Ordered mesoporous nanosilica derived from coal gasification fly ash for adsorption of Rhodamine B

Coal gasification fly ash (CGFA) is a kind of solid waste produced by coal gasification， and its output in⁃ creases with the development of coal gasification. The traditional utilization way of coal gasification fly ash for build⁃ ing materials is limited by the weakening demand of building materials market. Therefore， it is urgent to seek for high value⁃added utilization technology. Preparation of silicon⁃based materials using CGFA as silicon source can achieve a high value⁃added utilization of CGFA， but the low extraction efficiency of silicon and the poor properties of the pre⁃ pared materials limit the application of this technology. Based on this， an ordered mesoporous nanosilica was prepared via a two⁃step method⁃microwave assisted alkali⁃dissolution process (microwave method) followed by hydrothermal synthesis， using Shell pulverized coal gasification fly ash as silicon source. The differences between microwave method and conventional hydrothermal method in silicon extraction efficiency and the performance of SiO2 derived from the ex⁃ tracted solution obtained by the two methods were compared. The structure， morphology， functional groups and surface potential of the prepared nanosilica were characterized by small⁃angle XRD， N2 adsorption / desorption， TEM， FTIR， zeta potential， and its adsorption performance for Rhodamine B (RhB) was studied. The results show that the micro⁃ wave method is efficient and can selectively extract silicon from coal gasification fly ash. The extraction efficiency of Si reached up to 20.2% within 30 min when the alkali to ash ratio (the mass ratio of NaOH to coal gasification fly ash) was 0.5， and no other impurity ions dissolved except for a low concentration of Al， which is conducive to the synthesis of high⁃purity nanosilica. When the alkali to ash ratio was in the range of 0. 5 - 1. 5， under the same conditions， the concentration of Si extracted by microwave method is 2.4-3.3 times as high as that of hydrother⁃ mal method. The prepared nanosilica was a highly ordered mesoporous material， with a specific surface area of 1 069 m2 /g， pore volume of 1.02 cm3 /g， pore diameter of 2-6 nm， and zeta potential of -63.2 mV. The material has good adsorption capacity for RhB in wastewater. More than 90% of RhB in the solution was removed by mSiO2 within 10 min at room temperature， and the maximum removal rate was 96%， which was 1.9 times as high as that of SiO2 derived from the conventional hydrothermal extraction solution. The excellent adsorption performance of mSiO2 is attributed to its porous structure and abundant negative surface charges. The used nanosilica could easily be regenerated by calcination. The excellent adsorption capacity and the facile regeneration process would endow the ordered mesoporous nanosilica broad application prospects in the adsorption field.