SorptionDesorption

Sorption-desorption is a term used to describe reversible exchange of substances between a solid phase and a liquid phase, in which a contaminant or solute can accumulate on the surface or within the solid (sorption) and can later be released back into the liquid (desorption). Sorption encompasses adsorption, where molecules adhere to a surface, and absorption, where molecules diffuse into the bulk of a phase. The process is central to the fate and transport of contaminants in soils, sediments, activated carbon, and other porous materials.

Mechanisms include physical sorption, driven by van der Waals forces and electrostatic interactions, and chemical sorption, involving specific chemical reactions such as complexation, ion exchange, or covalent bonding. Sorption is influenced by surface area, porosity, mineralogy, organic matter, pH, ionic strength, and competing solutes. Equilibrium relationships are often described by sorption isotherms such as Langmuir or Freundlich models; desorption may follow different paths, leading to hysteresis and partial irreversibility.

Kinetics of sorption-desorption involve rapid surface uptake followed by slower intraparticle diffusion. Environmental conditions can shift desorption behavior; for example, changes in pH, salinity, or the presence of competing species can mobilize previously sorbed contaminants.

Applications include risk assessment, soil and wastewater remediation, and treatment design using sorbents like activated carbon, clays, or biochars. Measuring sorption-desorption through batch tests and desorption experiments helps predict contaminant mobility, bioavailability, and long-term environmental impact.