Effect Of Structure And Composition On The Degradation Of Azo Dyes By Zero-Valent Iron

In the paper,amorphous zero-valent iron based（G-ZVI）powders were prepared through melt-spinning method and ball-milling methods,and then the crystalline zero-valent iron based（C-ZVI）powders were prepared through ball-milling methods.The degradation efficiency and mechanism to methyl orange were evaluated.X-ray diffraction（XRD）was used to characterize and analyze the structural characteristics of zero-valent iron samples before and after degradation.Field emission scanning electron microscopy（SEM）were used to characterize the surface morphologies of the zero-valent iron sample powders before and after degradation.Energy dispersive spectrometer（EDS）was used to characterize the element types of zero-valent iron samples before and after degradation.The particle size and distribution of the obtained zero-valent iron sample powders were measured using laser particle size analyzer.X-ray photoelectron spectroscopy（XPS）was used to characterize the superficial elements,chemical valence before and after degradation.ICP-MS was used to measure the amount of iron ion after degradation.In this paper,these effects including amorphous Fe⁰ content,structure of Fe⁰,the type and content of non-metallic elements,the solution environment（pH value and temperature）and the cycling degradation efficiency of ZVI were systematically investigated.The results show that the G-ZVI powders with good dispersion and controllable particle size can be prepared by melt spinning and then ball-milling,and the average particle size of the powder can be kept at 10-30μm.The surface morphology of the powder is rugged.They have good degradation effect to methyl orange.The Fe₇₀Si₁₀B₂₀G-ZVI with the lowest iron content（30 h ball milling）has the highest degradation efficiency,and the first-order degradation rate constant k=0.062 min^-1,half-life t_1/2=11.2min.However,Fe₈₄Si₆B₁₀ with the highest iron content had the worst degradation efficiency,and k=0.025 min^-1,half-life t_1/2=27.7min,which is only 40%.Of that for Fe₇₀Si₁₀B₂₀ G-ZVI.Fe₇₅Si₁₀B₁₅ and Fe₇₈Si₉B₁₃ G-ZVI have a degradation efficiency between Fe₇₀Si₁₀B₂₀0 and Fe₈₄Si₆B₁₀.The ZVI powders are covered with petal-like oxidation products after degradation,which hinders the continuation of the degradation reaction.FeSiB,FeSi and FeB powders with 10-20μm particle size can be prepared through ball-milling.It was found that G-ZVIs and C-ZVIs with the same composition have a condiderable degradation efficiency.The degradation efficiency of C-ZVIs increased with the increase of B content.With the highest degradation efficiency,Fe43.64B56.36C-ZVI could degrade methyl orange by 99%in 50 min.The first-order degradation rate constant k=0.213 min^-1,half-life t_1/2=3.3 min.The k and t_1/2 for Fe90B10 C-ZVI are0.068 min^-1and 10.2 min,respectively.The k and t_1/2 for Fe70B30 C-ZVI are 0.134 min^-1 and 5.2 min,respectively.Si had a lower positive effect on degradation than that of B.The degradation efficiency of Fe70Si30 C-ZVI,which has k=0.046 min^-1 and t_1/2=15.1min,was lower than that of Fe70B30 C-ZVI.Fe43.64B56.36 C-ZVI can degrade methyl orange in acid and neutral environments.With the decrease of pH,the degradation efficiency of Fe43.64B56.36C-ZVI is improved.When the pH value was 4 and 6,the degradation half-life of methyl orange was 1 and 2 minutes,respectively.The activation energy of degradation to methyl orange for Fe43.64B56.36 C-ZVI wasΔE=19.33 kJ/mol,which was much lower than that of other zero-valent irons and confirm its excellent degradation property.The degradation efficiency of Fe₇₀Si₁₀B₂₀ G-ZVI and Fe43.64B56.36 C-ZVI can maintain 90%after 20 times and 16 times cyclic utilization,respectively,which indicates that G-ZVI and C-ZVI both have a good cycling stability.There are nano-scale heterogeneity in G-ZVI.Therefor iron-rich clusters and iron-poor clusters are typical and ubiquitous,which constitute galvanic cells and promote the rapid loss of electrons in Fe⁰.As for the boron-containing C-ZVI,ball-milling makes the hard boron particles embedded into the soft iron particle,and can also form the iron-rich and iron-poor regions that constitute galvanic cells.Once the crystalline Fe⁰ begins to lose electrons,the electron loss rate of the crystalline Fe⁰ is comparable to that of the loosely structured amorphous Fe⁰,due to the defects in the crystalline state itself.Therefore,the C-ZVIs have a considerable degradation efficiency conparing to G-ZVI with the same composition.This study provides an efficient,low cost,reusable and commercial method for the degradation of azo dye wastewater.