Adsorbent: Definition, Characteristics, Types and Examples

Adsorption is the ability of all solid things to attract molecules from the liquids or gases with which they come into contact to their surfaces. Adsorbents are solids that are used to adsorb dissolved chemicals or gases, and adsorbate is a term that refers to the molecules that are adsorbed by solids. Let's look at the case of charcoal. In gas masks, charcoal acts as an adsorbent, removing pollutants and poisons from the air or stream.

Key Takeaways: Adsorbent, Adsorption, Molecules, Emulsion, Catalysis, Crystals, Liquid, Solid, Amorphous liquids

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Adsorption

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Adsorption is the collection of molecules on a solid's internal or external surface, or on a liquid's surface. The collection of molecules by the external or internal surface (walls of capillaries or fissures) of solids or the surface of liquids is referred to as adsorption. You may be perplexed by the terms adsorption and absorption. The process of permeating the substance of blocks of amorphous liquids or solids, or the actual inner crystals, is known as absorption. When a solid absorbs a liquid or gas without defining a specific absorption or absorption method, this is referred to as sorption.

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Characteristics of Adsorption

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Adsorbents having a hydrodynamic radius of 0.25 to 5 mm are commonly utilised in the form of spherical pellets, rods, mouldings, or monoliths. They must be abrasion resistant, thermally stable, and have tiny pore sizes, resulting in a larger exposed surface area and hence a higher adsorption capacity. The adsorbents must also have a specific pore structure that allows gaseous vapours to move quickly.

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Types of Adsorption

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The majority of industrial adsorbents are divided into three categories:

Oxygen-containing compounds — These include materials like silica gel and zeolites, which are often hydrophilic and polar.

Carbon-based compounds — These include materials like activated carbon and graphite, which are typically hydrophobic and nonpolar.

Polymer-based chemicals — Depending on the functional groups in the polymer matrix, they can be polar or nonpolar.

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Example of Adsorbents

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Some of the commonly used adsorbents are:

Silica gel: Silica gel is an amorphous form of SiO₂ that is chemically inert, non-toxic, polar, and dimensionally stable (400 °C or 750 °F). It is formed by reacting sodium silicate with acetic acid, then going through a variety of post-processing steps like aging, pickling, and so on. The pore size distributions produced by these post-treatment procedures are diverse. Silica has been used in drying of process air (such as oxygen and natural gas) as well as adsorb heavy (polar) hydrocarbons from natural gas.

Zeolites: Zeolites are crystalline aluminosilicates with a repeating pore network that release water at high temperatures. They can be natural or manmade. In nature, zeolites are polar. They're made via ion exchange with particular cations (Na⁺, Li⁺, Ca²⁺, K⁺, NH⁴⁺) after hydrothermal synthesis of sodium aluminosilicate or another silica source in an autoclave. Zeolite cages typically have a channel diameter of 2 to 9 mm. The crystals are dried after the ion exchange process, and then pelletized with a binder to make macroporous pellets. Drying of process air, CO₂ removal from natural gas, CO removal from reforming gas, air separation, catalytic cracking, and catalytic synthesis and reforming are all applications for zeolites. Aluminum-free silica sources or deallumination of aluminum-containing zeolites are used to make non-polar (siliceous) zeolites. Delayumination is accomplished by processing zeolite with steam at high temperatures, often above 500 °C (930 °F). The aluminum-oxygen connections are broken by the high temperature heat treatment, and the aluminium atom is ejected from the zeolite framework.

Activated carbon: Activated carbon is a porous, amorphous material made up of microcrystallites with a graphite lattice that is typically sold as small pellets or powder. It's non-polar and inexpensive. One of its most significant disadvantages is that it interacts with oxygen at moderate temperatures (over 300 °C). Microporous type I behaviour in an activated carbon nitrogen isotherm. Carbonaceous materials such as coal (bituminous, subbituminous, and lignite), peat, wood, and nutshells can be used to make activated carbon (e.g., coconut). Carbonization and activation are the two processes of the manufacturing process. Activated carbon is commonly used for waste gas (and wastewater) treatment, as well as the adsorption of organic compounds and non-polar adsorbates.

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Things to Remember

• Adsorption is the ability of all solid things to attract molecules from the liquids or gases with which they come into contact to their surfaces.
• Adsorbents are solids that are used to adsorb dissolved chemicals or gases.
• The surface phenomena of adsorption is an exothermic process. It is temperature dependent, and the process will increase and reduce gradually at first.
• Adsorbent must be abrasion resistant, thermally stable, and have tiny pore sizes, resulting in a larger exposed surface area and hence a higher adsorption capacity.
• The adsorption technique is used to do heterogeneous catalysis. To separate noble gases, charcoal can be employed as an absorbent. The adsorption phenomenon is used in chromatographic analysis. Stable emulsions are formed through adsorption in syrups and cosmetics.

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