Adsorption and Absorption

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
Aqueous Phase
Napthalene dissolved in aqueous phase
Coating of organic matter
Solid Surface
Reactive surface site
Adapted from Fundamentals of Environmental
Engineering, Mihelcic
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.

12
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 sites
• carbon is non-polar, good for adsorbing non-polar
  compounds

15
Activated Carbon Picture
Source solomon.bond.okstate.edu/thinkchem97/exper
iments/lab7.html
16
Types

• PAC Powdered activated carbon
• A fine powder, lt 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 filter
• do not regenerate
• Typical dose 5 mg/L
• Used to remove taste and color

18
How do we use GAC?

• Water Treatment
• As filter media to assist in taste and odor
  removal

Water Head
Activated Carbon Bed
Sand Bed
Under drain
19
GAC Use (cont.)

• Clean contaminated groundwater
• Counter flow
• dirtiest GAC contacts dirtiest water
• continuous or batch addition of fresh GAC
  removal of dirty GAC

20
Gas Station Site
Clean Water
Dirty Water
Drums of Activated Carbon
Groundwater Pumping Well
Contaminated Aquifer
21
Gas Station Site
First Drum gets dirty fastest
Drums of Activated Carbon
Groundwater Pumping Well
Contaminated Aquifer
22
Gas Station Site
Add clean drum at end Pull first drum
Drums of Activated Carbon
Groundwater Pumping Well
Contaminated Aquifer
23
Single Tank
24
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.

26
Experiment
SP1
Column 1
Column 2
Column 3
2.3 m
SP3
SP4
SP2
27
Breakthrough Curve
Column 2
Column 1
Column 3
1.0
0.9
Cout/Co
0.1
0
0
10
40
50
60
20
30
Time (days)
Cout concentration exiting a column
28
Bed-Depth Service Time
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/time A Column
cross-sectional area, area Mac A(1/a)UWac AZ
Adsorption Zone length l length of column
No. of columns required (AZ / l) 1 then
round UP
29
Experiment
AZ 2.5 m
SP1
Column 1
Column 2
Column 3
2.3 m
SP3
SP4
SP2
Time Step 0
30
Experiment
AZ 2.5 m
SP1
Column 1
Column 2
Column 3
2.3 m
SP3
SP4
SP2
Time Step 1
31
Experiment
AZ 2.5 m
SP1
Column 1
Column 2
Column 3
2.3 m
SP3
SP4
SP2
Time Step 2
32
Experiment
AZ 2.5 m
SP1
Column 1
Column 2
Column 3
2.3 m
SP3
SP4
SP2
Time Step 3
33
Experiment
AZ 2.5 m
SP1
Column 1
Column 2
Column 3
2.3 m
SP3
SP4
SP2
Time Step 4
34
Experiment
AZ 2.5 m
SP1
Column 1
Column 2
Column 3
2.3 m
SP3
SP4
SP2
Time Step 5
35
Experiment
AZ 2.5 m
SP1
Column 1
Column 2
Column 3
2.3 m
SP3
SP4
SP2
Time Step 6
36
Experiment
AZ 2.5 m
SP1
Column 1
Column 2
Column 3
2.3 m
SP3
SP4
SP2
Time Step 7
37
Breakthrough Curve
Column 2
Column 1
Column 3
1.0
0.9
Cout/Co
0.1
0
0
10
40
50
60
20
30
Time (days)
Cout concentration exiting a column
38
Bed-Depth Service Time
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/time A Column
cross-sectional area, area Mac A(1/a)UWac AZ
Adsorption Zone length l length of column
No. of columns required (AZ / l) 1 then
round UP