| A series of Mg/Al layered double hydroxides(LDH) containting Ce(Ⅳ) were synthesized at differert conditions and calcined at different temperature and their dericatives,which are called calcined layered double hydroxides(CLDH,also called mixed oxides) were obtained.Both LDH and CLDH were characterized by XRD,FT-IR, TEM,SEM,TG-DTA,Potentiometric Titration and N2 adsorption/desorption suface area. The ability of Mg/Al/Ce mixed oxides to remove Cr(Ⅵ) from aqueous solutions was investigated.All the adsorption experiments were carried out as afunction of calcining temperature,contant of Ce(Ⅳ),cantact time,pH and temperature of aqueous solutions, concentration of adsorbate,adsorbent,competitive adsorption of inorganic aninous.The adsortive dynamics and thermodynamic and Mechanism were also studied.It was found that:(1) Crystallinity of Mg/Al/Ce-CO3-LDH synthesized using co-Precipitation method at pH=10.3±0.2 is higher than that of those synthesized at other conditions.When the molar ratio of n(Ce)/[n(Al)+n(Ce)]is higher than 0.4,the samples cantains other phase in adition to LDH phase.With the increase of contant of Ce,the suface of the samples first increases and then decreases and the pH of point at zero charge(pHPZC) increases.The layeded structure disappears when calcined at high temperature and the material of LDHs tranform into CLDHs.(2) Mg/Al/Ce-CLDH are effective in removal Cr(Ⅵ) from aqueous solutions. Ce0.3CLDH calcined at 500℃show capability in adsorbing Cr(Ⅵ).For 50 ml solutions of Cr(Ⅵ) with concentration of 50 mg/L,adsorption using Ce0.1-CLDH-500 as adsorbate reaches equilibrium within 90min and the absorption capacity is higher at pH 4-6 and temperature 25℃.The best dosage of adsorbate is 1.5 g/L.Inorganic anions, especially those with more negative charges have side effects on the adsorption of Cr(Ⅵ) on CLDH.(3) Study of Absorption kinetics showed that adsorption of Cr(Ⅵ)on Ce0.1-CLDH-500 follows Lagergren's second order equation.(4) Isothems of adsorption of Cr(Ⅵ) on Ce0.1-CLDH-500 fits well with Freundlich Isothems model.The saturated adsorption capacity reaches 89.365 milligram Cr(Ⅵ) per gram of Ce0.1-CLDH-500 and the incorporation of Ce(Ⅳ) favors the adsorption of Cr(Ⅵ).(5) Investigatory of thermodynamic revealed that the Gibbs free energy(△G) is negative and the adsorption is spontaneous;Enthalpy(△H) is negative,which revealed that adsorption is exothermic and high temperature reverse the adsorption of Cr(Ⅵ) on Ce0.1-CLDH-500;Entropy change is also negative,which argueed that the adsorption of Cr(Ⅵ) on Ce0.1-CLDH-500 lowers the DOF of the systerm.(6) The regeneration of Ce0.1-CLDH-500 after adsorping Cr(Ⅵ) showed lower adsorptive capacity of Cr(Ⅵ).(7) It was known from the XRD patterns that the structural rebuilding of CLDH plays an important role in adsorption and only partial of CLDH recoveres layer structure.The FT-IR spectras confirms that Cr(Ⅵ)incorporated into the interlayer region of recovered CLDH...
|
PDF Full Text Request