Removal Of Heavy Metals From Aqueous Solution By Spent Layered Double Hydroxides After Dye Adsorption

The wastewater of dye industry,a serious pollution to the environment,has always been one of the most difficult industrial wastewater to treat at home and abroad.Recently,adsorption method has attracted much attention due to its high selectivity and low-cost,but how to deal with the materials and achieve regeneration after adsorption of the dye has become the focus of research.Layered double hydroxide?LDH?shows a broad application prospect in the adsorption of the dye because of the special memory effect and anionic exchange ability.In this paper,we used spent Mg/Al-LDH and Fe₃O₄@LDH after congo red adsorption to synthesize the LDO/C and Fe₃O₄@LDO/C composite material.The composites were characterized and adsorbed three heavy metals by batch adsorption experiments to evaluate the adsorption performance.The results are shown as follows:The Mg/Al-LDH and Fe₃O₄@LDH were synthesized by low saturation coprecipitation method,adsorbed congo red dye to equilibrium as spent adsorbents,and then calcinated to yield LDO/C and Fe₃O₄@LDO/C composite material.The LDO/C and Fe₃O₄@LDO/C were characterized by FTIR,XRD,SEM,TG,EDS and BET characterization,the results showed that the synthesized composite materials had feature of diffraction peaks Mg?Al?O and C,the Fe₃O₄@LDO/C composite also had the characteristics of Fe₃O₄ diffraction peaks.When the LDO/C and Fe₃O₄@LDO/C composites were dispersed in water,the Mg/Al-LDH characteristic diffraction peaks appeared again,this indicated that they recovered back to the original structure of LDH because of its memory effect.We conducted the adsorption experiments of Cu²⁺,Cd²⁺and Pb²⁺by the LDO/C composites.The results show that the adsorption efficiency of the LDO/C for the heavy metals was higher.The best dosage of adsorbent all were 0.05 g,and the adsorption equilibrium time were 300 min,60 min and 500 min for Cu²⁺,Cd²⁺and Pb²⁺,respectively.The adsorption rate followed the order of Pb²⁺< Cu²⁺< Cd²⁺and adsorption kinetics data followed the intra-particle diffusion model,and also conformed to the pseudo-second-order model.The equilibrium adsorption isotherm data were well fitted the Langmuir model for Cu²⁺and Cd²⁺,the Freundlich and Langmuir model for Pb²⁺.In addition,the maximumadsorption capacity of Cu²⁺,Cd²⁺and Pb²⁺were 13.64,15.72 and 10.31 mmol/g,respectively.We performed the adsorption experiments of Cu²⁺,Cd²⁺and Pb²⁺by the Fe₃O₄@LDO/C composite.The results show that the adsorption capacity of Fe₃O₄@LDO/C was significantly higher than that of Fe₃O₄@LDH and CR/Fe₃O₄@LDH?the composite of Fe₃O₄@LDH after adsorption of congo red?.The kinetic data of Fe₃O₄@LDO/C composite for Cu²⁺,Cd²⁺and Pb²⁺adsorption showed good correlation to the intra-particle diffusion equation and the pseudo-second-order model.The adsorption isotherm data were in line with the Langmuir model for the heavy metals adsorption,and the maximum adsorption capacity of Cu²⁺,Cd²⁺and Pb²⁺were 3.96,4.76 and 1.86 mmol/g,respectively.Furthermore,the Fe₃O₄@LDO/C can be quickly and easily separated from the solution by magnetic separation technology.