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 degrees 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 gamma-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 degrees 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 degrees 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. (C) 2012 Elsevier Inc. All rights reserved.