Sorbents are chemicals or materials that can capture liquids or gases. Absorbents incorporate a substance throughout the body of the absorbing material, e.g. polypropylene which absorbs oil. Adsorbents adhere substances over the surface of the adsorbing material, e.g. activated carbon.
A desiccant is a specific type of sorbent used to reduce humidity or moisture. They are hygroscopic meaning they have the ability to attract and hold water molecules.
Absorbents and Adsorbents can be categorized by specific material types. Here are some types of absorbents and adsorbents:
- Activated alumina is an adsorbent made of aluminum oxide (Al2O3). It is used as a desiccant for drying gases and air and as a fluoride filter for drinking water. It has specific use as a silica gel replacement in certain environments due to its thermal shock resistance and physical constancy when immersed in water.
- Activated carbon is roasted organic material (coconut shell, bone, wood) that forms porous granules. It is a versatile and inexpensive adsorbent that comes in many sizes and has a range of applications from gas, water, and metal purification to air filtration.
- Calcium sulfate is a natural mineral that is chemically stable, and readily retains its captured moisture. It costs little but also has a low adsorbency capacity and is best suited for small moisture capture operations or laboratory use.
- Calcium oxide is a slow but strong and high capacity desiccant also known as quicklime. It is caustic and expands as it adsorbs and does so over several days. It is most effective in high humidity environments.
- Clay or clay silicates are natural mineral absorbents and adsorbents that are used as spill cleaning agents, sealants, and packing materials because they are inexpensive, inert, and have a quick capture rate. However they begin to desorb at temperatures above 120°F.
- Molecular sieves or zeolites are naturally occurring adsorbents with uniform pore size that can be tuned to be highly selective. They are used as dehumidifiers and air purifiers due their high retention and adsorption capacities even at high temperatures. Zeolites are often combined with activated carbon for combined effectiveness.
- Organic polymers are chains of repeating carbon based molecules used as adsorbents in size-exclusion chromatography and gas separation processes with high retention power and selectivity. Most do not require disposal and the regeneration process is environmentally friendly.
- Silica gel or silicon dioxide is a common desiccant used in food preservation, humidity control, and various medical devices. It has a higher water absorption capacity than clay silicates, it is very inert, and it can be regenerated through heating.
When selecting adsorbents and absorbents, the most important properties to consider are the selectivity, surface area, and regeneration ability.
- Selectivity is the amount of specificity a sorbent has in the materials that it can capture. A very unselective sorbent captures many substances and a selective sorbent only removes specific ones.
- Surface area is the amount of material a sorbent has available for contact and determines the capacity of material a sorbent can capture, the capture rate of the sorbent, and the retention rate of the substance within the sorbent.
- Regeneration is the ability of a sorbent to be reused after capturing to its capacity. Many desiccants can be heat treated in order to regenerate the material after reaching water capture capacity.
Other factors to consider when selecting sorbents are bulk density, chemical inertness, and ease of application.
Applications and Industries
Sorbents have usefulness in a large number of fields. The most common uses are for spill management, cleaning, pollution control, dehumidification, and drying. However, applications for these substances span many industries including electronics, pharmaceuticals, food processing, sanitation, packaging, energy production, and waste management.