22 

 1.1.3.3.2 Mechanisms of Sulfate Adsorption 



Sulfate, as are anions such as phosphate and silicate, may be ad- 

 sorbed by nonspecific or specific adsorption mechanisms. Nonspecific 

 adsorption involves the coulombic attraction of negatively charged 

 species to positively charged sites on soil colloids or metal oxide 

 surfaces where they are held in the Stern layer or as countericns in 

 the diffuse part of the electrical double layer (Gast, 1977: Bohn et 

 al. , 1979) . This type of adsorption involves a hydrolysis mechanism 

 for the development of surface sites for anion adsorption. The number 

 of sites formed depends on the pH and the type of surface but not on 

 the type of anion (Hingston et al. , 1967) . 



Some anions may be specifically adsorbed on mineral surfaces. 

 The mechanism involved is frequently referred to as ligand exchange 

 (Gast, 1977). Specific adsorption coordinates the adsorbing anion with 

 the metallic cation such that the anion is not easily replaced. This 

 involves the displacement of a coordinated OH or HO on the surface 

 of metal oxides or hydroxides and forms partly covalent bonds with the 

 structural cations (Hingston et al. , 1967). Mineral surfaces contain- 

 ing partially coordinated oxygen atoms and broken edges of layer sili- 

 cates are the sites for specific adsorption in soils. 



The exact mechanism of sulfate adsorption is not fully understood. 



Chang and Thomas (1963) proposed the following mechanism to explain 



their observation that the quantity of anions held by a Cecil subsoil 



increased with time. 



clay-Al-(OH) + SO?" -»■ clay-Al[(OH) (S0~) ] + 20H~ . 

 y 4 y-z -4 z 



Divalent sulfate ions replace OH from A1(0H) or Fe(OH) coatings 

 on the clay surface and substitute for them. The replaced hydroxyl 



