Magnetic chitosan biopolymer as a versatile adsorbent for simultaneous and synergistic removal of different sorts of dyestuffs from simulated wastewater

It is still challenging to treat wastewater bearing diverse sorts of organic dyestuffs via one-step adsorption. Herein, a structurally simple magnetic chitosan (CS) composite (Fe3O4-CS) was innovatively proposed as a versatile bioadsorbent for simultaneous removal of anionic and cationic dyes from their mixture. Results showed that resulting composite was a prominent adsorbent toward Methyl Orange (MO, an anionic dye) in its single dye solution: the adsorption process could be better described by pseudo-second-order kinetics model and Langmuir isotherm equation, and its saturated adsorption capacity reached up to 638.6 mg g(-1). However, no obvious adsorption was observed for Methylene Blue (MB, a cationic dye) in its individual dye solution no matter how the adsorption conditions were changed. Interestingly, in MO/MB mixed dye solution, MB removal was significantly enhanced by the co-existing MO and its adsorption capacity was proportional to concentration of coexisting MO, and its adsorption was highly independent on initial solution pH over pH range of 3-10. Whereas MO's removal was almost not affected by the presence of MB. Such results may be due to that free MO molecules or MO dimers in aqueous solution were preferentially adsorbed by electrostatic attraction on the adsorbent, then the adsorbed MO molecules pull MB via multiple mechanisms (such as electrostatic attraction, pi-pi stacking, etc.) to achieve simultaneous and synergistic removal of MO and MB. The excellent reusability combined with its unique magnetic separation property makes Fe3O4-CS a promising candidate adsorbent for treating simulated dyeing effluents bearing both cationic and anionic dyes.