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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: [email protected]
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
PRABA KARAN et al.: OIL SPILL CLEANUP BY STRUCTURED FIBRE ASSEMBLY
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
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 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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