Study On Simultaneous And Synergistic Removal Of Anionic And Cationic Dye From Aqueous Solution By Using Magnetic Chitosan Composite Adsorbent

In recent years,a large amount of organic dye wastewater discharged from dye production,textile,papermaking,leather and other industries has caused serious water pollution problems worldwide and caused great harm to the water environment on which humans depend.Therefore,an effective technique for dye contamination is necessary before it is discharged into the water.At present,there are a variety of methods for treating dye wastewater.Among them,the adsorption method is considered to be one of the most promising candidate technologies for the treatment of a single dye because of its high removal efficiency,low cost,simple operation and no secondary pollution.However,different types of dyes usually coexist in actual wastewater,and the different dyes vary greatly in charge and structure.It is still very challenging to treat wastewater bearing diverse sorts of organic dyes simultaneously via one-step adsorption.Herein,glutaraldehyde was used as the cross-linking agent,and the magnetic chitosan nanocomposite（Fe₃O₄-CS）was prepared by chemical cross-linking.The structure of the adsorbent was characterized in a variety of ways,and the adsorption performance and mechanism of methyl orange（MO,an anionic dye）and methylene blue（MB,a cationic dye）in its single and mixed system were studied.Finally,the practical application potential and recycling performance of the adsorbent in simulated wastewater were analyzed.The research results obtained are as follows:（1）The successful synthesis of the Fe₃O₄-CS magnetic nanocomposite adsorbent was demonstrated by characterizing and analyzing the microstructure,crystal phase structure,functional groups,thermogravimetry,specific surface area,pore characteristics,and magnetic properties of the prepared materials.（2）The results of adsorption experiments in a single pollutant solution show that Fe₃O₄-CS was a prominent adsorbent toward MO,and the adsorption process could be better described by pseudo-second-order kinetic model and Langmuir isotherm model,the adsorption of MO was a spontaneous and exothermic process,and its saturated adsorption capacity reached up to 638.6 mg g^-1.However,no obvious adsorption was observed for MB in its individual dye solution no matter how the adsorption conditions（adsorbent dosage,solution p H,ionic strength）were changed.（3）In the MO/MB mixed dye solution,MB removal was significantly enhanced by the co-existing MO and its adsorption capacity was proportional to the concentration of coexisting MO.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,π-πstacking,etc.）to achieve simultaneous and synergistic removal of MO and MB from their mixtures.（4）The magnetic nanocomposite Fe₃O₄-CS also has excellent reusability.In simulated wastewater,satisfactory removal can still be achieved after three adsorption-desorption cycles.In addition,its unique magnetic separation performance makes it become a promising candidate adsorbent for treating simulated dyeing effluents bearing both cationic and anionic dyes.Thus,this work provides an important idea of treating complex wastewater with different sorts of pollutants using a structurally simple adsorbent.