The Preparation And Enhanced Cr(Ⅵ) Removal Performance Of Surface Fluorizated Zero-valent Iron

With the rapid economic development,the problems of environmental pollution have been raising concerns,especially the water pollution caused by heavy metals which originated from anthropogenic sources such as industrial wastes.As a representative,the hexavalent chromium(Cr(Ⅵ))is of high toxicity,carcinogenicity and mutagenicity,and is widely detected in surface water and groundwater.Therefore,it is urgent to control the Cr(Ⅵ)pollution in water.At present,the most common approach to remediate the Cr(Ⅵ)pollution is to reduce Cr(Ⅵ)into relatively less toxic Cr(Ⅲ).Micron zero-valent iron(mZVI),with high reducibility,low price and environment-friendly features,has become a popular material for the remediation of various pollutants.However,due to the existence of a passivation layer at the outer surface,mZVI is faced with low reactivity for Cr(Ⅵ)removal because of the limited electron transfer between mZVI and Cr(VI).This paper aimed to modify mZVI with different types of fluorides to enhance the electron transport and Cr(Ⅵ)removal.The details were described as follows:Part Ⅰ:Surface inorganic fluorinated zero-valent iron(F-mZVI)was prepared by dipping mZVI into 0.4 mol/L sodium fluoride solution for 1 hour,and the resulting F-mZVI was tested for Cr(Ⅵ)removal.Surprisingly,the rate constant of Cr(Ⅵ)removal with F-mZVI increased by 6 times when compared with that of the pristine mZVI.Scanning electron microscope(SEM)and X-ray diffraction(XRD)indicted that the morphology or the phase-structure of F-mZVI did not change compared to mZVI.X-ray photoelectron spectroscopy(XPS)analysis proved that the mZVI surface was successfully modified with fluorine element by replacing the surficial hydroxyl groups.Moreover,the Fe 2p high-resolution XPS results showed that the surficial Fe(Ⅱ)ratios of F-mZVI increased to 52.3%relative to ZVI of 42.I%,suggesting that the fluorine modified on the surface of mZVI accelerated the electron-transfer from iron core to the surface.Then,discrete fourier transform(DFT)calculation confirmed the fluorine modification could increase the electron cloud density on the surface of mZVI,thereby improving the Cr(Ⅵ)reductively removal.Part Ⅱ:Compared with inorganic fluoride ions,organic fluoride has a stronger electron-withdrawing ability.Therefore,we chose trifluoroacetic acid to modify mZVI(TFA-mZVI)and compared their Cr(Ⅵ)removal performance.Excitingly,TFA-mZVI showed superior Cr(Ⅵ)removal capacity over both F-mZVI and mZVI.SEM and XRD indicted that the morphology or phase-structure of TFA-mZVI were not changed either.Attenuated total reflection fourier transform infrared spectroscopy(ATR-FTIR)and DFT proved that TFA was modified on the surface of mZVI by forming the monodentate inner-sphere surface complexes.H2 generation and atmosphere experiments indicated that TFA modification could facilitate the electrons’ transfer to Cr(Ⅵ)rather than H2O or O2,thus could improve the electron selectivity of mZVI toward Cr(Ⅵ)reduction.Meanwhile,water contact angle tests further confirmed TFA modification reduced the hydrophilicity of mZVI surface,responsible for the minimized water adsorption and electron consumption.To summarize,we proved in this paper that the Cr(Ⅵ)removal performance of the zero-valent iron could be significantly improved by surface fluorination treatment.The mechanism investigation indicated that the improvement of Cr(Ⅵ)removal was mainly due to the enhanced electron transfer ability of the iron core thanks to the strong eletron-withdrawing properties of the surficial fluorine.The change of surface properties such as hydrophobicity also helps by improving the electron selectivity toward Cr(Ⅵ)removal.These results provide a new pathway for zero-valent iron depassivation and offer the fundamental support for the application of surface fluorinated zero-valent iron in real water pollution treatment.