Azo dye decolorization by ZVI under circum-neutral pH conditions and the characterization of ZVI corrosion products

This study examined the effects of initial solution pH (pH_i), ZVI dosage, initial Trypan Blue (TB) concentration ([TB]₀), and background electrolytes (NaCl and NaNO₃) on the rate and extent of dye decolorization. TB azo dye ([TB]₀ = 90 μM) was almost completely removed in 1320 min at pH_i 4, while only 54% at pH_i 10. The effects of Cl⁻ and NO₃⁻ as a common major anion (10 mM) were contrasting on the efficiency of ZVI decolorization. The former accelerated the decolorization presumably due to impeding surface passivation of secondary Fe (oxyhydr)oxides by forming dissolved Fe–Cl complexes. On the contrary, the latter promoted the formation of secondary oxide layers resulting in the declining the ZVI reactivity. The XRD spectra of reacted ZVI particles suggested that lepidocrocite was initially formed as the ZVI corrosion products, which gradually transformed to magnetite. FT-IR spectroscopy revealed the decolorization processes as the destruction of N[dbnd]N bond in TB dye structure, followed by the formation of free aromatic amine groups ([sbnd]NH₂) after 1320 min of reaction with ZVI. The experimental results demonstrated that the novel ZVI treatment system could be a potential and promising alternative technique to remove TB dye by reductive decolorization treatment processes.