Adsorption

Adsorption

·        The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption

·        Adsorption is essentially a surface phenomenon.

·        In adsorption, the substance is concentrated only at the surface.

·        Adsorbate-The molecular species or substance, which concentrates or accumulates at the surface is termed adsorbate.

·        Adsorbent- the material on the surface of which the adsorption takes place is called adsorbent.

·        Desorption- It is the process of removing an adsorbed substance from a surface on which it is adsorbed.

·        O2, H2, Cl2, NH3 gases are adsorbed on the surface of charcoal.

·        In a solution of an organic dye, say methylene blue, when animal charcoal is added and the solution is well shaken, it is observed that the filtrate turns colourless. The molecules of the dye, thus, accumulate on the surface of charcoal, i.e., are adsorbed.

·        Aqueous solution of raw sugar, when passed over beds of animal charcoal, becomes colourless as the colouring substances are adsorbed by the charcoal.

·        The air becomes dry in the presence of silica gel because the water molecules get adsorbed on the surface of the gel.

·        Charcoal, silica gel, metals such as Ni, Cu, Ag, Pt and colloids are some adsorbents.

 

·        Sorption: When adsorption and absorption take place simultaneously, it is called sorption.

·        Adsorption of hydrogen over Pt is called occlusion.

Distinction between Adsorption and Absorption

Adsorption

Absorption

Accumulation of the molecular species (ions, atoms, or molecules) at the surface of the materials rather than in the bulk especially on solids or liquids is known as adsorption

Accumulation of substances (gas, liquids, or dissolved solids) throughout the bulk of the material, especially liquids and gases is called absorption

The adsorbent has vacant spaces that stimulate the adhesion of particles onto the surface

Absorption occurs due to the availability of molecular spaces and the nature of the particles

It is a surface phenomenon

It is a bulk phenomenon

The adsorbate (adsorbed material) remains attached to the adsorbent with either Van der Wall’s forces or covalent bonds

The absorbate (absorbed material) remains in the absorbent without having any chemical interactions with the absorbent

Adsorption is an exothermic process

Absorption is an endothermic process

It is a temperature-dependent process. Low temperature favors adsorption

It is not affected by temperature

The adsorbate is more concentrated on the surface than the other parts of the adsorbent after adsorption

The concentration of the absorbate in the absorbent is uniform after absorption

The adsorption steadily increases and reaches equilibrium eventually

The absorption process occurs uniformly

Adsorbed material (adsorbate) can be separated by passing new substance through the surface of the adsorbent (adsorbing material), which replaces the previously adsorbed material

Absorbing material (absorbate) can be separated into different phases based on its chemical interaction with the phases

Examples include water purification, gas masks, chromatographic analysis, etc

Examples include cold storage, ice production, etc

Mechanism of Adsorption

The molecules or atoms that make up the majority of the adsorbent are symmetrically surrounded by other atoms or molecules. As a result, it will have no net attractive force. However, since the surface molecules are not symmetrically surrounded, they have some residual force due to the valence force. Adsorption occurs at the surface of the adsorbent due to the residual force remaining at the surface.

Types of Adsorption

·        Physical adsorption

·        Chemical adsorption

·        

physisorption

Physical adsorption, also known as physisorption, is due to weak Van der Waals forces between adsorbate and adsorbent. For example, adsorption of gases like hydrogen or nitrogen on the surface of charcoal, etc.

chemisorption

Chemical adsorption, also known as chemisorption, is due to strong chemical forces (bonding) between adsorbate and adsorbent. For example, adsorption of hydrogen or nitrogen on the surface of adsorbent like ferrous catalyst at high temperatures, etc.

 

Physical adsorption

Chemical adsorption or chemisorption

If the adsorbate is held on a surface of adsorbent by weak van der Waals’ forces, the adsorption is called physical adsorption or physisorption.

 If the forces holding the adsorbate are as strong as in chemical bonds, the adsorption process is known as chemical adsorption of chemisorption.

It is non-specific.

It is highly specific.

It is reversible.

It is irreversible.

It depends on the nature of gas. More easily liquefiable gases are adsorbed readily.

Easily liquefiable gases like NH3, CO2, gas adsorbed to greater extent than H2 and He.

Higher the critical temperature of gas, more will be the extent of adsorption.

 It also depends on the nature of gas. Gases which can react with the adsorbent show chemisorption.

 The extent of adsorption increases with increase in surface area, e.g. porous and finely divided metals are good adsorbents.

It also increases with increase in surface area.

It has low enthalpy of adsorption (20 – 40 kJ /mol).

It has  high enthalpy heat of adsorption (180 – 240 kJ/mol).

Low temperature is favourable.

High temperature is favourable.

No appreciable activation energy is needed.

High activation energy is sometimes needed.

It results into multimolecular layers on adsorbent surface under high pressure.

It results into unimolecular layer.

 

Adsorption Isotherms

The variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature can be expressed by means of a curve is termed as adsorption isotherm.

Freundlich Adsorption isotherm: Freundlich gave an empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature.

 

Where x- mass of the gas adsorbed on mass m of the adsorbent and the gas at a particular temperature k and n depends upon the nature of gas

Taking logarithm on both sides, we get,

If we plot a graph between log  and log P, we get a straight line.

Observation

(i) At low pressure, the graph is almost straight line which indicates that x/m is directly proportional to the pressure.

x/m pz

x/m = Kp

where K is constant.

(ii) At high pressure, the graph becomes almost constant which means that x/m becomes independent of pressure. This may be expressed as:

x/m = constant

x/m p0

x/m = K p0

(iii) Thus, in the intermediate range of pressure, x/m will depend upon the power of pressure which lies between 0 to l i.e., fractional power of pressure (probable range 0.1 to 0.5).

This may be expressed as

x/m p1/n

x/m = Kp1/n

where n can take any whole number value which depends upon the nature of adsorbate and adsorbent. The above relationship is also called Freundlich’s adsorption isotherm.

Limitations of Freundlich Isotherm

Freundlich isotherm only approximately explains the behavior of adsorption. The value of 1/n can be between 0 and 1, therefore the equation holds good only over a limited range of pressure.

·         When 1/n = 0, x/m is constant, the adsorption is independent of pressure.

·         When 1/n =1, x/m = k P, i.e. x/m  P, adsorption is directly proportional to pressure.

 

Experimental results support both of the above-mentioned conditions. At high pressure, the experimental isotherms always seem to approach saturation. Freundlich isotherm does not explain this observation and therefore, fails at high pressure.

The Freundlich isotherm was followed by two other isotherms – Langmuir adsorption isotherm and BET adsorption isotherm. Langmuir isotherm assumed that adsorption is monolayer in nature whereas BET isotherm assumed that it is multi-layer.

Adsorption Isobars

With the increase in temperature at constant pressure the extent of adsorption (x/m) will decrease. The graph between the extent of adsorption and temperature at constant pressure is called adsorption isobar.

In case of chemisorption, the adsorption initially increases with rise in temperature and then decreases. Like all chemical reactions, some activation energy is required for chemisorption.

a) At low temperature, x/m is small.

b) As temperature is increased the molecules of the adsorbate gain energy and become equal to activation energy so that proper bonds are formed with the adsorbent molecules.

c) Therefore, initially amount of gas adsorbed increases with rise in temperature. Further

increase of temperature will increase the energy of molecules which have already been adsorbed.

d) This would increase the rate of desorption and, therefore, decrease the extent of adsorption.

The adsorption isobar graphs can be used to distinguish between physical and chemical adsorptions. In physical adsorption, there is a regular decrease as temperature increases. However, in chemisorption, there is an initial increase and then it decreases.

Adsorption from Solution Phase

Adsorption from solution phase is a process in which the molecules or ions of a solute are attracted and attached to the surface of a solid, liquid or gaseous substance, called an adsorbent. The adsorption process occurs due to the difference in concentration of the solute molecules between the bulk solution and the adsorbent surface.

Adsorption from solution phase is a complex process that depends on various factors, such as the nature of the solute and the adsorbent, the temperature, pressure, and the chemical environment of the system. The process of adsorption from solution phase can be classified into two types, namely physical adsorption and chemical adsorption.

Physical Adsorption from Solution Phase

Physical adsorption, also known as physisorption, is a reversible process in which the solute molecules are attracted to the surface of the adsorbent by weak van der Waals forces. The process of physical adsorption from solution phase is similar to that from gas phase, but the concentration of the solute in the solution is the driving force for the adsorption.

In physical adsorption, the adsorbent surface provides a surface area where the solute molecules can attach themselves through weak attractive forces. The strength of the attractive forces depends on the nature of the solute and the adsorbent surface. The adsorption capacity of the adsorbent increases with increasing surface area and concentration of the solute in the solution.

Chemical Adsorption from Solution Phase

Chemical adsorption, also known as chemisorption, is a type of adsorption in which the solute molecules are attached to the adsorbent surface by chemical bonds. The process of chemical adsorption from solution phase is similar to that from gas phase, but the solute molecules must undergo chemical reactions with the adsorbent surface to form chemical bonds.

In chemical adsorption, the adsorbent surface provides a specific chemical environment that promotes the chemical reaction between the solute and the adsorbent surface. The chemical adsorption process is irreversible and results in the formation of a new chemical compound on the surface of the adsorbent.

Applications of Adsorption from Solution Phase

Adsorption from solution phase has various practical applications in industries, such as wastewater treatment, separation and purification of chemicals, and gas purification. The process of adsorption is used to remove impurities from the solution or gas phase by attaching them to the surface of an adsorbent material. Adsorption is also used to separate and purify chemicals based on their different adsorption properties.

 

Applications of Adsorption

Adsorption has a wide range of applications in various fields due to its ability to separate, purify, and remove impurities from liquids, gases, and solids. Here are some of the applications of adsorption:

1.     Water purification: Activated carbon is used to purify drinking water and remove impurities, such as chlorine, organic compounds, and unpleasant odors and flavors.

2.     Gas purification: Adsorption is used to remove impurities, such as moisture, carbon dioxide, and sulfur dioxide, from industrial gases, including natural gas, biogas, and hydrogen.

3.     Separation and purification of chemicals: Adsorption is used to separate and purify chemicals based on their different adsorption properties. This technique is commonly used in the pharmaceutical, food, and petrochemical industries.

4.     Catalysis: Adsorption is an essential step in heterogeneous catalysis, where a solid catalyst is used to speed up chemical reactions by adsorbing reactant molecules onto its surface.

5.     Gas storage: Adsorption is used for gas storage in porous materials, such as zeolites and activated carbon. The gas molecules are adsorbed onto the surface of the porous material, which increases the storage capacity of the material.

6.     Environmental remediation: Adsorption is used to remove pollutants and contaminants from soil and water. This technique is used in soil remediation, where adsorbents are used to remove organic and inorganic pollutants from contaminated soil.

7.     Energy storage: Adsorption is used in energy storage devices, such as batteries and supercapacitors. The adsorbent material is used to store energy by adsorbing charged particles, such as ions and electrons, onto its surface.