1. Adsorption and ion exchange
TVM 4145 Vannrenseprosesser / unit processes
Adsorption and
ion exchange
Prof. TorOve Leiknes

2. Definition:
Process in which solute molecules (adsorbate) become attached to a solid surface (adsorbent) under the attracting influence of surface forces. It is primarily a surface phenomena

3. Adsorption processes in water treatment
Activated carbon most used in water treatment
PAC: Powder activated carbon
GAC: Granular activated carbon In
Out
GAC
Sand 8
PAC

4. Adsorption processes:
Mass transport from bulk liquid phase to the surface of solid material.
1. Physical adsorption:
Van der Waals forces
Hydrogen binding
Hydrophobic binding
 T
2. Chemical adsorption: A A2
 T A
3. Exchange adsorption:
 T

5. Adsorption equilibrium:
adsorption rate  desorpsjon rate q C
q =
mass adsorbed
weight of adsorbent
Langmuir og Freundlich developed equilibrium isotherms

6.  1-   = q/Qo KA(1-)C KA(1-)C = KD KD  (KA C+ KD) = KA C
Langmuir Isotherm (1916):
Assumptions:
1. Given a set number of adsorption sites
2. Each site can only hold one molecule
3. All sites have the same binding energy 
1-
 = q/Qo 
At equilibrium:
KA(1-)C
KA(1-)C = KD
KD
 (KA C+ KD) = KA C

7. Substitute f and rewrite;q C
1/C
1/q
Freundlich Isoterm :

8. Adsorption kinetics: 1 Transport mechanisms: 23 4
Transport mechanisms:
1. convection in the bulk liquid
2. Molecular diffusion (film diffusion)
3. Pore diffusion
4. Adsorption rate
In evaluating the adsorption mechanisms one must distinguish which of the respective transport mechanisms is limiting for the process.

9. Film diffusion: q C* Cb =0
Pore diffusion: Cb r r

10. Activated carbon: Surface area ca. 600-1500 m2/g Pore radius:
pyrolysis
Pore radius:
1 nm to 25 nm
99% of surface area is inside the pore structure
PAC - powder, d < 0,1 mm
GAC - granular, d = 0,42 - 2,38 mm

11. Ion exchange Reversible exchange process of ions
Q, Co
Q, C
Reversible exchange process of ions
between the liquid phase and a natural
or artificial ion exchange media

12. Ion exchange media: Structure:
Matrix
(polystyrene cross-linked
with divinylbenzene)
Structure:
Gel media – varying degree of cross-linking
where the matrix is filled with liquid
Macro-porous media - structure has pores
Isoporous media - uniform pore size
Micro media - pulverized media

13. Exchange mechanisms: For example: Hardness removal No ion exchangeB- A-
No ion exchange
Ion exchange
For example:
Hardness removal

14. Equilibrium conditions:
Separation factor:
NB!
For exchange of ions with same charge the equilibrium constant and separation factor are the same

15. Selectivity:
Ion exchange media will have different capacities based on the ions;
Sulphonic media (-SO3-)  Fe+ > Al3+ > Ca2+
Carboxylate media (-COO-)  H+ > Ca2+ > Mg2+ > K+ > Na+
The ion exchange media’s selectivity is derived by the equilibrium constant
Where: X is fraction of ions in the respective phases

16. Ion exchange isotherms :
1.0 XA XB
Ks>1
Ks<1

17. ”Break through” curves:
1.0
CB=0
CB= CBo
Complete removal
Partial removal
1. Temperature
2. Media size (diameter)
3. Rate of flow-through (Q)
4. Selectivity coefficient
Factors that affect operation:

18. + Utnyttelsesgrad: Calculation: Utilization factor = CB/CBo CB/CBo
1.0
CB/CBo
Calculation:
1.0
1.0
CB/CBo
CB/CBo
Utilization factor = +

19. Typical arrangement: Operation: Q : 16-50 m3/h.m2 QB QC0 QR
Regeneration:
QB : m3/h.m2
tB : 20 min
QR: 4 m3/h.m2
h > 0.8m QC QR QB