Study On Preparation Of Magnetic Mg-Al Mixed Oxides And Its Adsorption Of Methyl Orange And Chromium(Ⅵ)

The Co Fe2O4/Mg Al-LDH composite was prepared by hydrothermal and coprecipitation method in this study, and then was calcined at 450℃ for 4h to obtain Co Fe2O4/Mg Al-LDO. The Co Fe2O4/Mg Al-LDO was characterized by XRD, SEM,BET, TG-DSC, FTIR, VSM technologies and so on. Effect of calcination temperature,solution initial p H value, adsorbent dosage, adsorption time, solution initial concentration, coexist anions and react temperature on Methyl orange(MO) and Cr(Ⅵ)adsorption by Co Fe2O4/Mg Al-LDO in batch experiments were investigated systematically, and the regeneration performance of the adsorbent was studied. In addition, the adsorption kinetics, thermodynamics and mechanism of Methyl orange and Cr(Ⅵ) adsorption by Co Fe2O4/Mg Al-LDO was also discussed in this study.The results of characterization showed the combination of Co Fe2O4 and Mg Al-LDH was a physical process. After calcination, the characteristic diffraction peaks of Mg Al-LDH disappeared, OH- and CO32- were removed completely, the layer structure of Co Fe2O4/Mg Al-LDH was destroyed and the magnetic Mg-Al mixed oxides was synthesized. Compared with Co Fe2O4/Mg Al-LDH, Co Fe2O4/Mg Al-LDO possessed a better thermal stability, more developed pore structure, and a larger specific surface area, which was up to 192.82m2/g. Co Fe2O4/Mg Al-LDO also had a good magnetic property, and the saturation magnetization was 11.10eum/g. Test of isoelectric point suggested that the isoelectric point was 11.96.The results of batch experiments indicated the removal efficiency of MO and Cr(Ⅵ)using Co Fe2O4/Mg Al-LDO were higher than 97% at calcination temperature450-550℃. The initial p H value had little influence on adsorption of MO, but had a significant effect on Cr(Ⅵ) adsorption, both of the removal efficiency were higher96% within p H=4-6. The optimal adsorbent dosage were 0.2g/L and 1.2g/L for initial MO concentration of 100mg/L and initial Cr(Ⅵ) concentration of 50mg/L,respectively. Coexist anions had different influences on removal of MO and Cr(Ⅵ),and the interfering effect followed the order: NO3-<Cl-<SO42-<PO43-, and with the concentration increased, the inhibition got more obvious. The adsorption of MO using Co Fe2O4/Mg Al-LDO could reach equilibrium within 120 and 360 min when the initial concentration was 100 and 200mg/L, respectively. And the equilibrium were attained at 90, 120 and 240 min for the Cr(Ⅵ) initial concentration of 50, 100 and 200mg/L,respectively. Besides that, low react temperature would facilitate removal of MO, but was unfavorable for adsorption of Cr(Ⅵ). The maximum adsorption capacities were1195.92 and 86.05mg/g at the temperature of 298 K. The results of regeneration test showed that the adsorption capacity of MO was also up to 234.88mg/g and the removal efficiency of Cr(Ⅵ) was 60.76% by regenerated Co Fe2O4/Mg Al-LDO after three adsorption-reuse cycles.The study on adsorption kinetics and thermodynamics indicated the adsorption process of MO and Cr(Ⅵ) by Co Fe2O4/Mg Al-LDO fitted pseudo-second-order equation and Langmuir isothermal model well. The adsorption process might involved with chemical sorption, and the intraparticle diffusion was not the only rate controlling step, the process was also controlled by external diffusion such as liquid film diffusion and so on. In addition, the adsorption reaction of MO and Cr(Ⅵ) by Co Fe2O4/Mg Al-LDO were exothermic and endothermic in nature, and both of the processes were spontaneous. The theoretical saturated adsorption capacity of MO and Cr(Ⅵ) were 1219.51, 86.95mg/g which were calculated by Langmuir equation,respectively.The study on adsorption mechanism revealed that the basal spacing of Mg Al-LDH increased after MO and Cr(Ⅵ) were adsorbed, MO and Cr(Ⅵ) intercalated into layered structure instead of CO32-. And this further suggested the adsorption of MO and Cr(Ⅵ) mainly relied on “memory effect” of Co Fe2O4/Mg Al-LDO, that was to say,the adsorbent rebuilt its original layered structure by uptaking MO and Cr(Ⅵ) from aqueous solution to achieve the purpose for removal of MO and Cr(Ⅵ). Moreover, the adsorption mechanism also might involved with ion-exchange interaction between MO or Cr(Ⅵ) and CO32- which inserted into interlayer of Co Fe2O4/Mg Al-LDO.In a word, compared with other adsorbents, the Co Fe2O4/Mg Al-LDO composite prepared in this study not only possessed a higher adsorption capacity, but provided a rapid separation from aqueous solution by an outer magnetic field. So the adsorbent will have good practical value and application prospect in water treatment.