Recent Advances in Sulfidated Zerovalent Iron for Contaminant Transformation

Abstract

2021 marks 10 years since controlled abiotic synthesis of sulfidated nanoscale zerovalent iron (S-nZVI) for use in site remediation and water treatment emerged as an area of active research. It was then expanded to sulfidated microscale ZVI (S-mZVI) and together with S-nZVI, they are collectively referred to as S-(n)ZVI. Heightened interest in S-(n)ZVI stemmed from its significantly higher reactivity to chlorinated solvents and heavy metals. The extremely promising research outcomes during the initial period (2011−2017) led to renewed interest in (n)ZVI-based technologies for water treatment, with an explosion in new research in the last four years (2018−2021) that is building an understanding of the novel and complex role of iron sulfides in enhancing reactivity of (n)ZVI. Numerous studies have focused on exploring different S-(n)ZVI synthesis approaches, and its colloidal, surface, and reactivity (electrochemistry, contaminant selectivity, and corrosion) properties. This review provides a critical overview of the recent milestones in S-(n)ZVI technology development: (i) clear insights into the role of iron sulfides in contaminant transformation and long-term aging, (ii) impact of sulfidation methods and particle characteristics on reactivity, (iii) broader range of treatable contaminants, (iv) synthesis for complete decontamination, (v) ecotoxicity, and (vi) field implementation. In addition, this review discusses major knowledge gaps and future avenues for research opportunities.

Molecular Structure and Sulfur Content Affect Reductive Dechlorination of Chlorinated Ethenes by Sulfidized Nanoscale Zerovalent Iron

“…Nanoscale zerovalent iron (NZVI) has been extensively studied and deployed for in situ remediation of groundwater contaminated with chlorinated ethenes (CEs), − which are widely used as industrial solvents and raise environmental concerns. , However, the poor selectivity of NZVI toward the target contaminant (e.g., trichloroethene (TCE)) versus water impedes its penetration into the remediation market. ,, Sulfidized NZVI (SNZVI) has emerged as a better alternative to NZVI, which significantly increases the selectivity and reactive lifetime by 2 orders of magnitude as compared with NZVI and expands the categories of reactive contaminants. − A rapidly growing body of literature has been developed to explore a facile synthesis of highly reactive SNZVI and the structure–property–performance relationships of SNZVI, predominantly using TCE as the target CE. , …”

“…Sulfidation can make the NZVI materials with highly variable properties (e.g., improved hydrophobicity, enhanced electron transfer, and inhibited H adsorption), possibly resulting in different reactive sites (Fe or S sites) and species (electrons or atomic H) and reaction pathways for the degradation of different contaminants. ,, The degree of sulfidation (i.e., [S/Fe] particles , defined as the measured S/Fe molar ratio in the particles), regulated by changing the S and Fe doses, is one of the most critical factors affecting the reactivity and selectivity of SNZVI for various contaminants. ,,− Simply altering the sulfidation degree can cause significant changes in material properties and hence changes the effects on the contaminant removal. However, previous studies on the dechlorination of different CEs by SNZVI only used one degree of sulfidation, and the dechlorination might be affected by sulfidation to different extents (even oppositely) even for the same CE. − Thus, a comprehensive investigation on the degradation of CEs by SNZVI with different sulfidation degrees is required for a better understanding of how sulfidation exerts different effects on the degradation of these CEs.…”

Wet Milling of Zerovalent Iron in Sulfide Solution: Preserving and Securing the Metallic Iron

“…The formation of the FeS x outer layer on ZVI during its wet milling in a sulfide solution implicates that the method may equally be applicable to the sulfidation of ZVI, which also coats an FeS x outer layer on ZVI ,,, and thus improves its capability in contaminant degradation, such as increasing selectivity, reactivity, and hydrophobicity of the ZVI and reducing particle aggregation. ,,,, As to the existing sulfidation method, a sulfidation of nanoscale ZVI is based on a chemical reduction of Fe 2+ /Fe 3+ ion using NaBH 4 and is conducted via the addition of a sulfur source (typically Na 2 S or Na 2 S 2 O 4 ) during or after the reduction; , the sulfidation of microscale ZVI is conducted by the dry milling of mZVI powder with elemental sulfur in an argon atmosphere or mixing Na 2 S and mZVI after the breakdown of oxide shell of mZVI with buffer or acid . With its past experiences in industrial-scale applications, − the wet milling strategy studied herein may provide a scalable scheme for large-scale ZVI sulfidation.…”

Simultaneous Sequestration of Humic Acid-Complexed Pb(II), Zn(II), Cd(II), and As(V) by Sulfidated Zero-Valent Iron: Performance and Stability of Sequestration Products

“…For example, acid washing, H 2 pretreatment, ultrasonication, weak magnetic fields, phosphorylation, , and so forth are all reported to be able to accelerate ZVI corrosion either by disrupting the intrinsic oxide coating or modulating the spatial distribution of secondary authigenic iron (hydr)­oxides. Recently, by incorporating sulfur into ZVI and thus constructing an iron sulfide (FeS x ) shell on the ZVI surface, sulfidation pretreatment was proposed as an alternative approach to enhancing the performance of ZVI and has gained increasing interest since its inception. − As a semiconductor, FeS x has higher electron conductivity than that of iron oxides, which can thus facilitate electron transfer and ZVI corrosion under oxic conditions. ,, Besides the improved ZVI reactivity, the presence of reduced sulfur species (e.g., S 2– ) in sulfidated ZVI (S-ZVI) was also found to be able to provide an additional metal sequestration route by forming insoluble metal sulfide precipitates . Thus, all these benefits make S-ZVI a promising candidate for remediation of HMs. − …”