24 

 Raj an (1978) studied sulfate adsorption on hydrous alumina and 

 offered a refined mechanism for specific adsorption. He discounted 

 sulfate adsorption as HSO. because, under experimental conditions (and 



•4 



2- 



conditions in most soils) , sulfate exists as SO. . He proposed that 



2- 

 sulf ate is adsorbed as SO? across two Al atoms , forming a six-membered 



ring. At low sulfate concentrations, water is displaced from the 

 positive sites; as the concentration increases, increasing proportions 

 of OH groups are displaced from neutral sites (Fig. 1) . His proposed 

 mechanism is in agreement with experimental findings that the final 

 surface, after adsorption, carries close to a zero charge, that the 

 relationship between sulfate adsorbed and charge neutralized is curvi- 

 linear, and that the process is time dependent. 



Allophanic and highly weathered tropical soils possess extremely 

 high sulfate adsorbing capacities when compared to mineral soils of 

 temperate regions (Chao et al., 1962b; Bornemisza and Llanos, 1967; 

 Hanson et al. , 1970). Andepts from Mexico, Colombia, and Hawaii were 

 found to adsorb 10 to 20 meq of sulfate per 100 g in surface soils ard 

 15 to 60 meq sulfate per 100 g in subsoils (Gebhardt and Colemen, 1974). 

 Minerals associated with these soils are primarily amorphous silicates 

 and hydroxides and oxyhydroxides of Fe and Al. Experimental ZPC's for 

 some pure minerals that may be associated with high sulfate adsorbing 

 soils are listed below (Yoon et al. , 1979) : 



