What is activated carbon?

     Activated carbon is a material, which has a very high surface area, and can adsorb and remove contaminants from contaminated water or air. It can be made from carbon-rich materials such as plants (coal, wood, husks) or animals (bones), or fibers (synthetic polymers, cellulose).

What is “activate”?

     “Activate” in activated carbon refers to a high specific surface area. Activated carbon is made by expanding and adding numerous small pores on the surface of the char to increase the surface area. The high surface area also means that activated carbon can effectively adsorb organic molecules.

A”d”sorption and A”b”sorption

     Adsorption is the adhesion of one substance on the surface of another substance, while absorption is the drawing of one substance into the interior of another substance. In simple terms, absorption is like milk in glass, and adsorption is milk on glass after pouring out. Therefore, when the surface area is higher, the more substances can be adsorbed.

The principle of activated carbon filtration

     The principle of activated carbon filtration is to use its high surface area to bring excellent adsorption capacity to remove organic odor molecules.

The adsorption is divided into physisorption and chemisorptions.

     Physisorption is the use of Van der Wall’s force between molecules and solid molecules to attract the two together. Chemisorptions involves the formation of chemical bonds such as covalent bonds and ionic bonds, which make the adsorbate recombine into new substances on the solid surface.

     Taking the filtration of drinking water as an example, in order to avoid the threat of microorganisms, water plants add chlorine gas to the water to form hypochlorous acid and destroy the microorganisms, which is the so-called “disinfection process”. There may be unreacted hypochlorous acid and THMs produced by the reaction of hypochlorous acid with microorganisms or with other organic substances in the water after disinfection.

     Using activated carbon for filtration can utilize the high surface area of ​​activated carbon for physisorption to adsorb THMs and other organic substances; at the same time, the surface active functional groups of activated carbon will react with highly active hypochlorous acid to achieve the effect of chemisorptions.

Chemical equation I：

C* + HOCl → C*O + H+ + Cl–

C* + OCl– → C*O + Cl–

C*：Funcional groups of activated carbon

     However, the activities of chloramine and hypochlorous acid are different, so the general activated carbon is not effective in removing chloramine. The activated carbon after special modification treatment can strengthen the surface active functional groups on the carbon (surface-modified activated carbon), thus increasing the ability to remove chloramines.

Chemical equation II：

Reduction

C* + NH2Cl + H2O → NH3 + H+ + Cl- + C*O

Catalytic Decomposition

C*O + 2NH2Cl → C* + 2H+ + 2Cl- + H2O + N2

How to make an activated carbon?

     The formation of activated carbon basically requires three steps: the first step is carbonization, which is formed by burning carbon-rich raw material at high temperature without oxygen to form “Char”. The second step is to heat up by exposure to steam to form the polycyclic aromatic compound structure and create more pores, also known as activation. The last step is wash, which is removing ash by acid washing or water washing and adjust the activated carbon to the appropriate pH value.

Raw material

     There are many substances that can be made into activated carbon, and almost all carbon-containing materials can be used as raw materials for activated carbon under appropriate conditions. Plants are more economical and most commonly used as granular activated carbon, materials such as wood, coconut shell and coal are often used.

     Different types of plants have different fibrous tissues and cell wall structures, therefore, different forms of pore structure and pore size and distribution will be produced after carbonization. Different pores and pore sizes also affect their ability to adsorb molecules of different sizes.

     In general, coconut shell charcoal has a large proportion of small pores, so it also has a relatively high specific surface area, which is very suitable for the adsorption of small organic molecules; compared with coconut shell carbon, charcoal has more mesopore and more suitable for the adsorption of colored dye molecules; and organic compounds with large molecular weights such as protein molecules and macromolecules are suitable for adsorption by coal-based carbon with a large proportion of large pores.

     Comparison of activated carbon from different raw materials

     Definition of pore size

Activated carbon in life

     In our life, we know how to use activated carbon to remove odors and acetaldehyde, which are volatile organic compounds (VOCs) that are harmful to humans, in renovated homes.

     During World War I, activated carbon was already used for military purposes. In order to resist poison gas attacks (mustard gas, phosgene, and chlorine gas), granular activated coconut shell carbon has strong adsorption properties, and so it was quickly developed as an adsorbent, and was made mass produced into a canister in gas masks.

     Earlier in the late 18th century, the sugar industry began to explore the use of charcoal in an attempt to decolorize syrup. It may not have been able to master the technology to control the porosity in charcoal at that time, so the effect was not ideal. It was not until the beginning of the 19th century that animal bones were gradually developed into granular bone char as an effective decolorizing agent, which could adsorb and remove the impurities with color produced in the sugar making process.

     The application of activated carbon can also be seen in the whiskey industry. The traditional technology developed in the 19th century, using maple charcoal to filter freshly distilled spirit to adsorb and remove larger molecules such as fusel oils, ester and unpleasant-smelling, and then barreled and matured. This process is known as the Lincoln County Process, and the process of filtering with maple charcoal is the key difference between bourbon and Tennessee whiskey.

     Today, the excellent adsorption capacity of activated carbon has long been developed and applied in the treatment of drinking water resources. The taste of disinfected drinking water is often unpleasant and may contain the carcinogenic disinfection by-product THMs and other potentially harmful organic molecules. The high surface area of ​​activated carbon can effectively deal with these annoying problems and provide a pleasant and safe drinking water experience.