Activated carbon adsorption
Adsorption is a process where a solid is
used for removing a soluble substance from the water. In this process
active carbon is the solid. Activated
carbon is produced specifically so as to achieve a very big internal
surface (between 500 - 1500 m2/g). This big internal surface
makes active carbon ideal for adsorption. Active carbon comes in two variations:
Powder Activated Carbon (PAC) and Granular Activated Carbon (GAC). The
GAC version is mostly used in water treatment, it can adsorb the following
soluble substances:
Datasheet Active Carbon
- Adsorption of
organic, non-polar substances such as:
- Mineral oil
- BTEX
- Poly aromatic
hydrocarbons (PACs)
- (Chloride)
phenol
- Adsorption of
halogenated substance: I, Br, Cl, H en F
- Odor
- Taste
- Yeasts
- Various fermentation
products
- Non-polar substances
(Substances which are non soluble in water)
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Examples from active carbon in different
processes:
- Ground water purification
- The de-chlorination
of process water
- Water purification
for swimming pools
- The polishing of treated
effluent
Process description:
Water is pumped in a
column which contains active carbon, this water leaves the column through
a draining system. The activity of an active carbon column depends on
the temperature and the nature of the substances. Water goes through the
column constantly, which gives an accumulation of substances in the filter.
For that reason the filter needs to be replace periodically. A used filter
can be regenerated in different ways, granular carbon can be regenerated
easily by oxidizing the organic matter. The efficiency of the active carbon
decreases by 5 - 10% 1). A small part of the active carbon is destroyed
during the regeneration process and must be replaced. If you work
with different columns in series, you can assure that you will not have
a total exhaustion of your purification system.
Description of adsorption:
Molecules from gas or liquid phase will
be attached in a physical way to a surface, in this case the surface is
from the active carbon. The adsorption process takes place in three steps:
- Macro transport: The
movement of organic material through the macro-pore system of the active
carbon (macro-pore >50nm)
- Micro transport: The
movement of organic material through the meso-pore and micro-pore system
of the active carbon (micro-pore <2nm; meso-pore 2-50nm)
- Sorption: The physical
attachment of organic material on the surface of active carbon in the
meso-pores and micro-pores of the active carbon
The activity
level of adsorption is based on the concentration of substance in the
water, the temperature and the polarity of the substance. A polar substance
(= a substance which is good soluble in water) cannot or is badly removed
by active carbon, a non-polar substance can be removed totally by active
carbon. Every kind of carbon has its own adsorption isotherm (see figure
1) and in the water treatment business this isotherm is definite by the
function of Freundlich.
The function of Freundlich: 
x/m = adsorbed substance per gram active carbon
Ce = concentration difference (between before and after)
Kf, n = specific constants
The second curve from active carbon (see
figure 2) shows the exhausting of a filter. Normally we place a UV-disinfections
unit after the active carbon column.
What
is the difference between adsorption and absorption??
When a substance is attached to a surface
is is called adsorption, is this case the substance is attached to the
internal surface of active carbon. When a substance is absorbed in a different
medium it is called absorption. When a gas is taken in a solution it is
called absorption.
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Figure
1 gives a specific adsorption isotherm for active carbon. On the horizontal
axis you can find the concentration and on the vertical axis you can find
the necessary quantity of carbon. You
can use this kind of figures to optimize you column.
Source figure 1:
http://www.aapspharmscitech.org/scientificjournals/
pharmscitech/volume2issue1/056/manuscript.htm |
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Figure
2 tells about the exhaustion during usage of you column. Point C3
the column starts to break trough and near C4 your column is not
purifying anymore. Between point C3 and C4 you need to regenerate
you column.
Source
figure 2: http://www.activated-carbon.com |
Factors that influence the performance of active carbon in water:
- The
type of compound to be removed. Compounds with high molecular
weight and low solubility are better absorbed.
- The
concentration of the compound to be removed. The higher the concentration,
the higher the carbon consumption.
- Presence
of other organic compounds which will compete for the available
adsorption sites.
- The
pH of the waste stream. For example, acidic compounds are better
removed at lower pH.
According
to this we can classify some chemicals by their probability of being
efficiently adsorbed by active carbon in water:
1.-
Chemicals with very high probability of being adsorbed by
active carbon:
| 2,4-D
|
Deisopropyltatrazine
|
Linuron
|
| Alachlor
|
Desethylatrazine
|
Malathion
|
| Aldrin
|
Demeton-O
|
MCPA
|
| Anthracene
|
Di-n-butylphthalate
|
Mecoprop
|
|
Atrazine
|
1,2-Dichlorobenzene
|
Metazachlor
|
| Azinphos-ethyl
|
1,3-Dichlorobenzene
|
2-Methyl
benzenamine |
| Bentazone
|
1,4-Dichlorobenzene
|
Methyl
naphthalene |
| Biphenil
|
2,4-Dichlorocresol
|
2-Methylbutane
|
| 2,2-Bipyridine
|
2,5-Dichlorophenol
|
Monuron
|
| Bis(2-Ethylhexyl)Phthalate
|
3,6-Dichlorophenol
|
Napthalene
|
| Bromacil
|
2,4-Dichlorophenoxy
|
Nitrobenzene
|
| Bromodichloromethane
|
Dieldrin
|
m-Nitrophenol
|
| p-Bromophenol
|
Diethylphthalate
|
o-Nitrophenol
|
| Butylbenzene
|
2,4-Dinitrocresol
|
p-Nitrophenol
|
| Calcium
Hypochloryte |
2,4-Dinitrotoluene
|
Ozone
|
| Carbofuran
|
2,6-Dinitrotoluene
|
Parathion
|
| Chlorine
|
Diuron
|
Pentachlorophenol
|
| Chlorine
dioxide |
Endosulfan
|
Propazine
|
| Chlorobenzene
|
Endrin
|
Simazine
|
| 4-Chloro-2-nitrotoluene
|
Ethylbenzene
|
Terbutryn
|
| 2-Chlorophenol
|
Hezachlorobenzene
|
Tetrachloroethylene
|
| Chlorotoluene
|
Hezachlorobutadiene
|
Triclopyr
|
| Chrysene
|
Hexane
|
1,3,5-Trimethylbenzene
|
| m-Cresol
|
Isodrin
|
m-Xylene
|
| Cyanazine
|
Isooctane
|
o-Xylene
|
| Cyclohexane
|
Isoproturon
|
p-Xylene
|
| DDT
|
Lindane
|
2,4-Xylenol
|
2.-
Chemicals with high probability of being adsorbed by active
carbon:
| Aniline
|
Dibromo-3-chloropropane
|
1-Pentanol
|
| Benzene
|
Dibromochloromethane
|
Phenol
|
| Benzyl
alcohol |
1,1-Dichloroethylene
|
Phenylalanine
|
| Benzoic
acid |
cis-1,2-
Dichloroethylene |
o-Phthalic
acid |
| Bis(2-chloroethyl)
ether |
trans-1,2-
Dichloroethylene |
Styrene
|
| Bromodichloromethane
|
1,2-Dichloropropane
|
1,1,2,2-Tetrachloroethane
|
| Bromoform
|
Ethylene
|
Toluene
|
| Carbon
tetrachloride |
Hydroquinone
|
1,1,1-Trichloroethane
|
| 1-Chloropropane
|
Methyl
Isobutyl Ketone |
Trichloroethylene
|
| Chlorotoluron
|
4-Methylbenzenamine
|
Vinyl
acetate |
3.-
Chemicals with moderate probability of being adsorbed by
active carbon*:
| Acetic
acid |
Dimethoate
|
Methionine
|
| Acrylamide
|
Ethyl
acetate |
Methyl-tert-butyl
ether |
| Chloroethane
|
Ethyl
ether |
Methyl
ethyl ketone |
| Chloroform
|
Freon
11 |
Pyridine
|
| 1,1-Dichloroethane
|
Freon
113 |
1,1,2-Trichloroethane
|
| 1,2-Dichloroethane
|
Freon
12 |
Vinyl
chloride |
| 1,3-Dichloropropene
|
Glyphosate
|
|
| Dikegulac
|
Imazypur
|
|
*(For
this chemicals active carbon is only effective in certain cases).
4.-
Chemicals for which adsorption with active carbon is unlikely
to be effective. However it may be viable in certain cases such
as for low flow or concentrations:
| Acetone
|
Methylene
chloride |
| Acetonitrile
|
1-Propanol
|
| Acrylonitrile
|
Propionitrile
|
| Dimethylformaldehyde
|
Propylene
|
| 1,4-Dioxane
|
Tetrahydrofuran
|
| Isopropyl
alcohol |
Urea
|
| Methyl
chloride
|
|
Factors that
influence the performance of active carbon in air:
- Type
of compound to be removed: In general compounds with a high molecular
weight, lower vapor pressure/higher boiling point and high refractive
index are better adsorbed.
- Concentration:
The higher the concentration, the higher the carbon consumption.
- Temperature:
The lower the temperature, the better the adsorption capacity.
- Pressure:
The higher the pressure, the better the adsorption capacity.
- Humidity:
The lower the humidity, the better the adsorption capacity.
If
you want to know if a certain chemical can be effectively removed
from air by active carbon, please contact
us.
More information about
the Regeneration of Active Carbon |
1) source: Wastewater Engineering; Metcalf
& Eddy; third edition; 1991; page 317 |
