Stability Of Zero-valent Iron Research Articles

Trade-off between sulfidated zero-valent iron reactivity and air stability: Regulation of iron sulfides by ammonium dihydrogen phosphate

The reactivity and stability of zero-valent iron (ZVI) and sulfidated zero-valent iron (S-ZVI) are inherently contradictory. Iron sulfides (FeSX) on the S-ZVI surface play multiple roles, including electrostatic adsorption and catalyzing reduction. We proposed to balance the reactivity and air stability of S-ZVI by regulating FeSX. Benefiting from the superior coordination and accelerate electron transport capabilities of phosphate, herein, eco-friendly ammonium dihydrogen phosphate (ADP) was employed to synthesize N, P, and S-incorporated ZVI (NPS-ZVI) and regulate the FeSX. Raman, FTIR, XPS, and density functional theory (DFT) calculations were combined to reveal that HPO42- acts as the main P species on the Fe surface. The superior reactivity of NPS-ZVI was quantified by kobs, kSA, and kM of Cr(VI), which were 210.77, 27.44, and 211.17-fold than ZVI, respectively. NPS-ZVI demonstrated excellent reusability, with no risk of secondary pollution. Critically, NPS-ZVI could effectively maintain FeSX stability under the combination of diffusion limitation and surface protection mechanisms of ADP. The superior reactivity of NPS-ZVI was attributed to the fact that ADP maintains FeSX stability and accelerates electron transport. This study provides a novel strategy in balancing the reactivity and air stability of S-ZVI and offers theoretical support for material modification.

Double-edged effect of frequent freeze-thaw on the stability of zero-valent iron after heavy metal remediation

Freeze-thaw cycles (FTCs) cause dynamic microscale changes in ions and solvents. During freezing, heavy metals adsorbed on zero-valent iron (M-ZVI) and protons are excluded by ice crystals and concentrated in the liquid-like grain boundary region. The high proton concentration in this region leads to the dissolution of the passivation layer of ZVI. To assess the environmental risks of M-ZVI during FTCs, this study evaluated the stability of M-ZVI in this scenario from both microscale and macroscale perspectives. The results showed that the dissolution of the passivation layer had a dual effect on the stability of M-ZVI, which depends on the by-products of M-ZVI. The dissolution of the passivation layer was accompanied by the leaching of heavy metals, such as Ni-ZVI, but it also enhanced the reactivity of ZVI, causing it to re-react with desorbed heavy metals. The stability of Cr-ZVI and Cd-ZVI was improved due to frequent FTCs. Furthermore, changes in the surrounding environment (water dipole moment, ion concentration, etc.) of ZVI affected the crystallization of Fe oxides, increasing the content of amorphous Fe oxide. As low-crystallinity Fe oxides could facilitate ion doping, Ni2+ was doped into Fe3O4 lattice during FTCs, which reduced the mobility of heavy metals. Contrary to traditional views that freezing temperatures slow chemical reactions, this study provides new insights into the application of iron-based materials in cold environments.

Green synthesis of zero valent iron using tannins to activate persulfate for sulfamethoxazole degradation

Majority zero-valent iron (ZVI) materials are prepared by reducing agents in liquid phase, resulting in the high environmental pollution and poor particle size distribution uniformity. Therefore, this study employed a green synthesis method to prepare ZVI. Tannins (TA) with phenolic hydroxyl groups that are characterized by strong reducing capacity were employed to synthesize ZVI (TA@ZVI). The dispersity and stability of ZVI was improved by TA, which inhibited the agglomeration of ZVI. Meanwhile, the specific surface area of TA@ZVI was higher than chemical prepared ZVI, increasing the reactive sites. The organic matter components enriched on TA could promote the adsorption of pollutants and complex with Fe(II/III) to enhance the reactivity of TA@ZVI. Also, the polyphenol structure in TA was oxidized to quinone, which facilitated electron transport. In order further test the performance of TA@ZVI, SMX was chosen as a target pollutant to study the oxidative degradation performance of TA@ZVI. SO4•− degraded about 16.4%–25.5% SMX and •OH degraded about 49.8%–63.9% SMX in the pH range of 4–6 while •OH played a dominant role in the neutral and alkaline conditions. Moreover, the presence of TA reduced Fe(III) to Fe(II) and promoted the release of Fe(II), providing a continuous source of •OH for the oxidative degradation of SMX. Besides, the conversion of Fe(II/III) was accelerated due to TA, which delayed the formation of passivation layer. Thus, TA enhanced the antioxidant capacity of ZVI. Generally, this study provided an environmental-friendly technology to synthesize and improve the reactivity of ZVI.

Core-shell magnetic Ag-molecularly imprinted composite for surface enhanced Raman scattering detection of carbaryl

Surface-enhanced Raman scattering (SERS) is a promising technique for rapid detection of pesticide residues. However, conventional SERS substrates require extraction processes which are time consuming and they also lack selectivity, stability and reproducibility. Herein, we present a multifunctional stable zero-valent iron based core-shell substrate. It combines magnetic separation, selective adsorption by molecular imprinting technique and sensitive detection of carbaryl by SERS. The core-shell substrate was successfully prepared by immobilizing silver on the surface of zero-valent iron microspheres. Subsequent molecular imprinting on the bimetallic magnetic silver microspheres ensured selective removal and detection. The substrate exhibited magnetization saturation of 8.89 emu/g providing efficient analyte separation. It showed high sensitivity and selectivity toward carbaryl detection to nanomolar concentration level. Linear regression models for peaks at Raman shift 1599 cm−1 and 2233 cm−1 demonstrated a good linear fit with R2=0.9738 and R2=0.8952 respectively. The composite was successfully applied on spiked water samples resulting in average recovery rate of 89%. The findings of this study demonstrate great substrate potential for application in separation and detection of trace quantities of chemical contaminants for environment safety and protection.

Fabrication and characterization of clay nanoscrolls and stable zerovalent iron using montmorillonite

Halloysite are naturally occurring nanotubes which due to unique structural properties have found tremendous applications. Here, we have described the fabrication of cylindrical-shaped clay nanoscrolls similar to natural halloysite nanotubes, first time from 2:1 type montmorillonite, with palladium and iron-loaded cellulose nanocrystals used as structural directing agents. Electrostatic interaction between negatively charged surface of clay sheets and γ-Fe2O3 resulted in the formation of clay nanoscrolls. Interestingly, change in the calcination environment of the process from normal atmospheric to nitrogen resulted in the formation of stable and ferromagnetic zero-valent iron (ZVI) on the clay support instead of the scrolls. The prepared clay nanoscrolls can be used in different application as an alternative of halloysite. Here, we checked the hydrogen adsorption capacity of nanoscrolls, which showed the comparative results reported in the literature for halloysite. Similarly, the clay nanoscrolls showed good H2O2 reduction ability in the alkaline medium. Permanent magnetic nature of ZVI improves induction heating, and thus can be applied in the biomedical field. Thus, a sustainable approach for fabrication of hallyosite nanotube alternate clay nanoscrolls and stable ZVI has been described here.