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Activated Carbon |
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Activated carbon is a form of charcoal. Charcoal is obtained by burning wood, nutshells, coconut husks, animal bones, and/or other carbonaceous (carbon-containing) materials.
Charcoal becomes activated by heating it with steam to approximately 1000°C in the absence of oxygen (O2). This treatment removes residual non-carbon elements and produces a porous internal microstructure having an extremely high surface area. A single gram of such material can have 400 to 1,200 square meters of surface area, 98% of it internal! To put this into perspective, the amount of carbon that can fit in a shot glass has an internal surface area that if spread out would more than cover a football field.
The chemical nature of amorphous carbon, combined with a high surface area and porosity, makes it an ideal medium for the adsorption and absorption of organic chemicals.
- Adsorption means that the chemical has an interaction with the surface of the material. Activated Carbon uses the physical adsorption process whereby attractive van der Waals forces pull the solute out of solution and onto its surface. Once the solute is bound to the carbon is it considered "removed" from the water.
- Absorption is often confused with adsorption but is not the same thing. When a substance is "absorbed" into another material, it actually penetrates the other structure and is incorporated into the host structure through pores (interstices).
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Activated carbon can remove organic materials from gas streams (air) or solutions (water.) The amount of material removed depends on the capacity of the activated carbon as well as the affinity of the material for the carbon. Typical uses are to remove odors from air and volatile organic compounds (VOC's) from liquids. This latter capacity makes activated carbon particularly suited for various water treatment purposes.
Activated charcoal is good at trapping other carbon-based molecules (organic" chemicals), many of which can impart objectionable tastes and odors to water. That is why the early Romans used charred wooden barrels to store water. It is also very effective at trapping other organic chemicals like pesticides and herbicides can be quite toxic. It is also good at trapping certain other chemicals like chlorine.
Many other chemicals are not attracted to carbon at all so they pass right through. This includes inorganics like sodium, lithium and nitrates, metals like iron, lead, and arsenic, and other compounds like alcohols, glycols, ammonia, and a variety of strong acids and bases. This means that an activated charcoal filter will remove certain impurities while ignoring others. It also means that, once all of the bonding sites are filled, an activated charcoal filter stops working. At that point you must replace the filter.
The capacity of activated carbon to adsorb a compound depends on several factors including temperature, molecular weight, and concentration of the compound in the stream. Certain carbons are better at adsorbing small molecules, and others are better at handling very large molecules.
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When a bed of carbon has reached its maximum capacity, breakthrough occurs. The carbon becomes saturated with adsorbate and the contaminate can be detected in the product water. The carbon must then be replaced or regenerated.
Dumping is a phenomenon that can occur when water above 104° is passed through the media. The high temperature causes collected contaminants to be released from the carbon back into the treated water stream. This is the reason carbon is not suitable for treating hot water supplies.
The amount of activated carbon in a filter is one of the most important characteristics affecting the amount and rate of pollutant removal. More carbon in a cartridge means more capacity for chemical removal and, therefore, leads to longer cartridge lifetime. This also means fewer cartridge changes and less chance of drinking contaminated water. Particle size will also affect the rate of removal; smaller AC particles generally show higher adsorption rates.
Clogging by rust, scale, sand or other sediments can be a problem with any activated carbon filter. A solution to this problem is the placement of foam or cotton filters (often called sediment or fiber filters) between the cartridge and incoming water. When these sediment filters become clogged, they need to be replaced or they can cause a drop in water pressure.
Design of any activated carbon filter must ensure that the filter is deep enough so that the pollutants will adsorb to the activated carbon in the time it takes the water to move through the filter. The appropriate
filter depth is dependent on the flow rate of water through the filter. The slower the flow rate, the better the removal.
The poor performance of faucet mounted devices is directly related to improper filter depth. They may make water taste slightly better, but such filters do little else to improve water's quality.
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Catalytic Activated Carbon |
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Catalytic Activated Carbon is a new carbon product that enhances the catalytic functionality of standard activated carbons. The surface of the carbon has been changed so that it attracts certain chemicals that are not normally adsorbed by carbon. When this happens, these chemicals are "broken apart," thereby creating other substances that are more readily adsorbed by the carbon bed.
For example, chloramine is a chemical that is now widely used to disinfect water supplies. Unlike chlorine, it is not readily adsorbed by carbon. That is why it tends to go right through a distiller or a reverse-osmosis system. When it passes through a bed of catalytic activated carbon however, it is attracted to the surface of the carbon particle and is broken apart into its two constituents of chlorine and ammonia. Each of these are then adsorbed by the carbon bed.
Catalytic activated carbon was developed because of the need for more efficient removal rates of chloramine and other difficult to remove chemicals from water. Although it is very expensive compared to other standard carbons, it works in applications where other carbons can't.
For a selection of activated carbon, catalytic activated carbon, and carbon block filters, click here.
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