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Adsorption

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Physical adsorption - van der Waals forces. Electrostatic adsorption ... Cation and Anion Adsorption. Cations. Anions. 0. 100. pH % adsorbed. Sorption edge ... – PowerPoint PPT presentation

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Title: 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 ?
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