EFFECTS OF SOME CHEMICAL TREATMENTS ON K/Ca CATION EXCHANGE SELECTIVITY OF LAYER SILICATES

Abstract

The effects of chemical treatments commonly employed to remove amorphous materials, on the K/Ca cation exchange selectivity (CES) of montmorillonite, kaolinite, chlorite, vermiculite, biotite, muscovite, and soil clays were studied. The K/Ca CES values for biotite and muscovite decreased appreciably upon peroxidation. Both the Na–dithionite–citrate–bicarbonate and boiling 0.5 N NaOH dissolution treatments resulted in a substantial increase in the K/Ca CES of biotite and muscovite. This was attributable to increases in the wedge-zone sites resulting from partial K-depletion of these minerals. Furthermore, the release of structural cations and their subsequent precipitation on mineral surfaces appears to have resulted in the "preferential occupation" of Ca-adsorbing sites and thus contributed to the lower selectivity of Ca ions. However, these two treatments affected the K/Ca CES of illite slightly. Acid NH4-oxalate extraction reduced the K/Ca CES values for all layer silicates investigated except chlorite for which the reverse was true. Selectivity of Ca and the CEC of illite were substantially increased by this treatment. The data suggested that the naturally occurring amorphous inorganic and organic constituents may have higher selectivity towards Ca relative to K. However, the degree of variation in the K/Ca CES seems to be further controlled by the amount, nature, and composition of amorphous materials and the mechanisms by which they are associated with the crystalline mineral components.