Hybrid lignin-silica as a green adsorbent towards methylene blue in batch and fixed-bed column

Abstract

The number of research regarding the ‘lignin-silica complex first’ approach focusing on the synthesis of a long polymer with SiO2 distributed on the surface has been increasing significantly in recent years. Despite being considered an abundant source for the recovery of this hybrid, black liquor has not been widely employed in the synthesis of lignin/silica-derived materials. In order to propose a solution to utilize the waste liquid from the cellulose production process instead of current synthesized silica-containing compounds in the synthesis of highly effective materials for environmental treatment, this study aims to produce a lignin-silica hybrid (LS) from black liquor generated from rice straw alkaline treatment via sol-gel process. The difference in the material characteristics determined by XRF, FT-IR, SEM, and isothermal nitrogen adsorption at 77K led to the different capacities in methylene blue (MB) adsorption. The SiO2 content in the material increased with respect to pH value, which resulted in a higher specific surface area (SBET). Specifically, the greater SBET belonged to LS recovered at pH = 9 (LS9) with a value of 166.5 m2/g. Additionally, the presence of numerous negatively charged groups (i.e., COO−, OH) and silanol in the LS structure resulted in a strong affinity towards MB, a cationic dye. LS9 exhibited a better performance in MB removal with a capacity of nearly 50 mg/g in comparison with the value of LS7, which was around 45 mg/g. Along with the proposed adsorption mechanism, kinetic adsorption, isothermal adsorption, and fixed-bed column adsorption were also investigated to interpret the adsorption processes.