Nanotechnology-Based Solutions for Oil Spills

Nanotechnology-Based Solutions for Oil SpillsNanotechnology-Based Solutions for Oil Spills

Technology Brief

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About the Cover: Towing Clean Boom.

A boat tows cleaned boom across Barataria Bay to replace soiled boom. A boom is a floating device used to contain oil on a body of water.

Image Courtesy: NOAA

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NANoteCHNology-BAseD solutIoNs for oIl sPIlls

Fig. 1: The Ixtoc I exploratory well blew out on June 3, 1979 in the Bay of Campeche off Ciudad del Carmen, Mexico. By the time the well was brought under control in 1980, an estimated 140 million gallons of oil had spilled into the bay. [Courtesy: The National Oceanic and Atmospheric Administration (NOAA), http://www.noaa.gov/]

Introduction The recent oil spill in the Gulf of Mexico is widely acknowledged to be among the worst ocean oil spills in world history. Inevitably, the spill has once again raised serious concerns worldwide about the likely environmental impact of such catastrophic oil spills caused by oil tanker accidents at sea or mishaps during loading and unloading of oil from tankers at seaports. Similar concerns are also associated with discharge of oil in areas around oil wells and oil storage facilities1. Such oil spills can cause havoc to marine ecology (sea birds, mammals, algae, coral, seagrass etc.), beside the health hazards to the human population located in nearby coastal zones. Moreover, the economic loss suffered by oil companies resulting from oil-spillage is enormous (for example, BP p.l.c. recently revealed that, as on Sept 3, 2010, the cost of its response to the devastating Gulf of Mexico oil spill disaster had amounted to $8 billion dollars2).

Numerous solutions have been proposed for dealing with the problem of oil spills. These include (i) use of microorganisms to “digest” the oil, (ii) mechanical means like skimmers, booms, pumps, mechanical separators etc., (iii) sorbents to remove oil from water through adsorption and/or absorption and (iv) use of chemical dispersants like detergents etc. Conventional

techniques are not adequate to solve the problem of massive oil spills. In recent years, nanotechnology has emerged as a potential source of novel solutions to many of the world’s outstanding problems. Although the application of nanotechnology for oil spill cleanup is still in its nascent stage, it offers great promise for the future. In the last couple of years, there has been particularly growing interest worldwide in exploring ways of finding suitable solutions to clean up oil spills through use of nanomaterials.

Importance of Wettability for Water - oil separationSolid surfaces with unique wettability characteristics combining superhydrophobicity (water contact angle (CA) greater than 1500) along with superoleophilicity (oil CA smaller than 5°), have attracted great deal of interest in the field of marine coatings, microfluidic devices, self-cleaning surfaces etc3-5. Such surfaces have especially significant potential for the possible separation of oil and water. The phenomenon of superhydrophobicity has evolved over millions of years in nature and manifests itself in examples such as lotus leaves or water strider legs. A superhydrophobic surface is one that can cause water droplets to bead off completely. Such surfaces exhibit water droplet

contact angles of 150° or higher. In addition, the contact angle hysteresis can be very low (the receding contact angle is within 5° of the advancing contact angle), producing a surface on which water droplets simply roll off. Consequently, a self-cleaning surface results since the rolling water droplets remove dirt and debris. Superhydrophobicity is an outcome of a combination of intrinsic hydrophobic properties of the material (chemical composition) that constitutes the surface, as well as the microscale and nanoscale roughness of that surface. The surface of the lotus leaf, for example, is textured with 3-10 micron-sized hills and valleys that are decorated with nanometer-size particles of a hydrophobic wax-like material. The

Y. R. Mahajan

Nanotechnology-Based Solutions for Oil Spills

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Fig. 2: Various approaches for oil spill cleanup using nanomaterials nanotechnologies

Aerogel Materials

Nanogel@aerogel6

Company: Cabot Corp., USA http://www.cabot-corp.com/Key features: Hydrophobic silica aerogel is non-toxic, lightweight (bulk density - 70 - 90 kg/m3 ), selectively absorbs oil (800 - 1000 g/100 g)

Aeroclay®

Company: Aeroclay Inc., USA http://www.aeroclayinc.com/

Key features: Aeroclay is made from clay and polymer blended mixture using freeze drying method. It is an eco-friendly, low cost and ultra light aerogel based on a unique patented technology developed at Case Western Reserve University

Maerogel®

Company: Maero Tech SDN BHD (MTSB), Malaysia, http://www.maerotech.com/Key features: Maerogel is a cost effective silica aerogel produced from rice husk based on the technology developed at the Universiti Teknologi Malaysia (UTM)

Nano Dispersants

gold Crew® oil spill Dispersant (osD)Company: Environmental Chemical Solutions (ECS), USA, http://www.goldcrew.net/Key features: Gold Crew is a water based nano dispersant. It encapsulates and emulsifies the oil at a nanoscale and enhances the biodegradation process

g-Marine osC-1809 oil & fuel spill Clean-uP!Company: Green Earth Technologies Inc., USA http://www.getg.com/Key features: It is a unique plant-derived biodegradable formulation, which rapidly emulsifies and encapsulates oil spills. The nano-colloidal suspension of oil in water can be easily “digested” by the bacteria present in the water

table 1. Products based on nanomaterials/nanotechnologies for oil spill remediation*

of about 20-25 mN/m spreads quickly over the surface, exhibiting superoleophilic property. Surfaces possessing such a unique combination of superhydrophobic and superoleophilic characteristics offer great potential to construct an implement such as a membrane or filter or sponge/foam to separate oil from a water-oil mixture. This is accomplished either by 1) surface modification/coating of an implement using nanotechnology or by 2) constructing a device like a nanosponge using a nanomaterial. Figure 2 schematically shows the various approaches employed for oil spill cleanup using nanomaterials while Table 1 presents a list of products based on nanomaterials/nanotechnologies and their manufacturers, along with the key product features for oil spill remediation. The present article specifically presents an overview of the above approaches.

hills and valleys ensure that the surface contact area available to water is very low while the hydrophobic nanoparticles prevent penetration of water into the valleys. The net result is that water cannot wet the surface and, therefore, forms nearly spherical water droplets, leading to superhydrophobic surfaces.

It is of interest to note that liquids possessing different surface tension, γlv exhibit varying wettability characteristics on the same surface. For example, a water droplet having γlv = 73 mN/m easily slips over the superhydrophobic surface of a lotus leaf, while a drop of oil with lower surface tension

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Verusol(r) Marine and VerusolVe™ Marine 200 HPCompany: VeruTEK Technologies Inc., USA http://www.verutek.com/Key features: VeruSOL(R) Marine is derived from plant materials and does not contain any solvents, alcohols or ethers. VeruSOLVE™ Marine 200 HP works via Surfactant-Enhanced Chemical Oxidation (S-ISCOTM) process. VeruSOLVE™ Marine products, combines the plant-oil based biodegradable surfactants which emulsify and disperse spilled oil with hydrogen peroxide that helps to break apart and destroy oil by oxidation while promoting biodegradation process.

Nanostructured Carbon

recam®

Company: SA Envitech s.r.l., Italy http://www.sa-envitech.com/Key features: It is a hydrophobic sorbent based on reactive nanostructured carbon (graphene cells/CNTs), having excellent capacity of sorbing oil (90 g/g)

High reactivity Carbon Mixture (HrCM) sorbentCompany: Golden Formula Holding Ltd, Ukraine http://www.goldenformula.org.uaKey features: It is a High Reactivity Carbon Mixture (HRCM) of graphene, nanotubes, nanorings, nanofractals and branched nanotubes. HRCM selectively sorbs oil from oil containing water (1 g of HRCM sorbs approximately 80 g of oil) which can be reclaimed by simple squeezing. After squeezing, HRCM looses sorbing ability to 30 – 40% (part of oil will remain in HRCM mass) but continues to ‘work’

Magnetic Composites

CleanMag®7 (A Co-polymer organic Product)Company: Environmental Magnetic Technologies, Greece, http://www.cleanmag.gr/ind1.htmKey features: It is a new magnetic, oleophilic and porous sorbent material which is able to sorb 100% oil without polluting the environment, unlike chemical dispersants or solidifiers. It is also non-toxic and can be recycled. The oil is quickly sorbed at a weight ratio of 1: 6-9, when it comes in contact with CleanMag

ecoMag®8

Company: N. C. Christodoulou (Nicosia, CY)Key features: EcoMag is a magnetic sorbent based on a porous inorganic matrix material like Perlite and a magnetic nanoscale component such as Fe3O4 magnetite, γ Fe2O3 maghemite or Ba-ferrite. It is used as a sorbent to preferentially sorb oil from spills in the sea, lakes etc. EcoMag sorbs the oil and forms

a floating oil mixture trapped into the pores of solid material. This “EcoMag-oil” mixture can be separated from the water either by batch magnetic separation technique or by a continuous magnetic separation technique with nearly 100% recovery efficiency

Magnetic foamsCompany: AMT&C LLC, Troitsk, Russia http ://www.amtc.ru/en/catalog/foam/Key features: Magnetic foams consist of hydrophobic liquid foams containing magnetic nanosize particles (barium ferrite or magnetically rigid compounds of type Nd-Fe-B, Sm-Co, Sm-Fe with particles sizing 2 – 30 nm stabilized with surfactant).Use of magnetic liquid foam can increase effectiveness of oil removal and its recovery from water or solid body surface. The on-site production in necessary amounts is achieved from compact source materials with the help of the stationary or portable installations. The foam-oil complex can be collected effectively with the aid of magnetic devices, which is then destroyed, and a separated magnetic component may be reused

Chemical-free technology

ozonix Nanobubble technologyCompany: Ecosphere Technologies Inc., USAEcosphere Technologies Inc., USA http://www.ecospheretech.com/Key features: This chemical free technology uses millions of nanosized bubbles to raise the oil to the surface of sea water and performs the cleanup operation. The critical steps of this technology are: ozone generation, hydrodynamic cavitation, acoustic cavitation and electro-chemical deposition

Products based on Naturally occurring Materials

NanoBionic Jute (Burlap)Company: NanoTek-USA http://www.nanotec-usa.com/Key features: Shredded jute is treated with hydrophobic NanoBionics Textile treatment. When this Bionic treated jute is deployed to the oil spill it selectively absorbs and traps the oil and can be subsequently removed. Alternatively, the large sheets of jute can be treated with oleophobic NanoBionics Textile treatment. These Bionic jute sheets can be deployed onto beaches to prevent the oil from seeping into the ground

NanoBionic-sandCompany: NanoTek-USA http://www.nanotec-usa.com/Key features: Ordinary beach sand is treated with

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NanoBionics Sand Guard treatment during which each grain of sand is covered with Bionic nanoparticles. This product is economical and also helps in cleanup of the oil spill. When the NanoBionic-Sand is applied to the oil spill, the oil gets attached to the sand and the oil-sand mixture is dropped to the bottom of the ocean

fibertect® Cotton soaking (fBt-Cs) (Cotton-based sorbent)Company: First Line Technology, LLC, USA http://www.firstlinetech.comKey features: Ordinary beach sand is treated with NanoBionics Sand Guard treatment during which each grain of sand is covered with Bionic nanoparticles. This product is economical and also helps in cleanup of the oil spill. When the NanoBionic-Sand is applied to the oil spill, the oil gets attached to the sand and the oil-sand mixture is dropped to the bottom of the ocean

sea reClaimtM, (scoria-based sorbent)Company: Eco Renascence LLCEco Renascence LLC http://www.ecorenascence.com/Norex Technology, LLC, USA http://www.norextechnology.comKey features: Nanotechnology has been applied using scoria as a raw material and developed a unique and cost effective sorbent, Sea ReClaimTM. Scoria, a low cost material, is a kind of volcanic rock containing many holes or vesicles

organozorb™, (Based on Aluminosilicates)Company: Dove Biotech Co. Ltd., Thailand http://www.dovebiotech.com/Blue Gold, USA, http://www.oil.bgnano.com/Key features: Organozorb™, which employs both nano- and bio-technology, is based on naturally occurring aluminosilicates. Organozorb™ contains Polyhexamethylene Biguanide (PHMB), biocide and microporous mineral surfactants impregnated with microbes, Bacillus subtillis. These surfactants emulsify and separate the hydrocarbon molecules into tiny micro emulsions, wherein they come in contact with the powerful built-in bacteria. The end-chain structure formed during this process greatly enhances its bioremediation and efficiently accomplishes the task of total cleanup and eradication of oil and hydrocarbon spillage

Polymer-based sorbents

Polymer Nanofiber-based sorbentCompany: Milliken & Company, USAMilliken & Company, USA http://www.milliken.comKey features: The hydrophobic polypropylene nanofiber absorbs oil and other hydrocarbons. This

product is 2-3 times more effective than the current market products. These nanofibers are capable of absorbing 40 to 50 times their weight in oil. The fibers are bunched together in the form of booms and floated on oil contaminated water. The fibers then absorb the oil and the tiny pores present in the fibers help in retaining the oil after it is absorbed. Subsequently, the fibers can be squeezed to remove the oil, and then reused

gigasorbCompany: Ekosorber Ltd., RussiaEkosorber Ltd., Russia http://www.ecosorber.ru/Key features: This product is based on non-woven multicomponent, thermo-bonded polymer fiber web with low density (0.02 g/cm3). Its oil absorption capacity/cycle is 36 kg for 1 kg of sorbent. Its rate of sorption is 2.6 kg/kg/min and can be re-used about 100 times. Gigasorb sorbent is made from Megasorb by immobilizing nanoparticles (10-50 nm) of activated carbon on the surface of fibers. Gigasorb exhibits 8.6 times higher rate of sorption of hydrocarbons (18.6 kg/kg/min) than that of Megasorb (38 kg/kg/min). It can be regenerated by squeezing between rolls and can be used up to 250 times without its degradation. Both the sorbents are used on floating guard booms and are capable of simultaneously performing two tasks, i.e., collection and containment

*Note: All indicated features in the above table are as per reports/claims made by the manufacturers.

Nanomaterials and technologies

Aerogels

Hydrophobic silica Aerogels

Silica aerogels are extremely light materials having a specific gravity as low as 0.025 g/cm3, the lowest thermal conductivity of any known solid material, high surface area and high porosity (90 to 95%). This makes them suitable for use in diverse applications, including oil/water separation. When an oil-water mixture is brought into contact with an aerogel, the oil is absorbed and gets separated from the water provided the aerogel is hydrophobic in nature.

In general, the production of superhydrophobic aerogels involves energy-intensive supercritical drying and use of expensive precursors like methyltrimethoxysilane (MTMS). Therefore, large scale production of aerogels suitable for typical oil spill cleanup operations at a reasonable cost still has remained a challenge9. However, alternative cost effective processes for the manufacture of aerogels are now being developed9, 10. One such process involves use of inexpensive sodium silicate based precursor for the production of hydrophobic silica

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aerogels by following rapid gelation combined with simultaneous surface modification and subsequent ambient pressure drying. This process is amenable for economic and large scale production of silica aerogel beads. Another process, also based on simple ambient pressure drying method and use of relatively low cost TEOS precursor, has been jointly developed by researchers at Yonsei University, Seoul, Korea and Shivaji University, Kolhapur, India9. In this process, superhydrophobic aerogels were synthesized by two-step sol-gel route prior to ambient pressure drying. Surface chemical modification was carried out with 5% hexamethyldisilazane (HMDZ) in hexane. It was found that the surface modified TEOS-based aerogel is able to absorb oils up to about 12 times its own weight.

Modification by incorporation of chemical functionality can yield materials with specific chemical properties, such as hydrophobicity. For example, both the methyl and the perfluoro functional groups exhibit excellent properties in this regard, and they have been incorporated into silica aerogels, providing a durable hydrophobic material usable for separation of organic materials from mixtures of organics and water. The addition of fluorine to the aerogel, either during the sol-gel processing or by vapor treatment of a dried aerogel, produces a nanoporous material with very high hydrophobic property11,12.

Recently, Hanyang University and E&B Nanotech Co., Korea have jointly developed lightweight, cost effective, hydrophobic, mesoporous and transparent silica aerogels10. This process comprises of producing silica aerogel beads by rapid gelation of colloidal silica sol. This is followed by surface modification of wet silica beads by trimethylchlorosilane, followed by ambient pressure drying. The process is highly economical as it uses a low-cost silica source, i.e. sodium silicate, and does not involve expensive supercritical CO2 drying. It is also amenable to large scale production of silica aerogel beads.

water. Clay aerogels are fabricated by a one-step process using a freeze drying technique. Hence, the production cost is expected to be much lower than that of silica aerogels produced by an expensive supercritical CO2 drying process. The clay-based aerogels13 were first made by Professor David. A. Schiraldi of Case Western Reserve University and the aeroclay technology has been licensed to Aeroclay Inc, a startup company located in Ohio, USA. Clay-based aerogel is a versatile material and, when fired at 8000C, undergoes chemical transformation into a hard and lightweight ceramic. It becomes highly flexible like rubber when mixed with latex.

Maerogel (rice Husk Derived Aerogel)

Although aerogels can make significant contributions in the efforts to clean up the oil spills, their high cost is a major barrier in their widespread adoption. Dr. Halimaton Hamdan (http://www.chem.utm.my/units/physical/halimaton/) from the Universiti Teknologi in Malaysia has developed a cost effective process to produce silica aerogels14, using rice husk (agricultural waste) as a feedstock that could reduce the production cost of aerogels by 80%. Rice husk has high silica content, which is the main constituent of these aerogels. In addition to potentially being able to produce aerogels at one-fifth the current cost, the above method also addresses the problem of rice husk waste disposal. Green Earth Aerogel Technologies (GEAT)15, a startup company based in Barcelona, Spain, is also engaged in the development of silica aerogels using rice husk as a feedstock material. Their technology is protected by International PCT GB2010/000903. Other organizations also involved in the preparation of rice husk derived aerogels are Tonji University, China, Nano High-Tech Co., Shaoxing, China, and Tsinghua University16,

17, China. In fact, the earliest discovery of rice husk

Fig. 3: Silica (99.99% purity) based aerogels from rice husk ash [Courtesy: Fortunato Cardenas, CTO and Founder, Green Earth Aerogel Technologies, Spain]

Aeroclays

Aeroclay (clay based aerogel) is a superlight sponge made by blending a mixture of clay with polymer and water in a mixer. The blended mixture is freeze dried to obtain an ultra-lightweight sponge composed of 96% air, about 2% clay and 2% polymer. Aeroclay, being oleophilic in nature, readily absorbs oil from a water-oil mixture leaving behind water. The absorbed oil can be recovered by squeezing the aeroclay sponge.

The aeroclay can be made into any form, including granules, sheets or blocks, and effectively works in either fresh- or salt-

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derived aerogels was made by Tsinghua University, when they filed their patent No. CN 1449997 A on April 24, 200316. Recently, Nayak and Bera from National Institute of Technology, Rourkela, India have also developed silica aerogel by an ambient pressure drying process using rice husk ash as a raw material18

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Nano Dispersants

One of the technologies used for oil spill response involves the use of chemical dispersants which contain surfactant molecules (surface-active agents) that migrate to the oil/water interface and reduce interfacial tension between oil and water. With the aid of wave energy, tiny oil droplets break away from the oil slick. These small droplets get dispersed into the water column and remain in suspension and, thereby, become a good source of food for the naturally occurring bacteria. The dispersants catalyse the biodegradation process leading to the removal of spilled oil.

Green Earth Technologies have come up with a nanotechnology-based solution to clean up the oil spills. Their G-MARINE OSC-1809 and Fuel Spill Clean-UP! is a unique plant-derived biodegradable formulation, which rapidly emulsify and encapsulate oil spills while being benign to the environment. To synthesize this nanodispersant, plant based proprietary ingredients are processed into a colloidal micelle (particle size 1-4nm) solution. The small particle size of micelle allows it to penetrate and break the long chain molecular bonds in oils and form a colloidal suspension when mixed with water. This nano-colloidal suspension of oil in water can be easily “digested” by the bacteria present in the water. Gold Crew® Oil Spill

hydrophobicity can be induced in these clays by modifying them with quaternary amines (a type of surfactant containing nitrogen ion) 19, 20 .The presence of these amines render the clay hydrophobic in nature. These organophilic clays (or organoclays) are very efficient in selectively adsorbing the organic contaminants or oil from water. Recent studies21 have shown that the Brazilian organo-clays are more effective adsorbent materials than natural clays for hydrocarbons and are capable of adsorbing toluene and gasoline up to about 9.24 and 8.90 times their weight, respectively. Untreated natural clay, on the other hand, is able to absorb only 1.72 and 2.12 times its weight of toluene and gasoline, respectively.

Magnetic Materials

Magnetic Nanocomposites

Magnetic particle technology finds a number of applications for environmental remediation and wildlife preservation. A variety of products have been developed, such as ferromagnetic sorbents based on iron coated with polystyrene22, polymer coated vermiculite-iron composites23, multiwall carbon nanotube/iron oxide composites, activated carbon/magnetite, poly(dimethylsiloxane)-coated hematite etc. Recently, researchers from Tongji University, China have developed a novel product based on magnetic exfoliated graphite as a new sorbent for oil spill remediation24. The magnetic nanocomposites exhibit superhydrophobic adsorption of oil or hydrocarbon pollutants combined with superior magnetic properties, which enables them to effectively and quickly remove oil from a water-oil mixture by the application of an external magnetic field. These composites are

COOH

COOH

LiOH

COOLi

COOLi

FeSO4 sintering

vinyl triethoxy-

silane

highlyhydrophobic

Fe2O3@C

Fe2O3@C

Fe2O3

COO-Fe-OOC

COO-Fe-OOC

Fe2O3

Highly hydrophobicFe2O3@C nanoparticles

Water

N

S

N

S

Oil

Water Water Water

Fig.4: (a) Schematic showing the preparation method for super-hydrophobic Fe2O3@ C nano-particles and (b) technique for the removal of oil from water surface through Fe2O3 @ C nano-particles under the action of an external magnetic field [Reprinted with permission from American Chemical Society, Zu et al., ACS Appl. Mater. Interfaces, 2 (2010) 3141–3146]

Dispersant (OSD), VeruSOL® Marine and VeruSOLVE™ Marine 200 HP (Table 1) are some other environmentally friendly, non-toxic and non-solvent based formulations, which also promote the formation of small oil droplets and, thereby, accelerate the biodegradation process.

Hydrophobic organoclays

Natural clays like bentonites contain metallic cations, which impart hydrophilic character to the clay. Therefore, they are not suitable sorbents for the removal of organic compounds. However,

a)

b)

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affinity between the resin and the hydrocarbon/oil. They have also studied the magnetic force and gravimetric oil removal capabilities of the material and it was demonstrated that the polymer nano-composite is capable of removing significant quantity of oil (up to 8.33 times the weight of the composite) from a water-oil mixture. It was also shown that the oil removal capability is the sum of two distinct effects arising from the magnetic force and the chemical affinity between the resin and the oil.

Magnetic Carbon Composites

T. Viswanathan from University of Arkansas has recently developed a magnetic carbon nano-composite from renewable resource for oil spill cleanup and recovery27. This composite is very effective in quick and efficient removal of oil from water. When the magnetic carbon nano-composite is added to the body of oil-contaminated water, the liquid hydrocarbon material dispersed in the body of water gets attached to the nano-composite, and forms a mixture, which can then be removed using a strong magnet. Subsequently, the removed oil can be recovered from the nano-composite after the magnetic field is removed.

Carbon-metal nano-composites are synthesized from renewable resources such as lignin (a major non-cellulosic constituent of wood that is commercially available as a by-product from the paper industry), tannin, lignosulfonate, tanninsulfonate or their mixtures. Typically, the lignosulfonate sodium salt reacts with metal sulfate (cobalt, nickel iron etc.) at about 900 C to convert it into the desired metal lignosulfonate. Then, it is subjected to microwave radiation to obtain carbon-metal nano-composite. This process is simple, cost effective and environment friendly.

organo-clays with Magnetic fe�o� Nanoparticles

Combining hydrophobic organo-clays with magnetic oxide particles enables them to absorb oil, coagulate into large lumps and be maneuvered under an application of magnetic field. Recently, researchers from National Taiwan University, Taipei and National Chung Hsing University, Taichung, Taiwan have successfully produced nanohybrids28 composed of magnetic iron oxide nanoparticles (FeNPs) embedded into the layered structure of the natural clay by in situ co-precipitation of Fe2+/Fe3+. To synthesize this material, first Na-montmorillonite (Na+- MMT) was modified by incorporation of FeNPs by in situ co-precipitation. These nanohybrids exhibited the ability to preferentially absorb crude oil up to 4 times its weight from a water-oil mixture, form agglomerated lumps and be easily removed under the application of magnetic field.

composed of a non-magnetic component in the form of porous materials/nanoparticles and a magnetic component based on ferromagnetic or ferrimagnetic nanoparticles such as iron, γ Fe2O3, Fe3O4 magnetite, strontium ferrites etc.

Hydrophobic Core-shell Magnetic fe2o� @ C Nanoparticles

One of the challenging problems associated with oil spills is control of the rapid spreading of oil over the surface of water and its efficient and quick removal to reduce its ecological impact over a wide area. In order to meet this challenge, Zhu et al. have developed core-shell Fe2O3@ C magnetic nano-particles, coated with polysiloxane layers that exhibit highly super-hydrophobic and super-oleophilic characteristics25.

Figure 4(a) schematically shows the method for the preparation of superhydrophobic Fe2O3@ C nanoparticles and Fig. 4(b) illustrates the technique for the removal of oil from water surface through Fe2O3 @ C nanoparticles under the action of an external magnetic field. These nanoparticles are unsinkable and, when brought into contact with the oil floating on the surface of water, quickly and selectively absorb the oil up to 3.80 times their own weight. Subsequently, the hydrocarbons/oil pollutants can be immediately removed through core-shell Fe2O3@ C nanoparticles by the application of an external magnetic field. These oil absorbent nanomaterials show remarkably high separation selectivity, excellent thermal stability up to 4500 C, high corrosion resistance and good recycle-ability. These particles can be regenerated by simple ultrasonic washing in ethanol for 5 minutes.

Magnetic Polymer Nanocomposites

Currently, considerable efforts are being directed towards the development of environmentally friendly polymers based on agro-based resources. These biodegradable eco-friendly plastics find numerous industrial applications. Among these “green” plastics, a biopolymer material, alkyd, is an attractive option as it can be easily synthesized by treating glycerin (crude glycerin is the by-product of bio-diesel) with aromatic/aliphatic diacids. Brazilian scientists have recently investigated the preparation method for magnetic polymer nano-composites utilizing alkyd resin as a polymer matrix26. The main objective of this study was to develop composites for cleanup and recovery of oil spills in water. The production process involved curing of alkyd resin with toluene diisocynate (TDI) in the presence of super-paramagnetic nanoparticles of γ- Fe2O3 maghemite, which were introduced in situ in the final polymer material. The researchers have also shown that the aromatic/aliphatic nature of the composite can be manipulated to tailor the chemical

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Nanowire Membranes

Massachusetts Institute of Technology (MIT) researchers have developed absorbent, superhydrophobic nanowire membranes for the selective absorption of oil from an oil-water mixture29. Using self assembly method, they have constructed free-standing membranes comprising inorganic nanowires capable of absorbing oil up to 20 times their weight. The research team led by Prof. Francesco Stellaci has synthesized a super-hydrophobic/oleophilic membrane material by coating MnO2 nanowires (prepared by hydrothermal method) with silicone, using a vapour deposition technique. This membrane has a paper towel like appearance. When this “paper towel” is dipped in a water-oil mixture, it selectively absorbs the oil. The membrane consists of a mesh of super-wetting interconnected capillaries and, through the resulting capillary action, its oil absorption capacity is significantly enhanced. The basic concept of its functioning is illustrated in Fig. 5 (a)- (c). These membranes are thermally stable up to 3800C and can withstand harsh conditions. They can be regenerated by ultrasonic cleaning followed by autoclaving and can be reused. They offer great promise as a commercially viable solution for cleaning oil spills.

By employing the abovementioned nano-wire mesh,

MIT’s SENSEable City Laboratory has recently createdSENSEable City Laboratory has recently created City Laboratory has recently created an autonomous oil-absorbing robot called Sea-swarm30. This prototype robot uses a conveyor belt covered with the oil absorbing nano-wire mesh. When Sea-swarm moves along the surface of water, the conveyor belt along with the nano mesh rotates, and selectively absorbs the water to do the cleaning job. These autonomous vehicles use very little energy (as low as about 100 watts), run for weeks and have the capacity to clean up several gallons of oil per hour.

Carbon Nanostructures

exfoliated graphite

One of the promising forms of carbon for cleaning of oil spills is exfoliated graphite. This material has excellent capacity to selectively absorb the heavy oil from a water-oil mixture. Studies carried out by Japanese researchers have shown that 1 gram of exfoliated graphite (bulk density= 6 kg/m3) is able to absorb 86 g of A-grade heavy oil and 76 g of crude oil, respectively, at a very rapid rate (in about 2 minutes)31. However, the sorption capacity and amount of recovered oil decreased drastically with increasing number of cycles. Other carbon materials, such as carbon felts based on activated carbon fibers and PAN based carbon fibers, exhibited lower sorption capacity limited

Water

Oil

(a)

(b) (c)

Fig. 5. (a) The mesh of nanowires provides a porous structure with high surface-to-volume ratio and is shown with increasing magnification from left to right. (b) The contact angle, θ, quantifies the wetting behaviour of a material and is a measurement of the angle between the surface and liquid/vapour interface. (c) The membrane surface displays heterogeneous wetting when in contact with water (top) but homogeneous wetting for oil (bottom). [Reprinted with permission from Macmillan Publishers Ltd., J. Yuan et. al., Nature Nanotechnology, 3 (2008) 332]

to about 20 g/g maximum as compared to exfoliated graphite, although carbon felts showed superior recycling performance in comparison with exfoliated graphite.

Carbon Nanotube sponge

Professors Anyuan Cao, Peking University and Dehai Wu, TsinghuaDehai Wu, Tsinghua University have recently made an important discovery of extremely light and tough carbon nano-tube sponge32, 33, which can selectively absorb the oil from the surface of water that offers a potential solution to the oil spill problem. The remarkable feature of this nano-sponge is that it can absorb oils and solvents up to 180 times its own weight, which is far greater than what could be achieved with current commercial absorbers used for oil spill recovery. These nano-sponges, being very light in weight (> 99% porosity and density of 5.8−25.5 mg/cm3), float on the surface of water and efficiently do the job of mopping up the spilled oil.

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Fig. 7: (a) The oil sorption capacities of VACNTs, exfoliated graphite (EG) and agglomerated CNTs and (b) recycling performance by a simple compression (after 10 cycles)

These CNT sponges are made of an interconnected network of multi-walled carbon nanotubes (Fig. 6). They are robust and highly flexible in nature, unlike silica aerogels which are brittle and fragile. Moreover, they are capable of deforming elastically under compression to large strains and are able to recover most of the strain elastically. They exhibit excellent fatigue properties under cyclic conditions. Whenever the sponges, in a densified state (pre-compressed), come in contact with liquids, they swell immediately, and selectively absorb a large amount of solvents and oils ranging from 80 to 180 times their own weight. The absorbed oil can simply be removed by squeezing the sponge or by direct burning in air. They also show excellent recyclability and can be used thousands of times.

Vertically Aligned CNts (VACNts) for Combating oil spills

VACNt-based Membrane filters: Researchers from SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Korea have developed a novel membrane filter based on vertically aligned multi-walled carbon nanotubes synthesized on a stainless steel mesh34. This filter exhibits super-hydrophobic and superoleophilic properties. The unique structure, consisting of needle like tubes with protruding sharp tips having micro-scale roughness combined with micro-scale pores, enhances both hydrophobic and oleophilic characteristics of the membrane. This novel filter offers significant potential as an effective solution for the separation of oil and water. Lee et al. from Michigan Technological University have also grown vertically aligned multi-walled carbon nanotubes on stainless steel mesh using thermal CVD35, which exhibited super-hydrophobic and superoleophilic characteristics. The researchers demonstrated that the filtration efficiencies of these SS-CNT meshes are greater than 80% when used as membranes for the filtration of water-in-oil emulsions. Further, they have studied the mechanism of filtration and found that the efficiency of filtration depends on initial droplet sizes of the oil present in the feed emulsion.

VACNt-based oil sorbents: Recently, studies have also been conducted to investigate the oil (kerosene

of exfoliated graphite (41 g/g) and agglomerated CNTs (10 g/g) as depicted in Fig. 7 (a). The higher oil sorption capacity of VCNTs mainly stems from large sized macro-pores present within its inter-tube spaces. As indicated in Fig. 7 (b), VCNTs also exhibit superior recycling capability than either agglomerated CNTs or EG, on account of their high mechanical strength, high compressibility and excellent resilience properties (springinesss).

graphene Worms and Nano Accordions Derived from tego

As previously discussed, expanded graphite (EG) isexpanded graphite (EG) is used as an absorbent for oil spill remediation. Recently, researchers from Princeton University have developed

Fig. 6: Black carpet-like appearance of soft and flexible CNT sponge (thickness: 3.5 mm, Area: 12 cm2, density: 5.8 mg/cm3) that can be rolled into tight scrolls without splitting. [Reprinted with permission from American Chemical Society, X. Gui et al. ACS Nano, 4 (2010) 2320]

oil) sorption and recovery characteristics of the vertically aligned carbon nanotubes by Chinese researchers36. For the sake of comparison, exfoliated graphite (EG) and agglomerated carbon nanotubes were included in their study. The vertically aligned CNTs (VACNTs) showed superior oil sorption capacity (69 g/g) in comparison with that

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a technique of synthesizing thermally exfoliated graphite oxides (TEGO), a type of graphene, which has a nano-scale worm or accordion like appearance37-40. TEGO offers great potential for oil spill remediation.

The TEGO is prepared by initially treating the natural graphite flakes with sulfuric acid, nitric acid and potassium chlorate to oxidize it to form graphite oxide (GO). Subsequently, it is heat treated in argon atmosphere at about 11000C for ~ 30sec. Rapid heating (~ 20000C/min) allows expansion of intercalated water in GO, resulting in a total volume expansion of 200-300 times and wrinkled platelets resembling worm and nano accordion structure. The TEGO typically has a surface area of about 1500 m2/g (against < 100 m2/g in the case of standard EG) and a bulk density of about 1.83 kg/m3 (corresponding to the sample having a surface area of 1164 m2/g). The high surface area of TEGO and its hydrocarbon surfaces makes it an excellent absorbent material for oil and organic liquids. The low bulk density of TEGO also makes it attractive in that the amount of liquid that can be imbibed on a weight basis can be high. Liquid loadings between 100 to 10,000 wt:wt of oil to TEGO can be achieved.

The accompanying bar chart (Fig. 8) indicates the oil absorption capacities (typical range in g/g) for several candidate nanomaterials along with their typical densities in mg/cm3. As can be seen, thermally

exfoliated graphite oxide (TEGO) has the highest oil absorption capacity (i.e., 100 to 10,000 g/g) among all the sorbent materials, which could be attributed to its nano-scale worm or accordion type structure based on graphene and its very low density. Untreated silica aerogel exhibits very low absorption capacity that is even lower than 0.1 g/g while CF3 functionalized aerogel (14-237 g/g) shows remarkable improvement in the oil absorption capacity due to its hydrophobic nature. Carbon nanotube sponges (80-180 g/g) are also excellent oil sorbents.

reCAM® technology

Recam® was invented by Agliettois of SA Envitech s.r.l., Italy. It is a novel reactive nanostructured carbon material, which is composed of graphene cells and carbon nanotubes41. It is hydrophobic in nature, with a density of about 0.025 g/cm3 and a specific surface area of 460 m2/g. It has a peculiar cellular structure comprising cells separated by thin strands of graphitic carbon. This unique structure of Recam® bestows it with distinctive characteristics suitable for contaminants removal. Within its structure, 10-20% multiwalled carbon nanotubes are also present. The product contains nanopores having diameters in the 0.3-2 nm range, with an apparent porosity of 54%. The material is quite stable up to 7000 C and chemically inert. It has enormous capacity to selectively absorb oil up to 90 times its own weight.

Fig. 8: Oil absorption capacity of various candidate nanomaterials (different types of oils studied with each material: activated carbon – diesel oil, CNT sponge – various organic solvents and oils, PU foam – paraffinic and napthenic oils, CF3 functionalised aerogel – crude oil, silica aerogel – crude oil, EG – different grades of heavy oil, NW – various types of oils, ambient pressure dried silica aerogel – diesel oil, TEGO – not known, cotton towel - not known)

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Recam® can be employed for oil cleanup in two ways, i.e. it can either be used as a barrier system to prevent the spread of oil or for removal of oil from sea water. In the first case, the material is sprinkled over the water surface to form a barrier, which does not allow the oil to pass through it. Recam®’s high affinity for the hydrocarbons, coupled with its hydrophobic characteristics, blocks the passage of water. In the second case, once the Recam® material is spread over the surface of the oil contaminated water, it starts attracting and securely entrapping the hydrocarbon molecules within its graphene cells, not allowing them to be released in the water or in the environment. Subsequently, the captured oil without any traces of water can be recovered by mechanically compressing the material. It is also possible to recover the oil, which is present

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deep underwater, by injecting Recam® material through pipes at different depths under the sea. The Recam® material starts attracting the oil, entraps it, and then rises to the surface of the water along with the captured oil, where it can be easily salvaged. Recam® is also an ecofriendly material that can be regenerated.

Micro- and Nano-tio2 for oil spill remediation

Oil spills generally result in contamination of seawater due to the dissolution of water-soluble crude oil fractions. This contaminated water, rich in dissolved hydrocarbons, is highly toxic in nature and can cause irreparable damage to the coastal ecosystem. Photocatalytic decomposition of the oil-contaminated water using nanoscale or microscale TiO2 particles can provide an effective solution to this problem42-

44. Pure TiO2 exhibits photocatalytic activity in the near ultraviolet range (< λ = 390nm), which covers only 4% of the solar radiation spectrum. Therefore, doping of TiO2 with transition metal cations such as Cr, Co, Mo, and V is necessary to enhance theCr, Co, Mo, and V is necessary to enhance the is necessary to enhance the photocatalytic efficiency. In addition, modificationmodification of TiO2 with nonmetals, such as B, C, N, S, and F is known to improve the photocatalytic response under visible light region45. Although TiOTiO2 has great potential for oil spill remediation on account of its outstanding photocatalytic properties, it has not been introduced in the market on a commercial scale. This is because, in powder form, it has a tendency to form agglomerates resulting in the reduction of its photocatalytic activity. Furthermore, its use in oil spill cleanup necessitates separation and recovery of TiOTiO2 nanoparticle containing suspensions because there is always a likelihood that the presence of residual nano-titania in seawater would be environmentally damaging.

To circumvent the above problem, it is necessary to immobilise the TiO2 by providing a suitable support43, which would help in avoiding the problem of clustering and agglomeration of TiO2 nanoparticles that leads to associated reduction in photocatalytic activity. Secondly, this approach would also obviate the need of separation and recovery of suspensions containing micro- or nano-sized TiO2 particles (which possibly cause toxicity if they remain in sea water) after their use.

One of the approaches being suggested is to combine carbon with nano/micro TiO2 in the form of coatings on exfoliated graphite or doping of TiO2 into expanded graphite46 etc. Such combination of TiO2 and carbon is able to perform a dual function, i.e. adsorption/absorption and decomposition, and plays a synergistic role in combating oil spill problems. Use of TiO2 in the form of nanotubes as well as aerogels, and fly ash cenospheres coated with TiO2 nanoparticles, also find applications in oil spill remediation.

Cotton Absorbent Pads/filters and filter Papers

Inexpensive, raw cotton waste is an amazing oil absorbing material that is also biodegradable in nature. It can soak up the oil up to 40 times its weight. Professor S. Ramkumar of The Institute of Environmental and Human Health (TIEHH), Texas Tech University is developing value-added cotton absorbent pads using non-woven materials and nanotechnology47. Recently, he chemically treated the raw cotton which enhances its oil absorbing capabilities to soak the oil up to 70 times its weight. He has also commercialized a product, “Fibertect”, combines oleophilic absorbent raw cotton with an activated carbon that has a capability to soak up the oil and also to contain obnoxious hydrocarbon vapours and prevent them from being released into the atmosphere. Fibertect, which can be reused up to 10 times, is not only biodegradable but has got the capability to disintegrate the absorbed oil due to the presence of naturally containing anaerobic bacteria.naturally containing anaerobic bacteria.

Professor Di Gao of University of Pittsburgh has recently developed a novel polymer coated cotton filter, which effectively separates oil from an oil–water mixture48. Gao’s filter is based on a proprietary polymer, which has a combination of hydrophilic and oleophobic properties. The cotton fabric is soaked in this special polymer and then dried in an oven or in open air to form the polymer coating. This filter readily allows water to pass through but blocks the oil.

A simple dip-coating method has been developed at Shanghai Jiao Tong University, China to produce a filter paper, which possesses both superhydrophobic and superoleophilic properties49. The coating process involves treating the commercially available filter paper with a mixture of polystyrene in a solution of toluene and superhydrophobic silica. The unique wetting characteristics of this filter paper allow it to separate liquids differing in their surface tension. For example, low surface tension liquids such as oil or ethanol can be selectively absorbed and separated from water. This modified filter paper is able to remove oil floating on a water surface or from aqueous emulsions, filter oil-water mixtures and reduce the water content in oil.

DAg-Peg lipids for remediation of oil Contamination:

Brian Charles Keller discovered that certain diacylglycerol-polyethyleneglycol (DAG-PEG) lipids such as PEG-12 GDO (glycerol dioleate) and PEG-12 GDM (glycerol dimyristate) can be used to remediate the oil spills50. He found that, when DAG-PEG lipids are added to the surface of an oil spill in water, the lipids entrap both water and oil from the immediate surroundings and disperse the contents and vesicles

1�

into a suspension. There is a spontaneous reaction and DAG-PEG begins to entrap water and available oil from the spill into thermodynamically stable vesicles. Figure 9 schematically shows the various components of a thermodynamically stable vesicle.

Once the oil is entrapped into vesicles, it remains inside the bi-layers of the vesicle and in suspension indefinitely, since the formed vesicles are thermodynamically stable until disrupted by high energy mechanical shear, by enzyme activity or by heat. This allows for easy clean-up either by washing with more water or with vacuum equipment. The resulting liposome suspension is water washable and can be cleaned off animals, including bird feathers, fur

sorption capacity, selectivity for organic solvents and oil, rate of sorption, tailored surface chemistry etc. As shown in the bar chart in Fig. 8, the nanoporous sorbents, namely thermally exfoliated graphite oxide (TEGO), CF3 functionalized aerogels and carbon nanotube sponges have outstanding oil absorption capacities. Recam® is another nanosorbent having excellent capacity to absorb the oil (90 g of oil per 1 g of Recam®)41. One of the most exciting developments in recent months is the discovery of the lightest ever, free-standing multiwall carbon nanotube aerogel51 having a density of 4 mg/cm3 (compared to 5.8-25.5 mg/cm3 for CNT sponge) and a surface area of 580 m2/g (surface area for pristine MWCNT is 241 m2/g). Therefore, the ultra light CNT-based aerogels are expected to have much greater potential as a sorbent for oil spill cleanup than CNT sponge.

Another important requirement for the sorbent/membrane/filter nanomaterial is its selectivity for a broad range of organic solvents and oils to effectively separate oil and water and thus clean up the oil spills. Conventional sorbents based on polypropylene, silicon-coated glass fibers, raw cotton etc. have a tendency to absorb both water and organic solvents, whereas nanomaterials such as CNT sponges, nanowire membranes, Recam®, hydrophobic core-shell magnetic Fe2O3 @ C nano-particles etc. selectively absorb oil from water-oil mixtures because of their unique combination of superhydrophobic and superoleophilic properties. One outstanding example of highly selective absorbent material is the recently developed highly oriented free-standing

Hydrophilic head group

Hydrophobicchain

Entrapped oilEntrapped water

Fig. 9: Schematic showing the various components of a thermodynamically stable vesicle

Fig.10: Schematic showing the key issues involved in successful implementation of nanotechnology for oil spill remediation

and scales. Additionally, the suspension can be broken and components can be separated using a variety of techniques. As DAG-PEG is recyclable and non-toxic in nature, this technique is useful for cleaning of oil spills in water as well as on land and is also applicable for either crude oil or refined oil.

factors in Implementation of Nanotechnology Based solutionsFor the successful implementation in a real world scenario, a number of factors should be carefully considered. The critical issues that need to be examined are given in the following schematic diagram.

Performance Criteria

A variety of nano-based products such as membranes, filters, sponges, chemical dispersants etc. play an important role in oil spill remediation. The performance of nanoporous materials for the above application is mainly determined by their

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membranes consisting of ammonium vanadium oxide (NH4V4O14) nanobelts52. This novel product possesses unique nano-scaled self-assembled architecture and its wetting behavior can be controlled, ranging from superhydrophobic (with coating of silicone) to superhydrophilic. This material is able to selectively absorb a variety of both polar as well as non-polar organic solvents. Another exceptional feature of this material is that it can even separate organic solvents having similar polarity, such as benzene and toluene.

An additional feature of nanomaterials like CNT sponges and aerogels, Recam®, Gigasorb etc. is that they are robust, highly flexible and capable of withstanding a number of large strain compressive cycles without affecting their properties. These attributes are highly beneficial for recovery of oil as well as regeneration and reuse of the material.

It is also important that any potential nanomaterial to be employed for oil spill cleanup should be able to float on water and should not sink over time, even if it absorbs the oil. In this respect, a number of materials such as CNT sponges, aerogels, TEGO, nanowire membranes, foams etc. are fully buoyant in nature and do not sink in water, irrespective of whether they are in contact with oil or not. The buoyant nature of nanomaterials would facilitate their removal and thus contamination of water can be completely avoided.

The rate of sorption of crude oil and petroleum products (i.e. number of kg of oil sorbed by one kg of sorbent in a single cycle per unit time, kg/kg/min) is also an important factor while selecting a sorbent material. In this context, it is instructive to note that incorporation of activated carbon nanoparticles into Megasorb (non-woven polymer material) enhances the sorption rate from 3.4 kg/kg/min to 18.6 kg/kg/min. This is a perfect example of how nanotechnology can help in improving the sorption rates of oils and liquid hydrocarbons.

Multifunctional synergies

Nanomaterials combined with different functional characteristics such as chemical (hydrophobicity, hydrophilicity and oleophilicity), magnetic, mechanical, photocatalytic properties due to their synergistic effect, have enhanced capability for more efficient removal of oil spills from water surface. One such example is a magnetic polymer composite26, wherein superparamagnetic nanoparticles of γ- Fe2O3 maghemite are incorporated within the matrix of an alkyd resin polymer. In this composite, the oil removal capability is the sum of two distinct effects arising from the magnetic force and the chemical affinity between the resin and the oil. Another fascinating product developed by Blue Gold USA is Organozorb™ (Table 1), which combines both nano- and bio-technology for bioremediation

of hydrocarbons, resulting in total cleanup and eradication of oil and hydrocarbon spillage. As has also been discussed earlier, nano-TiO2 is a highly efficient and low cost material having significant potential for photocatalytic degradation of water soluble crude oil fraction, and therefore a promising candidate to minimize the impact of crude oil compounds on contaminated waters. Recam® is a commercial product based on nanostructured carbon containing graphene cell and CNTs. SA Envitech s.r.l. company has ingeniously coupled Recam® (containing graphene)

with TiO2 nanoparticles (anatase) and developed a new and innovative product53, 54. This unique combination creates synergy between anatase and graphene and dramatically enhances the efficiency of hydrocarbon/oil removal from contaminated water through photocatalytic degradation process. The coupling of TiO2 with graphene enables easy transfer of electrons, which are released following activation by means of illumination, into the graphene. Furthermore, possibility of electron-hole recombination is also significantly reduced. Silica aerogels, too, have significant potential as a candidate material for selective adsorption of oil. However, since they are fragile and prone to cracking during the ambient pressure drying process, there have been attempts to reinforce it with fibers. Recently, Iranian scientists have successfully synthesized MWCNT (0.05 wt%) / silica aerogel composites by ambient pressure drying technique55. It was found that the nanocomposites are able to withstand compressive stresses, which in turn, prevent cracking during the drying process. Moreover, the addition of MWCNTs renders the aerogel hydrophobic in nature and results in increased surface area. The silica aerogels and their composites show excellent adsorption capacity for the removal of organic solvents from water. This is another interesting case of how incorporation of CNTs into aerogels can create synergy between strength and oil sorption property.

environmental factors

Oil spill remediation using engineered nanomaterials is a more effective option than conventional techniques as it leads to improved performance and response. The superior performance of nanomaterials can be attributed to their increased surface area and, in turn, higher reactivity as well as the possibility of in situ treatment. However, at the same time, materials at the nanoscale may pose new toxicological risks due to their greater biological activity and may have negative impact on human health when these nanoparticles are inhaled, absorbed through skin, or ingested. There is also some scientific evidence, showing that the nanoparticles can travel through the food chain from smaller to larger organisms. In addition, they could damage important microbes in the environment.

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Unfortunately, in recent times, there has been a kind of hysteria about the harmful effects of nanotechnology being propagated by certain sections of the media, NGOs and the general public due to misinformation and/or lack of understanding56. In view of these environmental concerns, there is a clear need to carry out detailed studies and generate requisite data on toxicological effects of nanomaterials. Such data generation is especially pertinent in case of nano-dispersants, which may have the potential to cause damage to organisms in water bodies. In dealing with nanostructured materials such as CNT nanosponge, nanowire membranes etc., it is essential to ensure that individual nanotubes, nanowires etc. are not detached from the product to contaminate the water. For example, nano-titania is an important photocatalytic material used for oil spill remediation but there is difficulty in its separation and recovery after use, and the unrecovered TiO2 nanoparticles may enter human or animal cells and show toxic effects. One methodology to avoid such an eventuality is to immobilize the titania on a support material, which renders nanoparticle recovery unnecessary. Exfoliated graphite doped with nano-TiO2 and Recam containing nano titania are two typical examples utilizing this approach. Another environmentally safe pathway is to employ a technology similar to Ozonix nanobubble technology, which is a chemical-free technology generating millions of nanosized bubbles to raise the oil to the surface of sea water to aid cleanup operation. As this technology does not employ any nanomaterials, the question of contamination of water does not arise at all.

engineering Aspects

Practical implementation of nanotechnology for oil spill remediation demands that the nanomaterials in the form of nanoparticles/ foams/sponges etc. be suitably engineered or packaged to facilitate their application. For example, raw cotton is a potential absorbent material for cleaning of oil spills. In practical application, it is made into a three-layer flexible, non-woven, decontamination system called as Fibertect47 (Table 1). The Fibertect can be used in the form of a boom. Another example is TEGO, which is a powdery material that has to be contained in large porous sacks made from polypropylene or polyethylene fabric or porous film, alternatively TEGO can be co-processed with a polymer binder in the form of a foam sheet. This open cell structure of the foam allows contact between the oil and the TEGO surfaces. The advantage of this system is that the absorbent system can be rolled for storage. Another practical example is MIT’s seaswarm robot incorporating oil absorbing nanowire mesh. It is capable of autonomously navigating the surface of the ocean to collect surface oil and process it on

site. Hydrophobic CF3- functionalized aerogels are integrated into devices and, for this purpose, aerogels are incorporated into solid support like fiberglass, alumina, cotton, wool carbon foam etc. This is usually done by dipping the support material in either the powdered aerogel, or in a slurry of the aerogel in a solvent, or by any other coating technique. For example, discs of fiberglass can be dipped into a solution of 15 wt % CF3 -aerogel in acetone, two times and vacuum dried between dips to form a device.

Cost Aspects and Commercial Viability

The commercial viability of the technology is critically dependent on the cost of a product, which includes the raw material cost and the manufacturing cost. This is especially relevant in case of massive oil spills requiring huge amount of material for cleanup operation. Aerogel is one of the prime candidates for the mitigation of oil spills; however, its high cost is a major inhibiting factor for its widespread adoption. Currently, the typical cost of aerogel57 produced by supercritical drying is about $ 2870 per kg while the aerogel made from discarded rice husks is reported to cost only $ 276 per kg, thereby making the latter a commercially viable proposition. Another interesting option is to use clay-based aerogel (Aeroclay), which uses only clay, polymer and water as raw materials; hence, its production cost is expected to be much lower than that of silica aerogels. Furthermore, it is also highly flexible in nature (unlike silica aerogel), which is a desirable property for recovering the absorbed oil by simply compressing the aerogel. Carbon nanostructured materials such as CNT sponges, VACNTs, Recam, and HRCMs are also emerging materials having great potential for oil spill cleanup and recovery on account of their outstanding properties. As these are mainly based on carbon nanotubes and graphene, economics is certain to play a major role in determining their commercial viability. Currently, MWCNTs are priced at $100-150/kg58 and expected to reduce to $ 10-20/kg in the near future. The present price of graphene nano platelets is $ 385-525/kg and it is predicted that nanoplatelets could be produced at $ 11 per kilogram56. Therefore, it is likely that these technologies could become commercially viable solutions for oil spill cleanup in the future. Apart from the production costs, the economic viability of a technology for oil spill cleanup also depends on other performance factors such as sorption capacity, rate of sportion of crude oil, regeneration factor, recyclability, cost of disposal of the material etc. In this context, the company Ecosorber (http://www.ecosorber.ru/) has carried out a comparative study between a conventional low cost sorbent, Peat, and Gigasorb (sorbent based on nanotechnology), to ascertain their real cost for oil spill remediation. Given the relevance of the above mentioned performance factors, the

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coefficient of efficiency was determined to be 0.0216 and 192.41, for peat and Gigasorb, respectively. In the case of peat, notwithstanding its low cost and ready availability, in order to collect 50 tons of spilled oil one would need 17,700 tons of peat worth 8.64 million dollars, whereas to collect the same amount of oil only 5.6 tons of Gigasorb worth 0.131 million dollars would be needed. Furthermore, the spent sorbents have to be recycled by an incineration process. For this purpose, the cost of inputs required for burning of the peat would be 7.05 million dollars in addition to 8.64 million dollars spent for the cleaning operation whereas, in the case of Gigasorb, only an additional 0.0127 million dollars would have to be spent for burning of the sorbent. This is an apt example of how nanotechnology can help in improving the effectiveness of sorbents and, thereby, improve the economic viability of oil spill cleanup operation.

Nano sorbents based on naturally occurring materials like cotton, jute, wood, etc. are potentially low cost products and also biodegradable. Moreover, they are also a renewable resource. Mineral products such as sand, perlite, exfoliated graphite and activated carbon also have the advantage of economy. Therefore, there have been attempts to nano-engineer these materials to enable them to selectively absorb the oil when applied to an oil spill. As reported earlier (Table 1), NanoBionic jute (Burlap) and NanoBionic sand are two such examples where nanotechnology has been applied to improve the absorption efficiency of naturally occurring low-cost materials, namely jute and sand, respectively. Another low cost material is scoria, which is a kind of volcanic rock containing many holes or vesicles. As shown in Table 1, Eco Renascence LLC has applied the nanotechnology using scoria as raw material and developed a unique and cost effective sorbent, Sea ReclaimTM for oil reclamation.

Conclusions and future DirectionsThis article provides a general overview of the wide variety of nanomaterials and technologies that offer significant promise for oil spill cleanup and recovery. It is quite evident from the foregoing discussion that nano-materials have enormous potential to provide innovative solutions for oil spill cleanup by virtue of their unique structure, superior properties and outstanding performance. However, for their successful implementation as commercially viable technologies, one needs to carry out a thorough study on the engineering aspects, environmental issues, scalability and cost analysis.

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material”, Chinese Patent CN1317188C, May 23, 2007

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y. r. MahajanCentre for Knowledge Management of Nanoscience & technology(A Project of ARCI, Department of Science & Technology, Govt. of India)12-5-32/8, Vijayapuri Colony, Tarnaka Secunderabad-500 017, A.P., IndiaTel: 040 2700 7032, E-mail: [email protected] Web: www.ckmnt.com

Nanotechnology-Based Solutions for Oil Spills

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