23 



ions react with H in solution which may have resulted from cation 



exchange involving a similar mechanism. They explain that sulfate 

 adsorption is increased as the pH is lowered because the replaced 

 hydroxyl ions are more effectively neutralized. As the pH increases, 

 cation affinity increases also, and this results in the replacement of 

 more Al and more hydrolysis. They used this model to explain the pH 

 and time-dependent processes which had been observed in sulfate adsorp- 

 tion. 



Hingston et al. (1972) studied specific anion adsorption by 

 goethite and gibbsite and found that where the acid (in this case, 

 H SO.) was fully dissociated, specific adsorption occurred only to the 

 extent of the positive charge of the surface. Little specific adsorp- 

 tion was found at pH values greater than the zero point of charge (ZPC) 

 of the mineral. Maximum adsorption would occur when the pH = pKa. At 

 this pH both the amounts of anion (dissociated acid) available for 

 ligand exchange and the amounts of proton donor (undissociated acid) 

 capable of neutralizing liberated OH are greatest. Onlv the mono- 

 valent species, HSO , could be specifically adsorbed without creating 



additional negative charge at the surface. Their mechanism involves 



2- 

 SO accepting a proton from the surface at pH values near the pK 



of the acid: 



1+ ^ ~io -io 



A1-0H 2 



+ so; * — Ai-OH 



+ HSO, ^= Al-KSO, 

 4 4 



+ OH 



2- 

 One mole of SO adsorbed as HSO neutralizes one equivalent of sur- 



face charge. Gebbardt and Coleman (1974) agreed with the mechanism of 



Hingston et al. by concluding that sulfate was adsorbed as HSO, . 



