Synthesis And Characterization Of Sulfidated Nanoscale Zero-valent Iron Particles And The Removal Of Heavy Metals

With the rapid development of industrialization and urbanization in China,the problem of heavy metal pollution in the environment has become increasingly prominent,such as mercury（Hg）,nickel（Ni）,lead（Pb）,copper（Cu）,cadmium（Cd）and chromium（Cr）,among them,Cd is the most serious heavy metal pollutant.Heavy metal elements have strong migration,enrichment and hiding,and can enter the human body through air,water,food chain and other ways,and pose a great threat to people’s health.Techniques such as precipitation,membrane separation and ion exchange have been applied for heavy metals removal.However,these technologies exist some deficiencies,such as costly and low efficiency.Therefore,it appears that exploration of cost-effective and eco-friendly technologies are still necessary for the removal of heavy metals.In recent years,sulfidated nanoscale zero-valent iron（S-nZVI）has been attracted much attentions from scholars due to its high selectivity,reactivity and stability for the removal of target pollutants.However,the reaction between pollutants and S-nZVI is a surface-mediated process,the sulfidation reagents,synthesis method and S/Fe molar ratio can all affect the surface properties and reactivity of S-nZVI.Hence,the synthesis of S-nZVI still needs to be further optimized.In this study,firstly Cd was chosen as targeted pollutant,the S-nZVI screening can be achieved through the change of synthesis conditions,such as sulfidation chemicals,synthesis procedure,and S/Fe molar ratio.To elucidate the influence of sulfur on the microstructure of S-nZVI and the mechanism between Cd and S-nZVI,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,spherical aberration correction scanning transmission electron microscopy（Cs-STEM）,X-ray diffraction spectroscopy（XRD）,X-ray photoelectron spectroscopy（XPS）,Raman spectroscopy（Raman）were performed on S-nZVI before and after reacting with Cd（Ⅱ）.Moreover,the effects of Cd（Ⅱ）initial concentration,S-nZVI dosage,ions(Mg²⁺,NO₃^-),and organic matter（HA,EDTA）on Cd（Ⅱ）removal were also investigated.The mechanism between S-nZVI and other heavy metals was also illustrated.The main research conclusions are shown in the following:（1）The screening and characterization of S-nZVI:the removal efficiency of Cd（Ⅱ）by presynthesized S-nZVI with Na₂S as the sulfidation reagent was much higher than other methods and 0.75 was the optimal S/Fe molar ratio.SEM,TEM,and Cs-STEM results indicated that the freshly prepared S-nZVI had no obvious core-shell structure and was aggregtated flocs in shape and with a size of ca.50-100 nm.XRD and Raman results showed that S-nZVI particles as synthesized consisted of the heterogeneous mixtures of Fe⁰,Fe₃O₄and amorphous FeS.（2）The removal of Cd（Ⅱ）by S-nZVI:Cs-STEM,Raman,and XPS results revealed that unlike nZVI which mainly adsorbs Cd（Ⅱ）at the surface,the dominant reaction mechanism between Cd（Ⅱ）and S-nZVI is the replacement of Fe in FeS in the particle and form CdS.The reaction between Cd（Ⅱ）and S-nZVI followed a pseudo-second-order adsorption model.The pseudo-second-order rate constant（k₂）decreased by increasing initial Cd（Ⅱ）concentration,and increased by increasing the dosage of S-nZVI.Mg²⁺,NO₃^-,HA and EDTA negatively affected Cd（Ⅱ）removal at low concentration,but the inhibition would be weakened as the concentration increased,and high concentration of HA and EDTA even promoted the removal of Cd（Ⅱ）.S-nZVI had an optimized Cd removal capacity of 495 mg/g,which is significantly higher than that of other adsorbents and iron-based materials.（3）The removal of other heavy metals by S-nZVI:XRD,Raman,and XPS results showed that the removal mechanism of heavy metals（Hg（Ⅱ）,Ag（I））with strong affinity to sulfur by S-nZVI was to replace Fe in FeS and combine with S to form metal sulfide.As for Cu（Ⅱ）,Pb（Ⅱ）,Zn（Ⅱ）and Ni（Ⅱ）,in addition to substitution,complexation with surface groups of S-nZVI was also included.The removal of Zn（Ⅱ）and Ni（Ⅱ）was dominated by the complexation of surface groups.S-nZVI particles would be oxidized into different iron oxides,S⁰ and SO₄^2-after reacting with different heavy metals.