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The Production and Characterisation of Activated Carbons A Review.

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Dev.Chem. Eng. Mineral Process., 6(5),pp.231-261, 1998.
The Production and Characterisation of
Activated Carbons: A Review
S. J. Allen, L. Whitten
Dept. of Chemical Engineering, The Queen's University of Belfast,
Stranmillis Road, Belfast BT9 5AG, Northern Ireland, UK
and
G. McKay*
Department of Chemical Engineering, The Hong Kong University of
Science and Technology, Clear Water Bay, Kowloon, HONG KONG
The uses of active carbons and carbon chars for liquid and gas phase applications
has been presented. Their application for decolourising, water treatment, solvent
recoveiy, militaly uses, nuclear industry, air purification, domestic uses, precious
metal recovery and catalysis, have been outlined. A detailed review of the production
and manufacture of activated carbonsfiom a wide range of carbonaceous sources is
given. Methods of activation are discussed based on a range of chemicals and how
these chemicals influence surface activity, pore size and pore distribution.
The
techniques for characterising activated carbons are reviewed in detail and include
porosimetry, sorptometry, topography, pore size distnbution, isotherms and suflace
area measurements.
The specrfic results of some active carbons derived @om
lignocellulosic materials (peat and lignite) are also presented.
*Authorfor correspondence.
23 1
S.J,Allen, L Whitten and G. McKay
Introduction
The removal of contaminants out of ,.quids and gases is a process which has received
considerable attention throughout the twentieth century. However, the interest in h s
topic is not confined to more recent years. As long ago as 1550 BC the Egyptians
used charcoal to purify oils and early ocean going ships stored drinking water in
wooden barrels, the inside of which had been charred.
The earliest date at which the adsorptive powers of carbons and charcoals were
d e f ~ t e l yrecognised was in 1773 when Scheele described experiments with gases.
In 1786 Lowitz noticed the decolourising effects of charcoal on solutions. A few
years later, wood char was employed to purify cane sugar and in 1808, was applied to
the developing sugar beet industry. The discovery by Figers in 1811 of the greater
decolourising power of bone char, led to its swift adoption by the sugar refiners. At
first, pulverised bone char was applied on a single use and discard basis, but limited
supplies made regeneration necessary.
Active carbon is a generic term referring to highly porous carbonaceous materials.
Activated carbon can be from any carbon containing material by thermal
decomposition or pyrolysis followed by activation with steam or carbon &oxide at
high temperatures (700-1 l0OOC). During the 19th century, many studies were made
to develop decolourising carbons from other source materials. Bussy in 1822 heated
blood with potash and produced a carbon with 20 to 50 times the decolourising power
of bone char. Blood char so produced was used for years in many laboratory studies.
In 1865, Hunter reported the gas-adsorbing power of coconut char. In the same year
Stenhouse prepared a decolourising char by heating a mixture of flour, tar and
magnesium carbonate. In another process Winser and Swindells heated paper mill
wastes, and many of these processes are similar to processes now in industrial use,
however they were not developed until long after the time of their discovery.
Reasons for this are found in manufacturing problems that could not have been dealt
with at the time, such as the corrosive nature of most activation con&tions, whch
require special structural materials not then available, and the unavailability of precise
control instrumentation, which has been developed only relatively recently. AnotheI
232
The production and characterisation of activated carbons: A review
major factor which retarded the development of these special carbons was the absence
of apparent needs for a more powefil adsorptive carbon than bone char. However
early suggestions for a market, which have now become important, were made by
Lipscombe, who, in 1862 prepared a carbon to purify potable water, and Stenhouse in
1854, who described the forerunner to the modem gas mask.
The use of carbon molecular sieves in gas separation, in particular oxygen and
nitrogen, has grown steadily in the past years (Sircar et al., 1996). These carbons are
characterised by a high adsorption capacity and by the size of the micropores which
must be of the same order of magnitude as the adsorbate molecules (Stoekli et al.,
1996). Depending on the ability of a molecule to diffuse into the micropore volumes
dxtates the ability of carbon molecular sieves to effect separations of gas mixtures.
The creation of a large sorptive capacity in the micropores is achieved by physical
or chemical activation but this also leads to wide micropore openings. The reduction
in width of micropore openings can be acheved by the deposition of carbon resulting
from the crackmg of hydrocarbons adsorbed in the micropores via heat treatment
(Stoekli et al., 1996) or through mild gasification (Verma and Walker, 1993). As a
consequence there is a reduction in pore size and pore volume thus decreasing
adsorption capacity. As a result, the use of activated carbon fibres as molecular
sieves is increasing. These fibres often have a greater adsorption rate and larger
capacity than typical granular activated carbons when used in gas filtration (Suzuki,
1994). The key commercial applications of molecular sieve-type activated carbons in
the gas separation and purification industry have been reviewed (Sircar et al., 1996).
Characterisation of Chars and Carbons
(i) Sorbent Characterisation
Activated carbons are complex materials which, as mentioned previously, cannot be
characterised by chemical analysis or structural formula. Every carbon is unique with
its physical and chemical characteristics a direct result of its parent material, any pretreatments used, and the temperature and nature of the activation whether it be
233
S,J. Allen. L. Whitten and G. McKay
chemical, physical or a combination of both.
However, standard methods of
characterisation have been established according to, for example pore classification,
surface area or surface topography, in order to compare carbons and assess their
adsorptive potential.
The use of activated carbon for wastewater treatment and potable water
purification has been the topic of a number of papers in the Advances in Chemistry
Series (ACS) between 1960 and 1997; a particularly relevant volume was on the
application of granular activated carbon (McGuire and Suffet, 1983) to water
treatment.
(ii) Topography
Topographical stumes of the external surface of activated carbon gives an indication
of the porous nature of the carbon's exterior. The external pores are generally large
pores, normally mesopores and macropores depending on the parent material, and are
important in terms of adsorption as they are transport pores allowing the passage of
the adsorbing species, from the adsorptive, to the adsorbents internal porous structure.
Scanning electron microscopy (SEM) is a very effective and accurate method often
used for surface investigations. SEM gives a three-dimensional picture of the surface
structure. Zulkamain et al. (1993) found that activated carbons from mangrove wood
have surface pores of 20-40pm diameter whereas cross-sectioned samples showed
internal pore distributions of 80-100pm, 10-20pm and 1-2pm. Stoeckli (1989)
reported on using High-Resolution Transmission Electron Microscopy (HRTEM) to
confirm findings in adsorption experiments regarding the hsorganised nature of
microporous carbons. Using a selective technique developed by O b e r h known as
"dark-field", the carbons' interplaner spacings were determined from the
micrographs. Figure 1 shows SEM pictures of new peat and of the peat char based on
30 minutes charring at 400°C.
234
The production and characterisation of activated carbons: A review
Figure I. Scanning electron micrographs of raw peat, and of peat char - after
charringfor 30 mintues.
235
S.J. Allen, L. Whitten and G.McKay
(iii) Porosimehy
Porosimetry is a study related to the structure of a medium. Mercury porosimetry is
used extensively as a means of characterising porous media and powders. By far the
most important aspect of mercury porosimetry is its use for determining pore size
dmibution.
The concept of using mercury penetration was fust suggested by
Smetana in 1842 to show the porous nature of wood. Then Washbum in 1921, and
independently in 1941 by Loisy, proposed a method by which pore size distribution
could be determined from pressure-volume data for mercury penetration. However, it
was not until a paper was published in 1945 by Ritter and Drake that experimental
methods and data were described for this procedure.
Mercury porosimetry works on the principle that, as mercury is a non-wetting
liquid towards most substances (having a contact angle with mercury greater than
90"), it will not penetrate into openings without pressure being applied. If the solid to
be investigated is placed in a container with a tapered capillary end, evacuated and
filled to a predetermined level with mercury, when pressure is applied the mercury
level will fall as it is pushed into the pores. The decrease in the mercury level is
directly related to the pressure exerted and from this the volume-pressure
porosimetric curve can be constructed.
Winslow (1978) raised the issue that intrusion could be destructive to the pore
structure. For accuracy and to study microporous structures, increasingly higher
pressures are being used. He concluded that little or no damage was incurred with
strong materials however, weaker structures were susceptible at high pressure to pore
redistribution. Van Brake1 et al. (1981) also raised the question of mechanical
deformation of the sample being tested. From the findings of Moscou and Lub
(1981), Dees and Polderman (1981), and Spitzer (1981), they concluded that
additional porosity creation or mechanical deformation depends on the sample
preparation.
Ritter and Drake showed that, with a mercury pressure-volume relationshlp, on
retraction by a reduction in pressure a proportion of mercury remains in the pores
resulting in a &splay of hysteresis. Hysteresis has been attributed to two causes;
236
The production and characterisation of acrivated carbons: A review
(a) contact angle hysteresis in wluch the advancing contact angle is different to the
receeding angle and (b) structural hysteresis or ink-bottle hysteresis arising from the
non-cylindrical nature of pores in real porous bodies (cylindrical capillaries do not
display hysteresis).
Due to structural hysteresis, surface areas obtained from
porosimetry will never be accurate.
Although there are problems linked with the accuracy of porosimetry data, the
method provides a characteristic of pore space geometry, h s information combined
with a comparative method can be used as a means of sorbent characterisation. The
standard methods used today are still based on Washburn’s original theory and have
been described in detail (Bradley et al., 1989). Porosimetry is used frequently and
accurately as a characterisation tool in the catalysis industry.
(iv) Sorptometry
Gas sorption analysis and the interpretation of its physical and chemical frndings is by
far the most widely used and investigated method of sorbent characterisation. In
general, physisorption is used to determine the surface area, pore hstribution and
pore volume of a sorbent whereas chemisorption is used to assess the area of any
active surface component.
Initially the physical adsorption (and desorption) isotherm is determined for the
sorbent to be characterised. Thls is generally determined by nitrogen adsorption at
the liquid nitrogen temperature of 77K, however, other small molecule adsorptivities
have also been used, Ar (77K), Kr (77K), Xe (77K), alkanes (298K), CO, (195K) and
0, (90K). The shape of the isotherm provides an indication of the nature of the pore
structures in the sorbent make-up.
Table 1 presents a comparison of pore characteristics for different models for an
activated carbon treated with 50% ammonium molybdate and charred at 6OO0C for 30
minutes.
237
S.J. Allen, L. Whitten and G.McKay
Table 1.
Pore and surface characteristics of a chemically treated peat-based
activated carbon.
Adsorption
Model
Monolayer
Volume
Surface Area
Micropore
Volume
cm3g-'
m2gl
cm3g-'
0.094
408
BET
386
I
Langmuir
0.112
487
476
D-R
0.165
0.169
The pore size distribution of several carbonaceous adsorbents is shown in Figure 2.
A more detailed example of pore size distribution is presented in Figure 3, comparing
raw lignite with a lignite-based activated carbon using steam activation.
Nitrogen adsorption data is a good source of information which can be used to
yield information regarding the pore size distributions of the carbons.
The application of these methods by Brunnauer et al. (1940)yield information on
pore radii and pore size distributions. For an industrial application of a carbon to be
successful, the value of h s type of physical characterisation cannot be understated.
The monolayer capacity of the adsorbent will give an accurate and reproducible value
for the surface area of the carbon in question.
Langmuir values for monolayer capacity are found to be slightly hgher than those
obtained using BET. The micropore volume as determined by the
G,
and D-R
models are generally in good agreement. Surface areas are similar for Langmuir and
D-R models whilst BET and
from the results in Table 1.
238
a, surface areas are somewhat lower. This can be seen
The production and characterisation of activated carbons: A review
Figure 2. ?he micro,meso and macropore size distribution of dfferent akorbenrs.
Figure 3. Pore disfn'butionof lignite and a s t e m activated adsorbent.
239
S.J.Allen, L. Whitten and G.McKay
Figure 4. Isothenn classifcation.
240
The production and characterisation of activated carbons: A review
(v) Isotherms
An isotherm is a pictorial representation of the equilibrium adsorptioddesorption
process. It shows the equilibrium relationship, at constant temperature or pressure,
and the adsorbed quantity. Brunauer et al. (1940) noted that although isotherms were
dfferent for all sorbents and sorbates investigated, common shapes of isotherms were
observed. Six isotherm types are generally used as a basis for classification, and
these are shown in Figure 4.
Type I reversible isotherms are characteristic of microporous solids as seen for
example in activated carbons or zeolites. Uptake reaches a limiting value as p/p,
tends to 1, indicating that uptake is dependent on micropore volume accessibility
rather than surface area. Typical systems include nitrogen on charcoal at -1 83OC and
many dyes on activated carbon at 2OOC.
Type 11 isotherms are common to non-porous or macroporous solids. They
indicate unrestricted monolayer-multilayer adsorption, such as the sorption of
nitrogen on iron catalysts at - 195'C.
Type I11 isotherms are uncommon and are usually associated with water vapour
adsorption. Generally in this type of adsorption, the adsorbent-adsorbate interactions
are weak as compared with the adsorbate-adsorbate interactions. An example is
bromine on silica gel at 79'C.
Type IV isotherms are recognised by the presence of a hysteresis loop. This type
of adsorption is observed frequently with many mesoporous industrial adsorbents.
The adsorption of n-decane on vycor has been used to study isotherm hysteresis.
Type V is related to the Type 111isotherm and is very uncommon. Type VI isotherms
are seen with uniform non-porous surfaces and represent stepwise multilayer
adsorption.
(vi) Surface Area Determination
Specific surface area is a key parameter which is used to characterise porous solids.
Surface area determination relies on the accurate knowledge of the average area
241
S.J. Allen, L. Whitten and G. McKay
(a,,,, molecular cross-sectional area), occupied by the adsorbate molecule in a
complete monolayer. Thus:
A,(BET) = n,.a L.a,
and
a, (BET) = A, (BET) / m
where A,(BET) and a,(BET) are the total and specific surface areas respectively, m is
the mass of adsorbent, and L is the Avogadro constant.
To form a monolayer the adsorbate molecules must be physically adsorbed,
however, it is known that physical adsorption generally involves multilayer
adsorption. Multilayer adsorption commences at pressures below those required to
complete the monolayer formation thus preventing an exact determination of the
monolayer capacity from the experimental data. Brunnauer, Emmett and Teller
(BET, 1940) proposed a simple model to account for multilayer physical adsorption
and hence to determine the monolayer capacity for surface calculation. The BET
equation is written:
where V is the volume of gas adsorbed at pressure p; po is the saturated vapour
pressure of the liquid at the temperature of the experiment; V,,, is the volume
equivalent to an adsorbed monolayer; and C is the BET constant equal to
exp (H,-H,)/RT; H, is the molar enthalpy of adsorption in the first layer; and H, is the
molar enthalpy of condensation. In h s form, a linear plot can be achieved and C and
V, easily determined from the linear slope and intercept.
The derivation of the BET equation results from many over simplifications and
idealisations and in fact its linear form is only linear in the relative pressure range of
0.05-0.35.In spite of its limitations, the BET method is still the most widely used
method for surface determination in sorbent characterisation. Some indication of the
applicability of the BET model as a method for surface determination may be
242
The production and characterisation of activated carbons: A review
obtained from a comparison of data obtained with different sorbates on the same
porous material.
(vii) Pore Distribution
The porosity of a solid is related to its texture and refers to the pore space in the
material, i.e. the fraction of the bulk volume that is occupied by pore or void.space.
Pore size dstribution groups pores with respect to their diameters.
Industrial Applications of Activated Carbons
1. Uses of Chars and Carbons
The uses of activated carbon are widespread, with each particular application
requiring a specific grade of carbon. The various applications can broadly be split
into two categories, (i) liquid-phase applications and (ii) gas-phase applications.
2. Liquid-Phase Applications
Activated carbons for use in liquid-phase applications differ from gas-phase carbons
primarily in pore size &sttibution. Liquid-phase carbons have significantly more
pore volume in the macropore range, whlch permits liquids to d f h s e more rapidly
into the mesopores and micropores. The larger pores also promote greater adsorption
of large molecules, either impurities or products, in many liquid-phase applications.
Liquid-phase activated carbon can be applied either in a powder, granular or
shaped form. The average size of powdered carbon particles is 15-25 pm, and they
are most often used in batch applications. Granular or shaped-carbon particle size is
usually 0.3-3.0 mm, and these carbons are more suitable for use in continuous flow
systems.
243
S.J. Allen, L Whitten and G.McKay
2a. Batch Systems
Batch stirred vessels are most oflen used in treating material with powdered activated
carbon. The type of carbon, contact time, and amount of carbon vary with the desired
degree of purification. The efficiency of activated carbon may be improved by
applying continuous, counter-current, carbon-liquid flow with multiple stages.
2b. Continuous Systems
Granular and shaped carbons are used generally in continuous systems where the
liquid to be treated is passed through a fixed bed. Compounds are adsorbed by the
carbon bed in the adsorption zone. As carbon in the bed becomes saturated with
adsorbates, the adsorption zone moves in the direction of flow, and breakthrough
occurs when the leading edge of the adsorption zone reaches the end of the column.
3. Gas-Phase Applications
Gas-phase applications of activated carbon include separation, gas storage, and
catalysis. Although only about 20% of activated carbon production is used for gasphase applications, these products are generally more expensive than liquid-phase
carbons and account for about 40% of the total value of shpments. Most of the
activated carbon used in gas-phase applications is granular or shaped.
Separation processes comprise the main gas-phase applications of activated
carbon. These usually exploit the differences in the adsorptive behaviour of gases
and vapours on activated carbon, on the basis of molecular weight and size. For
example, organic molecules with a molecular weight greater than about 40 are readily
removed from air by activated carbon. Recent uses include odour removal and gas
purification; also the use of carbon molecular sieves for gas separation processes.
4. Water Treatment
A major use of activated carbon is in the treatment of both potable and effluent water.
Potable water primarily from rivers can be contaminated by sewage, effluent
244
The production and characterisation of activated carbons: A review
discharge, algae breakdown, etc.
Th~scan be rendered drinkable by chemical
treatment, but it may be left with an undesirable taste or odour (Cheremisinoff, 1993).
Activated carbons can be used as a fmal polishmg treatment to remove such tastes or
odours. Activated carbon is never a primary treatment, but is normally used in
conjunction with a suitable chemical treatment.
Industrial effluent containing toxic or harmful contaminants can be treated with
activated carbon to avoid sea or river pollution. Also effluents from detergent
manufacturers, refiners, electroplating and chemical manufacturers, and from
manufacturers of insecticides, herbicides and fungicides, can effectively be treated
with activated carbon.
Activated carbon is also a very effective dechlorinator, and therefore is used by
manufacturers of soft drinks and beers where water has been superchlorinated to
render it sterile.
5. Decolourising
Activated carbon developed out of the decolourising industry where it was first used
in decolourising sugar. It is still extensively used for this application. It is also used
as a decolouriser in the purification of edible oil, foodstuffs, pharmaceutical
intermediates and chemicals such as esters, surfactants and alcohols. The wine and
spirit industry uses activated carbon to ensure that whte wine, vodka and bacardi,
etc., reach the consumer with the correct appearance and flavour.
6. Solvent Recovery
Industries such as paints, adhesives, transparent film, printing, rubber, plastics, dry
cleaning, chemicals and textiles, increasingly use hydrocarbon solvents during the
manufacture of their products.
These solvents must be recovered for both
environmental and economic reasons.
For this purpose activated carbons are
particularly effective, and adsorb most solvents for any concentration above 1000
ppm. The method involves passing solvent-laden air through a bed of carbon until
245
S.J. Allen, L. Whitren and G.McKay
the bed is saturated. At this point the air stream is redirected to another bed and the
first bed is stripped of the solvent by passing steam at 105'C through the bed in the
reverse direction to air flow. The solvent is then separated from the aqueous phase,
and the carbon bed goes into a drylng cycle to prepare it again for the solvent-laden
air stream.
Solvents recovered by this method include chloroform, carbon
tetrachloride, acetone, pentane, methyl ethyl ketone, tetrahydrofuran, white spirit,
benzene, toluene, xylene, petroleum and ether.
7. Military Uses
Activated carbon was first used in World War I to protect against chlorine gas. Most
of the world's armed forces now use activated carbon to protect against attack by
toxic gases, such as mustard gas, during conflicts. Other such gases include hydrogen
cyanide, cyanogen chloride, phosphine and arsine. These carbons are put to use in
personal gas masks, shelter filters and inlet filters for tanks and armowed cars. A
further use is in military suiting where combat uniforms contain a layer of carbonimpregnated material under the outer cover. This protects the wearer from splashmg
of liquid gas after a toxic gas canister has exploded.
8. Nuclear Reactors
In the western world, all nuclear power reactors have activated carbon ventilators
installed as a precaution against radioactive iodine leaks from the core or heat
exchanger systems. Special carbons impregnated with potassium iodide or potassium
tri-iodide are commonly used for the purpose.
Also a potential problem with
radioactive krypton and xenon was highlighted as a result of the Three Mile Island
accident in 1979. To cope with this, special off-gas delay beds were designed so that
passage of radioactive krypton and xenon through the bed could be delayed, until
such time as the radiation hazard could have decayed to an acceptable level. These
types of bed are now being fitted to many nuclear reactors.
246
The production and characterisation of activated carbons: A review
9. Air Treatment
There are a large number of applications in which activated carbon is used to remove
noxious contaminants from air streams. The commonest include the ventilation of
areas where large numbers of people congregate, such as airports, hospitals,
submarines, office blocks and theatres. It is essential that toxic contaminants, which
result from industrial activity, such as mercury vapour, sulphur dioxide and hydrogen
sulphde, must be removed with maximum efficiency. In these instances a highly
efficient impregnated carbon is usually employed.
10. Domestic Uses
In recent years there has been a large increase in the number of uses to which
activated carbon can be put in the home. These include cooker hoods, fiidge deodourisers, air purifiers, deep fat fiyer cartridges, Odour Eater foot insoles and
cigarette filters.
11. Precious Metal Recovery
Activated carbon can adsorb trace quantities of gold and silver from a cyanide
solution. The process has been known since the 1930s, but it is only since the 1970s
that its full commercial implications have been recognised. Today its usage is widely
practised and is increasing at a rapid rate. The carbon-in-pulp (CIP)process for the
recovery of gold in an agitated, counter-current, multistage extraction process has
been developed and used successfully in recent years.
12. Catalysis
Many chemical reactions require a catalyst to improve efficiency, accordingly, in
many cases activated carbon's large surface area is used as a support for such
catalysts, thus further improving efficiency.
Examples of its use include the
production of chloro-fluoro carbons, terephthalic acid, vinyl chloride and sulphuryl
chloride.
247
S.J. Allen, L. Whitten and G.McKay
13. Miscellaneous Uses
Numerous relatively low-volume activated carbon uses make up a small proportion of
carbon consumption. Small carbon filters are used in households for purification of
tap water. Oils,dyes, and other organics are adsorbed on activated carbon in dry
cleaning recovery systems. Electroplating solutions are treated with carbon to
remove organics that can produce imperfections when the thin metal layer is
deposited on the substrate. Medical applications include removal of toxins from the
blood of patients with artificial kidneys and oral ingestion into the stomach to recover
poisons or toxic materials. Other uses include ELCD (evaporative loss control
device) canisters, gas desulphurisation, de-ozonisation, condensate de-oiling and
carbon dioxide purification.
Production and Manufacture of Activated Carbons
Despite the commercial importance of activated carbon, little is known about the raw
materials that are suitable as precursors for activated carbons, or the manufacturing
processes, and why one precursor makes a product superior for a particular use and
apparently unsuitable for another.
(a) Precursor Materials
Activated carbon can be produced from any carbonaceous material and until recently
anthracite and bituminous coals have been the major sources. However, today the
range of precursor materials is diverse and widespread, being influenced by the need
to produce low-cost carbons.
Environmental awareness has also shaped the
manufacturing of activated carbons by introducing the concept that normal everyday
waste materials, such as agncultural by-products and old tyres, are potential sources
of activated carbon. Today the sources of activated carbon are extensive and are ever
expandmg. Some of the source materials that have been investigated are outlined in
Table 2.
248
The production and characterisation of acrivated carbons: A review
Table 2. Source materials studied for activated carbon production.
Bagasse
Kelp and seaweed
Beet-sugar sludges
Lampblack
Blood
Leather waste
Bones
Lignin
Carbohydrates
Lignite
Cereals
Molasses
Coal
Nut shells
Coconut shells
Oil shale
Coffee beans
Peat
Corncobs and corn stalks
Petroleum acid sludge
Cottonseed hulls
Petroleum coke
Distillery waste
Potassium ferrocyanide residue
Fish
Pulp-mill waste
Blue Dust
k c e hulls
Fruit piths
Rubber waste
Gmphite
Sawdust
Wood
(b) Production Methods
The methods for producing activated carbons are nearly as widespread as their
potential uses and sources materials. However, the four basic steps common to most
methods are: raw material preparation, pelletising, low-temperature carbonisation,
and activation. There are two types of activation which are used to impart a porous
structure within a starting material of relatively low surface area, namely thermal or
chemical activation. Physical or thermal activation which, after initial treatment and
pelletising, involves carbonisation at 400-5OO0Cto eliminate the bulk of the volatile
matter, and then partial gasification using a mild oxidising gas such as CO,, steam or
flue gas at 800-1000°C to develop the porosity and surface area. l k s activation
249
S.J. Allen, L. Whitten and G.McKay
process is usually carried out in a fxed bed, however, in recent years fluidised beds
have also been utilised. The second method of chemical activation involves the
incorporation of inorganic additives or metallic chlorides into the precursor before
carbonisation. The action of these additives degrades and dehydrates the cellulosic
materials present during carbonisation at 250-650°C. L i e , usually the raw material
that is blended with activators such as sulphuric acid, phosphoric acid, zinc chloride
or potassium duocyanate or other vegetable matter, is carbonised at temperatures up
to 900OC.
The activated carbons produced have a unique surface property.
Cheremisinoff and Ellerbusch (1978) noted that the high specific area of activated
carbon (typically in the order of 1000m2/g; De John, 1976) is nonpolar or only
slightly polar due to surface oxide groups and inorganic impurities.
(c) Char Formation
Chars are formed when the precursor material is carbonised or pyrolysed usually in
the absence of air. Pyrolysis of lignocellulosic materials liberates most of the volatile
non-carbon elements, normally hydrogen, oxygen and nitrogen, from the precursor
matrix resulting in a non-graphitisable char with a rigid carbon skeleton, made up of
aromatic sheets and strips. Chars can be produced over a range of temperatures and
charring times, each carbonisation condition resulting in a sorbent with unique
adsorption properties. Zulkamain et al. (1993) investigated the effects of heating
times and temperatures on carbons prepared from mangrove wood. They noted
optimum conditions of preparation at 5OO0C for 3 hours, producing a carbon which
had a high iodine number of 503 m2/g,greater than that of a coconut carbon produced
under the same conditions. The pore sizes of the resultant carbon were larger than
those of coconut shell (40pm). Drozhalina et al. (1984) reported on the influence of
brown coal being added to peat in variable fractions prior to carbonisation at 800-
900OC. They found that the strength of the carbon was improved by the inclusion of
brown coal, porosity developed, and as temperature increased a parallel increase in
micropore and macropore volume was observed. Chars prepared under specific
carbonisation conditions can be utilised successfully, without further processing,
250
The production and characterisation of activated carbons: A review
however, their adsorptive potential, i.e. surface area, porosity and surface oxide
groups, can be greatly enhanced by chemical modification or physical activation.
Chaney (1919, 1923) suggested that chars to be used in physical activation should be
prepared at temperatures below 6OO0C, however, McBain (1936) produced an
activated carbon from a sugar char carbonised at 900°C.
(d) Physical Activation
Physical or thermal activation has traditionally taken place in two stages,
carbonisation and activation, however, in recent years, there has been a tendency to
perform the two processes in a single kiln such as a rotary furnace or fluidised bed.
The carbonisation step is performed to render the precursor in a suitable form for
oxidation or activation, by dehydrating the starting material. During carbonisation
the carbons atoms rearrange themselves into graphte-lke structures and the resultant
char has an increased fned carbon content (80% or hgher is desirable).
The carbons extended surface area and porosity are developed during the
activation or oxidation stage.
Activation is the controlled gasification, by an
oxidising gas such as steam, C 0 2 or air, at elevated temperatures, typically 800-
llOO°C. Initially the active oxygen in the activating agent bums away the tany
pyrolysis off-products trapped within the pores, initialising the porosity development,
then the microporous structure is developed as the oxi&sing agent bums away the
more reactive areas of the carbon skeleton. It is desirable that the burning-out of the
carbon skeleton, to create new pores, initiates and occurs within the particle interior
and not from the exterior surface as t h ~ swould result in an overall loss of porosity
and surface area. The chemical and physical nature of physically activated carbons is
very dependent on the precursor, the oxidising agent employed, the temperature of
activation and the degree of activation. The activating agent employed, in physical
processes, has been investigated by many authors. Steam is preferable to carbon
dioxide and much better than air for activating many chars. Utilising steam for
activation requires a high temperature of activation to provide rapid oxidation,
however, temperatures above 1OOO°C impair adsorptive power. Work conducted by
251
S.J. Allen, L. Whitten and G.McKay
Gamer and Packer (1980) showed that activation with steam at 850°C for up to 2%
hours produced activated carbons with significantly increased activity, although, the
yield of product was seen to decrease with increasing time. Gergova et al. (1994)
prepared a range of steam activated-carbons, between 600-700°C for 1-3 hours, from
a range of agrrcultural by-products. They reported common trends within the carbons
produced such as surface area and porosity increase with increasing temperature and
time. They also noted that the resultant carbon propemes were very dependent on the
precursor material. Activation is performed to open up a porous structure within the
adsorbent. Gergova et al. (1994) along with other authors Garner and Packer (1980),
Kaloc et al. (1996) and McDougall(l991) have shown that porosity development is
mainly in the form of micro and mesopores.
Carbon dioxide activation is a relatively new concept and is performed at
temperatures between 800-9OO0C. Rodriguez-Reinoso and Molina-Sabio (1992)
reported that activation with CO, opens and widens microporosity and that at high
temperatures the exterior of the particles are rapidly burnt off. Increased porosity is
accompanied by a parallel increase in surface area, see Lu and Chung (1996), Sollars
et al. (1990), Lopez et al. (1990) and Jankowska (1996). Sollars et al. (1990) showed
that tyre rubber was a potential source of CO, activated carbon. They concluded that
the rate of carbon loss appeared linear in the range of activation times employed and
that optimum activation conditions were found to be 95OoC for 400 minutes,
producing a meso-microporous carbon with good phenol adsorption.
Comparing the effectiveness of steam and COz as activating agents is difficult
because factors other than the oxidising gas influence the final product. Smisek and
Cerney (1970) state that carbonaceous gasification by steam and CO, occurs
accordmg to an endothermic reaction and therefore can be accurately controlled.
Although both reactions proceed endothermically they occur at different rates. Park
et al. (1996) showed that steam activation was about three times more rapid than C 0 2
activation. Pashchenko et al. (1996) also reported a decrease in reaction rate with
CO, activation on carbon containing wastes, nearly 2 times less than that of steam.
The resultant pore structure is also dependent on the activator employed. Zietek et al.
252
The production and churacterisation of activated carbons: A review
(1996) showed that pore volumes increased with both activators but that steam
activation resulted in microporous structures and C02 resulted in larger micropores
and mesoporous structures.
When air or oxygen as used as the activating agent problems are encountered due
to the exothermic nature of the reaction of carbon with a& (oxygen). The rapidity of
this reaction makes it difficult to control, resulting in excessive bum-off and reducing
the product yield. Despite the operational difficulties several authors have published
their findings on air activation. Buczek et al. (1995) investigated air as an activator
and also 10% oxygen in nitrogen for carbons from hard coal. Their results showed
that texturally little changed and very little increase in surface area or micropore
volume was observed, as compared to a steam activated carbon. Gomez-Corzo et al.
(1996) prepared air activated carbons from cherry stones. From their experiments it
was observed that carbon yield and pore volume decreased with increased activation
time, however, the pore structure remained greater than for carbons produced by N,
treatments. Work conducted by Jankowska (1996) prepared a carbon from Polish
anthracite at 5OO0C in 21% oxygen, and showed that oxygen activation produced
carbons with low surface areas and pore volumes. He concluded that activity
decreases in the order of steam > carbon dioxide > oxygen.
It is interesting to note that Lu and Chung (1996) used O3 activation prior to heat
treatment and CO, activation, to enhance the chemical and physical properties of the
fmal product. They observed a slight decrease in surface area, due to the removal of
surface microcracks, but also noted an increase in mesopore size.
(e) Chemical Activation
Chemical activation has the advantage that it is a single-stage process. Chemical
activation has been associated with imparting surface area and pore development
withm the carbon structure for many years. Inorganic materials, such as chalk, lime,
sulphuric acid, calcium chloride and zinc chloride, to name but a few, have been
incorporated into precursors prior to pyrolysis resulting in carbons with enhanced
pore structures and hence large surface areas. In wet-chemical processes, to date, a
253
S.J. Allen, L. Whitten and G.McKay
wide variety of chemicals have been suggested for activation including phosphoric
acid (the most popular agent), chloride salts of magnesium, femc iron and
aluminium, s o d i m carbonate, and sodium and calcium hydroxide.
The only
common trait between all these chemicals is their strength as dehydrating agents.
As mentioned, ZnClz is a popular activator and is employed regularly on an
industrial scale in Europe and Japan. Rodriguez-Reinoso and Molina-Sabio (1992)
conducted extensive work into the mechamsm of ZnC1, activation and the physical
and chemical nature of the resultant carbons.
They reported that during
impregnation, the chemical activator reaches the interior of the precursor and causes
hydrolysis reactions to occur. These are noted by weight loss, exit of volatiles,
weakening of the structure, and increased elasticity. Accompanying the hydrolysis,
particle swelling is observed. During carbonisation the incorporated ZnC1, prevents
pyrolysis products, such as tars, forming and thus increases the yield of the product.
Further explanation for the increased yield of product comes from considering the
action of the chloride on the elemental make-up of the precursor and its derived
carbon. ZnC1, tends to cause hydrogen and oxygen atoms to be stripped away as
water, rather than as hydrocarbons or as oxygenated organic compounds, leaving the
carbon skeleton largely untouched. On the physical and chemical characteristics,
Rodriguez-Reinoso and Molina-Sabio ( 1992) concluded that carbons with a well
developed porous structure, mainly meso and microporous, can be produced by ZnC1,
incorporation. The nature of the carbon is dependent on the amount of chemical
incorporated.
As the percentage increases its distribution within the particles
becomes less ordered, resulting in a pore distribution with a heterogeneous nature.
KOH activation has been shown to successfully increase the surface area and pore
volume of active carbons according to the relative amount of KOH incorporated into
the precursor. Otowa et al. (1995) showed that an active carbon, with a surface area
in excess of 3000m2/g, could be prepared by mixing petroleum coke with excess
KOH prior to pyrolysis. Hu et al. (1996) produced a range of chemically activated
carbons from walnut shells. They concluded that surfaces and porosity can be
specifically tailored using KOH activation, the pore size and distribution depending
254
The production and characterisation of activated carbons: A review
on the soak times and impregnation ratios utilised. Interesting results were reported
by Gonzalez et al. (1996) who compared preparing carbons from olive stones, firstly
by pre-treating with KOH and secondly by treating pre-carbonised olive stones with
KOH followed by further heat treatment to activate them. They found that in the
original olive stone there is a high increase in pore volume as a consequence of
hydrolysis during impregnation, the increase being much smaller in the carbons from
the carbonised stones.
Use of potassium carbonate dates back to early carbons produced from blood,
where 8 parts of dried blood was mixed with 1 part of K,C03 and carbonised at 8OO0C
to form blood char. More recently Hayashi et al. (1996) compared the activation
potential of K,CO, to Na,CO, and NaOH when producing carbons from bean curd
refuse. They found that K2C03works effectively as an activator yielding high surface
areas in two temperature ranges, below 4OO0C and above 80OoC. Results for Na,CO,
were promising with increased surface areas being observed, however, NaOH was
seen to be a very intense activator, in some instances resulting in zero product yield.
Butuzova and Krzton (1996) showed that brown coal is a better receptor to NaOH
activation than bean-curd refuse, with enhanced adsorption characteristics observed
without any mass loss.
Many other chemicals have been forwarded as potential activation agents, the
main ones are: ammonium salts, borates, boric acid, calcium oxide, cyanides, ferric
and ferrous compounds, hydrochloric acid, manganese dioxide, nickel salts, nitric
acid and sulphur.
Allen and Balasundaram (1995) reported on chemically activating Northern
Ireland lignite with ZnC1, and iron nitrate and sulphate. On chemical activation
alone, they concluded that iron nitrate gave the best performance with the reactivity
decreasing in the sequence iron nitrate > zinc chloride > iron sulphate. Some results
from this work are shown in Table 3.
The type and amount of chemical used in the pre-treatment of peat has a
substantial effect on the extent of the development of the pore structure within the
carbon. Begin et al. (1996) investigated the influence of Feel, graphite intercalation
255
S.J. Allen, L Whitten and G.McKay
compound (GIC) on the pyrolysis of coal tar pitch. Results showed that Lewis acid
FeC1, released out of the graphite layer during heating promotes polycondensation
and dehydration reactions enhancing carbon yield, also catalytic activity was noted on
the mesopore growth. US patent number 4,149,994 (1979) describes a chemical
process in which no carbonisation occurs. Activation is performed by addmg polar
compounds to coal powders, containing non-polar groups bonded with polar groups,
and drying at a temperature of 120'C. These carbons perform well in wastewater
treatment.
Table 3. Peat activated carbons.
~~
256
Chemical
Additives
Surface Area
(BET) rn'g-'
Pore Specific Volume
cm3g-' x lo-,
10% ZnC1,
198
1.1
20% ZnC1,
557
2.3
10% Fe,(SO,),
70
5.4
20% Fe2(S0,),
243
1.8
10% FeSO,
89
5.2
20% FeSO,
412
2.8
The production and characterisation of activated carbons: A review
@ Comparing and Combining Physical and Chemical Processes
Activated carbons can be produced by either physical or chemical activation (or a
combination of both) to produce sorbents with well distributed porosity and high
surface areas. However, the mechanisms of activation are very different for both
processes and the resultant carbons possess very different chemical and physical
properties.
The first very visible differences between physical and chemical processes are the
number of stages required for activation and the temperature at whch activation takes
place. Chemical activation is conducted in one step whereas physical activation is
normally a two-step process of carbonisation and activation. The temperatures
employed in chemical activation (200-800°C) are lower than those required for
physical activation (typically 800-1 100°C), this in itself results in different bum-off
rates of the carbon skeleton and often it is reported that the carbon yields in chemical
activation are greater than those for physical activation. As a result of thls
temperature difference, chemically activated carbons lack the semi-conductor-redox
properties that are seen in physically activated carbons.
ms is a
result of the
r e s ~ c t i o nof formation of graphte zones due to the low temperatures used in
chemical processes.
Another significant difference relates to the pore-size distribution. RodnguezReinoso and Molina-Sabio (1 992) reported that chemical activation of lignocellulosic
precursors with ZnC1, resulted in the same micropore volume as CO, activation but
with a higher yield. They found that as a consequence of chemical activation the
porosity is more developed, and the result is that it leads to carbons having similar
micropore volumes but with larger mesopore volumes. Physical activation with CO,
was reported to open and widen the microporosity, but at high temperatures there was
an ablation of the exterior of the particle resulting in low carbon yields. Lopez et al.
(1996) investigated the variable effects of CO, and ZnC1, activation on carbons from
wood monolith. In contrast to chemical activation, physical activation resulted in
smaller surface areas, smaller yields and significant pore differences.
Pore
investigations implied that micropore and mesopore size distributions were closer in
257
S.J. Allen, L. Whitten and G. McKay
chemical activation as opposed to the physical activation. They concluded that pores
can be classified and attributed to the particular activation used, wide micropores and
narrow mesopores being associated with chemical activation and narrow micropores
and wide mesopores being characteristic of physical activation.
Chemical activation has great potential for providing a flexible method for
preparing activated carbons with effective control of pore distribution. However, the
ramifications and possibilities are fiuther heightened when gasification of these
carbons, with steam or CO,, etc., is considered.
Low-temperature chemical
modification by HClO, and high-temperature physical activation were combined by
Lynubchik et al. (1996) to produce activated carbons from anthracite. They proposed
that the chemical modification step lays the foundations of the primary pore structure.
Allen and Balasundaram (1995) reported similar findings when producing activated
carbons from lignite.
Although inconclusive, evidence suggests that chemical
treatment alone determines the pore structure and that gasification merely cleans these
pores out. Baranchrkova et al. (1984) steam activated a peat char in the presence of
AlCl,. They showed that incorporating 5% AlCl, into the peat resulted in a good
microporous carbon after bum-off with steam. Leboda et al. (1996) showed that
calcium incorporated into a variety of precursors, by ionic exchange or in solution as
calcium acetate, acts as a catalyst in porosity development. It was found that in the
presence of CaCO, steam activation caused changes in both the micro and
mesoporous structure.
The extent to how successful a precursor will be for producing activated carbon is
often gauged by its ash content. Flynn et al. (1988) stated that, due to its high ash
content, Northern Ireland lignite was unsuitable for producing activated carbon. This
problem can be overcome by demineralising, with mineral acids, to remove the
majority of inorganic matter which ashes on carbonisation. Demineralisation is a
chemical modification process and is often performed in industrial processes prior to
pyrolysis and gasification. US patents 4,149,994 and 4,149,995 by Murty (1979 a, b)
describe the demineralisation of brown coal by both concentrated and dilute H,SO,,
H,PO, and HC1 to reduce the volatile content and thereby increasing the futed carbon
258
The production and characterisation of activated carbons: A review
content, prior to steam activation. Recent studies by Kaczmarczuk et al. (1996)
showed that HC1 could be used successfully to demineralise brown coal prior to
carbonisation at 800°C and steam activation at 700OC. It was observed that activated
carbons derived from the demineralised parent coal possessed greater mechanical
strength and nearly double the surface area of those produced without
demineralisation. Also, whilst mesopores were dominating in the activated carbons
from the parent coal, micropores dominated the activated carbons from the
demineralised parent coal.
Conclusions
Lignocellulosic materials, in particular peat and ligmte, have considerable potential in
the future production of hgh-value-added activated carbons. Recent research is
concentrating on understanding the effects of chemical additives and pre-treatment on
the quality (sorption capacity, activity, porosity and mechanical strength) of activated
carbons and chars produced from lignocellulose-based materials.
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261
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