Green synthesis of robust chitosan-based hydrogel beads for Cr(VI) removal from water

Chitosan-based adsorbents have been widely regarded as promising candidates for water purification. However, the potential environmental burdens, such as excessive organic solvent release and high energy consumption, associated with their manufacturing stage are largely overlooked. Herein, we propose a relatively facile and green synthesis strategy for constructing a robust chitosan-based hydrogel bead (CHB-M) with high positive charge density. Specifically, the preparation process was conducted at room temperature under air atmosphere, without the use of organic solvents like cyclohexane, heating energy or inert gas. More importantly, the charge density and bead size of CHB-M can be finely tuned by simply changing the introduced cationic content and dripping method, respectively. The resultant CHB-M not only exhibited high mechanical strength but also manifested outstanding adsorption performance toward anionic pollutant Cr(VI) with an adsorption capacity of 234.99 mg/g (at pH 3.0 and room temperature for 24 h), superior to that of most previously reported adsorbents. Furthermore, a continuous flow adsorption system using CHB-M as a filter material was operated for 1440 h to purify micropolluted water. The system consistently achieved over 99 % of Cr(VI) removal efficiency with a treatment capacity of 24 L/g, demonstrating the potential of CHB-M for practical applications. Energy and economic analyses revealed the high energy efficiency and economic benefits of the synthesis process. The proposed green strategy for chitosan-based hydrogel synthesis provides a new perspective on developing truly environmentally friendly adsorbents for sustainable water purification.