Efficient Fe(III)/Fe(II) cycling mediated by L-cysteine functionalized zero-valent iron for enhancing Cr(VI) removal

Role of Cr(VI) in the efficient removal of Cd(II) from aqueous solutions using Fe3S4 in a Cd(II)/Cr(VI) binary system

In this study, the synthesized magnetic greigite (Fe3S4) was utilized to simultaneously eliminate Cd(II) and Cr(VI) from aqueous solution. The removal efficiency (96.6 %) of Cd(II) in the binary (Cd(II)/Cr(VI)) system was superior to that (79.2 %) of the single (Cd(II)) system, accompanied by a negligible difference in the removal efficiency of Cr(VI) (∼100 %) between the single Cr(VI) and binary systems. The adsorption process of Cd(II) onto Fe3S4 closely followed the pseudo-first order kinetic model and Freundlich isotherm, suggesting that the adsorption of Cd(II) primarily involved multi-molecular layer physical adsorption. Characterization results confirmed that the surface hydroxyl groups on Fe3S4 played a significant role in the Cd(II) adsorption in the single system. In contrast, in the binary system, the adsorption of Cd(II) was predominantly attributed to surface sulfide groups at pH 3.8, while hydroxyl groups were the primary contributors to Cd(II) adsorption at pH 6.8. Additionally, at pH 5.3, both sulfide and hydroxyl groups contributed equally to Cd(II) adsorption. This pH-dependent mechanism for Cd(II) removal was resulted from the fact that Cr(VI) could not only influence the solution pH and adjust the isoelectric point of Fe3S4 to enhance the electrostatic attraction between Cd(II) and Fe3S4, but also diminish the concentrations of Fe species to eliminate their competitive adsorption with Cd(II). This study is significantly important for understanding the enhanced removal mechanisms of Cd(II) in the presence of Fe3S4, particularly with the coexistence of Cr(VI), as well as for the simultaneous remediation of both cationic and anionic pollutants from wastewater using iron sulfides.

A green sulfidated micro zero-valent iron based-hydrogel for the synergistic removal of heavy metal cations and anions in groundwater

Heavy metal cations and anions contaminated groundwater was a big challenge to water resource safety. Herein, a green sulfidated micro zero-valent iron-based hydrogel (SA-S-mZVI) was synthesized using sodium alginate biomass for the simultaneous removal of heavy metal cations (Cu(II), Pb(II), Cd(II)) and anions (Cr(VI)). The sulfur modification and incorporation of sodium alginate hydrogel facilitated the efficient and sustainable removal of both single and multi-heavy metals. The co-existing heavy metal cations benefited the removal of Cr(VI), and heavy metals were mostly transformed into stable precipitates. The presence of organic substance and ions slightly affected the removal of heavy metals. Long-term column experiments (240 days) showed that SA-S-mZVI maintained over 99.9 % removal efficiency for heavy metal cations and anions, without adverse impacts on the groundwater environment. This study provided new insights into the development of eco-friendly, long-lasting zero-valent iron-based hydrogels for in-situ remediation of heavy metals-contaminated groundwater.

Surface-bound Fe(0) and Fe(II) mediated by 2-picolinic acid functionalized zero-valent iron for highly Cr(VI) removal

Electron transfer of zero-valent iron (ZVI) is significantly impeded by its oxide layer, and limiting its removal of pollutants. In this study, 2-picolinic acid (PA) and ZVI were co-ball milled to improve electron transfer in ZVI (PA-ZVIbm), and used for the removal of heavy metal Cr(VI). Characterization analysis showed that the presence of electron-rich groups on the surface of PA-ZVIbm promoted the transfer of electrons from the Fe(0) core to the surface, and the surface Fe(0) and Fe(II) contents increased from 1.1 % to 6.3 % and from 60.2 % to 72.9 %, respectively, effectively reducing Cr(VI) through an electron transfer mechanism. Theoretical calculations showed that the modification of PA enhanced the adsorption of Cr(VI) on the ZVI surface, and the adsorption energy decreased from -3.561 eV to -5.119 eV. PA-ZVIbm showed strong advantages in the removal of Cr(VI), with a reaction rate constant and adsorption capacity 17 and 13 times that of ZVIbm, respectively, and a conversion rate of 100 %. Moreover, PA-ZVIbm showed excellent performance over a wide pH range (3-10) and under different coexisting ions, while being cost-effective and having low environmental risks. This study explored the relationship between ZVI surface modification and performance, and provided new insights into the modification of ZVI using small molecule oxygen-containing organic acids.

Cysteine-Facilitated Cr(VI) reduction by Fe(II/III)-bearing phyllosilicates: Enhancement from In-Situ Fe(II) generation

Structural Fe in phyllosilicates represents a crucial and potentially renewable reservoir of electron equivalents for contaminants reduction in aquatic and soil systems. However, it remains unclear how in-situ modification of Fe redox states within Fe-bearing phyllosilicates, induced by electron shuttles such as naturally occurring organics, influences the fate of contaminants. Herein, this study investigated the processes and mechanism of Cr(VI) reduction on two typical Fe(II/III)-bearing phyllosilicates, biotite and chlorite, in the presence of cysteine (Cys) at circumneutral pH. The experimental results demonstrated that Cys markedly enhanced the rate and extent of Cr(VI) reduction by biotite/chlorite, likely because of the formation of Cr(V)-organic complexes and consequent electron transfer from Cys to Cr(V). The concomitant production of non-structural Fe(II) (including aqueous Fe(II), surface bound Fe(II), and Cys-Fe(II) complex) cascaded transferring electrons from Cys to surface Fe(III), which further contributed to Cr(VI) reduction. Notably, structural Fe(II) in phyllosilicates also facilitated Cr(VI) reduction by mediating electron transfer from Cys to structural Fe(III) and then to edge-sorbed Cr(VI). 57Fe Mössbauer analysis revealed that cis-coordinated Fe(II) in biotite and chlorite exhibits higher reductivity compared to trans-coordinated Fe(II). The Cr end-products were insoluble Cr(III)-organic complex and sub-nanometer Cr2O3/Cr(OH)3, associated with residual minerals as micro-aggregates. These findings highlight the significance of in-situ produced Fe(II) from Fe(II/III)-bearing phyllosilicates in the cycling of redox-sensitive contaminants in the environment.

Reconsideration of the role of hydrogen peroxide in peroxymonocarbonate-based oxidation system for pollutant control

Advanced oxidation processes that utilize peroxymonocarbonate (HCO4-), generated in-situ through the reaction of HCO3- and H2O2, are employed for the removal of pollutants in water. Nevertheless, the precise role of H2O2 in these processes remains a subject of debate. This study established a HCO4--based oxidation system using NaHCO3 and H2O2 for the degradation of acetaminophen and investigated the activation mechanisms of coexisting oxidants. Under thermal activation conditions, the OO bond in HCO4- (HOOCOO-) was more readily cleaved than the OO bond in the co-existing oxidant H2O2 (HOOH), leading to the generation of reactive oxygen species (ROS). Based on kinetics and ROS evaluation, H2O2 primarily served to form HCO4- rather than converting to ·OH or O2, with HCO4- acting as the primary oxidant for degradation through the formation of CO3·-and ·OH. In this oxidation system, H2O2 utilization efficiency for ·OH production reached 27.34 %, ·OH yield reached 24.15 % and acetaminophen degradation efficiency realized 83 % at 60 °C with 20 mM HCO3- and 20 mM H2O2. The apparent activation energy of acetaminophen degradation and HCO4- activation were calculated as 90.83 kJ mol-1 and 18.81 kJ mol-1, respectively. Moreover, a novel CO2-derived HCO4--based system led to a comparable acetaminophen degradation efficiency of 82 % and a higher kobs of 0.028 min-1. The system optimization and ROS evaluation suggest that high concentration of H2O2 inhibited the degradation and quenched CO3·- and ·OH to yield ·O2- and 1O2. Furthermore, EPR analysis and quenching experiments indicate that CO3·- was mainly responsible for acetaminophen degradation. This work provides fundamental understanding of the HCO4--based oxidation system.

Heterogeneous activation of self-generated H2O2 by Pd@UiO-66(Zr) for trimethoprim degradation: Efficiency and mechanism

The Fenton reaction is recognized as an effective technique for degrading persistent organic pollutants, such as the emerging pollutant trimethoprim (TMP). Recently, due to the excellent reducibility of active hydrogen ([H]), Pd-H2 has been preferred for Fenton-like reactions and the specific H2 activation of Pd-based catalysts. Herein, a heterogeneous Fenton catalyst named the hydrogen-accelerated oxygen reduction Fenton (MHORF@UiO-66(Zr)) system was prepared through the strategy of building ships in the bottle. The [H] has been used for the acceleration of the reduction of Fe(III) and self-generate H2O2. The systematic characterization demonstrated that the nano Pd0 particle was highly dispersed into the UiO-66(Zr). The results found that 20 mg L-1 of TMP was thoroughly degraded within 90 min in the MHORF@UiO-66(Zr) system under conditions of initial pH 3, 30 mL min-1 H2, 2 g L-1 Pd@UiO-66(Zr) and 25 μM Fe2+. The hydroxyl radical as well as the singlet oxygen were evidenced to be the main reactive oxygen species by scavenging experiments and electron spin resonance. In addition, both reducing Fe(III) and self-generating H2O2 could be achieved due to the strong metal-support interaction (SMSI) between the nano Pd0 particles and UiO-66(Zr) confirmed by the correlation results of XPS and calculation of density functional theory. Finally, the working mechanism of the MHORF@UiO-66(Zr) system and the possible degradation pathway of the TMP have been proposed. The novel system exhibited excellent reusability and stability after six cyclic reaction processes.

Tea saponin co-ball milled commercial micro zero-valent iron for boosting Cr(VI) removal

Tea saponins (TS), a natural biosurfactant extracted from tea trees, were co-ball milled with commercial micro zero-valent iron (mZVI) to produce TS modified mZVI (TS-BZVI) for efficient hexavalent chromium (Cr(VI)) removal. The findings demonstrated that TS-BZVI could nearly remove 100% of Cr(VI) within 2 h, which was 1.43 times higher than that by ball milled mZVI (BZVI) (70%). Kinetics analysis demonstrated a high degree of compatibility with the pseudo-second-order (PSO), revealing that TS-BZVI exhibited a 2.83 times faster Cr(VI) removal rate involved primarily chemisorption. Further, X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) measurements indicated that the TS co-ball milling process improved the exposure of Fe(II) and Fe(0) on mZVI, which further promoted the Cr(VI) reduction process. Impressively, the introduction of TS increased the hydrophobicity of ZVI, effectively inhibiting the H2 evolution by 95%, thus improved electron selectivity for efficient Cr(VI) removal. Ultimately, after operating for 10 days, a simulated permeable reactive barrier (PRB) column experiment revealed that TS-BZVI had a higher Cr(VI) elimination efficiency than BZVI, indicating that TS-BZVI was promising for practical environment remediation.

Effect of pyrite on the treatment of chlorophenolic compounds with zero-valent iron-Fenton process under uncontrolled pH conditions: reaction mechanism and biodegradability

This current study explored the effect of pyrite on the treatment of chlorophenolic compounds (CP) by Fenton process with micron-sized zero-valent iron (ZVI) as the catalyst. The experiments were conducted in batch reactors with 100 mg L-1 CP, 0-0.02 M H2O2, and variable pyrite and ZVI doses (0-1 g L-1). Our findings show that while the reactor with 1 g L-1 ZVI as the only catalyst achieved only 10% CP removal efficiency due to rapid ZVI surface passivation and ZVI particle aggregation, the CP removal efficiency increased with increasing pyrite dose and reached 100% within couple of minutes in reactors with 0.8 g L-1 pyrite and 0.2 g L-1 ZVI. The CP removal was mainly driven by the oxidative treatment of CPs with some strong radicals such as hydroxyl radicals (•OH) while the adsorption onto the catalyst surface was only responsible for 10 to 25% of CP removals, depending on the type of CP studied. The positive impact of pyrite on CP removal by the ZVI/H2O2 system could be attributed to the ability of pyrite to (1) create an acidic environment for optimum Fenton process, (2) provide support material for ZVI to minimize ZVI particle agglomeration, and (3) stimulate iron redox cycling for improved surface site generation. Following oxidative Fenton treatment, the degradation intermediate products of CPs, including some aromatic compounds (benzoquinone, hydroquinone, etc.) and organic acids (e.g., acetic acid), became more biodegradable in comparison to their mother compounds. Overall, the treatment systems with a mixture of ZVI and pyrite as catalyst materials could offer a suitable cost-effective technology for the treatment of wastewater containing biologically non- or low-degradable toxic compounds such as chlorophenols.

Enhanced Cr(VI) elimination from water by goethite-impregnated activated carbon coupled with weak electric field

A weak electric field (WEF, 2 mA cm-2) was employed to promote Fe(III)/Fe(II) cycle on goethite-impregnated activated carbon (FeOOH@AC) filled in a continuous-flow column for enhanced Cr(VI) elimination from water. Surficial analysis and Cr species distribution showed that α-FeOOH of 0.2-1 μm was successfully synthesized and evenly loaded onto AC. Electron transfer from WEF to α-FeOOH was facilitated with AC as electron shuttles, thereby boosting Fe(III) reduction in the α-FeOOH. The generated Fe(II) reduced Cr(VI) and the resultant Cr(III) subsequently precipitated with OH- and Fe(III) to form Cr(OH)3 and (CrχFe1-χ)(OH)3. Therefore, the WEF-FeOOH@AC column exhibited a much lower Cr(VI) migration rate of 0.0018 cm PV-1 in comparison with 0.0037 cm PV-1 of the FeOOH@AC column, equal to 104 % higher Cr(VI) elimination capacity and 90 % longer column service life-span. Additionally, under different Cr(VI) loadings by varying either seepage velocities or influent Cr(VI) concentrations, the WEF-FeOOH@AC column maintained 1.0-1.5 folds higher Cr(VI) elimination and 0.9-1.4 folds longer longevity than those of the FeOOH@AC column owing to the interaction between FeOOH@AC and WEF. Our research demonstrated that WEF-FeOOH@AC was a potential method to promote Cr(VI) elimination from water and offer an effective strategy to facilitate Fe(III)/Fe(II) cycle in iron oxides.