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

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

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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