CE 529 Hazardous Waste Management
Carbon Adsorption
Dr. S. K. Ong
______________________ of a substance involves the accumulation at the interface between two phases. The
molecule that accumulates or adsorbs at the interface is called ________________ and the solid on which adsorption
occurs is the _________________.
Activated carbon is most commonly used as the adsorbent for the removal of a wide range of toxic organic
compounds from industrial wastewater and hazardous waste.
Activated carbon comes in two forms: ________________ and _____________________.
Granular activated carbon
Common sizes are US standard mesh of ___________ (_____________ mm) and _________________
(____________________ mm). Uniformity coefficient is usually ______________.
Apparent density = _____________________ kg/m3 (dry activated carbon per unit volume of activated
carbon, including volume of voids between grains.
Particle density (wetted in water) = __________________________ kg/m3
Surface areas – several ________________ to more than ___________ m2/g
Pores classified as follows:
Micropores < _________ nm diameter
Macropores > __________ nm diameter
Intermediate pores between ______________________ nm.
Pore volumes range from ____________________ mL/g.
Powdered activated carbon – commercially available with __________ % passing through ____________ mesh
(______________ m) sieve.
GAC and PAC are made from _________________________________________________________________.
The process involves ______________________________ or _________________________ (conversion of the raw
materials to char - done in the absence of air at temperature < __________________ o C) followed by
_________________________ or oxidation (to develop the internal pores and surface characteristics – use of
oxidizing gases such as steam and CO2 between ___________________o C).
Adsorption may be viewed as taking place in the following steps:
 __________________________________ – adsorbates are transported from the bulk solution to the
boundary layer of water surrounding the adsorbent particle. Transport is by diffusion in quiescent water or
by turbulence in a mixing tank or adsorption tower
 _________________________________ – adsorbates are transported by molecular diffusion through the
stationary layer of water that surrounds adsorbent particles when water is flowing past them. The distance
of transport and thus the time for this step is determined by the rate of flow past the particle: the higher the
rate of flow, the shorter is the distance.
 ________________________________– adsorbates are transported through the adsorbent’s pores to
available adsorption sites – occurs by molecular diffusion through the solution in the pores
(__________________) or by diffusion along the adsorbent surface after adsorption takes place
(________________________).
 __________________________ – interaction of the adsorbate with the surface of the adsorbent – forces
involved include hydrogen bonds, dipole-dipole interactions, van der Walls forces and covalent bonds.
Factors affecting adsorption equilibria
 _______________________________________________:
- the amount of adsorption is proportional to the amount of surface area accessible to the adsorbate.
- In addition the size of the adsorbate will also determine which surface within the different pores sizes will
be available for sorption. For example, large molecules such as fulvic and humic acids will not penentrate
the micropores as compared to the smaller phenol molecules.
____________________________________:
- increased amounts of surface oxygen decreased the affinity of the carbon for nonpolar compounds such as
aromatic compounds (benzene and phenol).
 ________________________________:
- the hydrophobic nature of the adsorbate plays a role on the affinity of the activated carbon for the
adsorbate. Generally, adsorption onto GAC from water increases as the adsorbate’s solubility decreases.
- The size of the molecule plays a role. As molecular size increases, the rate of diffusion within the carbon
decreases, especially as the molecular size approaches that of the particle pore diameter. However, larger
molecules are more easily adsorbed.
Freundlich model is typically used to characterize single solute adsorption properties of GAC.
Single solute isotherms are useful for providing rough estimates of adsorption capacity. See attached
Table.
GAC Process Design Criteria
In a design, the minimum ______________________ needed and the ________________of carbon needed
to provide the necessary removal for a given volume of water treated. The following design information
are needed:
 ___________ (both average and maximum)
 Surface loading rate (SLR):
___________________________________
 Empty Bed Contact Time (EBCT) = ________________________________
Where Q = flow rate
A = area of the column
 GAC Depth – selected based on required breakthrough
Properties
SLR (m3/m2/hr)
Typical
Range
EBCT (mins)
can be as high as 4 hours for high concentrations
GAC depth (m)
 Type of GAC – GAC made from different materials have different sorption capacities and characteristics
 GAC Usage rate or carbon usage rate (CUR) - indicates the mass of carbon required per unit volume of
water treated.
CUR = mass of activated carbon in column/volume of water treated to breakthrough, V B
This probably the most difficult to estimate without doing batch adsorption studies or pilot column studies.
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Batch Adsorption Studies
Isotherms can be used to obtain a rough estimate of activated carbon loading and bed life.
Where
Co
C1
(qe)o
Y
= influent concentration
= average effluent concentration for entire column run
= mass adsorbed (mg/g) when Ce = Co
= bed life (volume of water that can be treated per unit volume of carbon)
=
units [(mg/g GAC  (g/L)/ (mg/L)]
Example: Estimate the bed life and carbon usage rate for a GAC adsorber that is to remove 10 g/L of
bromoform from solution, GAC = 500 g/L
1. Freundlich isotherm constants for bromoform are K = ________ and 1/n = _____________
2. Using the Freundlich equation qe = KCe1/n
(qe)o = 20 (0.01)0.52
= ____________________ mg/g
3. Assuming C1 = 0, gives
Y = (qe)o  GAC /(Co – C1) = ______________ = ___________________ L H2O/L GAC
4. CUR
= (Co – C1)/(qe)o
= _________________ = ______________________ g GAC/L H2O
Limitations
 for a given breakthrough concentration, the entire GAC bed adsorber may not be in
equilibrium with the breakthrough concentration
 the approach does not take into consideration competition amongst adsorbates
Column Studies for Design
 Different types of small-scale column tests have been developed to obtain data that can be used to
estimate the performance of large carbon adsorbers. Several different procedures have developed, for
example,
Rosene et al. developed the high pressure mini-column (___________) technique while Crittenden et al.
developed the rapid small-scale column test (________________) to estimate the breakthrough curve for
the treatment of a given adsorbate.
Several approaches have been developed to analyze the breakthrough curve to obtain information for the
design of GAC adsorbers.
____________________________ (BDST) Method
- In this approach, breakthrough curves for different bed depths are obtained.
- The breakthrough curves are plotted with the normalized concentrations (C/C o) as the y-axis and the
breakthrough time on the x-axis.
- Based on the breakthrough curves, horizontal lines are drawn at the desired breakthrough concentrations
or at different normalized concentrations (usually 0.1 and 0.9). The time needed to reach these
concentrations for different bed depths are obtained.
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- The bed depth to service time is plotted with the service time (for breakthrough concentration or at 0.1
and 0.9 normalized concentrations) as the y-axis against each depth (x-axis). Based on this plot, a straight
line is drawn through the data (breakthrough concentration or 0.1 or 0.9 normalized concentrations).
d1
d2
d3
90%
C/Co
Breakthrough
ts2
ts1
ts3
Time (days)
Figure 1
Min. EBCT
90%
Re
kg/d
Break
through
ts
(d)
Theoretical min, Re
d
EBCT (min)
Figure 2
Figure 3
- Using only the breakthrough concentrations as an example, a regression line can be obtained. The line is
given by
where
ts
d
a
No
Co
V
B
K
CB
= operating time for a bed depth d
= bed depth
= slope of the plot = No/(Cov) (units = time/length)
= adsorptive capacity of carbon (mass/vol)
= initial concentration of solute (mass/vol)
= linear flow velocity of feed to bed (length/time)
= intercept = [1/KCo][ln{(Co/CB) - 1}] (units = length)
= rate constant (vol of liquid/mass of carbon-time)
= desired breakthrough concentration (mass/vol)
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Note that the weakest part of this equation is not knowing the K value. However, using the slope, a, the
adsorptive capacity for the given flow rate may be estimated.
The intercept of the y-axis is an indication of the critical or minimum bed depth, D min needed. The
horizontal difference between the exhaustion line (C/Co = 90%) and the breakthrough line is defined as the
height of the adsorption zone (D).
The velocity of the adsorption zone as it moves down the column and the time for a column to be exhausted
is given by:
Adsorption velocity (m/h) = __________________
Exhaustion time (hr) = ________________________
The carbon utilization rate = _______________________________________
Scaling up:
The number of columns needed (based on the height of adsorption zone) is
n = ____________________
For a different flow rate
a' = _______________
b is not modified
For a different feed concentration a' = _______________
b' = ______________________________
Another approach in using the above plots is to compute the minimum contact time from:
- To determine the carbon exhaustion rate, the carbon exhaustion rate data was plotted against the EBCT to
form the operating line. The overall rate of carbon exhaustion rate R e for a given bed depth is calculated
from
- Based on this plot, the designer can choose any point on the operating line for a proper design taking into
consideration the costs associated with larger bed depths and rate of carbon regeneration at each depth.
Kinetic Approach
Need a breakthrough curve and the curve is modeled using:
where
C = effluent solute concentration
Co = influent solute concentration
K1 = rate constant
qo = maximum solid phase concentration of the sorbed solute
M = mass of sorbent in test
V = throughput volume
Q = flow rate
Rearranging we have
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This is a straight line plot of ln(C/Co -1) vs V
With slope = -K1Co/Q and intercept = K1qoM/Q
Co is known, Q is known, M is known, K1 and qo can be found. Use new Q and C desired and determine M
needed.
Note that this method suffers from scaling up problems.
ln
(Co/C
-1)
C/Co
Volume (V)
Volume V
Dynamic Models
Several dynamic models have been developed that can be used to predict breakthrough curves and therefore
assist in the design of carbon bed adsorbers. These models include the _____________________________(PSDM)
and the ________________________________________ (HDSM) by Crittenden and Weber.
Based on the mass transfer of the adsorbates through film diffusion and pore diffusion, the following mass
transfer equations can be written:
In the liquid phase
C
C
 2 C 3( 1   )k f
 v
 Dh

( C  Cs )
t
x
R
x 2
In the solid phase
s
C p
t
 s
q ave D p  s   2 C p  Ds  s   2 q ave 
 2
r

r
t
r 
r 
r 
r
r 2 r 
Mass transport in the liquid phase must equal to the uptake by the solids phase
3( 1   )k f
R
where
C
v
Dh
s
R
Cs
( C  Cs ) 
 s ( 1   ) q ave

t
= bulk liquid concentration
= liquid velocity
= hydrodynamic dispersion coefficient
= solid phase density
= carbon particle radius
= liquid phase concentration at the carbon surface
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qave
Cp
s
Dp
Ds
= average solid phase concentration
= liquid phase concentration in the carbon pores
= solid phase porosity
= pore diffusion coefficient
= surface diffusion coefficient
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