Allophanes anion exchange

Figure 3.17. Dependence of cation and anion exchange capacity of smectite and allophane on solution pH, measured by the adsorption of cations (Cs for smectite, Na for allophane) and the chloride anion. PZC, point of zero charge.

IiMURA [1961a], however, attributed anion exchange to the dissociation of hydroxide from aluminum hydroxide. In a further paper (Iimura [1961b]) he showed that drying allophane at 105°C results in an increase in the capacity to adsorb ammonium, but also in a remarkable decrease in anion adsorption, presumably due to the dehydration of the hydroxyl group attached to the aluminum of allophane. 

The rate of adsorption of humified plant residues on clay has been studied by Inoue and Wada [1968], who have found that allophane absorbs considerably more organic matter than layer silicate clays. Their adsorption curves may be explained by the reaction of humate groups on the anion exchange sites of allophane. It has also been observed that adsorbed organic matter tends to reduce the anion exchange capacity of allophane. 

The coulombic attraction for anions and cations due to the positive and negative charges of allophane that may arise from the dissociation of H" from Si—OH groups or OH from A1—OH groups between pH4 and 7. This is the normal cation exchange reaction that applies to all clay minerals. However, in the case of allophane the reaction is pH-dependent the lower the pH the lesser the amount of cation adsorbed, and vice versa for anions. 

---