What is the Properties of Activated Carbon? May 09 2013, 6 Comments
Activated carbon products can be characterized by physical properties and activity properties. Both physical and activity properties become important factors in the specification of commercial carbons for POU applications.
Important physical properties are surface area, product density; mesh size, abrasion resistance and ash content. In water treatment applications, carbon density is expressed as back washed and drained (BWD) or bulk density. This establishes the number of pounds of carbon required to fill a back washable filter, and is expressed in terms of pounds per cubic foot.
Mesh size (8x30, 12x40, 20x50, etc.) establishes the range of particle sizes and thus, the effective particle size that will be used in a filter. Particle size is an important parameter in specifying carbons for specific applications, affecting such operating conditions as pressure drop, filtration capabilities, backwash rate requirements and the rate of adsorption of contaminants. While a smaller particle size effects more pressure drop across a carbon bed, the rate of diffusion of an organic into the pore and its subsequent adsorption is significantly increased.
Another important characteristic that distinguishes different types of liquid phase carbons is abrasion resistance. Abrasion resistance refers to a carbon's ability to withstand degradation during handling and is expressed in terms of abrasion number. The higher the abrasion number, the more resistant the carbon is to abrasion.
The final important physical property of activated carbon is ash level, which reflects the purity of the carbon. It is the inorganic residue left after heating of the raw material. Common ash constituents of coal-based carbons are silica, alumina, iron, calcium and magnesium.
Activity characterizations are key indicators of a carbon's potential performance for removing contaminants from water. An important characterization tool used in determining the ability of a carbon to adsorb a particular adsorbent is the pore size distribution, which is usually depicted in the form of a curve (Figure 1). The pore size distribution is produced through adsorption of gases and liquids under pressure. It defines the avail- able pore volume of a carbon over three pore size regions: The micropore region (less than 100 Angstroms. in size), mesopore region (between 100 and 1,000 Angstroms), and macropore region (greater than 1,000 Angstroms).
The molecules encountered in gas phase are generally smaller and more mobile than those in liquid phase applications. Therefore, a gas phase carbon has the majority of its pores concentrated in the micropore region. There is a wider range of molecular sizes in liquid phase work (taste and odor, color bodies, organics, pesticides), and adsorbates are less mobile in water. This means a broader range of pore sizes must be available, both for ease of movement of adsorbates through the carbon pores and for adsorption of particular molecular sizes. Inexpensive tests have traditionally been used to approximate the distribution of pores available for a carbon as just described. These tests include the adsorption of a single standard reference adsorbate, and give the ability to distinguish activity characteristics of different carbons.
Iodine is the most common standard adsorbate and is often used as a general measurement of carbon capacity. However, because of its small molecular size, Iodine more accurately defines the small pore or micropore volume of a carbon and thus reflects its ability to adsorb low molecular weight, small substances. Iodine number is defined as the milligrams of Iodine adsorbed by one gram of carbon, and it approximates the internal surface area (square meters per gram).
Molasses number is a measure of the degree of de-colorization of a standard molasses solution. Because color pigments are large and cannot penetrate into small pores, the molasses number defines the large pore or macropore volume of a carbon. It is used as a relative guideline for measuring the capacity of a carbon for larger adsorbate molecules.