Adsorptive Removal Of Anionic Pollutants By Functionalization Modified Hydrotalcite

In recent years, hydrotalcite compounds(Layered Double Hydroxide, LDH), as a new type of adsorbents, have received wide attention since it has unique layered structure and physicochemical properties. However, little information is available on the use of functionalization modified hydrotalcite to adsorb anionic pollutants. In our paper, phosphate and chromate were selected as the target pollutants, different types of functionalization modified hydrotalcites were prepared to remove phosphate and chromate from aqueous solution by batch adsorption method. The main reserches are concluded as follows:(1) The Mg-Al-LDH and Zn-Al-LDH were synthesized by co-precipitation method. The prepared LDHs were characterized by XRD and BET surface area determination. The XRD patterns showed the characteristic basal reflections of hydrotalcite-like LDH materials. BET surface area of Zn-Al LDH was larger than that of Mg-Al LDH. Adsorption experiments were carried out as a function of LDHs dosage, contact time and initial pH of phosphate solution. 4 g/L Mg-Al-LDH and 1.6 g/L Zn-Al-LDH were selected as optimum dosage. In the strong alkaline environment, there is a steady decrease in the adsorption capacity. The adsorption of phosphate reached equilibrium quickly at about 40 min. The experimental data showed a good compliance with the pseudo-second-order kinetic model. The Freundlich and Langmuir models both described the adsorption isotherm data well. Zeta potential and FTIR analyses were used to elucidate adsorption mechanisms. The results indicated that phosphate species were adsorbed via electrostatic attraction, ligand exchange and ion exchange.(2) Fe3O4 microspheres were prepared via a modified solvothermal route. Three different magnetic core-shell Fe3O4@LDHs composites, Fe3O4@Zn-Al-, Fe3O4@Mg-Al- and Fe3O4@Ni-Al-LDH were prepared via a rapid coprecipitation method for phosphate adsorptive removal. The composites were characterized by XRD, FTIR, TEM, VSM and BET analysis. Batch experiments were conducted to study the adsorption behavior of phosphate. Optimal conditions for the phosphate adsorption were obtained: 2 g/L adsorbent, pH of 3 and contact time of 60 min. Proposed mechanisms for the removal of phosphate species onto Fe3O4@LDHs composites at different initial solution pH were showed. Using the KASRA model as a theoretical basis, the kinetic data was described better by the pseudo-second-order kinetic rate equation. It was clear that adsorption isotherm curves show a three-region behavior in the ARIAN model. The adsorption isotherms had a good fit with Langmuir model and the maximum adsorption capacity followed the order of Fe3O4@Zn-Al-LDH > Fe3O4@Mg-Al-LDH > Fe3O4@Ni-Al-LDH. Thermodynamic analyses indicated that the phosphate adsorption process was endothermic and spontaneous in nature. Magnetic Fe3O4@LDHs composites were easily separated from aqueous solution by the external magnetic field in 10 s.(3) Zn-Al-LDH and Fe3O4@Zn-Al-LDH were prepared via a coprecipitation method. Then the calcined hydrotalcites(Zn-Al-500-LDO and Fe3O4/Zn-Al-500-LDO) were obtained by thermal decomposition of their corresponding precursors and characterized by XRD and FTIR. The results showed that interlayer anions of CO32- were disappeared after calcination. However, calcined hydrotalcites could recover layered structure according to “memory effect”, which would improve the adsorption capactity of anions. Batch experiments were conducted to study the adsorption behavior of chromate. Optimal conditions for chromate adsorption were obtained: 2 g/L adsorbent, pH of 3 and contact time of 60 min. The kinetic data was described better by the pseudo-second-order kinetic rate equation. The adsorption isotherms had a good fit with Langmuir model. Thermodynamic analyses indicated that chromate adsorption process was endothermic and spontaneous in nature. The removal ratio of phosphate onto Zn-Al-500-LDO and Fe3O4/Zn-Al-500-LDO were higher after calcination, suggesting that Zn-Al-500-LDO and Fe3O4/Zn-Al-500-LDO were more potential for chromate removal from wastewater.This paper used different types of functionalization modified hydrotalcites as adsorbents to removal phosphate and toxicity chromate. The results obtained in these studies are expected to provide beneficial theoretical basis for anionic pollutants removal from wastewater.