Sulfidation Affects Hydroxyl Radicals Production Upon Magnetite Oxygenation And Its Application

Magnetite(Fe3O4)is a common iron oxide mineral distributed widely in underground environments.It contains both Fe(II)and Fe(III),and can act as a miniature geological battery,functioning as both an electron acceptor for charging and an electron donor for discharging without undergoing any biogeochemical transformations,thus influencing biogeochemical cycles and pollutant transportation and transformation.Underground environments are affected by various natural and anthropogenic activities(such as inputs of pollutants,microbial activity,fluctuations in water amount and hydrodynamic conditions),which can cause the transition from anaerobic to aerobic conditions.During the process of redox condition changes,redox-active substances(such as sulfides and O2)may react with Fe3O4,charging or discharging it.However,few studies have been carried out on the charging and discharging processes of Fe3O4 under redox fluctuating conditions.In this study,we investigated for the first time how sulfidation/oxidation conditions at neutral p H affect the charging and discharging of Fe3O4,as well as their influence on ·OH generation mechanisms.Different sulfur-to-iron ratios of sulfidated magnetite(S-Fe3O4)were synthesized under anaerobic conditions using chemical methods.The effects of sulfidation on the surface morphology,elemental composition and physicochemical properties of Fe3O4 were investigated by various solid-state characterization techniques and wet chemical analysis methods.Batch experiments were carried out to investigate the kinetics of hydroxyl radical(·OH)generation by S-Fe3O4 under oxidation conditions and its effect on pollutant removal.The effects of various environmental factors on ·OH generation were also studied.Quenching experiments and electron paramagnetic resonance spectroscopy were employed to explore the mechanism of S-Fe3O4 catalyzed generation of hydroxyl radicals.The main findings of this study are as follows:(1)Under anaerobic conditions,sulfidation can alter the surface morphology,elemental composition and physicochemical properties of Fe3O4.Sulfidation results in the uniform coverage of Fe3O4 surface with fine particles of iron sulfides;however,it does not change the crystal structure of Fe3O4.Sulfidation charges Fe3O4 by increasing the amount of divalent iron in it.As the [S/Fe] dose increases,the content of divalent iron in Fe3O4 increases(mainly amorphous Fe2+ with high activity).Sulfur species in Fe3O4 surface exists mainly in the form of S2-,S22-and S0,and the relative content of S species changes with the increase of the solid surface depth,with the content of S22-gradually increasing.In addition,sulfidation increases the stability of Fe3O4,decreases its particle size,increases its specific surface area and contact angle,enhances its electron transfer ability,and increases its ability to catalyze O2.(2)Under aerobic conditions,sulfur iron compounds on S-Fe3O4 surface transfer electron to Fe3O4(Fe2+)or directly to O2 for discharging,and catalyze the generation of ·OH from O2.The amount of ·OH generated by S-Fe3O4 is highly dependent on the [S/Fe] dose and is higher than that generated by Fe3O4.The higher the content of reducible S species in S-Fe3O4,the stronger the ability to activate O2 and generate ·OH.S-Fe3O4 has the optimal sulfur-to-iron ratio of 0.10 for ·OH generation under the mutual influence of Fe(II)and S.Sulfidation also enhances the ability of Fe3O4 to remove organic and inorganic pollutants.(3)Under neutral and organic conditions,the method of oxygen supply affects the ability of S-Fe3O4 to catalyze ·OH production.In addition,the synthesis of S-Fe3O4 under purer nitrogen,stronger reductive sulfur sources,higher amount of S-Fe3O4,smaller particle size,and more sulfidation times all contribute to enhancing the ability of S-Fe3O4 to catalyze ·OH production.Moreover,the experimental results indicate that different kinds of iron minerals can be enhanced to catalyze O2 to generate ·OH by sulfidation,among which goethite has the most significant effect.This study provides research basis for the generation of ·OH reactions in underground environments rich in sulfides and Fe3O4,and can also provide basic data and theoretical support for the application of sulfidated magnetite in environmental remediation.It is the first report on the study of sulfidation on the catalytic activity of Fe3O4 towards the production of·OH and could have significant implications for biogeochemical cycles and pollutant transportation and transformation.