KANEMITE (NAHSI2O5.3H(2)O) AND ITS HYDROGEN-EXCHANGED FORM

Kanemite, NaHSi2O5 . 3H(2)O, is able to exchange metal cations directly from aqueous solution. However, the degree of extraction is dependent upon whether kanemite or its hydrogen-exchanged form is the exchanger. Zinc cations, for example, are exchanged from basic solutions but the equilibrium extraction decreases with increasing acidity (at an equilibrium pH of 11.2, K-d = 1070 cm(3) g(-1), whereas at an equilibrium pH of 4.88, K-d = 70 cm(3) g(-1)). Kanemite acts as an effective ion exchanger but the hydrogen-exchanged form is less effective, exchanging only by surface sorption at pH 2. In order to explain these and other properties, high-resolution solid-state magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) has been used to probe the structure of kanemite and its hydrogen-exchanged form. In kanemite there are only Q(3) silicon-containing groups with extensive hydrogen bonding, and the interlamellar water molecules interact strongly with the silicate layers, In the hydrogen form, there is a dominance of Q(3), but there are also Q(2) and Q(4) silicons. There is no NMR evidence for extensive hydrogen bonding and the SiO-H group is essentially undissociated. There are dipolar interactions between the protons and the Q(3) silicon atoms with weaker interactions with the Q(2) and Q(4), When the H-form is heated to 400 degrees C, some of the Q(3) groups are involved in dehydroxylation reactions, as evidenced by the formation of Q(4) silicons, but there are still some Q(3) silicons remaining,and hence there are OH groups in the resulting material both from interlamellar water and Si-OH groups.