Allophanes cation exchange

Aomine, S. and Jackson, M.L., 1959. Allophane determination in Avdo soils by cation-exchange capacity delta values. Proc. Soil. Sci. Soc. Am., 23 210-214. 

In a later study Birrell [1961a] studied the adsorption of various cations in allophane and showed that the quantity of cation adsorbed is inversely related to the radius of the hydrated cation, confirming that the apparent cation exchange capacity of allophane was a surface area effect. A similar conclusion was reached by Egawa, Watanabe, and Sato [1959]. 

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. 

The absolute cation exchange capacity and the total acidity of allophane were determined (IiMURA [1965]) by measuring the consumption of barium hydroxide in aqueous suspension and by titration curves. Results gave values of several hundred meq/100 g for CEC and 11.5 to 11.9 for pK value. The concentration of silanol groups was calculated from these results to be 7 to 9 per nm, all of which react with alkalis. 

Y. Watanabe, and A. Sato, 1959. A study on cation-exchange capacity of allophane. Adv. 

Cation-exchange capacity and total acidity of allophane and volcanic ash soils. Clay... 

Udagawa, S., and T. Nakada, 1969. Molecular structure of allophane as revealed by its thermal transformation. Proc. Int. Clay Conf, Tokyo. 1 151-160. de Villiers, J. M., and M. L. Jackson, 1967. Cation-exchange capacity variations with pH in soil clays. Proc. Soil Sci. Soc. Amer. 31 473-476. 

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.

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