Activated carbons have been used for many years quite successfully for adsorptive removal of
impurities from exhaust gas and waste water streams. However, for cost-effective removal of certain
impurities contained in gases (such as hydrogen sulphide, mercury and ammonia).
The adsorption capacities and the feasible removal rates must be substantially boosted by
impregnation of the activated carbon by suitable chemicals.
When these chemicals are deposited on the internal surface of the activated carbon, the removal
mechanism also changes.
The impurities are no longer removed by adsorption but by chemisorption.
Three reasons for impregnating activated carbon may be defined, and relevant examples are given
below.
1 Optimization of existing properties of activated carbon:
Activated carbons are capable of catalytic oxidation of organic and inorganic compounds.
The property as oxidation catalyst can be boosted by, for example, impregnation with potassium
iodide acting as promoter. Potassium iodide-impregnated activated carbons are, in fact, already
used for catalytic hydrogen sulfide oxidation to elemental sulfur
2 Synergism between activated carbon and impregnating agent :
Mercury and sulfur are not normally converted to mercury sulfide at ambient temperature.
However, if the sulfur is distributed onto the internal surface of activated carbon, this reaction can
be run at low temperatures for mercury removal from gases.
3 Use of activated carbon as an inert porous carrier material :
Activated carbon is used as an inert porous carrier material for dist~buting chemicals on the
large internal surface, thus making them accessible to reactants. For example, activated carbon
impregnated with phosphoric acid is used for a~onia removal: HX PO1 + 3 NH3 + (NH,), PO, As well
as the pore radii distribution of the activated carbon to be imprecated, the chemical composition
and the quantity of the impregnation agents used and their distribution in the pore system are very
important.
Manufature of Impregnated AC :
For the manufacture of impregnated activated carbon, an activated carbon of suitable
quality for the particular application is impregnated with solutions of salts or other chemicals which,
after drying or other after-treatment steps, remain on the internal surface of the activated carbon.
Two methods of impregnation
a. soaking impregnation
b. spray impregnation
In that case the activated carbon is sprayed in a rotary kiln or in a fluidized bed under
defined conditions. The impregnated wet activated carbon needs to be dried in an appropriate
installation (e.g. a rotary kiln or fluidized-bed drier).
After the drying step, most of the impregnated activated carbons can be used industrially.
In some applications the impregnation agents are present in the form of hydroxides, carbonates,
chromates or nitrates and must be subjected to thermal after-treatment at higher temperatures
(150 to 200°C) to decompose the anions.
According to the application, various activated carbons (pellets, granular and powdered qualities)
are impregnated with suitable organic or inorganic chemicals.
Homogeneous distribution of the impregnating agents on the internal surface of an activated carbon
is important. Furthermore, blocking of the micropores and macropores should be avoided in order
to keep the impregnation agent accessible for the reactants.
Example :
sulfur-impregnated activated carbon for mercury removal and demonstrated that the
sulfur is predominantly distributed in the micropore system and that no pores are blocked. (If
impregnated agents occupies macro pore/meso pore , pores will get blocked.)
Figure shows a comparison of the micropore volume of the initial activated carbon quality D
4714 and the same activated carbon quality impregnated with 15% sulfur (D 47/4 + S). The
impregnation reduced the micropore system’s surface from 742 to 579m*g-‘. Thus, not only does
the chemisorption of mercury by sulfur take place, but the adsorptive removal of further gas
impurities can also be achieved.
Products and application fields:
Gas purification
Civil and military gas protection
Catalysis
Application in gas masks, room filters
Application fields:
and respiratory apparatus filters:
Application in catalysis:
Hydrogen sulfide
Sulfur dioxide
Vinyl acetate synthesis
Mercaptan
Hydrogen chloride
Vinyl chloride synthesis
Mercury
Hydrogen fluoride
Vinyl fluoride synthesis
Ammonia
Nitrogen oxide
Amine
Amine
Acid gases (HCI, SOA, HF, HCN)
Hydrogen sulfide
Arsine
Mercury
Phosphine
Radioactive iodine
Aldehyde
Radioactive methyl iodide
Radioactive iodine
Phosgene
Radioactive methyl iodide
Hydrogen cyanide
Nitrogen oxide
Chlorine
Arsine
Sarin and other nerve gases
1. potassium iodide impregnation :
I.
II.
Potassium iodide, as promoter of the oxidation catalyst ‘activated carbon’, allows
catalytic oxidation of hydrogen sulfide to sulfur.
phosphine (PH,) to phosphoric acid:
2PH3 + 402 -> H3PO4.
III.
potassium iodide-impregnated activated carbon can be used for removal of Mercury
from gases.
mercury removal:
In contrast to all other metals, mercury is in the liquid state at room temperature
and has a relatively high vapour pressure of 171 Pa (15 mg m -3) at 20°C. Apart from
problems caused by the toxic properties of mercury and its other environmental hazards,
traces of mercury can poison many industrial catalytic processes. For mercury removal from
gases, wet processes (oxidizing gas dabbing) as well as dry processes (adsorption
processes) are in operation.
Commercial qualities of impregnated activated carbon:
Impregnation
Chemicals
Sulfuric acid
Phosphoric acid
Potassium carbonate
Iron oxide
Potassium iodide
Quantity
(wt %)
Activated
carbon‘
2-25
10-30
F 1-4 mm 0
F 1—4 mm 0
F 1—4 mm 0
F 1—4 mm 0
F 1—4 mm 0
10-20
10
1-5
Triethylene diamine (TEDA)
2—5
F 1—2 mm la
Examples for application
Ammonia, amine, mercuryAmmonia,
amine
Acid gases (HCI, HF, SO„ H, S, NO2 ), Carbon disulfide
H2 S, mercaptan, COS
H2 S, PHA, Hg, AsH3, radioactive gases/radioactivemethyl
iodide
Radioactive gases/radioactive methyl iodide
G &-16 mesh
Sulfur
Potassium permanganate
Manganese IV oxide
Silver
10-20
5
0.1-3
Zinc oxide
Chromium-copper-silvar salts
10
10—20
F 1—4 mm 0, GF
3 + 4 mm 0
Mercury
H2 S from oxygen-lacking gases
G 6-16 mesh
Aldehyde
F 3 + 4 mm 0
G &-30 mesh
F 1—4 mm ja
F 0.8-3 mm 0
G 12—30 mesh
G 6-16 mesh
Mercury II chloride
10-15
F 3 + 4 mm 0
Zinc acetate
Noble metals
(palladium, platinum)
15—25
1—5
F 3 + 4 mm 0
F, G, P
F: phosphine, arsina
G: domestic drinking water filters (oligodynamic effect)
Hydrogen cyanide
Civil and military gas protection
Phosgene, chlorine, arsine
Chloropicrin, sarin and other nerve gases
Vinyl chloride synthesis
Vinyl fluoride synthesis
Vinyl acetate synthesis
Organic synthesis, hydrogenation
F = pelletized activated carbon
G = granulated activated carbon
P = powdered activated carbon
0 = pellet diameter
H2S Adsorption:
Activated carbons are frequently used for gas adsorption because of their high surface area, porosity,
and surface chemistry where H2S can be physically and chemically adsorbed.
Most activated carbon tested is in granular form, called Granular Activated Carbon (GAC).
Activated carbon can come in two forms:
Unimpregnated:
Unimpregnated activated carbon removes hydrogen sulfide at a much slower rate
because activated carbon is only a weak catalyst and is rate limited by the complex reactions
that occur.
A typical H2S adsorption capacity for unimpregnated activated carbons is 20 mg H2S/g
of activated carbon.
One of the simple ways to regenerate the activated carbon is to wash it with water.
As of yet, the complete mechanisms by which H2S is removed using activated carbon
are not well understood. It is accepted that removal occurs by both physical and chemical
mechanisms. One chemical removal mechanism is caused by the presence of heteroatoms at
the carbon surface.
Impregnated:
Impregnation refers to the addition of cations to assist as catalysts in the adsorption
process (Bandosz, 2002). Removal capacities may vary greatly in on-site applications, as the
presence of other constituents (such as VOCs) may inhibit or enhance the removal capacity,
depending on other environmental conditions.
The caustic addition has a catalytic effect by oxidizing the sulfide ions to elemental
sulfur until there is no caustic left to react.
A typical H2S adsorption capacity for impregnated activated carbons is 150 mg H2S/g
of activated carbon
The cations added to impregnated activated carbon:( Usually caustic compounds)
1.
2.
3.
4.
5.
6.
sodium hydroxide (NaOH)
potassium hydroxide (KOH), which act as strong bases that react with H2S and immobilize it.
sodium bicarbonate (NaHCO3).
sodium carbonate (Na2CO3).
potassium iodide (KI).
potassium permanganate (KMnO4).
Drawbacks of Caustic Impregnaions:
 The addition of caustics lowers the ignition temperature and therefore the material can self-ignite and
is considered hazardous.
 The addition of caustics to activated carbon increases the costs of production.
 Because of the high cost of activated carbon, it is desirable to “wash” or “clean” the activated carbon
in order to regenerate it so that it will regain some of its ability to remove H2S.
 The caustic additions to impregnated activated carbon cause H2S to be oxidized to elemental sulfur,
which cannot be removed from the activated carbon by washing with water and therefore costs of
H2S removal are increased due to the need to purchase more adsorbent.
NaOH impregnated activated carbon:(For H2S Removal)
In a study by Bagreev and Bandosz (2002), it was tested for its H2S removal capacity. Four different
types of activated carbon were used and different volume percentages of NaOH were added. The results
showed that with increasing amounts of NaOH added, the H2S removal capacity of the activated carbons
increases. This effect occurred until maximum capacity was reached at 10 vol% NaOH. This result was the
same regardless of the origin of the activated carbon, and was even the same when activated alumina was
used. This result implies that the amount of NaOH present on the surface of the material is a limiting factor
for the H2S removal capacity in NaOH impregnated activated carbons.