Indian Journal of Fibre & Textile Research

Vol. 36, June 2011, pp. 190-200

Review Article

Oil spill cleanup by structured fibre assembly

C Praba Karana, R S Rengasamy & Dipayan Das

Department of Textile Technology, Indian Institute of Technology, New Delhi 110 016, 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 reservoirs1. Oil is one of the most

important sources of energy and is also used as raw

material for synthetic polymers and chemicals

worldwide2-4

. Oil has been a part of the natural

environment for millions of years1.

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 environment3,5-10

. 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 environment11

.

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 processes12

. 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 carcinogens4.

Oil is a very complex mixture of many different

chemicals13

and a mixture of components consists of

different hydrocarbons that range from a light gas

(methane) to heavy solids with differing properties14

.

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

agglomeration1,7,13,15,17

.

The volatile components present in oil evaporate

quickly. Some of the medium-sized polycyclic

aromatic hydrocarbons are slightly soluble. Some of

_________ a To whom all the correspondence should be addressed.

E - mail: prabaiitd@gmail.com

PRABA KARAN et al.: OIL SPILL CLEANUP BY STRUCTURED FIBRE ASSEMBLY

191

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 hours15

.

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 oil18

.

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 there19

.

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 1.5-3.0 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

biodegradability20

.

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

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 normally1.

Sorbent is an insoluble material or mixture of

materials used to recover liquids through the

mechanisms of absorption or adsorption, or both21

.

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 molecule21, 22

.

2.1 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 fluid21, 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.

Fig. 2 — Mechanism of adsorption24

Fig. 1 — Weathering process of oil from the sea surface1,7,13,17, 20

INDIAN J. FIBRE TEXT. RES., JUNE 2011

192

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 balanced24

. The adsorption

of molecules forms layers, one over the previously

adsorbed layer. Chemical adsorption happens from a

changing electron among adsorbate and surface of

adsorbent24

.

2.2 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 liquids25

. 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 pressure25,26

.

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 oil10,24,27

.

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 alight28

. 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 toxic29

. 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 decades28

.

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 techniques4. Moreover,

the application of sorbent materials is an attractive

method for combating of oil spill pollution mainly

due to the lower costs and high effectiveness30

.

4 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 out16,18,26,32,33

. One of the most

economical and efficient methods for combating oil

spills is oil sorption by sorbents9,18,30,33

. Once this

change is achieved, the removal of the oil by removal

of the sorbent structure is not difficult18,33

. 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 recycled31

.

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 prevented6. 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 action34

.

The capillary action in the fibre assembly occurs

between the fibres and/or within the fibres. Micro

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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 fibres16,34

.

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 oil3,4,8,18,35-38

. 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 mineral7,16,30,33,37,39

. Among

synthetic products, polypropylene and polyurethane

foams are the most widely used sorbents for oil

spill cleanup because of their highly oleophilic

and hydrophobic properties7,9,28,33,36,37

. 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

surface36,40,41

. 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 cost8,31,40

. Natural sorbents, if used

effectively, can thus be more efficient than

synthetic products12,42

.

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 capacity9. Nearly

6.35 kg of clay absorbs 3.78 liter oil. Clay is not

hydrophobic, sinks in water and creates large volumes

to transport43

. The limitations of the mineral products

and organic synthetic products have led to the recent

interest in developing alternative materials, especially

biodegradable9. 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 materials9.

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 capacity18

. Pore size is

an important parameter for absorbent materials as it

affects the rate at which a fluid flows into or thorough

a capillary network26

.

The oil with higher viscosity tends to have higher

initial sorption ratio3, 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 period38

.

The high viscosity of heavy oil significantly affects

the capillary penetration of oil into the small pores of

sorbent material7. 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 oils37

. 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 capacity3.

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 0.9-10

micrometer shows better oil retention than the needle-

punched fibres of the diameter 19 micrometer3.

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 pores7. 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 viscous25

. The

rate of absorption varies with the thickness of the oil.

INDIAN J. FIBRE TEXT. RES., JUNE 2011

194

Light oils are soaked up more quickly than heavy

ones31

. When the thickness of oil film increases, the

sorption capacity increases7.

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 oil31

.

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 materials43

. 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 wettability44

. 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:

ku P

µ= − ∇ …(1)

where u is the average velocity; µ, the Newtonian

viscosity of the fluid; P∇ , the pressure gradient; and

K, the permeability of the porous structure32,45

.

The fibre liquid surface attraction force causes the

liquid to wet the fibres and is determined by both

fibre surface properties and liquid properties26,43

.

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

medium43

. 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 array46

. 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:

2 cosγ θp

rc= …(2)

where rc is the capillary radius; γ, the surface tension

of the liquid; and θ, the contact angle at the liquid-

solid-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 as25,43,47-49

:

pLeq

ρ gi= …(3)

where Leq is the equilibrium height; p, the capillary

pressure; ρi, 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 tube50

, as shown

below:

2

8

r ∆pcqηL

= …(4)

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 as47

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195

1(1 ) d

f ff

VTC A α

W ρ W= − + − … (5)

where C is the liquid absorption capacity; A, the

contact area between fibre assembly and liquid; T and

Wf, the thickness and mass of the fibre assembly; ρf, the density of the dry fibre; Vd, 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 tube51

. Ninety holes each of

1 mm diameter were made at equidistance. The fibre

packing fraction was varied from 0.01 to 0.09. If,

mf and ρf 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:

Φ

mf

Vfρ

= …(6)

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/m3,

1320 kg/m3, 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

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

hydrocarbons16

. 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

Fig. 3 — Perforated PVC tube used for making fibrous assembly51

Fig. 4 — ESEM image of adsorption of engine oil by

cotton fibres52

Table 1—Physical properties of oils

Property Engine oil (HD) Diesel oil

Surface tension, mN/m

(at 20°C) 31 25

Density, g/cc

(at 20°C) 0.90 0.82

Kinematic viscosity

mm2/s (at 20°C) 130 2

INDIAN J. FIBRE TEXT. RES., JUNE 2011

196

adsorption by interaction between surface waxes and

oil is only responsible for 20-30% of the oil sorbed by

cotton20

. 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 lumen9.

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 cellulose54-55

. Kapok fibre is significantly

hydrophobic and does not get wet with water34

. The

wax cutin content in the kapok fibre is larger

than that in cotton20

. 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) polypropylene51,53

Fig. 6 — SEM image of fibres (a) kapok, (b) milkweed, and (c) polypropylene51,53

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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 Xiao56

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/cm3. 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 materials56

.

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 lumen9.

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 0.8-1.0 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 (7.2 × 10-4

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 1.5-2.0 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 environment9.

9.3 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 fibre33

. It is reported that

the mean diameter and mean wall thickness of milkweed

fibre are 22.4 µm and 1.27 µm respectively57

. 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 environments3.

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 fibres17,19

. 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).

INDIAN J. FIBRE TEXT. RES., JUNE 2011

198

In all the cases the polypropylene fibre shows

highest sorption followed by kapok. The diameters of

polypropylene, cotton, milkweed and kapok fibres are

19, 21, 29 and 23 µm respectively. Polypropylene

being finer provides smaller capillaries in the fibre

assembly. The gravity effect of rising liquid into the

fibre assembly is less with finer pore; hence most of

the pores are filled into PP fibre assembly, improving

its sorption capacity.

9.5 Other Materials

Acylated cotton can be used as sorbent in oil spill

removal application. It has higher sorption value than

the other natural materials. But the degradation time is

much higher for acylated cotton. Flax is a natural

cellulosic fibre having a large amount of surface wax

and a lumen, and therefore should have the same

mechanism as that of cotton and milkweed36

.

Recycled wool-based nonwoven material can also be

used for the sorption of oil58

.

10 Test Methods for Oil Sorbents 10.1 Test for Sorption Capacity

In a simple test, 40 g of oil is placed in 400 mL of

demineralized water (pH 6.20) inside 800 mL glass

beaker. One gram of dry material is then placed in the

beaker and shaken in a laboratory shaker at a

frequency of 110 cycles / min for 15 min. Water

content is determined by distillation using a mixture

of toluene and xylene (20/80 v/v) as a solvent, in

accordance with ASTM D95-83 (ref. 59). This test

can also be done for sea water and without water

medium. The oil sorption capacity (q) is determined

by the following relationship:

( )f o w

o

m m mq

m

− += …(7)

where mf is the weight of the wet material after

draining (g); mo, the initial weight of the material (g);

and mw, the water content in the material (g).

To study oil sorption capacity of sorbents without

the water medium, a simple procedure is used.

Sample oil (50 g) is placed in a 1000 mL glass beaker,

and the sorbent is immersed in the bath. After shaking

and draining the amount of oil sorbed (g) per g of the

sorbent (S) is determined by weighing38

, using the

following relationship:

so s o

s s

W -W WS= =

W W …(8)

where WS is the weight of fresh sorbent sample (g);

WSO, the weight of sorbent saturated with oil product

(g); and WO, the weight of oil product retained into

sorbent matrix30

(g). 10.2 Test for Oil Sorption Rate and Absorbency

Gravimetric absorbency testing system (GATS) is

used to record absorbency or wicking as a function of

time of absorption. The liquid is supplied from a

reservoir through a tube to the sample stage. The

sorbent or fabric is initially at level with the oil on the

reservoir. Liquid from the reservoir penetrates into the

sorbent due to wicking/capillary sorption. The change

in the mass of the reservoir can be recorded as a

function of time. Sorption rate and absorbency (g/g)

can be computed60

. 10.3 Test for Oil Retention

To determine oil retention of an oil sorbent sample,

the sample (5 × 5 cm2) is placed in 150 mL of oil for

15 min. The sorbent is then removed and vertically

hung, where upon the adsorbed oil begin to drip from

the sorbent. The weight of the material is measured

after 15, 30, 60, 120, 300 and 1800s of drainage. The

amount of oil retained is determined as the difference

between the weight of the wet material after drainage

and the initial weight of the material23, 61

. 10.4 Test for Recovery of Sorbed Oil and Reusability of

Sorbents

In this test method, artificial sea water (500 mL) is

placed in a 1000 mL glass beaker and 50 g of crude

oil is then added to it. One gram of dry material is

then placed in the beaker and shaken in a laboratory

shaker at a frequency of 110 cycles / min for 15 min.

The sorbent with oil is squeezed between two rollers

at a pressure of 98 N/cm before it is reweighed to

determine the amount of recovered oil. The squeezed

sorbent is again used in the sorption process as before.

The efficiency of sorbent reusability is determined by

oil sorption capacity of each sorbent after repeated

sorption and desorption cycles23,38

.

10.5 Test for Water Uptake and Buoyancy

Static and dynamic conditions are the two methods

available for the evaluation of water uptake and

buoyancy of sorbent. In static conditions, the sorbent

as received (5 × 5 cm) is placed in a beaker filled with

~7.5 cm deep layer of demineralized water. After

15 min and 24 h, observations are done and the

sorbent is removed from water. In dynamic

conditions, 4 samples (5×5 cm) are placed in a

PRABA KARAN et al.: OIL SPILL CLEANUP BY STRUCTURED FIBRE ASSEMBLY

199

container half-filled with demineralized water. The

container is sealed and mounted on the shaker, resting

on its side. Shaking is carried out at a frequency of

110 cycles/min for 15 min. In both cases, any sorbent,

which does not remain floating on the water is

considered to fail the test. Then the water uptake

percentage is calculated26,61

.

11 Conclusion It has been found that there is a great potential for

sorbents made from natural fibres to replace synthetic

oil sorbents. There is indeed a great scope in realizing

a clean environment free from pollution with the use

of natural fibre based oil sorbents. But still the

commercialization of natural fibre based material

from milkweed and cotton is a great challenge due to

difficulty in forming these fibres to a predetermined

structure like polypropylene melt blown oil sorbents.

Concerted efforts are needed to structure suitable fibre

assemblies for their use as environment- friendly oil

sorbents.

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