Activated carbon as an alternative treatment for AIDS
The word activated in the
name is sometimes replaced with active. Due to its high degree of
microporosity, just 1 gram of activated carbon has a surface area in excess of
500 m2 (about one tenth the size of a football field), as
determined typically by nitrogen gas absorption. Sufficient activation for useful applications may come
solely from the high surface area, though further chemical treatment often
enhances the adsorbing properties of the material. Activated carbon is usually
derived from charcoal.
Production
Activated carbon is carbon produced
from carbonaceous source materials like nutshells, peat, wood, coir, lignite, coal and petroleum pitch.
It can be produced by one of the following processes:
- Physical reactivation:
The precursor is developed into activated carbons using gases. This is
generally done by using one or a combination of the following processes:
- Carbonization:
Material with carbon content is pyrolyzed
at temperatures in the range 600–900 °C, in absence of oxygen (usually in
inert atmosphere with gases like argon
or nitrogen)
- Activation/Oxidation: Raw material or carbonized
material is exposed to oxidizing atmospheres (carbon dioxide,
oxygen, or steam) at temperatures above 250 °C, usually in the
temperature range of 600–1200 °C.
- Chemical activation:
Prior to carbonization, the raw material is impregnated with certain
chemicals. The chemical is typically an acid,
strong base,
or a salt (phosphoric acid,
potassium
hydroxide, sodium hydroxide, calcium chloride,and zinc
chloride 25%). Then, the raw material is
carbonized at lower temperatures (450–900 °C). It is believed that the
carbonization / activation step proceeds simultaneously with the chemical
activation. Chemical activation is preferred over physical activation
owing to the lower temperatures and shorter time needed for activating
material.
Classification
Activated carbons are complex
products which are difficult to classify on the basis of their behaviour,
surface characteristics and preparation methods. However, some broad
classification is made for general purpose based on their physical
characteristics.
Powdered
activated carbon (PAC)
A micrograph
of activated charcoal under bright field illumination on a light microscope.
Notice the fractal-like
shape of the particles hinting at their enormous surface area. Each particle in
this image, despite being only around 0.1 mm wide, has a surface area of
several square metres. The entire image covers a region of approximately 1.1 by
0.7 mm, and the full resolution version is at a scale of 6.236 pixels/μm.
Traditionally, active carbons are
made in particulate form as powders or fine granules less than 1.0 mm in
size with an average diameter between .15 and .25 mm.[2]
Thus they present a large surface to volume ratio with a small diffusion
distance. PAC is made up of crushed or ground carbon particles, 95–100% of
which will pass through a designated mesh sieve.
Granular activated carbon is defined as the activated carbon retained on a
50-mesh sieve (0.297 mm) and PAC material as finer material, while ASTM classifies particle sizes corresponding to an 80-mesh sieve
(0.177 mm) and smaller as PAC. PAC is not commonly used in a dedicated
vessel, due to the high head loss that would occur. PAC is generally added directly to other
process units, such as raw water intakes, rapid mix basins, clarifiers, and
gravity filters.
Granular activated carbon (GAC)
Granular activated carbon has a
relatively larger particle size compared to powdered activated carbon and
consequently, presents a smaller external surface. Diffusion of the adsorbate
is thus an important factor. These carbons are therefore preferred for all absorption
of gases and vapors as their rate of diffusion are faster. Granulated carbons
are used for water treatment, deodorization and separation of components of flow system.
GAC can be either in the granular form or extruded. GAC is designated by sizes
such as 8×20, 20×40, or 8×30 for liquid phase applications and 4×6, 4×8 or 4×10
for vapor phase applications. A 20×40 carbon is made of particles that will
pass through a U.S. Standard Mesh Size No. 20 sieve (0.84 mm) (generally
specified as 85% passing) but be retained on a U.S. Standard Mesh Size No. 40
sieve (0.42 mm) (generally specified as 95% retained). AWWA (1992) B604
uses the 50-mesh sieve (0.297 mm) as the minimum GAC size. The most
popular aqueous phase carbons are the 12×40 and 8×30 sizes because they have a
good balance of size, surface area, and head loss characteristics.
Extruded activated carbon (EAC)
Extruded activated carbon combines
powdered activated carbon with a binder, which are fused together and extruded
into a cylindrical shaped activated carbon block with diameters from 0.8 to
130 mm. These are mainly used for gas phase applications because of their
low pressure drop, high mechanical strength and low dust content. GAC is also
used in rapid mix basins and for waste-water treatment in general.
Bead activated carbon (BAC)
Bead activated carbon is made from
petroleum pitch and supplied in diameters from approximately 0.35 to
0.80 mm. Similar to EAC, it is also noted for its low pressure drop, high
mechanical strength and low dust content, but with a smaller grain size. Its
spherical shape makes it preferred for fluidized bed applications such as water
filtration.
Impregnated carbon
Porous carbons containing several
types of inorganic impregnate such as iodine, silver, cations such as Al, Mn, Zn, Fe, Li, Ca have also been prepared for
specific application in air pollution
control especially in museums and galleries. Due to antimicrobial/antiseptic
properties, silver loaded activated carbon is used as an adsorbent for
purification of domestic water. Drinking water can be obtained from natural
water by treating the natural water with a mixture of activated carbon and Al(OH)3, a flocculating agent.
Impregnated carbons are also used for the adsorption of H2S and thiols. Adsorption rates for H2S
as high as 50% by weight have been reported.
Polymer coated carbon
This is a process by which a porous
carbon can be coated with a biocompatible polymer to give a
smooth and permeable coat without blocking the pores. The resulting carbon is
useful for hemoperfusion. Hemoperfusion is a treatment technique in which large
volumes of the patient's blood are passed over an adsorbent substance in order
to remove toxic substances from the blood.
Other
Activated carbon is also available
in special forms such as cloths and fibres. The "carbon cloth" for
instance is used in personnel protection for the military.
Properties
A gram of activated carbon can have
a surface area in excess of 500 m2, with 1500 m2 being
readily achievable.[3]
Carbon aerogels, while
more expensive, have even higher surface areas, and are used in special
applications.
Activated carbon, as viewed by an
electron microscope
Under an electron microscope, the high surface-area structures of activated carbon are
revealed. Individual particles are intensely convoluted and display various
kinds of porosity; there
may be many areas where flat surfaces of graphite-like
material run parallel to each other, separated by only a few nanometers or so.
These micropores
provide superb conditions for adsorption
to occur, since adsorbing material can interact with many surfaces
simultaneously. Tests of adsorption behaviour are usually done with nitrogen gas at 77
K under high vacuum, but in everyday terms activated carbon is perfectly
capable of producing the equivalent, by adsorption from its environment, liquid
water from steam at 100 °C
and a pressure of 1/10,000 of an atmosphere.
James Dewar, the scientist after
whom the Dewar (vacuum flask) is named, spent much time studying activated carbon and
published a paper regarding its absorption capacity with regard to gases.[4]
In this paper, he discovered that cooling the carbon to liquid nitrogen
temperatures allowed it to absorb significant quantities of numerous air gases,
among others, that could then be recollected by simply allowing the carbon to
warm again and that coconut based carbon was superior for the effect. He uses
oxygen as an example, wherein the activated carbon would typically adsorb the
atmospheric concentration (21%) under standard conditions, but release over 80%
oxygen if the carbon was first cooled to low temperatures.
Activated carbon does not bind well
to certain chemicals, including alcohols, glycols, strong acids and bases, metals and most inorganics,
such as lithium, sodium, iron, lead, arsenic, fluorine, and boric acid.
Activated carbon does adsorb iodine very well and in fact the iodine number,
mg/g, (ASTM D28 Standard Method test) is used as an indication of total
surface area.
Carbon monoxide is not well adsorbed
by activated carbon. This should be of particular concern to those using the
material in filters for respirators, fume hoods or other gas control systems as
the gas is undetectable to the human senses, toxic to metabolism and
neurotoxic.
Substantial lists of the common
industrial and agricultural gases absorbed by activated carbon can be found
online.[5]
Activated carbon can be used as a
substrate for the application of various chemicals to improve the adsorptive
capacity for some inorganic (and problematic organic) compounds such as hydrogen sulfide
(H2S), ammonia (NH3), formaldehyde (HCOH), radioisotopes iodine-131(131I)
and mercury (Hg). This property is known as chemisorption.
Iodine number
Many carbons preferentially absorb
small molecules. Iodine number is the most fundamental parameter used to characterize
activated carbon performance. It is a measure of activity level (higher number
indicates higher degree of activation), often reported in mg/g (typical range
500–1200 mg/g). It is a measure of the micropore content of the activated
carbon (0 to 20 Ã…,
or up to 2 nm)
by adsorption of iodine from solution. It is equivalent to surface area of
carbon between 900 m²/g and 1100 m²/g. It is the standard measure for liquid
phase applications.
Iodine number is defined as the
milligrams of iodine adsorbed by one gram of carbon when the iodine concentration in the
residual filtrate is 0.02 normal. Basically, iodine number is a measure of the
iodine adsorbed in the pores and, as such, is an indication of the pore volume
available in the activated carbon of interest. Typically, water treatment
carbons have iodine numbers ranging from 600 to 1100. Frequently, this
parameter is used to determine the degree of exhaustion of a carbon in use.
However, this practice should be viewed with caution as chemical interactions
with the adsorbate may affect the iodine uptake giving false results. Thus,
the use of iodine number as a measure of the degree of exhaustion of a carbon
bed can only be recommended if it has been shown to be free of chemical
interactions with adsorbates and if an experimental correlation between iodine
number and the degree of exhaustion has been determined for the particular
application.
Molasses
Some carbons are more adept at
adsorbing large molecules. Molasses
number or molasses efficiency is a measure
of the mesopore content of the activated carbon (greater than 20 Ã…, or larger than 2 nm) by adsorption of molasses from
solution. A high molasses number indicates a high adsorption of big molecules
(range 95–600). Caramel dp (decolorizing performance) is similar to molasses
number. Molasses efficiency is reported as a percentage (range 40%–185%) and
parallels molasses number (600 = 185%, 425 = 85%). The European molasses number
(range 525–110) is inversely related to the North American molasses number.
Molasses Number is a measure of the
degree of decolorization of a standard molasses solution that has been diluted
and standardized against standardized activated carbon. Due to the size of
color bodies, the molasses number represents the potential pore volume
available for larger adsorbing species. As all of the pore volume may not be
available for adsorption in a particular waste water application, and as some
of the adsorbate may enter smaller pores, it is not a good measure of the worth
of a particular activated carbon for a specific application. Frequently, this
parameter is useful in evaluating a series of active carbons for their rates of
adsorption. Given two active carbons with similar pore volumes for adsorption,
the one having the higher molasses number will usually have larger feeder pores
resulting in more efficient transfer of adsorbate into the adsorption space.
Tannin
Tannins are a
mixture of large and medium size molecules. Carbons with a combination of macropores
and mesopores adsorb tannins. The ability of a carbon to adsorb tannins
is reported in parts per million concentration (range 200 ppm–362 ppm).
Methylene blue
Some carbons have a mesopore (20 Ã… to 50 Ã…, or 2 to 5 nm) structure
which adsorbs medium size molecules, such as the dye methylene blue.
Methylene blue adsorption is reported in g/100g (range 11–28 g/100g).
Dechlorination
Some carbons are evaluated based on
the dechlorination half-value length, which measures the chlorine-removal
efficiency of activated carbon. The dechlorination half-value length is the
depth of carbon required to reduce the chlorine level of a flowing stream from
5 ppm to 3.5 ppm. A lower half-value length indicates superior performance.
Apparent density
Higher density provides greater
volume activity and normally indicates better quality activated carbon.
Hardness/abrasion number
It is a measure of the activated
carbon’s resistance to attrition. It is important indicator of activated carbon
to maintain its physical integrity and withstand frictional forces imposed by
backwashing, etc. There are large differences in the hardness of activated
carbons, depending on the raw material and activity level.
Ash content
It reduces the overall activity of
activated carbon. It reduces the efficiency of reactivation. The metal oxides
(Fe2O3) can leach out of activated carbon resulting in
discoloration. Acid/water soluble ash content is more significant than total
ash content. Soluble ash content can be very important for aquarists, as ferric
oxide can promote algal growths. A carbon with a low soluble ash content should
be used for marine, freshwater fish and reef tanks to avoid heavy metal
poisoning and excess plant/algal growth.
Carbon tetrachloride activity
Measurement of the porosity of an
activated carbon by the adsorption of saturated carbon tetrachloride vapour.
Particle size distribution
The finer the particle size of an
activated carbon, the better the access to the surface area and the faster the
rate of adsorption kinetics. In vapour phase systems this needs to be
considered against pressure drop, which will affect energy cost. Careful
consideration of particle size distribution can provide significant operating
benefits.
Examples of adsorption
Heterogeneous catalysis
The most commonly encountered form
of chemisorption in industry, occurs when a solid catalyst interacts
with a gaseous feedstock, the reactant/s. The adsorption of reactant/s to the
catalyst surface creates a chemical bond, altering the electron density around
the reactant molecule and allowing it to undergo reactions that would not
normally be available to it.
Adsorption refrigeration
Adsorption
refrigeration and heat pump cycles rely on the
adsorption of a refrigerant gas into an adsorbent at low pressure and
subsequent desorption by heating. The adsorbent acts as a "chemical
compressor" driven by heat and is, from this point of view, the "pump"
of the system. It consists of a solar collector, a condenser or heat-exchanger
and an evaporator that is placed in a refrigerator box. The inside of the
collector is lined with an adsorption bed packed with activated carbon adsorbed
with methanol. The
refrigerator box is insulated filled with water. The activated carbon can
adsorb a large amount of methanol vapours in ambient temperature and desorb it
at a higher temperature (around 100 degrees Celsius). During the daytime, the
sunshine irradiates the collector, so the collector is heated up and the methanol is
desorbed from the activated carbon. In desorption, the liquid methanol adsorbed
in the charcoal heats up and vaporizes. The methanol vapour condenses and is
stored in the evaporator.
At night, the collector temperature
decreases to the ambient temperature, and the charcoal adsorbs the methanol
from the evaporator. The liquid methanol in the evaporator vaporizes and
absorbs the heat from the water contained in the trays. Since adsorption is a
process of releasing heat, the collector must be cooled efficiently at night.
As mentioned above, the adsorption refrigeration system operates in an
intermittent way to produce the refrigerating effect.
Helium gas can also be 'pumped' by
thermally cycling activated carbon 'sorption pumps' between 4 kelvins and higher temperatures. An example of this is to provide
the cooling power for the Oxford Instruments AST series dilution refrigerators.
3He vapour is pumped from the surface of the dilute phase of a
mixture of liquid 4He and its isotope 3He. The 3He
is adsorbed onto the surfaces of the carbon at low temperature (typically
<4K), the regeneration of the pump between 20 and 40 K returns the 3He
to the concentrated phase of the liquid mixture. Cooling occurs at the
interface between the two liquid phases as 3He 'evaporates' across
the phase boundary. If more than one pump is present in the system a continuous
flow of gas and hence constant cooling power can be obtained, by having one
sorption pump regenerating while the other is pumping. Systems such as this
allow temperatures as low as 10 mK (0.01 kelvin) to be obtained with very few
moving parts.
Applications
One major industrial application
involves use of activated carbon in the metal finishing field. It is very
widely employed for purification of electroplating solutions. For example, it
is a main purification technique for removing organic impurities from bright
nickel plating solutions. A variety of organic chemicals are added to plating
solutions for improving their deposit qualities and for enhancing properties
like brightness, smoothness, ductility, etc. Due to passage of direct current
and electrolytic reactions of anodic oxidation and cathodic reduction, organic
additives generate unwanted break down products in solution. Their excessive
build up can adversely affect the plating quality and physical properties of
deposited metal. Activated carbon treatment removes such impurities and
restores plating performance to the desired level.
Analytical chemistry applications
Activated carbon, in 50% w/w
combination with celite, is used as stationary phase in low-pressure
chromatographic separation of carbohydrates (mono-, di- trisacchardes) using
ethanol solutions (5–50%) as mobile phase in analytical or preparative
protocols.
Environmental applications
Activated carbon is usually used in
water filtration systems. In this illustration, the activated carbon is in the
fourth level (counted from bottom).
Carbon adsorption
has numerous applications in removing pollutants
from air or water streams both in the field and in industrial processes such
as:
In 2007, West-Flanders University
(in Belgium) began research in water treatment after festivals.[6]
A full scale activated carbon installation was built at the Dranouter
music festival in 2008, with plans to utilize the
technology to treat water at this festival for the next 20 years.[6]
Activated charcoal is also used for
the measurement of radon concentration in air.
Medical applications
It is thought to bind to poison and
prevent its absorption by the gastrointestinal tract. In cases of suspected poisoning, medical personnel
administer activated charcoal on the scene or at a hospital's emergency department. Dosing is usually empirical at 1 gram/kg of body mass
(for adolescents or adults, give 50–100 g), usually given only once, but
depending on the drug taken, it may be given more than once. In rare situations
activated charcoal is used in Intensive Care to filter out harmful drugs from
the blood stream of poisoned patients. Activated charcoal has become the
treatment of choice for many poisonings, and other decontamination methods such
as ipecac-induced emesis or stomach pumping
are now used rarely.
Activated charcoal for medical use
While activated carbon is useful in
acute poisoning, it has been shown to not be effective in long term
accumulation of toxins, such as with the use of toxic herbicides.[7]
Mechanisms of action:
Incorrect application (e.g. into the
lungs) results in pulmonary aspiration which can sometimes be fatal if immediate medical treatment
is not initiated.[8]
The use of activated charcoal is contraindicated
when the ingested substance is an acid, an alkali, or a petroleum product.
For pre-hospital (paramedic)
use, it comes in plastic tubes or bottles, commonly 12.5 or 25 grams,
pre-mixed with water. The trade names include InstaChar, SuperChar, Actidose,
Charcodote, and Liqui-Char, but it is commonly called activated charcoal.
Ingestion of activated charcoal
prior to consumption of alcoholic beverages appeared to reduce absorption of ethanol into the
blood. 5 to 15 milligrams of charcoal per kilogram of body weight taken at the
same time as 170 ml of pure ethanol (which equals to about 10 servings of an
alcoholic beverage), over the course of one hour, seemed to reduce potential blood alcohol content.[9]
Yet other studies showed that this is not the case, and that ethanol blood
concentrations were increased because of activated charcoal use.[10]
Tablets or capsules of activated charcoal are used in many countries as an over-the-counter drug to treat diarrhea, indigestion,
and flatulence.[12]
Previous versions of this article have claimed that evidence exists that it is
effective in treating irritable
bowel syndrome (IBS),[13],
but the reference study given did not use activated carbon (or activated
charcoal), rather tablets of non-activated charcoal. It has also been used to
prevent diarrhea in cancer patients who have received irinotecan.[14]
It can interfere with the absorption of some medications, and lead to
unreliable readings in medical tests such as the guaiac card
test.[15]
Activated charcoal is also used for bowel preparation by reducing intestinal
gas content before abdominal radiography
to visualize bile and
pancreatic and renal stones. A type of charcoal biscuit has also been marketed as a pet
care product.[16]
Fuel storage
Research is being done testing
various activated carbons' ability to store natural gas
and hydrogen gas. The porous material acts like a sponge for different types
of gasses. The gas is attracted to the carbon material via Van der Waals forces. Some carbons have been able to achieve bonding energies of
5–10 kJ per mol. The gas may then be desorbed when subjected to higher
temperatures and either combusted to do work or in the case of hydrogen gas
extracted for use in a hydrogen fuel cell. Gas storage in activated carbons is an appealing gas
storage method because the gas can be stored in a low pressure, low mass, low
volume environment that would be much more feasible than bulky on board
compression tanks in vehicles. The United
States Department of Energy has
specified certain goals to be achieved in the area of research and development
of nano-porous carbon materials. As of yet all of the goals are yet to be
satisfied but numerous institutions, including the ALL-CRAFT program[17],
are continuing to conduct work in this promising field.
Gas purification
Filters with activated carbon are
usually used in compressed air and gas purification to remove oil vapors, odors, and other hydrocarbons
from the air. The most common designs use a 1 stage or 2 stage filtration
principle in which activated carbon is embedded inside the filter media.
Activated charcoal is also used in spacesuit
Primary
Life Support Systems. Activated charcoal filters are
used to retain radioactive gases from a nuclear boiling water reactor turbine
condenser. The air vacuumed from the condenser contains traces of radioactive
gases. The large charcoal beds adsorb these gases and retain them while they
rapidly decay to non-radioactive solid species. The solids are trapped in the
charcoal particles, while the filtered air passes through.
Chemical
purification
Activated carbon is commonly used to
purify solutions containing un-wanted colored impurities such as during a
recrystallization procedure in Organic Chemistry.
Distilled alcoholic beverage purification
Activated carbon filters can be used
to filter vodka and whiskey of organic
impurities which can affect color, taste, and odor. Passing an organically
impure vodka through an activated carbon filter at the proper flow rate will
result in vodka with an identical alcohol content and significantly increased
organic purity, as judged by odor and taste.[citation needed]
Mercury scrubbing
Activated carbon, often impregnated
with iodine or sulfur, is widely used to trap mercury emissions from coal-fired
power stations, medical incinerators,
and from natural gas at the wellhead. This carbon is a specialty product costing
more than US$4.00 per kg. However, it is often not recycled.
Disposal in the USA after absorbing mercury
The mercury laden activated carbon
presents a disposal dilemma.[18]
If the activated carbon contains less than 260 ppm mercury, Federal regulations
allow it to be stabilized (for example, trapped in concrete) for landfilling.[citation
needed] However, waste containing greater than 260 ppm is
considered to be in the high mercury subcategory and is banned from landfilling
(Land-Ban Rule).[citation
needed] It is this material which is now accumulating in warehouses
and in deep abandoned mines at an estimated rate of 1000 tons per year.[citation
needed]
The problem of disposal of mercury
laden activated carbon is not unique to the U.S. In the Netherlands this
mercury is largely recovered[19]
and the activated carbon is disposed of by complete burning.
Regeneration
The regeneration of activated
carbons involves restoring the adsorptive capacity
of saturated activated carbon by desorbing adsorbed contaminants on the
activated carbon surface.
Thermal regeneration
The most common regeneration
technique employed in industrial processes is thermal regeneration.[20]
The thermal regeneration process generally follows three steps[21]:
- Adsorbent drying at approximately 105 °C
- High temperature desorption and decomposition (500–900°C)
under an inert atmosphere
- Residual organic gasification by an oxidising gas
(steam or carbon dioxide) at elevated temperatures (800°C)
The heat treatment stage utilises
the exothermic
nature of adsorption and results in desorption, partial cracking and polymerization
of the adsorbed organics. The final step aims to remove charred organic residue
formed in the porous structure in the previous stage and re-expose the porous
carbon structure regenerating its original surface characteristics. After
treatment the adsorption column can be reused. Per adsorption-thermal
regeneration cycle between 5–15 wt% of the carbon bed is burnt off resulting in
a loss of adsorptive capacity.[22]
Thermal regeneration is a high energy process due to the high required
temperatures making it both an energetically and commercially expensive
process.[21]
Plants that rely on thermal regeneration of activated carbon have to be of a
certain size before it is economically viable to have regeneration facilities
onsite. As a result it is common for smaller waste treatment sites to ship
their activated carbon cores to a specialised facility for regeneration,
increasing the process' already significant carbon footprint.[23]
Activated carbon used in consumer
devices such as oil deep fryers or air and water filters can similarly be
reactivated using commonly available heating appliances such as a baking oven,
toaster oven, or simply a propane torch.[citation needed] The carbon is
removed from any paper or plastic containers that could melt or ignite, and
heated to vaporize and/or burn off contaminants.
Other regeneration techniques
Current concerns with the high
energy/cost nature of thermal regeneration of activated carbon has encouraged
research into alternative regeneration methods to reduce the environmental
impact of such processes. Though several of the regeneration techniques cited
have remained areas of purely academic research, some alternatives to thermal
regeneration systems have been employed in industry. Current alternative
regeneration methods are:
Source:wikipedia
Description: Activated carbon has been known
since the first,
has the ability
to absorb
undesirable
compounds. In dirty water when mixed with active carbon, then the smell will be absorbed. Also on diarrheal disease is usually given
a drug that
contains
activated carbon.
From the
description
above, made the drafting of AIDS research. Where problems are found "the extent to which the ability
of activated
carbon to absorb the HIV virus in the blood."
Research
scheme, starting from the researchers propose
the proposal
"The impact
of active
carbon
in people with
HIV" to the health care team. If approved then performed
a medical
act
to
persons with HIV
by paramedics.
By: Muchsin Faisol Effendie
