Research On Adsorption Of Heavy Metal Anion From Aqueous Solution By Mg-Al-LDHs And Its Calcined Products

Mg/Al layered double hydroxides (LDHs) was synthesized by co-precipitation method and its calcined products (CLDHs) was prepared by heating LDHs at773K in this article, and discusses the influence factors of LDHs synthesis, the XRD and SEM characterization of LDHs and CLDHs indicate that they have complete structure, high crystallinity, single crystal and good layer structure; after calcination, layer structure of LDHs was destroyed and appeared diffraction peak of mixed MgAl(O).Adsorption of chromate (CrO42-) from aqueous solution on Mg/Al layered double hydroxides (LDHs) and calcined Mg/Al layered double hydroxides (CLDHs) was studied in this paper. Experiments were carried out as a function of different Mg/Al molar ratio, adsorbent dosage and initial concentration of chromate, and discussed the kinetics and thermodynamics to study the adsorption property. The results indicate that the LDH2is suitable for adsorbent of low concentration of chromate, CLDH2is suitable for adsorbent of high concentration of chromate; In293-313K temperature range, Maximum uptake capacity of LDH2and CLDH2increases with the rise of temperature, the both adsorption kinetics datas fit the pseudo-first-order model well, isotherms for adsorption chromate by LDH2and CLDH2at different solution temperatures were well described using the Langmuir equations; Gibbs’ free energy change of adsorption on LDH2is-0.12～-0.53KJ/mol, and Gibbs’ free energy change of adsorption on CLDH2is-1.16～-2.98KJ/mol, suggesting both are spontaneous physical adsorption processes, and adsorption on CLDH2has more impetus; enthalpy changes of chromate adsorption on LDH2and CLDH2are5.86KJ/mol and25.59KJ/mol, both positive, indicating the endothermic nature of the process, So as the adsorption temperature rises, the adsorption capacity gradually increases; entropy changes of chromate adsorption on LDH2and CLDH2are20.45J/(mol·K) and91.02J/(mol·K), showing that the adsorption processes are entropy increase processes.This paper also studied the adsorption of vanadate (VO43-) on calcined Mg/Al layered double hydroxides (CLDHs), and investigated the effects of different Mg/Al molar ratio, adsorbent dosage, contact time and initial concentration of solution, and further investigated the adsorption kinetics and thermodynamics process. The results show that the CLDH2is the effective adsorbent, adsorption kinetics and thermodynamics researches suggest that the adsorption of vanadate on CLDH2fit the pseudo-first-order model and Langmuir isotherms equations better; In293-313K temperature range, Gibbs’ free energy change of adsorption on CLDH2is-2.74～-5.77kJ/mol, indicating that the adsorption is spontaneous physical process; enthalpy change of vanadate adsorption on CLDH2is41.72KJ/mol, positive, suggesting that the adsorption process is endothermic, So, with the adsorption temperature rising, the adsorption capacity tends to increase, entropy change of vanadate adsorption on CLDH2is151.5J/(mol·K), positive, showing that the adsorption is entropy increase process.