A quantitative study of the effects of particle' properties and environmental conditions on the electron efficiency of Pd and sulfidated nanoscale zero-valent irons

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.

Reductive dechlorination of 2,4-dichlorophenol by using MWCNTs-Pd/Fe nanocomposites prepared in the presence of ultrasonic irradiation

The research on developing a purification technology for 2,4-dichlorophenol (2,4-DCP) polluted water with high efficiency and the low energy consumption is crucial for achieving several Sustainable Development Goals (SDGs). In order to achieve these goals, MWCNTs-Pd/Fe nanocomposites were prepared by Fe nanoparticles modified with multi-walled carbon nanotubes (MWCNTs) and palladium (Pd) in the presence of ultrasonic irradiation. The MWCNTs-Pd/Fe nanocomposites were characterized by using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and X-Ray Diffraction (XRD), and others. Characterization results confirmed that the MWCNTs-Pd/Fe was successfully prepared, with the particle size of 80 nm and the specific surface area of 89.5 m2/g confirmed. We studied the reductive dechlorination of 2,4-Dichlorophenol (2,4-DCP) by MWCNTs-Pd/Fe nanocomposites under different conditions, and the optimized experimental results were found when the Pd loading was 0.4 %, the pH was 3, and the temperature was 30 °C. The phenol yield increased from 76.5 % (without ultrasonic irradiation) to 92.3 % (with ultrasonic irradiation) in 300 min and the 2,4-DCP removal rate reached 98.7 % under the optimal conditions. Therefore, ultrasonic irradiation enhanced the performance of MWCNTs-Pd/Fe nanocomposites for 2,4-DCP removal. We also established the degradation mechanism of chlorophenol by analyzing the intermediates, and proposed the degradation kinetics model. The degradation of 2,4-DCP followed the pseudo-first-order kinetics with the rate constant of 0.05988 min-1. Also, this study demonstrated the potential of using ultrasonic irradiation to improve the properties and recovery of MWCNTs-Pd/Fe nanocomposites, contributing to achievement of the Sustainable Development Goals (SDGs), including SDG-3, SDG-6.

Carboxymethyl cellulose stabilization induced changes in particle characteristics and dechlorination efficiency of sulfidated nanoscale zero-valent iron

Polymer stabilization, exemplified by carboxymethyl cellulose (CMC), has demonstrated effectiveness in enhancing the transport of nanoscale zero-valent iron (nZVI). And, sulfidation is recognized for enhancing the reactivity and selectivity of nZVI in dechlorination processes. The influence of polymer stabilization on sulfidated nZVI (S-nZVI) with various sulfur precursors remains unclear. In this study, CMC-stabilized S-nZVI (CMC-S-nZVI) was synthesized using three distinct sulfur precursors (S2-, S2O42-, and S2O32-) through one-step approach. The antioxidant properties of CMC significantly elevated the concentration of reduced sulfur species (S2-) on CMC-S-nZVIs, marking a 3.1-7.0-fold increase compared to S-nZVIs. The rate of trichloroethylene degradation (km) by CMC-S-nZVIs was observed to be 2.2-9.0 times higher than that achieved by their non-stabilized counterparts. Among the three CMC-S-nZVIs, CMC-S-nZVINa2S exhibited the highest km. Interesting, while the electron efficiency of CMC-S-nZVIs surged by 7.9-12 times relative to nZVI, it experienced a reduction of 7.0-34% when compared with S-nZVIs. This phenomenon is attributed to the increased hydrophilicity of S-nZVI particles due to CMC stabilization, which inadvertently promotes the hydrogen evolution reaction (HER). In conclusion, the findings of this study underscores the impact of CMC stabilization on the properties and dechlorination performance of S-nZVI sulfidated using different sulfur precursors, offering guidance for engineering CMC-S-nZVIs with desirable properties for contaminated groundwater remediation.

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.

Sulfidated microscale zero-valent iron/reduced graphene oxide composite (S-mZVI/rGO) for enhanced degradation of trichloroethylene: The role of hydrogen spillover

Atomic hydrogen (H*) has long been thought to play an important role in the dechlorination of trichloroethylene (TCE) by carbon-supported zero-valent iron (ZVI), which offers an alternative pathway for TCE dechlorination. Herein, we demonstrate that the reductive dechlorination of TCE by sulfidated microscale ZVI (S-mZVI) can be further enhanced by promoting the formation of H* through the introduction of reduced graphene oxide (rGO). The completely degradation of 10 mg/L TCE can be achieved by S-mZVI/rGO within 24 h, which was 3.3 times faster than that of S-mZVI. The change in the distribution of TCE degradation products over time suggests that the introduction of rGO leads to a change in the dechlorination pathway. The percentage of ethane in the final products of TCE degradation by S-mZVI/rGO was 34.3 %, while that of S-mZVI was only 21.9 %. The electrochemical tests confirmed the occurrence of hydrogen spillover in the S-mZVI/rGO composite, which promoted the reductive dechlorination of TCE by H*. Although the S-mZVI/rGO composite had stronger hydrogen evolution propensity than S-mZVI, the S-mZVI/rGO composite still exhibited higher electron utilization efficiency than S-mZVI thanks to the increased utilization of hydrogen.