Salt and surfactant induced changes in physicochemical properties of cellulose aerogels



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Cellulose aerogels are a special type of cellulose materials prepared using sol-gel process and supercritical drying techniques. They have attracted considerable attention in various fields including wastewater treatment, energy storage, biomedical, etc. due to the combination of natural properties of cellulose such as biodegradability, biocompatibility, and material characteristics of aerogels such as high porosity, lightweight, and high specific surface area. However, application in each field is guided by specific requirements. Specifically, porosity and specific surface area are the most meaningful characteristics in such applications. While porosity is very important for adsorption and thermal insulation applications, mechanical properties are important for construction applications. Therefore, the development of cellulose aerogels with special characteristics is the prime requisite for efficient applications in different areas. Fortunately, cellulose aerogels offer a unique advantage of tailoring their physicochemical properties including nanostructure and chemical functions, thanks to the abundant hydroxyl groups on their surface and the possibility of adding different additives during different stages of their applications. In this study, alkali and alkaline metal chloride salts with different cationic radii (LiCl, NaCl, and KCl, MgCl2, and CaCl2) and surface-active agents (surfactants) were used to manipulate the specific surface area and pore characteristics of cellulose aerogels. The effect of salt and surfactant concentration and types on the sol-gel transition of cellulose solutions and the physicochemical properties of the aerogels developed thereof were studied. In addition, the aerogels were used for dye adsorption after cationic functionalization to study the impact of their physicochemical properties on dye adsorption. From this study, the following major conclusions were made. • Salts concentration had a significant effect on the gelation time and specific surface area of the cellulose aerogels. The gelation time of the cellulose solution decreased progressively with an increase in salt (NaCl) concentration whereas the specific surface area and porosity increased with increase in NaCl concentration. More than 80% improvement in specific surface area and about 5% improvement in porosity were observed by adding more than 5% NaCl. • Cellulose solutions with different concentrations (0.5 – 10%) of salts having different cationic radii (LiCl, NaCl, KCl, MgCl2, and CaCl2) showed different tendencies to form intact monoliths upon regeneration. The tendency to form monoliths decreased with increase in cationic radii. The salts with smaller cationic radii (LiCl and MgCl2) significantly reduced the sol-gel transition time thereby inducing faster gelation of cellulose solutions compared to those with larger cationic radii (KCl). Moreover, salts with smaller cationic radii led to aerogels with higher specific surface area and lower porosity whereas salts with larger cationic radii led to lower specific surface area and higher porosity. • Surfactant accelerated the sol-gel process in the cellulose solution. There was a gradual decrease in sol-gel transition time with an increase in surfactant concentrations. The aerogels prepared with surfactants had higher porosity, high specific surface area, and highly fibrillated well-developed pore structures. Furthermore, the surfactant with higher HLB values imparted higher specific surface area and better thermal stability than the one prepared using surfactant with lower HLB values. • Adsorption experiments were carried out to remove C.I. Reactive Blue – 19 (RB-19), an anionic dye, from aqueous solution using three different cationized cellulose aerogel monoliths having different specific surface areas and pore characteristics: control, cationized aerogels prepared by adding KCl and MgCl2 were used for adsorption. In addition, dye adsorption by cationized cellulose aerogel powder was also performed. The aerogels with higher specific surface area (ACel-MgCl2) adsorbed anionic dye faster than those with the lower specific surface area (Control and ACel-KCl). The dye adsorption by cellulose aerogel powder was significantly faster than corresponding monoliths. Dye adsorption experiments showed that the adsorption capacity depends mostly on particle arrangement and in part on morphological properties of cellulose aerogels. In summary, this study showed a simple approach to manipulate the physicochemical properties of cellulose aerogels which could be used as a means to tailor these characteristics for specific applications.

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Cellulose Aerogels, Salts, Surfactants, Specific Surface Area, Porosity, Dye, Adsorption