Removal of oil and organic solvents from water using cellulosic aerogel prepared from rice straw

Biological wastes such as lignocellulosic materials are on high attention for the purification of contaminated water due to their special properties, such as renewability, biodegradability, and low costs. However, despite its many benefits, disadvantages such as hydrophilic structure, low adsorption capacity, and the lack of flotation have limited the vast application of these natural materials. Therefore, the necessity of producing novel adsorbents with significant capacity such as aerogels has been concerned. Cellulose aerogels have good performance due to their special properties such as low density, high specific surface area and high porosity. In this paper, the effectiveness of cellulose aerogel adsorbents for removing oil and organic solvents from water has been studied. After the extraction of long cellulose micro-nano fibrils from rice straw by chemical and mechanical processes, cellulosic aerogels are fabricated and their efficacy for water pollution remediation was evaluated. The high porous structure and very low density of these cellulosic aerogels are highly desired for the adsorption of organic solvents and oils.

In this research, the rice straw was washed, dried, milled and completely passed through the mesh. The straw wax was extracted using a mixture of toluene/ethanol solution. In order to separate lignin, de-waxed rice straw was combined with a desired amount of sodium chlorite solution in an acidic environment. After de-lignification, the dry specimen was combined with potassium hydroxide solution until the hemicellulose and silica were totally removed from the fibers and pure cellulose fibers remained. After extraction of cellulose, suspension of cellulosic fibers was homogenized using high speed homogenization and the suspension was then treated with high-intensity ultrasonic apparatus with a frequency of 20 kHz. In order to produce aerogels, the suspensions of cellulose micro-nanofibres containing different amount of crosslinking agent were prepared in various concentrations. The suspensions were dried in a freeze-dryer. Finally, using a chemical vapor deposition method (CVD) with trimethoxysilane, the surface of cellulose aerogel became hydrophobic. The water adsorption capacity of cellulose aerogels in the presence of various concentrations of the crosslinker was determined to optimize the concentration of the crosslinker. Furthermore, the density and porosity of cellulosic aerogels were investigated at various cellulose contents. The structure and morphology of aerogels were evaluated using a scanning electron microscope. The application of these cellulosic aerogel in elimination of water pollution was also studied.

In the pre-freezing process, the use of liquid nitrogen resulted in the rapid freezing of cellulose suspension which produced highly porous structure of aerogels. Adding a crosslinking agent improved the wet strength and stability of the cellulose aerogel structure. As the concentration of cellulose fibers increased, the density of cellulose aerogel increased and simultaneously the porosity decreased. The scanning electron microscopic images of cross-sectional area of aerogels represented a porous structure with a pore size of one to several hundred micrometers, distributed throughout isotropic structure. The results of the water contact angle test revealed a mean value of 151.7 ° for the coated sample, indicating the super-hydrophobicity of the cellulose aerogel. The hydrophobic cellulose aerogel had a highly desirable adsorption capability in the range of 130-69 g.g-1 for different types of oil (pump oil, crude oil, paraffin, diesel oil) and organic solvents (toluene, acetone, dimethylformamide, and chloroform).

Generally, very low density and high porous hydrophobic cellulose aerogel obtained from rice straw through series of chemical and mechanical treatment, had substantial adsorption capacity for organic solvents and oils.