Loading Of Chitosan On Montmorillonite And Adsorpiton Of Metal Ions On The Composite

During recent years,synthesis and application of montmorillonite and chitosan based materials have attracted researcher`s interestes.These "green" and low-cost natural materials could meet many requirements for the adsorbents.The montmorillonite-chitosan composite could provide hydrophobicity and active groups to enhance the performances of montmorillonite in wastewater treatment.However,the interpretations in literature on the adsorption of chitosan-metals and composite-metals were conflicting and confusing somewhere,the mechanisms of the adsorption/intercalation of chitosan on/in montmorillonite were not well understood,and the application needed to be deeply studied.In this study,low molar mass（LC）and medium molar mass chitosan（MC）were selected to intercalate a sodium montmorillonite（Mt）.The adsorption isotherm of LC and MC on the Mt,and the p H dependency of chitosan adsorption were measured.The composites with different initial mass ratios of Mt to chitosan were used for characterization of the composites.The interlayer spacings and sludge settling ratios were investigated with X-ray diffraction to obtain the best procession of the synthesis.The adsorption of Cu²⁺ on the composite and Mt toward different p H and concentrations of acetate ligand were investigated.To get more information of the multielements adsorption on the composite,the adsorption of Cu²⁺,Pb²⁺and Cd²⁺ on the composite were investigated.Pseudo-first,pseudo-second and intraparticle diffusion models were used to fit the kinetic adsorption of the single and binary metal system.Thermodynamics parameters,such as the changes of enthalpy,entropy and free Gibbs energy of the metals adsorption were calculated from the thermodynamics adsorption at different temperature.Langmuir and Freundlich equations were used to analyze the mechanism of isotherm adsorption.The in-situ attenuated total reflection Fourier transform infrared spectroscopy was conducted to record the spectra during the adsorption process of Cu²⁺ on the adsorbents and the groups changes with p H perturbations.The two-dimension correlation analysis（2D-COS）was performed with program “shige” and “Noda” rules.The results showed:（1）The best sludge-settling ratio was obtained when the Mt was dispersed in the chitosan solution and the mass ratio of Mt to chitosan was 10: 1.With hydrochloric acid as the solvent of chitosans,increasing p H from 3 to 5.5 enhanced the adsorption capacity of both LC and MC on the Mt.When the equilibrium concentration of chitosans approached 300 mg/L at p H 5.5,the amount of adsorbed chitosan reached the highest level of 203 mg/g for LC and 190 mg/g for MC,respectively.When the Mt was saturated with the chitosan,the amino groups on the chitosan were bonded with the Mt and there was likely a monolayer of the chitosan in the interlayer space of Mt,which was confirmed by Fourier transform infrared spectra and XRD patterns,respectively.（2）The Langmuir adsorption isotherm indicated the maximum Cu²⁺ adsorption capacity by chitosan-Mt composite was greater than that by Mt.While the Mt had better affinity than composite with Cu²⁺ at low concentration of Cu²⁺.Acetate ligand at different p H and concentrations displayed the influences on the adsorption.With the analysis by programs Medusa and Visual Minteq,the effect of acetate ligand on the copper ions was presented and the effect on the surface of absorbents was investigated.The co-existed acetate increased Cu²⁺ adsorption on the composite and Mt at low ligand concentration（0.08 mmol/L）or low p H（<4.0）,and presented negative effect on the adsorption by the composite and Mt at high ligand concentration（4 mmol/L）or high p H（>4.5）.Due to the chelating ability of amino,the Mt-chitosan composite was better than Mt at various p Hs and with the existence of acetate ligand.（3）the affinity sequence and the capacity of the adsorption toward the 3 cations is Pb²⁺>Cu²⁺>Cd²⁺.One of the cations could improve the adsorption of the other coexisted in the binary system,except the Pb²⁺-Cd²⁺.The adsorption of the 3 cations could be better fitted with pseudo-second order equation than the others,indicated chemisorption is the rate controlling step.The metals adsorption are both heterogeneous and exothermic reaction on the surface of the composite,and Pb²⁺,Cu²⁺ adsorption are spontaneous reaction at 25-50 oC.（4）The adsorption of Cu²⁺ both on composite and chitosan were due to the performance of amino group on C2 and the hydroxyl group on C3 and C6 position in the chitosan or chitosan on the composite.When the p H was 3.0,the amino acted as –NH3+ and had weak relationship with the Cu²⁺,and mainly the hydroxyl group was bonded with Cu²⁺.When the p H increased to 5.0,the amino groups changed their form from –NH3+ to –NH2,chelated with Cu²⁺ and freed part of the hydroxyl.The adsorbed amount of Cu²⁺ mainly relied on the interaction with amino,so did the increased adsorption amount from p H 3 to 5.This study proves the mechanism of the bonding between Mt and chitosan,and provides further understanding on the synthesis of the composite.The efficiency and mechanism of the metals adsorption on the composite are identified with adsorption experiments and in-situ Fourier transform infrared spectroscopy analysis,which prefers scientic suggestion for the application.In the future,the synthesis of Mt-chitosan could be modified to improve the abilities and stabilities of the composite in the removing of heavy metals in different media.