AdsorptionDesorption

Adsorption is the process by which atoms, ions, or molecules from a gas, liquid, or dissolved phase adhere to a solid surface, forming a thin film. Desorption is the reverse process, where adsorbed species leave the surface and re-enter the bulk phase.

Adsorption can be physical (physisorption) or chemical (chemisorption). Physisorption involves weak van der Waals forces, is usually reversible, and depends strongly on temperature and available surface area. Chemisorption involves stronger chemical bonds, can be more selective, and is often less easily reversible, potentially altering the surface.

Isotherms describe how adsorption varies with pressure or concentration at a constant temperature. The Langmuir isotherm models monolayer adsorption on a homogeneous surface with finite capacity. The Freundlich isotherm describes adsorption on heterogeneous surfaces and can accommodate multilayer uptake. The BET model extends to multilayer adsorption on porous materials and is widely used for surface area estimation.

Desorption mechanisms are the reverse of adsorption and can be driven by increasing temperature, lowering pressure, changing the solvent, or altering chemical conditions. Desorption is essential for regenerating adsorbents and enabling repeated use.

Thermodynamics and kinetics are central to adsorption studies. A negative Gibbs free energy change indicates spontaneous adsorption under given conditions. Kinetic models, such as pseudo-first-order and pseudo-second-order equations, describe uptake rates and help infer mechanisms. Enthalpy changes help distinguish physical from chemical adsorption.

Key factors influencing adsorption include surface area and porosity, pore size distribution, adsorbent structure, temperature, pressure or concentration, solution pH, ionic strength, and the presence of competing species.

Applications span environmental remediation (activated carbon, zeolites, metal-organic frameworks for dyes, metals, and organics), gas storage and separation, catalysis, chromatography, and sensing. Regeneration strategies include thermal desorption, solvent washing, and chemical treatment, with performance varies across cycles.