Study On Adsorption Performance For Phosphate In Aqueous Solution By Cerium Oxide And Its Magnetic Nanocomposites

In the past few decades,eutrophication has been recognized as the most serious water pollution problem for many natural water bodies largely due to the excessive phosphate present in wastewater discharged from municipal or industrial plants.Thus it is imperative to remove phosphate from wastewater before discharge to protect against eutrophication.For the treatment of wastewater,the adsorption method has proved to be one of the most effective technologies for the removal of phosphate because of its simple process,stable operation,low cost and high adsorption capacity.Among the various types of adsorbents,cerium oxide,which is a relatively cheap and abundant rare earth oxide,has been widely used to adsorb a variety of harmful anions in wastewater such as fluoride,arsenate and bichromate due to its good resistance against acid and base,high specific surface area and wide availability.However,the research and application of phosphate removal by cerium oxide is very limited at present,thus limiting the potential function of cerium oxide on the prevention of water eutrophication.In this paper,cerium oxide was used as the object of study,and two kinds of cerium oxide-based adsorbent were synthesized.The phosphate adsorption performance and mechanism on cerium oxide was studied systematically,and the difficult problem about solid-liquid separation of cerium oxide in powder form was solved successfully,hoping to provide a new material and a new idea for the advanced purification of phosphate in wastewater.Firstly,a novel cerium oxide nanoadsorbent was synthesized by a simple precipitation method,using cerium chloride as inorganic cerium source and CTAB as surfactant.The crystalline phase,micromorphology,pore structure and other physical properties of the prepared adsorbent were characterized by X-ray diffractometry?XRD?,transmission electron microscopy?TEM?,scanning electron microscopy?SEM?and nitrogen adsorption-desorption instrument.The phosphate adsorption performance of the prepared adsorbent was investigated through the batch adsorption experiments.Characterization results indicated that the cerium oxide nanoadsorbent was composed of cerium oxide nanoparticles with the size of about 3-6nm,and possessed a large BET surface area of 236.75m2/g and many mesopores of 2.5nm.The results of adsorption experiments showed that the adsorption process of phosphate by cerium oxide nanoadsorbent was quite fast,and the adsorption kinetic data could be best fitted with the pseudo-second-order kinetic model.The adsorption isotherm data could be better described with the Langmuir isotherm model and the maximum adsorption capacity was about 83.64mg/g.The phosphate adsorption by cerium oxide nanoadsorbent was pH-dependent and the phosphate removal decreased with the increase of solution pH,while the higher ionic strength slightly promoted the phosphate adsorption.The coexisting anions such as Cl-and SO42-could enhance the adsorption of phosphate whereas HCO3-had interfering effect on the phosphate adsorption.The saturated adsorbent could be effectively regenerated using a 0.5M NaOH solution.The results of Zeta potential and FTIR spectroscopy suggested that the electrostatic attraction and the substitution of Ce-OH groups by phosphate ions were the main adsorption mechanism.Secondly,a novel magnetic nanocomposite adsorbent?Fe₃O₄@SiO₂@CeO₂?was fabricated by loading cerium oxide onto the surface of Fe₃O₄@SiO₂ magnetic nanoparticles to solve the difficult problem about solid-liquid separation of cerium oxide nanoadsorbent.The composition,structure,porosity and magnetic property of the magnetic composite were characterized by X-ray diffractometry?XRD?,transmission electron microscopy?TEM?,nitrogen adsorption–desorption instrument and vibrating sample magnetometry?VSM?.The phosphate removal performance of the synthesized adsorbent was also investigated through the batch adsorption studies.Characterization results confirmed that cerium oxide was successfully assembled on the surface of Fe₃O₄@SiO₂ magnetic nanoparticles,and the Fe₃O₄@SiO₂@CeO₂ magnetic nanocomposite possessed a typical core-shell structure with a relatively high BET surface area of 194.86m2/g,accessible mesopores of 2.6nm,and good magnetic properties,which could be rapidly separated from the aqueous solution within 30 s with the assistance of a permanent magnet.The results of adsorption experiments showed that the Fe₃O₄@SiO₂@CeO₂ magnetic nanocomposite still had an excellent performance in adsorbing phosphate,and the adsorption kinetic data could also be better described with the pseudo-second-order kinetic model.The adsorption isotherm data showed a good compliance with the Langmuir isotherm model and the maximum adsorption capacity of the Fe₃O₄@SiO₂@CeO₂ magnetic nanocomposite was found to be 64.07mg/g.The phosphate adsorption by the magnetic composite adsorbent also decreased with the increase of solution pH,while the increase of ionic strength slightly enhanced the phosphate removal.The increase of Cl-and SO42-concentration in the solution could promote the adsorption of phosphate by the magnetic composite adsorbent but high concentration of HCO3-was unfavorable for the phosphate adsorption.The study of the adsorption-desorption cycles of phosphate over the synthesized adsorbent showed that the Fe₃O₄@SiO₂@CeO₂ magnetic nanocomposite possessed excellent regeneration ability and reusability.The analyses of Zeta potential and FTIR spectroscopy also indicated that the electrostatic attraction and the replacement of Ce-OH groups by phosphate ions were both responsible for the phosphate adsorption on cerium oxide.