Cellulose-based aerogels prepared by freeze-drying for selective adsorption of cationic/anionic dyes

Nanocellulose (NC) aerogels with positively and negatively charged groups were prepared for selective dyes adsorption. The 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidation was employed to prepare carboxylated nanocellulose (CNC)-NC aerogels for cationic dyes adsorption. Polyethyleneimine (PEI) was physically crosslinked with CNC to prepare CNC-PEI aerogels with amino groups for anionic dyes adsorption. Directional freezing was adopted to construct mono-directional pores for intensifying mass transfer, and dielectric-aided microwave heating was used to intensify heat transfer for freeze-drying. The results showed that the proposed drying method could reduce more than 40% of drying time. The maximum adsorption capabilities of prepared aerogels were 778.70 mg⋅g−1 for Crystal violet (CV) and 543.81 mg⋅g−1 for Methyl orange (MO). Adsorption kinetics and isotherm experiments were conducted to analyze adsorption mechanism. The aerogels showed excellent adsorption selectivity in hybrid dyes solution and satisfactory reusability in 10-cycle operations, possessing an outstanding promise for dyeing wastewater treatment.