Facile preparation and application of magnetic chitosan/fly ash composite as a hybrid biosorbent for the effective removal of direct dyes

A novel magnetic chitosan (CTS)/fly ash (FA) hybrid composite (Fe3O4@CTS/FA) was fabricated and characterized by various analytical techniques, and directly used for anionic dye removal from water as a low-cost biosorbent. The effects of some key parameters on adsorption, adsorption mechanism and reusability of Fe3O4@CTS/FA were investigated. The results show that Fe3O4@CTS/FA with sensitive magnetic responsiveness and pore structure displays strong adsorption performance for Direct Blue 86 (DB 86) and Direct Black 19 (DB 19) dyes, and the matching adsorption capacities can reach 225 and 199 mg/g, which are approximately 8.1 and 6.3 times higher than that of FA. The dye removal efficiency is almost 100 % at pH 2.0 and 25℃, and still reach 91.6 and 95.1 % with increasing pH to 8.0, respectively. The Pseudo-second-order kinetic and Langmuir models perfectly elucidate the spontaneous adsorption processes controlled by intraparticle diffusion and adsorption behaviour, respectively. Adsorption mechanism primarily involved electrostatic attraction, various H-bonding at multiple sites and n-π bonding. The dye-adsorbed composite can be rapidly and completely recovered from water by an external magnetic field and recycled at least three times. Fe3O4@CTS/FA would be a promising biosorbent with high cost-effectiveness, simplicity of preparation and implementation, easy recovery and reusability.