Carbon Adsorption

Adsorption is the process of removing chemicals from a solution by accumulating the chemicals on a solid surface. Engineers refer to this as the transition from an aqueous environment to a solid environment. Large wastewater treatment plants have more than one method for removing hazards from wastewater; adsorption supplements these treatments because of its efficiency and ease of use. Sustainable wastewater treatment plants, small rural plants on a strict budget, and sustainable homes act as ideal places to use adsorption for cleaning wastewater.

Three types of adsorbents, substances that adsorb materials from water, work in treatment: activated carbon, synthetic long-chain compounds called polymers, and silica-based adsorbents. Polymers and silicabased compounds are expensive, but inexpensive activated carbon has been important in wastewater treatment for many years. Activated carbon consists of any material with high carbon content, which is treated to increase its capacity to draw contaminants from water. Also important, activated carbon holds onto the contaminants and does not release them back into the water.

Makers of activated carbon use any of the following high-carbon materials as precursors: almond, coconut, or walnut hulls; wood; bone; or coal. The activation process contains the following two steps: (1) heating the material to 700°F (371°C) to burn off hydrocarbons and make a material called char; and then (2) activating the char by exposing it to steam and carbon dioxide to create a porous structure. The extensive pore system in the carbon greatly increases surface area and thus makes the material much more efficient as an adsorbent. Activated carbon tends to contain the following pore sizes:

1. macropores—greater than 25 nanometers (nm)
2. mesopores—greater than 1 nm and less than 25 nm
3. micropores—less than 1 nm

 Wastewater treatment systems use either of two types of activated carbon: (1) powdered activated carbon (PAC) that contains small particles (less than 0.075 mm diameter), or (2) granular activated carbon (GAC) that contains larger particles of about 0.1–2.4 mm diameter. Smaller particle sizes also increase the adsorbing surface area compared with large particles.

Carbon adsorption works by exposing the solid surface to a flowing liquid. Contaminants drift toward the solid-liquid interface located adjacent to the solid adsorbent. Contaminant molecules diffuse through this static interface and enter the particles’ pores where they attach to sites called available adsorption sites, which are spaces on the adsorbent’s surface that are unclaimed and available to catch and hold onto a contaminant. Because water flows in and around the adsorbent’s pores, the method is often called carbon filtration. A variety of chemical interactions create the force that holds a contaminant on the surface: charge, covalent bonding, hydrogen bonding, or van der Waals forces. Covalent bonds occur when two atoms share electrons, while van der Waals forces develop when adjacent atoms share a weak attraction to each other because of an electromagnetic field.

Carbon adsorption offers an inexpensive and easy-to-maintain water cleaning method for sustainable homes, especially for treating gray water. The carbon can be reactivated once it has filled up with contaminants and no longer has many available adsorption sites remaining. Reactivation uses the same process as that used to activate char, that is, heating. For home systems, homeowners can simply replace the activated carbon filter every six months to a year after installing it rather than reactivate a used filter.