Adsorption is the accumulation of molecules, atoms, or ions at an interface, producing a concentration different from that in the surrounding bulk phase. It commonly occurs when gases or dissolved substances attach to solid surfaces, although liquid interfaces can also exhibit adsorption. The material providing the surface is the adsorbent, and the retained substance is the adsorbate. Adsorption differs from absorption, in which substances enter the bulk of another phase. This distinction remains important in porous solids: adsorption on internal pore walls is still a surface phenomenon. (goldbook.iupac.org)
Physical and chemical adsorption
Two principal mechanisms are distinguished. Physisorption, or physical adsorption, involves intermolecular interactions, especially van der Waals forces, without a substantial change in the electronic orbital structure of the interacting species. It can produce a single molecular layer, multiple layers, or extensive filling of narrow pores. Its behavior therefore depends on both surface interactions and pore geometry. (goldbook.iupac.org)
Chemisorption, or chemical adsorption, involves formation of a chemical bond between the adsorbate and the surface. The directly bonded species occupy a surface monolayer, although additional physically adsorbed material may occur above it. Chemical adsorption is central to the terminology and mechanisms of heterogeneous catalysis, where surface-bound species participate in reactions. The distinction between physical and chemical adsorption concerns the nature of bonding, rather than simply whether a substance can subsequently be removed. (goldbook.iupac.org)
Desorption is the reverse process: the amount of adsorbed substance decreases. It may be induced by changing pressure or temperature, or by introducing another substance that binds more strongly and displaces the original adsorbate. Reversible adsorption and desorption provide the basis for regenerating many industrial adsorbents. (old.goldbook.iupac.org)
Equilibrium and adsorption isotherms
An adsorption isotherm relates the equilibrium amount adsorbed to gas pressure—or, for adsorption from solution, to dissolved concentration—at a fixed temperature. Uptake is commonly reported per unit mass of adsorbent. Isotherms permit comparisons between materials, but meaningful comparison requires matching the adsorbate, temperature, pressure range, and reporting units. (old.iupac.org)
The Langmuir model describes monolayer adsorption on an idealized uniform surface. For a gas, its common form is
[ q=\frac{q_{\mathrm{m}}bp}{1+bp}, ]
where (q) is uptake, (q_{\mathrm{m}}) is the limiting monolayer capacity, (p) is equilibrium pressure, and (b) is an affinity parameter. At low pressure, the equation approaches a linear relationship; at high pressure, it approaches saturation. These limits follow from the model and need not describe every real porous material. (doi.org)
The Freundlich isotherm, often written (q=Kp^{1/n}) for gases, supplies an empirical alternative. Unlike the Langmuir equation, this power-law expression has no finite saturation limit. Consequently, extrapolating it far beyond the measured range can give unrealistic uptake. More elaborate models account for surface heterogeneity, molecular interactions, or pore filling; choosing a model requires attention to its physical assumptions as well as its numerical fit. (adsorption.nist.gov)
Adsorbents and porous structure
Many practical adsorbents are porous materials whose internal surfaces greatly exceed their external surfaces. Activated carbon is manufactured from carbon-rich feedstocks such as coal, wood, and coconut shells through treatments that create or enlarge pores. Its adsorption properties vary with feedstock and manufacturing conditions, so different carbons do not necessarily retain the same substances equally well. (epa.gov)
Zeolites are another important class, used in selective gas adsorption and purification. Their behavior illustrates the relationship between adsorption and molecular sieving: retention depends not only on interaction strength but also on access to the pore system. Adsorption research also examines metal–organic frameworks and other engineered porous solids as potential separation and storage materials. (energy.gov)
Gas adsorption is a major method of measuring accessible surface area and pore structure. The Brunauer–Emmett–Teller (BET) method estimates surface area from multilayer adsorption data and an assumed molecular cross-sectional area. Its result is model-dependent rather than a direct geometric measurement. Strong or weak surface interactions and overlap between monolayer and multilayer formation can complicate interpretation. (tsapps.nist.gov)
Separation and treatment processes
In chemical engineering, adsorption is used to separate components that differ in their affinity for a solid. Pressure-swing adsorption loads a bed at relatively high pressure and regenerates it at lower pressure. Multiple beds operated in alternating stages can provide a continuous purified stream. Hydrogen purification and separation of oxygen from air are established examples. (energy.gov)
Adsorption also supports carbon capture. A bed retains carbon dioxide until its useful loading capacity is approached; another bed can then take over while the first is regenerated. Pressure, vacuum, and temperature swings represent different ways of releasing the retained gas. Process performance depends on the usable difference between loading and regeneration conditions, not merely the maximum uptake measured in a laboratory. (energy.gov)
In water treatment, granular activated carbon removes various organic compounds, including substances responsible for taste and odor. Competing adsorbates can reduce capacity for a target contaminant. As a bed becomes exhausted, contaminants increasingly pass through—a condition called breakthrough—and the carbon must be replaced or regenerated. Adsorption transfers contaminants to another phase; their subsequent recovery, treatment, or disposal remains a separate operational requirement. (epa.gov)