Study On The Preparation Of Magnetic Bentonite Material And Its Adsorption Of Heavy Metal Ions And Regeneration

Bentonite is a kind of natural mineral material.Due to its unique physical and chemical properties with excellent adsorption performance,it showed a broad application prospect for adsorption treatment of heavy metal wastewater.At present,most researchers are focusing on studying the adsorption performance and mechanism of bentonite materials used for the removal of heavy metal ions.However,bentonite is difficult to precipitate and separate from wastewater as its suspension,which seriously restrict its application in water treatment field.Moreover,the bentonite after adsorbing pollutants is possibly to cause secondary pollution.This means that the pollutants have not been successfully disposed after they are absorbed by bentonite adsorbent.Desorption regeneration and reuse of the bentonite is a sustainable development direction worthy of attention and research.In this paper,hydroxyl-aluminum pillared bentonite?H-Al-B?and aluminum pillared bentonite?Al-B?were successfully prepared with the raw material of sodium bentonite?Na-B?.Magnetic bentonite?M-B?was prepared by loading magnetic Fe₃O₄ particles onto the surface of Al-B with solvent thermal synthesis method.The adsorption property,adsorption mechanism and desorption regeneration of modified bentonites were studied systematically.This study will provide a new economic adsorbent material and some theoretical basis for treating and controlling heavy metal ions pollution.The chemical composition,microscopic morphology and magnetic properties of all modified bentonite samples were characterized by methods of FT-IR,XRD,Zeta potential,SEM,BET specific surface area and VSM.The results indicated that the basic skeleton structure of bentonite was not destroyed after being modified to H-Al-B.Keggin ions of hydroxyl-aluminum pillaring agent entered into the bentonite layers.The porosity and roughness of the bentonite surface increased greatly.The magnetic Fe₃O₄ spherical particles were evenly distributed on the surface of A-B with particle sizes of 20¹00 nm.The specific surface area and micropore area of the H-Al-B were all significantly increased and affected by the preparation conditions of the Al³⁺/Na-B ratio.At the Al³⁺/Na-B ratio of 6 mol/kg,the BET specific surface area,Langmir specific surface area and microporous area of H-Al-B increased from 49.56,66.31 and 11.35 m²/g to 247.52,324.59 and 181.61 m²/g compared with Na-B,respectively.The specific surface area and microporous area of Al-B were seriously affected by the preparation conditions of the heat treatment temperature.The BET specific surface area,Langmir specific surface area and microporous area of the Al-B prepared at 350?were the largest at 206.71,271.09 and 144.49 m²/g,respectively.The prepared M-B showed superparamagnetic with high magnetization saturation value.After modification,the adsorption performance of bentonite on heavy metal ions was significantly improved and affected by the preparation conditions.The preparation condition of the Al³⁺/Na-B ratio showed great influence on the adsorption property of H-Al-B and Al-B.H-Al-B and Al-B prepared with the Al³⁺/Na-B ratio of 6 mol/kg showed better adsorption performance on Cr⁶⁺and Pb²⁺ions,respectively.After being prepared to Al-B by heat treatment,the adsorption effect of it on Cr⁶⁺ions was reduced,the adsorption effect on Pb²⁺ions was improved and influenced by the heat treatment temperature.The Al-B prepared at treatment temperature of 350?showed better adsorption performance on Pb²⁺ions.The M-B prepared with Fe³⁺/Al-B ratio of 6 mol/kg showed better adsorption performance on Cr⁶⁺ions and Pb²⁺ions.The adsorption mechanisms and application scopes of H-Al-B and M-B on Cr⁶⁺ions,Al-B and M-B on Pb²⁺ions,M-B on Hg²⁺ions were further studied.The results showed the adsorption effect was influenced by pH,temperatures,dosages of modified bentonite and initial ion concentrations.The adsorption mechanisms were analyzed through adsorption kinetics,adsorption isothermal model and thermodynamics in detail.The results showed the adsorption process of Cr⁶⁺,Pb²⁺and Hg²⁺ions on the modified bentonite were fitted well by the pseudo-second-order models at different experimental temperatures.The adsorption rate was controlled by both particle diffusion and liquid membrane diffusion.When the Langmuir,Freundlich and D-R model were used to fit the adsorption processes of Cr⁶⁺,Pb²⁺and Hg²⁺ions onto the modified bentonite,the linear correlation coefficients R² were all relatively high.It showed that all three models can be used to describe the adsorption processes,indicating the adsorption process included both monolayer chemical adsorption and heterogeneous surface adsorption.The values of separation factor R_L and Freundlich constant n indicated that the adsorption on the surface of modified bentonite was relatively favorable.The fitting via D-R model showed that ion exchange reaction played an important role in the adsorption process.Thermodynamics study indicated that the adsorption process of Cr⁶⁺,Pb²⁺and Hg²⁺ions on modified bentonite was a spontaneous endothermic adsorption process driven by increasing of entropy.Increasing the temperature was beneficial to the adsorption of Cr⁶⁺,Pb²⁺and Hg²⁺ions.The mechanism of adsorption is very complicated,both physical and chemical adsorption played important roles in the adsorption process.However,the action degree of chemical adsorption and physical adsorption were different in all adsorption processes.The effect of physical adsorption on adsorption of Cr⁶⁺ions by M-B was stronger than that of adsorption by H-Al-B;the effect of chemical adsorption on adsorption of Pb²⁺ions by M-B was stronger than that of adsorption by Al-B.The M-B showed good performance on desorption regeneration and recycling.The desorption effect was affected by the desorption time,temperature,pH,amount and concentration of desorption solution.The desorption rate reached 64.85%using 50 ml/g and0.5 mol/L NaCl desorption liquid at pH=3.0,35°C for the first time after adsorbing Cr⁶⁺ions.The adsorption removal rate remained above 96.58%and the desorption rate remained at around 84.95%after 6 times cycles.The desorption rate reached 90.21%using 50 ml/g and 1.00 mol/L NaNO₃ desorption liquid at pH=3.0,35°C for the first time after adsorbing Pb²⁺ions.The adsorption removal rate remained above 96.58%and the desorption rate remained at around 84.95%after 6 cycles.The process of Cr⁶⁺and Pb²⁺ions desorbed from M-B can be described by pseudo-second order models,double constant models and Elovich models.The process of desorption can be divided into surface desorption stage with faster rate at earlier stage and diffusion stage with slower rate at later stage.Langmuir isotherm model can be used to describe the desorption equilibrium of Cr⁶⁺and Pb²⁺ions desorbing from M-B.After multiple adsorption–desorption cycles,M-B composites still maintained stable physical and chemical structures.The spherical Fe₃O₄ particles were evenly distributed on the bentonite surface without falling off significantly.The typical laminar structure of bentonite was not significantly changed.The porosity and roughness of the surface increased slightly.