1 adsorption and absorption l adsorption »process by which a solute accumulates at a solid-liquid...
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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
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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
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Adsorption to a pore
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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
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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)
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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
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Reactive (Active) surface site
A location on the surface of the adsorbent where the physical/chemical attraction is favorable
Reactive surface site
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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
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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
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Adsorbates of Interest
Taste and Odor (major interest)
Synthetic Organic Compounds (SOC)» Aromatic solvents (benzene, toluene)» Chlorinated aromatics » Pesticides, herbicides» Many more
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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