ADSORPTION

1. ADSORPTION
LIA LAILA, S.FARM., M.SC., APT.
PHARMACEUTICAL TECHNOLOGY DEPARTMENT
FACULTY OF PHARMACY
UNIVERSITY OF SUMATERA UTARA

2. ADSORPTION IS THE PROCESS THROUGH
WHICH A SUBSTANCE, ORIGINALLY
PRESENT IN ONE PHASE, IS REMOVED
FROM THAT PHASE BY ACCUMULATION AT
THE INTERFACE BETWEEN THAT PHASE
AND A SEPARATE (SOLID) PHASE.

3. ADSORBENT, ADSORPT DAN
ADSORBATE

4. • PHYSISORPTION → ABSENCE OF CHEMICAL BONDS THE
MOLECULE RETAINS ITS GAS PHASE ELECTRONIC
STRUCTURE, ALTHOUGH SOME DISTORTION IS STILL
POSSIBLE. THE BINDING ENERGY DEPENDS ON THE
POLARIZABILITY AND ON THE NUMBER OF ATOMS INVOLVED
OF THE ATOMS AND VARIES BETWEEN FEW MEV (LIGHT
GASES) AND SEVERAL EV (LARGE ORGANIC MOLECULES)
• CHEMISORPTION → STRONGER PERTURBATION OF THE
MOLECULAR ELECTRONIC STRUCTURE WITH FORMATION OF
CHEMICAL BONDS WITH THE SUBSTRATE. ENERGIES
TYPICALLY OF SEVERAL EV

6. FACTORS AFFECTING
ADSORPTION
THE MOST IMPORTANT FACTORS AFFECTING ADSORPTION
ARE:
• SURFACE AREA OF ADSORBENT.
LARGER SIZES IMPLY A GREATER ADSORPTION CAPACITY.
• PARTICLE SIZE OF ADSORBENT.
SMALLER PARTICLE SIZES REDUCE INTERNAL DIFFUSIONAL
AND MASS TRANSFER LIMITATION TO THE PENETRATION OF
THE ADSORBATE INSIDE THE ADSORBENT (I.E., EQUILIBRIUM
IS MORE EASILY ACHIEVED AND NEARLY FULL ADSORPTION
CAPABILITY CAN BE ATTAINED). HOWEVER, WASTEWATER
DROP ACROSS COLUMNS PACKED WITH POWDERED
MATERIAL IS TOO HIGH FOR USE OF THIS MATERIAL IN
PACKED BEDS. ADDITION OF POWDERED ADSORBENT MUST
BE FOLLOWED BY THEIR REMOVAL.

7. • CONTACT TIME OR RESIDENCE TIME.
THE LONGER THE TIME THE MORE COMPLETE THE
ADSORPTION WILL BE. HOWEVER, THE EQUIPMENT WILL BE
LARGER.
• · SOLUBILITY OF SOLUTE (ADSORBATE) IN LIQUID
(WASTEWATER).
SUBSTANCES SLIGHTLY SOLUBLE IN WATER WILL BE MORE
EASILY REMOVED FROM WATER (I.E., ADSORBED) THAN
SUBSTANCES WITH HIGH SOLUBILITY. ALSO, NON-POLAR
SUBSTANCES WILL BE MORE EASILY REMOVED THAN POLAR
SUBSTANCES SINCE THE LATTER HAVE A GREATER AFFINITY
FOR WATER.

8. • AFFINITY OF THE SOLUTE FOR THE ADSORBENT (CARBON).
THE SURFACE OF ACTIVATED CARBON IS ONLY SLIGHTLY
POLAR. HENCE NON-POLAR SUBSTANCES WILL BE MORE
EASILY PICKED UP BY THE CARBON THAN POLAR ONES.
• NUMBER OF CARBON ATOMS.
FOR SUBSTANCES IN THE SAME HOMOLOGOUS SERIES A
LARGER NUMBER OF CARBON ATOMS IS GENERALLY
ASSOCIATED WITH A LOWER POLARITY AND HENCE A
GREATER POTENTIAL FOR BEING ADSORBED (E.G., THE
DEGREE OF ADSORPTION INCREASES IN THE SEQUENCE
FORMIC-ACETICPROPIONIC-BUTYRIC ACID).

9. • SIZE OF THE MOLECULE WITH RESPECT TO SIZE OF THE
PORES.
LARGE MOLECULES MAY BE TOO LARGE TO ENTER SMALL
PORES. THIS MAY REDUCE ADSORPTION INDEPENDENTLY OF
OTHER CAUSES.
• DEGREE OF IONIZATION OF THE ADSORBATE MOLECULE.
MORE HIGHLY IONIZED MOLECULES ARE ADSORBED TO A
SMALLER DEGREE THAN NEUTRAL MOLECULES.
• PH.
THE DEGREE OF IONIZATION OF A SPECIES IS AFFECTED BY
THE PH (E.G., A WEAK ACID OR A WEAK BASIS). THIS, IN
TURN, AFFECTS ADSORPTION.

10. ADSORPTION ISOTHERM
• THE ADSORPTION EQUILIBRIUM RELATES Q TO C. THE
EQUILIBRIUM IS A FUNCTION OF THE TEMPERATURE.
THEREFORE, THE ADSORPTION EQUILIBRIUM RELATIONSHIP
AT A GIVEN TEMPERATURE IS TYPICALLY REFERRED TO AS
ADSORPTION ISOTHERM, I.E.:
Q = F (C)
WHERE:
• Q = MASS OF SPECIES ADSORBED/MASS OF ADSORBENT
(I.E., EQUILIBRIUM CONCENTRATION OF ADSORBABLE
SPECIES IN SOLID ADSORBENT)
• C = EQUILIBRIUM CONCENTRATION OF ADSORBABLE
SPECIES IN SOLUTION

11. TYPES OF ADSORPTION
ISOTHERMS
• DIFFERENT ADSORBATES-ADSORBENTS EXHIBIT
DIFFERENT TYPES OF EQUILIBRIUM RELATIONSHIPS (I.E., THE
FUNCTION Q = F(C) MAY TAKE DIFFERENT MATHEMATICAL
FORMS)
• IT HAS BEEN FOUND THAT FOR MOST OF THE CASES OF
IMPORTANCE IN WASTEWATER TREATMENT THE FUNCTION Q
= F(C) TAKES THE FORM OF ONE OF THE FOLLOWING
ISOTHERMS:
• - LANGMUIR ISOTHERM
• - BRAUNER-EMMET-TELLER (BET) ISOTHERM
• - FREUNDLICH ISOTHERM

12. ADSORPTION MODEL FOR LANGMUIR
ISOTHERM
THE ASSUMPTIONS MADE IN THE DERIVATION OF THE
LANGMUIR MODEL ARE:
• ADSORPTION IS A REVERSIBLE PROCESS
• THE ADSORBED LAYER IS MADE UP OF A SINGLE LAYER OF
MOLECULES
• THE ADSORBED MOLECULES DO NOT MOVE ON THE
SURFACE OF THE ADSORBENT. HOWEVER, THEY CAN BE
LOST BACK TO THE SOLUTION
• THE ENTHALPY OF ADSORPTION IS THE SAME FOR ALL
• MOLECULES INDEPENDENTLY OF HOW MANY HAVE BEEN
ADSORBED

13. SCHEMATIC OF LANGMUIR
ADSORPTION MODEL

14. DERIVATION OF LANGMUIR
ISOTHERM
• IN THE LANGMUIR MODEL (1918) ADSORPTION IS ASSUMED
TO BE A DYNAMIC PROCESS. AT EQUILIBRIUM THE NUMBER
OF MOLECULES BEING ADSORBED WILL BE EQUAL TO THE
NUMBER OF MOLECULES LEAVING THE ADSORBED STATE,
I.E.:

15. DERIVATION OF LANGMUIR
ISOTHERM

22. SCHEMATIC OF BET ADSORPTION
MODEL

30. Log C
Log q

33. SURFACE CHEMICAL REACTIONS
• (A) CORROSION REACTIONS
• IN CORROSION, ADSORBATES REACT DIRECTLY WITH THE
SUBSTRATE ATOMS TO FORM NEW CHEMICAL SPECIES. THE
PRODUCTS MAY DESORB FROM THE SURFACE (VOLATILIZATION
REACTION) OR MAY REMAIN ADSORBED IN FORMING A CORROSION
LAYER. CORROSION REACTIONS HAVE MANY INDUSTRIAL
APPLICATIONS, SUCH AS DRY ETCHING OF SEMICONDUCTOR
SURFACES.
• AN EXAMPLE OF A VOLATILIZATION REACTION IS THE ETCHING OF SI
BY FLUORINE. IN THIS CASE, FLUORINE REACTS WITH THE SI
SURFACE TO FORM SIF4 GAS. NOTE THAT THE CRYSTALLINITY OF
THE REMAINING SURFACE IS ALSO SEVERELY DISRUPTED BY THIS
REACTION.
• AN EXAMPLE OF CORROSION LAYER FORMATION IS THE OXIDATION
OF FE METAL TO FORM RUST. IN THIS CASE, NONE OF THE
PRODUCTS ARE VOLATILE, BUT THE CRYSTALLINITY OF THE
SURFACE IS DISRUPTED AS THE BULK OXIDE FORMS.

34. • (B) CRYSTAL GROWTH REACTIONS
• THE GROWTH OF SOLID FILMS ONTO SOLID SUBSTRATES
ALLOWS FOR THE PRODUCTION OF ARTIFICIAL STRUCTURES
THAT CAN BE USED FOR MANY PURPOSES.
• FOR EXAMPLE, FILM GROWTH IS USED TO CREATE PN
JUNCTIONS AND METAL–SEMICONDUCTOR CONTACTS
DURING SEMICONDUCTOR MANUFACTURE, AND TO
PRODUCE CATALYTIC SURFACES WITH PROPERTIES THAT
ARE NOT FOUND IN ANY SINGLE MATERIAL.
• LUBRICATION CAN BE APPLIED TO SOLID SURFACES BY THE
APPROPRIATE GROWTH OF A SOLID LUBRICATING FILM.
• FILM GROWTH IS ALSO USED TO FABRICATE QUANTUM-
WELLS AND OTHER TYPES OF LAYERED STRUCTURES THAT
HAVE UNIQUE ELECTRONIC PROPERTIES.

35. • THE THREE BASIC TYPES OF FILM GROWTH REACTIONS ARE:
A. PHYSICAL VAPOUR DEPOSITION (PVD)
IN PVD, AN ATOMIC GAS IS CONDENSED ONTO A SURFACE
FORMING A SOLID.
B. CHEMICAL VAPOUR DEPOSITION (CVD)
IN CVD, A MOLECULAR GAS DISSOCIATES UPON
ADSORPTION. SOME OF THE DISSOCIATION FRAGMENTS
SOLIDIFY TO FORM THE MATERIAL, WHILE OTHER
DISSOCIATION FRAGMENTS ARE EVOLVED BACK INTO THE
GAS PHASE.
C. MOLECULAR BEAM EPITAXY (MBE)
IN MBE, CAREFULLY CONTROLLED ATOMIC AND/OR
MOLECULAR BEAMS ARE CONDENSED ONTO A SURFACE IN
THE PROPER STOICHIOMETRY IN ORDER TO GROW A
DESIRED MATERIAL

37. (C) CATALYTIC REACTIONS
IN A CATALYTIC REACTION, THE REACTANTS FIRST
ADSORB ONTO THE SURFACE AND THEN REACT WITH
EACH OTHER TO FORM VOLATILE PRODUCT(S). THE
SUBSTRATE ITSELF IS NOT AFFECTED BY THE REACTION,
BUT THE REACTION WOULD NOT OCCUR WITHOUT ITS
PRESENCE.
• TYPES OF CATALYTIC REACTIONS INCLUDE EXCHANGE,
RECOMBINATION, UNIMOLECULAR DECOMPOSITION, AND
BIMOLECULAR REACTIONS.