Even Incorporation of Nitrogen into Fe0 Nanoparticles as Crystalline Fe4N for Efficient and Selective Trichloroethylene Degradation

Nonmetallic modified zero-valent iron for remediating halogenated organic compounds and heavy metals: A comprehensive review

Zero Valent Iron (ZVI), an ideal reductant treating persistent pollutants, is hampered by issues like corrosion, passivation, and suboptimal utilization. Recent advancements in nonmetallic modified ZVI (NM-ZVI) show promising potential in circumventing these challenges by modifying ZVI's surface and internal physicochemical properties. Despite its promise, a thorough synthesis of research advancements in this domain remains elusive. Here we review the innovative methodologies, regulatory principles, and reduction-centric mechanisms underpinning NM-ZVI's effectiveness against two prevalent persistent pollutants: halogenated organic compounds and heavy metals. We start by evaluating different nonmetallic modification techniques, such as liquid-phase reduction, mechanical ball milling, and pyrolysis, and their respective advantages. The discussion progresses towards a critical analysis of current strategies and mechanisms used for NM-ZVI to enhance its reactivity, electron selectivity, and electron utilization efficiency. This is achieved by optimizing the elemental compositions, content ratios, lattice constants, hydrophobicity, and conductivity. Furthermore, we propose novel approaches for augmenting NM-ZVI's capability to address complex pollution challenges. This review highlights NM-ZVI's potential as an alternative to remediate water environments contaminated with halogenated organic compounds or heavy metals, contributing to the broader discourse on green remediation technologies.

Single-Atom Iron Can Steer Atomic Hydrogen toward Selective Reductive Dechlorination: Implications for Remediation of Chlorinated Solvents-Impacted Groundwater

Atomic hydrogen (H*) is a powerful and versatile reductant and has tremendous potential in the degradation of oxidized pollutants (e.g., chlorinated solvents). However, its application for groundwater remediation is hindered by the scavenging side reaction of H2 evolution. Herein, we report that a composite material (Fe0@Fe-N4-C), consisting of zerovalent iron (Fe0) nanoparticles and nitrogen-coordinated single-atom Fe (Fe-N4), can effectively steer H* toward reductive dechlorination of trichloroethylene (TCE), a common groundwater contaminant and primary risk driver at many hazardous waste sites. The Fe-N4 structure strengthens the bond between surface Fe atoms and H*, inhibiting H2 evolution. Nonetheless, H* is available for dechlorination, as the adsorption of TCE weakens this bond. Interestingly, H* also enhances electron delocalization and transfer between adsorbed TCE and surface Fe atoms, increasing the reactivity of adsorbed TCE with H*. Consequently, Fe0@Fe-N4-C exhibits high electron selectivity (up to 86%) toward dechlorination, as well as a high TCE degradation kinetic constant. This material is resilient against water matrix interferences, achieving long-lasting performance for effective TCE removal. These findings shed light on the utilization of H* for the in situ remediation of groundwater contaminated with chlorinated solvents, by rational design of earth-abundant metal-based single-atom catalysts.

Impacts of Perfluoroalkyl Substances on Aqueous and Nonaqueous Phase Liquid Dechlorination by Sulfidized Nanoscale Zerovalent Iron

Per- and poly fluoroalkyl substances (PFASs) are often encountered with nonaqueous phase liquid (NAPL) in the groundwater at fire-fighting and military training sites. However, it is unclear how PFASs affect the dechlorination performance of sulfidized nanoscale zerovalent iron (S-nFe0), which is an emerging promising NAPL remediation agent. Here, S-nFe0 synthesized with controllable S speciation (FeS or FeS2) were characterized to assess their interactions with PFASs and their dechlorination performance for trichloroethylene NAPL (TCE-NAPL). Surface-adsorbed PFASs blocked materials' reactive sites and inhibited aqueous TCE dechlorination. In contrast, PFASs-adsorbed particles with improved hydrophobicity tended to enrich at the NAPL-water interface, and the reactive sites were re-exposed after the PFASs accumulation into the NAPL phase to accelerate dechlorination. This PFASs-induced phenomenon allowed the materials to present a higher reactivity (up to 1.8-fold) with a high electron efficiency (up to 99%) for TCE-NAPL dechlorination. Moreover, nFe0-FeS2 with a higher hydrophobicity was more readily enriched at the NAPL-water interface and more reactive and selective than nFe0-FeS, regardless of coexisting PFASs. These results unveil that a small amount of yet previously overlooked coexisting PFASs can favor selective reductions of TCE-NAPL by S-nFe0, highlighting the importance of materials hydrophobicity and transportation induced by S and PFASs for NAPL remediation.

Particle-Scale Understanding of Arsenic Interactions with Sulfidized Nanoscale Zerovalent Iron and Their Impacts on Dehalogenation Reactivity

Co-occurrence of organic contaminants and arsenic oxoanions occurs often at polluted groundwater sites, but the effect of arsenite on the reactivity of sulfidized nanoscale zerovalent iron (SNZVI) used to remediate groundwater has not been evaluated. Here, we study the interaction of arsenite [As(III)] with SNZVI at the individual-particle scale to better understand the impacts on the SNZVI properties and reactivity. Surface and intraparticle accumulation of As was observed on hydrophilic FeS-Fe0 and hydrophobic FeS2-Fe0 particles, respectively. X-ray absorption spectroscopy indicated the presence of realgar-like As-S and elemental As0 species at low and high As/Fe concentration ratios, respectively. Single-particle inductively coupled plasma time-of-flight mass spectrometry analysis identified As-containing particles both with and without Fe. The probability of finding As-containing particles without Fe increased with the S-induced hydrophobicity of SNZVI. The interactions of SNZVI materials with coexisting arsenite inhibited their reactivity with water (∼5.8-230.7-fold), trichloroethylene (∼3.6-67.5-fold), and florfenicol (∼1.1-5.9-fold). However, the overall selectivity toward trichloroethylene and florfenicol relative to water was improved (up to 9.0-fold) because the surface-associated As increased the SNZVI hydrophobicity. These results indicate that reactions of SNZVI with arsenite can remove As from groundwater and improve the properties of SNZVI for dehalogenation selectivity.

Tailoring Fe0 Nanoparticles via Lattice Engineering for Environmental Remediation

Lattice engineering of nanomaterials holds promise in simultaneously regulating their geometric and electronic effects to promote their performance. However, local microenvironment engineering of Fe0 nanoparticles (nFe0) for efficient and selective environmental remediation is still in its infancy and lacks deep understanding. Here, we present the design principles and characterization techniques of lattice-doped nFe0 from the point of view of microenvironment chemistry at both atomic and elemental levels, revealing their crystalline structure, electronic effects, and physicochemical properties. We summarize the current knowledge about the impacts of doping nonmetal p-block elements, transition-metal d-block elements, and hybrid elements into nFe0 crystals on their local coordination environment, which largely determines their structure-property-activity relationships. The materials' reactivity-selectivity trade-off can be altered via facile and feasible approaches, e.g., controlling doping elements' amounts, types, and speciation. We also discuss the remaining challenges and future outlooks of using lattice-doped nFe0 materials in real applications. This perspective provides an intuitive interpretation for the rational design of lattice-doped nFe0, which is conducive to real practice for efficient and selective environmental remediation.

A multiple Kirkendall strategy for converting nanosized zero-valent iron to highly active Fenton-like catalyst for organics degradation

Nanosized zero-valent iron (nZVI) is a promising persulfate (PS) activator, however, its structurally dense oxide shell seriously inhibited electrons transfer for O-O bond cleavage of PS. Herein, we introduced sulfidation and phosphorus-doped biochar for breaking the pristine oxide shell with formation of FeS and FePO4-containing mixed shell. In this case, the faster diffusion rate of iron atoms compared to shell components triggered multiple Kirkendall effects, causing inward fluxion of vacancies with further coalescing into radial nanocracks. Exemplified by trichloroethylene (TCE) removal, such a unique "lemon-slice-like" nanocrack structure favored fast outward transfer of electrons and ferrous ions across the mixed shell to PS activation for high-efficient generation and utilization of reactive species, as evidenced by effective dechlorination (90.6%) and mineralization (85.4%) of TCE. [Formula: see text] contributed most to TCE decomposition, moreover, the SnZVI@PBC gradually became electron-deficient and thus extracted electrons from TCE with achieving nonradical-based degradation. Compared to nZVI/PS process, the SnZVI@PBC/PS system could significantly reduce catalyst dosage (87.5%) and PS amount (68.8%) to achieve nearly complete TCE degradation, and was anti-interference, stable, and pH-universal. This study advanced mechanistic understandings of multiple Kirkendall effects-triggered nanocrack formation on nZVI with corresponding rational design of Fenton-like catalysts for organics degradation.

Tetrabromobisphenol A transformation by biochar supported post-sulfidated nanoscale zero-valent iron: Mechanistic insights from shell control and solvent kinetic isotope effects

Post-sulfidated nanoscale zero-valent iron with a controlled FeS_X shell thickness deposited on biochar (S-nZVI/BC) was synthesized to degrade tetrabromobisphenol A (TBBPA). Detailed characterizations revealed that the increasing sulfidation degree altered shell thickness/morphology, S content/speciation/distribution, hydrophobicity, and electron transfer capacity. Meanwhile, the BC improved electron transfer capacity and hydrophobicity and inhibited the surface oxidation of S-nZVI. These properties endowed S-nZVI/BC with highly reactive (∼8.9-13.2 times) and selective (∼58.4-228.9 times) over nZVI/BC in TBBPA transformation. BC modification improved the reactivity and selectivity of S-nZVI by 1.77 and 1.96 times, respectively. The difference of S-nZVI/BC in reactivity was related to hydrophobicity and electron transfer, particularly FeS_X shell thickness and morphology. Optimal shell thickness of ∼32 nm allowed the maximum association between Fe⁰ core and exterior FeS_X, resulting in superior reactivity. A thicker shell with abundant networks increased the roughness but decreased the surface area and electron transfer. The higher [S/Fe]_surface and [S/Fe]_particle were conducive to the selectivity, and [S/Fe]_particle was more influential than [S/Fe]_surface on selectivity upon similar hydrophobicity. The solvent kinetic isotope effects (SKIEs) exhibited that increasing [S/Fe]_dose tuned the relative contributions of atomic H and electron in TBBPA debromination but failed to alter the dominant debromination pathway (i.e., direct electron transfer) in (S)-nZVI/BC systems. Mechanism of electron transfer rather than atomic H contributed to higher selectivity. This work demonstrated that S-nZVI/BC was a prospective material for the remediation of TBBPA-contaminated groundwater.

Cations facilitate sulfidation of zero-valent iron by elemental sulfur: Mechanism and dechlorination application

The solid-solid reaction of microscale zero-valent iron (mZVI) with elemental sulfur (S0) in water can form sulfidated mZVI (S-mZVI) with high reactivity and selectivity. However, the inherent passivation layer of mZVI hinders the sulfidation. In this study, we demonstrate that ionic solutions of Me-chloride (Me: Mg2+, Ca2+, K+, Na+ and Fe2+) can accelerate the sulfidation of mZVI by S0. The S0 with S/Fe molar ratio of 0.1 was fully reacted with mZVI in all solutions to form unevenly distributed FeS species on S-mZVIs as confirmed by SEM-EDX and XANES characterization. The cations depassivated the mZVI surface by driving the proton release from the surface site (FeOH) and resulting in localized acidification. The probe reaction test (tetrachloride dechlorination) and open circuit potential (EOCP) measurement demonstrated that Mg2+ was most efficient in depassivating the mZVI and therefore promoting sulfidation. The decrease of surface proton for hydrogenolysis on the S-mZVI synthesized in MgCl2 solution also inhibited the formation of cis-1,2-dichloroethylene by 14-79% compared to other S-mZVIs during trichloroethylene dechlorination. In addition, the synthesized S-mZVIs exhibited the highest reduction capacity reported so far. These findings provide a theoretical basis for the facile on-site sulfidation of mZVI by S0 with cation-rich natural waters for sustainable remediation of contaminated sites.

Functional materials contributing to the removal of chlorinated hydrocarbons from soil and groundwater: Classification and intrinsic chemical-biological removal mechanisms

Chlorinated hydrocarbons (CHs) are the main contaminants in soil and groundwater and have posed great challenge on the remediation of soil and ground water. Different remediation materials have been developed to deal with the environmental problems caused by CHs. Remediation materials can be classified into three main categories according to the corresponding technologies: adsorption materials, chemical reduction materials and bioaugmentation materials. In this paper, the classification and preparation of the three materials are briefly described in terms of synthesis and properties according to the different types. Then, a detailed review of the remediation mechanisms and applications of the different materials in soil and groundwater remediation is presented in relation to the various properties of the materials and the different challenges encountered in laboratory research or in the environmental application. The removal trends in different environments were found to be largely similar, which means that composite materials tend to be more effective in removing CHs in actual remediation. For instance, adsorbents were found to be effective when combined with other materials, due to the ability to take advantage of the respective strengths of both materials. The rapid removal of CHs while minimizing the impact of CHs on another material and the material itself on the environment. Finally, suggestions for the next research directions are given in conjunction with this paper.

Degradation of Chloroform by Zerovalent Iron: Effects of Mechanochemical Sulfidation and Nitridation on the Kinetics and Mechanism

Chloroform (CF) is a widely used chemical reagent and disinfectant and a probable human carcinogen. The extensive literature on halocarbon reduction with zerovalent iron (ZVI) shows that transformation of CF is slow, even with nano, bimetallic, sulfidated, and other modified forms of ZVI. In this study, an alternative method of ZVI modification─involving simultaneous sulfidation and nitridation through mechanochemical ball milling─was developed and shown to give improved degradation of CF (i.e., higher degradation rate and inhibited H₂ evolution reaction). The composite material (denoted as S-N(C)-ZVI) gave synergistic effects of nitridation and sulfidation on CF degradation. A complete chemical reaction network (CRN) analysis of CF degradation suggests that O-nucleophile-mediated transformation pathways may be the main route for the formation of the terminal nonchlorinated products (formate, CO, and glycolic polymers) that have been used to explain the undetected products needed for mass balance. Material characterizations of the ZVI recovered after batch experiments showed that sulfidation and nitridation promoted the formation of Fe₃O₄ on the S-N(C)-ZVI particles, and the effect of aging on CF degradation rates was minor for S-N(C)-ZVI. The synergistic benefits of sulfidation and nitridation on CF degradation were also observed in experiments performed with groundwater.

Iron nitride nanoparticles for rapid dechlorination of mixed chlorinated ethene contamination

Sulfidation and, more recently, nitriding have been recognized as promising modifications to enhance the selectivity of nanoscale zero-valent iron (nZVI) particles for trichloroethene (TCE). Herein, we investigated the performance of iron nitride (Fe_xN) nanoparticles in the removal of a broader range of chlorinated ethenes (CEs), including tetrachloroethene (PCE), cis-1,2-dichloroethene (cis-DCE), and their mixture with TCE, and compared it to the performance of sulfidated nZVI (S-nZVI) prepared from the same precursor nZVI. Two distinct types of iron nitride (Fe_xN) nanoparticles, containing γ'-Fe₄N and ε-Fe_2-3N phases, exhibited substantially higher PCE and cis-DCE dechlorination rates compared to S-nZVI. A similar effect was observed with a CE mixture, which was completely dechlorinated by both types of Fe_xN nanoparticles within 10 days, whereas S-nZVI was able to remove only about half of the amount, most of which being TCE. Density functional theory calculations further revealed that the cleavage of the first C-Cl bond was the rate-limiting step for all CEs dechlorinated on the γ'-Fe₄N(001) surface, with the reaction barriers of PCE and cis-DCE being 29.9, and 40.8 kJ mol^-1, respectively. Fe_xN nanoparticles proved to be highly effective in the remediation of PCE, cis-DCE, and mixed CE contamination.