The Role of Reduced Surface Sulfur Species in Se(VI) Removal #WorldResearchAwards


Introduction

Sulfidized nano zero-valent iron (S-nZVI) has emerged as a highly effective material for the reductive removal of toxic oxyanions from aqueous environments. Its enhanced performance compared to pristine nZVI is attributed to improved electron selectivity and conductivity between the Fe(0) core and target contaminants. Sulfidation modifies the particle surface, introducing reactive sulfur species that fundamentally alter redox behavior. While S-nZVI has been widely studied for contaminant removal, the specific role of these sulfur species has remained insufficiently explored. Understanding their contribution is essential for optimizing S-nZVI design and predicting its behavior in both engineered and natural sulfidic systems.

Mechanism of Se(VI) Reduction by S-nZVI

The reduction of Se(VI) by S-nZVI occurs at significantly enhanced rates, up to ten times faster than with non-sulfidized nZVI. This acceleration is strongly influenced by the molar S/Fe ratio and selenium concentration, highlighting the importance of surface chemistry. The sulfidized layer facilitates efficient electron transfer from the Fe(0) core, enabling rapid reduction of high-valence selenium species. These findings demonstrate that sulfidation not only protects the iron core from rapid passivation but also actively participates in redox reactions.

Role of Reactive Sulfur Species

Sulfidation generates multiple reactive sulfur species, including surface-bound FeS, FeSx, and elemental sulfur (S⁰). Experimental evidence shows that these species are not merely passive coatings but active reductants. Acid volatile sulfide (AVS), in particular, directly contributes to the reduction of Se(IV) to Se(0). The oxidation of FeS to polysulfide species during this process confirms that sulfur plays a chemically reactive role, reshaping our understanding of S-nZVI reactivity.

Spectroscopic Evidence of Reaction Pathways

Advanced spectroscopic techniques, including XANES and XPS, revealed that elemental selenium (Se⁰) accounts for approximately 90% of the reduction products. These analyses also identified the concurrent loss of FeS and formation of Fe(II) polysulfides and S⁰ on the particle surface. Such observations provide strong molecular-level evidence linking sulfur oxidation to selenium reduction, validating the proposed reaction pathways and highlighting the complexity of surface-mediated redox processes.

Mass Balance and Quantitative Contributions

Mass balance calculations indicate that 9–15% of the total Se(0) formation originates from the oxidation of FeS to FeSx. Although Fe(0) remains the dominant electron donor, this quantitative contribution from sulfur species is significant and previously overlooked. These results emphasize that sulfur-mediated pathways can meaningfully influence contaminant transformation efficiency, especially under conditions of higher sulfidation.

Environmental and Research Implications

The findings demonstrate that surface-bound iron sulfide species are critical reactants in S-nZVI systems and likely play similar roles in natural environments undergoing sulfidation. This insight has important implications for groundwater remediation, sediment geochemistry, and the design of next-generation reactive materials. Recognizing sulfur species as active participants rather than inert coatings opens new research directions focused on tailoring sulfidation chemistry to enhance contaminant removal performance.

Global Particle Physics Excellence Awards


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#SulfidizedIron #NanoZeroValentIron #SeleniumReduction #EnvironmentalRemediation #ReactiveSulfurSpecies #WaterTreatmentResearch #EnvironmentalChemistry #Nanomaterials #RedoxProcesses #GroundwaterRemediation #SurfaceChemistry #XANES #XPS #IronSulfide #OxyanionRemoval #AdvancedMaterials #SustainableRemediation #ChemicalEngineering #EnvironmentalScience #worldresearchawards

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