| Owing to their small particles size, large surface area and high reactivity, nanoscale zero valent iron (nZVI or Fe0) particles have been widely applied in heavy metals, chlorine-containing organics and nitrate polluted areas remediation. Unfortunately, precisely because of these characteristics, Fe0 could rapid react with its surrounding media, resulted in formation of an oxidation layer. Also, agglomeration of Fe0 particles hinders the reaction between Fe0 and contaminants, thus decreases their reactivity. In order to solve these problems, several stabilized Fe0 systems were synthesized in this study, and their efficiencies on Cr(VI) removal were investigated.In stabilized MWCNTs-Fe0 system, Fe0 particles could be loaded on the surface of MWCNTs to avoid agglomeration. Compared to bare Fe0 and active carbon-Fe0 system, Cr(VI) removal efficiency by MWCNTs-Fe0 within 2 h was increased from 47% and 59% to 98%. The result also suggested that MWCNTs to Fe0 ratio 10:1 is appropriate to achieve high removal efficiency. Environmental factors exhibited strong influences:efficiency decreased from 100% to 90% when pH value increased from 5.0 to 9.0; increase of initial Cr(VI) concentration and ionic strength led to lower removal efficiency; most co-existing ions hindered Cr(VI) removal by competition for active surface; and low concentration of natural organic materials (NOM) facilitated the stabilization of Fe0, while high concentration of NOM resulted coagulation of Fe0 particles and decreased their removal capacity. The robust Cr(VI) removal achieved by MWCNTs-Fe0 system thanks to numerous carbon-iron galvanic cells formed in aqueous solution, benefiting electrons transfer from Fe0 to Cr(VI).In stabilized Fe3O4-Fe0 system, small Fe3O4 coupled with Fe0 through magnetic force to separate chained Fe0 particles. The suitable mass ratio of Fe3O4to Fe0 is 12:1, which could remove 96.4% of 20 mg/L Cr(VI) within 2 h. Moreover, for increased magnetism, separation of Fe3O4-Fe0 system from treated solution could be easily accomplished by placing them in an external magnetic field. In the meantime, the tolerance of Fe3O4-Fe0 system toward fluctuant environment parameters had been improved, insignificant effects were observed when changing ionic strength, or concentration of co-existing ions and NOM. However, pH was still a critical factor, and maximum adsorption capability decreased from 100 mg/g to 29.43 mg/g with pH increase from 3.0 to 8.0. Synergistic effect also existed in this system:the surface Fe(Ⅱ) of Fe3O4 could reduce Cr(Ⅵ) and oxidize to Fe(Ⅲ), then gain electrons from wrapped Fe0 to reduce back to Fe(II), providing another pathway for electron transfer. Reaction kinetics fitted well with the pseudo second-order adsorption model, and adsorption behavior could be described by the Langmuir and Freundlich model.To better utilization of carbon and magnetic materials, stabilized magnetic graphene-Fe0 system were designed. For halved Fe0 mass concentration, these new system could still remove 83.3% 40 mg/L Cr(Ⅵ) within 2 h. Experimental results suggested low temperature and pH were favored, and compared to FeO4-Fe0 system, the maximum adsorption capability greatly increased to 66.2 mg/g under pH 8.0. Kinetics study also verified the predominant mechanism and the rate-controlling step of removal process was more likely be adsorption rather than reduction by obeying pseudo second-order adsorption model. Negative ΔG and ΔH indicated spontaneous tendency and exothermic nature of Cr(Ⅵ) removal by magnetic graphene-Fe0 system.Stabilized immobilized Fe0 system embedded Fe0-Fe3O4 nanocomposites into polyvinyl alcohol (PVA)/sodium alginate (SA) beads, which not only eased the burden of separation, but also alleviated fears about potential risk of nano material. But along with the increased mass transfer resistance, removal rate of Cr(Ⅵ) by immobilized Fe0 system definitely decreased, and completely removal time extended to 18 h.5.0 wt% PVA with 1.5 wt% SA was the optimal proportion to maintain the best condition of immobilized beads, and the followed acidification and reduction pre-treatments were also critical to ensure high mechanical strength and Cr(Ⅵ) removal. Decreasing pH value and initial Cr(Ⅵ) concentration both benefited final removal efficiency. The used beads could be separated and regenerated through another pre-treatment process, and maintained high efficiency in cyclic utilization. |
