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1
Adsorption and Absorption
Adsorption» Process by which a solute accumulates at
a solid-liquid interface
Absorption» partitioning of solute into a solid material
(at molecular level)
Sorption = Adsorption + Absorption
2
Solid Surface
Napthalene dissolved in aqueous phase
Coating of organic matter
Reactive surface site
Adsorption Absorption
Adapted from Fundamentals of Environmental Engineering, Mihelcic
Aqueous Phase
3
Adsorption to a pore
4
Terms Adsorbate
» Substance removed from liquid phase Adsorbent
» Solid phase on which accumulation occurs
Example» color can be removed from water using
activated carbon. Color is the adsorbate, activated carbon is the adsorbent
5
Physical Adsorption
Electrostatic attraction » oppositely charged particles
Dipole-Dipole Interaction » Attraction of two Polar Compounds
Polar compounds have an unequal distribution of charge (e.g., one end of molecule has slight + charge, the other a - charge)
6
Physical (cont.) Hydrogen Bonding
» special case of dipole-dipole interaction, involves hydrogen atom with slightly positive charge
Vander Waals Force » Weak attraction caused when close
proximity of two non-polar molecules causes change in distribution of charges, setting up a slight dipole-dipole attraction
7
Reactive (Active) surface site
A location on the surface of the adsorbent where the physical/chemical attraction is favorable
Reactive surface site
8
Another way to look at adsorption
Molecules prefer to be in lower energy state
If molecule can attain lower energy state by “sticking” to a solid surface, it will.» E.g., hydrophobic compounds
9
Equilibrium
At equilibrium, the chemical of concern will be found… » Dissolved in aqueous phase AND» Adsorbed to solid phase adsorbent
Adsorption is Reversable» add more to aqueous phase - get more
adsorption» reduce concentration in aqueous phase,
get desorption
10
Adsorbates of Interest
Taste and Odor (major interest)
Synthetic Organic Compounds (SOC)» Aromatic solvents (benzene, toluene)» Chlorinated aromatics » Pesticides, herbicides» Many more
11
Adsorbates of Interest (cont.)
Humic substances» large natural organics, often color forming, with
molecular weights ranging from few hundred to hundred thousands. Adsorption properties vary widely.
C
H
H H
H
Halomethanes can be formed when water containing humic substances is chlorinated.
Methane molecule with halogens (Cl, Br,...) substituted for H’s. Some are carcinogens.
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Adsorbates of Interest (cont.)
Some metals» antimony, arsenic, silver, mercury,...
Viruses
Other inorganics» Chlorine, Bromine
13
Adsorbents
Activated Carbon» Will remove all of the adsorbates
mentioned above (to varying degrees)» by far most popular adsorbent
Synthetic resins
Zeolites» Clays with adsorptive properties
14
What is Activated Carbon?
Carbon that has been pyrolyzed (heated in a low oxygen environment)
» Burns off tar, volatizes off gases» Creates material with lots of pores, thus lots
of surface area 500 - 1000 m2/g
» Creates active adsorption sitescarbon is non-polar, good for adsorbing non-
polar compounds
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Activated Carbon Picture
Source: solomon.bond.okstate.edu/thinkchem97/experiments/lab7.html
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Types
PAC: Powdered activated carbon» A fine powder, < 0.05 mm dia.» As much as 100 acres of surface area / lb
Pore sizes down to 10 x10-7 m.
GAC: Granular activated carbon » 0.3 - 3 mm» Not as much surface area as PAC
17
How do we use PAC?
Water Treatment
» Add it to rapid mix unit, remove in filterdo not regenerate
» Typical dose ~ 5 mg/L
» Used to remove taste and color
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How do we use GAC?
Water Treatment» As filter media to assist in taste and odor
removal
Sand Bed
Activated Carbon Bed
Under drain
Water Head
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GAC Use (cont.)
Clean contaminated groundwater
» Counter flow
dirtiest GAC contacts dirtiest water
continuous or batch addition of fresh GAC & removal of dirty GAC
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Gas Station Site
Drums of Activated Carbon
Contaminated AquiferGroundwaterPumping Well
Dirty Water
Clean Water
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Gas Station Site
Drums of Activated Carbon
Contaminated AquiferGroundwaterPumping Well
First Drum gets dirty fastest
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Gas Station Site
Drums of Activated Carbon
Contaminated AquiferGroundwaterPumping Well
Add clean drum at endPull first drum
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Single TankDirty Water
Clean Water
Dirty GAC
Clean GAC
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Design Pass contaminated water through single or series of
columns» Use constant Loading Rate (flow/area) and Influent
Concentration
Record concentration at difference points along column(s) over time
Plot Breakthrough & Bed Service Time Curves
Determine width & velocity of Adsorption Zone» determine # of columns needed, amount of adsorbant
needed
25
Terms
Loading Rate» Flux through column, Flow / Area
Co
» Concentration in influent to 1st column Adsorption Zone
» Zone where majority of adsorption is occurring
» Defined as zone where concentration is between 10 and 90 % of Co.
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SP2 SP3
SP1
SP4
Experiment
Co
lum
n 1
Co
lum
n 2
Co
lum
n 3
2.3 m
27
Breakthrough Curve
0 10 20 30 40 50 600
0.1
0.9
1.0
Time (days)
Column 1 Column 2 Column 3
Cout/Co
Cout = concentration exiting a column
28
Bed-Depth Service Time
Se
rvic
e T
ime
(d
ays
)
Bed Depth (m)2.3 4.6 6.9
20
40
60
90 % Feed C
oncentra
tion
10 % Feed C
oncentra
tion
AZ
UWac = Unit weight of act. carbon, mass/volume
Mac = Activated carbon needed, mass/time
a = slope of service time lines, time/length
1/a = velocity of AZ, length/timeA = Column cross-sectional area,
areaMac = A(1/a)UWac
AZ = Adsorption Zone lengthl = length of column# = No. of columns required
# = (AZ / l) + 1 then round UP
29
SP2 SP3
SP1
SP4
Experiment
Co
lum
n 1
Co
lum
n 2
Co
lum
n 3
2.3 m
AZ = 2.5 m< 0.1
0.1 to 0.9>0.9
Time Step 0
30
SP2 SP3
SP1
SP4
Experiment
Co
lum
n 1
Co
lum
n 2
Co
lum
n 3
2.3 m
AZ = 2.5 m< 0.1
0.1 to 0.9>0.9
Time Step 1
31
SP2 SP3
SP1
SP4
Experiment
Co
lum
n 1
Co
lum
n 2
Co
lum
n 3
2.3 m
AZ = 2.5 m< 0.1
0.1 to 0.9>0.9
Time Step 2
32
SP2 SP3
SP1
SP4
Experiment
Co
lum
n 1
Co
lum
n 2
Co
lum
n 3
2.3 m
AZ = 2.5 m< 0.1
0.1 to 0.9>0.9
Time Step 3
33
SP2 SP3
SP1
SP4
Experiment
Co
lum
n 1
Co
lum
n 2
Co
lum
n 3
2.3 m
AZ = 2.5 m< 0.1
0.1 to 0.9>0.9
Time Step 4
34
SP2 SP3
SP1
SP4
Experiment
Co
lum
n 1
Co
lum
n 2
Co
lum
n 3
2.3 m
AZ = 2.5 m< 0.1
0.1 to 0.9>0.9
Time Step 5
35
SP2 SP3
SP1
SP4
Experiment
Co
lum
n 1
Co
lum
n 2
Co
lum
n 3
2.3 m
AZ = 2.5 m< 0.1
0.1 to 0.9>0.9
Time Step 6
36
SP2 SP3
SP1
SP4
Experiment
Co
lum
n 1
Co
lum
n 2
Co
lum
n 3
2.3 m
AZ = 2.5 m< 0.1
0.1 to 0.9>0.9
Time Step 7
37
Breakthrough Curve
0 10 20 30 40 50 600
0.1
0.9
1.0
Time (days)
Column 1 Column 2 Column 3
Cout/Co
Cout = concentration exiting a column
38
Bed-Depth Service Time
Se
rvic
e T
ime
(d
ays
)
Bed Depth (m)2.3 4.6 6.9
20
40
60
90 % Feed C
oncentra
tion
10 % Feed C
oncentra
tion
AZ
UWac = Unit weight of act. carbon, mass/volume
Mac = Activated carbon needed, mass/time
a = slope of service time lines, time/length
1/a = velocity of AZ, length/timeA = Column cross-sectional area,
areaMac = A(1/a)UWac
AZ = Adsorption Zone lengthl = length of column# = No. of columns required
# = (AZ / l) + 1 then round UP