Dye removal by biosorption using cross-linked chitosan-based hydrogels

Synthetic dyes are an important class of recalcitrant organic compounds that are often found in the environment as a result of their wide industrial use. There are estimated to be more than 100,000 commercially available dyes. These substances are common contaminants, and many of them are known to be toxic or carcinogenic. Colored effluents from the industry is perceived by the public as an indication of the presence of a dangerous pollution. Even at very low concentrations, dyes are highly visible—an esthetic pollution—and modify the aquatic life and food chain, as a chemical contamination. Dye contamination of water is a major problem worldwide, and the treatment of wastewaters before their discharge into the environment has become a priority. Dyes are difficult to treat due to their complex aromatic structure and synthetic origin. In general, a combination of different physical, chemical and biological processes is often used to obtain the targeted water quality. Nonetheless, there is a need to develop new removal strategies and decolorization methods that are more effective, acceptable for industrial use and ecofriendly. Currently, there is increasing interest in the application of biological materials as effective adsorbents for dye removal. Among all the materials proposed, cross-linked chitosan-based hydrogels are the most popular biosorbents. These polymeric matrices are the object of numerous fundamental studies. In this review, after a brief description of the use of chitosan in wastewater treatment and the basic principles of chitosan-based hydrogels and biosorption, we focus on some of the work published over the past 5 years. Overall, these polymeric materials have demonstrated outstanding removal capabilities for some dyes. They might be promising biosorbents for environmental purposes.