Effect of the modification sequence on the reactivity, electron selectivity, and mobility of sulfidated and CMC-stabilized nanoscale zerovalent iron

Unveiling the Role of Mackinawite and Fe0 Components in Arsenic(III) Removal by Sulfidated Zero-Valent Iron under Aerobic Conditions

In this study, we synthesized microscale sulfidated zerovalent iron (S-mZVI) with controllable mackinawite (FeS) content up to nearly 100 wt % and investigated the roles of FeS and Fe0 for arsenite (As(III)) sequestration under aerobic conditions. Batch experiments show that FeS and Fe0 contents determine the kinetics and longevity of As removal by S-mZVI, respectively. The Fe0/FeS galvanic cell accelerates the consumption of Fe0 by dissolved oxygen (DO) while preserving FeS to preferentially remove As as sulfide, which is protected from oxidation by Fe0. In column studies, mZVI and S0 were mixed in sand to form S-mZVI in situ. Results of sequential extraction of reacted S-mZVI particles from different column zones and run stages further indicate that As formed as sulfide by S-mZVI, which was then oxidized by DO after Fe0 depletion to form As-iron (hydr)oxides. X-ray absorption near-edge structure characterization confirmed that As sulfide is mainly in the form of realgar (As4S4). S-mZVI (2 wt % of column sand) reduces total As from 2 mg/L to 10 μg/L, up to 300 bed volumes, with a residence time of 70 min. In situ synthesis and cost advantages make S-mZVI a promising method to address As contamination issues worldwide.

Recent advances in sulfidized nanoscale zero-valent iron materials for environmental remediation and challenges

Over the past decade, sulfidized nanoscale zero-valent iron (S-nZVI) has been developed as a promising tool for the remediation of contaminated soil, sediment, and water. Although most studies have focused on applying S-nZVI for clean-up purposes, there is still a lack of systematic summary and discussion from its synthesis, application, to toxicity assessment. This review firstly summarized and compared the properties of S-nZVI synthesized from one-step and two-step synthesis methods, and the modification protocols for obtaining better stability and reactivity. In the context of environmental remediation, this review outlined an update on the latest development of S-nZVI for removal of heavy metals, organic pollutants, antibiotic resistance genes (ARGs), and antibiotic resistant bacteria (ARB) and also discussed the underlying removal mechanisms. Environmental factors affecting the remediation performance of S-nZVI (e.g., humic acid, coexisting ions, S/Fe molar ratio, pH, and oxygen condition) were highlighted. Besides, the application potential of S-nZVI in advanced oxidation processes (AOP), especially in activating persulfate, was also evaluated. The toxicity impacts of S-nZVI on the environmental microorganism were described. Finally, the future challenges and remaining restrains to be resolved for better applicability of S-nZVI are also proposed. This review could provide guidance for the environmental remediation with S-nZVI-based technology from theoretical basis and practical perspectives.

Transport of nanoscale zero-valent iron in saturated porous media: Effects of grain size, surface metal oxides, and sulfidation

Knowledge of the fate and transport of nanoscale zero-valent iron (nZVI) in saturated porous media is crucial to the development of in situ remediation technologies. This work systematically compared the retention and transport of carboxymethyl cellulose (CMC) modified nZVI (CMC-nZVI) and sulfidated nZVI (CMC-S-nZVI) particles in saturated columns packed with quartz sand of various grain sizes and different surface metal oxide coatings. Grain size reduction had an inhibitory effect on the transport of CMC-S-nZVI and CMC-nZVI due to increasing immobile zone deposition and straining in the columns. Metal oxide coatings had minor effect on the transport of CMC-S-nZVI and CMC-nZVI because the sand surface was coated by the free CMC in the suspensions, reducing the electrostatic attraction between the nZVI and surface metal oxides. CMC-S-nZVI displayed greater breakthrough (C/C₀ = 0.82-0.90) and higher mass recovery (84.9%-89.3%) than CMC-nZVI (C/C₀ = 0.70-0.80 and mass recovery = 70.9%-79.6%, respectively) under the same experimental conditions. A mathematical model based on the advection-dispersion equation simulated the experimental data of nZVI breakthrough curves very well. Findings of this study suggest sulfidation could enhance the transport of CMC-nZVI in saturated porous media with grain and surface heterogeneities, promoting its application in situ remediation.