Preparation Of SiO₂-coated Iron Nanoparticles And Degradation Of2,4-DCP Of Refractory Organics

Chlorinated organic compounds (COCs) have a wide range of uses. For example,2,4-dichlorophenol (2,4-DCP) is an important organic intermediate, and is widely used in themanufacture of pesticides, pharmaceuticals, lubricants, dyestuffs, and in the synthesis of otherchemicals. Chlorophenols(CPs) in water causes serious problems due to their toxicity arehighly mutagenic or carcinogenic and adverse effects on the human. Removing thesecontaminants from water is a sighificant challenge because of ever-increasing pollution andthe shortage of high quality fresh water. Recently, a chemical reduction method usingzero-valent iron (ZVI) has been widely studied for contaminated wastewater treatment.Earlier studies showed that this is a promising approach for chlorophenol removal because itis cheap, highly efficient, and environmentally friendly. However, using nZVI for treatingchlorinated organics has many drawbacks. For example, the high surface energy makesoxidation of nZVI in the atmosphere easy, which means that the rate constant for thedechlorination of chlorophenol is low, and this decreases the nZVI reducing abilities. In thisstudy, SiO2-coated nZVI using a one-step liquid-phase reduction method was synthesized andthe removal efficiency for2,4-DCP and the oxidation resistance of the SiO2-coated ironparticles was investigated. The effects of SiO2-coating preparation conditions on particlesurface texture, size, reduction efficiency, and antioxidation abilities were also investigated.The degradation characteristics of SiO2-coated nZVI and uncoated nZI were compared withthose of uncoated nZVI.The preparation conditions of SiO2-nZVI were investigated. In the present work, a novelmethod for in situ synthesis of SiO2-coated iron nanoparticles was investigated. The maincharacteristic of SiO2coating of nZVI is that improvements in the oxidation resistance,reducing capacity, coating, and stability of nZVI are accomplished in one step. SiO2-coatingtechnology provides an amorphous mesh structure, stops oxygen migration and nZVI surfaceoxidation, and gives improved pollutants degradation abilities. Core–shell SiO2-coated ironnanoparticles were synthesized using a one-step St ber method in the presence and absenceof PEG400. The2,4-dichlorophenol degradation and anti-oxidation abilities of thenanoparticles were investigated. The effects of isopropanol/H2O ratio, NaOH dosage, tetraethyl orthosilicate (TEOS) dosage, and reaction time used in the synthesis wereinvestigated. The results showed that the nanoparticles were stable in the presence of PEG400.A comparison of the removal rates of2,4-dichlorophenol using pure iron nanoparticles andusing SiO2-coated iron nanoparticles showed an improvement of about40%usingSiO2-coated iron nanoparticles, suggesting that SiO2-coated iron nanoparticles improve theantioxidation abilities and reducing capacity of iron nanoparticles; such nanoparticles couldhave wide applications in chlorophenol degradation. The optimum conditions for preparationcondition of SiO2-nZVI: isopropanol/H2O ratio1:2, NaOH dosage4mL, tetraethylorthosilicate (TEOS) dosage2mL, and reaction time4h.The characterizations of SiO2-nZVI were investigated. The SiO2-coated ironnanoparticles were characterized using transmission electron microscopy, scanning electronmicroscopy, X-ray diffraction, and Fourier-transform infrared spectroscopy. RepresentativeTEM and SEM images of SiO2-coated nZVI are shown. It can be seen that the bare nZVIparticle size was about80-100nm. Moreover, from the TEM images, we can clearly see thenanoparticles coated with a transparent film. The SEM images shows that the SiO2-coatednZVI were spherical and formed linear chains in space, and the surfaces of the coatedparticles were smooth and uniform. FTIR spectra of the nZVI, SiO2-nZVI and SiO2nanoparticles are shown that the presence of O-H stretching and bending vibrations showTEOS hydrolysis to produce SiO2containing hydroxyl groups, and reveal bonding of SiO2and nZVI by hydrogen, the Si-O-Si stretching vibrations reveal that the surfaces were coatedwith SiO2. The XRD patterns of analytically pure SiO2, freshly synthesized nZVI, and freshlysynthesized SiO2-coated nZVI samples indicated that pure Fe was prepared using the presentprocedures. The diffuse and broad peaks show that the SiO2from the hydrolysis process andSiO2nanoparticles are amorphous. TG was employed to examine the oxidation stability ofnZVI and SiO2-coated nZVI. SiO2-coated nZVI shows the weight loss of the first step due towater and solvent volatilize and the two produts of oxidation of nZVI were Fe2O3and Fe3O4was certified contemporary.Comparison of rates of2,4-DCP removal by nZVI and SiO2-coated nZVI duringdegradation process.2,4-DCP reaction conditions and changes in solution ion concentrationswith reaction progress were investigated. The effects of FeSO4·7H2O dosage,2,4-DCP concentration, pH, and co-existing ions(cations Al3+,Mn2+,Cu2+and anions Cl–,PO43–,NO3–) onthe degradation activity were investigated. The results showed that the efficiency ofSiO2-coated nZVI far exceeded that of uncoated nZVI for removal of2,4-DCP from aqueoussolutions. The best removal conditions were obtained when the FeSO4·7H2O dosage of theSiO2-coated nZVI was about3.0g/L, the2,4-DCP concentration was150mg/L, and the pHwas about2.72at25°C in a volume of100mL.Compared with those using uncoated nZVI,2,4-DCP reaction conditions and changes insolution ion concentrations with reaction progress were investigated. Changes in pH, Fe2+andFe3+concentration, and UV-Vis spectra with reaction time were analyzed. Changes in thesurface properties of the uncoated and SiO2-coated nZVI were characterized using scanningelectron microscopy (SEM), energy dispersive X-ray emission spectra (EDX), andFourier-transform infrared spectroscopy (FTIR). The redox reaction progressed on the nZVIsurface for uncoated nZVI and on the SiO2amorphous coating for coated nZVI. Theamorphous SiO2coatings obtained on the nZVI surface distinctly increased their oxidationresistance.Comparison of degradation products with progress of2,4-DCP degradation by nZVI andSiO2-coated nZVI. The reaction mechanism of the uncoated and SiO2-coated nZVI which hadthe same degradation path that the products were4-CP,2-CP and phenol but different masstransfer mode, was discussed. When nZVI was used alone, the nanoparticles easily andrapidly captured aqueous2,4-DCP easily due to their large surface area. The adsorbed2,4-DCP was partly reduced by electrons and Fe2+on the nanoparticles surface to produce2-CP and Fe3+, respectively, and were finally released back into the solution for phenoldegradation. Then, nZVI surface caused oxidation, which passivated the nZVI surface andreduced the internal electron supply of the particles. nZVI is known to easily react with wateror oxygen in its surrounding media, which results in the formation of an oxidation layer thathinders further reaction. For SiO2-coated nZVI, electrons, Fe2+, and Fe3+were absorbed oninner coating, and2,4-DCP and O2were then adsorbed on the outer coating. Because theoxygen was molecular phase, it was more difficult for it to migrate in the amorphousmicropores of the SiO2coating than the other ions, which increased the active time forreaction. At the same time, the layer strongly adsorbed Fe2+and Fe3+and slowed their migration from the inner to the outer surface, delaying the rate of iron oxide formation andnanoparticle passivation.