| Chitosan(CS),a chemical derivative obtained through alkaline deacetylation of chitin and found naturally in certain fungal cell walls,is one of the high-performance materials with advantages of widespread availability,environmental friendliness,and biodegradability.Moreover,CS has been also considered as an ideal adsorbent because of the abundant free amino and hydroxyl groups on its backbone.Various pollutants such as numerous metal ions and many dyes could be easily bound to these groups due to chelating effects or/and electrostatic attraction.Moreover,on the basis of structure-activity relationship of materials,various functional groups could be flexibly grafted onto chitosan for effective adsorption of target pollutants.However,chitosan-based adsorbents could not be separated from water rapidly and easily after achieving adsorption equilibrium in practical applications because of their extremely small size.The magnetic separation technique has recently been proven to be an effective method to accelerate the process of separation and is widely applied in water treatment.After being impregnated with magnetism,the separability of the adsorbents is greatly enhanced under external magnetic fields.However,different kinds of pollutants usually coexist in real wastewater.Evidently,removing and separating the target pollutants in their individual pure form efficiently through the selectivity of certain adsorbent is significant for recycling use and avoiding secondary pollution.However,most traditional adsorbents,including chitosan,have no evident selectivity to certain individual pollutant.Accordingly,different kinds of effective functional groups have been introduced onto adsorbents by chemical modification to improve the selectivity.In this work,a chitosan-graft-polyacrylamide magnetic composite microsphere(CS-PAM-MCM)and a quaternary ammonium salt modified chitosan magnetic composite(CS-CTA-MCM)were both prepared by a simple method.The former adsorbent showed enhanced selective removal of in Cu2+,Pb2+,and Hg2+mixed system;while the latter one could selective removal of methyl orange(MO)from MO and chromium(Cr(?))mixture at suitable pHs.Moreover,to further improving the selectivity and adsorption efficiency,a porous chitosan-based adsorbent from CS-CTA-MCM was parepared which owns enhanced,rapid,and selective adsorption of MO from MO and Cr(VI)mixture.These results have been drawn as shown below:(1)A novel magnetic composite microsphere based on polyacrylamide(PAM)grafted chitosan and silica-coated Fe3O4 nanoparticles(CS-PAM-MCM)was successfully synthesized by a simple method.The prepared CS-PAM-MCM was applied as an efficient adsorbent for the removal of Cu2+,Pb2+,and Hg2+ from aqueous solutions in respective single,binary,and ternary metal systems.Compared with chitosan magnetic composite microsphere(CS-MCM)without modification,CS-PAM-MCM showed improved adsorption capacity for each metal ion and highly selective adsorption for Hg2+from Pb2+ and Cu2+.This improvement is attributed to the formation of stronger interactions between Hg2+and the amide groups of PAM branches for chelating effects.The adsorption isotherms of Hg/Cu and Hg/Pb binary metal systems onto CS-PAM-MCM are both well-described by extended and modified Langmuir models,indicating that the removal of the three aforementioned metal ions may follow a similar adsorption manner;that is,through a homogeneous monolayer chemisorption process.Furthermore,these magnetic adsorbents could be easily regenerated in EDTA aqueous solution and reused virtually without any adsorption capacity loss.(2)A quaternary ammonium salt modified chitosan magnetic composite adsorbent(CS-CTA-MCM)was prepared by combination of Fe3O4 nanoparticles.CS-CTA-MCM was employed for the removal of Cr(VI)and MO from water in respective single and binary systems.Compared with chitosan magnetic adsorbent(CS-MCM)without modification,CS-CTA-MCM shows evidently improved adsorption capacity for both pollutants ascribed to the additional quaternary ammonium salt groups.Based on the adsorption equilibrium study,MO bears more affinity to CS-CTA-MCM than Cr(VI)causing a considerable extent of preferential adsorption of dye over metal ions in their aqueous mixture.However,at weak acidic solutions,Cr(VI)adsorption is evidently improved due to more efficient Cr(VI)forms,i.e.dichromate and monovalent chromate,binding to this chitosan-based adsorbent.Thus chromium could be efficient removal together with MO at suitable pH conditions.The adsorption isotherms and kinetics indicate that CS-CTA-MCM for Cr(VI)and MO both follow a homogeneous monolayer chemisorption process.This magnetic adsorbent after saturated adsorption could be rapidly separated from water and easily regenerated using dilute NaOH aqueous solutions then virtually reused with little adsorption capacity loss.(3)A porous chitosan-based adsorbent(porous CS-CTA)has been obtained conveniently just by removal of embedded Fe3O4 from previously well-prepared CS-CTA-MCM.Compared with CS-CTA-MCM and CS-CTA without porous structure,porous CS-CTA shows not only evidently improved and rapid adsorption of aforementioned two pollutants due to its larger specific surface area but also enhanced selective adsorption of MO over Cr(VI)in their aqueous mixture at neutral and alkaline conditions ascribed to the stronger affinity of MO to porous CS-CTA than Cr(VI).Interestingly,at weak acidic solutions,chromium could be efficient removal with MO together due to more efficient Cr(VI)forms binding to this chitosan-based adsorbent.The porous micro-structure of CS-CTA results in its higher adsorption efficiency in removal of those two pollutants.Above all,the modified chitosan-based magnetic adsorbents not only exhibited enhanced adsorption performances for metal ions and dyes but also showed evidently selectivity for target pollutants,which is significant in real applications.These advantages make the chitosan-based adsorbents have great application potentials in the field of wastewater treatment. |
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