Adsorption and desorption of cationic malachite green dye on cellulose nanofibril aerogels

Ultra-light aerogels have been assembled from cellulose nanofibrils into hierarchically macroporous (several hundred μm) honeycomb cellular structure surrounded with mesoporous (8-60 nm) thin walls. The high specific surface (193 m2/g) and surface carboxyl content (1.29 mmol/g) of these aerogels were demonstrated to be highly capable of removing cationic malachite green (MG) dye from aqueous media. The rapid MG adsorption was driven by electrostatic interactions and followed a pseudo-second-order adsorption kinetic and monolayer Langmuir adsorption isotherm. At a low 1:5 mg/mL aerogel/MG ratio, both initial MG adsorption rate (2.3-59.8 mg g−1 min−1) and equilibrium adsorption capacity (53.0-203.7 mg g−1) increased with increasing initial MG concentrations from 10 to 200 mg/L, reaching a maximum adsorption of 212.7 mg g−1. The excellent dye removal efficiency was demonstrated by complete MG removal through four repetitive adsorptions at a low 1:5 mg/mL aerogel/MG ratio and 10 mg/L dye concentration as well as 92% MG adsorption in a single batch at one order of magnitude higher10:5 mg/mL aerogel/MG ratio and 100 mg/L dye concentration. The adsorbed MG in aerogels could be desorbed in aqueous media by increasing ionic strength, demonstrating facile recovery of both dye and aerogel as well as the robust capability of this aerogel for repetitive applications.