Preparation Of Adsorption Material Based On Chitosan And Adsorption Of Heavy Metal Ions

Landfill leachate is rich of humic acid,ammonia nitrogen and other substances,creating conditions for its agricultural resources,but the presence of heavy metals in leachate can cause potential harm to human health.In this paper,an economical and efficient adsorption method is adopted for the removal of heavy metals.Chitosan（CS）is use as a low-cost and environmentally friendly biological polysaccharide,because it contained a large number of free-NH₂ and-OH and others active to removal to heavy metals.Therefore with CS as the matrix,using self-made nanometer magnetic iron oxide（NFe Ms）and polyethylene glycol（PEG）to prepare adsorbent.CS-NFe Ms gel ball and CS-NFe Ms-PEG gel ball are prepar by embedding method and embedding-gel method.CS-NFe Ms and CS-NFe Ms-PEG as the research object,two kinds of absorbents Cr（Ⅵ）and Ni（Ⅱ）as the goal to remove content.Static adsorption experiments are conduct to explore the influence of factors such as p H,dosage,adsorption time,initial concentration and temperature on the removal heavy metals in a single system and a binary system.The effects of recycling property of materials and leachate on adsorption in the binary system are investigated.At the same time,the modified adsorbent is characteriz by FTIR,SEM,EDSand XRD to explore the modification mechanism and adsorption mechanism.The main results are as follows:（1）The dissolution effect of CS and the dispersion effect of NFe Ms are dissolved with different concentrations of glacial acetic acid.When the volume fraction of glacial acetic acid is 0.25%and 0.5%,CS is not completely dissolved and NFe Ms is not dispersed.CS is completely dissolved but NFe Ms is easy to settle when the volume fraction is 1.5%,CS is completely dissolved and NFe Ms is evenly dispersed when the volume fraction is 1%.The optimal mass ratio of CS to NFe Ms is 3:2 by the control variable method.The optimal mass ratio of CS-NFe Ms-PEG is obtained by orthogonal experiment at the level of 2 factors and 5,and determine the mass ratio CS:NFe Ms:PEG=3:1:1.5.At the same experimental conditions,CS and CS-NFe Ms and CS-NFe Ms-PEG to Cr（Ⅵ）and Ni（Ⅱ）adsorption performance increase in turn.Characterization showed that NFe Ms is uniformly embedded,and cross-linked between PEG and CS.To sum up,the modification is successful.（2）In a single system experiment,the optimal p H values to Cr（Ⅵ）and Ni（Ⅱ）adsorption of the two materials are 3 and 7.The removal rate of Cr（Ⅵ）and Ni（Ⅱ）increased with the increase of adsorbent dosage.The kinetic model fitting indicates that CS-NFe Ms is mainly chemical adsorption of Cr（Ⅵ）and Ni（Ⅱ）.The adsorption of Cr（Ⅵ）and Ni（Ⅱ）by CS-NFe Ms-PEG is a combination of chemical and physical adsorption.The model fitting of intra-particle diffusion shows that adsorption is controlled by boundary layer diffusion,internal diffusion and chemisorption.The model fitting of Langmuir and Freundlich adsorption isotherms showed that the adsorption of Cr（Ⅵ）and Ni（Ⅱ）by CS-NFe Ms is dominated by single-layer chemisorption.CS-NFe Ms-PEG shows that both single-layer chemical adsorption and multi-layer physical adsorption.As the initial concentration of Cr（Ⅵ）and Ni（Ⅱ）increased the adsorption capacity is increased of the two materials.The adsorption effect of temperature is opposite of Cr（Ⅵ）and Ni（Ⅱ）.The thermodynamic fitting results show that the adsorption of Cr（Ⅵ）is a spontaneous exothermic process,while of Ni（Ⅱ）is a spontaneous endothermic process.（3）The adsorption effect of p H is consistent with a single and binary system of Cr（Ⅵ）and Ni（Ⅱ）.The equilibrium time of CS-NFe Ms and CS-NFe Ms-PEG is 300min and 280min respectively.Compared with the single system,the removal rate of adsorbent dosage in the experiment changed little.It is because of the synergistic effect Cr（Ⅵ）and Ni（Ⅱ）with opposite charges are prone to electrostatic shielding,which will increase additional adsorption sites in the binary system.However,there is competition between Cr（Ⅵ）and Ni（Ⅱ）,and the synergistic and competitive effects will cancel each other out.CS-NFe Ms is a stronger adsorption of Ni（Ⅱ）than Cr（Ⅵ）,because Ni（Ⅱ）has a smaller ion radius so more likely to enter the active site inside the gel sphere through surface pores.CS-NFe Ms-PEG showed high adsorption selectivity of Cr（Ⅵ）.This is because CS-NFe Ms-PEG has a three-dimensional network structure,which weakens the influence of the radius of heavy metal ions on the adsorption performance.At the same time,the acidic reaction conditions make the surface of the gel ball positively charged,and strengthen the binding of Cr（Ⅵ）in the negative valence state.The experimental results of desorption recirculation and leachate show that the two materials have excellent recycling properties and development prospects.It is recommended to adjust p H from small to large in practical engineering applications.（4）The FTIR characterization show that adsorption of Cr（Ⅵ）mainly is supported-NH₂and–OH^-,but Ni（Ⅱ）is adsorbed by-NH₂,while both Fe-O bond and–C-O-C-participate adsorption.FTIR results show that two modified materials have a large specific surface area,CS-NFe Ms-PEG has a three-dimensional network structure especially.EDS results show that heavy metals are combined with materials.XRD shows that the crystallinity of the two materials is obviously weakened compared with CS,which is conducive to the adsorption of the materials.（5）According to the adsorption mechanism,there are a large number of free-NH₂ on the CS molecule,and the free electron dipoles held by N atoms and Ni（Ⅱ）form stable chelates through chelation and mechanism.And-NH₂ can borrow hydrogen bonds from ortho-OH to form a network structure,then the molecule can have a stable coordination effect with Ni（Ⅱ）.Covalent bonds can be formed with Ni（Ⅱ）in the-OH group and-C-O-C-group of NFe Ms through sharing electron pairs.Adsorption of Cr（Ⅵ）is carried out under acidic conditions,and many of-NH₂ is transformed into-NH³⁺.It can be combined with Cr（Ⅵ）through electrostatic interaction and hydrogen bond interaction,and the lone pair electrons of N atom in-NH₂ can be invested into the 3d empty orbital of Cr（Ⅵ）forming coordination bond.At the same time,the unique magnetic characteristics of NFe Ms can improve the spatial ordering of molecular arrangement,increase the coordination sites and reduce the internal mass transfer process,so enhancing the removal effect of Cr（Ⅵ）and Ni（Ⅱ）.