Influence of synthesis conditions on mercury adsorption capacity of propylthiol functionalized SBA-15 obtained by Co-condensation
Thiol functionalized mesoporous silicas for aqueous mercury removal by adsorption have been synthesized by co-condensation of 3-mercaptopropyltrimetoxysilane (MPTMS) and tetraethoxysilane (TEOS) in acidic medium with the block copolymer Pluronic 123 as structure directing agent. The influence of two synthesis parameters, the time of TEOS prehydrolysis and the molar ratio x = MPTMS / (MPTMS + TEOS), on both the textural properties and mercury adsorption capacities have been investigated. Materials were characterized by low-angle XRD, nitrogen adsorption-desorption, 29Si NMR, TGA-DTA, transmission electron microscopy and elemental chemical analysis. Adsorbents are mostly mesostructured SBA-15 type silicas with suitable textural properties for mercury adsorption and exhibit thermal stability up to at least 285ºC. The amount of propylthiol incorporated to the materials is in accordance with the MPTMS / TEOS proportion in the synthesis medium (up to 4.1 mmol of S per gram of adsorbent). We have found that it is neccessary to employ a minimal amount of TEOS in order to preserve the mesoporous structure in the resulting material when a high quantity of propylthiol groups is incorporated. Prehydrolysis of silica precursor (TEOS) for 45 minutes before MPTMS addition leads to succesful results. Longer prehydrolysis times can improve the textural and structural properties of materials but the amount of propylthiol incorporated and the mercury adsorption capacity are not superior. Experimental isotherms show that materials are efficient adsorbents of mercury at low aqueous concentration with maximum adsorption capacity as high as 4.1 mmol of Hg(II) per gram of adsorbent.
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