Scientists are describing what may be a “complete solution” to cleaning up oil spills — a superabsorbent material that sops up 40 times its own weight in oil and then can be shipped to an oil refinery and processed to recover the oil. Their article on the material appears in ACS’ journal Energy & Fuels.
T. C. Mike Chung and Xuepei Yuan point out that current methods for coping with oil spills like the 2010 Deepwater Horizon disaster are low-tech, decades-old and have many disadvantages. Corncobs, straw and other absorbents, for instance, can hold only about 5 times their own weight and pick up water, as well as oil. Those materials then become industrial waste that must be disposed of in special landfills or burned.
Their solution is a polymer material that transforms an oil spill into a soft, solid oil-containing gel. One pound of the material can recover about 5 gallons of crude oil. The gel is strong enough to be collected and transported. Then, it can be converted to a liquid and refined like regular crude oil. That oil would be worth $15 when crude oil sells for $100 a barrel. “Overall, this cost-effective new polyolefin oil-SAP technology shall dramatically reduce the environmental impacts from oil spills and help recover one of our most precious natural resources,” the authors said.
This paper discusses a novel approach that may provide a complete solution to combating oil spills. The technology is centered on a cross-linked polyolefin terpolymer (x–OS–DVB), containing 1-octene, styrene, and divinylbenzene units, which is an oil superabsorbent polymer (oil–SAP) with aliphatic and aromatic side chains that have similar solubility parameters (oleophilic and hydrophobic properties), with the hydrocarbons in crude oil. Some x–OS–DVB terpolymers, with desirable morphology (amorphous, low Tg, and high free volume) and lightly cross-linked (complete network) structure, show rapid oil absorption and swelling to reach a capacity 45 times their weight. The capacity of oil uptake (swelling) is inversely proportional to the cross-linking density. The combination of selective oil absorption (without water) and tough mechanical strength offers buoyancy, stability, and easy recovery on water surfaces. The recovered oil-swelled gel, containing more than 98% oil and 2% x–OS–DVB, is suitable for regular oil-refining processes (an economic, no waste, and no pollutant approach). The bulk side chains in x–OS–DVB result in a relatively low ceiling temperature for depolymerization and zero heating residue at 450 °C, well below the first distillation step (>600 °C) in oil refining. Furthermore, polyolefins are the most inexpensive polymeric material, with a large production capability around the world. Overall, this cost-effective new polyolefin oil–SAP technology shall dramatically reduce the environmental impacts from oil spills and help recover one of our most precious natural resources.
In light of the 2010 BP oil spill in the Gulf of Mexico, more than 20 years after the Exxon Valdez disaster in Alaska, we still have no effective technology for removing, recovering, and cleaning up oil spills or oil slicks from the surfaces of water bodies and shorelines. Despite the government’s “all hands on deck” approach to combating the oil spill, currently used methods (booms and skimmers, dispersants, and in situ burning) are decades old, low-technology, and manpower-intensive, some with unknown environmental consequences. On the basis of these current methods, most spilled oil is wasted and becomes pollutants in our air and waters. Furthermore, any small fraction that is actually recovered generates a large quantity of solid and liquid waste itself, from tons of soiled boom and other oily waste. It is treated as industrial waste and buried at specially designated dumps, some near residential neighborhoods. In fact, oil spill accidents around the world are actually more frequent than the few highly publicized cases in United States. As shown in Figure S1 of the Supporting Information, every few years, there has been a major oil spill, as a result of storage tanks and cracked pipes, oil tanker collisions or wrecks, and even from war with delivery-destroying oil facilities. The Exxon Valdez spilled 11 million gallons of oil into the Prince William Sound, but even that case did not make the top 10 list of the largest oil spills (the smallest spill on the list was 4 times larger than that of Exxon Valdez). Indeed, more than 30 oil spills were measured as larger and more devastating in the past 40 years.
There have been some reports discussing the sorption of crude oils, which is an attractive approach to the actual removal of spilled oil. Two general sorption mechanisms include adsorption (surface sorption) mechanism by employing porous or fibrous (high surface area) materials and absorption (matrix sorption/swelling) mechanism using cross-linked oleophilic and hydrophobic materials.(1) The economic and environmental concerns surrounding oil spills encouraged many researchers to investigate natural sorption materials, such as multifarious inorganic porous products (i.e., clay, talc, zeolites, silica aerogel, calcium fly ash, etc.),(2, 3) and organic biodegradable products (straw, hull, corncob, peat moss, sugar cane bagasse, wood/cotton fibers, wool-based materials, silkworm cocoon waste, etc.).(4-10) However, most of them show limited oil sorption capacity and also absorb water, thus making the recovered solids unsuitable for calcinations; most of them end up in the landfills. Meltblown polypropylene (PP) pads and booms(11, 12) are the most commonly used oil sorbent materials, adsorbing oil in their interstices via capillary action. Because of the weak oil–substrate interaction (adsorption mechanism), the fiber-based adsorbers exhibit many disadvantages, including failure to maintain oil of low viscosity and easy rebleeding of adsorbed oil under a slight external force. .
[toggle title=”Usage of hydrophobic alkyl acrylate oil absorbents.” height=”auto”]
There are also some reports disclosing the usage of hydrophobic alkyl acrylate oil absorbents,(13-16) such as cross-linked styrene/acrylate, 1-octene/acylate, and octadecene/maleic anhydride co-polymers. However, these resins contain some hydrophilic polar groups and require an additional procedure for the cross-linking reaction after co-polymerization. In addition, this method has the drawback of a long absorbing time, especially for aliphatic hydrocarbon components. Some synthesized rubbers,(17-22) such as polybutadiene, butyl rubber, styrene–butadiene rubber (SBR), and ethylene propylene diene monomer (EPDM), were also modified (grafting and cross-linking) to achieve the network structure for oil absorption. However, the solution cross-linking is hardly controlled; they usually require extensive solvent extraction to remove the soluble fraction. The resulting sol-free materials possess various degrees of cross-linking density, reducing the overall oil swelling capability. Some methods, i.e., milling, electric-spinning, and foaming of the oil absorbents to increase surface area, have also been applied to improve oil-absorbing speed. However, many materials,(23-25) similar to that of meltblown PP, only physically adsorb oil at the surface by capillary action. Recently, several papers applied the high surface area materials, including nanowire membranes,(26) nanocellulose aerogels,(27) and carbon nanotube aerogels,(28) to increase oil adsorption capacity. The treatment of the recovered solid materials is always a major concern, including waste disposal, recyclability, and biodegradability. Overall, it is still a major scientific challenge to identify a suitable oil superabsorbent polymer (oil–SAP) material that can offer a comprehensive solution for combating future oil spills.
Materials and Instrumentation
All O2– and moisture-sensitive manipulations were carried out inside an argon-filled vacuum atmosphere drybox. 1-Octene, styrene, and p-divinylbenzene (Sigma-Aldrich) were distilled over CaH2 under argon. Toluene (Wiley Organics) was dried via refluxing over sodium metal for 48 h under argon. TiCl3·AA (AA = activated by aluminum metal), AlEt2Cl (25 wt % in toluene) (Sigma-Aldrich), hexane, benzene, cyclohexane, and petroleum (VWR) were used as received. Gasoline (octane number of 98) and diesel were purchased from a service station of the Shell Oil Company. Meltblown PP pads were kindly provided by Newpig Corporation in Pennsylvania.
All 1H spectra were recorded on a Bruker AM-300 instrument in chloroform-d at room temperature. The thermal properties of the polymers were measured by differential scanning calorimetry (DSC) using a Perkin-Elmer DSC-7 instrument controller with a heating and cooling rate of 20 °C/min under nitrogen. The molecular weight of the polymer was determined using Waters gel permeation chromatography (GPC). The columns used were Phenomenex Phenogel of 105, 104, 103, and 500 Å. A flow rate of 0.7 mL/min was used, and the mobile phase was tetrahydrofuran (THF). Narrow molecular-weight polystyrene standards were used to estimate the molecular weight.
Synthesis of 1-Octene/Styrene/Divinylbenzene (OS–DVB) Terpolymers
In a typical 1-octene/styrene/DVB terpolymerization reaction, 50 mL of toluene, 5 mL of 1-octene, 5 mL of styrene, and 0.2 mL of DVB were charged into a Parr 450 mL stainless autoclave equipped with a mechanical stirrer in an argon-filled drybox. The reactor was sealed, moved out of the drybox, and then purged with nitrogen gas at 25 °C. About 0.101 g of TiCl3(AA) and 4 mL of AlCl2Et (25 wt % in toluene) were added under nitrogen pressure to initiate the polymerization. After agitation for 3 h at elevated temperature (45 °C), the reaction was terminated by adding 100 mL of dilute HCl solution in methanol. The polymer was isolated by filtration, washed completely with methanol, and then dried under vacuum for 8 h. About 4 g of OS–DVB terpolymer was obtained. The terpolymers were completely soluble in common organic solvents, including toluene, THF, and decalin. They were analyzed by 1H NMR and GPC to determine their molecular structures.
Oil Absorption Evaluation
Oil absorption tests were conducted following the standard method [American Society for Testing and Materials (ASTM) F726-06] using various oils. Typically, a piece of polymer around 0.2 g was put into gasoline. After a certain time, the sample was picked up with a tweezer and laid on a filter paper to remove the excess gasoline before weighing it on a balance. Oil absorbency (g/g) was calculated by the weight ratio between the absorbed oil and the original dried materials. To study swelling kinetics, the above measurements were carried out from time to time. In addition, the absorption study was also extended to the individual crude oil components, including alkanes (such as heptane), cycloalkanes (such as cyclohexane), and aromatic hydrocarbons (toluene and xylene). Evidently, these amorphous x–OS–DVB-based oil–SAP samples are very effective at absorbing both aliphatic (linear and cyclic) and aromatic hydrocarbon molecules, and their capacities are basically controlled by cross-linking density (swelling ability), lower cross-linking density higher swelling, and absorption capacity.
Results and Discussion
Our research approach has been focused around the design of a new oil–SAP that can simultaneously exhibit a combination of several essential properties, including (a) high oil absorption capacity, (b) fast kinetics, (c) no water absorption, (d) buoyancy and good mechanical strength for easy recovery from the water surface, (e) recovered oil/oil–SAP mixture suitable in a regular oil-refining process, and (f) cost-effectiveness. The study has been centered on new polyolefin-based oil–SAP polymers that are petroleum downstream products, having similar solubility parameters (oleophilic and hydrophobic properties), with the hydrocarbon components in crude oil and low production cost. With a suitable polyolefin structure and morphology, the polymer shall rapidly absorb oil (without water) and swell its volume to accommodate a large quantity of absorbed oil. The resulting oil/oil–SAP mixture (pure hydrocarbons) will float on the water surface with good stability for easy recovery. In addition, it is highly desirable that the oil–SAP polymer can be thermally decomposed back to low-molecular-weight hydrocarbons (monomers) at <600 °C during the first refining (distillation) step. Therefore, the recovered oil/oil–SAP mixtures shall be suitable for regular oil-refining processes and resolving concerns regarding disposal, recyclability, biodegradability, and environmental issues caused by the initial oil spill.
As illustrated in Scheme 1, a series of amorphous OS–DVB terpolymers (I) were systematically synthesized using a common heterogeneous Ziegler–Natta catalyst that shows suitablity for preparing high-molecular-weight OS–DVB terpolymers with good control of composition and random structure and only engaging mono-enchainment for DVB. As will be discussed, a lightly cross-linked oil–SAP material (an oil-swellable network) is crucial for achieving a high oil absorption capability, which requires a high-molecular-weight OS–DVB terpolymer (I) with few DVB cross-linker units in every polymer chain. The resulting OS–DVB terpolymer (I), containing some pendent styrene moieties (thermal cross-linkers), is completely processable (soluble) in forming various product sizes and shapes (II). However, upon thermal heating (>220 °C), it becomes a completely insoluble network structure (III) by engaging in a Diels–Alder [2 + 4] interchain cycloaddition reaction between two pendent styrene units in the adjacent polymer chains.(28-30) This solid-state cross-linking reaction (effective and without any byproduct) eliminates the requirement of an expensive solution removal of hydrocarbon-soluble fractions shown in many papers, in which the cross-linking reactions were usually carried out in dilute solutions with a considerable amount of intrachain coupling activities.
Table 1 summarizes the experimental results of the terpolymerization reaction with 1-octene, styrene, and DVB using a heterogeneous Ziegler–Natta catalyst [i.e., TiCl3(AA)/AlCl2Et]. This traditional Ziegler–Natta catalyst shows an effective incorporation of both 1-octene and styrene co-monomers and mono-enchainment of DVB to form the homogeneous OS–DVB terpolymer (I) solution with a rather narrow molecular weight and composition distribution. In general, the terpolymerization results are quite consistent with the previous observation in the co-polymerization reactions between 1-octene and styrene(31) and 1-octene and p-methylstyrene.(32) Although 1-octene exhibits higher reactivity than styrenic monomers (styrene and p-methylstyrene), the styrenic co-monomers can be homogeneously incorporated in the co-polymer with high molecular weight and a broad range of co-polymer compositions using heterogeneous Ziegler–Natta catalysts. Figure S2 in the Supporting Information shows a typical GPC curve of the terpolymer (run A-1) with high molecular weight (Mw 330 000 g/mol) and relatively narrow molecular weight distribution (Mw/Mn 2.1). As will be discussed, the DSC curves also show a sharp Tg that is proportional to the styrene content and the styrene and DVB contents are directly proportional to the monomer feed ratios. The high OS–DVB terpolymers, containing more than 20 mol % aromatic units (styrene and DVB), have been prepared without any detectable cross-linking reaction. All resulting OS–DVB terpolymers were completely soluble in common hydrocarbon solvents, such as hexane and toluene, at ambient temperature.
Table 1. Synthesis and Oil Absorption Evaluation of Cross-Linked 1-Octene/Styrene/DVB Terpolymersa
aPolymerization condition: TiCl3(AA)/AlCl2Et = 0.101 g/4 mL (25 wt % in toluene), 50 mL of toluene, and 45 °C for 3 h. Cross-linking condition: 240 °C for 2 h.
bAbsorption time = 24 h
cA, 1-octene; B, styrene; and C, DVB.
dDetermined by 1H NMR spectra.
eMeasured by GPC with a standard polystyrene calibration curve.
fAfter the thermal cross-linking reaction, the gel content was determined from the toluene-insoluble part after Soxhlet extraction.
gCommercial meltblown PP pad (adsorption mechanism).
Figure 1 shows a typical 1H nuclear magnetic resonance (NMR) spectrum of OS–DVB terpolymer (run A-5). There are several aliphatic proton chemical shifts at 0.8 ppm, corresponding to CH3 in the 1-octene units, and a band between 0.9 and 1.7 ppm, corresponding to CH2 and CH in the polymer backbone and 1-octene side chains. In the expanded region, there are three minor bands around 5.2 and 5.7 ppm (CH═CH2) and 6.7 ppm (CH═CH2), corresponding to the vinyl groups in the pendent styrene groups (after mono-enchainment of DVB units). There is also a broad aromatic proton band between 6.9 and 7.4 ppm (C6H4) from both styrene and DVB units. The integrated intensity ratio between all three vinyl protons from the DVB units and the phenyl protons from both styrene and DVB units determines the styrene and DVB mole ratio. The mole ratio of 1-octene was determined by the CH3 chemical-shift intensity at 0.8 ppm.
Figure 2 shows thermal properties of the OS–DVB terpolymers in Table 1, including differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) measurements. On the left panel of Figure 2, all DSC curves exhibit only one sharp Tg transition in the flat baselines. There was no detectable melting point up to 200 °C. The Tg (−80 °C) of poly(1-octene) linearly increases with its aromatic co-monomer content. All OS–DVB terpolymers (Table 1) exhibit low Tg values (< –50 °C), even with 25 mol % of aromatic (styrene and DVB) content. The combination implies a homogeneous terpolymer microstructure with a completely amorphous morphology and high free volume. The right panel of Figure 2 shows the TGA curve of an OS–DVB sample (run A-1). Under an inert atmosphere, the OS–DVB terpolymer starts its thermal decomposition at 300 °C and rapidly decreases its weight around 400 °C. At 450 °C, the OS–DVB terpolymer was completely decomposed without any residue, indicating the formation of volatile small hydrocarbon molecules. On the basis of the mass spectrum, the main volatile components are the 1-octene and styrene monomers, with their derivatives having a molecular size below C20. Overall, the thermal decomposition profile resembles those of poly(1-octene) and polystyrene homo-polymers. The bulky side chains in 1-octene and styrenic monomer units weaken the C–C bonds along the backbone, which may result in chain scission and the subsequent free-radical-mediated depolymerization(33-35) at a relatively low pyrolysis temperature. It is interesting to note that the OS–DVB terpolymer (I) shall engage in rapid cross-linking activities during the thermal process (>200 °C). However, the resulting cross-linking structure appears to have little effect on the overall thermal degradation.
Before oil absorption evaluation, the OS–DVB terpolymer (I) solids were divided in 1/4 in. particles and then heated at 240 °C under N2 for 2 h to complete the cross-linking reaction. The resulting x–OS–DVB samples (III) were subjected to a vigorous solvent extraction by refluxing toluene for 36 h to examine any soluble fraction that was not fully cross-linked into the network structure. Evidently, all x–OS–DVB samples in Table 1 show no detectable soluble fraction, even in the terpolymer (run A-1) with only 0.3 mol % DVB units (about 5 DVB units per polymer chain), indicating a very efficient thermal cross-linking reaction under a polymer melt state condition. The extensive chain entanglement among many polymer chains significantly enhances the interchain Diels–Alder [2 + 4] cycloaddition reaction between two pendent styrene units from adjacent polymer chains. The high-molecular-weight terpolymer with narrow molecular weight and composition distributions certainly also helps in achieving the complete network structure.
The resulting x–OS–DVB terpolymers (runs A-1–A-5) were subjected to contact with various oils and pure hydrocarbons to understand their oil absorption capability and kinetics. Because crude oil is predominantly a mixture of aliphatic and aromatic hydrocarbons with various molecular weights, the exact molecular composition varies widely from formation to formation. Thus, we decided to examine a broad range of petroleum products, including some representative of pure aliphatic and aromatic compounds. Table 1 summarizes the experimental results with several individual aliphatic and aromatic hydrocarbons (i.e., hexane, cyclohexane, benzene, and toluene) and some common petroleum products (i.e., gasoline, petroleum, and diesel). Figure 3 compares the oil uptake versus time for five x–OS–DVB samples (runs A-1–A-5) with a crude containing approximately 70% volatile light oils and 30% nonvolatile heavy oils.
Seemingly, the oil absorbency and swelling capacity in x–OS–DVB is largely controlled by cross-linking density, reversely proportional to the cross-linking density. There is a very minor dependence upon the absorbates, either the aliphatic or aromatic hydrocarbons, or even the mixed oil products. All of the x–OS–DVB samples contain both aliphatic and aromatic side chains with similar mole ratios but with different cross-linking densities. The lowest cross-linking density of x–OS–DVB (run A-1), with 82.3/17.4/0.3 1-octene/styrene/DVB mol %, exhibits a high absorbent capacity and the largest degree of swelling, forming a softer and more cohesive gel formation. Conversely, the x–OS–DVB sample with high cross-link density (run A-5) shows lower absorbent capacity and swell. The gel strength is firmer and can maintain particle shape, even under modest pressure.
Figure 4 compares the oil absorption performance of the x–OS–DVB sample (run A-1) with a state-of-the-art meltblown PP pad that is fabricated from a nonwoven fibrous PP textile with a highly crystalline polymer structure and porous morphology (high surface area). They were examined side by side for comparison. The meltblown PP pads (adsorption mechanism) show rapid oil adsorption in their interstices by capillary action, saturating at 10 times the weight of uptake without any visible volume enlargement. The adsorption mechanism happens only on the PP fiber surfaces (not inside the crystalline matrix), which is advantageous with fast kinetics but with limited capacity, and the weak oil–PP interaction results in some adsorbed oil rebleeding under a slight external force. On the other hand, the lightly cross-linked oil–SAP sample (run A-1), with amorphous morphology, gradually absorbs oil in its matrix, increasing its weight by more than 10 times within 10 min and reaching 40 times its weight after 12 h. Its oil sorption capacity is superior (>4 times) to that of the state-of-the-art meltblown PP pad.
Figure 5 shows the action of this x–OS–DVB sample (run A-1) during the oil absorption. The starting 1/4 in. oil–SAP sample (Figure 5a) can effectively absorb crude oil from the water surface with a large expansion of its volume (>40 times). Subsequently, the resulting oil/oil–SAP mixture floating on the water surface (Figure 5b) is ready for collection and can be picked up with a tweezer without leaking oil (Figure 5c). The combination of good mechanical strength (cross-linked elastic structure) and a strong affinity between the oil and polymer matrix ensure its structure integrity and oil absorption stability. In a real-world context, it shall also be stable under ocean environments (waves, wind, sunlight, etc.) and easily removed from the water surface. Had this material been applied to the top of the leaking well head in the Gulf of Mexico during the 2010 spill, this oil–SAP could have effectively transformed the gushing brown oil into a floating gel for easy collection and minimized the pollution consequences to our air and waters.
In addition to effective oil recovery, the resulting oil-swelled gel mixture can be treated as crude oil, suitable for regular refining processes (distillation and cracking). The mixtures contain no water and have nearly the same composition as the original crude oil. During refinery, the minor component (2–3%) of x–OS–DVB polymer will be thermally decomposed back to <C20 hydrocarbon molecules (typically existing in crude oil) without residue (Figure 2), well below the typical crude-oil-refining temperature (>600 °C). This process has multifarious benefits: it eliminates the concern surrounding solid waste disposal, recyclability, and biodegradability and maintains our reservations of natural resources. Furthermore, polyolefin products are the most inexpensive of polymeric materials and are capable of large-scale production around the globe. With conservative estimates, the production cost of new oil–SAP material comes below $2/lb in large-scale industrial production. A total of 1 lb of oil–SAP, with a 40 times absorption capacity, can recover more than 5 gallons of spilled oil (currently treated as a pollutant instead of a useable resource) to regular crude oil that is worth more than $12 (on the basis of $80/barrel).
In this study, we have developed a new oil–SAP technology based on polyolefin-based x–OS–DVB terpolymers that can be effectively prepared by a conventional Ziegler–Natta catalyst and then thermally cross-linked into the oil–SAP materials. The combination of oleophilic and hydrophobic properties with amorphous morphology, high free volume, and a cross-linked network offers a desirable matrix for oil absorption and swelling. The oil uptake is inversely proportional to the cross-linking density. Oil uptake with up to 45 times that of the polymer weight and fast kinetics has been observed in a lightly cross-linked x–OS–DVB terpolymer. Overall, this new oil–SAP technology exhibits a combination of benefits in oil recovery and cleanup, including (i) high oil absorption capability, (ii) fast kinetics, (iii) easy recovery from the water surface, (iv) no water absorption, (v) no waste in natural resources, and (vi) being cost-effective and economically feasible. This new oil–SAP technology shall fundamentally address the multiple issues created by mega oil spills, whether seen from an environmental or economic standpoint.
The authors declare no competing financial interest.
The authors gratefully acknowledge the financial support of this work by a National Science Foundation (NSF) Research Grant (CBET-1067554) and commercial feasibility study by Ben Franklin Foundation under its Translational Research in Energy Support Program.
Deepwater Horizon Oil Spill flowed unabated for three months in 2010.
Largest accidental marine oil spill in the history of the petroleum industry.
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