Adsorption And Adsorption Mechanism Of Mg-Al Hydrotalcite On Cd(Ⅱ)

Layered double hydroxide (LDH) is a new functional material with excellentperformances and wide applications in environmental and water treatment. WhenLDH was used as the adsorbent, the optimal conditions of adsorption of Cd(Ⅱ) byLDH were as followed:0.1g LDH was added in100mL Cd(NO3)2solution with aninitial concentration of100mg/L at40°C for180min, and the equilibrium adsorptionamount was49.0mg/g. Ratio of Cd(Ⅱ) elimination increased with the increment ofpH. The calcined sample (LDO) had higher adsorption capacity to Cd(Ⅱ) than LDH,and The optimum calcinating temperature was500°C. The elimination andadsorption capacity of Cd(Ⅱ) could reach91.2%and638.4mg/g respectivelyunder the optimal conditions of the concentration of LDO1.0g/L, the initialconcentration of Cd(Ⅱ)700mg/L, the adsorption temperature60°C andadsorption time120min. But compared with LDH, the initial pH had little effect onadsorption of Cd(Ⅱ) to LDO. LDO could return to the layered structure by “memoryeffect” after water treatment, and it used very large amounts of OH–, which wouldlead to the increment of pH. Forms of adsorption included surface physical adsorption,surface complexation and chemical precipitation, especially reconstruction couldpromote the adsorption of Cd(Ⅱ).The optimal volume fraction of glycerin was15percent to LDH modification(G-LDH), and the Ratio of Cd(Ⅱ) elimination increased by17percent compared tothe unmodified LDH. The modified LDH still retained typical hydrotalcite structure,and had smaller plastic size, narrower distribution of particle size and larger surfacearea. The calcinated G-LDH (G-LDO) possessed larger specific surface area, largerproe size distiributionand and larger proe volume, which would facilitate chemicalprecipitation of Cd(Ⅱ).The result of adsorption kinetics showed that the adsorption of Cd(Ⅱ) by LDOwas in good agreement with Lagergren pseudo-second order kinetic equation andLangmuir absorption isotherms equations. The endothermic process was basicallycontrolled by the chemical adsorption, and the apparent activition energy was37.10kJ/mol. The result of adsorption thermodynamic (ΔH022.84kJ/mol) showed that theadsorption mainly was composed of ligands exchange and chemical binding. TheGibbs free energy (ΔG) was a negative and its absolute value increased as thetemperature increased, and it showed that the adsorption reaction was a spontaneous chemical process. The adsortion entropy was a positive and it proved that this processwas inreversible. The adsorption capacity of Cd(Ⅱ) was875.00mg/g by LDO underconditions of temperature60oC and initial pH5.8.The mixed adsorption of Cd(Ⅱ) and P by LDH and LDO showed that LDH andLDO could promoted the Cd(Ⅱ) adsorption. The Cd(Ⅱ) adsorption capacity increasedwith P concentration. The adsorption capability was seriously influenced by addingorders of P, the results demonstrated that the adsorption of heavy metals need moreenergy.