Fabrication of aerogels from cellulose nanofibril grafted with β-cyclodextrin for capture of water pollutants

Interactions at the molecular and surface chemistry are some of the key factors that determine the adsorption capacity of pollutants and emerging contaminants in porous materials. As filtration-based purification of water sources expands, the generation of green materials, such as biopolymers, is the priority. However, to increase the removal capacity, modification of natural polymers appears necessary. Nanomaterials, especially bio-based materials like cellulose nanofibrils, inherently have large surface areas as a consequence of their high aspect ratios. Their capacity to modulate the interactions with contaminants present in water can be modulated by incorporating selective active points, such as hydrophobic cavities, that can further improve their overall adsorption capability. A bio-based material that can fulfil this requirement is β-cyclodextrin, a cyclic oligosaccharide with seven glucose units, which provides an easy grafting strategy onto cellulose due to structural affinity. Another advantage of using cellulose nanofibril is their film formability, aerogels, and hydrogels without the need of harsh chemicals or processes. In this work, an oligosaccharide with a hydrophobic centre – β-cyclodextrin – was immobilized onto bleached softwood cellulose nanofibrils, and then used to generate high surface area aerogels with a density of 175 kg/m3 and porosities above 88%. Charge density titration, Fourier transform infrared with attenuated total reflectance (FTIR-ATR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and atomic force microscopy (AFM) characterization techniques were used to assess the successful modification of the fibrils. Inductive coupled plasma mass spectroscopy (ICP-MS) was used to determine the lack of trace chlorine in the material from the grafting process while scanning electron microscopy (SEM) and dynamic vapor sorption (DVS) were used to determine porosity and surface area of the aerogels. The adsorption capacity was tested with two molecules of different natures: a cyanotoxin (microcystin-LR) and a dye (methylene blue), using high-performance liquid chromatography with a UV detector (HPLC–UV) and UV–vis spectroscopy, respectively. The adsorption in equilibrium for CNF-CD aerogels was calculated to be 0.078 mg/g of microcystin-LR and 3.46 mg/g of methylene blue, enlightening its possible use to improve water quality.