Adsorption

Adsorption is a process that occurs when a liquid or gas (called adsorbate) accumulates on the surface of a solid or liquid (adsorbent), forming a molecular or atomic film (adsorbate). It is different from absorption, where a substance diffuses into a liquid or solid to form a "solution". The term sorption encompasses both processes, while desorption is the reverse process.

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. In a bulk material, all the bonding requirements (be they ionic, covalent or metallic) of the constituent atoms of the material are filled. But atoms on the (clean) surface experience a bond deficiency, because they are not wholly surrounded by other atoms. Thus it is energetically favourable for them to bond with whatever happens to be available. The exact nature of the bonding depends on the details of the species involved, but the adsorbed material is generally classified as exhibiting physisorption or chemisorption.

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: $\frac{x}{m}=kP^{\frac{1}{n}}$ , 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. The function has an asymtotic maximum. As the temperature increases, the adsorbed quantity rises more slowly and more pressure is required to achieve the maximum.

Langmuir isotherm

In 1916, Irving Langmuir published a new isotherm for gases adsorbed on solids, which retained his name. It is an empirical isotherm derived from a proposed kinetic mechanism. It is based on four hypotheses:

The surface of the adsorbent is uniform, that is, all the adsorption sites are equal.
Adsorbed molecules do not interact.
All adsorption occurs through the same mechanism.
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. The fourth condition is the most troublesome, as often more molecules can adsorb on the monolayer, but this problem is solved by the BET isotherm.

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, $\theta$ , as the fraction of the adsorption sites occupied, in the equilibrium we have

K=\frac{k}{k_{-1}}=\frac{\theta}{(1-\theta)P}

\theta=\frac{KP}{1+KP}.

For very low pressures $\theta\approx KP$ and for high pressures $\theta\approx1$

$\theta$ is difficult to measure experimentally; usually, the adsorbate is a gas and the adsorbed quantity is given at standard temperature and pressure (STP) volume per gram of adsorbent. Therefore, if we call v_mon the STP volume of adsorbate required to form a monolayer on the adsorbant (per gram of adsorbent too), $\theta = \frac{v}{v_\mathrm{mon}}$ and we obtain an expression for a straight line:

\frac{1}{v}=\frac{1}{Kv_\mathrm{mon}}\frac{1}{P}+\frac{1}{v_\mathrm{mon}}.

Through its slope and y-intercept we can obtain v_mon and K, which are constants for each adsorbent/adsorbate pair at a given temperature. v_mon is related to the number of adsorption sites through the ideal gas law. If we assume that the number of sites is just the whole area of the solid divided into the cross section of the adsorbate molecules, we can easily calculate the surface area of the adsorbent. Surface area of adsorbents depends on their structure, the more pores they have, the greater the area, which has a big influence on reactions on surfaces.

If more than one gas adsorbs on the surface, we call $\theta_E$ the fraction of empty sites and we have

\theta_E=\frac{1}{1+\sum_{i=1}^n K_iP_i}

and

\theta_j=\frac{K_jP_j}{1+\sum_{i=1}^n K_iP_i}

where i is each one of the gases that adsorb.

Frumkin isotherm

Frumkin isotherm is an extension of Langmuir isotherm. It states that adsorbed molecule do interact and affects further adsorption by either repulsion or attraction of molecules.

\delta G_{Frumkin}=\delta G_{Langmuir}-2g\Gamma_i

BET isotherm

Often molecules do form multilayers, that is, some are adsorbed on already adsorbed molecules and the Langmuir isotherm is not valid. In 1938 Stephan Brunauer, Paul Emmett and Edward Teller developed an isotherm that takes into account that possibility. The proposed mechanism is now:

A_(g) + S AS

A_(g) + AS A₂S

A_(g) + A₂S A₃S and so on

The derivation of the formula is more complicated than Langmuir's (see links for complete derivation). We obtain:

\frac{x}{v(1-x)}=\frac{1}{v_\mathrm{mon}c}+\frac{x(c-1)}{v_\mathrm{mon}c}.

x is the pressure divided into the vapour pressure for the adsorbate at that temperature, v is the STP volume of adsorbed adsorbate, v_mon 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. The biggest step in BET isotherm is to consider that the successive equilibria for all the layers except for the first are equal to the liquefaction 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:

\left( \frac{\partial \ln K}{\partial \frac{1}{T}} \right)_\theta=-\frac{\Delta H}{R}.

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 $\Delta H_\mathrm{ads}=\Delta H_\mathrm{liq}-RT\ln c$ , 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. The adsorbents must also have a distinct macropore structure which enables fast transport of the gaseous vapours.

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 (< 400 °C) amorphous form of SiO₂. It is prepared by the reaction between sodium silicate and sulphuric acid, which is followed by a series of after-treatment processes such as aging, pickling, etc. These after treatment methods results in various pore size distributions on its surface.

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. Zeolites are polar in nature.

They are manufactured by hydrothermal synthesis of sodium aluminosilicate in an autoclave followed by ion exchange with certain cations (Na+, Li+, Ca++, K+). The channel diameter of zeolite cages usually ranges from 2-9 Å. This process is followed by drying of microcrystals, which are palletized with a binder, to form macropores and thermally activated at a temperature of 6500C.

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. This is done by treating the zeolite with steam at elevated temperatures, greater than 500° (1000 °F). This high temperature heat treatment breaks the aluminum-oxygen bonds and the aluminum atom is expelled from the zeolite framework.

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. They are non-polar and cheap. 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 manufacturing process consists of two phases, carbonization and activation. The carbonization process includes drying and then heating to separate by-products, including tars and other hydrocarbons, from the raw material, as well as to drive off any gases generated. The carbonization process is completed by heating the material at 400–600 °C in an oxygen-deficient atmosphere that cannot support combustion.

The carbonized particles are “activated” by exposing them to an activating agent, such as steam at high temperature. The steam burns off the decomposition products from the carbonization phase to develop a porous, three-dimensional graphite lattice structure. The size of the pores developed during activation is a function of the time that they are exposed to the steam. Longer exposure times result in larger pore sizes. The most popular aqueous phase carbons are bituminous based because of their hardness, abrasion resistance, pore size distribution, and low cost, but their effectiveness needs to be tested in each application to determine the optimal product.

Activated carbom is used for adsorption of organic substances and non-polar adsorptives and it is also usually used for waste gas (and waste water) treatment. It is the most widely used adsorbent. Its usefulness derives mainly from its large micropore and mesopore volumes and the resulting high surface area.

Activated carbons are complex products which are difficult to classify on the basis of their behaviour, surface characteristics and preparation methods. However, some broad classification is made for general purpose based on their physical characteristics.

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. Thus they present a large internal surface with a small diffusion distance. PAC is made up of crushed or ground carbon particles, 95–100% of which will pass through a designated mesh sieve or sieves. 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. PAC is not commonly used in a dedicated vessel, owing to the high headloss that would occur. PAC is generally added directly to other process units, such as raw water intakes, rapid mix basins, clarifiers, and gravity filters.

Granulated activated carbon

Granulated activated carbon have a relatively larger size of particles compared to powdered activated and consequently, present a smaller external surface. Diffusion of the adsorbate is thus an important factor. These carbons are therefore preferred for all adsorption of gases and vapours as their rate of diffusion are faster. Granulated carbons are used for water treatment, deodorisation and separation of components of flow system. GAC can be either in the granular form or extruded. GAC is designated by sizes such as 8x20, 20x40, or 8x30 for liquid phase applications and 4x6, 4x8 or 4x10 for vapour phase applications. A 20x40 carbon is made of particles that will pass through a U.S. Standard Mesh Size No. 20 sieve (0.84 mm) (generally specified as 85% passing) but be retained on a U.S. Standard Mesh Size No. 40 sieve (0.42 mm) (generally specified as 95% retained). AWWA (1992) B604 uses the 50-mesh sieve (0.297 mm) as the minimum GAC size. 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. These spheres are contacted with solution naphtha, which extracts naphthalene and creates a porous structure. These porous spheres are then heated at temperatures between 100 and 4000C in the presence of an oxidising gas containing about 30% of oxygen by weight. The oxidised spheres are then heated between 150 and 7000C in the presence of ammonia to introduce nitrogen into spheres which are then activated in steam or CO₂. The spheres have high mechanical strength and excellent SO₂ and NO₂ adsorption capacity.

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. The resulting carbon is useful for homoperfusion.

Properties of activated carbon

Iodine Number

It is most fundamental parameter used to characterize activated carbon performance. It is measure of activity level (higher number indicates higher degree of activation). It is measure of the micropore content of the activated carbon (0 – 20 Å). It is equivalent to surface area of activated carbon in sq m/g between 900 – 1100. It is standard measure for liquid phase applications.

Apparent Density

Higher density provides greater volume activity and normally indicates better quality activated carbon.

Hardness/Abrasion Number

It is a measure of the activated carbon’s resistance to attrition. It is important indicator of activated carbon to maintain its physical integrity and withstand frictional forces imposed by backwashing etc.

Ash Content

It reduces the overall activity of activated carbon. It reduces the efficiency of reactivation. The metals (Fe2O3) can leach out of activated carbon resulting in discoloration. Acid/water soluble ash content is more significant than total ash content

Examples of adsorption

Heterogeneous catalysis

The most commonly encountered form of chemisorption in industry, occurs when a solid catalyst interacts with a gaseous feedstock, the reactant/s. The adsorption of reactant/s to the catalyst surface creates a chemical bond, altering the electron density around the reactant molecule and allowing it to undergo reactions that would not normally be available to it.

Adsorption refrigeration

Adsorption 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 adsorbent acts as a ‘chemical compressor” driven by heat. It consists of a solar collector, a condenser or heat-exchanger and an evaporator that is placed in a refrigerator box. The inside of the collector is lined with an adsorption bed packed with activated carbon absorbed with methanol. The refrigerator box is insulated filled with water. 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. In desorption, the liquid methanol adsorbed in the charcoal heats up and vaporizes. The methanol vapor condenses and is stored in the evaporator.

At night, the collector temperature decreases to the ambient temperature, and the charcoal adsorbs the methanol from the evaporator. The liquid methanol in the evaporator vaporizes and adsorbs the heat from the water contained in the trays. Since adsorption is a process of releasing heat, the collector must be cooled efficiently at night. As mentioned above, the adsorption refrigeration system operates in an intermittent way to produce the refrigerating effect.

Surface Enhanced Raman Spectroscopy ^*

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 - characterisitc of the molecule which is adsorbed. If the surface plasmon wave of the enhancing surface is of a specific frequency on the excitation laser used super enhancement can be achieved and is known as SERRS - Surface Enhanced Raman Resonance Spectroscopy.

Adsorption in viruses

Adsorption is the first step in the viral infection cycle. The next steps are penetration, uncoating, synthesise (transcription if needed, and translation), and release. The virus replication cycle is similar if not the same for all types of viruses. Factors such as transcription, may or may not be needed if the virus is able to integrate its genomic information in the cell's nucleus or if the virus can replicate itself directly within the cell's cytoplasm.

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