Chemical Engineering July 2017 - 38

Coconut AC
Lignite AC
100
75
50
25
FIGURE 5. This diagram shows a comparison
of lignite- and coconut-based activated
carbon performance with 50 mg/L
natural organic material (NOM)
5
PCB-118 conc. (ng/L)
Cwater = equilibrium concentration of
adsorbate in solution
Kf and n are constants for a given HOC
and carbon at room temperature
The partition
coefficient
10
15
Factors affecting performance
Factors that affect the performance of
activated carbons can come from the
specifics of the application, or from the
activated carbon itself.
Application-related factors. Several
factors can aid or hinder adsorption.
The principal factor is the molecular size
of the compound being adsorbed. Activated
carbon adsorption increases as
the size of the molecule being adsorbed
increases. Certain types of functional
groups on the impurity molecule can
also affect its adsorbability.
Because the physical adsorption
(Ccarbon /
Cwater) can be calculated from Freundlich
equation at a given equilibrium
concentration.
If a single compound is being adsorbed
by activated carbon, all pores
of a suitable size are available for adsorption
and the amount of material adsorbed
is the maximum achievable by
the particular carbon.
If several adsorbate molecules are
present, there can be competition for
the adsorption sites. In that situation,
the larger impurity molecules will block
some of the smaller pores. This may reduce
the carbon's ability to adsorb the
smaller-sized adsorbates.
Chemical adsorption. Chemical adsorption
refers to the direct reaction of
the adsorbed molecule with an active
site on the carbon surface. As a result of
the reaction, a chemical bond is formed
between the adsorbate and the carbon
surface. The active sites on the carbon
surface involved in chemisorption are
mainly functional groups that contain
oxygen and alter the electron balance
of the carbon surface. If the molecule
being adsorbed is chemically bound to
the carbon surface (shared electrons),
the process is termed chemisorption.
If the adsorbed molecule resides only
temporarily on the carbon surface, picks
up an electron, and then leaves the carbon
surface, the process is described as
catalytic conversion. This is the mechanism
responsible for the dechlorination
reaction used by bottlers to convert free
chlorine in water to chlorides.
36
process is an equilibrium reaction, the
concentration of the molecule to be adsorbed
strongly affects the amount adsorbed,
as described by the adsorption
isotherm. The solubility of the adsorbed
compound is also important, with lower
solubility resulting in greater adsorption.
Adsorption rate increases with temperature.
However, desorption rate also
increases with temperature and it is not
possible to predict the net effect in liquidphase
applications. The principal reason
for adsorbing at elevated temperatures in
the liquid phase is to lower viscosity and
increase diffusion rate. In gas-phase applications,
adsorption always decreases
with higher temperature.
Activated-carbon-related factors. In
adsorption applications, the most critical
performance parameter is the distribution
of pore size and volume. Since
adsorption occurs almost exclusively in
pores just a few times larger than the adsorbate
molecule, it is the pore volume
within this size range that determines
adsorption capacity. The highly porous
nature of activated carbon gives rise to
surface areas as large as 3,000 m²/g.
Pore sizes are classified as micro-,
meso- or macropores, according to the
conventions of the International Union
of Pure and Applied Chemisty (IUPAC;
iupac.org). Micropores with widths less
than 2 nm are useful for adsorbing small
molecules, especially in vapor applications.
Mesopores, ranging in width from
2 to 50 nm, are in the right size range
to adsorb many contaminant materials.
Macropores greater than 50 nm in
width have minimal adsorption capacity,
but are critical in determining adsorption
kinetics within the particle. The
larger pores provide transport paths for
molecules to diffuse into the mesopores
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JULY 2017
AC capacity NOM/
AC capacity DI, %
http://www.iupac.org http://WWW.CHEMENGONLINE.COM
Chemical Engineering July 2017

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