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water selectivity and reusability for oil spill cleanup
Marine Pollution Bulletin xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Oil sorbents with high sorption capacity, oil/water selectivity and reusability for oil spill cleanup Daxiong Wu, Linlin Fang, Yanmin Qin, Wenjuan Wu, Changming Mao, Haitao Zhu ⇑ Qingdao University of Science and Technology, Qingdao 266042, PR China
a r t i c l e
i n f o
a b s t r a c t
Keywords: Oil sorbent Oil/water selectivity Sorption capacity Reusability Oil-spill cleanup
A sorbent for oil spill cleanup was prepared through a novel strategy by treating polyurethane sponges with silica sol and gasoline successively. The oil sorption capacity, oil/water selectivity, reusability and sorption mechanism of prepared sorbent were studied. The results showed that the prepared sorbent exhibited high sorption capacity and excellent oil/water selectivity. 1 g of the prepared sorbent could adsorb more than 100 g of motor oil, while it only picks up less than 0.1 g of water from an oil–water interface under both static and dynamic conditions. More than 70% of the sorption capacity remained after 15 successive sorption–squeezing cycles, which suggests an extraordinary high reusability. The prepared sorbent is a better alternative of the commercial polypropylene sorbent which are being used nowadays. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
prepared via a one-step electrospinning process, such as polyvinyl chloride/polystyrene fibers (Zhu et al., 2011), were reported to have very high oil sorption capacity up to 146 g g1. However, these electrospun oil sorbents have poor reusability. Once they are compressed to remove oil, they will lose most of their sorption capacities and it is difficult to recover. Another kind of newly developed oil sorbents is carbon base sponges and aerogels. For example, Gui et al. reported a type of carbon nanotube sponges with sorption capacities of about 125 and 143 g g1 for pump oil and diesel oil, respectively (Gui et al., 2010). Sun et al. (2013) and Hu et al. (2013) reported that graphene aerogels not only had extremely high sorption capacities but also were reusable. The reported aerogels kept their initial shape and absorption capability after more than 10 cycles of compression. However, the oil/ water selectivity of these graphene aerogels has not yet been reported. Furthermore, these materials are still synthesized in a lab scale. The research for high performance oil sorbents at low cost and scalable production remains a challenging issue and needs urgent attention. In the current paper, we report the preparation of ideal oil sorbents that can meet the three mentioned criteria. Polyurethane (PU) sponges are chosen as raw materials because they are durable, cheap, available in large scale, and above all, they have huge sum of connected holes that can provide very high oil sorption capacity. Furthermore, polyurethane sponges have excellent elastic property and can recover to the initial state after many times of compression. The only obstacle is that polyurethane sponges are hydrophilic, thus surface modifications are required to improve the
Oil-sorbent plays an important role in many fields such as oil spill cleanup, oil/water separation and environmental remediation (Gui et al., 2010; Yuan et al., 2008; Su et al., 2005; Whitfield, 2003; Moura and Lago, 2009; Fingas, 2000; Adebajo et al., 2003). Disasters such as the oil spill in Mexico Gulf in 2010 remind us again the importance of oil-sorbents in spilled oil cleanup and environmental remediation (Kerr et al., 2010; Aguilera et al., 2010). High oil sorption capacity, low water pickup, and excellent reusability are the most important criteria for selecting oil sorbent (Adebajo et al., 2003). To date, several types of materials, such as inorganic sorbents, natural organic sorbents, and synthetic organic sorbents, have been extensively studied (Adebajo et al., 2003; Gurav et al., 2010; Carmody et al., 2007; Abdullah et al., 2010; Srinivasan and Viraraghavan, 2008; Annunciado et al., 2005; Ceylan et al., 2009; Duong and Burford, 2009; Lin et al., 2010; Wei et al., 2003; Zhang et al., 2009). However, it is still a challenge to get ideal oil sorbents that could meet all the three criteria. Generally, the oil sorption capacities of inorganic sorbents and natural organic sorbents are only tens of grams per gram of sorbent and the oil/water selectivity is basically not very high. Commercial polypropylene (PP) oil sorbents have good oil/water selectivity because of their oleophilic–hydrophobic properties, but their oil sorption capacities are only 15–25 g g1. Recently, oil sorbents made of polymer fibers ⇑ Corresponding author. Tel.: +86 532 84022676; fax: +86 532 84022814. E-mail address: [email protected] (H. Zhu). http://dx.doi.org/10.1016/j.marpolbul.2014.05.005 0025-326X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wu, D., et al. Oil sorbents with high sorption capacity, oil/water selectivity and reusability for oil spill cleanup. Mar. Pollut. Bull. (2014), http://dx.doi.org/10.1016/j.marpolbul.2014.05.005
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D. Wu et al. / Marine Pollution Bulletin xxx (2014) xxx–xxx
oil/water selectivity. In this work, polyurethane sponges are treated with SiO2 sol and subsequently gasoline to develop hydrophobic surface. Then their performances as oil sorbents are characterized in terms of oil sorption capacity, oil/water selectivity, reusability and oil retaining ability. Particularly, the oil sorption mechanism is studied by way of investigating the microstructures, the fiber surface property and the contact angles of the prepared sorbent. 2. Materials and methods 2.1. Preparation of sorbents Polyurethane sponges were obtained from Jinhua Duxiu Technology Co. Ltd. In a typical preparation process, the sponges were cut into blocks of 45 20 15 mm, and then immersed completely into an aqueous SiO2 sol containing 0.5 wt.% of SiO2 nanoparticles. After being soaked in the SiO2 sol for 30 min, the sponges were centrifuged to remove liquid, dried naturally, and then immersed completely into gasoline for another 15 min. The sponges were then centrifuged and dried at room temperature to get the final oil sorbents. Scanning electron microscopy (SEM) images were taken on a FESEM-6700 field-emission microscope. An OCA20 contact angle system was applied to measure the contact angle of the obtained sorbent with water or motor oil. 2.2. Sorption experiments Three types of oils including motor oil, peanut oil and diesel were employed to study the sorption capacity of the sorbents. The sorption experiments were conducted in dynamic simulated system and static system at 20 ± 4 °C. In dynamic simulated system, different amount of oil was poured into four 5 L glass beakers, containing 800 mL tap water each, to obtain oil films of 1–4 mm in thickness on water surface, respectively. The mixture of oil and water were constantly agitated (500 rpm) by magnetic stirrer. Then blocks of sorbent were weighed and placed in each of the glass beakers, respectively. After 30 min of sorption, the sorbents were taken out and allowed to drain for 2 min. The saturated sorbents were squeezed and the recovered liquids were centrifuged to separate water from oil according to the standard method D400781 (ASTM-1998a). Thus the amounts of sorbed water and sorbed oil were determined. The sorption capacity of oil and water can be calculated using Eq. (1),
C s;o ¼ mo =mi ; or C s;w ¼ mw =mi
Fig. 1. Sorption capacities of the prepared sorbents at static and dynamic conditions (motor oil at oil–water interface).
films of different thickness on the top of tap water, under both static and dynamic conditions. Taken the reasonable experimental fluctuation into account, the sorption capacity of motor oil was determined to be around 103 ± 3 g g1 under static condition and 106 ± 3 g g1 under dynamic condition, which showed less relevant to the thickness of oil films. In comparison, water pick up under both static and dynamic conditions was measured to be less than 0.1 g per gram sorbents, not affected by the thickness of oil films. The results suggest high oil sorption capacity for both thick oil films and thin oil films. As the thickness of oil films is not relevant to the oil sorption capacity, it should be emphasized that the sorbents are especially applicable at the final stage of oil spill cleanup when there is very thin oil film remained and it is difficult to handle with traditional methods. The results also indicate that the sorbents have not only high sorption capacity, but also have very good oil/water selectivity, which means the water pickup is negligible in comparison to the oil sorption. This is of great significance because it suggests a very high efficiency of a single sorption cycle when the sorbents pick up large amount of oil without further efforts required for oil–water separations. The sorption capacities of the prepared sorbents for different kinds of oils determined by putting the sorbents on 3 mm oil film at oil–water interface under static condition are presented as Fig. 2, together with the data of commercial PP sorbents measured under identical conditions. The sorption capacities of the prepared
ð1Þ
where Cs,o and Cs,w represent oil and water sorption capacity (g g1) of the sorbent, mi is the initial weight (g) of the sorbent, mo is the weight (g) of the sorbed oil, and mw is the weight (g) of the sorbed water. Four independent experiments were conducted to get average value for each test. In static system, the magnetic stirrer was turned off while keeping all other experimental parameters unchanged as in the dynamic system. The saturated sorbents were squeezed and then reused as sorbent. The sorption–squeezing procedure was repeated many times under identical conditions to evaluate the reusability of the sorbent. 3. Results and discussions 3.1. Sorption capacity and oil/water selectivity of the sorbents Fig. 1 shows the sorption capacity of motor oil and water measured when sorbents were placed in beakers containing motor oil
Fig. 2. Sorption capacities of the prepared sorbents and commercial PP sorbents for various oils (3 mm oil film at oil–water interface, static condition).
Please cite this article in press as: Wu, D., et al. Oil sorbents with high sorption capacity, oil/water selectivity and reusability for oil spill cleanup. Mar. Pollut. Bull. (2014), http://dx.doi.org/10.1016/j.marpolbul.2014.05.005
D. Wu et al. / Marine Pollution Bulletin xxx (2014) xxx–xxx
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Fig. 3. Reusability and oil retaining ability of the prepared sorbents.
Fig. 4. SEM images of (a) skeleton and (b) fiber surface of the initial PU sponge; (c) skeleton and (d) fiber surface of the prepared sorbents.
sorbents for motor oil, peanut oil and diesel are determined to be 103 ± 3, 108 ± 4 and 95 ± 3 g g1, respectively. In comparison, the sorption capacities of the commercial PP sorbents for motor oil, peanut oil and diesel are determined to be 10 ± 1, 11 ± 1 and 6 ± 1 g g1, respectively. For motor oil and peanut oil, the sorption capacities of the prepared sorbents are about 10 times higher than that of the commercial PP sorbents; while for diesel, it is 15 times higher. It should be noted that water pickup is negligible (less than 0.1 g) for both the commercial PP sorbents and the prepared sorbents. 3.2. Reusability of the sorbents Fig. 3a shows the varying values of sorption capacities of the prepared sorbents for motor oil at oil–water interface, when the sorbents are subjected to successive sorption–squeezing cycles. In general, oil sorption capacities of the sorbents reduce gradually from 103 to 76 g g1 after 15 cycles, with about 70% oil sorption capacity remained. Water pickup is negligible (less than 0.1 g) in each cycle. Assuming that the sorbents are reused 15 times successively, the accumulated sorption capacity of 1 g sorbents can be easily calculated to be 1350 g oil. Base on the calculation, it only requires less than 0.75 tons sorbents to clean 1000 tons of spilled oil. On the contrary, it may require about 100 tons of commercial
PP sorbents to collect 1000 tons of spilled oil as the commercial PP sorbents show relatively lower sorption capacity of about 10 g g1 and are not reusable. Thus it is of great economic significance to apply the prepared sorbents in oil spill cleanup process. Another concern about sorbents is their retaining ability of the sorbed oil. An excellent retaining ability guarantees that the sorbed oil will not be lost while handling the sorbents. When the prepared sorbents are saturated with motor oil and hung under gravity circumstance, their gross mass remains almost unchanged within a long duration. When a weight of 200 g is put on the top of an oil saturated sorbent (20 17 15 mm), the sorbed oil only starts to be squeezed out from the sorbents (Fig. 3b). It stands for a pressure of 7.7 kPa, which is easy to reach on a squeezer, but will unlikely emerge during transportation and other handling process. 3.3. Oil sorption mechanism To understand the excellent sorption capacity and oil/water selectivity of the prepared sorbents, the microstructures of the sorbents and the contact-angles of the sorbents with oil and water are investigated. Under observations on a field-emission scanning electron microscope (FESEM-6700), the geometry of the prepared sorbents shows little difference to that of the initial PU sponge, with huge sum of interconnected hexagonal holes providing
Please cite this article in press as: Wu, D., et al. Oil sorbents with high sorption capacity, oil/water selectivity and reusability for oil spill cleanup. Mar. Pollut. Bull. (2014), http://dx.doi.org/10.1016/j.marpolbul.2014.05.005
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D. Wu et al. / Marine Pollution Bulletin xxx (2014) xxx–xxx
accommodation for large amount of sorbed oil (Fig. 4a and c). The significant change is related to surface properties of the fibers forming the initial PU sponge (Fig. 4b) and the prepared sorbents (Fig. 4d). The fibers of the initial PU sponge show a smooth surface, while the surface roughness of the fibers in the prepared sorbents increases remarkably with a lot of silica nanoparticles attached. The contact-angle images of the initial PU sponge, only SiO2 sol treated sponge, only gasoline treated sponge and the prepared sorbent (treated with SiO2 sol and gasoline) with water and motor oil are shown in Fig. 5. For the initial PU sponges, a droplet of water (4 lL) can completely spread into the pores of the sponges within one second and no contact angle can be measured (Fig. 5a), while the oil (2 lL) contact-angle is about 118° (Fig. 5b), exhibiting the intrinsic hydrophilicity of the initial PU sponges. After the SiO2 sol treatment, a water droplet can also completely spread into the sponge within one second (Fig. 5c) and the oil contact-angle is about 128° (Fig. 5d). If the sponge was treated only with gasoline, the spreading rate of water droplet into sponge is reduced but a water droplet can still completely spread into the sponge within three seconds (Fig. 5e). An oil droplet can spread slowly into the sponge within 6 s (Fig. 5f). If the sponge is treated with SiO2 sol and subsequently gasoline (the prepared sorbent), a water droplet cannot spread into the sponge and the water contact-angle is about 126° while an oil droplet can spread into the sponge within two seconds. The results suggest that it has changed from a highly hydrophilic surface to a hydrophobic surface after successive treatments with silica sol and gasoline. Generally, the wettability of a solid surface depends on two factors: the topographical microstructure and the surface chemical composition (Barthlott and Neinhuis, 1997; Zhang et al., 2013). Due to the smooth fiber surface and the polar groups of –CO and –NH, the initial PU sponge exhibits hydrophilic and oleophobic properties. After the treatment with silica sol, a lot of silica nanoparticles attached on the surface of the fibers and the roughness increased obviously. Through the modification with gasoline, the surface chemical composition changed.
Thus the prepared sorbent has good hydrophobic and oleophilic properties. Accordingly, significant changes can be found between the data of sorption capacity before and after treatments. The oil and water sorption capacity of the untreated initial PU sponge at dynamic oil–water interface with 3 mm oil film (motor oil) are determined to be around 25 and 95 g g1, respectively. The results illustrate a poor oil/water selectivity of the initial PU sponge. On the contrary, the prepared sorbents show remarkable improvement in oil/water selectivity with the corresponding data determined to be around 103 and 0.1 g g1 under the identical conditions. Thus the improvement in oil/water selectivity verifies the successful modification in surface properties of the prepared sorbents. 4. Conclusions In summary, the results show a new sorbent with the performance of high oil sorption capacity, oil/water selectivity and reusability. Its sorption capacity for motor oil is 10 times higher than that of the commercial PP sorbent and its water pickup is negligible. It can be recovered with simple squeezing process and reused for more than 15 times. It is also available in large scale and expected to become promising candidate for spilled oil cleanup applications to replace the commercial PP sorbent which is being widely used nowadays. The understanding of oil sorption mechanism obtained in the current work is significant to the research and design of new oil sorbents. Acknowledgments This work was supported by the National Natural Science Foundation of China (51172117), Natural Science Foundation of Shandong Province (ZR2010EM035, ZR2013EMM003) and Foundation of Qingdao Science and Technology (13-1-4-148-jch). References
Fig. 5. Water contact-angle images of the initial PU sponge (a), sponge treated with SiO2 sol (c), sponge treated with gasoline (e) and the prepared sorbent (g), oil contact-angle images of the initial PU sponge (b), sponge treated with SiO2 sol (d), sponge treated with gasoline (f) and the prepared sorbent (h).
Abdullah, M.A., Rahmah, A.U., Man, Z., 2010. Physicochemical and sorption characteristics of Malaysian Ceiba pentandra (L.) Gaertn as a natural oil sorbent. J. Hazard. Mater. 177, 683–691. Adebajo, M.O., Frost, R.L., Kloprogge, J.T., Carmody, O., Kokot, S., 2003. Porous materials for oil spill cleanup: a review of synthesis and absorbing properties. J. Porous Mater. 10, 159–170. Aguilera, F., Mendez, J., Pasaro, E., Laffon, B., 2010. Review on the effects of exposure to spilled oils on human health. J. Appl. Toxicol. 30, 291–301. Annunciado, T.R., Sydenstricker, T.H.D., Amico, S.C., 2005. Experimental investigation of various vegetable fibers as sorbent materials for oil spills. Mar. Pollut. Bull. 50, 1340–1346. Barthlott, W., Neinhuis, C., 1997. Purity of the sacred lotus, or escape from contamination in biological surface. Planta 202, 1–8. Carmody, O., Frost, R., Xi, Y., Kokot, S., 2007. Surface characterisation of selected sorbent materials for common hydrocarbon fuels. Surf. Sci. 601, 2066–2076. Ceylan, D., Dogu, S., Karacik, B., Yakan, S.D., Okay, O.S., Okay, O., 2009. Evaluation of butyl rubber as sorbent material for the removal of oil and polycyclic aromatic hydrocarbons from seawater. Environ. Sci. Technol. 43, 3846–3852. Duong, H.T.T., Burford, R.P., 2009. Effect of foam density, oil viscosity, and temperature on oil sorption behavior of polyurethane. J. Appl. Polym. Sci. 99, 360–367. Fingas, M., 2000. The Basics of Oil Spill Clearnup. CRC Press, NewYork, pp. 89–107. Gui, X., Wei, J., Wang, K., Cao, A., Zhu, H., Jia, Y., Shu, Q., Wu, D., 2010. Carbon nanotube sponges. Adv. Mater. 22, 617–621. Gurav, J.L., Rao, A.V., Nadargi, D.Y., Park, H.H., 2010. Ambient pressure dried TEOSbased silica aerogels: good absorbents of organic liquids. J. Mater. Sci. 45, 503– 510. Hu, H., Zhao, Z., Wan, W., Gogotsi, Y., Qiu, J., 2013. Ultralight and highly compressible graphene aerogels. Adv. Mater. 25, 2219–2223. Kerr, R.A., Kintisch, E., Schenkman, L., Stokstad, E., 2010. Gulf oil disaster: five questions on the spill. Science 328, 962–963. Lin, C., Hong, Y.J., Hu, A.H., 2010. Recycling of waste tyre rubber into oil absorbent. Waste Manage. 30, 263–267. Moura, F.C., Lago, R.M., 2009. Catalytic growth of carbon nanotubes and nanofibers on vermiculite to produce floatable hydrophobic ‘‘nanosponges’’ for oil spill remediation. Appl. Catal. B-Environ. 90, 436–440.
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D. Wu et al. / Marine Pollution Bulletin xxx (2014) xxx–xxx Srinivasan, A., Viraraghavan, T., 2008. Removal of oil by walnut shell media. Bioresour. Technol. 99, 8217–8220. Su, D.S., Chen, X., Weinberg, G., Klein-Hofmann, A., Timpe, O., Hamid, S.B.A., Schlögl, R., 2005. Hierarchically structured carbon: synthesis of carbon nanofibers nested inside or immobilized onto modified activated carbon. Angew. Chem. Int. Ed. 44, 5488–5492. Sun, H., Xu, Z., Gao, C., 2013. Multifunctional, ultra-flyweight, synergistically assembled carbon aerogels. Adv. Mater. 25, 2554–2560. Wei, Q.F., Mather, R.R., Fotheringham, A.F., Yang, R.D., 2003. Evaluation of nonwoven polypropylene oil sorbents in marine oil-spill recovery. Mar. Pollut. Bull. 46, 780–783.
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Whitfield, J., 2003. How to clean a beach. Nature 422, 464–466. Yuan, J., Liu, X., Akbulut, O., Hu, J., Suib, S.L., Kong, J., Stellacci, F., 2008. Superwetting nanowire membranes for selective absorption. Nat. Nanotechnol. 3, 332–336. Zhang, Y., Wei, S., Liu, F., Du, Y., Liu, S., Ji, Y., Yokoi, T., Tatsumi, T., Xiao, F.S., 2009. Superhydrophobic nanoporous polymers as efficient adsorbents for organic compounds. Nano Today 4, 135–142. Zhang, X., Li, Z., Liu, K., Jiang, L., 2013. Bioinspired multifunctional foam with selfcleaning and oil/water separation. Adv. Funct. Mater. 23, 2881–2886. Zhu, H., Qiu, S., Jiang, W., Wu, D., Zhang, C., 2011. Evaluation of electrospun polyvinyl chloride/polystyrene fibers as sorbent materials for oil spill cleanup. Environ. Sci. Technol. 45, 4527–4531.
Please cite this article in press as: Wu, D., et al. Oil sorbents with high sorption capacity, oil/water selectivity and reusability for oil spill cleanup. Mar. Pollut. Bull. (2014), http://dx.doi.org/10.1016/j.marpolbul.2014.05.005
Contents lists available at ScienceDirect
Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Oil sorbents with high sorption capacity, oil/water selectivity and reusability for oil spill cleanup Daxiong Wu, Linlin Fang, Yanmin Qin, Wenjuan Wu, Changming Mao, Haitao Zhu ⇑ Qingdao University of Science and Technology, Qingdao 266042, PR China
a r t i c l e
i n f o
a b s t r a c t
Keywords: Oil sorbent Oil/water selectivity Sorption capacity Reusability Oil-spill cleanup
A sorbent for oil spill cleanup was prepared through a novel strategy by treating polyurethane sponges with silica sol and gasoline successively. The oil sorption capacity, oil/water selectivity, reusability and sorption mechanism of prepared sorbent were studied. The results showed that the prepared sorbent exhibited high sorption capacity and excellent oil/water selectivity. 1 g of the prepared sorbent could adsorb more than 100 g of motor oil, while it only picks up less than 0.1 g of water from an oil–water interface under both static and dynamic conditions. More than 70% of the sorption capacity remained after 15 successive sorption–squeezing cycles, which suggests an extraordinary high reusability. The prepared sorbent is a better alternative of the commercial polypropylene sorbent which are being used nowadays. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
prepared via a one-step electrospinning process, such as polyvinyl chloride/polystyrene fibers (Zhu et al., 2011), were reported to have very high oil sorption capacity up to 146 g g1. However, these electrospun oil sorbents have poor reusability. Once they are compressed to remove oil, they will lose most of their sorption capacities and it is difficult to recover. Another kind of newly developed oil sorbents is carbon base sponges and aerogels. For example, Gui et al. reported a type of carbon nanotube sponges with sorption capacities of about 125 and 143 g g1 for pump oil and diesel oil, respectively (Gui et al., 2010). Sun et al. (2013) and Hu et al. (2013) reported that graphene aerogels not only had extremely high sorption capacities but also were reusable. The reported aerogels kept their initial shape and absorption capability after more than 10 cycles of compression. However, the oil/ water selectivity of these graphene aerogels has not yet been reported. Furthermore, these materials are still synthesized in a lab scale. The research for high performance oil sorbents at low cost and scalable production remains a challenging issue and needs urgent attention. In the current paper, we report the preparation of ideal oil sorbents that can meet the three mentioned criteria. Polyurethane (PU) sponges are chosen as raw materials because they are durable, cheap, available in large scale, and above all, they have huge sum of connected holes that can provide very high oil sorption capacity. Furthermore, polyurethane sponges have excellent elastic property and can recover to the initial state after many times of compression. The only obstacle is that polyurethane sponges are hydrophilic, thus surface modifications are required to improve the
Oil-sorbent plays an important role in many fields such as oil spill cleanup, oil/water separation and environmental remediation (Gui et al., 2010; Yuan et al., 2008; Su et al., 2005; Whitfield, 2003; Moura and Lago, 2009; Fingas, 2000; Adebajo et al., 2003). Disasters such as the oil spill in Mexico Gulf in 2010 remind us again the importance of oil-sorbents in spilled oil cleanup and environmental remediation (Kerr et al., 2010; Aguilera et al., 2010). High oil sorption capacity, low water pickup, and excellent reusability are the most important criteria for selecting oil sorbent (Adebajo et al., 2003). To date, several types of materials, such as inorganic sorbents, natural organic sorbents, and synthetic organic sorbents, have been extensively studied (Adebajo et al., 2003; Gurav et al., 2010; Carmody et al., 2007; Abdullah et al., 2010; Srinivasan and Viraraghavan, 2008; Annunciado et al., 2005; Ceylan et al., 2009; Duong and Burford, 2009; Lin et al., 2010; Wei et al., 2003; Zhang et al., 2009). However, it is still a challenge to get ideal oil sorbents that could meet all the three criteria. Generally, the oil sorption capacities of inorganic sorbents and natural organic sorbents are only tens of grams per gram of sorbent and the oil/water selectivity is basically not very high. Commercial polypropylene (PP) oil sorbents have good oil/water selectivity because of their oleophilic–hydrophobic properties, but their oil sorption capacities are only 15–25 g g1. Recently, oil sorbents made of polymer fibers ⇑ Corresponding author. Tel.: +86 532 84022676; fax: +86 532 84022814. E-mail address: [email protected] (H. Zhu). http://dx.doi.org/10.1016/j.marpolbul.2014.05.005 0025-326X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wu, D., et al. Oil sorbents with high sorption capacity, oil/water selectivity and reusability for oil spill cleanup. Mar. Pollut. Bull. (2014), http://dx.doi.org/10.1016/j.marpolbul.2014.05.005
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D. Wu et al. / Marine Pollution Bulletin xxx (2014) xxx–xxx
oil/water selectivity. In this work, polyurethane sponges are treated with SiO2 sol and subsequently gasoline to develop hydrophobic surface. Then their performances as oil sorbents are characterized in terms of oil sorption capacity, oil/water selectivity, reusability and oil retaining ability. Particularly, the oil sorption mechanism is studied by way of investigating the microstructures, the fiber surface property and the contact angles of the prepared sorbent. 2. Materials and methods 2.1. Preparation of sorbents Polyurethane sponges were obtained from Jinhua Duxiu Technology Co. Ltd. In a typical preparation process, the sponges were cut into blocks of 45 20 15 mm, and then immersed completely into an aqueous SiO2 sol containing 0.5 wt.% of SiO2 nanoparticles. After being soaked in the SiO2 sol for 30 min, the sponges were centrifuged to remove liquid, dried naturally, and then immersed completely into gasoline for another 15 min. The sponges were then centrifuged and dried at room temperature to get the final oil sorbents. Scanning electron microscopy (SEM) images were taken on a FESEM-6700 field-emission microscope. An OCA20 contact angle system was applied to measure the contact angle of the obtained sorbent with water or motor oil. 2.2. Sorption experiments Three types of oils including motor oil, peanut oil and diesel were employed to study the sorption capacity of the sorbents. The sorption experiments were conducted in dynamic simulated system and static system at 20 ± 4 °C. In dynamic simulated system, different amount of oil was poured into four 5 L glass beakers, containing 800 mL tap water each, to obtain oil films of 1–4 mm in thickness on water surface, respectively. The mixture of oil and water were constantly agitated (500 rpm) by magnetic stirrer. Then blocks of sorbent were weighed and placed in each of the glass beakers, respectively. After 30 min of sorption, the sorbents were taken out and allowed to drain for 2 min. The saturated sorbents were squeezed and the recovered liquids were centrifuged to separate water from oil according to the standard method D400781 (ASTM-1998a). Thus the amounts of sorbed water and sorbed oil were determined. The sorption capacity of oil and water can be calculated using Eq. (1),
C s;o ¼ mo =mi ; or C s;w ¼ mw =mi
Fig. 1. Sorption capacities of the prepared sorbents at static and dynamic conditions (motor oil at oil–water interface).
films of different thickness on the top of tap water, under both static and dynamic conditions. Taken the reasonable experimental fluctuation into account, the sorption capacity of motor oil was determined to be around 103 ± 3 g g1 under static condition and 106 ± 3 g g1 under dynamic condition, which showed less relevant to the thickness of oil films. In comparison, water pick up under both static and dynamic conditions was measured to be less than 0.1 g per gram sorbents, not affected by the thickness of oil films. The results suggest high oil sorption capacity for both thick oil films and thin oil films. As the thickness of oil films is not relevant to the oil sorption capacity, it should be emphasized that the sorbents are especially applicable at the final stage of oil spill cleanup when there is very thin oil film remained and it is difficult to handle with traditional methods. The results also indicate that the sorbents have not only high sorption capacity, but also have very good oil/water selectivity, which means the water pickup is negligible in comparison to the oil sorption. This is of great significance because it suggests a very high efficiency of a single sorption cycle when the sorbents pick up large amount of oil without further efforts required for oil–water separations. The sorption capacities of the prepared sorbents for different kinds of oils determined by putting the sorbents on 3 mm oil film at oil–water interface under static condition are presented as Fig. 2, together with the data of commercial PP sorbents measured under identical conditions. The sorption capacities of the prepared
ð1Þ
where Cs,o and Cs,w represent oil and water sorption capacity (g g1) of the sorbent, mi is the initial weight (g) of the sorbent, mo is the weight (g) of the sorbed oil, and mw is the weight (g) of the sorbed water. Four independent experiments were conducted to get average value for each test. In static system, the magnetic stirrer was turned off while keeping all other experimental parameters unchanged as in the dynamic system. The saturated sorbents were squeezed and then reused as sorbent. The sorption–squeezing procedure was repeated many times under identical conditions to evaluate the reusability of the sorbent. 3. Results and discussions 3.1. Sorption capacity and oil/water selectivity of the sorbents Fig. 1 shows the sorption capacity of motor oil and water measured when sorbents were placed in beakers containing motor oil
Fig. 2. Sorption capacities of the prepared sorbents and commercial PP sorbents for various oils (3 mm oil film at oil–water interface, static condition).
Please cite this article in press as: Wu, D., et al. Oil sorbents with high sorption capacity, oil/water selectivity and reusability for oil spill cleanup. Mar. Pollut. Bull. (2014), http://dx.doi.org/10.1016/j.marpolbul.2014.05.005
D. Wu et al. / Marine Pollution Bulletin xxx (2014) xxx–xxx
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Fig. 3. Reusability and oil retaining ability of the prepared sorbents.
Fig. 4. SEM images of (a) skeleton and (b) fiber surface of the initial PU sponge; (c) skeleton and (d) fiber surface of the prepared sorbents.
sorbents for motor oil, peanut oil and diesel are determined to be 103 ± 3, 108 ± 4 and 95 ± 3 g g1, respectively. In comparison, the sorption capacities of the commercial PP sorbents for motor oil, peanut oil and diesel are determined to be 10 ± 1, 11 ± 1 and 6 ± 1 g g1, respectively. For motor oil and peanut oil, the sorption capacities of the prepared sorbents are about 10 times higher than that of the commercial PP sorbents; while for diesel, it is 15 times higher. It should be noted that water pickup is negligible (less than 0.1 g) for both the commercial PP sorbents and the prepared sorbents. 3.2. Reusability of the sorbents Fig. 3a shows the varying values of sorption capacities of the prepared sorbents for motor oil at oil–water interface, when the sorbents are subjected to successive sorption–squeezing cycles. In general, oil sorption capacities of the sorbents reduce gradually from 103 to 76 g g1 after 15 cycles, with about 70% oil sorption capacity remained. Water pickup is negligible (less than 0.1 g) in each cycle. Assuming that the sorbents are reused 15 times successively, the accumulated sorption capacity of 1 g sorbents can be easily calculated to be 1350 g oil. Base on the calculation, it only requires less than 0.75 tons sorbents to clean 1000 tons of spilled oil. On the contrary, it may require about 100 tons of commercial
PP sorbents to collect 1000 tons of spilled oil as the commercial PP sorbents show relatively lower sorption capacity of about 10 g g1 and are not reusable. Thus it is of great economic significance to apply the prepared sorbents in oil spill cleanup process. Another concern about sorbents is their retaining ability of the sorbed oil. An excellent retaining ability guarantees that the sorbed oil will not be lost while handling the sorbents. When the prepared sorbents are saturated with motor oil and hung under gravity circumstance, their gross mass remains almost unchanged within a long duration. When a weight of 200 g is put on the top of an oil saturated sorbent (20 17 15 mm), the sorbed oil only starts to be squeezed out from the sorbents (Fig. 3b). It stands for a pressure of 7.7 kPa, which is easy to reach on a squeezer, but will unlikely emerge during transportation and other handling process. 3.3. Oil sorption mechanism To understand the excellent sorption capacity and oil/water selectivity of the prepared sorbents, the microstructures of the sorbents and the contact-angles of the sorbents with oil and water are investigated. Under observations on a field-emission scanning electron microscope (FESEM-6700), the geometry of the prepared sorbents shows little difference to that of the initial PU sponge, with huge sum of interconnected hexagonal holes providing
Please cite this article in press as: Wu, D., et al. Oil sorbents with high sorption capacity, oil/water selectivity and reusability for oil spill cleanup. Mar. Pollut. Bull. (2014), http://dx.doi.org/10.1016/j.marpolbul.2014.05.005
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accommodation for large amount of sorbed oil (Fig. 4a and c). The significant change is related to surface properties of the fibers forming the initial PU sponge (Fig. 4b) and the prepared sorbents (Fig. 4d). The fibers of the initial PU sponge show a smooth surface, while the surface roughness of the fibers in the prepared sorbents increases remarkably with a lot of silica nanoparticles attached. The contact-angle images of the initial PU sponge, only SiO2 sol treated sponge, only gasoline treated sponge and the prepared sorbent (treated with SiO2 sol and gasoline) with water and motor oil are shown in Fig. 5. For the initial PU sponges, a droplet of water (4 lL) can completely spread into the pores of the sponges within one second and no contact angle can be measured (Fig. 5a), while the oil (2 lL) contact-angle is about 118° (Fig. 5b), exhibiting the intrinsic hydrophilicity of the initial PU sponges. After the SiO2 sol treatment, a water droplet can also completely spread into the sponge within one second (Fig. 5c) and the oil contact-angle is about 128° (Fig. 5d). If the sponge was treated only with gasoline, the spreading rate of water droplet into sponge is reduced but a water droplet can still completely spread into the sponge within three seconds (Fig. 5e). An oil droplet can spread slowly into the sponge within 6 s (Fig. 5f). If the sponge is treated with SiO2 sol and subsequently gasoline (the prepared sorbent), a water droplet cannot spread into the sponge and the water contact-angle is about 126° while an oil droplet can spread into the sponge within two seconds. The results suggest that it has changed from a highly hydrophilic surface to a hydrophobic surface after successive treatments with silica sol and gasoline. Generally, the wettability of a solid surface depends on two factors: the topographical microstructure and the surface chemical composition (Barthlott and Neinhuis, 1997; Zhang et al., 2013). Due to the smooth fiber surface and the polar groups of –CO and –NH, the initial PU sponge exhibits hydrophilic and oleophobic properties. After the treatment with silica sol, a lot of silica nanoparticles attached on the surface of the fibers and the roughness increased obviously. Through the modification with gasoline, the surface chemical composition changed.
Thus the prepared sorbent has good hydrophobic and oleophilic properties. Accordingly, significant changes can be found between the data of sorption capacity before and after treatments. The oil and water sorption capacity of the untreated initial PU sponge at dynamic oil–water interface with 3 mm oil film (motor oil) are determined to be around 25 and 95 g g1, respectively. The results illustrate a poor oil/water selectivity of the initial PU sponge. On the contrary, the prepared sorbents show remarkable improvement in oil/water selectivity with the corresponding data determined to be around 103 and 0.1 g g1 under the identical conditions. Thus the improvement in oil/water selectivity verifies the successful modification in surface properties of the prepared sorbents. 4. Conclusions In summary, the results show a new sorbent with the performance of high oil sorption capacity, oil/water selectivity and reusability. Its sorption capacity for motor oil is 10 times higher than that of the commercial PP sorbent and its water pickup is negligible. It can be recovered with simple squeezing process and reused for more than 15 times. It is also available in large scale and expected to become promising candidate for spilled oil cleanup applications to replace the commercial PP sorbent which is being widely used nowadays. The understanding of oil sorption mechanism obtained in the current work is significant to the research and design of new oil sorbents. Acknowledgments This work was supported by the National Natural Science Foundation of China (51172117), Natural Science Foundation of Shandong Province (ZR2010EM035, ZR2013EMM003) and Foundation of Qingdao Science and Technology (13-1-4-148-jch). References
Fig. 5. Water contact-angle images of the initial PU sponge (a), sponge treated with SiO2 sol (c), sponge treated with gasoline (e) and the prepared sorbent (g), oil contact-angle images of the initial PU sponge (b), sponge treated with SiO2 sol (d), sponge treated with gasoline (f) and the prepared sorbent (h).
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Please cite this article in press as: Wu, D., et al. Oil sorbents with high sorption capacity, oil/water selectivity and reusability for oil spill cleanup. Mar. Pollut. Bull. (2014), http://dx.doi.org/10.1016/j.marpolbul.2014.05.005