Mesoporous/microporous silica materials: Preparation from natural sands and highly efficient fixed-bed adsorption of methylene blue in wastewater

The chemical and mineral compositions, particle size, hydrothermal dissolubility of four silica sands with different origins, including two sands from crushed quartz stones, one from a desert and one from a sea bed deposit, and their availability to synthesize nanoporous silica have been carefully analyzed and compared. The results indicate that irrespective of the chemical composition and crystallinity, the four sands show similar dissolution trends in alkaline solutions under hydrothermal conditions, that is, the dissolubilities of the sands increase with treatment temperature, time and the alkaline concentration. The main difference in the dissolubilities of the four sands is correlated with the differences in the sand particle size. The larger the size, the smaller the dissolubility. The dissolved silica solutions were successfully used as silica source to prepare mesoporous/microporous silicas without significant difference. The yield of the synthesized silicas decreases sharply when the pH of the dissolved silica sand solution was adjusted to be higher than 9.5. The meso- and micro-silicas prepared at pH of 9.5 and 11 using the cheaper sand from a desert were evaluated in the fixed-bed adsorption of methylene blue (MB) from aqueous solution. It is found that both silicas have good fixed-bed performance toward MB. And the adsorption well fits the Thomas model. The column packed with the microporous silica has longer breakthrough and exhaustion times than with the mesoporous one. (C) 2017 Elsevier Inc. All rights reserved.