Research On The Factors Affecting The Preparation Process Of Sulfidated Zero Valent Iron And Its Phase Transition

Zero valent iron(ZVI)has been widely used for contaminants removal due to its low cost,easy availability,and a great ability of electron donation.However,the inherent passivation film of ZVI and its low reactivity because of the accumulation of corrosion products on the ZVI surface during the reaction with contaminants restrict the potential application of ZVI for water and soil remediation.In order to improve the reaction efficiency of zero valent iron,researchers have proposed many improved techniques,such as acid washing,ultrasound-pretreatment,bimetallic,nanotechnology,weak magnetic field and sulfidation to improve its reactivity.The discovery of the sulfidated zero valent iron(S-ZVI),which greatly improves the reactivity and electron selectivity of ZVI,which has become a research hotspot in recent years.The presence of the FeSx phase dominating on the surface of the S-ZVI,on one hand,prevents the formation of any inert iron oxides;and on the other hand,effectively promotes the electron transfer due to its inherent conducting nature,yielding an overall durable and highly efficient reductive reaction.Therefore,the incorporation of S is very critical in the preparation of S-ZVI.Different factors such as sulfidation method,sulfidation agent types,S/Fe,Fe sources,etc.have different influences on the surface morphology characteristics of S-ZVI,the structure and form of FeSx,and the content of S,and there are also great differences in the removal efficiency of pollutants.In this experiment,influencing factors(sulfidation time,pH in the preparation system,aging,etc.)in the preparation process of S-ZVI and the removal of hexavalent chromium(Cr(VI))in water were systematically studied.The structure and morphology of the synthesized S-ZVI were investigated by SEM-EDS,BET,XPS and other solid phase characterization.The results showed that both of the sulfidation time and the pH of the preparation system could change the morphology and structure of the surface of S-ZVI material and affect the content of S on the surface of ZVI.The results of Cr(VI)removal experiments showed that the optimal removal efficiency could be achieved when the sulfidation time was 24 h and the pH value of the preparation system was 6.In addition,the obtained S-ZVI was aged in the air,and it was found that aging could change the properties of S-ZVI material,affect the phase composition and content on its surface,and lead to the decrease of FeS content.However,even the S-ZVI material most affected by aging has a higher reaction rate with Cr(VI)than ZVI,which indicated that the presence of even a small amount of S could improve the reactivity of ZVI.Changes of the influencing factors during the preparation process may affect the distribution of iron(hydr)oxides on the surface of zero valent iron,and affect the subsequent sulfidation process.In this experiment,the correlation between iron(hydr)oxides and sulfides in the passivation layer of zero valent iron was explored.Herin,we applied X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),and scanning electron microscopy and energy dispersive X-ray spectroscopy(SEM-EDS)to characterize the aged zero valent iron(AZVI)samples and the aged zero valent iron of sulfidation(S-AZVI).Results revealed that the morphological structures and compositions of the iron(hydr)oxides changed with aging process.In particular,the content of γ-FeOOH peaked at 24 h and disappeared after sulfidation,demonstrating an opposite trend with that of FeS.According to Fe K-edge X-ray absorption spectroscopy(XANES)analyses,the valence state of the AZVI and S-AZVI samples did not change as the aging time increased,indicating that there were phase transformation instead of content increasing in various iron(hydr)oxides.This further confirmed that rather than iron(hydr)oxides content,γ-FeOOH in the passivation layer were a key factor for sulfidation and a necessary intermediate that required to be formed firstly and then converted to FeS.These characteristics gave a fully interpret the the effect of aging on the removal of Cr(Ⅵ)by S-ZVI.These characteristics gave a fully interpret the the effect of aging on the removal of Cr(VI)by S-ZVI.Compared to the pristine ZVI(Pri-ZVI)system and S-ZVI system,the removal rate of Cr(Ⅵ)by the S-AZVI system increased by 38.7-189.8 times and 2.1-10.5 times,respectively.This research bring new insights into modified iron-based materials and contaminants removal that can guide the design of reasonable and effective S-ZVI materials suitable for wastewater treatment.