Sorption technologies are used in water treatment to clarify water and improve its organoleptic quality by removing dissolved organic substances and gases.
In technology of water purification activated carbon and other (often artificial sorbents) are used to reduce water color and odor. In particular, they are effective against natural humic acids, a wide range of industrial organic compounds, including phenols, organochlorines, effectively reduce the chlorine content, the indicators of total organic carbon. These processes allow it to be used in the sugar, food and alcohol industries, in complex chemical production. Some craftsmen use activated carbon to purify moonshine from distillate oils.
In the case of liquids, gases, and water, the term adsorption applies. It is the process of absorption of gases, vapors, substances from a solution or gas mixture by the surface layer of a solid (liquid). These absorbents are called adsorbents and the substance absorbed is called an adsorbate.
Modern sorbents for water treatment
The mechanism of adsorption
Let us take activated carbon as an example to illustrate this process. If you examine its particle under a microscope you can see many protrusions, depressions and peaks that greatly increase its surface and form porosity. There are several types of pores that are classified by size: macro, meso and micro.
There are two types of adsorption – physical and chemical.
Physical is a process which occurs due to the presence of special (vanderwaal) interactions that cause molecules to be attracted to the surface of the adsorbent. This physical process is reversible – under certain conditions (temperature increase, change in the composition of water, etc.) a part of the sorbed substance may return to the water.under certain conditions (increase in temperature, change in the composition of water, etc.) may return to water. Thanks to this, the activated carbons can be partially regenerated by backwashing.
If the process is complicated by a chemical reaction, then chemical adsorption (chemisorption) occurs; it is not characterized by reverse reactions. It means, that if the substances are fixed on the surface, they will not return to the water.
It is noteworthy that both processes run simultaneously and are not separated in time.
The figure shows in detail the difference between the two mechanisms.
The most popular sorbent for water purification is activated carbon. It is obtained from natural plant materials (wood, coconut shells, apricot kernels, etc.).) or minerals (bituminous and hard coal). Technology of obtaining is based on combustion in special furnaces and further activation.
Quantachrome Instruments Gas Sorption Show
Activation is a process of opening closed pores in the coal structure.
There are three methods of activation:
- treatment with solutions of certain substances and subsequent high-temperature treatment without air access (to exclude combustion);
- treatment with hot steam and/or carbon dioxide at 800 – 850 o C;
- Treatment with hot steam with a limited amount of air to burn a certain amount of coal.
Value of specific surface area of pores at the best grades of activated carbons can reach 1800-2200 m² per 1 g of coal.
Modern sorbents for water treatment
Nowadays activated carbon is used for sorption treatment of water. Granular forms are usually used because they are more convenient to use. Depending on the quality of raw materials and production technology, specific surface area of such coals can reach 800 – 1300 m²/g.
One of the largest manufacturers is the American company Calgon Carbon. The products are delivered to Ukraine. Predominantly in water treatment technologies are used coals derived from coconut shells.
The chart shows what the global distribution of activated carbon production looks like.
It is worth noting that there are alternatives to coal. These include new synthetic sorbents such as HumiSorb, a component of the multifunctional mixture Ecomix, which can comprehensively purify water from organic compounds, hardness salts and iron.
Modern water purification equipment is performed as cylinder filters with a capacity of 0.6 to 25 – 30 m.cb/h. The capacity of such systems is determined by the volume of activated carbon and, accordingly, by the size of the filter.
The principle of operation of such systems is that the water, previously cleaned of mechanical particles from the pipeline through a distribution system is fed into the filter housing (cylinder) and distributed over the thickness of the load. Then it is led through a central pipe to the consumer.
After a certain time the system is regenerated. It involves backwashing the load at high speed with a reverse flow of water.
Depending on the water quality, high-quality activated carbon can last for more than five years if used properly.
After this period, the material is replaced. On an industrial scale, it can be thermally regenerated by baking in furnaces at 800 – 850 °C to remove organic particles and reactivate.
Another modification of the carbon filter is the cartridge trunk filter. It consists of a classical BB20 or BB10 flask, which is installed in a pipeline. In the role of the filter element is installed a cartridge of granular activated carbon or carbon block of compressed powder. The principle of operation can be seen in the figure below.
These solutions have a rather short life span (50 to 60 m).cube for the BB20 filter) and must be replaced when it is exhausted. Enough water treatment for an apartment with 2 to 3 residents for about 3 months. Widely used in domestic and drinking water production (bottling stations, kiosks, etc.).).
Activated carbon has been used for water purification for a very long time, the first mentions of it are found in ancient India, and the Romans used it for water purification. At the end of the 18th century people began to use activated carbon already for industrial purposes in pharmacological and sugar industry. And already in the twentieth century it was widely used in water treatment for water clarification.
We have tried to explain the essential points about the sorption process. If you still have questions, we will be happy to answer them in the comments.