Qu'est-ce que l'adsorption ?
What is adsorption?
Adsorption is member of the family of the removal/separation techniques like stripping and extraction. Adsorption is no filtration technique.
There are 2 methods of adsorption: physisorption and chemisorption. Both methods take place when the molecules in the liquid (or gas) phase become attached to the surface of the solid as a result of the attractive forces at the solid surface (adsorbent), overcoming the kinetic energy of the liquid contaminant (adsorbate) molecules. Physisorption occurs when, as a result of energy differences and/or electrical attractive forces (weak van der Waals forces), the adsorbate molecules become physically fastened to the adsorbent molecules. This type of adsorption is multilayered and reversible (regeneration by desorption). This is normally what happens with activated carbon. Chemisorption is when a chemical compound is produced by the reaction between the adsorbed molecule and the adsorbent, one molecule thick and irreversible.
Adsorption occurs in phases: transport in the liquid, diffusion through the water-film, transport through the (macro-) pores and the graphite plates, and the physical bound (exothermal 8–40kJ/mol).
Activated carbon is an organic adsorber with non-polar surfaces, making the adsorption of inorganic electrolytes difficult and the adsorption of organics easily effected. Other adsorbents are e.g. zeolites, activated alumina, silica and clay.
Characteristics of adsorbents are the adsorption-capacity, selectivity, possibility of regeneration, reaction velocity, compatibility and price.
The adsorption capacity (mg/g) is affected by the physical and chemical characteristics of the adsorbent (surface area, pore size, diameter, chemical composition) and adsorbate (molecular size, vapour pressure and boiling point, liquid density, molecular polarity, chemical composition), the concentration of the adsorbate in the liquid phase (solution), the characteristics of the phase (pH, temperature, pressure, humidity) and the contact time.
Adsorbability is depending on the different groups and bonds – some general rules:
| • | (C6H6)x+1 | > | (C6H6)x | > | –C≡C– | > | –C=C– | > | –C–C |
| • | –COOH | > | –C–O | > | –C–O–C– | > | –C–OH | > | –C–H |
| • | –C–I | > | –C–Br | > | –C–Cl | > | –C–H | > | –C–F |
| • | –C–SH | > | –C–CH3 | > | –C–NH2 | > | –C–OH | > | –C–F |