Synthesis and adsorption characteristics of hollow spherical allophane nano-particles

We synthesized three allophanes from precursors by a hydrothermal reaction at 100 °C for 48 h. The precursors were formed from the solutions of Na4SiO4 and AlCl3⋅ 6H2O at different Si/Al molar ratios (0.5, 0.75, 1.0). The nanostructure of the synthetic allophanes was compared with that of a natural allophane from New Zealand by using X-ray diffractometry, energy dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy, thermogravimetry/differential thermal analysis, 29Si and 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR), field emission electron microscopy, and pore-size distribution based on the Cranston–Inkley method. The propensity of the allophanes to adsorb adenine and adenosine-5′-monophosphate (5′-AMP) was assessed by batch experiments. The adsorption data were fitted by the Freundlich equation and the adsorption parameters were discussed in relation to the properties of the natural and synthetic allophanes. The adsorption capacity (Kf) of the natural allophane for 5′-AMP was three times that for adenine. The average Kf value of the three synthetic allophanes for 5′-AMP was more than twice that of the natural allophane, possibly due to the higher purity of the synthetic allophanes.

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► The synthetic allophanes had pores and wall structure similar to natural allophanes.
► The Si/Al ratio of the allophanes influenced the adsorption of 5’-AMP.
► The adsorption of 5’-AMP was considerably larger than on adenine.
► The hollow spherule and perforation played a significant role in the adsorption.