Adsorption surface phenomenon

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Adsorption

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The concentration of gases, liquids or dissolved substances (adsorbate) on the surface of solids (adsorbent)

Reverse of adsorption is called desorptionSorption is combination of both

The surface of a solid shows a strong affinity for molecules that come into contact with it.Certain solid materials concentrate specific substances from a solution onto their surfaces.

Adsorption

Physical adsorption (physisorption): Physical attractive forces (van der Waals forces)e.g. Carbon, Activated aluminaAdsorption

PhenomenonChemical adsorption (chemisorption): the adsorbed molecules are held to the surface by covalent forces.(little application in ww treatment)

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Sorbent Materials• Activated Carbon• Activated Alumina

• Silica Gel• Molecular Sieves (zeolite)

Polar and Non-polar adsorbentsProperties of Activated Carbon

Bulk Density 22-34 lb/ft3

Heat Capacity 0.27-0.36 BTU/lboF

Pore Volume 0.56-1.20 cm3/g

Surface Area 600-1600 m2/g

Average Pore Diameter 15-25 Å

Regeneration Temperature (Steaming)

100-140 oC

Maximum Allowable Temperature

150 oC

Factors effecting Carbon Adsorption

• Concentration of adsorbate in the liquid phase (solution)• Characteristics of the liquid phase

(pH, temperature)• Contact time• Increasing solubility of the solute in the liquid carrier

decreases adsorbability• Branched chains are usually more adsorbable than

straight chains

Factors effecting Carbon Adsorption

• Substituent groups (hydroxyl, amino, carbonyl groups, double bonds)

• Molecules with low polarity are more sorbable than highly polar ones.

ADSORPTION ISOTHERM

• A mathematical equation, which describes the relationship between pressure (p) of the gaseous adsorbate and the extent of adsorption at any fixed temperature, is called adsorption isotherm.

• The extent of adsorption is expressed as mass of the adsorbate adsorbed on one unit mass of the adsorbent.

• Thus, if x g of an adsorbate is adsorbed on m g of the adsorbent, then

Extent of adsorption = x/m

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Freundlich adsorption isotherm(Freundlich adsorption isotherm is obeyed by the adsorptions where the adsorbate forms a

monomolecular layer on the surface of the adsorbent.

x/m = kp1/n (Freundlich adsorption isotherm) or

log x/m = log k + 1/n log P

where x is the weight of the gas adsorbed by m gm of the adsorbent at a pressure p, thus x/m represents the amount of gas adsorbed by the adsorbents per gm (unit mass), k and n are constant at a particular temperature and for a particular adsorbent and adsorbate (gas), n is always greater than one, indicating that the amount of the gas adsorbed does not increase as rapidly as the pressure.

(ii) At low pressure, the extent of adsorption varies linearly with pressure. x/m p'∝(ii) At high pressure, it becomes independent of pressure. x/m p∝ 0

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Drawbacks of Freundlich

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it fails at high pressure of the gas

Langmuir - adsorption isotherms

(a) Adsorption takes place on the surface of the solid only till the whole of the surface is completely covered with a unimolecular layer of the adsorbed gas.

(b) Adsorption consists of two opposing processes, namely Condensation of the gas molecules on the solid surface and Evaporation (desorption)of the gas molecules from the surface back into the gaseous phase.

(c) The rate of condensation depends upon the uncovered (bare) surface of the adsorbent available for condensation. Naturally, at start when whole of the surface is uncovered the rate of condensation is very high and as the surface is covered more and more, the rate of condensation progressively decreases. On the contrary, the rate of evaporation depends upon the covered surface and hence increases as more and more of the surface is covered ultimately an equilibrium will be set up at a stage when the rate of condensation becomes equal to the rate of evaporation (adsorption equilibrium).

(d) The rate of condensation also depends upon the pressure of the gas since according the kinetic theory of gases, the number of molecules striking per unit area is proportional to the pressure.

Mathematically, x/m = ap/1+bp, where a and b are constants and their value depends upon the nature of gas (adsorbate), nature of the solid adsorbent and the temperature. Their values can be determined from the experimental data.

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It relates the coverage or adsorption of molecules on a solid surface to gas pressure or concentration of a medium above the solid surface at a fixed temperature. The equation was developed by Irving Langmuir in 1916. The equation is stated as:

θ is the fractional coverage of the surface, P is the gas pressure or concentration, α is a constant.

The constant α is the Langmuir adsorption constant and increases with an increase in the binding energy of adsorption and with a decrease in temperature.

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Langmuir - adsorption isotherms

P/x= 1/K’ + 1/k’’

At low pressure

The factor P/k’’ nay be ignored and the isotherm assumed the form

x= K’P

At high pressure the factor 1/k’’ may be ignored and isotherm become

X= k’’

It indicated at low pressure x is proportional to P where as at high pressure it is independent of P because adsorbent surface is completely covered with a unimolecular layer of gas.

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Langmuir equation

Application of adsorption(1) In the preparation of gas masks using activated charcoal to avoid

poisonous gases like etc.

(2) Froth floatation method used for concentration of sulphide ores is based on adsorption.

(3) Silica gel can be used to remove moisture and to control humidity.

(4) Ion exchange method used to soften water is based on adsorption. 

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(5) Adsorption chromatography is used to purify and separate pigments, harmones etc.

(6) Charcoal powder can remove coloured impurities from sugar or any colored solution

(7) Charcoal is used for making high vacuum.

(8) The cleaning action of soaps and detergents.

(9) Formation of stable emulsions in cosmetics and syrups etc.

(10) In heterogeneous catalysis.

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