Hydroxyl modification of silica aerogel: An effective adsorbent for cationic and anionic dyes

Hydrophobic silica aerogels are attractive and highly efficient adsorbent for oil wastewater. However, the hydrophobicity seriously affects their dispersibility in water and limits the application for cationic and anionic dyes adsorption. Herein, a green sol-gel method was developed to prepare silica aerogels by using rice husk ash as silica source. The results demonstrate that heat treatment significantly impacts surface functional groups and pores of silica aerogels, which can be easily adjusted by changing heat treatment temperature. Cationic (Methyl blue, MB) and anionic (Congo red, CR) dyes were used to evaluate the adsorption ability of modified silica aerogels. The maximum MB and CR adsorption capacities of silica aerogel treat at 500℃ (SA-500) reached to 428.9 and 437.2 mg/g, respectively, which was achieved by simple control of pH. Besides, the adsorption of MB and CR by the SA-500 obeys the pseudo-second-order adsorption model. Electrostatic attraction and hydrogen bonds are critical for the adsorption of MB and CR. Furthermore, the adsorption efficiency of SA-500 for MB and CR was still as high as 95.5 % and 89.5 %, respectively, after 10 cycles of adsorption/desorption. Thus, SA-500 shows a promising application in removing both cationic and anionic dyes from wastewaters.