Surface Properties of Allophane, Halloysite, and Imogolite

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Abstract
The adsorption of sodium, chloride, and phosphate ions by allophane, imogolite, and halloysite has been studied in relation to the surface structure of the mineral samples. The high adsorption of phos- phate (>200/zmole/g) and chloride (10-30 meq/100 g at pH 4) by allophane is ascribed to the small particle size of allophane, its high surface area (-800 m2/g), and the presence at the surface of A1-OH-A1 groups and defect sites. In contrast, halloysite has a relatively large particle size and a Si-O-Si surface. Accord- ingly, the adsorption of phosphate (5-10/xmole/g) and chloride (1 meq/100 g) by halloysite is very much lower as compared with allophane. Phosphate adsorption by halloysite is also related to particle mor- phology and the number of edge sites. Thus, a sample consisting entirely of spheroidal particles adsorbed only 5/xmole/g at a solution concentration of 1  10 4 M, whereas the tubular types of comparable surface area adsorbed 7-10/zmole/g at the same concentration. This is because spheroidal halloysite particles have few, if any, edge sites at which phosphate can adsorb. The relative degree of order and hydration of hal- loysite, as indicated by infrared spectroscopy, also affects phosphate adsorption. However, this factor is apparently less important than particle morphology and surface structure. Although imogolite also has an A1-OH-A1 surface, it contains relatively few defect sites where phosphate can adsorb. Consequently, much less phosphate (120/zmole/g) was adsorbed as compared with allophane.