Charcoal is a highly effective filtering material, but the type used in filtration systems is not the briquettes used for grilling; it is a specialized substance called activated carbon. Its filtering capability stems from a massive internal surface area that traps contaminants as water or air passes through it. The carbon material is processed to create a microscopic “sponge” structure, which allows it to excel at binding certain types of impurities.
Creating Activated Carbon
Activated carbon begins as a carbon-rich raw material, such as wood, coconut shells, or coal. The first step is carbonization, where the raw material is heated to temperatures typically between 400°C and 800°C in an environment with very little or no oxygen. This high-temperature treatment drives off non-carbon components like water and tars, leaving behind a carbon-rich char and creating an initial, rudimentary pore structure.
The char must then undergo a second step called activation to develop the extensive network of tiny pores required for filtration. This can be achieved through physical or chemical activation methods. Physical activation uses hot gases, such as steam or carbon dioxide, at temperatures ranging from 800°C to 1000°C, which selectively burn away carbon atoms to enlarge and create a vast number of new pores. Chemical activation involves impregnating the char with a dehydrating agent like phosphoric acid before heating it to lower temperatures, typically between 400°C and 700°C, which also results in a highly porous structure.
The Principle of Adsorption
The filtering power of activated carbon is based on the physical process of adsorption, where molecules of a contaminant adhere to the surface of the carbon. In contrast, absorption is when one substance soaks up another, distributing the substance throughout its volume.
The effectiveness of activated carbon is a direct result of its enormous internal surface area, which can exceed 1,200 square meters per gram. The mechanism that holds the contaminants to the surface is primarily a weak electrical attraction known as Van der Waals forces. These forces are short-ranged and additive, meaning the massive surface area, coupled with the close proximity of the carbon walls, creates a strong collective attraction for molecules.
The pore structure itself further defines the carbon’s filtering capabilities, as the pores are categorized by size. Micropores, which are less than 2 nanometers wide, are where the majority of the adsorption occurs and are suited for trapping smaller molecules. Mesopores accommodate medium-sized molecules and act as channels for larger contaminants to reach the smaller pores. This intricate, porous architecture and the non-specific attractive forces are the scientific foundation for activated carbon’s ability to pull impurities from a fluid.
What Activated Carbon Removes
Activated carbon is particularly effective at removing organic compounds from water and air. These include many chemicals that cause unpleasant tastes and odors, such as chlorine, which is a common disinfectant in municipal water supplies. The carbon converts chlorine molecules into harmless chloride ions through a process known as catalytic reduction, greatly improving the water’s palatability.
It excels at trapping Volatile Organic Compounds (VOCs), which are often found in industrial solvents and cleaning agents. Specific examples of organic contaminants removed include pesticides, herbicides, benzene, and trichloroethylene. This makes activated carbon a standard component in many point-of-use water filters and aquarium filtration systems, where it removes organic waste and medications.
Filtration Limitations
Despite its effectiveness against organic compounds, activated carbon cannot remove every type of contaminant. It is ineffective against small, dissolved inorganic substances and some heavy metals unless specifically treated. This includes common water quality issues like nitrates, total dissolved solids, and most dissolved minerals that cause water hardness.
Activated carbon is not a standalone solution for pathogens in water. It does not reliably eliminate microbial contaminants such as bacteria and viruses. For water with a high concentration of these substances, a different approach, such as reverse osmosis or specialized disinfection methods, is required to achieve complete purification.