Which term describes the binding to a surface rather than absorption?

• A. Absorption
• B. Aeration
• C. Adsorption
• D. Advanced waste treatment

Answer: (C)

Binding to a surface is adsorption. In adsorption, molecules cling to the exterior of a solid or liquid rather than moving into its interior, which is what absorption would involve. This surface adherence is the key idea: contaminants are held on the surface of a medium like activated carbon or a porous mineral, making them removable from water without penetrating the bulk of the material. Adsorption can be physical, driven by weaker forces such as van der Waals attractions, or chemical, involving stronger bonds. In wastewater treatment, adsorption is used to remove organics and other pollutants by binding them to the surface of treatment media. The other terms don’t describe this surface-binding process: aeration is about supplying air, and advanced waste treatment refers to overall treatment processes rather than the binding mechanism.