Adsorption Fundamentals

8/12/2019 Adsorption Fundamentals

1/16

The process of adsorption involves separation of a substance from one phase

accompanied by its accumulation or concentration at the surface of another. The processcontinues until equilibrium is established between the amount of solid-bound adsorbate and

the remaining portion in the solution. The adsorbing phase is the adsorbent, and thematerial concentrated or adsorbed at the surface of that phase is the adsorbate.

Adsorption phenomena are operative in most natural physical, biological, and chemicalsystems, and adsorption operations employing solids such as activated carbon and

synthetic resins are used widely in industrial applications and for purification of waters

and wastewaters.

Similar to surface tension, adsorption is a consequence of surface energy.

Whatever the nature of the forces holding a solid together, it can be regarded as producing a field of force

around each ion, atom, or molecule. At the surface of the solid, these forces cannot suddenly disappear and

thus reach out in space beyond the surface of the solid. Due to these unsaturated and unbalanced forces, the

solid has a tendency to attract and retain on its surface molecules and ions of other substances with which it

comes into contact. Thus, when a solid surface comes in contact with a gas or a liquid, the concentration ofthe gas or liquid is always greater on the surface of the solid than in the bul gas or liquid phase. This leads

to the gas or liquid getting adsorbed on the solid surface. The e!act nature of the bonding depends

on the details of the species involved, but the adsorbed material is generally classified as

e!hibiting physisorption or chemisorption.

4.2 PHYSICAL ADSORPTION AND CHEMICAL ADSORPTION

Traditionally, adsorption is classified according to the magnitude of adsorption forces.

Wea interactions "#$%&mol' analogous to those between molecules in liquids give riseto physical adsorption. "(ritannica')hysisorption or physical adsorption is a type ofadsorption in which the adsorbate adheres to the surface only through *an der Waals

"wea intermolecular' interactions, which originates from the interactions between

induced, permanent or transient electric dipoles. The individuality of adsorbate andadsorbent are preserved. +t is multilayer process and the molecules are not site specific.

hemisorption is a type of adsorption whereby a molecule adheres to a surface throughthe formation of a chemical bond "ionic or covalentbond' depending on the reactive

chemical compounds used "e.g. corrosion'. +n chemisorption,there is a transfer or sharing

of electron, or breaage of the adsorbate into atoms or radicals, which are bound

separately. Strong interactions "#$%&mol' similar to those found between atoms withina molecule "covalent bonds' give rise to chemical adsorption or chemisorption. The

molecules are site specific and the process is monolayer.
http://en.wikipedia.org/wiki/Ionic_bondhttp://en.wikipedia.org/wiki/Ionic_bondhttp://en.wikipedia.org/wiki/Covalent_bondhttp://en.wikipedia.org/wiki/Ionic_bondhttp://en.wikipedia.org/wiki/Covalent_bond


2/16

4.3 APPLICATION OF ADSORPTION

The fundamental practical applications of adsorption and related areas are-

separation and purification of liquid and gas mi!tures, bul chemicals, isomers and air/ drying gases and liquids before loading them into industrial systems/

removal of impurities from liquid and gas media/

recovery of chemicals from industrial and vent gases/ and

water purification.

The commercial adsorption processes for separating gas and liquid mi!tures

accomplished due to selective adsorption of certain substances from their mi!tures. Thesame idea is true for purification of gas and liquid mi!tures and drying of some industrial

gases. 0or those purposes, the pore system of adsorbents used is sufficiently wide to

enable fast diffusion/ separation is caused mainly by selective adsorption that depends

upon the van der Waals forces between the adsorbent and the constituents of the gas orliquid mi!tures. The above processes are ma1or unit operations in the chemical and

petrochemical industries. "adsorption theory to practice- Dabrowsi'

Table 4.1: Compar!o" o# P$%!&al a"' C$em&al A'!orp(o"

Sl. No P$%!&al A'!orp(o" C$em&al A'!orp(o"

2 *an der Waal3s adsorptionActivated adsorption

"ionic or covalent bond'

4 5eat of adsorption6 7 cal&mol 5eat of adsorption6 4$-2$$ cal&mol

8 Adsorption occurs only attemperature less than the boiling

point of the adsorbate

Adsorption can occur even at higher

temperature

#9o activation energy is involved in

the adsorption process Activation energy may be involved

7Activation occurs in mono as well

as in multi layers

Activation occurs mostly in mono

layer


3/16

:

;uantity adsorbed per unit mass is

high i.e. entire surface isparticipating

;uantity adsorbed per unit mass is

low i.e. only active surface sites areimportant

orris "2?:8'

while referring to the rate limiting step of organic materials uptae by granulated

activated carbon in the rapidly mi!ed batch system propose the term @+ntra-particleTransport which comprises of surface diffusion and molecular diffusion. Several

researchers have shown that surface diffusion is the dominant mechanism and is a rate-

determining step. A functional relationship common to most of the treatments of intra-

particle transport is that the uptae varies almost proportionally with the square root oftime.

4.4.1 STAGES IN ADSORPTION PROCESS

Adsorption is thought to occur in three stages, as the adsorbate concentration increases.

S(a)e I: 0irstly, a single layer of molecules builds up over the surface of the adsorbent.This monolayer may be chemisorbed and is associated with a change in free energy that

is a characteristic of the forces that hold it.


4/16

S(a)e II: As the fluid concentration is further increased, the layers are formed by

physical adsorption. The number of layers formed is limited by the siBe of the pores.

S(a)e III: 0inally, for adsorption from the gas phase, capillary condensation may occurin which capillaries become filled with condensed adsorbate, when its partial pressure

reaches a critical value relative to the siBe of the pore.

4.* ADSORPTION DIFF+SION ST+DY

The mathematical treatment of (oyd et al., "2?#


5/16

+n addition, the adsorption of adsorbate at surface sites "step #' is usually very rapid, and

thus offers negligible resistance in comparison to steps 4 and 8. Thus, these processesusually are not considered to be the rate-limiting steps in the sorption process.

+n most cases, steps "4' and "8' may control the sorption phenomena. 0or the remaining

two steps in the overall adsorbate transport, three distinct cases may occurCase +C e!ternal transport resistance internal transport resistance

ase ++C e!ternal transport resistance internal transport resistance

ase +++C e!ternal transport resistance E internal transport resistance+n cases + and ++, the overall rate is governed by the film diffusion and diffusion in the

pores, respectively. +n case +++, the transport of solute to the boundary may not be

possible at a significant rate, thereby, leading to the formation of a liquid film with a

concentration gradient surrounding the adsorbent particles.Fsually, e!ternal transport is the rate-limiting step in systems which have "a' poor phase

mi!ing, "b' dilute concentration of adsorbate, "c' small particle siBe, and "d' high affinity

of the adsorbate to adsorbent. +n contrast, the intra-particle step limits the overall sorption

for systems that have "a' a high concentration of adsorbate, "b' a good phase mi!ing, "c'large particle siBe of the adsorbents, and "d' low affinity of the adsorbate to adsorbent.

4., ADSORPTION E-+ILIRI+M ST+DY:

When a solid surface is e!posed to a gas, the molecules of the gas strie the surface of the solid. Some of

the striing molecules stic to the solid surface and become adsorbed while the others rebound. +nitially the

rate of adsorption is large as the whole surface is bare but as more and more of the surface becomes

covered by the molecules of the gas, the available bare surface decreases and so does the rate of adsorption.5owever, the rate of desorption, which is the rate at which adsorbed molecules rebound from the surface,

increases because desorption taes place from the covered surface. As time passes, the rate of adsorption

continues to decrease while the rate of desorption increases until an equilibrium is reached between the rate

of adsorption and the rate of desorption. At this stage the solid is in adsorption equilibrium with the gas or

liquid, and the rate of adsorption is equal to the rate of desorption. +t is a dynamic equilibrium because the

number of molecules sticing to the surface is equal to the number of molecules rebounding from the

surface. 0or a given adsorbate-adsorbent system, the equilibrium amount adsorbedx&m is a function of

pressure and temperature/ i.e.

"#.2'

wherex/m is the amount adsorbed per unit mass of the adsorbent at the equilibriumpressurep, and T is the temperature of adsorption. "bansal'

The relationship between the amount of adsorbate adsorbed and the adsorbate

concentration remaining in solution is described by an isotherm. The adsorption isotherm

can be depicted by plotting solid phase concentration against liquid phase concentration

graphically. To optimiBe the design of adsorption system for the removal of adsorbate, it


6/16

is important to establish the most appropriate correlation for the equilibrium curve

GHataye et al.,4$$IJ.

Kquilibrium isotherms are measured to determine the capacity of the adsorbent. *arious

isotherm equations lie those of Hangmuir, 0reundlich have been used in the literature to

describe the equilibrium characteristics of adsorption from liquid solutions. Adsorptionisotherm is a graphical plot showing distribution of contaminants between the adsorbed

phase and solution phase at the dynamic equilibrium.>any theoretical and empirical

models have been developed to represent the various types of adsorption isotherms.Hangmuir, 0reundlich, (runauer-Kmmet-Teller "(KT', etc. are most commonly used

adsorption isotherm models for describing the dynamic equilibrium. The isotherm

equations used for the study are described follows "Duong D. Do 2??I'.

4.6.1 ADSORPTION ISOTHERMS

(runauer "S.(runauer, H.S. Deming, W.K. Deming, and K. %. Teller, %.Am. hem. Soc. :4, 2


7/16

The adsorption isotherm is the most e!tensively employed method for representing the equilibrium states of

an adsorption system. +t can give useful information regarding the adsorbate, the adsorbent, and the

adsorption process. +t helps in the determination of the surface area of the adsorbent, the volume of the

pores, and their siBe distribution, the heat of adsorption, and the relative absorbability of a gas or a vapor ona given adsorbent. Several adsorption isotherm equations such as Hangmuir, 0reundlich, Temin have been

derived.

4.6.1.1 Langmuir IsothermHangmuir isotherm describes adsorbate-adsorbent systems in which the e!tent of adsorbate coverage is limited to

one molecular layer at or before a relative pressure of unity is reached. Although the isotherm, proposed

originally by Hangmuir "2?2I', is more usually appropriate for the description of chemisorption "when an ionic

or covalent chemical bond is formed between adsorbent and adsorbate', the equation is nevertheless obeyed at

moderately low coverages by a number of systems and can, moreover, be readily e!tended to describe the

behaviour of binary adsorbate systems.

Hangmuir isotherm model is based on the following assumptionC

2. Surface is homogeneous, that is adsorption energy is constant over all sites

4. Adsorption on surface is localised, that is adsorbed atoms or molecules are

adsorbed at definite, localised sites

8. Kach site can accommodate only one molecule or atom.

The adsorption isotherm derived by Hangmuir for the adsorption of a solute from a liquid

solution is

eA

eAme

CK

CKQQ

+=

2or

+

=

memAe QCQKQ

2222"#.2'

Where,

eQ 6 Amount of adsorbate adsorbed per unit amount of adsorbent at

equilibrium/

mQ 6 Amount of adsorbate adsorbed per unit amount of adsorbent required for

monolayer adsorption "limiting adsorbing capacity'/

AK 6 onstant related to enthalpy of adsorption/

eC 6 oncentration of adsorbate solution at equilibrium.

4.6.1.2 Freundlich Isotherm

The 0reundlich isotherm is derived by assuming a heterogeneous surface with a non-uniform distribution of heat of adsorption over the surface. The heat of adsorption in


8/16

many instances decreases in magnitude with increasing e!tent of adsorption. This decline

in heat of adsorption is logarithmic, implying that adsorption sites are distributed

e!ponentially with respect to adsorption energy. This isotherm does not indicate anadsorption limit when coverage is sufficient to fill a monolayer.


9/16

The equation that describes such isotherm is 0reundlich +sotherm, given as

neFe CKQ2

=or eFe C

nKQ ln

2lnln += "#.4'

Where, FK and n are the constants/

eC 6 the concentration of adsorbate solution at equilibrium.

The 0reundlich equation is most useful for dilute solutions over small concentration

ranges. +t is frequently applied to the adsorption of impurities from a liquid solution onto

activated carbon. A high M0and high Nn3 value is an indication of high adsorptionthroughout the concentration range. A low M0and high Nn3 indicates a low adsorption

throughout the concentration range. A low Nn3 value indicates high adsorption at strong

solute concentration "9emr et al., 4$22'.

4.6.1.3 Temkin IsothermTemin and )yBhev considered the effects of some indirect adsorbate&adsorbentinteractions on adsorption isotherms and suggested that because of these interactions theheat of adsorption of all the molecules in the layer would decrease linearly with coverage.

The Temin isotherm has been used in the following formC

'" ee CALnb

RTq = or '"'" ee CLn

b

RTALn

b

RTq += "#.8'

Where, ( 6b

RT

The adsorption data can be analyBed according to equation. A plot of qeversus lnCe

enables the determination of the constantsA andB. The constantB is related to the heatof adsorption "9emr et al., 4$22'.

4.6.2 ERROR ANALYSIS

The use of =4 is limited to solving linear forms of isotherm equations, which measures

difference between e!perimental data and theoretical data in linear plots only, but not the

errors in isotherm curves.

)urely, from a comparison of the correlation coefficients "=4 values' for the lineariBed

models, it can be seen that higher weightage is given to the higher Cevalue data points,

thus giving a better fit correlation to the higher Cevalue data points. Due to the inherentbias resulting from lineariBation, error functions of non-linear regression basis are

employed to evaluate the isotherm constants and compare them with the less accurate

lineariBed analysis values. Three different error functions of non-linear regression basinwere employed in this study to find out the best-fit isotherm model to the e!perimental

equilibrium data. The values of error functions used in the adsorption of phenol using

banana peel activated carbon are given in table :.4.


10/16

4.6.2.1 The Hybrid Fractional Error Function H!"#I$%

5O(=+D is given as

"#.#'

This error function was developed ")orter and >cMay, 2???' to improve the fit of the

A=K method at low concentration values. +nstead of n as used in A=K, the sum of thefractional errors is divided by "n-p' where p is the number of parameters in the isotherm

equation. The values of 5O(=+D error functions are given in Table :.4, for activated

carbon from banana peels.

4.6.2.2 &ar'uardt(s )ercent *tandard $e+iation &)*$%

>)SD has been used by a number of researchers in the field "Seidel and Pelbin, 2?I?'totest the adequacy and accuracy of the model fit with the e!perimental data. +t has some

similarity to the geometric mean error distribution, but was modified by incorporating the

number of degrees of freedom. This error function is given as

=

n

iimease

calcemease

q

qq

pn 2

4

,

,, '"22$$ "#.7'

4.6.2.3 *um o, the *'uares o, the Errors **E%

The Sum of the Squares of the Krrors "SSK' function is given as

"#.:'

Where, qe,calcis equilibrium capacity obtained by calculating from the model "mg&g'

qe,e!pis e!perimental data of the equilibrium capacity "mg&g'

n is the number of data points

4.6.2.4 *um o, the -bsolute Errors *-E%

The Sum of the Absolute Krrors "SAK' function is given as

"#.


11/16

4.6.2. -+erage #elati+e Error -#E%

The Average =elative Krror "A=K' function attempts to minimiBe the fractional errordistribution across the entire concentration range

The Average =elative Krror "A=K' function is given as

"#.I'


qe,e!pis e!perimental data of the equilibrium capacity "mg&g'

n is the number of data points.

4.6.2.6 /hi0*'uare Error Function

The hi-square test statistic is basically the sum of the squares of differences between the

e!perimental data and data obtained by calculating from models, with each squared

difference divided by the corresponding data obtained by calculating from the models.The equivalent mathematical statement isC

4

,e!p ,4

,

" 'e e calc

e calc

q q

q

= "#.?'


qe,e!pis e!perimental data of the equilibrium capacity "mg&g'.

+f data from the model are similar to the e!perimental data, 4 will be a small number,

while if they differ,4 will be a bigger number. Therefore, it is necessary to analyBe thedata set using the non-linear hi-square test to confirm the best-fit isotherm for the

sorption system.

3./ ADSORPTION PRACTICES

Adsorption systems are run either on batch or on continuous basis. The following

te!t gives a brief account of both types of systems as in practice.

3./.1 ATCH ADSORPTION SYSTEMS

+n a batch adsorption process, the adsorbent is mi!ed with the solution to be

treated in a suitable reaction vessel for the stipulated period of time, until the

concentration of adsorbate in solution reaches an equilibrium value. Agitation is

( ),e!p ,2 ,e!p

2$$ n e e calc

i ei

q qARE

n q=

=


12/16

generally provided to ensure proper contact of the two phases. After the equilibrium is

attained the adsorbent is separated from the liquid through any of the methods available

lie filtration, centrifugation or settling. The adsorbent can be regenerated and reused

depending upon the need.

3./.2 CONTIN+O+S ADSORPTION SYSTEM

The continuous flow processes are usually operated in fi!ed bed adsorption columns.

These systems are capable of treating large volumes of waste wasters and are widely used

for treating domestic and industrial wastewaters. They may be operated either in the up

flow columns or down flow column. ontinuous counter current columns are generally

not used for wastewater treatment due to operational problems.

0luidiBed beds have higher operating costs hence are not common in use. Wastewater

usually contains several compounds which have different properties and which are

adsorbed at different rates. (iological reactions occurring in the column may also

function as filter bed, retaining solids entering with the feed. As a result of these and

other complicating factors, laboratory or pilot plant studies on specific wastewater to be

treated should be carried out. The variables to be e!amined include type of adsorbent,

liquid feed rate, solute concentration in feed and height of adsorbent bed.

3.0 FACTORS CONTROLLIN ADSORPTION

The amount of adsorbate adsorbed by an adsorbent from aqueous solution is

depend upon a number of factors which are discussed below.

9ature of Adsorbent

Adsorbent dose

p5 of Solution

ontact Time

+nitial oncentration of Adsorbate

Temperature


13/16

Degree of Agitation.

3.0.1 NAT+RE OF ADSORENT

The adsorption capacity of an adsorbent depends upon its physicochemical

characteristics, specific surface area and its affinity to adsorbate. Adsorption capacity is

directly proportional to the e!posed surface of the adsorbent. 0or the non-porous

adsorbents, the adsorption capacity is inversely proportional to the particle diameter

whereas for porous material it is practically independent of particle siBe. 5owever, for

porous substances particle siBe affects the rate of adsorption "Srivastava et al., 4$2$'.

3.0.2 ADSORENT DOSE

The removal increases rapidly with an increase in the adsorbent dose. An increase in the

sorption with an increase in the adsorbent dose can be attributed to the increase in the

mesoporous surface area available for sorption and hence, the availability of more

adsorption sites. 5owever, the unit adsorption decreases with an increase in dose "m'.

The decrease in sorption capacity per unit weight of adsorbent is because of the fact that

an increase in the sorbent dose at a constant concentration and volume leads to the

saturation of sorption sites through the sorption process "Shula et al., 4$$4/ Ou et al.,

4$$8/ Hataye et al., 4$$:/ Hataye et al., 4$$


14/16

3.0.4 CONTACT TIME

+n physical adsorption, most of the adsorbate species are adsorbed within a short

interval of contact time. 5owever, strong chemical binding of adsorbate with adsorbent

requires a longer contact time for the attainment of equilibrium. Available adsorptionresults reveal that the uptae of adsorbate species is fast at the initial stages of the

contact period, and thereafter, it becomes slower near the equilibrium. +n between these

two stages of the uptae, the rate of adsorption is found to be nearly constant. This may

be due to the fact that a large number of active surface sites are available for adsorption at

initial stages and the rate of adsorption is a function of available vacant site. During the

course of adsorption, the concentration of the available vacant sites decreases and the

repulsion between solute molecules on the surface and solution increases thereby

reducing the adsorption rate "Srivastava et al., 4$2$'.

3.0.* INITIAL CONCENTRATION OF ADSORATE

A given mass of adsorbent can absorb only a fi!ed amount of adsorbate. So the

initial concentration of the adsorbate in the solution is very important. The Q removal

in adsorption decreases with an increase in adsorbate initial concentration. (ut the

adsorption capacity of the adsorbent increases with an increase in initial concentration

because, the resistance to the uptae of solute from the solution decreases with an

increase in the solute concentration also the driving force is increases with increasing

concentration "Hataye et al. 4$$I'.

3.0., TEMPERAT+RE

Temperature dependence of adsorption is of comple! nature. Adsorption

processes are generally e!othermic in nature and the e!tent and rate of adsorption in most

cases decreases with increasing temperature. This trend may be e!plained on the basis ofrapid increase in the rate of desorption or alternatively e!plained on the basis of He-

hatelierRs principle. Some of the adsorption studies show increased adsorption with

increasing temperature. This increase in adsorption is mainly due to an increase in

number of adsorption sites caused by breaing of some of the internal bonds near the

edge of the active surface sites of the adsorbent. When the adsorption process is


15/16

controlled by the diffusion process, then the sorption capacity increases with an increase

in temperature due to endothermicity of the diffusion process. An increase in temperature

results in an increased mobility of the ions and a decrease in the retarding forces acting

on the diffusing ions. This results in enhancement in the sorption capacity of the

adsorbents "Shrivastav et al., 4$2$'.

3.0./ DEREE OF AITATION

Agitation in batch adsorption is important to ensure proper contact between the adsorbent

and the adsorbate in the solution. At lower agitation speeds, the stationary liquid film

around the particle is thicer and the resistance to transport is large. Thus, the process is

e!ternal mass transfer controlled. With the increase in agitation "or proper mi!ing' this

film thicness decreases and the film resistance to mass transfer gets reduced and after a

certain agitation speed, the process becomes intraparticle diffusion controlled. Whatever

is the e!tent of agitation, the solution inside the pores remains unaffected and hence, for

intraparticle mass transfer controlled processes agitation has no effect on the rate of

adsorption.

The adsorption isotherm is the most e!tensively employed method for representing

the equilibrium states of an adsorption system. +t can give useful information regarding

the adsorbate, the adsorbent, and the adsorption process. +t helps in the determination ofthe surface area of the adsorbent, the volume of the pores, and their siBe distribution, the

heat of adsorption, and the relative absorbability of a gas or a vapor on a given adsorbent.

"(ansal and Poyal'


16/16

Documents

Adsorption Fundamentals