Synthesis Of Calcium-based Nanoscale Zero-valent Iron Composites And The Application For Removal Of Heavy Metal Ions In Water

Heavy metal pollution in water is one of the most serious pollution problems in the world.Due to their toxicity,non-biodegradation and bioaccumulation,heavy metal ions exist in the environment are harmful to ecosystem and human safety.Therefore,remediation of heavy metal pollution in water has been the key of environmental protection work.Nanoscale zero-valent iron?nZVI?is a kind of nano-material with strong reduction ability,which has been widely concerned in recent years.Owing to their advantages such as easy availability,low toxicity,high activity,nZVI has been widely applied in many fields.It has been widely used in remediation of various pollutants in wastewater,such as organic chlorine pesticides,organic dyes,antibiotics,nitrates,organic halogenates,heavy metals and so on.However,similar to other inorganic nanoparticles,freshly prepared nZVI is generally tend to aggregate into large clusters due to the interparticle interactions such as magnetic interactions and van der Waals force,leading to the decreased reactivity and reusability in practical applications.Therefore,it is of great significance for the development of zero-valent iron to restrict the aggregation of nZVI particles and allow them to apply on a much larger scale.In the past 15 years,researchers have developed various modification techniques for zero-valent iron to overcome these shortcomings.Among them,the idea of incorporating nZVI particles onto support materials has been identified,which not only enhances stability of nZVI,but also maximizes the advantages of both support materials and nZVI.Previously reported supports for nZVI include a variety of organic and inorganic materials.In this paper,two kinds of green,non-toxic calcium-based materials were used to synthesize two kinds of nZVI composites,and the composites were used to remove Pb?II?,Cr?VI?,Cd?II?ions in aqueous solution,respectively.The effects of various factors on the removal efficiency were systematically investigated,and the possible reaction mechanism was discussed.The main research contents and conclusions were as follows:1.Two kinds of supported nZVI composites were prepared.Firstly,some micro scale CaCO₃ were synthesized in direct precipitation method by changing the reaction conditions.Among them,the rod-like calcium carbonate with the best morphology and properties was selected as the carrier to synthesize the nZVI@CaCO₃ composite.Secondly,hydroxyapatite,which is harmless to human body and has a large specific surface area,was selected to synthesize HAP/nZVI composites.The morphology,structure and properties of the two composites were studied by XRD,TEM,SEM,EDS,BET-N₂ and other characterization methods.The characterization results show that the CaCO₃ exhibits a fagot-like structure,and Fe⁰ particles are well dispersed on CaCO₃ with an average diameter of around 15 nm.And the mean surface area of nZVI@CaCO₃ was twice as large as the bare nZVI.The aggregation of nZVI in the prepared HAP/nZVI composite was effectively inhibited,and the specific surface area of the HAP/nZVI composite was 85.06 m²/g,which was much higher than that of the bare nZVI.2.Pb?II?ion was selected as the target pollutant to study the properties of nZVI@CaCO₃ composites,and the effects of iron loading,initial concentration,pH,temperature and reaction time on the removal efficiency of Pb?II?ions were investigated.The results show that the Pb?II?removal capacity of nZVI@CaCO₃ is as high as 3828 mg/g,which is much greater than that by neat CaCO₃?2209 mg/g?and bare nZVI?1308 mg/g?.Owing to the loading of Fe?50%?in nZVI@CaCO₃,a significant synergistic effect is observed between CaCO₃ and nZVI in high performance removal of lead.A possible removal mechanism is proposed from a comparative investigation on the surface valence state and nZVI@CaCO₃phase before and after absorption of Pb?II?.The combination of CaCO₃ and nZVI is a combination of biologically compatible,green,cheap and efficient environmental functional materials.Considering the advantages of improved dispersibility,easy preparation and remarkable performance,the as-prepared nZVI@CaCO₃ has the potential to become a promising remediation material for Pb?II?polluted water.3.The properties of HAP/nZVI composites were studied by adsorption experiments of Cr?VI?and Cd?II?ions.The effects of dosage,temperature,initial solution concentrations and pH were investigated systematically,and the kinetic model was used to understand the adsorption process Cr?VI?and Cd?II?on the HAP/NZVI.And in acidic condition,Cr?VI?was easier to remove,but Cd?II?was easier to remove in neutral or alkaline conditions.And it is clear that the pseudo-second-order equation was more appropriate to describe the kinetics of Cr?VI?and Cd?II?by HAP/NZVI,indicating that Cr?VI?and Cd?II?removal was predominated through a chemical adsorption.