. 2 for Poly C12, as an example. The resonances at 68 and 66 ppm indicated that CH2–OH carbons have different chemical environments. The signal at 57 ppm showed (–CH2–O– Si) carbons. The signals between 41 and 33 ppm illustrated that – CH and CH2 of the cycloaliphatic ring carbons originated from the UNOXOL™ monomer. With different chemical environments, (–CH2–) aliphatic linear carbons originating from the alkoxysilane crosslinking agents were observed between 30 and 23 ppm.

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OH O O Si O

HO

HO

OH

Condensation Polymerization

OH

-

o

160 C, Argon

CH3

O O Si O

O O

Si

O

O Si O O

Ethoxy End Group

O Si O O

Crosslinking Point

O O Si O

O O

O O

O O Si

Si

O

O

O Si O

Si O

O O Si O

O O

Repeating Unit

O

Si

O O

O

O O Si O

O

O Si O

O O Si O

O

Si

Si

O

O Si O O O O Si O

O HO

O

Hydroxyl End Group

Scheme 2. Schematic representation of proposed poly(alkoxysilane) organogel network structure for Poly C6.

(a)

Fig. 1. FTIR spectra of poly(alkoxysilane) organogels.

Carbons of –CH3 of ethoxy end groups (–O–CH2–CH3), and alkoxysilane end groups (–Si–(CH2)11–CH3) were determined by the resonances at 18 and 14 ppm respectively. The signal between 10 ppm is evidence of carbons from the (CH2–Si–O) group (Fig. 2).

Fig. 2. The solid state CPMAS (a)

(b)

13

C NMR and (b)

29

Si NMR spectra of Poly C12.

Solid state 29Si CPMAS NMR spectra have been often used to assign the chemical environment around the Si atoms. The resonances at approximately 67 and 60 ppm indicate that ethoxy and hydroxy groups are found at the end of the chain due to the

Please cite this article in press as: Ozan Aydin, G., Bulbul Sonmez, H. Hydrophobic poly(alkoxysilane) organogels as sorbent material for oil spill cleanup. Mar. Pollut. Bull. (2015), http://dx.doi.org/10.1016/j.marpolbul.2015.05.033

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silicons of groups that came from –Si–OCH2CH3 and –Si–OH. These results clearly indicated the incorporation of alkoxysilanes into the polymeric gels (Fig. 2). Thermogravimetric analysis (TGA) was used to study the thermal properties of the synthesized organogels. TGA curves do not show weight loss below 100 °C, indicating that no water or ethanol was retained in the gels. TGA shows the organogels have good thermal stability with a thermal decomposition temperature over 300 °C (Fig. 3). With reference to the mass loss at onset for each organogel, their individual thermal stability is arranged; Poly C12 > Poly C8 > Poly C18 > Poly C10 > Poly C6 > Poly C4 = Poly C16 > Poly C2. The weight residue remaining at 900 °C is regarded as the real silica content and was 20.6% in Poly C2, 19.6% in Poly C4, 16.3% in Poly C6, 10.7% in Poly C8, 13.3% in Poly C10, 11.5% in Poly C12, 10.9% Poly C16, 14.8% Poly C18. 3.3. Soluble fraction of poly(alkoxysilane) organogels During the synthesis of organogels, soluble polymer chains are not attached to the polymeric network. These structures constitute soluble fractions (SFs) which can negatively influence the absorbency of the organogels. Before determining oil absorbency, the SF must be removed from the polymeric network to prevent the negative influence of the organogels’ oil absorbency. For that purpose, a known quantity of dried organogel was added to the solvent (DCM), and the SFs were extracted three times over a 24 h period, at room temperature. Then, the organogels were dried in a vacuum at room temperature for 48 h. The SF values were determined to be 17.8% in Poly C2, 8.9% in Poly C4, 16.9% in Poly C6, 28.7% in Poly C8, 23.9% in Poly C10, 22.4% in Poly C12, 9.1% in Poly C16, 14.8% in Poly C18. The percentage of the extracted SF depends on the type and concentration of the monomers and crosslinking agents. In the next step, the absorption capacities of organogels were determined by a swelling test in oils at room temperature. All of the swelling data was obtained by averaging at least four measurements for each polymer sample. 3.4. Oil absorbency of poly(alkoxysilane) orgonogels It has been reported that the oil absorptivity of materials ultimately depends on the bulkiness and the length of their alkyl substituent (Jang and Kim, 2000). The length of the alkyl substituent increases the hydrophobicity of the polymer as a result of the interaction by the van der Waals force between oil and polymer and the oil absorption of polymer increase.

Fig. 3. TGA thermograms of poly(alkoxysilane) organogels.

Alkoxysilane crosslinking agents with different alkyl chain lengths were used to examine the effect of hydrophobicity (Scheme 1) on the absorption capacity of the organogels. The synthesized poly(alkoxysilane) organogels were insoluble in oils such as DCM, THF, hexane, benzene, toluene, gasoline, euro diesel and also crude oil, however, in these liquids they have good swelling abilities. The absorption capacities of the poly(alkoxysilane) organogels for oils such as dicloromethane, tetrahydrofuran, hexane, benzene, toluene, gasoline, diesel and also crude oil were investigated, and the results are presented in Fig. 4. All of the poly(alkoxysilane) organogels have a very high swelling capacity for these liquids. The present investigation aims to show the effect of alkyl chain length on the sorbent properties of poly(alkoxysilane) organogels. Their absorption capacities were ranked as follows: Poly C12 > Poly C10 > Poly C8 > Poly C6 > Poly C4  Poly C16 > Poly C2 > Poly C18. This ranking indicates that the alkyl chain length of alkoxysilane crosslinking agents plays an important role in the capacity of the poly(alkoxysilane) organogels’ oil absorbency. Increasing number of carbons of the alkyl chain length of alkoxysilane crosslinking agent resulted with higher swelling capacity because of hydrophobicity of the polymer increase. While low oil absorbency was observed for organogels synthesized from ethyltriethoxysilane with the short alkyl chain, the highest oil absorbency was found in organogels synthesized from the dodecyltriethoxysilane crosslinking agent. As shown in Fig. 4, the highest oil absorbency values are 1060% for DCM, 868% for THF, 792% for benzene, 792% for toluene, 652% for gasoline, 389% for euro diesel and 295% for hexane. Nevertheless, organogels synthesized from hexadecyltriethoxysilane and octadecyltriethoxysilane crosslinking agents possess low oil absorbency, albeit with a longer alkyl chain. Because an extension of the alkyl group caused chain entanglement and then diffusion of oil molecules are blocked in the network. In other words, the oil-polymer interaction decreases because of these phenomena. Therefore, the solvent absorbency of the organogels was irregularly reduced when the number of carbon atoms exceeded 12. In Table 1, a comparison of the maximum sorption capacity of the UNOXOL-based poly(alkoxysilane) with other, previously reported sorbents was listed. In previous work we have examined absorbent properties of the poly(alkoxysilane) gels based on 1,3 benzenedimethanol and different alkoxysilane monomers. When we compared the swelling values of these two series of polymers,

Fig. 4. Oil absorbency of poly(alkoxysilane) organogels.

Please cite this article in press as: Ozan Aydin, G., Bulbul Sonmez, H. Hydrophobic poly(alkoxysilane) organogels as sorbent material for oil spill cleanup. Mar. Pollut. Bull. (2015), http://dx.doi.org/10.1016/j.marpolbul.2015.05.033

7

G. Ozan Aydin, H. Bulbul Sonmez / Marine Pollution Bulletin xxx (2015) xxx–xxx Table 1 The highest absorption capacities of organogels based on different ethoxysilane crosslinking agents and diols. Polymer

Type of oil

Absorption capacity (%)

Reference

Diol monomers

Ethoxysilane monomers

UNOXOL™

TEOS

DCM THF Benzene

204 143 137

Sonmez et al. (2011)

BDM

ETES

DCM THF Benzene DCM THF Benzene

433 405 148 725 584 416

Kizil et al. (2015)

DCM THF Benzene DCM THF Benzene

554 488 235 1060 868 792

DDTES

UNOXOL™

ETES

DDTES

we noted a higher oil absorption capacity for poly(alkoxysilane) organogels synthesized from a cyclo aliphatic glycol (UNOXOL™) than an aromatic glycol (1,3-benzenedimethanol). When the oil absorbency of the poly(alkoxysilane) organogel-based DDTES and BDM (with the highest oil absorbency in the series) was compared to the oil absorbency of the poly(alkoxysilane) organogel-based DDTES and UNOXOL™ (with the highest oil absorbency in the series) increased by a factor of 1.5-fold for DCM, 1.5-fold for THF 1.9-fold for benzene, as can see from Table 1. As we expected for the synthesis of poly(alkoxysilane) organogels, when UNOXOL™ was used instead of 1,3-benzenedimethanol, p–p interaction of chains of polymer eliminated thus, holes of polymer network increased. Also, cycloaliphatic diol monomer is an isomer of 1,3 and 1,4 cyclohexanedimethanol and this positively influenced swelling ability and the values consequently increased. Ultimately, oil absorption of poly(alkoxysilane) organogels increased due to oil molecules diffusing more easily. Similarly, when comparing the swelling ability of a polymer, which was synthesized from the condensation of TEOS with UNOXOL with this present work, the latter has higher values than the former. This is because when the TEOS-derived alkyltriethoxysilanes (having 3-functional ethoxy groups) were used instead of TEOS (having 4-functional ethoxy groups) the oil absorbency of poly(alkoxysilane)s was increased due to the decreasing of the density of crosslinking points. When we compared the oil absorbency values with our previous work, oil absorbency of the poly(alkoxysilane) organogel-based on ETES (having the shortest alky chain) and UNOXOL™ increased by a factor of 2.7-fold for DCM, 3.4-fold for THF and 1.7-fold for benzene. If we compared the oil absorbency of poly(alkoxysilane) organogel-based on DDTES and UNOXOL™ (having the highest oil absorbency in the poly(alkoxysilane) series), we showed that it increased by a factor of 5.2-fold for DCM, 6.1-fold for THF and 5.8-fold for benzene. Thus, improved absorption properties can be obtained by both decreasing the crosslinking density and also increasing the hydrophobic interaction by alkyl groups. We can easily conclude from these results that poly(alkoxysilane) gels have a better oil absorption capacity than poly(orthosilicate) organogels. After observing the swelling properties of different organic solvents and petroleum-derived oils, to examine the crude oil sorption ability of the gels, Poly C12 was chosen as a representative example and the crude oil sorption capacity of the polymer was determined by directly immersing it in crude oil without any water and noting its absorption capacity to be about 420%. Sorption selectivity between oils and water is an important parameter for

This study

the cleaning of water from oil by using a sorbent. For that reason, the oil absorption capacities of Poly C12 was also investigated in oil/water, oil/natural seawater, oil/lake water and also oil/simulated seawater. The amount of water sorption is also shown, which allows an indication of sorbent selectivity between oils and water (Fig. 5). When the gels were placed on the surface of the crude oil and water mixture, they floated on the surface and did not submerge. The sorbents’ ability to float on the surface of the water before and after absorption is important for practical applications. Fig. 5a shows that the absorption capacity of Poly C12 was not change either the oil was taken from the water surface or not and also the swelling capacity of the polymer did not change what kind of water in the present. As can be seen from Fig. 5, Poly C12 completely absorbed oil from the water surface without absorbing the water itself – any that remained and the swollen gels and crude oil could then be easily separated by filtration (Fig. 5b–d). This implies that poly(alkoxysilane) organogels can be effectively used to remove crude oil from the water surface without any change in its capacity to absorb oil. 3.5. Effect of reaction time on the soluble fraction and oil absorbency By changing the reaction time from 2 h to 7 h and keeping other conditions constant, the influence of reaction time on the DCM absorbency is shown in Table 2. As reaction time increased from 2 h to 7 h, the absorbency of DCM decreased irregularly for Poly C16. It can be explained that as reaction times increase the crosslinking degree of organogels increase, the holes of the polymer network decrease and the subsequent diffusion of solvent molecules is blocked in the network. Similarly, an increasing reaction time resulted with decrease in soluble fraction. 3.6. Absorption and desorption kinetics of poly(alkoxysilane) organogels For applications involving absorbent materials with high absorption capacities, a fast absorption rate is also an important parameter. In the same way, easy desorption of the absorbed oil is also required. For that purpose the absorption and desorption kinetics of the polymers in DCM – which was the most absorbable solvent – were investigated (Fig. 6). For kinetic studies, the solvent absorption capacities were measured by immersing a known amount of dry gel in a solvent and then noting the weight of the swollen gel at different time intervals (5 min, 15 min, 30 min, 45 min, 1 h, etc.). The results showed that all the synthesized

Please cite this article in press as: Ozan Aydin, G., Bulbul Sonmez, H. Hydrophobic poly(alkoxysilane) organogels as sorbent material for oil spill cleanup. Mar. Pollut. Bull. (2015), http://dx.doi.org/10.1016/j.marpolbul.2015.05.033

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G. Ozan Aydin, H. Bulbul Sonmez / Marine Pollution Bulletin xxx (2015) xxx–xxx

Fig. 5. (a) Crude oil absorption capacity of Poly C12, (b–d) Process of separating oil from water surface by using the Poly C12.

Table 2 The effect of reaction time on oil absorbency of Poly C16. Reaction time (h)

Oil absorbency (%)

Soluble fraction (%)

2 3 4 5 6 7

579 347 342 184 152 149

9.1 8.3 8.4 6.3 2.8 2.7

crosslinked polymers had fast and high solvent uptake capabilities. The saturation time of each of the synthesized polymers was found to be around 10–50 min. The sorption and diffusion of oils through cross-linked polymer networks has been a subject of great interest (Errede, 1986; Poh et al., 1987). Information about the mechanism of absorption can be acquired from the study of absorption kinetics. In Fig. 7, a linear relationship is observed for all the polymers which are evaluated from the plot of t/h versus t from Eq. (4) and it confirms that the swelling follows second order kinetics. The maximum oil

Fig. 6. Absorption kinetics of poly(alkoxysilane)s in DCM at room condition.

absorption capacity (Qmax) and k2 values can be obtained by intercepting the linear plots and experimental data provided in Fig. 7 where correlating coefficient values (R2) are greater than 0.999 for all polymers presented in Table 3. The solvent retention kinetics for the organogels was studied by investigating the evaporation of DCM swollen gels in air at room temperature as a function of time. As shown in Fig. 8, gels released more than 95% absorbed DCM within 20–25 min. The remaining DCM was released in 35 min. After this process, poly(alkoxysilane) organogels can be used again and the results also showed that the poly(alkoxysilane) organogels can be used in the recovery of volatile, organic compounds. 3.7. Reusability of poly(alkoxysilane) organogels In addition to these results, the recyclability of absorbents and the preservation of their ability to efficiently remove pollutants also play an important role in environmental applications because of economic usages. In this respect, the swelling behavior of the

Fig. 7. t/Q versus t plot for poly(alkoxysilane) organogels.

Please cite this article in press as: Ozan Aydin, G., Bulbul Sonmez, H. Hydrophobic poly(alkoxysilane) organogels as sorbent material for oil spill cleanup. Mar. Pollut. Bull. (2015), http://dx.doi.org/10.1016/j.marpolbul.2015.05.033

G. Ozan Aydin, H. Bulbul Sonmez / Marine Pollution Bulletin xxx (2015) xxx–xxx Table 3 Parameters from second order kinetic model for poly(alkoxysilane) organogels. Polymer Poly Poly Poly Poly Poly Poly Poly Poly

C2 C4 C6 C8 C10 C12 C16 C18

Qmax 555.56 709.22 751.88 740.74 1036.27 1138.95 578.03 377.36

k (gg1 min1) 4

9.729  10 11.492  104 31.815  104 5.748  104 4.122  104 4.122  104 32.010  104 68.848  104

R2 0.99987 0.99993 0.99999 0.99995 0.99971 0.99982 0.99996 0.99998

9

temperature in one step without using a catalyst. We further investigated some factors on their usage as an absorbent for oils and crude oil. All the synthesized poly(alkoxysilane) organogels have a high absorption capacity and also possess excellent reusability properties. The poly(alkoxysilane) organogels were thermally stable at temperatures of less than 300 °C. The highest oil absorbency was about 295% for hexane, 389% for euro diesel, 652% for gasoline, 792% for benzene, 792% for toluene, 868% for tetrahydrofuran (THF) and 1060% for dichloromethane (DCM), the Poly C12 based on UNOXOL™ and dodecyltriethoxysilane. The crude oil absorption capacity of Poly C12 was determined directly through its immersion in crude oil and found to be about 428%. As the length of the alkyl chain in the alkoxysilane monomers increased, so did the oil absorbency. All the synthesized polymers, which reached about 95% of their maximum capacity in approximately 10–50 min in DCM had fast and high absorption capabilities; poly(alkoxysilane) organogels also released the absorbed DCM within 20–55 min in standard room conditions. The reusability of the absorbents was quantitatively investigated, demonstrating that absorbents can be used at least nine times. Poly(alkoxysilane) organogels would be ideal candidates for usable absorbents in oil–water separation or the removal of organic solvents from water. The high oil capacity and excellent oil/water selectivity of poly(alkoxysilane) gel make it an attractive sorbent for oil spill cleanup.

Acknowledgments Fig. 8. Desorption kinetics of poly(alkoxysilane)s in DCM at room condition.

We thank the Scientific and Technological Research Council of Turkey-TUBITAK for the support of this work through Grant TBAG/111T098, the Turkish Petroleum Refineries Co.-TUPRAS for supplying of crude oil samples and the Dow Chemical Company, USA for supplying mixture of 1,3-1,4-cyclohexanedimethanol (UNOXOL™) monomer.

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Fig. 9. Reusability of Poly C6 for DCM.

Poly C6 organogel in DCM was selected as a model absorbate. The ability of the swelled gels to undergo several cycles of swelling and deswelling is shown in Fig. 9. The absorption and desorption processes were repeated nine times and the relevant results showed that over this period, the solvent absorption capacities of the gel did not change and the poly(alkoxysilane) organogels could be used without any loss of absorption capacity (Fig. 9). Hence, poly(alkoxysilane) organogels are expected to find some practical application owing to their interesting reusability and capacity to retain solvents, together with high solvent absorbencies.

4. Conclusions We synthesized poly(alkoxysilane) organogels via a condensation reaction under solvent-free conditions at a moderately high

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