Absorbents and Adsorbents Information

Silica gelThere is often confusion between absorbents and adsorbents. Simply put, absorbents work by taking up liquid or gas into spaces within and throughout the material. Adsorbents accumulate liquid or gas on the surface of a solid material. Absorption involves dissolution or diffusion. In absorbent materials, the atoms, molecules, or particles are taken internally in its interstice, or chambers, like in a sponge. Adsorption is a consequence of surface energy; atoms on the outer surface experience a bond shortage because they are not completely surrounded by others. As a result, they favor energy-bonding with what is nearby. 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.

Types

Absorbents and adsorbents can be categorized by specific material types. They are differentiated by chemical makeup and physical structure. 

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 and 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 to 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.

Molecular sieveSpecifications

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

Adsorbents and absorbents are useful in a large number of applications and industries. Specifically, adsorbents and absorbents are used for the concentration and recovery of media or the concentration of a solution in a mixed phase. Processes include dewatering, drying/dehydration, moisture and humidity control, and spill control and containment. They protect electronics, medical products, food, and other sanitary goods. Adsorbents and absorbents are frequently used for spill control, containment, and remediation tasks. The use of adsorbents and absorbents span many industries, including semiconductors, pharmaceuticals, food processing, sanitation, packaging, energy production, pollution control, and waste management. Absorbents and adsorbents make it possible for food and other products to last longer in damp environments as well as help in cleanup of minor to major spills of potentially hazardous materials.

Related Information

Engineering360—Super-Absorbent Material Developed to Clean Oil Spills

IEEE Spectrum—Carbon Capture Starts From Coal Plant, Advances in Lab

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Desiccants / CC BY-SA 3.0 | GOKLuLe / CC BY-SA 3.0