Activated
carbons are different because of the starting material and manufacturing
methods. These raw materials establish the general characterizes, and
differences will exist in the finished product.
Domestically,
most carbons are manufactured from coals. The base raw material and
pretreatment steps prior to activation can affect many of the physical and
activity characteristics of activated carbon. These different properties make
some carbons more suited than others for specific applications.
Bituminous
coal activated carbons have a broad range of pore diameters. Since these
carbons have both a fine and wide pore diameter, they are well-suited for
general dechlorination and the removal of a wider variety of organic chemical
contaminants from water, including the larger color bodies. Coconut-based
carbon tends to exhibit greater microporosity, which is more suited for removal
of low concentrations of organics such as in drinking water applications. This
property can be deduced when comparing iodine numbers on the activated carbons.
Carbons with higher iodine numbers will tend to have larger surface area;
therefore, they will have higher capacity for comparatively weakly adsorbed
organics. On the other hand, carbons with lower iodine numbers may still have
wider pores, which could be favored for removal of large organic molecules.
There are some applications where color removal will be better facilitated by a
reactivated carbon as opposed to a high iodine carbon.
Another
comparative factor is the hardness of the carbon. For instance, the abrasion
resistance of activated carbons can be important if the carbon is to be used in
an application where frequent back-washing will be required. As mentioned
above, coconut carbons have a higher abrasion number than bituminous coal-based
carbons and would be expected to experience less attrition over time in this
type of an application.
Density
can also be a major consideration for specific applications. As the table below
shows, the densities of activated carbons vary with the raw material. Fewer
pounds of carbon with a low density will fit into a given container as compared
to a carbon with a high density. This is significant because, while a container
may require less carbon weight of a low-density carbon to make a volume fill,
its contaminant removal performance may be severely reduced as compared to a
higher density carbon.
Ash
content can play an important role in applications for water treatment. The
water soluble ash fraction may be liberated on contact with the activated
carbon; this may lead to undesirable effects, such as imparting cloudiness to
the water. Some applications with water having low pH can also liberate acid
soluble ash and can actually impart color, such as when coal-based carbons are
exposed to low pH water and iron is eluted from the carbon, imparting a
yellowish-orange color to the effluent water. The table above summarizes these
comparative properties.
While
activated carbon is very useful for applications such as municipal water
treatment, it is important for the user to solicit the product information and
pricing from the activated carbon provider, ensuring that the best possible
choice is made for the application. In this way, although a number of carbons
may be good candidates for the application, the one that may offer the best
cost-effective solution is the one that is used.
