Which statement correctly defines adsorption and not absorption?

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

Answer: (C)

Adsorption is the process where molecules stick to the surface of a solid or liquid, forming a film on the exterior rather than entering the material itself. Absorption, on the other hand, means the substance is taken up into the interior of another material and distributed throughout its volume. In wastewater treatment, adsorption describes how contaminants attach to the surface of adsorbent materials like activated carbon, removing pollutants without dissolving into the bulk. The other terms describe different processes—air is exchanged with the liquid in aeration, and advanced wastewater treatment refers to a broader set of treatment steps—so they don’t define adsorption.