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.

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 sitescarbon is non-polar, good for adsorbing non-

polar compounds

15

Activated Carbon Picture

Source: solomon.bond.okstate.edu/thinkchem97/experiments/lab7.html

16

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

18

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

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

Drums of Activated Carbon

Contaminated AquiferGroundwaterPumping Well

Dirty Water

Clean Water

21

Gas Station Site

Drums of Activated Carbon

Contaminated AquiferGroundwaterPumping Well

First Drum gets dirty fastest

22

Gas Station Site

Drums of Activated Carbon

Contaminated AquiferGroundwaterPumping Well

Add clean drum at endPull first drum

23

Single TankDirty Water

Clean Water

Dirty GAC

Clean GAC

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

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