The ability of activated carbon to absorb a contaminant depends on which factors?

• A. The particle size of the contaminant
• B. The color of the water
• C. Temperature and pH of water
• D. The chemical nature of the contaminant, the pH of the water, and the contact time

Answer: (D)

Activated carbon works by interactions between the contaminant and the carbon surface, so what matters most is how the contaminant behaves in water and how long it has to interact with the carbon. The chemical nature of the contaminant—its polarity, whether it’s hydrophobic or hydrophilic, and its functional groups—determines how strongly it will be attracted to the nonpolar, porous carbon. The pH of the water can change the contaminant’s charge or form (for many pollutants) and can also alter the surface charge of the carbon, which can either enhance or hinder adsorption depending on the species present. Finally, contact time matters because adsorption is both a kinetic and equilibrium process; enough time allows molecules to diffuse into the carbon’s pores and approach the adsorption capacity of the bed.

Other factors like the particle size of the contaminant or the water color aren’t the primary drivers of adsorption. Temperature can influence adsorption as well, but the combination of the contaminant’s chemical nature, the water’s pH, and how long the contaminant is in contact with the carbon best explains why adsorption occurs for a given contaminant.