| Great advances have been achieved in the use of zero-valent iron (ZVI) in contaminants removal from water in the past few years. However, the conventional ZVI technology has several limitations, including low reactivity, easy deactivation, and narrow working pH. To improve the reactivity of ZVI, a general strategy is to incorporate a second metal such as Pd, Cu, Ni, Ag, and Pt, onto ZVI to form a bimetallic symtem. Among the second metal introduced, Pd, Pt, and Ag are noble metals, and Cu, Ni, and Pb are toxic heavy metals. It is thus desirable to investigate the catalytic effect of some cost effective alternatives to these metals.The low cost and low toxic Fe/Al bimetallic particles were synthesized through a direct element replacement reaction and used in the removal of aqueous Cr(VI) and As(III). Brunauer-Emmett-Teller analysis method, X-ray diffraction, Vibrating-sample magnetometry, Scanning electron microscopy and energy dispersive spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy were employed to characterize the Fe/Al bimetallic particles. Batch experiments were carried out to explore the effect of the Fe/Al mass ratio, Fe/Al bimetal dosage, and initial pH on the removal of aqueous Cr(VI) and As(III). In addition, the concentrations of released iron and aluminum ions were determined to examine the stability of the bimetallic particles. Finally, the Cr(VI) and As(III) removal mechanism by Fe/Al bimetallic particles were proposed through the analysis of the characterization of the bimetallic particles before and after the reaction.The main achievements of this study are as follows:(1) The core of these mesoporous and ferromagnetic Fe/Al bimetallic particles was Al, and the planting Fe covered on the surface of Al. The size of the bimetal was about 20-30 μm, and the size of the Fe was about 200-400 nm. With different FeSO4 concentration used in the synthesis,the Fe/Al mass ratio and the structure of the bimetallic particles are different. The bimetallic particle with lower Fe/Al mass ratio is solid ball like, while the one with higher Fe/Al mass ratio is porous ball like.(2) The Fe/Al bimetallic particles exhibited a fast and highly efficient removal of Cr(Ⅵ) over a wide pH range from 3.0 to 11.0. Higher Fe/Al mass ratio, higer Fe/Al bimetal dosage and acidic and neutral conditions are beneficial to the Cr(Ⅵ) removal. The Fe/Al bimetal exhibited a good stability and the released iron and aluminum ions remained in a low concentration, especially in the removal of Cr(Ⅵ) in the pH range from 3.0 to 7.0.(3) All the synthesized Fe/Al bimetallic particles exhibited a great removal capability of As(Ⅲ) and total As under all tested pH values. The Fe/Al bimetal exhibited a good stability and the the problem of the leaching of metal ions form bimetallic particles was controllable, especially in the removal of As(Ⅲ) in the pH range from 5.0 to 7.0.(4) Galvanic cell effect is the main reasons for the enhanced removal of Cr(Ⅵ) by Fe/Al bimetallic particles, which is also the reason for the fast and highly efficient removal of Cr(Ⅵ) over a wide pH range.In the process of Cr(Ⅵ) removal by Fe/Al bimetallic particles, Cr(Ⅵ) can obtain the electron provided by the ZVI and zero-valent aluminum (ZVAl) directly or through the galvanic cell reaction between Fe and Al and be reduced to Cr(Ⅲ). Then the reduced Cr(Ⅲ) was finally immobilized via the formation of Cr(OH)3 and CrxFe1-x(OH)3 on the surface of Fe/Al bimetal.(5) The removal of As(Ⅲ) is a complicated process including surface adsorption and oxidation, and intraparticle reduction. The oxidation of As(Ⅲ) to As(Ⅴ) mainly occurred on the surface, while the reduction of As(Ⅲ) and As(Ⅴ) mainly occurred in the intraparticle. The As(Ⅴ) and As(Ⅲ) can obtain the electron provided by the ZVI and ZVAl directly or through the galvanic cell reaction between Fe and Al. In addition, there might also be diffusion of As(0) across the thin oxide layer enclosing Fe(0) core, resulting in increase of the content of the As(0) in the intraparticle. |
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