1 Indian Journal of Fibre & Textile Research Vol. 36, June 2011, pp Review Article Oil spill cleanup by structured fibre assembly C Praba Karan a, R S Rengasamy & Dipayan Das Department of Textile Technology, Indian Institute of Technology, New Delhi , India Received 8 March 2010; revised received and accepted 18 June 2010 Oil is one of the important sources of energy in the modern industrial world. It has to be transported from the source of production to many places across the globe through oceans and inland transport. During transportation the chance of oil spillage over the water body occurs due to accidents or by deliberate action during war time and this causes environmental pollution. Sorbents made from structured fibre assembly are found to be the best material to clean up oil spill. The oil sorption and retention behavior of sorbents are influenced by the material and structure of the sorbents and oil physical characteristics. For sustainable environment, disposal of used sorbents is a major issue. In this context, the naturally available biodegradable materials have great potential than the synthetic ones. This paper reviews about oil spill cleanup with special emphasis on the phenomenon of oil sorption, methods of oil spill cleanup, characteristics of oil sorbent materials, fluid flow through fibrous materials, types of fibre materials envisaged for making sorbents and test methods for oil sorbents. Keywords: Cotton, Kapok, Milkweed, Natural fibres, Oil spill, Polypropylene, Porosity, Sorbents 1 Introduction Oil is a naturally occurring substance. The organic residues from the decay of plants and animals are converted by heat and pressure into petroleum, migrating upwards, sometimes over extensive areas, either to reach the surface or be occasionally trapped in to become oil reservoirs 1. Oil is one of the most important sources of energy and is also used as raw material for synthetic polymers and chemicals worldwide 2-4. Oil has been a part of the natural environment for millions of years 1. There has been increasing demand for oil supply in the modern industrial world. Oil spill occurs over the seas, water bodies and land surfaces due to tanker disasters, wars, operation failures, equipment breaking down, accidents, and natural disasters during the production, transportation, storage and use of oil. Oil spills into land, river or ocean and imposes a major problem for the environment 3,5-10. So, it is necessary to clean the water or land immediately after the oil spill. The impact of marine oil spills to coastal environments and marine resources has over the past decade created increased public and government awareness and concern to preserve and protect the marine environment 11. a To whom all the correspondence should be addressed. E - mail: When oil comes in contact with water, it forms oil-in-water emulsion or floating film that needs to be removed before it is discharged into the environment. Even very low concentrations of oils can be toxic to microorganisms responsible for biodegradation in conventional sewage processes 12. Removal of crude oil and petroleum products that are spilled at sea is a serious problem of the last decades. Another important threat to the environment comes from polycyclic aromatic hydrocarbons which are known to affect a variety of biological processes and can be potent cell mutagens and carcinogens 4. Oil is a very complex mixture of many different chemicals 13 and a mixture of components consists of different hydrocarbons that range from a light gas (methane) to heavy solids with differing properties 14. When oil is spilled on water or on land, the physical and chemical properties of oil change progressively. This process is referred to as weathering 1,7,14-16, i.e. these physico-chemical changes enhance oil dissolution in sea water. The weathering process (Fig. 1) includes evaporation, dissolution, dispersion, photochemical oxidation, microbial degradation, adsorption onto suspended materials and agglomeration 1,7,13,15,17. The volatile components present in oil evaporate quickly. Some of the medium-sized polycyclic aromatic hydrocarbons are slightly soluble. Some of
2 PRABA KARAN et al.: OIL SPILL CLEANUP BY STRUCTURED FIBRE ASSEMBLY 191 1,7,13,17, 20 Fig. 1 Weathering process of oil from the sea surface the products, which are degraded by sun and microorganisms, are highly soluble. Weathering rates are not constant but are usually highest in the first few hours 15. In practice, cleaning up an oil spill is a difficult economical problem. It is uneconomical to store large quantities of sorbents materials that are used to clean up the oil spill and their disposal. The use of sorbents made from organic materials does not cause additional problems in the disposal of the spilled oil 18. The production of bio fuel from the used organic sorbent materials could be one solution to improve our preparedness for oil spills. In normal situations the biomaterial yield is utilized in a coastal bio power plant. When an oil spill occurs the bio fuel is used as adsorption material instead of being burned. In the case of marine oil spills, the adsorption material is transported to the coastal area near the spill and spread on the oil using a flat-bottomed boat. After sorption the oil-saturated sorbent material is shipped back to the bio power plant and burned there 19. Many researchers have demonstrated that unscoured and unbleached natural fibres such as milkweed, kapok, and cotton have great potential as sorbents in oil spill cleanup over commercially available synthetic materials. Use of these natural fibres resulted in times greater oil sorption, depending on the nature of the studies, than commercial polypropylene fibres or mats which is mostly used for oil sorption application. Partial or complete replacement of synthetic sorbents by natural sorbents could offer other benefits such as biodegradability 20. The cleaning of oil spill is defined with two terminologies, namely clean and recovery. Clean, in the context of an oil spill, may be defined as the Fig. 2 Mechanism of adsorption 24 return to a level of petroleum hydrocarbons that has no detectable impact on the function of an ecosystem. Recovery of an ecosystem is characterized by the re-establishment of a biological community in which the plants and animals characteristic of that community are present and functioning normally 1. Sorbent is an insoluble material or mixture of materials used to recover liquids through the mechanisms of absorption or adsorption, or both 21. The objective of this study is to review research work done on oil spill clean up, oil sorption behavior of fibre based sorbents and test methods for oil sorbents. 2 Oil Sorption Phenomenon Sorption is the common term used for both absorption and adsorption. These terms are often confused with other. Absorption is the incorporation of a substance from one state into another (e.g. liquids being absorbed by a solid or gases being absorbed by water). Adsorption is the physical adherence or bonding of ions and molecules onto the surface of another molecule 21, Adsorption Adsorbents are natural or synthetic materials of microcrystalline structure, whose internal pore surfaces are accessible for selective combinations of solid and solute. It is an insoluble material that is coated by a liquid on its surface including pores and capillaries without the solid swelling more than 50% in excess fluid 21, 23. Adsorption occurs in three steps. Firstly, the adsorbate diffuses from the major body of stream to the external surface of the adsorbent particle (Fig. 2). Secondly, the adsorbate migrates from the relatively small area of the external surface to the pores within each adsorbent particle. The bulk of adsorption usually occurs in these pores because there is the majority of available surface area. Thirdly, the contaminant molecule adheres to the surface in the pore.
3 192 INDIAN J. FIBRE TEXT. RES., JUNE 2011 Adsorption at a surface is the result of binding between the individual atoms, ions, or molecules of an adsorbate and the surface of adsorbent. The adsorption process can be classified into physical adsorption and chemical adsorption. Physical adsorption happens when the contacting molecules of adsorbate and adsorbent are held together by Van der Waals force. Van der Waals force is an attractive force between molecules because the suffusion electrons are not balanced 24. The adsorption of molecules forms layers, one over the previously adsorbed layer. Chemical adsorption happens from a changing electron among adsorbate and surface of adsorbent Absorption Absorption is a process where one substance permeates another. The phenomenon is generally limited to systems where there is affinity between the liquid and the absorbent. Absorbency is a phenomenon characterized by the mode and the extent of transport of liquid into an absorbing material. The main driving force for the transport of the bulk of the liquid into a material is the capillary pressure. Capillary pressure can be defined as the pressure difference existing across the interface separating two immiscible liquids 25. The capillary force comes from the intrinsic liquid attraction capacity of the material and the overall driving force can be enhanced by a secondary force like gravity or pressure 25,26. 3 Methods of Oil Spill Cleanup Physical, chemical and biological methods are used to clean up oil spill. Sorbents, skimmers, boomers etc are used to physically remove oil. In situ burning, dispersion and solidifiers are used to chemically clean up oil 10,24,27. The use of booms to contain and concentrate floating oil prior to its recovery by specialized skimmers is often seen as the ideal solution, since if effective, it would remove the oil from the marine environment. In situ burning is the alternative method of oil removal. Because of the logistical difficulties of picking up oil from the sea surface and storing it prior to final disposal on land, an alternative approach involves concentrating the oil in special fireproof booms and setting it alight 28. Oil dispersion is the second best solution, in such case dispersants (chemicals) are generally used. The dispersed oil droplets remain in the water and are decomposed by the action of bacteria, sunlight, or both. However, the dispersants may cause other problems because they are somewhat toxic 29. Dispersant chemicals work by enhancing the natural dispersion of the oil into the sea. The application of oil-degrading bacteria and nutrients to contaminated shorelines to enhance the process of natural degradation has generated considerable interest for more than two decades 28. Among existing techniques for the removal of oil and polycyclic aromatic hydrocarbons (PAHs) from water, the use of sorbents is generally considered to be one of the most efficient techniques 4. Moreover, the application of sorbent materials is an attractive method for combating of oil spill pollution mainly due to the lower costs and high effectiveness Oil Sorbents Sorbents are the materials that soak up liquids. They can be used to recover oil through the mechanisms of absorption, adsorption or both. Absorbents allow oil to penetrate into pore spaces in the material they are made of, while adsorbents attract oil to their surfaces but do not allow it to penetrate into the material. To be useful in combating oil spills, sorbents need to be both oleophilic and hydrophobic (water-repellant) 31. Oil sorbents are able to concentrate and transform liquid oil to the semi solid or solid phase, which can then be removed from the water and handled in a convenient manner without significant oil draining out 16,18,26,32,33. One of the most economical and efficient methods for combating oil spills is oil sorption by sorbents 9,18,30,33. Once this change is achieved, the removal of the oil by removal of the sorbent structure is not difficult 18,33. Although they may be used as the sole cleanup method in small spills, sorbents are most often used to remove final traces of oil. Once sorbents have been used to recover oil, they must be removed from the water and properly disposed of on land or cleaned for re-use. Any oil that is removed from sorbent materials must also be properly disposed of or recycled 31. Sorbents are easily storable, inexpensive and can be handled by untrained personnel. They remove oil by absorption to their hydrophobic surfaces. Thus, the spreading, dispersion or sedimentation of spilled oil can be prevented 6. In a majority of the sorbents there exists a continuous phase on which sorption takes place by hydrophobic interaction and a capillary region in which sorption occurs by capillary action 34. The capillary action in the fibre assembly occurs between the fibres and/or within the fibres. Micro
4 PRABA KARAN et al.: OIL SPILL CLEANUP BY STRUCTURED FIBRE ASSEMBLY 193 pores on the wall of the natural fibres and hollow lumen available in fibres like cotton, flax milkweed and kapok are examples of pores within fibres 16,34. 5 Characteristics of Oil Sorbent Materials The characteristics of an ideal sorbent material used for oil spill cleanup include hydrophobicity or oleophilicity, high uptake capacity and high rate of uptake of oil, buoyancy, and retention over time, durability in aqueous media, reusability, biodegradability, and recovery of oil 3,4,8,18, Of course, it is very difficult to achieve all these in a single material, still these are all need to be considered before selecting the sorbents. Oil sorbent materials can be categorized into three major classes, namely organic synthetic, organic vegetable and inorganic mineral 7,16,30,33,37,39. Among synthetic products, polypropylene and polyurethane foams are the most widely used sorbents for oil spill cleanup because of their highly oleophilic and hydrophobic properties 7,9,28,33,36,37. Polypropylene nonwoven sorbents have high oil sorption capacity and low water uptake, and hence these sorbents are ideal materials for oil recovery from the water surface 36,40,41. Despite the superior oil sorption properties their poor biodegradability makes them less attractive compared to some natural oil sorbents. However, in this process most of the used sorbents end up in landfills and incineration, which either produces another source of pollution or increases the oil recovery cost 8,31,40. Natural sorbents, if used effectively, can thus be more efficient than synthetic products 12,42. The mineral products used as oil sorbents include perlite, exfoliated graphite, vermiculites, organoclay, zeolite, silica aerogel, and diatomite. Most of them have poor buoyancy and oil sorption capacity 9. Nearly 6.35 kg of clay absorbs 3.78 liter oil. Clay is not hydrophobic, sinks in water and creates large volumes to transport 43. The limitations of the mineral products and organic synthetic products have led to the recent interest in developing alternative materials, especially biodegradable 9. Natural agro-based products such as milkweed and cotton have greater potential for oil spills cleanup as they are able to absorb significantly more oil compared to the commercial synthetic sorbent materials 9. 6 Oil Sorption and Retention Husseien et al 7. studied the effect of different properties of oils and sorbents on sorption and retention behavior. Study summarized the effect of oil type, sorbent dosage, oil film thickness, temperature and reusability. The surface area of the sorbent, pore size present in the sorbent, and shape & strength of fibres constituting the sorbent and oil type, are the factors that affect oil sorption capacity 18. Pore size is an important parameter for absorbent materials as it affects the rate at which a fluid flows into or thorough a capillary network 26. The oil with higher viscosity tends to have higher initial sorption ratio 3, 38. Higher absorption may be that the fibres have greater sorption capacity to adsorb and hold the oil of higher viscosities. Since the oil is more viscous as well as heavy, having a higher specific gravity, it goes on to the fibre and moves into the interior of the fibrous mass. In the case of low viscosity oil with low specific gravity, the oil quickly moves into the fibrous mass as well as on to the surface but desorbs easily during the drainage period 38. The high viscosity of heavy oil significantly affects the capillary penetration of oil into the small pores of sorbent material 7. It is also evident from the Darcy s law. When the oil is more viscous, pores may become obstructed and therefore sorption capacity decreases. The opposite is found for sorbents made from kapok and cotton fibres, because thin oil films develop among fibres, favoring higher oil uptake for more viscous oils 37. So the sorbent construction plays an important role in the retention behavior of a sorbent. The sorbent with a higher porosity has a higher initial oil pickup, but poor retention capacity 3. Wei et al. 3 analyzed the effect of different materials and their structure on oil sorption and retention properties. It was observed that spun bonded nonwoven having high porosity (94.5%) has high desorption rate. Fibre diameters play vital role in influencing the oil retention. For the same porosity of nonwoven, melt-blown fibre with diameters micrometer shows better oil retention than the needlepunched fibres of the diameter 19 micrometer 3. The oil sorption capacity increases with increasing temperature. This increase may be due to the decrease in the oil viscosity at higher temperature to be suitable to penetrate pores 7. But the viscosity of the liquid to be absorbed determines the rate of fluid absorption to a greater extent than the ultimate absorption capacity of the material. A fluid with a higher viscosity takes a higher amount of time in penetrating through a pore when compared to liquids that are less viscous 25. The rate of absorption varies with the thickness of the oil.
5 194 INDIAN J. FIBRE TEXT. RES., JUNE 2011 Light oils are soaked up more quickly than heavy ones 31. When the thickness of oil film increases, the sorption capacity increases 7. The weight of recovered oil can cause a sorbent structure to sag and deform. When it is lifted out of the water, it can release oil that is trapped in its pores. During recovery of absorbent materials, lighter and less viscous oil may be lost from the pores more easily than heavier, more viscous oil 31. Changes in fibre properties when wet by the liquid can significantly affect liquid movement and retention properties. Fibre swelling not only increases liquid retention in the fibres at the expense of the capillary liquid capacity in pores, but also complicates the pore structure. Reduction in wet fibre strength and resiliency can lead to collapsed pores and lowering of the liquid holding capacity. All these factors present challenges to quantitative prediction and interpretation of capillary liquid transport phenomena in fibrous materials 43. The sorption capacity decreases with the number of dosage. 7 Fluid Flow through Fibrous Materials The transport of fluids in porous material is controlled by the structure of the porous medium and the molecular interaction with the pore wall, also known as wettability 44. During wetting the pores turn from a gas-dominant state to a liquid-dominant state. Fibrous structure is regarded as a kind of porous medium, most research on this phenomenon uses Darcy s law to relate rate of fluid flow (u) into porous medium 26,32, as shown below: k u = P (1) µ where u is the average velocity; µ, the Newtonian viscosity of the fluid; P, the pressure gradient; and K, the permeability of the porous structure 32,45. The fibre liquid surface attraction force causes the liquid to wet the fibres and is determined by both fibre surface properties and liquid properties 26,43. Liquid movement in any porous medium is driven by capillary action, which is governed by the liquid properties, liquid-medium surface interaction, and geometric configurations of the pore structure in the medium 43. This wicking of liquids in a textile fabric takes place mainly through a capillary system composed of the individual fibres in more or less parallel array 46. In the case of sorbents made from nonwoven, the fibre arrangements are random resulting in a 3-D network of capillaries aligned in all directions. The magnitude of the capillary pressure is commonly given by the following Laplace equation as applied to idealized capillary tubes: 2γcosθ p = (2) r c where r c is the capillary radius; γ, the surface tension of the liquid; and θ, the contact angle at the liquidsolid-air interface. When equilibrium is reached, the upward capillary driving force p equals the weight of the column of liquid; the net force on the liquid is zero and the rising of the liquid stops. The equilibrium capillary rise can be expressed as 25,43,47-49 : p L eq = (3) ρ i g where L eq is the equilibrium height; p, the capillary pressure; ρ i, the density of the liquid; and g, the acceleration due to gravity. For a circular capillary, Hagen-Poiseuille law for laminar flow through pipes can be used. The law states that the volumetric flow rate is proportional to the pressure drop gradient along the tube 50, as shown below: r 2 p q = c (4) 8ηL where q is the volumetric flow rate; η, the fluid viscosity; p, the net driving pressure; and L, the wetted length of the tube. 7.1 Absorbency When a fibre assembly absorbs liquid, the term liquid absorbency (C) can be used to evaluate the potential of that assembly in absorbing the liquid. This could be one of the measures of evaluating the efficiency of oil sorbent. The liquid absorbency is defined as the mass of the liquid absorbed by the sorbent of unit mass. The liquid absorbency of a fibre assembly is based on determining the total interstitial space available for holding fluid per unit dry mass of fibre and it is expressed as 47
6 PRABA KARAN et al.: OIL SPILL CLEANUP BY STRUCTURED FIBRE ASSEMBLY 195 T 1 V C = A + (1 α) d (5) W f ρ W f f where C is the liquid absorption capacity; A, the contact area between fibre assembly and liquid; T and W f, the thickness and mass of the fibre assembly; ρ f, the density of the dry fibre; V d, the amount of fluid diffused into the structure; and α, the ratio of increase in volume of a fibre upon wetting to the volume of fluid diffused into the fibre. 8 Sorption Capacity of Loose Fibre Assembly Fibrous assemblies were prepared from kapok, milkweed, cotton and polypropylene fibres by filling them into a circular PVC tube with 2 cm diameter and 2.5 cm length with required fibre volume fractions. Perforated PVC tube used for making fibrous assembly is shown in Fig. 3 (ref. 51). Holes were made on the surface of the PVC tube for the easy flow of liquids inside the tube 51. Ninety holes each of 1 mm diameter were made at equidistance. The fibre packing fraction was varied from 0.01 to If, m f and ρ f are the mass and density of fibre assembly and v is the volume of the fibre assembly, then the volume fraction of fibres in the fibrous assembly is given by the relationship as shown below: m f Φ = (6) ρ f V Prior to filling the fibres into the tube, kapok and milkweed fibres were opened by hand. Trash particles and lumps were removed. For cotton and polypropylene fibres, the fibre assemblies were made from drawn sliver. The densities of milkweed, kapok, cotton and polypropylene are 1480 kg/m 3, 1320 kg/m 3, 1520 kg/m 3 and 920 kg/m 3 respectively. The oils used for the test are high density engine oil (HD) and low density oil (diesel). The physical properties of the oils are given in Table 1. 9 Fibre Materials used for Oil Sorption 9.1 Cotton Fibre Oil sorption by raw cotton fibres could be explained by one or a combination of the following mechanisms such as adsorption by interactions between waxes on fibre surface and oils, adsorption by physical trapping on the fibre surface through its Fig. 3 Perforated PVC tube used for making fibrous assembly 51 Fig. 4 ESEM image of adsorption of engine oil by cotton fibres 52 Table 1 Physical properties of oils Property Engine oil (HD) Diesel oil Surface tension, mn/m (at 20 C) Density, g/cc (at 20 C) Kinematic viscosity mm 2 /s (at 20 C) irregular surface morphology; capillary action by diffusion of oil through the cuticle to the fibre lumen; and capillary action through its hollow lumen from fibre ends. It is plausible to presume that oil sorption by absorption into the pores in the secondary wall would be negligible with cotton fibres because of the hydrophilic nature of cellulose molecules. Since waxes are generally deposited in the cuticle of cotton, attractions between oil and the fibre surface are present through their hydrophobic interaction and Van der Waals forces, because both are hydrocarbons 16. Figure 4 (ref. 52) shows the oil sorption in cotton fibres. Organic solvents such as trichloroethane and trichloroethylene remove the surface wax. This reduces the oil sorption of cotton fibres by 20-30% compared to grey cotton. This indicates that
7 196 INDIAN J. FIBRE TEXT. RES., JUNE 2011 adsorption by interaction between surface waxes and oil is only responsible for 20-30% of the oil sorbed by cotton 20. The oil sorption capacity of cotton loose fibre assembly packed in PVC tube for various fibre volume fraction is shown in Fig. 5 (refs 51, 53). 9.2 Kapok Fibre Kapok [Ceibapentandra (L) Gaertn] fibre is an agricultural product which has high oil absorbency characteristics. The kapok fibre is fluffy, lightweight, non-allergic, non-toxic, resistant to rot and odorless. It has rich oiliness and is inelastic to be spun. It is conventionally used as stuffing for bedding, upholstery, life preservers and other water-safety equipment because of its excellent buoyancy due to the presence of high proportion of air-filled lumen 9. Kapok fibres typically comprise 64% cellulose, 13% lignin and 23% pentosan (non starch polysaccharide). Besides these constituents, they also contain waxy cutin on the fibre surface which makes them water repellent notwithstanding they are mainly composed of cellulose Kapok fibre is significantly hydrophobic and does not get wet with water 34. The wax cutin content in the kapok fibre is larger than that in cotton 20. It has a hollow tubular structure (or lumen shown in the Fig. 6) with external diameter of 16.5 ± 2.4 µm, internal diameter of 14.5 ± 2.4 µm, and length of 25 ± 5 µm. This indicates that 77% of the fibre volume is lumen. Kapok fibre is characterized by having a high level of acetyl groups (about 13.0%). Usually cell walls of plants contain about 1-2 % of acetyl groups attached to Fig. 5 Sorption capacity of fibre assemblies (a) cotton, (b) kapok, (c) milkweed, and (d) polypropylene 51,53 Fig. 6 SEM image of fibres (a) kapok, (b) milkweed, and (c) polypropylene 51,53
8 PRABA KARAN et al.: OIL SPILL CLEANUP BY STRUCTURED FIBRE ASSEMBLY 197 non-cellulosic polysaccharides. The density of kapok fibre wall material is 1.31 g cm -3 and its ash content is 0.78% (refs 9, 54, 55). Shi and Xiao 56 analysed the fine structure of kapok fibre using TEM and showed that it has a huge lumen and a thin cell wall. The cell wall constituted a distinctive five-layered cell wall structure with a compacted cuticle, an interlaced network structure, two layers of closely packed and aligned parallel fibril structures, and an inner skin. The huge lumen provided the kapok fibre with a very low density of 0.29 g/cm 3. The thin cell wall enabled the kapok fibre to be compressed more easily. The subtle structure of the cell wall prevents other little particles from entering into the lumen. These structural features show that the kapok fibres are suitable for buoyancy materials and stuffing materials 56. Water cannot easily penetrate into the lumen due to the presence of negative capillary entry pressure arising from the large contact angle (>90 ) between water and kapok fibre wall, and the large surface tension against air in the lumen 9. The kapok fibre extracted with diethylether followed by alcohol benzene shows the same results for the sorption capacity as observed with the original fibre (untreated fibre). The fibre is quite fine (8-10 µm diameters) and has a homogeneous hollow tube shape with a wall thickness of µm. It was observed that a significant amount of oil was diffused in the hollow tubes (8-10 µm) of the kapok fibre, suggesting that water cannot penetrate the tube because of its high surface tension ( N/cm at 20 C against air) 34. Kobayashi et al. 54 examined a hollow cellulosic kapok fibre. According to their results, the oil sorption of kapok fibre used in a mat, block, band, or screen was approximately times greater than that of polypropylene mat, which sorbs 11.1 g of B-heavy oil and 7.8 g of machine oil in water. The sorption capacity of loose kapok fibre assembly packed in PVC tube for various fibre volume fraction is shown in Fig. 5 (refs 51, 53). The sorption capacity of kapok fibre reduces with the number of dosage (or repeated usage). Because during the sorption, the numerous liquid bridges form between neighboring fibres due to interfacial interaction between liquid surface and fibre surface which leads to pulling the fibres together. As a result, the fibre assemblies become more compact and reduce the total effective pore volume of the fibre assemblies for subsequent uses. Additionally, residual oil trapped in the lumen of kapok fibres could not be fully drained out from the dead end of lumen formed by the folded fibres. This also lowers the oil sorption capacity of used sorbent. Fibres can be recovered from discarded bedding, upholstery and life preservers for reuse as oil sorbents. The sorbents made from kapok fibres can be ultimately taken for biomass energy recovery. Thus its use leaves no secondary waste to the environment Milkweed Fibre One gram of milkweed floss can sorb 40 g of light crude oil at room temperature. The milkweed fibres range in diameter from 20 µm to 50 µm. This exceptionally high oil sorption by milkweed fibre can be explained by the presence of large amount of wax on the fibre surface, 3% compared with the 0.448% wax content of cotton, and the larger and non-collapsing lumen of the fibre, which provides more void volume to absorb oil. The wall thickness of milkweed is only 10% of the total diameter of the fibre 33. It is reported that the mean diameter and mean wall thickness of milkweed fibre are 22.4 µm and 1.27 µm respectively 57. The SEM image of the milkweed fibre is shown in Fig. 6. The sorption capacity of loose milkweed fibre assembly packed in PVC tube for various fibre volume fraction is shown in Fig. 5 (refs 51, 53). 9.4 Polypropylene Fibre Nonwoven polypropylene sorbents are consolidated fibrous materials, which are different from the conventional textile fabrics. These fibrous webs, which contain small pores, facilitate the transport of liquids into the sorbents and retain the liquids after sorption. Nonwovens polypropylene sorbents would, therefore, appear to be ideal materials for oil-spill recovery in marine environments 3. The oil sorption property of polypropylene fibre is entirely based on the pores available between fibres. The mechanism for oil sorption by cotton fibre is controlled by adsorption on the fibre surface and capillary action through its lumen. On the contrary, oil sorption of polypropylene is through capillary bridges between fibres 17,19. So it is entirely based on porosity and structure of the sorbent materials. The scanning electron microscopy (SEM) photograph of polypropylene fibres is shown in Fig. 6 (refs 51, 53). The sorption capacity of loose polypropylene fibre assembly packed in PVC tube for various fibre volumr fraction is shown in Fig. 5 (refs 51, 53).