In a granular activated carbon (GAC) filter, what phenomenon is primarily responsible for the removal of organic compounds?

In a granular activated carbon (GAC) filter, the primary phenomenon responsible for the removal of organic compounds is adsorption. Adsorption is the process where organic molecules adhere to the surface of the GAC material. GAC is a highly porous material with a large surface area, typically ranging from 500 to 1500 square meters per gram. This large surface area provides numerous sites for organic compounds to attach to. Organic compounds are attracted to the GAC surface through various mechanisms, including van der Waals forces, hydrophobic interactions, and electrostatic interactions. Hydrophobic interactions are particularly important, as many organic contaminants in water are non-polar and tend to be more attracted to the non-polar surface of GAC than to the polar water molecules. As water flows through the GAC filter, organic molecules diffuse to the GAC surface and are adsorbed, effectively removing them from the water. Over time, the adsorption capacity of the GAC is exhausted as the available surface sites become occupied. At this point, the GAC must be replaced or regenerated to restore its adsorption capacity. Biological activity can also play a role in the removal of organic compounds in GAC filters, particularly in older filters where a biofilm has developed on the GAC surface. However, adsorption is the dominant removal mechanism, especially in the initial stages of filter operation. For example, GAC filters are commonly used to remove disinfection byproducts (DBPs) such as trihalomethanes (THMs) and haloacetic acids (HAAs) from drinking water. These organic compounds are effectively adsorbed onto the GAC surface, reducing their concentration in the treated water.