Applications of Adsorption

Adsorption underlies an enormous range of practical applications, of which thirteen representative examples are collected here.

1. Gas masks. During World War I, both British and American forces used charcoal-based gas masks to protect soldiers from poisonous gases, since activated charcoal was found to be one of the most effective adsorbents available for trapping toxic gas molecules before they could be inhaled.

2. Vacuum creation and gas drying/purification. Tait and Dewar used activated charcoal to create high vacuum inside vessels, by letting the charcoal adsorb away the residual gas molecules. For drying and purifying gases such as $\text{CO}_2$, $\text{N}_2$, $\text{Cl}_2$, $\text{O}_2$ and He, alumina and silica gel are employed as adsorbents; in a blast furnace, silica gel is likewise used to dry the incoming air.

3. Softening of hard water. One of the most important uses of adsorption is water softening using Permutit, a material that adsorbs $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ ions from hard water onto its surface via an ion-exchange reaction: $\text{Na}_2\text{Al}_2\text{Si}_2\text{O}_{12} + \text{CaCl}_2 \rightarrow \text{CaAl}_2\text{Si}_2\text{O}_{12} + 2\text{NaCl}$. Once the Permutit is exhausted (loaded with $\text{Ca}^{2+}$/$\text{Mg}^{2+}$), it is regenerated by washing it with a concentrated solution of ordinary common salt, which drives the exchange back the other way: $\text{CaAl}_2\text{Si}_2\text{O}_{12} + 2\text{NaCl} \rightarrow \text{Na}_2\text{Al}_2\text{Si}_2\text{O}_{12} + \text{CaCl}_2$.

4. Ion exchange resins. Ion exchange resins work entirely on the principle of adsorption and are used to demineralise water by passing it successively through a cation-exchange column and an anion-exchange column: the cation resin exchanges hardness-causing $\text{Ca}^{2+}$/$\text{Mg}^{2+}$ for $\text{H}^+$ (e.g. $\text{RSO}_3\text{H} + \text{Ca}^{2+}/\text{Mg}^{2+} \rightarrow (\text{RSO}_3)_2\text{Ca}(\text{Mg}) + 2\text{H}^+$), while the anion resin exchanges dissolved anions for $\text{OH}^-$, and the released $\text{H}^+$ and $\text{OH}^-$ simply combine to give pure water.

5. Petroleum refining and vegetable oil refining. Fuller's earth and silica gel adsorb coloured and undesirable impurities during the refining of petroleum and vegetable oils.

6. Decolourisation of sugar. Sugar prepared from molasses carries coloured impurities, which are removed by adding animal charcoal, a decolourising adsorbent, which adsorbs away the colorants and leaves clean, white sugar.

7. Chromatography. This widely used separation technique relies mainly on the differential adsorption of the components of a mixture onto the surface of an adsorbent; because different components adsorb to different extents, they can be separated, identified, detected and estimated with high sensitivity even when present only in microscopic (trace) quantities.

8. Catalysed reactions. Catalysis is itself an important branch of surface chemistry, resting fundamentally on the adsorption of reactant molecules onto the catalyst's surface. In the Haber process, iron catalyses the synthesis of ammonia from $\text{N}_2$ and $\text{H}_2$ ($\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3$), with molybdenum acting as a promoter; in the hydrogenation of vegetable oil to make vanaspati, finely divided nickel catalyses the addition of hydrogen across the oil's double bonds.

9. Qualitative analysis. When blue litmus solution is added to an $\text{Al}^{3+}$ solution, a red colour appears (reflecting the acidic nature of the aluminium salt solution); on subsequently adding ammonium hydroxide, a characteristic blue LAKE forms, caused by the blue litmus compound being adsorbed onto the surface of the freshly precipitated $\text{Al(OH)}_3$.

10. Medicine. Many drugs act by adsorbing onto specific body tissues, which is part of how they exert their curative effect. …