Synthesis Of Mg/Mn/Al Composite Oxides And Its Properties For Adsorption Of Cr(Ⅵ)

In this study, co-precipitation was used to produce Mg/Mn/Al-LDHs and itscomposite oxide （Mg/Mn/Al-0.5-LDO-450） was obtained by high temperature （450°C）roasting in muffle furnace. The adsoption mechanism of the composite oxide to Cr（VI）in water was studied,and it was characterized by XRD, FT-IR, TG-DSC, SEM. Usingbatch adsoption experiments, effects of Mg/Mn ratio, calcination temperature,calcination time, pH value, adsorbent dosage, initial concentration, adsorption timeand competitive adsorption of inorganic anionous on Cr（VI） removal wereinvestigated. In addition, the kinetic and thermodynamic properties of the Cr（VI）adsorbed on Mg/Mn/Al-0.5-LDO-450, the regeneration and mechanism were alsoinvestigated.The Mg/Mn/Al-LDHs and Al/Mg/Mn-0.5-LDO-450were characterized by XRD,FT-IR, TG-DTA and SEM. The results showed: Mg/Mn/Al-LDHs had strong typicaldiffraction peaks of hydrotalcite, the best crystalline effect was acheieved atn（Mg）/n（Mn）5:1; Its layered structure was destroyed and its laminate was colla-psedafter calcinated at450°C and eventually it formed a composite oxidematter; TheFT-IR, TG-DSC and SEM characterization results further validated the syntheticproduct has the characteristics of hydrotalcite-like compounds; The surface area ofMg/Mn/Al-0.5-LDHs and Mg/Mn/Al-0.5-LDO-450are87.503m2/g and177.642m2/g;The isoelectric point of Mg/Mn/Al-0.5-LDO-450is11.76.Using the batch experiments, this study investigated the effect of the preparationfactors and experimental conditions on adsoption of Cr（VI）. The experimental resultsshowed: The adsorption effect of Cr（VI） gradually decreased with the increase ofmanganese content; The best adsoption of Cr（VI） was achieved at the calcinationtemperature between450℃to550℃; Calcination time had little influence on theadsorption effect of Cr（VI）; The CO₃^2-and water in the layers basically weredeprivated after calcinated at450℃for2hours and the adsorption effect of Cr（VI） wasessentially unchanged; Joining the manganese can improve its ability to adapt the pHchange and the variation of solution initial pH had little influence on the Cr（VI）adsorption on the Mg/Mn/Al-0.5-LDO-450; When the initial concentration of Cr（VI）was100mg/L, the best adsorbent dosage was2g/L; When the initial concentration ofCr（VI） rose from50mg/L to120mg/L, the removal efficiency of Mg/Mn/Al-0.5-LDO -450decreased from99.9%to67.2%, and the adsoption capacity of the unit mass ofadsorbent increased from23.3mg/g to64.9mg/g; The adsorption procedure can bedivided into two steps: the first four hours was the fast adsorption stage in which theremoval efficiency increased with adsorption time approximately linear; With theincrease of adsorption time, the adsorption capacity essentially unchanged after ninehours. The higher the anions concentration and valence of different anions and thesmaller of ionic radius for the same kind of price anons, the greater impact on theremoval efficiency; The order of the anions on the adsorption effect was PO₄^3-> SO₄^2->Cl-> NO₃^-.The regeneration experimental study showed: The order of regeneration effectwas Na₂CO₃>NaOH>NaNO3>H2O. The regeneration rate was increased by increasingthe concentration of actified solution, but it became stable after the concentration ofactified solution was greater than0.1mol/L. When the concentration of actifiedsolution was0.1mol/L, its regeneration rate was63.2%. With the increase of theregeneration times, the adsorption removal efficiency gradually decreased.The rate ofabsorption ability decreased from98.3%to64.9%after the first regeneration. The rateof absorption ability was47.4%after the third regeneration, indicating might bereused. When using Na₂CO₃as actified solution, the pseudo-second order kineticscould better describe the kinetic data of desorption experiments, and its correlationcoefficient R₂>0.99. When the actified solution’s concentrations were0.01mol/L,0.05mol/L and0.05mol/L respectively, and the maximum desorption were29.1mg/g,32.36mg/g and33.00mg/g respectively.The adsorption equilibrium curves of the Cr（VI） on Mg/Mn/Al-0.5-LDO-450could be well described by pseudo-second order kinetic and its correlation coefficientR₂>0.999. With the increase of the reaction temperature, the adsorption capacityslighly increased. The theoretical adsorption capacity which obtained by fittingparameters was slightly larger than that obtained by experiment. The data for Cr（VI）adsorption were well fitted to the Langmuir adsorption isotherm. Thermodynamicparameters showed that the adsorption of Cr（VI） on Mg/Mn/Al-0.5-LDO-450wasspontaneous and endothermic. The absolute value of G0increased with the risingof temperature, showing that this adsorption is prone to happen at higher temperature.The activation energy was27.368kJ/mol which could primarily prove that theadsorption of Cr（VI） on it was chemical adsorption.The mechanism of removal of Cr（VI） using Mg/Mn/Al-0.5-LDO-450relies mainly on its structure memory effect to recove layer structure by adsorptionchromate ions in solution, so as to acheieve the purpose of adsorption of Cr（VI）. Inaddition, surface adsorption may also plays a role in.