An adsorption `isotherm`

Kesetimbangan Adsorpsi
• Adsorpsi vs Absorpsi
– Adsorption is accumulation of molecules on a surface
(a surface layer of molecules) in contact with an air or
water phase
– Absorption is dissolution of molecules within a phase,
e.g., within an organic phase in contact with an air or
water phase
Adsorpsi
FASE I
‘FASE’ 2
absorpsi (“partitioning”)
FASE I
FASE 2
Pgas  K H caq
Henry’s Law
“Jargon” Adsorpsi
Cu2+
Cu2+
Cu2+
Cu2+
Cu2+
Cu2+
Cu2+
Cu
2+
Cu2+
Adsorbed species present
at an overall concentration
of cCu(ads)
Dissolved
adsorbate, at
concentration cCu(aq)
Cu2+
Cu2+
Cu2+
Adsorbent, in
suspension at
concentration
csolid
Adsorbed species,
with adsorption
density q mg Cu per
g solid or per m2
Surface area per
gram of solid is the
specific surface area
 mg adsorbed 
 mg adsorbed 
 g solid per 
ci ,ads 
  qi 
 csolid 

per
L
of
solution
per
g
adsorbent
L
of
solution






Penyebab Adsorpsi
• Ketidaktertarikan pada fasa air –
‘Hydrophobicity’
• Daya tarik permukaan sorben
– van der Waals forces: daya tarik fisik
– electrostatic forces (interaksi muatan di
permukaan
– chemical forces (ex: ikatan hidrogen)
Adsorben di Alam & Sistem
Rekayasa
• Sistem Alamiah
– Sedimen
– Tanah
• Sistem Rekayasa
– Karbon Aktif
– Metal oxides (iron and aluminum as
coagulants)
– Ion exchange resins
– Biosolid
Sistem Rekayasa
• Karbon aktif (gusus fungsi kimia)
– Adsorption of organics (esp. hydrophobic)
– Redusi - Oksidasi
• Metal oxides (surface charge depends on pH)
– Adsorpsi bahan organik alam (NOM)
– Adsorpsi bahan anorganik (kations & anion)
• Ion exchange resins
– Kation and anion
– Penghilang kesadahan (Ca2+, Mg2+)
– Pengilang Arsen (berbagai spesies bermuatan
negatif), NO3-, Ba2+
Permukaan Metal Oksida
Coagulants form precipitates of Fe(OH)3 and Al(OH)3
which have –OH surface groups that can adsorb humics
and many metals
Humic substances where R is organic
Ion Exchange Resins
2R--Na+ + Ca2+  R2-Ca + 2Na+
R+-Cl- + H2AsO4-  R+- H2AsO4- + Cl-
Isoterm Adsorpsi
Add Same Initial Target Chemical Concentration, Cinit, in each
Control
Different activated carbon dosage, Csolid, in each
 mg  cinit - c fin  mg/L 
q fin 

csolid  g/L 
 g 
An adsorption ‘isotherm’ is a q vs. c
relationship at equilibrium
Example. Adsorption of benzene onto activated carbon has been reported to obey
the following Freundlich isotherm equation, where c is in mg/L and q is in mg/g:
0.533
qbenz  50.1 cbenz
A solution at 25oC containing 0.50 mg/L benzene is to be treated in a batch
process to reduce the concentration to less than 0.01 mg/L. The adsorbent is
activated carbon with a specific surface area of 650 m2/g. Compute the required
activated carbon dose.
Solution. The adsorption density of benzene in equilibrium with ceq of 0.010 mg/L
can be determined from the isotherm expression:
0.533
qbenz  50.1 cbenz
 4.30 mg/g
A mass balance on the contaminant can then be written and solved for the
activated carbon dose:
ctot ,benz  cbenz  qbenz c AC
0.50  0.010   4.30 mg/g  cAC
cAC  0.114 g/L  114 mg/L
Example If the same adsorbent dose is used to treat a solution containing 0.500
mg/L toluene, what will the equilibrium concentration and adsorption density be?
The adsorption isotherm for toluene is:
0.365
qtol  76.6 ctol
Solution. The mass balance on toluene is:
ctot ,tol  ctol  qtol c AC
0.50  ctol   76.6 ctol 0.365   0.114 g/L 
ctol  3.93x10-4 mg/L