Preparation Of Magnetic Carboxymethyl Chitosan-based Microspheres And Its Application In The Removal Of Cr(Ⅵ) And Cu(Ⅱ)

Heavy metal-contaiminanted wastewater not only pose a threat to the environment,but also damages the health of living organisms and triggers various diseases of humans and animals.Carboxymethyl chitosan（CCS）is one of the common derivatives of chitosan.There are some problems with the unmodified CCS,for example,the amounts of functional groups are limited,the adsorption capacity for some pollutants and the separability of CCS are limited,which affect the further application of CCS.In view of these problems,magnetic carboxymethyl chitosan-based microspheres（PD-Fe₃O₄@CCS）was prepared,and its characterization,adsorption performance,and mechanisms were studied.The main research goals and conclusions as follows:（1）Magnetic carboxymethyl chitosan microspheres（Fe₃O₄@CCS）were prepared by mixing CCS and Fe₃O₄ in water-cyclohexane mixed suspension,and used glutaraldehyde as a crosslinking agent,PDMC was grafted to prepare PD-Fe₃O₄@CCS.（2）The morphology,structure and chemical characteristics of PD-Fe₃O₄@CCS and Fe₃O₄@CCS were characterized by SEM,FTIR,XPS,XRD,VSM,and TGA.PD-Fe₃O₄@CCS not only showed uniform spherical shape,and superparamagnetism,but also met the requirements of magnetic separation.Besides,it have good thermal stability,contains more organic groups,and kept the crystal structure of Fe₃O₄ during the grafting process.The zeta potential of PD-Fe₃O₄@CCS was positive and higher than Fe₃O₄@CCS.The results showed that PDMC was successfully grafted on PD-Fe₃O₄@CCS,and the polymerization mainly occurred on the amino and hydroxyl groups of CCS.（3）The adsorption of Cr（Ⅵ）by PD-Fe₃O₄@CCS was studied.The pH value had a great influence on the adsorption capacity of PD-Fe₃O₄@CCS for Cr（Ⅵ）.The adsorption capacity of PD-Fe₃O₄@CCS decreased with increasing pH value.Compared with Fe₃O₄@CCS,the adsorption capacity of PD-Fe₃O₄@CCS had been significantly improved,and the maximum adsorption capacity was 152.73 mg/g.The results of the adsorption isotherm fitting showed that the Langmuir model was the best one for fitting the experiment data,indicating that the reaction was monolayer adsorption.As for the kinetics,PSO model was the most relevant kinetic model,indicating that the rate-limiting step for Cr（Ⅵ）adsorption by PD-Fe₃O₄@CCS was chemisorption.The co-existiing ions of SO₄^2-,NO₃^-and Cl^-could reduce the removal ratio of PD-Fe₃O₄@CCS on Cr（Ⅵ）.The removal efficiency of PD-Fe₃O₄@CCS of Cr（Ⅵ）was more than 99%under different water conditions（tap water,minzhu lake,simulated wastewater）.After 5 adsorption-desorption cycles,the adsorption capacity still remained 83.7%of initial adsorption capacity.FTIR and XPS characterization of PD-Fe₃O₄@CCS and PD-Fe₃O₄@CCS loaded with Cr（Ⅵ）,showed that the main adsorption mechanisms are electrostatic interaction and ion exchange.（4）The Cu（Ⅱ）adsorption by PD-Fe₃O₄@CCS was studied.The adsorption capacity of PD-Fe₃O₄@CCS for Cu（Ⅱ）increased with the pH increase,and the maximum adsorption capacity was 45.39 mg/g.The results of the adsorption isotherm showed that the Freundlich model had the highest correlation coefficient,indicating a multilayer adsorpyion.Besides,PSO was the most relevant kinetic model,indicating that the rate-limiting step for Cu（Ⅱ）adsorption by PD-Fe₃O₄@CCS is chemisorption.Na⁺,Ca²⁺,K⁺had little effect on Cu（Ⅱ）adsorption,even at low concentration these ions had slight promotion effect.The removal efficiency of PD-Fe₃O₄@CCS for Cu（Ⅱ）was more than99%under different water conditions（tap water,minzhu lake,simulated wastewater）.After 5 adsorption-desorption cycles,the adsorption capacity still remained 84.89%.FTIR and XPS characterization of PD-Fe₃O₄@CCS and PD-Fe₃O₄@CCS loaded with Cu（Ⅱ）,showed that the adsorption mechanisms were chelation and electrostatic interaction.（5）The removal ratio of Cr（Ⅵ）and Cu（Ⅱ）in the binary system by PD-Fe₃O₄@CCS was studied.Compared with the removal ratio in single system,the removal ratio of Cu（Ⅱ）was improved,while that of Cr（Ⅵ）was reduced in the binary system.Under three different sources（tap water,minzhu lake,and simulated wastewater）,the removal ratio of Cr（Ⅵ）and Cu（Ⅱ）by PD-Fe₃O₄@CCS（1 g/L）in the binary system were all above99%.Comparing the adsorption capacity of PD-Fe₃O₄@CCS with other chitosan-based biosorbents for Cr（Ⅵ）and Cu（Ⅱ）,the result showed that the adsorption capacity of PD-Fe₃O₄@CCS is relatively high.PD-Fe₃O₄@CCS can efficiently remove Cr（Ⅵ）and Cu（Ⅱ）,and thus has good application potential.