Sorption onto Colloidal, Suspended, or Settled Sediments

1 Sorption onto Colloidal, Suspended, or Settled Sediments 

We use here the term sorption as the retention of a compound on the surface of a solid particle that removes it from the aqueous medium. This phenomenon affects the composition of water by transferring the compound or ion from the aqueous medium to a solid (mainly a sediment in suspension or a colloid). Then, it may no longer be present in water, especially if the sediment settles. Sorption may be identified and associated with adsorption, surface precipitation, surface complexation, and/or ion exchange (or even absorption). 

Sorption is a surface phenomenon determined by the surface charges and those of the ions surrounding it. One or more of the following mechanisms can be involved in the removal of species by sorption (i) mechanical entrapment, (ii) absorption, (iii) physical sorption, or (iv) chemical sorption on the surface of the solid particle. Physical adsorption (which is weaker than its chemical counterpart) occurs through Van der Waals forces and it is generally reversible and instantaneous. On the other hand, chemical adsorption or chemisorption occurs through the formation of chemical bonds at specific sites. This is closely related to ion exchange processes and complexation. 

Surface functional groups in the suspended particles determine the type of process that will take place. In the case of inorganic colloidal particles (e.g., clays) the main functional groups are silanol (=Si — OH) and aluminol (=AI — OH), whereas in metal oxides or hydroxides the functional groups are (=M — OH). These groups may become protonated or deprotonated, depending on the pH of the aqueous medium, by sorption of H+ or OH- ions as follows  

Surface ionization and complexation models account for metal sorption reactions at these 

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