Synthesis And Characterization Of Functionalized Core-shell Fe Nanoparticles And Application For Cr(Ⅵ) Removal In Groundwater

Chromium, with its great economic importance for industrial use, is widely usedin a variety of industrial processes. It is one of the important pollutants found in soiland groundwater. Chromium exists in the environment primarily in two valence states:Cr（Ⅵ） and Cr（III）. Cr（Ⅵ） is a toxic, carcinogenic substance to human and animals.Furthermore, Cr（Ⅵ） has high solubility in aqueous phase over almost the entire pHrange and it is only weakly sorbed onto inorganic surfaces, thus it is quite mobile inthe natural environment. On the contrary, Cr（III） is nontoxic and an essential humannutrient, which does not readily migrate in groundwater since it usually precipitatesas Cr（OH）3or as mixed Fe（III）–Cr（III）（oxy）hydroxides.In situ chemical reduction of Cr（Ⅵ） by zero-valent iron nanoparticles （nZⅥ）represents a potentially more effective, lower cost alternative to other remediationtechniques. However, there are still serious technical challenges associated with itsuse. For example, nZⅥ prepared using traditional methods tend to either agglomeraterapidly or react quickly with the surrounding media （e.g., dissolved oxygen or water）,resulting in rapid loss in reactivity. Also, they are essentially not transportable ordeliverable in soils, and thus, cannot be used for in situ applications. Several methodshave been proposed to solve this problem. One approach is attaching nZⅥ to supportmaterial such as resin and carbon. Another approach is to directly modify the surfacesof individual nZⅥ by adding poly acrylic acid, starch and carboxymethylcellulose. Astabilizer can enhance dispersion of nanoparticles through electrostatic repulsion andstatic hindrance.However, the cost of general used support materials is high. And the choice ofmodifier can affect the activity, stability and cost of nanoiron. To solve theseproblems, a suitable support and modifier were chosen in this study. The core-shellsilica fume-supported Fe0（SF-Fe） nanoparticles and SiO₂-coated Fe0（Fe@SiO₂）nanoparticles were prepared. These nanoparticles had high activity and mobility. Themain contents are as follows:（1）Fe0nanoparticles supported on commercial and friendly silica fume weresynthesized by borohydride reduction of ferric chloride in the presence of a support material. Transmission electron microscopy, X-ray diffraction and Fourier transforminfrared were used to characterize the SF-Fe. The feasibility of using this SF-Fe forreductive immobilization of hexavalent chromium Cr（Ⅵ） in soil and groundwaterwas studied. The effect of silica fume percentage on Cr（Ⅵ） removal by SF-Fe wasinvestigated, and was compared with those of unsupported Fe0nanoparticlescontaining the same amount of iron as SF-Fe. By analysis of TEM micrographs, itcan be seen that, the nanomaterial had core-shell structure and approximatelyspherical Fe was dispersed well on the surface of silica fume. The composite particlesize range was on the order of0.15-0.45μm, and the Fe size ranged from20nm to110nm. FTIR study indicated that hydroxyl group was the anchor for silica fume oniron which was accountable for the stability of Fe0nanoparticles. Compared withunsupported Fe0, SF-Fe was significantly more active in Cr （Ⅵ） removal especiallyin84wt%silica fume loading. The experiment results showed that the removal ratioof Cr（Ⅵ） by SF-Fe was22.55%higher than that of unsupported Fe0.（2）Without using aqueous ammonia and a surface modifier, a facile one-stepmethod was developed to fabricate Fe0nanoparticles coated with a SiO₂shell（Fe@SiO₂） by a modified St ber method combined with an aqueous reductionmethod. The Fe@SiO₂was prepared by directly adding KBH₄to a mixed solution oftetraethylorthosilicate （TEOS） and anhydrous ferric chloride. The structure andmorphology of the as-synthesized powders were investigated by X-ray powderdiffraction, energy dispersion analysis of X-ray, transmission electron microscopy,ultraviolet-visible absorption spectroscopy, Fourier-transform infrared spectrometryand X-ray photoelectron spectroscopy. The XRD and EDAX analysis had showedthat the amorphous SiO₂was prepared by the KBH₄. The TEOS amount and KBH₄injection speed had influence on Fe@SiO₂structure. For preparation of0.015g bettercoated Fe0nanoparticles, the perfect TEOS dose was0.1mL and the optimal KBH₄injection speed was5mL min-1. The TEM images showed that under this optimumcondition the prepared Fe@SiO₂had a distinct core-shell structure. One or two Fe0nanoparticles （25nm in diameter） were homogeneously coated by a porous SiO₂shell（9nm）. With an increase in the amount of added TEOS the Fe0nanoparticles hadbetter dispersion and the thickness of the SiO₂coating increased gradually. Lower KBH₄injection speed was preferable to assemble Fe0nanoparticles. The feasibility ofusing the prepared Fe@SiO₂for the reductive immobilization of Cr（Ⅵ） in water wasstudied. Compared with uncoated Fe0nanoparticles, Cr（Ⅵ） removal by Fe@SiO₂increased greatly. The removal ability of the prepared Fe@SiO₂was the highest atTEOS dosage of0.1mL and KBH₄injection speed of5mL min-1. The highestremoval ability of Fe@SiO₂was466.67mg·g-1and it was only76.35mg·g-1foruncoated Fe0nanoparticles.（3） Batch experiments were conducted to evaluate the influences ofinterference factors on Cr（Ⅵ） reduction by the SF-Fe and Fe@SiO₂. The rate ofreduction of Cr（Ⅵ） to Cr（III） can be expressed by a pseudo-first-order reactionkinetics. In order to study the role of silica fume and SiO₂shell, the reaction productwas analyzed by XPS.（4）The column experiments had showed that at certain hydraulic conditions,the Cr（Ⅵ） removal rate gradually slowed down as time went on. However, thenanoiron was not used up. The removal ability of Fe@SiO₂and SF-Fe were only65,12mg·g-1respectively, which were much lower than batch experiment. This may bethat the nanomaterials were heavily aggregated by drying and was not grinded.The mobility of SF-Fe and Fe@SiO₂through sediments was tested usingvertical and horizontal column transport experiments. The experiments resultsconfirmed that the SF-Fe and Fe@SiO₂moved more effectively through model soilsthan unsupported Fe0nanoparticles. It was indicated that51.50%and38.29%ofSF-Fe were eluted from the vertical and horizontal columns under the specifiedconditions, respectively. And88.03%and61%of Fe@SiO₂were eluted from thevertical and horizontal columns, respectively. Increasing the solution ionic strengthwould decrease the mobility of SF-Fe and Fe@SiO₂due to the increased attachmentto sand grains. However, the mobility of Fe0nanoparticles was enhanced by thepresence of humic acid.In summation, this study was the first time to use silica fume as a support for Fe0nanoparticles, and the prepared core-shell nanomaterials had high dispersion andactivity. A facile and friendly one-step method to fabricate monodispersed core-shellFe@SiO₂nanocomposites was discussed. Borohydride was acted not only as a reductant for iron salt but also a catalyst for hydrolysis and polycondensation reactionof TEOS. The study of mobility of SF-Fe and Fe@SiO₂in vertical and horizontalcolumn filled with quartz sand was a meaningful effort. It not only served as areference for the research methods, but also provided theory guide for the practicalapplication of nanoiron technology.