Mechanisms Of Chromium And Copper Removal With Nanoscale Zero-valent Iron And Iron Oxides/Hydroxides

The surge of industrial activities has intensified more environmental problems as seen forexample in the deterioration of several ecosystems due to the accumulation of dangerouspollutants,such as heavy metals. Heavy metals are still being used in various industries due totheir technological importance. Yet, untreated or imperfect treatment of waste products fromthese industries will carry other serious issues to human health and environment. Aside fromthe environmental damage, human health is likely to be affected as the presence of heavymetals beyond a certain limit brings serious hazards to living organisms. For instance, Cr (VI),Cd²⁺, Cu²⁺and Ni²⁺ions have been proven to cause kidney damage, liver damage or Wilsondisease and dermatitis or chronic asthma. Therefore, heavy metal pollution treatment isurgent.Iron based materials, including iron hydroxides and zero-valent iron are usedsuccessfully for remediation of a variety of environmental pollutants. Goethite and hematiteare always considered to be important environmental materials which can be functioned asadsorbents for heavy metals. They widely exist in soil particles and rock, aquatic sediment.Due to large specific surface area and more active sites, the use of zero-valent iron (Fe⁰),especially nanoscale zero-valent iron (N-Fe⁰), as reactive media for treatment of heavymetals and for remediation of contaminated soil and groundwater has been extensivelyinvestigated. Nanoparticles are defined as particles with size in the range of1to100nm atleast in one of the three dimensions. Because of this very small size scale, they possess animmense surface area per unit volume, a high proportion of atoms in the surface and nearsurface layers, and the ability to exhibit quantum effects. The resulting unique properties ofnanoparticles can not be anticipated from a simple extrapolation of the properties of bulkmaterials. The synthesized nanoparticles have to be surface modified in most cases, in orderto passivate and stabilize them since their nanoscale renders them chemically very reactiveand/or physically aggregative. The nanoparticles are also surface functionalized in order tomeet the needs of specific applications.In this research work, the wastewater containing Cr (VI) was treated through contrastresearch by N-Fe⁰, α-Fe₂O₃and α-FeOOH synthesized in the lab. On the bases of theexperimental results, the intensive efforts have been made to coat and protect Fe⁰nanoparticles from agglomeration and air oxidation. The influences of pH, initialconcentrations of heavy metal ions, temperature and the dosages of iron bases materials onthe Cr (VI) and Cu²⁺ions removal were investigated. The kinetics and mechanisms of Cr (VI)and Cu²⁺ions removal were also discussed in this research work. The main details and results were presented:Microcrystalline precipitates of hematiteα-Fe₂O₃and goethite α-FeOOH were preparedfrom ferric ions. The N-Fe⁰nanoparticles were prepared with FeCl₃·6H₂O and NaBH4byliquid reduction, and the P-Fe⁰and T-Fe⁰nanoparticles were modified by2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA) and TH-904respectively with thesame method. The X-ray diffraction (XRD), scanning electron microscope (SEM),transmission electron microscope (TEM) and Fourier transform infrared spectroscopy (FTIR)were conducted to characterize the structure of these products. The results show that hematiteα-Fe₂O₃and goethite α-FeOOH had good crystallinity and excellent performance. The-PO₃H₂group on the P-Fe⁰nanoparticle surface and the-COOH group on the T-Fe⁰nanoparticle surface were detected by FTIR spectroscopy, which demonstrated themodification for N-Fe⁰nanoparticles were successful. TEM images illustrated that theaverage diameters of produced P-Fe⁰and T-Fe⁰nanoparticles were about73and64nmrespectively.The experiment results on Cr (VI) removal by N-Fe⁰, P-Fe⁰and T-Fe⁰nanoparticlesshowed that the removal efficiencies by P-Fe⁰and T-Fe⁰nanoparticles were higher than thoseby N-Fe⁰nanoparticles, as well as the reaction kinetics apparent rate constants. The findingsby hematite α-Fe₂O₃and goethite α-FeOOH demonstrated that the goethite α-FeOOH wasbetter than hematite α-Fe₂O₃on Cr (VI) removal. Furthermore, all the iron nanoparticles(N-Fe⁰, P-Fe⁰and T-Fe⁰) were greatly superior to α-Fe₂O₃and α-FeOOH on Cr (VI) removal.Mechanism analysis indicated that the Cr (VI) uptake by the iron nanoparticles (N-Fe⁰, P-Fe⁰and T-Fe⁰) was mainly via a redox by Fe (0) particles, but the one by hematite α-Fe₂O₃andgoethite α-FeOOH was primarily of adsorption.The experiment results on Cu²⁺ions removal by N-Fe⁰, P-Fe⁰and T-Fe⁰nanoparticlesshowed that the Cu²⁺ions removal efficiencies by P-Fe⁰and T-Fe⁰nanoparticles were higherthan those by N-Fe⁰nanoparticles, as well as the adsorption rate constants. The efficiencies ofCu²⁺ions removal for different iron nanoparticles were P-Fe⁰>T-Fe⁰>N-Fe⁰, which obtainedthrough investigating the influencing factors, such as pH, Cu²⁺ions initial concentration,temperature, as well as analyzing the kinetics and mechanisms. The rate of Cu²⁺ionsadsorption kinetics on the iron nanoparticles followed the pseudo-second order equation. Themechanism was mainly of redox by Fe(0) nanoparticles. XPS results showed that Fe(0)particles was oxided to Fe₂O₃å’ŒFeOOH and the Cu²⁺ions was reduced to Cu0å’ŒCu2O afterreaction.The nanoscale zero-valent iron was modified by PBTCA and TH-904. There is a-PO₃H₂group and three–COOH groups in a PBTCA molecule, and many–COOH groups in aTH-904molecule. On one hand, the-PO₃H₂and–COOH groups on the P-Fe⁰and T-Fe⁰ nanoparticle surfaces increased the distances and, therefore, weaken the aggregation forcesbetween the iron nanoparticles, on the other hand, the hydrolysis of PBTCA and TH-904molecules in the solution changed the charge distribution on the iron nanoparticle surfaces,which increased the electrostatic repulsion hence hindering the interaction between ironnanoparticles. The modified iron nanoparticles can achieve good dispersibility when theelectrostatic stabilize effect and space steric hindrance stabilize effect act simultaneously. Thegood dispersibility of iron nanoparticles played an important role in increasing theefficiencies of Cr (VI) and Cu²⁺ions removal.