Surface silylation of mesoporous/macroporous diatomite (diatomaceous earth) and its function in Cu(II) adsorption: The effects of heating pretreatment

The calcined product of naturally occurring porous diatomite was previously assumed to be structure-destroyed or at least surface-reconstructed and therefore unsuitable for surface silylation. The present study indicates that the porosity of the mesoporous/macroporous diatomite remains intact after calcination at temperatures as high as 800 °C, and the surface silylation of diatomite is achievable even for diatomite calcined at high temperatures. The interface interactions between the hydroxyl species of diatomite and γ-aminopropyltriethoxysilane (APTES) are significantly affected by heating pretreatment. Physically adsorbed water was largely preserved in diatomite at a low heating temperature, leading to the strong hydrolysis of APTES and the subsequent oligomerization between the hydrolyzed APTES species. Under heating at high temperature (800 °C), the isolated silanols initially covered by water molecules were exposed and available for the direct grafting of APTES, forming a grafting-dominant structure with high thermal stability (540 °C). The grafting-dominant diatomite had a much higher Cu(II) adsorption than the oligomerization-dominant type, because the coordination between the copper and nitrogen was stronger in the former case. These results demonstrate that heating pretreatment plays a key role in the surface silylation of diatomite, and that Cu(II) adsorption is highly dependent on the surface structure of the silylated diatomite.