Adsorbent

An adsorbent is a solid material that presents a large surface area and porous structure to attract, hold, and store molecules from a gas or liquid on its surface, a process known as adsorption. Adsorbents differ from absorbents, which take up substances into a bulk phase; adsorption is a surface phenomenon.

Key properties include high surface area, controlled porosity, and chemical stability in the target environment. Pore size distribution (micropores, mesopores, macropores) and surface chemistry determine capacity and selectivity. Common adsorbents include activated carbon, silica gel, alumina, zeolites, clays, and ion-exchange resins. More recently, metal-organic frameworks and porous polymers have expanded options.

Adsorption mechanisms include physical adsorption (physisorption) driven by van der Waals forces and chemical adsorption (chemisorption) involving bond formation. Temperature, pressure, and moisture influence capacity and kinetics. Adsorbents are used in gas purification (removing CO2, water vapor, hydrocarbons), air and odor control, water treatment (organic contaminants, heavy metals), catalysis support, and separation processes such as dehydration and desalination by adsorption.

Regeneration and reuse are common: adsorbents can often be restored by heating (thermal desorption), reduced pressure, or solvent washing, enabling multiple cycles. Selection criteria include surface area, pore structure, chemical compatibility with the feed, mechanical strength, thermal stability, resistance to fouling, regeneration efficiency, and cost.

Limitations include finite capacity, ageing that reduces performance, and selectivity limitations in mixed feeds. Proper design and pretreatment help maximize service life and economic viability.