d. Adsorption

7/26/2019 d. Adsorption

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In liquid phase adsorption process, molecules,

ions or atoms in a liquid is diffused to the

surface of a solid, where they bond with the

solid surface through physical attractive

forces, ion exchange, and chemical binding

(Rangabhashiyam et al., 2013).

London, van der waals and electrostatic forces

Covalent bonding, ionic bonding

Physisorption

Chemisorption

SCHOOL OF CHEMICAL ENGINEERING,UNIVERSITI SAINS MALAYSIA


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The solid, which is insoluble in the liquid, is

the adsorbent. The components being

adsorbed are called solutes in the liquid and

form the adsorbate upon adsorption on the

solid.



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Micropore < 2 nm

Mesopore 2 < 50 nm

Macropore > 50 nm



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Plate 2.1 SEM image of activated carbon produced from banana

frond (Foo et al., 2013)



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Commercial adsorbercontinuous



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Physical properties of adsorbents

Different forms: Granules, beads, small pellet.

Different sizes: 0.112 mm

Porous materials



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MOF

Graphene???Ordered

Mesoporous

Carbon

2000sHydrogel

Aerogel

XerogelCryogelSilica

Clay

Bio

1900sActivated

Carbon

1880s

Char3000BC

Figure 1: Materials evolution map

Template CarbonizationKyotani, 1984

MCM-n family - Kresge, 1992



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Table 2.3 Methylene blue adsorption capacity on various sorbent media

Adsorbents Adsorption capacity

(mg.g-1)

Reference

Diatomite-templated carbon

505.10 (Liu et al., 2013)

Carboxylic acid functionalized

mesoporous silica

373 (Chang et al., 2013)

Graphene 204.08 (Liu et al., 2012b)

Graphene/magnetite composite 43.82 (Ai et al., 2011)

Rejected tea 156.00 (Nasuha et al., 2010)

Activated clay minerals 558.00 (El Mouzdahir et al.,

2010)

Vetiver root activated carbon 526.00 (Altenor et al., 2009)

CMK-3 ordered mesoporous

carbon

207.90 (Asouhidou et al., 2009)

H2SO4modified activated carbon 16.43 (Karagz et al., 2008)

Norit SA3 Activated carbon 91.00 (Yener et al., 2008)

Rattan dust activated carbon 294.12 (Hameed et al., 2007a)

Algae Gelidium 104.00 (Vilar et al., 2007)

Activated charcoal 519.75 (Iqbal and Ashiq, 2007)SCHOOL OF CHEMICAL ENGINEERING,UNIVERSITI SAINS MALAYSIA


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Equilibrium Curve



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Linear relationship is described by Henrys

Law. q = Kc

Freundlich isothermheterolayer adsorption

Langmuir isothermmonolayer adsorption



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According to Crini, comprehensive study on adsorption

isotherms is very important for designing and optimizing

batch adsorption process because the isotherms representthe behaviour of the adsorbates when interacted with the

adsorbents (Crini, 2008).

The adsorption isotherm provides an important correlation

between the mass of adsorbate adsorbed per unit weight ofadsorbent with the liquid-phase equilibrium concentration of

the adsorbate (Lata et al., 2007).



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Two-Parameter Isotherms

Langmuir model applies to homogeneous adsorption,which each molecule possess constant enthalpies andsorption activation energy and postulates notransmigration of the adsorbate in the plane of the

adsorbent surface (Prez-Marn et al., 2007, Kundu andGupta, 2006).

The model assumes monolayer adsorption where oneadsorbate can only attached on the specific identical

sites, with no lateral interaction and steric hindrancebetween the adsorbed molecules, even on adjacentsites (Vijayaraghavan et al., 2006).



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Freundlich has a practical application in

describing the non-ideal and reversibleadsorption of heterogoneous sytem.

This empirical model can be applied to

multilayer adsorption, with non-uniform

distribution energy on the adsorbent surface.



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The empirical equation of Freundlich is given in Equation 2.3

as:

(2.3)

where KF is the Freundlich constant (L.mg-1), and 1/n is the

heterogeneity factor.

1/n is a measure of the deviation from linearity of theadsorption between 0 and 1. If the value of 1/nis equal to 1,

the adsorption is linearreduce to HenrysLaw.

If 1/n < 1, this implies for physisorption; if 1/n > 1, this

indicates for chemisorption.

The more heterogeneous the surface, the closer 1/nvalue is

to 0.

n

eFe CKq /1



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Tempkin and Pyzhev takes into consideration

that the heat of adsorption of all molecules in

the layer decreases linearly with coverage due

to the effects of indirect adsorbate/adsorbentinteractions (Hosseini-Bandegharaei et al.,

2013). The non-linear form of Tempkin

equation is given as (Equation 2.4):(2.4)

)( eTT

e CKLnb

RTq



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Tempkin constants, KTand bT can be determined by

plotting a non-linear curve; qeversus Ce. R and T are

the universal gas constant (8.314 J.(K.mol)-1). and

temperature (K), respectively. KT is the equilibriumbinding constant (L.mol-1) corresponding to the

maximum binding energy and subsequently,

constant BTrelated to the heat of adsorption can be

solved through the following Equation 2.5:(2.5)

T

Tb

RTB



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Dubinin-Raduskevich (D-R) is one of the isotherms that widely

used to characterize liquid-phase adsorption process. It is

originally developed to emulate the experimental data ofsubcriticals vapors adsorption onto micropore solids based on

pore filling mechanism (Dubinin and Radushkevich, 1947).

The model does not assume a heterogeneous surface and

neither constant adsorption potential. The non-linear form of

D-R equation is presented as follow (Equation 2.6):

))]1

1ln([exp( 2

e

DDe

C

RTBqq (2.6)



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The constant, BD is corresponded to the mean free energy, E

of sorption/mole of the adsorbate as it is migrated to the

surface of the solid from infinitedistance in the solution and itcan be solved using the following relationship (Ho et al.,

2002). E can be calculated according to the Equation 2.7 as

follow:

(2.7)

D-R model is useful to distinguish between physical or

adsorption process based on the amount of calculated E.

Amount of E less than 8 kJ.mol-1indicates for possible physicaladsorption while 8-16 kJ.mol-1 could be a sign for

chemisorption (Asgari et al., 2013)

DBE 2

1



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Three-parameter Isotherms

Sips

Koble-Corrigan

Toth Redlich-Peterson

Etc.



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In class example



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Langmuir Plot

y = 0.0148x + 9.6511

R = 0.7841

0

5

10

15

20

25

0 200 400 600 800 1000

1/q

1/c



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Freundlich Plot

y = 0.2184x - 0.7183

R = 0.9941

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

-3.5 -3 -2.5 -2 -1.5 -1 -0.5 0

Logq

Log C



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Batch Adsorption



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In class example



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Column Adsorption

Fixed bed column is one of the most widely

employed method in the field of liquid and gas

adsorption.

The technique is desirable due to:

continuous nature of the process,

high efficiency adsorbent utilization,

less number of equipment,

smaller operation area and

cost savings.



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The performance and characteristics of

column operation can be determined by

analyzing the breakthrough curve.

The typical breakthrough curve as the ratio of

the effluent concentration (Ce) to the influent

concentration (Ci) versus time or throughput

volume is shown in Figure 2.7.



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Figure 2.7: Typical breakthrough profiles in column operation

(Barros et al., 2013)



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After a lapse of time, a breakthrough will occur at time, tb

with determined breakthrough concentration, Cb and the

effluent concentration continue to increase till reaching

saturation at time, ts.

Saturation point is a condition where the effluent

concentration becomes equal to the feed concentration

increases with time.

The breakthrough time is normally assumed when

Cb/C0reached 0.1; while the saturation point is defined ideally

when Cb/C0reach 1.0 (generally at 0.90-0.95) (Asberry et al.,

2014, Unuabonah et al., 2010, Singh et al., 2009).

However, under certain condition, some researchers wouldalso consider breakthrough point of Cb/C0 = 0.5(Wu et al.,

2012)



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Scale Up Adsorption Column



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In Class Example 3



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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6 7

c/c0

t, hourtb td

cb

cd

A1 A2



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Commercial operation

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d. Adsorption

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