Absorption is a process in which atoms, molecules, or ions enter into a bulk phase – liquid or solid material. Meanwhile, adsorption is the process in which gas, liquids, or dissolved solids loosely adhere or stick to the surface of another substance – solid or liquid.
Absorption and adsorption are both sorption processes — referring to a physical or chemical process through which one substance attaches to another. However, they differ mainly on the driving force.
The driving force for adsorption is unsaturated forces at the solid surface which can form bonds with the adsorbate. These forces are typically electrostatic or van der Waals interactions (reversible). Stronger interactions involve direct electron transfer between the sorbate and the sorbent (irreversible). Meanwhile, the driving force for absorption is the concentration difference between the two distinct phases.
Absorption involves the mass transfer of particles across the bulk phase of another material. The substance that gets absorbed is called absorbate, while the substance that absorbs is called the absorbent. There are two types of absorption depending on the nature of the interaction between the absorbate and absorbent – physical and chemical.
This involves a chemical reaction between the absorbate and absorbent. This process is useful in purifying chemical mixtures and preventing unwanted components from entering the final product.
An example of this is the acid-gas removal in the petroleum industry. Natural gas is purified by passing it through an aqueous solution of ethanolamine. Generally, ethanolamine is used as a “scrubber” to remove acidic pollutants from waste gas streams (such as H2S and CO2) by chemical absorption.
This involves a gas-liquid mass transfer that occurs at the phase boundary. This process depends on physical properties like solubility, temperature, and pressure. It is also a non-reactive process e.g., when oxygen present in the air dissolves in water.
The video below shows what happens inside industrial absorption columns, particularly in scrubber systems used to separate air pollutants from the flue gas
Adsorption involves the adhesion of particles unto the surface of a substance only. The adsorbed substance is called the adsorbate, while the substance unto which it is adsorbed is known as an adsorbent. Based on the strength of interaction between the adsorbate and adsorbent, there are two types of adsorption – physisorption and chemisorption.
Adsorbate molecules are held at the surface of the adsorbent by weak intermolecular forces. Due to its weak binding caused by Van der Waals interactions, physisorption is considered a reversible process, and generally, a multilayer formation is observed in this process. It is also characterized by low enthalpy of adsorption.
This is highly specific in nature and takes place only if there is a chemical bonding between adsorbent and adsorbate. Contrary to physisorption, only monolayer formation is observed. Due to its strong driving forces, chemisorption is irreversible and is characterized by high enthalpy of adsorption.
The video below shows how activated carbon, a widely used adsorbent in the industry, separates impurities by surface adhesion.
Activated carbon is used in adsorption processes because it is a highly porous material, has variable surface chemical composition, and provides a large surface area to which contaminants may adsorb.
Chromatographic techniques are based on four different sorption mechanisms — surface adsorption, partition, ion exchange, and size exclusion. Chromatography carbon is based on adsorption mechanisms involving solute molecules being bonded directly to the surface of the stationary phase.
Ion-exchange is neither an adsorption process nor absorption. It has a different mechanism of action involving a chemical reaction in which free mobile ions of a solid (the ion exchanger), are exchanged for different ions of similar charge in solution.
Yes, adsorption and absorption can happen simultaneously. This occurrence is known as sorption. A typical example is a sponge dipped in water. Water molecules form a film on the sponge’s layer, and after some time, start getting inside the sponge’s bulk.
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