Abstract

Samples of allophane, iron oxide coated allophane and iron oxide and organic matter coated allophane, and samples of similar synthetic aluminosilicates showed a remarkable K selectivity in K-Ca binary cation exchange equilibria, carried out at 25°C and studied through the Gaines-Thomas thermodynamic approach. The cation exchange capacity (CEC) of synthetic compounds is 3 to 12 times higher than CEC of equivalent natural samples. Both synthetic and natural samples showed the same selectivity sequence, related with the surface coating present. The thermodynamic constants of equilibria (Ke) of aluminosilicate and iron oxide coated aluminosilicate are almost one order higher than those of equivalent natural samples, which is consistent with the higher selectivity for K showed by synthetic samples. On the other side, synthetic and natural iron oxide and organic matter coated samples showed similar Ke values.