Kanemite (NaHSi2O5·3H2O) and its hydrogen-exchanged form

Abstract

Kanemite, NaHSi₂O₅·3H₂O, 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 cm³ g^–1, whereas at an equilibrium pH of 4.88, K_d=70 cm³ 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 Q³ 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 Q³, but there are also Q² and Q⁴ silicons. There is no NMR evidence for extensive hydrogen bonding and the SiO-H group is essentially undissociated. There are Bipolar interactions between the protons and the Q³ silicon atoms with weaker interactions with the Q² and Q⁴. When the H-form is heated to 400 °C, some of the Q³ groups are involved in dehydroxylation reactions, as evidenced by the formation of Q⁴ silicons, but there are still some Q³ silicons remaining, and hence there are OH groups in the resulting material both from interlamellar water and Si-OH groups.