Adsorption

1
Adsorption
H
H
H
H
O
O
H
H
O
O
Colloid
H
H
H
O
O
H
H
H
O
O
H
H
H
H
O
H
O
H
H
H
H
H
H
H
O
O
O
O
X
X
H
H
O
O
H
H
H
H
H
H
2
Types of Adsorption
?S(H2O)m X(H2O)nq ? ? SX(H2O)pr (q-r)H
(mn-p)H2O

• Physical adsorption - van der Waals forces
• Electrostatic adsorption - electric charge
• Chemical adsorption - bond to solid

3
Linear Distribution Coefficient
4
Freundlich Isotherm
5
Langmuir Isotherm
6
Comparison of Isotherms
Kd 1.5 Freundlich K 1.5, n 0.9 Langmuir K
1.5, Cmax 30
7
Acid-Base Equilibria

• Dissociate surface
• ?S?OH H ? ?S?OH2
• Protonate surface
• ?S?OH ? ?S?O- H
• pH where the net surface charge is zero is the
  zero point of charge ZPC

8
ZPC
?S?OH2 ? ?S?OH H
?S?OH ? ?S?O- H
9
Surface Titration
At any point of the titration the Proton
condition is given by
The net number of moles of bound OH- is given by
The mean charge is given by
10
Surface Titration
If we know the surface area S
The mean charge is given by
If ions other than H are absorped then
11
ZPC - Isoelectric Point
ve surface charge
-ve surface charge
Cback elec 0.001 0.003 0.01 0.03
pHZPC
CB
CA
12
Typical Isoelectric Points
13
Specific Adsorption
H
H
Non-specific
Substrate
O
H
H
O
O
Outer Shell
H
H

H
H
O
H
H
H
O
H
O
O
O

H
H
O
H
H
O
O

H
H
Inner Shell
H
H
O
H
H
Specific
H
H
14
Cation and Anion Adsorption
100
adsorbed
Cations
Anions
0
pH
Sorption edge
15
Electrostatic Adsorption Models

• Sorbing surface contains functional groups that
  react with sorbing solutes to form surface
  complexes
• Surface complexing can be described my
  mass-action equations (K) corrected for
  electrostatic effects

16
Electrostatic Adsorption Models

• Surface charge ? and electrical potential ? are
  consequences of chemical reactions involving the
  surface groups
• Binding constants are empirical parameters
  related to the intrinsic thermodynamic constants
  by the activity coefficient of the surface
  species.

17
Constant Capacitance Model
Counter ions
?1
Distance ?
OH O - OH OH2 OM L OH
?0
?0 ?
18
Diffuse Double- Layer Model

-

?d
-

-
Distance ?
OH O - OH OH2 OM L OH
?d

-

?0
?0 ?
19
Triple- Layer Model

-

?d
-

-
?d

-

??
Distance ?
??
OH O - OH OH2 OM L OH
-

?0
?0 ?