| Zero-valent iron(ZVI) is an inexpensive agent that can remove many common environmental contaminants. This study demonstrated that ZVI was effective for Se(IV) and Se(VI) removal. The kinetics and the involved mechanisms of Se(IV) and Se(VI) removal by zero valent iron(ZVI) were investigated in this study. The effect of magnetic ?eld(MF) on Se(IV) and Se(VI) removal by zero valent iron(ZVI) were investigated for the first time.Under the condition of solution mixed with a magnetic stirring bar, the effects of p H, dissolved oxygen(DO), initial selenite concentration(Se(IV)), ZVI dosage and particle size as well as reaction temperature on Se(IV) removal by ZVI were systematically investigated in this study. The kinetics of Se(IV) removal by z ero valent iron(ZVI) open to the air as a function of p H was investigated. The speci?c rate constants of Se(IV) removal by ZVI decreased from 0.467 min-1 to 0.037 min-1 as p H increased from 4.0 to 7.0. Se(IV) removal by ZVI was more favored under oxic conditions with higher reaction rate than under anoxic conditions, ascribing to the promoted ZVI corrosion rate in the presence of DO. Moreover, Se(IV) removal by ZVI was enhanced with increasing ZVI dosage and reaction temperature but decreased with increasing ZVI particle size. The removal rate of Se(IV) by ZVI experienced an increase and then a decrease with initial Se(IV) concentration ranging from 9.9 to 78.6 mg·L-1. To further describe the reaction rate, a pseudo-?rst-order kinetics was employed, and the calculated activation energy, by ?tting the rate constants at different temperatures, was determined to be 32.86 k J·mol-1. When ?xing other conditions, good linear correlation could be observed between pseudo-?rst-order reaction rate constants(kobs) and ZVI dosage. The positive correlation between the removal rate of Se(IV) and the generation rate of Fe(II) and the depression of Se(IV) removal in the presence of 1,10-phenanthroline indicated that both ZVI and adsorbed Fe(II) on ZVI surface contributed to the reductive removal of Se(IV). The soft X-ray STXM measurement con?rmed the adsorption of Fe(II) on the surface of ZVI and freshly formed ferric(hydr)oxides. Se(IV) was removed by adsorption followed by reduction to Se(0) on ZVI surface at p H 4.0-7.0, as revealed by XANES spectra. A core-shell structure was observed when ZVI reacted with Se(IV)-containing solution for 3 h at p H 6.0. Se(IV) was reduced to Se(0) and co-precipitated with the freshly formed Fe(III), forming the shell surrounding the iron core. After reaction for 24 h, the generated Se(0) was surrounded by multiple layers of Fe(III) oxides/hydroxides. SEM images and XRD patterns revealed that the corrosion products of ZVI at p H 6.0 transformed from amorphous iron hydroxides to lepidocrocite(γ-Fe OOH) as reaction proceeded. The ?nal corrosion products of ZVI contained both lepidocrocite and goethite at p H 5.0 while they were X-ray amorphous at p H 4.0 and 7.0. Compared with other methods for Se(IV) removal reported in literatures, reduction by ZVI was considered a promising technique, which could rapidly and effectively eliminate Se(IV) from waters.The presence of MF signi?cantly accelerated Se(IV) removal and extended the working p H range of ZVI from 4.0-6.0 to 4.0-7.2. The MF induced greater enhancement in Se(IV) removal by ZVI at lower initial Se(IV) concentrations. The in?uence of MF on Se(IV) removal by ZVI was associated with a more dramatic drop in ORP and a more rapid release of Fe2+ compared to the case without MF. SEM and XRD analysis revealed that MF accelerated the corrosion of ZVI and the transformation of amorphous iron(hdyr)oxides to lepidocroci te. XANES analyses showed that MF expedited the reduction of Se(IV) to Se(0) by ZVI at p H 6.0 when its initial concentration was ≤20.0 mg·L-1. Se(IV) dosed at 40.0 mg·L-1 was removed by ZVI via adsorption followed by reduction to Se(0) at p H 7.0 but via adsorption at 7.2 in the presence of MF. Regardless of MF, Se(IV) applied at 40.0 mg·L-1 was removed by reduction at p H 4.0-6.0. Employing MF to enhance Se(IV) removal by ZVI is a promising and environmental-friendly method since it does not need extra energy and costly reagents.Based on the magnetic field can greatly improve the reactivity of zero-valent iron, the MF induced greater enhancement in Se(VI) removal by ZVI was systematically investigated in this study. Although Se(VI) was difficult to be removal by zero-valent iron even lasted for 24 h without MF. However, Se(VI) can be quickly removed by ZVI following zero-order kinetics in the presence of a magnetic field. selenate shows relatively low adsorption onto iron oxide or iron hydroxide when the ZVI was depleted. The XRD and Fe-edge XANES spectra showed that the main corrosion products of ZVI were Fe3O4 and γ-Fe OOH. The linear component analysis of Se-edge XANES spectra showed that two mechanisms that contribute to the removal of Se(VI) from solution by ZVI. The ?rst mechanism was the reduction of Se(VI) to Se(IV), with the rapid adsorption of Se(IV) to oxyhydroxides. The second mechanism was the reduction of Se(VI) to Se(IV) then be further reduced to Se(0).A novel magnetic field(MF) to rejuvenate passivated ZVI material was found for the first time and applied to improve selenite removal from water, w hich was evaluated through kinetics experiments. A time series of aged ZVI was synthesized, and the composition and structure of the aged particles were investigated by XRD, SEM and Raman spectra. The sequestration of Se(IV) by zero-valent iron(ZVI) is strongly influenced by the coupled effects of aging ZVI and presence of a magnetic field(MF). ZVI aged at p H 6.0 with MES as buffer between 6 and 60 hours gives nearly constant rates of Se(IV) removal with MF, but with rate constants that are 10- to 100-fold greater than without. XANES analysis showed that applying MF changes the mechanism of Se(IV) removal by ZVI aged for 6-60 h from adsorption followed by reduction to direct reduction. The strong correlation between Se(IV) removal and Fe2+ release suggests direct reduction of Se(IV) to Se(0) by ZVI, in agreement with the XANES analysis.The numerical simulation of ZVI magnetization revealed that the MF influence on Se(IV) sequestration is associated mainly with the ferromagnetism of ZVI and the paramagnetism of Fe2+. In the presence of the MF, the Lorentz force gives rise to convection in the solution which narrows the diffusion layer and the field gradient force which tends to move paramagnetic ions(esp. Fe2+) along the higher field gradient at the ZVI particle surface, thereby inducing non-uniform depassivation and eventually localized corrosion of the ZVI surface. Therefore, the MF-induced improvement in Se(IV) removal by ZVI may be mainly attributable to the Lorentz force and magnetic ?eld gradient force. |
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