Adsorption is the physical or chemical process by which dissolved or suspended substances adhere to the surface of a solid material, rather than being absorbed into its internal structure. In stormwater management and water quality treatment, adsorption is a key mechanism for removing pollutants from runoff as water passes over or through soils, sediments, or engineered media.
During adsorption, molecules or particles in water are attracted to and held on the surface of solids by forces such as electrostatic attraction, van der Waals forces, or chemical bonding. Common adsorptive materials in stormwater systems include soil particles, organic matter, clay minerals, activated carbon, and engineered filter media. These materials provide large surface areas where pollutants can attach.
Adsorption is particularly important for removing dissolved contaminants that do not readily settle out through sedimentation. These include nutrients such as phosphorus, metals, hydrocarbons, and certain organic pollutants. For example, phosphorus often binds to soil particles or metal oxides, allowing it to be retained within a treatment system rather than transported to receiving waters.
In stormwater practices such as rain gardens, bioswales, permeable pavement systems, and constructed wetlands, adsorption works in combination with other processes like filtration, sedimentation, and microbial transformation to improve water quality. As stormwater infiltrates through soil or filter media, pollutants are captured on particle surfaces and retained within the system.
It is important to distinguish adsorption from absorption. In absorption, a substance is taken into the internal volume of another material, like a sponge soaking up water. In adsorption, the substance adheres only to the surface.
Simply put, adsorption is a critical process in stormwater treatment, enabling the removal and retention of a wide range of pollutants and contributing to the protection of downstream water resources.