Enhanced adsorption of methylene blue using modified coal gasification coarse slag

Abstract

Huge amount of coal gasification coarse slag (CGCS) is produced every year as a by product of the coal gasification process. Currently, most of the CGCS in China is disposed of in landfills, which not only occupies vast areas of land but also poses potential environmental risks, e.g., leaching of heavy metals and other pollutants contained wherein. Therefore, alternative approaches to achieve safe disposal and even resource utilization of CGCS are in urgent need.
In this study, we developed an experimental protocol to convert CGCS into mesoporous carbon─silicon composite (CGCS─CSC) from CGCS after ball mill (CGCS─BM) for the adsorbent of methylene blue (MB). The CGCS is firstly leached by an acid to remove the metal oxides and is then calcinated with an alkaline to activate the carbonaceous components and silica in the slag to form mesoporous carbon-silicon composite. The effects of various acids and calcination temperature on the pore structure were investigated. The structures, morphology, and properties of the as prepared CGCS─CSC were characterized using various analytical methods, e.g., XRD, BET, SEM, TEM, FTIR, TGA, and XRF. Then, the MB adsorption performance of the CGCS─CSC for MB was evaluated experimentally and compared with those predicted by the response surface methodology (RSM). Moreover, the kinetics and thermodynamics of the adsorption process were systematically analyzed.
The BET results reveal that the as-prepared CGCS─CSC exhibits a much higher BET surface area than CGCS─BM, i.e., 438 m2 /g and 116 m2 /g, respectively. On the other hand, the SEM images indicate that after the treatments, the morphology of CGCS─BM II was converted from a large flaky shape to granules with well─developed pores. The comparisons between the experimental and simulation results demonstrate that the RSM can accurately predict the removal capacity of CGCS─CSC for MB. Guided by the RSM, the maximum adsorption amount of MB (i.e., 556 mg/g) could be achieved when 0.5 g/L CGCS─CSC is added to a 300 mg/L MB solution at pH = 7, room temperature for 3 hours.
The kinetics study indicates that the adsorption process of MB using CGCS─CSC fits very well with the Langmuir model and follows the first─order kinetics. This suggests that the adsorption process of MB by CGCS─CSC is a monomolecular layer of physical adsorption. This is mainly caused by electrostatic interaction, i.e., via surface-active group (e.g., silica hydroxyl group) and hydrogen bonding (3450 and 1010 cm-1 ) that is evidenced by TGA and FTIR measurements, respectively. The TGA results indicate that amount of silica hydroxyl group on the as─prepared CGCS─CSC is between 24.0 and 26.5 /nm2 , much larger than those reported by other researchers.

Date of Award	Jul 2022
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Mengxia Xu (Supervisor), Tao Wu (Supervisor) & Xiang Luo (Supervisor)