Study On Characteristics Of Sulfate Adsorbents Based On Hydrotalcite-like Compounds

Sulfate ion is a kind of anion widely existing in natural waters and industrial wastewater.The discharge of wastewater containing sulfate ion without treatment can lead to the decrease of water quality,destroy the ecological balance and bring damage to the health of aquatic animals and plants and human beings.Recently,a variety of methods have been applied to remove sulfate ion from water including chemical precipitation,membrane processing,electro coagulation,ion exchange,adsorption and biological treatment.Compared to other methods,adsorption is simple to operate,high-efficiency,low-cost and less environmental pollution.In this research,a series of MgAl Fe hydrotalcite-like compounds and zinc aluminum hydrotalcite-like composites were synthetized by co-precipitation method.The calcined MgAl Fe hydrotalcite-like compounds and zinc aluminum hydrotalcite-like composites were used as adsorbents to remove sulfate ion in the solution.The crystalline state,surface morphology and composition of adsorbents were described via XRD、FT-IR、SEM and EDS elemental analysis.The adsorption characteristics and mechanisms were studied via adsorption influencing factors,adsorption kinetics and thernodynamics.The results show in following:（1）After 823 K calcination of MgAlFe hydrotalcite-like compounds,the calcined productions were used as adsorbents to remove sulfate ions in the solution.The analyses of XRD and SEM show the sheet structure of MgAl Fe hydrotalcite-like compounds are destroyed after calcination,and then recovered by rehydration with sulfate ions incorporated.The best molar ratio of Mg/Al/Fe is 3:0.8:0.2.The amount of sulfate ion is increasing as time goes on.The adsorption equilibrium reaches at 180 min.The change of acidity and alkalinity of aqueous solution can affect the adsorption efficiency.It performed best at pH 2.0.The coexisting ions can influence the adsorption amount in the order of H₂PO₄^-> CO₃^2-> Cl^-> NO₃^-.The pseudo-second order model is more suitable to describe the adsorption process.It suggests that the rate limiting step is the chemical adsorption.Freundlich model fit the experiment data well.It means that the adsorption is multilayered over the heterogeneous surface.Thermodynamic parameters indicate that the adsorption reaction is endothermic and spontaneous in nature.（2）The zinc aluminum hydrotalcite-like composites formed by the intercalation of glutamic acid,histidine and phenylalanine were respectively used as adsorbents to remove sulfate ions in aqueous solution.The results of XRD,FT-IR,SEM and EDS elemental analysis show the material are composites of zinc aluminum nitrate hydrotalcite-like compounds and zinc aluminum amino acid hydrotalcite-like compounds.The composition can affect the adsorption efficiency as following: phenylalanine > glutamic acid > histidine.With the increase of SO₄^2-initial concentration,the loading capacity improves and removal rate decreases.With the increase of acidity or alkalinity of aqueous solution,the adsorption amount decreased.The coexisting ions can influence the adsorption amount in the order of CO₃^2-> H₂PO₄^-> Cl^-≈ NO₃^-.The results of adsorption kinetics study shows that for different hydrotalcite-like composites,the rate limiting factor of adsorption reaction is different.The Freundlich model fit the experiment data well.It means that the adsorption is multilayered over the heterogeneous surface.ΔH and ΔS are positive.It indicates that the adsorption reaction is endothermic and the randomness increases during adsorption.Under the condition of different temperature,the sign of ΔG is different.It influences the spontaneous direction of adsorption reaction.The results show hydrotalcite-like composites adsorb sulfate ions mainly through ion exchange,electrostatic force and physical adsorption.