The extraction of a component from one phase into another phase, usually by chemical interaction (chemisorption) between the material adsorbed (the adsorbate) and the surface of the adsorbing material (the adsorbent). Sometimes, the adsorbate is incorporated into the structure of the adsorbent; an example is the adsorption of hydrogen gas by palladium, which can adsorb several hundred times its own volume of the gas.
Adsorption is a process that occurs when a gas or liquid or solute (called adsorbate) accumulates on the surface of a solid or more rarely a liquid (adsorbent), forming a molecular or atomic film (adsorbate).
Adsorption is operative in most natural physical, biological, and chemical systems, and is widely used in industrial applications such as activated charcoal, synthetic resins and water purification. Adsorption, ion exchange and chromatography are sorption processes in which certain adsorptives are selectively transferred from the fluid phase to the surface of insoluble, rigid particles suspended in a vessel or packed in a column.
Similar to surface tension, adsorption is a consequence of surface energy.
Adsorption isotherms
Adsorption is usually described through isotherms, that is, functions which connect the amount of adsorbate on the adsorbent, with its pressure (if gas) or concentration (if liquid).
The first isotherm is due to Freundlich and Küster (1894) and it is a purely empirical formula valid for gaseous adsorbates: , where x is the adsorbed quantity, m is the mass of adsorbent, P is the pressure of adsorbate and k and n are empirical constants for each adsorbant-adsorbate pair at each temperature.
Langmuir isotherm
In 1916, Irving Langmuir published a new isotherm for gases adsorbed on solids, which retained his name. At the maximum adsorption, only a monolayer is formed: molecules of adsorbate do not deposit on other, already adsorbed, molecules of adsorbate, only on the free surface of the adsorbent.
These four points are seldom true: there are always imperfections on the surface, adsorbed molecules are not necessarily inert, the mechanism is clearly not the same for the very first molecules as for the last to adsorb.
Langmuir suggests that adsorption takes place through this mechanism: A(g) + S ⇌ AS, where A is a gas molecule and S is an adsorption site.
The direct and inverse rate constants are k and k-1. If we define surface coverage, θ, as the fraction of the adsorption sites occupied, in the equilibrium we have
orFor very low pressures and for high pressures
θ is difficult to measure experimentally; Therefore, if we call vmon the STP volume of adsorbate required to form a monolayer on the adsorbant (per gram of adsorbent too), and we obtain an expression for a straight line:
Through its slope and y-intercept we can obtain vmon and K, which are constants for each adsorbent/adsorbate pair at a given temperature.
If more than one gas adsorbs on the surface, we call θE the fraction of empty sites and we have
and
where i is each one of the gases that adsorb.
Frumkin isotherm
Frumkin isotherm is an extension of Langmuir isotherm.
δGFrumkin = δGLangmuir − 2gΓiBET isotherm
Often molecules do form multilayers, that is, some are adsorbed on already adsorbed molecules and the Langmuir isotherm is not valid. The proposed mechanism is now:
A(g) + S ⇌ AS A(g) + AS ⇌ A2S A(g) + A2S ⇌ A3S and so onThe derivation of the formula is more complicated than Langmuir's (see links for complete derivation). We obtain:
x is the pressure divided into the vapour pressure for the adsorbate at that temperature, v is the STP volume of adsorbed adsorbate, vmon is the STP volume of the amount of adsorbate required to form a monolayer and c is the equilibrium constant K we used in Langmuir isotherm multiplied by the vapour pressure of the adsorbate.
Langmuir isotherm is usually better for chemisorption and BET isotherm works better for physisorption.
Adsorption enthalpy
Adsorption is an exothermic process because energy is liberated, therefore enthalpy is always negative. Adsorption constants are equilibrium constants, therefore they obey van 't Hoff's equation:
As can be seen in the formula, the variation of K must be isosteric, that is, at constant coverage. If we start from BET isotherm and assume that the entropy change is the same for liquefaction and adsorption we obtain ΔHads = ΔHliq − RTlnc, that is to say, adsorption is more exothermic than liquefaction.
Adsorbents
Characteristics and general requirements
The adsorbents are used usually in the form of spherical pellets, rods, moldings or monoliths with hydrodynamic diameter between 0.5 and 10 mm. They must have high abrasion resistance, high thermal stability and small micropore diameter, which results in higher exposed surface area and hence high capacity of adsorption.
Different types of industrial adsorbents used are:
Oxygen-containing compounds – hydrophilic / polar such as silica gel and hydrophilic zeolites
Carbon-based compounds – hydrophobic / non-polar such as activated carbon
Polymer-based compounds-polar/non-polar functional groups in porous polymer matrix
Silica Gel
Silica gel is a chemically inert, nontoxic, polar and dimensionally stable (<
Silica is also used for drying of process air (e.g. Oxygen, natural gas etc) and adsorption of higher (polar) hydrocarbons from natural gas.
Zeolites
Zeolites are natural or synthetic aluminum silicates which form a regular crystal lattice and release water at high temperature.
They are manufactured by hydrothermal synthesis of sodium aluminosilicate in an autoclave followed by ion exchange with certain cations (Na+, Li+, Ca++, K+).
Zeolites are applied in drying of process air (only traces), CO2 removal from natural gas, CO removal from reforming gas and air separation.
Non-polar zeolites are synthesized by dealumination of polar zeolites.
Non-polar zeolites are used in non-polar organics removal.
Activated carbon
They are highly porous, amorphous solids consisting of microcrystallites with a graphite lattice. One of their main drawbacks is that they are combustible
Activated carbon can be manufactured from carbonaceous material, including coal (bituminous, subbituminous, and lignite), peat, wood, or nutshells (i.e., coconut).
The carbonized particles are “activated” by exposing them to an activating agent, such as steam at high temperature.
Activated carbon is used for adsorption of organic substances and non-polar adsorptives and it is also usually used for waste gas (and waste water) treatment.
Activated carbons are complex products which are difficult to classify on the basis of their behaviour, surface characteristics and preparation methods.
Powdered activated carbon
Traditionally, active carbons are made in particular form as powders or fine granules less than 100 mm in size with an average diameter between 15 and 25 mm. Granular activated carbon is defined as the activated carbon being retained on a 50- mesh sieve (0.297 mm) and PAC material as finer material, while ASTM classifies particle sizes corresponding to an 80-mesh sieve (0.177 mm) and smaller as PAC.
Granulated activated carbon
Granulated activated carbon have a relatively larger size of particles compared to powdered activated and consequently, present a smaller external surface. These carbons are therefore preferred for all adsorption of gases and vapours as their rate of diffusion are faster. The most popular aqueous phase carbons are the 12x40 and 8x30 sizes because they have a good balance of size, surface area, and headloss characteristics.
Spherical activated carbon
These are made of small spherical balls wherein pitch is melted in the presence of naphthalene or tetorlin and converted into spheres.
Impregnated carbon
Porous carbons containing several types of inorganic impregnant such as iodine, silver, cation such as Al, Mn, Zn, Fe, Li, Ca have also been prepared for specific application in air pollution control especially in museums and galleries. Silver loaded activated carbon is used as an adsorbent for purifications of domestic water. Drinking water can be obtained from natural water by treating the natural water with a mixture of activated carbon and flocculating agent Al(OH)3. Impregnated carbons are also used for the adsorption of H2S and mercaptans.
Polymers coated carbon
This is a process by which a porous carbon can be coated with a biocompatible polymer to give a smooth and permeable coat without blocking the pores.
Properties of activated carbon
Iodine NumberIt is most fundamental parameter used to characterize activated carbon performance. It is equivalent to surface area of activated carbon between 900 m²/g and 1100 m²/g.
Apparent densityHigher density provides greater volume activity and normally indicates better quality activated carbon.
Hardness/abrasion numberIt is a measure of the activated carbon’s resistance to attrition.
Ash contentIt reduces the overall activity of activated carbon. Acid/water soluble ash content is more significant than total ash content
Examples of adsorption
Heterogeneous catalysisThe most commonly encountered form of chemisorption in industry, occurs when a solid catalyst interacts with a gaseous feedstock, the reactant/s.
Adsorption refrigerationAdsorption refrigeration and heat pump cycles rely on the adsorption of a refrigerant gas into an adsorbent at low pressure and subsequent desorption by heating. The inside of the collector is lined with an adsorption bed packed with activated carbon absorbed with methanol. The activated carbon can adsorb a large amount of methanol vapor in ambient temperature and desorb it at a higher temperature (around 100 degree Celsius). During the daytime, the sunshine irradiates the collector, so the collector is heated up and the methanol is desorbed from the activated carbon.
At night, the collector temperature decreases to the ambient temperature, and the charcoal adsorbs the methanol from the evaporator.
Surface enhanced Raman spectroscopy [SERS]SERS is totally dependent on the interactions between a usually metalic enhancing surface and the adsorbed analytes and leads to the amplification of the usually very weak emission of raman radiation—characteristic of the molecule which is adsorbed.
Adsorption in viruses
Adsorption is the first step in the viral infection cycle.
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