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Evaluation of Electrospun Polyvinyl Chloride/Polystyrene Fibers As Sorbent Materials for Oil Spill Cleanup Haitao Zhu,* Shanshan Qiu, Wei Jiang, Daxiong Wu, and Canying Zhang Qingdao University of Science and Technology, Qingdao 266042, People's Republic of China ABSTRACT: A novel, high-capacity oil sorbent consisting of polyvinyl chloride (PVC)/polystyrene (PS) fiber was prepared by an electrospinning process. The sorption capacity, oil/water selectivity, and sorption mechanism of the PVC/PS sorbent were studied. The results showed that the sorption capacities of the PVC/PS sorbent for motor oil, peanut oil, diesel, and ethylene glycol were 146, 119, 38, and 81 g/g, respectively. It was about 59 times that of a commercial polypropylene (PP) sorbent. The PVC/PS sorbent also had excellent oil/water selectivity (about 1000 times) and high buoyancy in the cleanup of oil over water. The SEM analysis indicated that voids among fibers were the key for the high capacity. The electrospun PVC/PS sorbent is a better alternative to the widely used PP sorbent for oil spill cleanup.
1. INTRODUCTION Oil spill accidents often happen during extraction, transportation, transfer, and storage.1 Oil spills will not only cause loss of energy source, but also have long-term damaging impacts on the ecological environment upon which our society relies.25 The recent Gulf of Mexico oil-spill reminds us again of the importance of oil spill cleanup and environmental remediation.2 Generally, methods dealing with oil spill are mechanical extraction, in situ burning, and bioremediation.6,7 One of the most economical and efficient methods for oil spill cleanup is mechanical extraction by sorbents.4,8,9 Oil sorbents are able to concentrate and transform liquid oil to the semi solid or solid phase, which can then be removed from the spilled area in a convenient manner.10,11 Properties of an ideal sorbent material for oil spill cleanup include oleophilicity and hydrophobicity, high oil sorption capacity, low water pickup (high oil/water selectivity), high buoyancy, being inexpensive and readily available.3,4,9 At present, oil sorbent materials can be categorized into three major classes: inorganic mineral products, organic natural products, and synthetic organic products.4,9,10 Among them, organic synthetic fibers, such as polypropylene (PP), play an important role in oil spill cleanup because of their oleophilichydrophobic properties, high oil/water selectivity, low density (high buoyancy), and readily available in large-scale.3,4,10,1213 However, the oil sorption capacities of these sorbents are generally only tens gram of oil per gram of sorbent. If the capacity of synthetic sorbent could be enhanced, then it would be very valuable for the oil spill cleanup. Electrospinning is a versatile technique to produce continuous fibers with diameters ranging from a few nanometers to a few micrometers.1417 Both the simplicity of the electrospinning scheme and controllable feature of electrospun fibers make this technique attractive in the preparation of oil sorbent.18 However, until now, there are no reports on the oil sorbent prepared by electrospinning process. r 2011 American Chemical Society
The aim of this work was to exploit a novel oil sorbent that can be used to absorb oil above water efficiently and with low-cost. Polystyrene (PS) and polyvinyl chloride (PVC) are two of the most widely used produced plastic. They are used in many fields because they are air-stable, durable, cheap, and available in largescale.19,20 For these reasons, in this work, PVC/PS sorbent was prepared by an electrospinning process. The sorption capacity and oil/water selectivity of PVC/PS sorbent were evaluated. For comparison, a commercial polypropylene (PP) material was also used in the sorption tests. In addition, based on SEM analysis, the oil sorption mechanism of PVC/PS sorbent was studied.
2. MATERIALS AND METHODS 2.1. Sorbent Preparation. PVC and PS particles were of industrial grade, and solvents were of analytical grade, which were all used without further treatment. The sample was prepared as follows: 0.3 g of PVC and 2.7 g of PS were added to 10 mL mixed solvent of N,N-dimethyl formamide (DMF) and tetrahydrofuran (THF) (the volume ratio of DMF and THF is 1:1). A homogeneous PVC/PS solution was obtained after stirring for 30 min. Then the solution was loaded into a syringe with a stainless-steel needle (the inlet diameter was about 1.2 mm). Electrospinning was conducted at room temperature (1823 °C) in atmosphere. The voltage applied to the needle was 2530 kV. The distance between the needle tip and collector was 1520 cm. The sample was named P1. Received: January 20, 2011 Accepted: April 13, 2011 Revised: April 13, 2011 Published: April 22, 2011 4527
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For comparison, a commercial nonwoven polypropylene sorbent, PP, obtained from Qingdao Guangming Co. (Qingdao, China) was used as a reference in the sorption experiments. 2.2. Types of Oils or Solvents. Four types of oils and solvents, namely motor oil, diesel, peanut oil, and ethylene glycol, were employed to investigate the sorption characteristics of PVC/PS sorbent. The physical properties of the investigated pollutants are given in Table 1. 2.3. Procedure for Oil absorption Tests. All tests were performed at 23 ( 4 °C In order to analyze the oil sorption capacity of PVC/PS sorbent in oil without water, 1.00 g of sorbent was placed on top of 300 mL of oil in a glass beaker. After 60 min of sorption, the oil was drained for 2 min and the wet sorbent was weighed. The oil sorption capacity of the sorbent was determined by following equation1:13 q ¼ ½mf ðmo þ mw Þ=mo
ð1Þ
where q is the sorption capacity (g/g), mf is the weight of the wet sorbent after 2 min of drainage (g), mo is the initial weight of the sorbent (g), and mw is the weight of the water (g). Certainly, in oil medium without any water, and mw is equal to zero. For determination of the relations between oil/water selectivity and oil thickness in the water surface, different volumes of the oil (motor oil, diesel, or peanut oil) were poured onto 800 mL of tap water in a series of 5 L glass beakers. The oil thicknesses were 3, 6, 9, 12, 15, 18, and 21 mm, respectively. After attaining a steady state condition, 1.00 g fiber (PVC/PS or PP) was placed in the beakers. After 60 min of sorption, the wet sorbent was removed, drained for 2 min, and weighed. The saturated sorbent was squeezed. Then the liquids recovered from the sorbent were centrifuged according to the standard method D400781 (ASTM, Table 1. Characteristics of Studied Oils/Solvents at Room Temperature a
oils
viscosity
density
and solvents
(Pa 3 s1)
(g 3 cm3)
producer
motor oil
0.277
0.875
SINOPEC Lubricants Co.,
peanut oil
0.055
0.907
Qingdao Changsheng Co.,
ethylene glycol
0.016
1.111
Tianjin Bodi Chemical Co.,
diesel
0.001
0.732
SINOPEC Changcheng Group Co., P. R. China
P. R. China P. R. China P. R. China.
a
The viscosity of studied pollutants was measured using a viscometer (Rheolab QC, Anton Paar) at room temperature. The shear rate was 200 s1.
1998 a). The oil capacity of the sorbent was determined by eq 1. For each sample, 3 to 5 independent sorption experiments were carried out, and then the average value and standard deviation were calculated. 2.4. Procedure for Buoyancy Test. The buoyancy test of the sorbent was experimentally simulated according to ref 21. (1) Static system: 40 mL motor oil (dyed with Oil Red) was poured into a beaker containing 800 mL water. After that, 0.3 g of the obtained sorbent was gently and evenly placed onto the oil surface. (2) Dynamic system: on the basis of the static system, dynamic buoyancy test experiment was carried out under constant steering (∼500 rpm) in a magnetic stirrer. 2.5. Characterization of Sorbent. The morphology of the electrospun PVC/PS fibers and PP sorbent were investigated with scanning electron microscopy (FESEM-6700). Prior to the measurement, the fibers were coated with a thin layer of gold. To assess a possible mechanism of oil sorption on PVC/PS fibers, the oil-saturated fibers were also analyzed by scanning electron microscopy. The preparation method of oil-saturated samples was as follows: 0.05 g P1 and 0.1 g PP were immersed into 10 mL motor oil, respectively. After 1 h, the oil-saturated samples were removed from oil, and coated with thin layer of gold. The bulk density of the obtained P1 sorbent was obtained as follows: some P1 sorbents were loosely filled in a columned box (about 175 cm3) without press, and the density was estimated by using the mass dividing the volume of the filled P1 sorbent. The density of PP sorbent was obtained by directly measuring the mass and volume of a sheet (10 10 0.5 cm) of PP sorbent.
3. RESULTS AND DISCUSSION 3.1. Characteristics of P1 and PP Sorbent. Figure 1a is the photo of sample P1. As shown in Figure 1a, the P1 sample exhibits cotton-like appearance. The bulk density of P1 is about 23 mg/cm3, which accounts for the porosity of the sorbent about 99.7% using a true density of 1.1 g/cm3 and 1.38 g/cm3 for PS and PVC, respectively. The SEM images of P1 are shown in the Figure 1b,c. The P1 sorbent consists of fibers with diameters of 1.5 to 3 μm. There are large amounts of interconnected voids among fibers. The size of voids is in the range of 2070 μm. The high magnification SEM image (Figure 1c) indicates that the surface of P1 fibers is rough, which is helpful for the adsorption and adhesion of oil on the fiber face. For comparison, the commercial PP sorbent was also characterized. As shown in Figure 2a, the PP sorbent is a cloth-like material. The bulk density of PP sorbent is 0.89 g/cm3, and the porosity of PP sorbent is about 90.2%. The SEM image (Figure 2b) shows that the diameters of PP fibers are in the range of 2030 μm. But there are conglutinations between two or several fibers.
Figure 1. Photo (a), low (b), and high (c) magnification SEM images of P1 sorbent. 4528
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Figure 2. Photo (a), low (b), and high (c) magnification SEM images of PP sorbent.
Figure 3. Maximum sorption capacities of P1 and PP for various oils or solvents.
The size of voids among fibers is about 70300 μm. The high magnification SEM image (Figure 2c) shows the surface of PP fibers is relatively smooth. The bulk density of P1 sorbent is lower than that of PP, which can lead to higher buoyancy over the water surface. Compared with PP sorbent, the P1 sorbent has a smaller fiber diameter and higher porosity. It is favorable to the adsorption and adherence of oil on the fiber surfaces and in the voids. Thus, P1 sorbent may have higher sorption capacity. 3.2. Oil Sorption from Pure Oil Medium. To investigate the maximum oil sorption capacity of P1 sorbent, the sorption test was performed in oil medium without any water. For comparison, the sorption of the commercial PP sorbent was also studied. Figure 3 shows the oil sorption capacities of the P1 and PP sorbent. The sorption capacities of PP for motor oil, peanut oil, ethylene glycol, and diesel are 19, 13, 15, and 8 g/g, respectively. Surprisingly, the sorption capacities of P1 sorbent for the above four organics are 146, 119, 81, and 38 g/g, respectively. It is about 59 times that of the commercial PP sorbent. The above result implies that electrospinning is an effective method to prepare oil sorbent with high capacity. For the four organics, the sorption capacity of the P1 sorbent increase in the following order: diesel < ethylene glycol < peanut oil < motor oil. The capacity of the PP sorbent also shows the same trend. Thus, for both P1 and PP sorbent, the sorption capacity is highest for motor oil, i.e., for the organics with highest viscosity. A high oil viscosity can induce two opposite effects: increasing sorption by improved the adherence of oil onto the fiber surface, and decreasing sorption by inhibiting the oil penetration into the interior of sorbent.3,9,22 For the obtained P1 and PP sorbents, both are macroporous materials, the former plays a leading role in sorption capacity. The observation is consistent with the results of other authors.3,23
3.3. Sorption in OilOverwater Bath. Sorption selectivity between oils and water is an important parameter for oil sorbent used in spill cleanup on the water surface. The oil sorption characteristics of P1 and PP in the oiloverwater baths containing different thicknesses of oil film (motor oil, peanut oil,and diesel) are illustrated in Figure 4. The amounts of water pickup are also shown, which allows indication of sorbent selectivity between oils and water. As shown in Figure 4, the oil sorption capacity of P1 sorbent increases with the increasing of oil film thickness until the P1 sorbent is saturated with oil. At low quantities of oil spilled over water, almost the entire amount of added oil can be picked up by the P1 sorbent. In oil/water mixture systems, the maximum oil capacities of P1 for motor oil, peanut oil, and diesel are 149 g/g, 107 g/g, and 37 g/g, respectively. Considering the experimental error, there is no difference in oil capacities for pure oil and oil/water mixture systems. However, for PP sorbent, the oil sorption capacity is almost a constant with varying the oil film thickness (321 mm). The water uptakes for P1 and PP sorbent are in the same order (∼0.1 to 0.2 g/g). Thus, compared with PP, P1 sorbent presents excellent oil/water selectivity (∼1000 times) due to its higher oil sorption capacity. The high oil capacity and excellent oil/water selectivity of P1 make it an attractive sorbent in oil spill cleanup, as it will reduce the mass of sorbent and reduce the volume of liquid pickup from the spilled site which has to be handled on board or on shore 4 Buoyancy is also an important parameter of oil sorbent for practical operation in oil spill cleanup over water. High buoyancy can keep the sorbent floating over the water surface before and after the oil sorption, which is helpful for the oil sorption and remove from the spilled area. In a static system, when placing P1 sorbent on the surface of oil over the water (Figure 5a), the sorbent floats on the surface and keeps sucking the oil (Figure 5b). After 60 min, the P1 sorbent could absorb all motor oil and still float on the water (Figure 5c). In dynamic systems, the test is carried out under constant steering (approximately 500 rpm) in a magnetic stirrer. As shown in Figure 5ef, the P1 sorbent also has good buoyancy and high sorption capacity in dynamic systems. The high buoyancy is due to its low density and oleophilic hydrophobic properties. 3.4. Oil Sorption Mechanisms. In order to study the sorption mechanism of PVC/PS fibers, oil-saturated P1 sorbent was observed with the SEM (Figure 6a). It is obvious that the voids among fibers are filled with oil, and the oil adheres to the surface of fibers firmly. There are no obvious interfaces between oil and fibers. From the SEM image (Figure 6a), it is clear that the oil is mainly retained in the voids. For PP sorbent, some voids are filled with oil (Figure 6b). But some voids among fibers are vacant, which are due to the escape of oil from voids when the sorbent was drained, with only oil films and drops adhering to surfaces of these fibers. 4529
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Figure 4. The sorportion capacities of P1 and PP vs oil film thickness. (a) motor oil, (b) peanut oil, and (c) diesel.
Figure 5. Cleanup of motor oil film (dyed with Oil Red) on water by the obtained P1 sorbent.
Figure 6. SEM images of P1 (a) and PP (b) after sorption of motor oil.
The mechanism of oil sorption by sorbents can be adsorption, absorption, capillary action, or a combination of these.8 For fibrous porous sorbent, the adsorption and capillary action are the main controlling mechanism. Therefore, many parameters, such as the fiber properties (fiber diameter, surface configuration, lipophilicity, special surface area, density, etc.), the oil properties (special gravity, viscosity), pore structures in sorbent (porosity, pore sizes, pore shape, etc.), as well as the interactions between oils with sorbents affect the oil sorption capacity. Some articles reported on the influences of fiber properties, oil properties, and
oil/sorbent interaction on the oil sorption capacity.4,24 However, only a few articles reported on the effect of pore structures, especially the porosity and pore size.23 For conventional fibrous sorbents, such as polypropylene fibers, milkweed, kapok, and cotton, the fibers diameter are usually in the range of tens to hundreds of micrometers.4,22,25,26 Compared with them, the electrospun P1 fiber has a smaller diameter (1.53 μm), thus leading to larger special surface area and larger oil adsorption on the fiber surfaces. More importantly, the electrospun P1 sorbent has very low density (23 mg/cm3) and very high porosity (∼99.7%). Therefore, huge numbers of voids among fibers provide large amounts of storage volume for sorbed oils, acting as an oil reservoir. In addition, the size of voids in P1 sorbent is in the appropriate range that the sorbed oil could be retained in voids rather than escaped from voids in the draining. While for the PP sorbent, a part of the oil flees from some voids due to large pore size. The above results indicate that high porosity and appropriated void size are the keys for the high capacity. However, the appropriated void size is determined by many parameters, such as oil viscosity and special gravity, fiber diameter and surface roughness, interaction between oil and fiber, interface tension force, etc. More detailed works are needed in the future. 4530
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Environmental Science & Technology The results in this work demonstrate that electrospinning is an effective method to prepare oil sorbent with high capacity. PVC/ PS sorbent is a better alternative to the widely used PP sorbent for oil spill cleanup due to its high oil sorption capacity, excellent oil/water selectivity, and high buoyancy. The findings provide new insight into fabricating low-cost and highly efficient oil sorbents, which will offer important opportunities in oil-spill cleanup applications.
’ AUTHOR INFORMATION Corresponding Author
*Phone: (086)532-84022676; fax: (086)532-84022787; e-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of China (50872061) and the Foundation of Qingdao Science and Technology (10-3-4-4-12-jch). Any use of products or firm names is for descriptive purposes only and does not imply endorsement by the Chinese Government. ’ REFERENCES (1) Fingas, M. F. The Basics of Oil Spill Cleanup; Lewis Publishers: London, 2001. (2) Schnoor, J. L. The gulf oil spill. Environ. Sci. Technol. 2010, 4833; DOI 10.1021/es101727m. (3) Ceylan, D.; Dogu, S.; Karacik, B.; Yakan, D. S.; Okay, S. O.; Okay, O. Evaluation of butyl rubber as sorbent material for the removal of oil and polycyclic aromatic hydrocarbons from seawater. Environ. Sci. Technol. 2009, 43, 38463852; DOI 10.1021/es900166v. (4) Lim, T. T.; Huang, X. Evaluation of kapok (Ceiba pentandra (L.) gaertn.) as a natural hollow hydrophobic-oleophilic fibrous sorbent for oil spill cleanup. Chemosphere 2007, 66, 955963; DOI 10.1016/ j.chemosphere.2006.05.062. (5) Lin, C.; Huang, C. L.; Shern, C. C. Recycling waste tire powder for the recovery of oil spills. Resour. Conserv. Recycl. 2008, 52, 11621166; DOI 10.1016/j.resconrec.2008.06.003. (6) Lin, Q.; Mendelssohn, I. A.; Carney, K.; Miles, S. M.; Bryner, N. P.; Walton, W. D. In-situ burning of oil in coastal marshes. 2. Oil spill cleanup efficiency as a function of oil type, marsh type, and water depth. Environ. Sci. Technol. 2005, 39, 18551860; DOI 10.1021/es0490626. (7) Whitfield, J. How to clean a beach. Nature 2003, 422, 464466; DOI 10.1038/422464a. (8) Radetic, M.; Ilic, V.; Radojevic, D.; Miladinovic, R.; Jocic D.; Jovancic, P. Efficiency of recycled wool-based nonwoven material for the removal of oils from water. Chemosphere 2008, 70, 525530; DOI 10.1016/j.chemosphere.2007.07.005. (9) Choi, H. M.; Moreau, J. P. Oil sorption behavior of various sorbents studied by sorption capacity measurement and environmental scanning electron microscopy. Microsc. Res. Tech. 1993, 25, 447455; DOI 10.1002/jemt.1070250516. (10) Choi, H. M.; Cloud, R. M. Natural sorbents in oil spill cleanup. Environ. Sci. Technol. 1992, 26, 772776; DOI 10.1021/es00028a016. (11) Teas, C.; Kalligeros, S.; Zanikos, F.; Stournas, S.; Lois, E. ; Anastopoulos, G. Investigation of the effectiveness of absorbent materials in oil spills clean up. Desalination 2001, 140, 259264; DOI 10.1016/ S0011-9164(01)00375-7. (12) Radetic, M. M.; Jocic, D. M.; Jovancic, P. M.; Petrovic, Z. L.; Thomas, H. F. Recycled wool-based nonwoven material as an oil sorbent. Environ. Sci. Technol. 2003, 37, 10081012; DOI 10.1021/es0201303. (13) Deschamps, G.; Caruel, H.; Borredon, M. E.; Bonnin, C. ; Vignoles, C. Oil removal from water by selective sorption on hydrophobic cotton fibers. 1. Study of sorption properties and comparison
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