Depassivation of aged Fe 0 by divalent cations: correlation between contaminant degradation and surface complexation constants

Decoding the divalent cation effect on sulfidation of zero-valent iron: Phase evolution and FeSx assembly

The decontamination ability of sulfidated zero-valent iron (S-ZVI) can be enhanced by the effective assembly of iron sulfides (FeSx) on neglected heterogeneous surfaces by liquid-phase precipitation. However, S-ZVI preparation with the usual pickling is detrimental to orderly interfacial assembly and leads to an imbalance between electron transfer optimization and electron storage. In this work, S-ZVI was prepared in solutions containing trace divalent cation, and it removed Cr(VI) up to 323.25 times higher than ZVI. This result is achieved by surface sites protonation of divalent cations regulating the phase evolution on the ZVI surface and inducing FeSx chemical assembly. Regulation of divalent cation and S(-II) content further promotes FeSx targeted assembly and reduces electron storage consumption as much as possible. The barrier for FeSx assembly is found to lie at the ZVI interface rather than in the deposition between FeSx. Chemical assembly at heterogeneous interfaces is a prerequisite for the ordered assembly of FeSx. In addition, S-ZVI prepared in simulated groundwater showed extensive preparation pH and universality for remediation scenarios. These findings provide new insights into the development of in-situ sulfidation mechanisms with particular implications for S-ZVI applied to soil and groundwater remediation by the regulation of heterogeneous interfacial assembly.

Dinitrosyl Iron Complex-Derived Nanosized Zerovalent Iron (NZVI) as a Template for the Fe-Co Cracked NZVI: An Electrocatalyst for the Oxygen Evolution Reaction

Nanosized zerovalent iron (NZVI) Fe@Fe3O4 with a core-shell structure derived from photocatalytic MeOH aqueous solution of dinitrosyl iron complex (DNIC) [(N3MDA)Fe(NO)2] (N3MDA = N,N-dimethyl-2-(((1-methyl-1H-imidazole-2-yl)methylene)amino)ethane-1-amine) (1-N3MDA), eosin Y, and triethylamine (TEA) is demonstrated. The NZVI Fe@Fe3O4 core shows a high percentage of zerovalent iron (Fe0 %) and is stabilized by a hydrophobic organic support formed through the photodegradation of eosin Y hybridized with the N3MDA ligand. In addition to its well-known reductive properties in wastewater treatment and groundwater remediation, NZVI demonstrates the ability to form heterostructures when it interacts with metal ions. In this research, Co2+ is employed as a model contaminant and reacted with NZVI Fe@Fe3O4 to result in the formation of a distinct Fe-Co heterostructure, cracked NZVI (CNZVI). The slight difference in the standard redox potentials between Fe2+ and Co2+, the magnetic properties of Co2+, and the absence of surface hydroxides of Fe@Fe3O4 enable NZVI to mildly reduce Co2+ and facilitate Co2+ penetration into the iron core. Taking advantage of the well-dispersed nature of CNZVI on an organic support, the reduction in particle size due to Co2+ penetration, and Fe-Co synergism, CNZVI is employed as a catalyst in the alkaline oxygen evolution reaction (OER). Remarkably, CNZVI exhibits a highly efficient OER performance, surpassing the benchmark IrO2 catalyst. These findings show the potential of using NZVI as a template for synthesizing highly efficient OER catalysts. Moreover, the study demonstrates the possibility of repurposing waste materials from water treatment as valuable resources for catalytic energy conversion, particularly in water oxidation processes.

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.

Enhancing zerovalent iron-based Fenton-like chemistry by copper sulfide: Insight into the active sites for sustainable Fe(II) supply

Zerovalent iron (ZVI)-based Fenton-like processes have been widely applied in degrading organic contaminants. However, the surface oxyhydroxide passivation layer produced during the preparation and oxidation of ZVI hinders its dissolution and Fe(III)/Fe(II) cycling, and restricts the generation of reactive oxygen species (ROS). In this study, copper sulfide (CuS) was found to effectively enhance the degradation of diverse organic pollutants in the ZVI/H₂O₂ system. Moreover, the degradation performance for the actual industrial wastewater (i.e., dinitrodiazophenol wastewater) in the ZVI/H₂O₂ system was impressively improved by 41% with CuS addition, and the COD removal efficiency could reach 97% after 2 h of treatment. Mechanism investigation revealed that the introduction of CuS accelerated the sustainable supply of Fe(II) in the ZVI/H₂O₂ system. Specifically, Cu(I) and reductive sulfur species (i.e., S^2-, S₂^2-, S_n^2- and H₂S (aq)) from CuS directly induced efficient Fe(III)/Fe(II) cycling. The iron-copper synergistic effect between Cu(II) from CuS and ZVI expedited Fe(II) generation from ZVI dissolution and Fe(III) reduction by formed Cu(I). This study not only elucidates the promotion effects of CuS on ZVI dissolution and Fe(III)/Fe(II) cycling in ZVI-based Fenton-like processes, but also provides a sustainable and high-efficiency iron-based oxidation system for removal of organic contaminants.

Aging and reactivity assessment of nanoscale zerovalent iron in groundwater systems

In this study, changes in the reactivity of nanoscale zerovalent iron (NZVI) in five different groundwater (GW) systems under anoxic and oxic conditions were examined over a wide range of aging time (0 - 60 d). p-nitrophenol (p-NP) was used as a redox-sensitive probe, whereas nalidixic acid (NA), a typical antibiotic found in the natural environment, was used as a sorbing compound. Investigation of the p-NP reduction in pure water systems showed that NZVI lost 41% and 98% of its reductive activity under anoxic and oxic conditions after 60 d, while enhancement of its reactivity was observed after short-term aging in GW (1 - 5 d), followed by a further decline. This behavior has been ascribed to the formation of secondary Fe(II)-bearing phases, including magnetite and green rust, resulting from NZVI aging in GW. Adsorption experiments revealed that GW-anoxic-aged NZVI samples exhibited a good affinity toward NA, and a greater NA adsorption (∼27 µmol g ^- 1) than that of pristine NZVI (∼2 µmol g ^- 1) at alkaline pH values. Surface complexation modeling showed that the enhanced adsorption of NA onto secondary minerals can be attributed to the Fe(II)-NA surface complexation. This considerable change in the reductive ability and the adsorption capacity of NZVI arising from groundwater corrosion calls for greater attention to be paid in assessment studies, where NZVI is injected for long-term remediation in groundwater.

Insights into the mechanisms of aqueous Cd(II) reduction and adsorption by nanoscale zerovalent iron under different atmosphere conditions

Nanoscale zero-valent iron (NZVI) can effectively remove and recover Cd(II) from aqueous solutions. However, the oxygen effects on Cd(II) removal by NZVI have been overlooked and not well studied. In this research, the Cd MNN auger lines obtained by X-ray photoelectron spectroscopy (XPS) revealed that Cd(II) adsorbed on the NZVI surface could be reduced to Cd(0) by the Fe(0) core under anaerobic conditions. With coexisting oxygen, the Cd(II) removal efficiency declined significantly, and Cd(II) reduction was inhibited by the thickened surface γ-FeOOH layer. Furthermore, the post-oxygen intrusion corroded the generated Cd(0) and led to the dramatic leaching of Cd(II) ions. According to the density functional theory (DFT) simulation, the adsorbed Cd(II) was preferably coordinated via a monodentate model on the surface of Fe₃O₄ and γ-FeOOH, which are the dominant surface species of NZVI under anaerobic and aerobic conditions, respectively. Thus, γ-FeOOH with doubly coordinated hydroxyl groups provided fewer adsorption sites than Fe₃O₄ for Cd(II) ions. Overall, the atmospheric conditions of subsurface remediation and wastewater treatment should be considered when applying NZVI for Cd(II) removal. Favorable atmospheric conditions would improve the efficiency and cost-effectiveness of NZVI-based technologies for the practical remediation of Cd(II) pollution.

Synthesis of Ag-Cu co-doping sponge iron-based trimetal for boosting simultaneous degradation of combined pollutants

To date, zero-valent iron (ZVI)-based technique has encountered a baffle, challenging simultaneous detoxification of refractory rhodamine B (RhB) and p-nitrophenol (PNP) possessing strong electronwithdrawing nitro-group. In this study, we synthesized Ag-Cu decorated sponge iron (s-Fe⁰)-based trimetal for simultaneous degradation of RhB and PNP. The results show that Cu-Ag co-doping s-Fe⁰ (s-Fe⁰-(Cu-Ag)) achieves approx. 90.6 % of maximized removal of RhB; the preferred s-Fe⁰-(5 wt%Cu-1 wt%Ag) assisted with 6 L/min aeration rate simultaneously declines RhB and PNP within 10 recycling tests; non-aeration process obtains a complete reduction of PNP as well as merely approx. 23.9 % removal of RhB. Moreover, the Cu-Ag microstructure covering s-Fe⁰-(Cu-Ag) has been characterized in detail. Furthermore, the electron spin resonance (ESR) spectra have been applied to investigate simultaneous generation of reactive oxygen species (ROS_s) and hydrogen radicals ([H]_abs) over s-Fe⁰-(Cu-Ag). To our best knowledge, this is the first study reporting the enhanced bifunctional catalysis of s-Fe⁰-(Cu-Ag)/O₂ for simultaneous degradation of RhB and PNP.

The role of nitrate in simultaneous removal of nitrate and trichloroethylene by sulfidated zero-valent Iron

Sulfidated zero-valent iron (S-ZVI) is commonly used to degrade trichloroethylene (TCE). The reactivity of S-ZVI is related to not only the properties of S-ZVI but also the geochemical conditions in groundwater, such as coexisted NO₃^-. Therefore, the effect of NO₃^- on TCE degradation by S-ZVI and its mechanism were systematically studied. 95.17% of TCE was degraded to acetylene, dichloroethene, ethene, ethane and multi‑carbon products via β-elimination by fresh S-ZVI that contained 85.31% Fe⁰ and 14.69% FeS in the presence of NO₃^-, demonstrating that NO₃^- did not affect the degradation pathway of TCE. While high concentration of NO₃^- (> 10 mg/L) competed for electrons at the Fe/FeO_x interface with degradation products, leading to a continuous rising of acetylene. Moreover, the rapid reduction of NO₃^- to NH₄⁺ (89.79%) at the Fe⁰ interface contributed to the release of 5.08 mM Fe²⁺ from S-ZVI, which promoted the formation of Fe₃O₄ with excellent electron conduction properties on the surface of S-ZVI. Accordingly, NO₃^- improved the degradation and electron selectivity of TCE by 51.07% and 2.79 fold, respectively. This study demonstrated that S-ZVI could remediate the contamination of NO₃^- and TCE simultaneously and the presence of NO₃^- could effectively enhance the degradation of TCE in groundwater.

Acid mine drainage treatment using zero-valent iron nanoparticles in biochemical passive reactors

Acid mine drainage (AMD) is the major effluent generated from metal and coal mines, causing serious ecological risks and degradation of aquatic habitats and surrounding soil quality. Biochemical passive reactors (BPRs) are an option for improving AMD affected water. This study investigates the effect of the size and concentration of zerovalent iron nanoparticles (nZVI) on the efficiency of batch BPRs during AMD remediation. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were also used as complementary techniques for the investigation of the changes in microbial cells and nZVI properties after the AMD remediation. The results from the batch experiment showed that the concentration of nZVI increases the pH and decreases ORP during AMD treatment, thus favouring the removal of metals. The results also suggest that metal sulfide precipitation occurred in all the batch with reactive mixture but was greater in reactors amended with nZVI of larger size. This study revealed that the presence of nZVI in the BPR leads to metal removal as well as the inhibition of sulfate-reducing bacteria (SRB) activity. Microscopy study indicated that the addition of nZVI creates a morphological change on certain microorganisms in which the cellular membrane was fully covered with nZVI, inducing cell lysis process. These results show that nZVI is a promising reactive material for the treatment of AMD in BPR systems.

Effects of Calcium on Arsenate Adsorption and Arsenate/Iron Bioreduction of Ferrihydrite in Stimulated Groundwater

The reduction and transformation of arsenic-bearing ferrihydrite by arsenate-iron reducing bacteria is one of the main sources of arsenic enrichment in groundwater. During this process the coexistence cations may have a considerable effect. However, the ionic radius of calcium is larger than that of iron and shows a low affinity for ferrihydrite, and the effect of coexisting calcium on the migration and release of arsenic in arsenic-bearing ferrihydrite remains unclear. This study mainly explored the influence of adsorbed Ca²⁺ on strain JH012-1-mediated migration and release of arsenate in a simulated groundwater environment, in which 3 mM ferrihydrite and pH 7.5. Ca²⁺ were pre-absorbed on As(V)-containing ferrihydrite with a As:Fe ratio of 0.2. Solid samples were analyzed by X-ray diffraction (XRD), scanning electron microscopic (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The results show that calcium and arsenate can synergistically adsorb on ferrihydrite due to the electrostatic interactions, and the adsorbed Ca²⁺ mainly exists on the surface through the outer-sphere complex. Adsorbed Ca²⁺ entering the stimulated groundwater was easily disturbed and led to an extra release of 3.5 mg/L arsenic in the early stage. Moreover, adsorbed Ca²⁺ inhibited biogenic ferrous ions from accumulating on ferrihydrite. As a result, only 12.30% Fe(II) existed in the solid phase, whereas 29.35% existed without Ca²⁺ adsorption. Thus, the generation of parasymplesite was inhibited, which is not conducive to the immobilization of arsenic in groundwater.

Enhanced reductive reactivity of zero-valent iron (ZVI) for pollutant removal by natural organic matters (NOMs) under aerobic conditions: Correlation between NOM properties and ZVI activity

While ubiquitous natural organic matters (NOMs) are capable of enhancing zero-valent iron (ZVI) performance under aerobic conditions, there is limited understanding of how the properties of NOMs affect the reactivity of ZVI towards contaminants removal. Here, the corresponding activity of ZVI under aerobic conditions was investigated in the presence of humic acid (HA), fulvic acid (FA), bovine serum albumin (BSA). It was found that three models of NOMs were all effective in promoting diatrizoate (DTA) reduction via depassivating ZVI. Interestingly, fast adsorption of NOM onto ZVI surface initially caused inconspicuous impact or visible inhibition on hydrophilic DTA reduction depending on their hydrophobicity. However, subsequent exposure of more reactive sites with high hydrophilicity arising from the detachment of surfaced NOM-associated iron oxide finally contributed to the enhanced consumption of Fe⁰ with the ability: HA > FA ≈ BSA, and 1-2 times increase in DTA removal kinetic rate following the order: HA > FA > BSA. It further revealed that there were two key factors in determining DTA removal under aerobic conditions, including the ability of NOMs to boost Fe⁰ consumption as contributed by their aromaticity degree and amino groups, and the hydrophobicity of NOMs to initially affect the property of ZVI surfaces.

Removal of heavy metal ions and polybrominated biphenyl ethers by sulfurized nanoscale zerovalent iron: Compound effects and removal mechanism

Sulfurized nanoscale zerovalent iron (S-nZVI) has been widely reported to be able to quickly remove heavy metals/persistent organic pollutants, but the limited understanding of the complicated removal process of heavy metals-organic combined pollutants restricts the application of S-nZVI. Here, we demonstrate that there is significant difference in the effectiveness of S-nZVI for removing single pollutant and complex pollutants. The removal kinetic constant (k_obs) of heavy metals by S-nZVI followed a sequence of Cr(VI)>Pb(II)>Ni(II)>Cd(II) with or without polybrominated diphenyl ethers (PBDEs). While the capacity of co-existing cations increasing the k_obs of PBDEs followed the order: Ni(II)>Pb(II)>Cd(II), and the co-existence of Cr(VI) anion inhibited the reduction of PBDE by S-nZVI because the generated Cr-Fe precipitate hindered the electron transfer. The de-passivation process on S-nZVI surface by Cd(II) ions slightly accelerated the transformation rate of electron. Nevertheless, the co-existing Pb(II) significantly accelerated the transformation of BDE-209 via the galvanic effect from the generated Pb⁰/Fe⁰ bimetal. Interestingly, the k_obs of BDE-47 in Ni(II)/S-nZVI system was 5.51 times higher than that of Pb(II)/S-nZVI system, implying that an atomic hydrogen mechanism dominated the reduction of BDE-47 by Ni(II)/S-nZVI. In conclusion, the results provided a deep comprehending of removal mechanism of heavy metal-organic complex pollutants by S-nZVI.

Fulvic Acid-Mediated Interfacial Reactions on Exposed Hematite Facets during Dissimilatory Iron Reduction

Although the dual role of natural organic matter (NOM) as an electron shuttle and an electron donor for dissimilatory iron (Fe) reduction has been extensively investigated, the underlying interfacial interactions between various exposed facets and NOM are poorly understood. In this study, fulvic acid (FA), as typical NOM, was used and its effect on the dissimilatory reduction of hematite {001} and {100} by Shewanella putrefaciens CN-32 was investigated. FA accelerates the bioreduction rates of hematite {001} and {100}, where the rate of hematite {100} is lower than that of hematite {001}. Secondary Fe minerals were not observed, but the HR-TEM images reveal significant defects. The ATR-FTIR results demonstrate that facet-dependent binding mainly occurs via surface complexation between the surface iron atoms and carboxyl groups of NOM. The spectroscopic and mass spectrometry analyses suggest that organic compounds with large molecular weight, highly aromatic and unsaturated structures, and lower H/C ratios are easily adsorbed on Fe oxides or decomposed by bacteria in FA-hematite {001} treatment after iron reduction. Due to the metabolic processes of cells, a significant number of compounds with higher H/C and medium O/C ratios appear. The Tafel curves show that hematite {100} possessed higher resistance (4.1-2.6 Ω) than hematite {001} (3.5-2.2 Ω) at FA concentrations ranging from 0 to 500 mg L^-1, indicating that hematite {100} is less conductive during the electron transfer from reduced FA or cells to Fe oxides than hematite {001}. Overall, the discrepancy in the iron bioreduction of two exposed facets is attributed to both the different electrochemical activities of the Fe oxides and the different impacts on the properties and composition of OM. Our findings shed light on the molecular mechanisms of mutual interactions between FA and Fe oxides with various facets.

Mechanistic role of nitrate anion in TCE dechlorination by ball milled ZVI and sulfidated ZVI: Experimental investigation and theoretical analysis

Mechanistic role of NO₃^- in trichloroethylene (TCE) dechlorination by ball milled, micro-scale sulfidated and unsulfidated ZVI (e.g., S-mZVI^bm and mZVI^bm) was explored through experiments and density functional theory (DFT) calculations. Sulfidation inhibited NO₃^- reduction by mZVI^bm as S weakened its interaction with NO₃^-. mZVI^bm reduced NO₃^- within 2 h. This just resulted in a short-term electron competition during the dechlorination process by mZVI^bm and hardly affected its sluggish dechlorination kinetics (complete TCE dechlorination in 11 d). On the contrary, NO₃^- suppressed TCE dechlorination by S-mZVI^bm. This was attributed to that inhibited NO₃^- reduction by S-mZVI^bm (40 % reduction in 6 h) induced continuous electron competition with TCE during the time span of its dechlorination by S-mZVI^bm. NO₃^- reduction was also observed to facilitate formation/crystallization of Fe₃O₄ on both ZVI particles, promoting dechlorination by mZVI^bm after 4 d while not taking effect to the S-mZVI^bm/TCE system, as its dechlorination time was too short for the surface of S-mZVI^bm to transform. This observation has important implication on groundwater remediation by ZVI or sulfidated ZVI PRBs under a scenario of upgradient anthropogenic release of NO₃^-.

Arsenic removal with zero-valent iron filters in Burkina Faso: Field and laboratory insights

Groundwater contaminated with geogenic arsenic (As) is frequently used as drinking water in Burkina Faso, despite adverse health effects. This study focused on testing low-cost filter systems based on zero-valent iron (ZVI), which have not yet been explored in West Africa for As removal. The active ZVI bed was constructed using small-sized iron nails, embedded between sand layers. Household filters were tested for nine months in a remote village relying on tube well water with As concentrations of 400-1350 μg/L. Daily filtered volumes were 40-60 L, with flow rates of ~10 L/h. In parallel, downscaled laboratory filter columns were run to find the best set-up for optimal As removal, with special attention given to the influence of input pH, flow rate and water/nail contact time. Arsenic removal efficiencies in the field were 60-80% in the first six months of operation. The laboratory experiments revealed that trapped air in the nail layer greatly lowered As removal due to preferential flow and decreased water/nail contact time. Measures taken to avoid trapped air led to a partial improvement in the field filters, but effluent As remained >50 μg/L. Similar structural modifications were however very successful in the laboratory columns, where As removal efficiencies were consistently >95% and effluent concentrations frequently <10 μg/L, despite inflow As >1000 μg/L. A constantly saturated nail bed and careful flow control is necessary for optimal As removal. Slow flow and longer pauses between filtrations are important for sufficient contact times and for transformation of brown amorphous Fe-hydroxides to dense magnetite with incorporated As(V). This preliminary study has shown that nail-based filters have the potential to achieve As removal >90% in a field context if conditions (filter bed saturation, flow rate, pauses between filtrations) are well controlled.

Efficient removal of tetrabromobisphenol A (TBBPA) using sewage sludge-derived biochar: Adsorptive effect and mechanism

Sewerage sludge-derived biochars (SSDBCs) with high adsorption capacity and excellent recyclability were synthesized to remove tetrabromobisphenol A (TBBPA) in aqueous system. Scanning electron microscopy, elemental mapping via energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy were used to characterize the morphology, composition, and microstructures. The maximum adsorption capacity of SSDBCs was about 87.02 mg g^-1 at 303 K and pH 7.5. The Langmuir isotherm demonstrated that the adsorption was mainly homogeneous and chemical processes. The kinetics of TBBPA removal well fitted the second-order dynamic model. Thermodynamic analysis showed that the adsorption was exothermic. The effect of π-π dispersive force and hydrogen bonding was proven as the main adsorption mechanism. Multiple cycle runs experiment revealed the excellent stability of recycled SSDBCs. This work provided a promising method of sludge resourceful treatment using an efficient, economic, cyclic, and convenient material for typical organic contaminant in the environment.

Effects of common groundwater ions on the transformation and reactivity of sulfidized nanoscale zerovalent iron

Sulfidized nanoscale zerovalent iron (S-nZVI) is an Fe-based reactant widely studied for its potential use for groundwater remediation. S-nZVI reactivity has been widely investigated testing various contaminants in various water matrices, but studies on S-nZVI corrosion behaviour and reactivity upon exposure to complex groundwater chemistries are limited. Here, we show that anoxic aging of S-nZVI for 7 days in the absence and presence of key groundwater solutes (i.e., Cl^-, SO₄^2-, Mg²⁺, Ca²⁺, HCO₃^-, CO₃^2-, NO₃^-, or HPO₄^2-) impacts Fe⁰ corrosion extent, corrosion product and reduction rates with trichloroethene (TCE). White rust was the dominant corrosion product in ultrapure water and in SO₄^2-, Cl^-, Mg²⁺ or Ca²⁺ solutions; green rust and/or chukanovite formed in HCO₃^- and CO₃^2- solutions; magnetite, formed in NO₃^- solutions and vivianite in HPO₄^2- solutions. The aged S-nZVI materials expectedly showed lower reactivities with TCE compared to unaged S-nZVI, with reaction rates mainly controlled by ion concentration, Fe⁰ corrosion extent, type(s) of corrosion product, and solution pH. Comparison of these results to observations in two types of groundwaters, one from a carbonate-rich aquifer and one from a marine intruded aquifer, showed that S-nZVI corrosion products are likely controlled by the dominant GW solutes, while reactivity with TCE is generally lower than expected, due to the multitude of ion effects. Overall, these results highlight that S-nZVI corrosion behaviour in GW can be manifold, with varied impact on its reactivity. Thus, testing of S-nZVI stability and reactivity under expected field conditions is key to understand its longevity in remediation applications.

Relative contribution of ferryl ion species (Fe(IV)) and sulfate radical formed in nanoscale zero valent iron activated peroxydisulfate and peroxymonosulfate processes

Activation of persulfates (i.e., peroxydisulfate (PDS) and peroxymonosulfate (PMS)) by nanoscale zero-valent iron (nZVI) is reported to be effective in oxidative treatment of environmental contaminants. It has been widely accepted in numerous literature that sulfate radical (SO₄^•-) formed from the decomposition of persulfates activated by aqueous Fe(II) released from nZVI corrosion is responsible for the oxidative performance in nZVI/persulfates systems. In this work, by employing methyl phenyl sulfoxide (PMSO) as a probe, we demonstrated that the activation of persulfates by nZVI through electron transfer led to SO₄^•- formation, while the homogeneous activation of persulfate by the released Fe(II) resulted in ferryl ion species (Fe(IV)) generation in nZVI/persulfates systems. Similarly, nanoscale zero-valent aluminum (nZVAl) and zinc (nZVZn) were also demonstrated to be able to donate electron to persulfates leading to SO₄^•- formation. However, the insulative aluminum oxide shell hindered the electron transfer leading to the poor persulfates decomposition, while the conductive iron and zinc oxide shell enabled the electron transfer process resulting in a continuous generation of SO₄^•-. Further, it was obtained that the relative contribution of SO₄^•- and Fe(IV) in nZVI/persulfates systems was independent of the initial concentration of nZVI and PDS, but was positively correlated with PMS concentration. In addition, the increase of pH from 3 to 7 led to the decrease of the relative contribution of Fe(IV), which was rationalized by the decrease of availability of aqueous Fe(II) at higher pH. Our findings not only shed lights on the nature of the reactive intermediate formed in the nZVI/persulfates systems, but also unprecedentedly distinguished the surface activation of persulfates from the homogeneous catalysis process.

Ferrous ion mitigates the negative effects of humic acid on removal of 4-nitrophenol by zerovalent iron

In this study, Fe²⁺ addition was employed to overcome the negative effects of humic acid (HA) on contaminant removal by zerovalent iron (ZVI), and its feasibility to improve electron efficiency of ZVI was also tested. HA at high concentrations suppressed the removal of 4-nitrophenol (4-NP) by ZVI, while the addition of 0.25-1.0 mM Fe²⁺ could greatly mitigate this inhibitory effect and enhance 4-NP reduction. Specifically, with a mixed-order model, global fitting results showed that the addition of Fe²⁺ increased the rate constant from 0.124 × 10^-2-0.219 × 10^-2 mM/min to 0.227 × 10^-2-0.417 × 10^-2 mM/min and shortened lag period from 19.7-47.9 min to 8.0-15.2 min for 4-NP removal. The mechanistic investigation revealed this trend could be explained by the following aspects: i) Fe²⁺ can facilitate the generation of Fe(II)-containing oxides, which can act as an electron mediator or direct electron donor for 4-NP reduction; ii) the presence of Fe²⁺ could lead to aggregation of HA particles and accordingly reduced its coverage on ZVI surface. But the results of respike experiments indicate that Fe²⁺ addition did not show remarkable effect on the electron efficiency of 4-NP by ZVI, which should be associated with that Fe²⁺ was not able to favor the enrichment of 4-NP on ZVI surface.

Reductive debromination of decabromodiphenyl ether by iron sulfide-coated nanoscale zerovalent iron: mechanistic insights from Fe(II) dissolution and solvent kinetic isotope effects

The mechanism that iron sulfide-coated nanoscale zero valent iron (S-nZVI) has better reduction activity towards organic pollutants than nanoscale zero-valent iron (nZVI) has long been debated. In this work, a systematic study was investigated to compare differences of main influences, BDE-209 degradation pathway, degradation kinetics and reduction mechanism of BDE-209 between nZVI and S-nZVI systems. The observed transformation rate of BDE-209 (k_obs) by S-nZVI was 58.3 and 7.1 times greater than that by S^2- and nZVI, respectively. The valence change of Fe and S on S-nZVI surface before and after BDE-209 degradation process based on XPS characterization confirmed that both Fe⁰ and iron sulfide were the reduction entity of the surface-mediated reaction. The presence of tetrahydrofuran (THF) promoted the surface contact of BDE-209 with S-nZVI, thus accelerating the BDE-209 degradation process. Compared with nZVI, the iron sulfide coated on the Fe⁰ core surface could not only greatly reduce unnecessary electron loss via Fe⁰ corrosion with water, but also accelerate the transmission of electrons from Fe⁰ core to organic pollutants according to Fe(II) dissolution and solvent kinetic isotope effects investigations. These findings help to clarify the synergistic degradation mechanism between Fe⁰ core and iron sulfide shell layer.

Sulfidation mitigates the passivation of zero valent iron at alkaline pHs: Experimental evidences and mechanism

Groundwater pH is one of the most important geochemical parameters in controlling the interfacial reactions of zero-valent iron (ZVI) with water and contaminants. Ball milled, microscale ZVI (mZVI^bm) efficiently dechlorinated TCE at initial stage (<24 h) at pH 6-7 but got passivated at later stage due to pH rise caused by iron corrosion. At pH > 9, mZVI^bm almost completely lost its reactivity. In contrast, ball milled, sulfidated microscale ZVI (S-mZVI^bm) didn't experience any reactivity loss during the whole reaction stage across pH 6-10 and could efficiently dechlorinate TCE at pH 10 with a reaction rate of 0.03 h^-1. Increasing pH from 6 to 9 also enhanced electron utilization efficiency from 0.95% to 5.3%, and from 3.2% to 22%, for mZVI^bm and S-mZVI^bm, respectively. SEM images of the reacted particles showed that the corrosion product layer on S-mZVI^bm had a puffy/porous structure while that on mZVI^bm was dense, which may account for the mitigated passivation of S-mZVI^bm under alkaline pHs. Density functional theory calculations show that covered S atoms on the Fe(100) surface weaken the interactions of H₂O molecules with Fe surfaces, which renders the sulfidated Fe surface inefficient for H₂O dissociation and resistant to surface passivation. The observation from this study provides important implication that natural sulfidation of ZVI may largely contribute to the long-term (>10 years) efficiency of TCE decontamination by permeable reactive barriers with pore water pH above 9.

Removal of heavy metals by aged zero-valent iron from flue-gas-desulfurization brine under high salt and temperature conditions

To achieve zero liquid discharge, the flue-gas-desulfurization (FGD) wastewater at coal-fired power plants can be concentrated into brine through thermal evaporation to maximize water reuse; however, the hot brine generated requires further treatment prior to disposal. To address this need, this study investigates the performance of aged, micron-sized zero-valent iron (ZVI) for heavy metal removal in simulated and real FGD hot brines, which was scarcely studied previously. The effects of temperature, pH, total dissolved solids, ZVI dosage, major cations, nitrate and sulfate on the reactivity of ZVI in the brines were evaluated. Among many factors, higher temperature and Mg²⁺ exert the dominant influence. At 80 °C, almost 100% of arsenate (1 mg/L) and chromate (1 mg/L) can be removed in <5 min using 4.17 g/L of ZVI in simulated brines, while selenate (25 mg/L) and cadmium (5 mg/L) can be completely removed within 30 min. Mg²⁺ ions naturally present in FGD brines account for the depassivation of aged ZVI. X-ray diffraction results suggest that green rust is the reactive intermediate for selenate and cadmium removal. Overall, this study demonstrates that ZVI is an effective material for removing heavy metals in hot FGD brines generated through thermal evaporation at power plants.

Heavy metals immobilization capability of two iron-based nanoparticles (nZVI and Fe3O4): Soil and freshwater bioassays to assess ecotoxicological impact

The contamination by heavy metals constitutes an environmental problem of great importance in the last decades, and demands of society for clean environments are increasingly evident. To achieve this goal, several strategies have appeared for the in situ remediation of soil contamination caused by heavy metals. This study evaluated two types of iron-based nanoparticles, zero-valent iron nanoparticles (nZVI) and Fe₃O₄ nanoparticles, for the effective immobilization of Furthermore, we conducted a set of ecotoxicological bioassays: Microtox® Test, Caenorhabditis elegans Test, and Phytoplankton Toxicity Tests, on selected soil and aquatic test organisms to both, i) evaluate the potential ecotoxicological risks associated with nanoparticles treatment, and ii) to define sensitive organisms to be used as suitable bioindicators of heavy metals pollution. The application of 5% nZVI significantly reduced the amount of bioavailable heavy metals, which was effective from an ecotoxicity point of view as a reduction of the toxicity of was observed. Among the bioassays used, C. elegans seems the most effective reference organism in detecting changes in the toxicity of and therefore, C. elegans was found to be a sensitive heavy metals pollution bioindicator. When the Combination index (CI) was obtained to determine combined heavy metals interactions, the results indicated that toxicity would be higher than that expected for Pb, Cd and Zn individually considered, due to the proved antagonistic interactions of those toxicants. The obtained results suggested that nZVI nanoparticles are susceptible to be used as a soil remediation strategy for heavy metal pollution, although a short reactive lifespan must be considered, and therefore its effectiveness in long periods remains to be elucidated.

Advances in Surface Passivation of Nanoscale Zerovalent Iron: A Critical Review

Nanoscale zerovalent iron (NZVI) is one of the most extensively studied nanomaterials in the fields of wastewater treatment and remediation of soil and groundwater. However, rapid oxidative transformations of NZVI can result in reduced NZVI reactivity. Indeed, the surface passivation of NZVI is considered one of the most challenging aspects in successfully applying NZVI to contaminant degradation. The oxidation of NZVI can lead to the formation of Fe^II-bearing phases (e.g., Fe^IIO, Fe^II(OH)₂, Fe^IIFe^III₂O₄) on the NZVI surface or complete oxidation to ferric (oxyhydr)oxides (e.g., Fe^IIIOOH). This corrosion phenomenon is dependent upon various factors including the composition of NZVI itself, the type and concentration of aqueous species, reaction time and oxic/anoxic environments. As such, the coexistence of different Fe oxidation states on NZVI surfaces may also, in some instances, provide a unique reactive microenvironment to promote the adsorption of contaminants and their subsequent transformation via redox reactions. Thus, an understanding of passivation chemistry, and its related mechanisms, is essential not only for effective NZVI application but also for accurately assessing the positive and negative effects of NZVI surface passivation. The aim of this review is to discuss the nature of the passivation processes that occur and the passivation byproducts that form in various environments. In particular, the review presents: (i) the strengths and limitations of state-of-the-art techniques (e.g., electron microscopies and X-ray-based spectroscopies) to identify passivation byproducts; (ii) the passivation mechanisms proposed to occur in anoxic and oxic environments; and (iii) the effects arising from synthesis procedures and the presence of inorganics/organics on the nature of the passivation byproducts that form. In addition, several depassivation strategies that may assist in increasing and/or maintaining the reactivity of NZVI are considered, thereby enhancing the effectiveness of NZVI in contaminant degradation.

Calcium hydroxide coating on highly reactive nanoscale zero-valent iron for in situ remediation application

Nano scale zero-valent iron (nZVI), a promising engineering technology for in situ remediation, has been greatly limited by quick self-corrosion and low mobility in porous media. Highly reactive nZVI particles produced from the borohydride reduction method were enclosed in a releasable Ca(OH)₂ layer by the chemical deposition method. The amount of Ca(OH)₂ coated on nZVI surface were well controlled by the precursor dosage. At moderate Ca(OH)₂ dosage (R_Ca/TFe = 0.25) condition, the increment of Fe⁰ content for the obtained nZVI/Ca-0.25 sample was observed. The interfacial reactions between the iron oxide shell and the Ca(OH)₂ saturated environment were delicately elucidated by the X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) spectrum. And the coverage of Ca(OH)₂ shell on spherical nZVI surface was found more complete and uniform for the nZVI/Ca sample obtained from the moderate precursor dosage condition (R_Ca/TFe = 0.25). The Ca(OH)₂ shell before dissolution was demonstrated owning the anti-corrosion capability to slow down the oxidation of Fe⁰ core in air, during ethanol storage and in aqueous environment. The mechanism of anti-corrosion capability for nZVI/Ca-0.25 particle was interestingly found to be attributed to the Ca(OH)₂ shell isolation and also be potentially due to the iron oxide shell phase transformation mediated by the outer Ca(OH)₂ shell. An improved trichloroethylene reduction performance was observed for nZVI/Ca-0.25 than bare nZVI. The mobility of nZVI/Ca particles in water-saturated porous media was moderately improved before shell dissolution.

Effects of Good's Buffers and pH on the Structural Transformation of Zero Valent Iron and the Oxidative Degradation of Contaminants

The presence of Good's buffers caused rapid ZVI corrosion and a dramatic release of Fe(II) leading to the Fe(II)-catalyzed transformation of ferrihydrite to lepidocrocite and/or the direct formation of lepidocrocite from the oxidation of Fe(II) in the pH range 4.0-6.2. In comparison, in the absence of Good's buffers, elution of Fe(II) was insignificant with ferrihydrite being the only Fe(III) oxyhydroxide detected following the oxidative transformation of ZVI. The rapid ZVI corrosion in the presence of Good's buffer is possibly due to either (i) disruption of the Fe oxide surface layer as a result of attack by Good's buffers and/or (ii) interaction of Good's buffer with the outer Fe oxide surface and surface-associated Fe(II)/Fe(III) causing the Fe oxide surface layers to be more porous with both these processes facilitating continuous O₂ access to the Fe(0) core and allowing the diffusion of Fe atoms outward. Our results further show that the deprotonated forms of Good's buffers and the surface charge of the Fe oxides formed at the ZVI surface strongly affect the sorption of the target compound (i.e., formate) and hence the oxidation of these compounds via surface-associated Fe(II)-mediated heterogeneous Fenton processes.

Rapid decomposition of diclofenac in a magnetic field enhanced zero-valent iron/EDTA Fenton-like system

In this study, significant synergistic degradation of antibiotic diclofenac (DCF) was demonstrated in a novel magnetic field (MF) enhanced zero-valent iron (ZVI)/EDTA Fenton-like system. Five operational parameters, namely, initial ZVI loading, pH, EDTA dosage, DCF concentration and reaction temperature, were investigated for their effects on the DCF degradation. OH was identified as the predominant reactive oxygen species for DCF degradation in ZVI/EDTA systems whether in the presence or absence of MF. DCF molecule can be oxidized by OH, attacking via the hydroxylation and substituted dechlorination of the chlorinated aromatic ring, as well as by dehydration between the N atom and the acetoxyl. It could also be directly dechlorinated by ZVI reduction simultaneously. The reaction mechanism and promotional role of MF in the MF/ZVI/EDTA system were proposed. It is suggested that MF mainly alters the heterogeneous ZVI surface-bond reactions and accelerates the surface corrosion depending on the presence of pristine iron oxides layer, but MF does not change the homogeneous iron cycle and the Fenton-like reactions.

Environmental transformations and ecological effects of iron-based nanoparticles

The increasing application of iron-based nanoparticles (NPs), especially high concentrations of zero-valent iron nanoparticles (nZVI), has raised concerns regarding their environmental behavior and potential ecological effects. In the environment, iron-based NPs undergo physical, chemical, and/or biological transformations as influenced by environmental factors such as pH, ions, dissolved oxygen, natural organic matter (NOM), and biotas. This review presents recent research advances on environmental transformations of iron-based NPs, and articulates their relationships with the observed toxicities. The type and extent of physical, chemical, and biological transformations, including aggregation, oxidation, and bio-reduction, depend on the properties of NPs and the receiving environment. Toxicities of iron-based NPs to bacteria, algae, fish, and plants are increasingly observed, which are evaluated with a particular focus on the underlying mechanisms. The toxicity of iron-based NPs is a function of their properties, tolerance of test organisms, and environmental conditions. Oxidative stress induced by reactive oxygen species is considered as the primary toxic mechanism of iron-based NPs. Factors influencing the toxicity of iron-based NPs are addressed and environmental transformations play a significant role, for example, surface oxidation or coating by NOM generally lowers the toxicity of nZVI. Research gaps and future directions are suggested with an aim to boost concerted research efforts on environmental transformations and toxicity of iron-based NPs, e.g., toxicity studies of transformed NPs in field, expansion of toxicity endpoints, and roles of laden contaminants and surface coating. This review will enhance our understanding of potential risks of iron-based NPs and proper uses of environmentally benign NPs.

Sustained molecular oxygen activation by solid iron doped silicon carbide under microwave irradiation: Mechanism and application to norfloxacin degradation

Sustained molecular oxygen activation by iron doped silicon carbide (Fe/SiC) was investigated under microwave (MW) irradiation. The catalytic performance of Fe/SiC for norfloxacin (NOR) degradation was also studied. Rapid mineralization in neutral solution was observed with a pseudo-first-order rate constant of 0.2239 min^-1 under 540 W of MW irradiation for 20 min. Increasing Fe/SiC rod and MW power significantly enhanced the degradation and mineralization rate with higher yield of reactive oxygen species (ROS). Fe shell corrosion and subsequent Fe^0/II oxidation by molecular oxygen with MW activation was the key factor for NOR degradation through two-electron-transfer by Fe⁰ under acidic conditions and single-electron-transfer by Fe^II under neutral-alkaline solution. Removal rate of NOR was significantly affected by solution pH, showing higher degradation rates at both acidic and alkaline conditions. The highest removal efficiencies and rates at alkaline pH values were ascribed to the contribution of bound Fe^II species on the Fe shell surface due to the hydroxylation of Fe/SiC. ·OH was the main oxidizing specie for NOR degradation, confirmed by density functional theory (DFT) calculations and radical scavenger tests. DFT calculations were conducted on the reaction/activation energies of the transition/final states of NOR/degradation products, combined with intermediate identification with high performance liquid chromatography coupled with a triple-quadruple mass spectrometer (HPLC-MS/MS), the piperazinyl ring was the most reactive site for ·OH attack, followed by further ring-opening and stepwise oxidation. In this study, Fe/SiC were proved to be an excellent catalyst for the treatment of fluoroquinolone antibiotics with MW activation.

Heavy metals removal using hydrogel-supported nanosized hydrous ferric oxide: Synthesis, characterization, and mechanism

A new polymer-supported hybrid adsorbent (HFO-P(TAA/HEA)) for highly efficient removal of Pb²⁺, Cu²⁺, Cd²⁺ and Ni²⁺ from wastewater was developed by supporting hydrous ferric oxide (HFO) nanoparticles onto a porous poly(trans-Aconitic acid/2-hydroxyethyl acrylate) hydrogel (P(TAA/HEA)) with in situ precipitation method. Swelling kinetics, scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), Fourier transform infrared spectra (FTIR) and X-ray photoelectron spectroscopy (XPS) were used for characterization of the prepared HFO-P(TAA/HEA). The characterization data demonstrated that the hybrid hydrogel HFO-P(TAA/HEA) was successfully fabricated, and swelling ability as well as thermal stability was promoted after loading. The results of batch equilibrium experiments indicated that pH and temperature significantly influenced the adsorption process and adsorption of heavy metals was better fitted to Langmuir and pseudo-second-order models. Selectivity of HFO-P(TAA/HEA) towards heavy metals was greatly improved under the calcium ions competition at higher concentration compared to P(TAA/HEA). Competitive adsorption evidenced the priority order in multifold metal species system was Pb²⁺>Cu²⁺>Ni²⁺>Cd²⁺. What's more, FTIR and XPS analyses manifested that heavy metals might mainly be adsorbed via inner sphere complexation. These findings revealed that hydrogel HFO-P(TAA/HEA) is a potential adsorption material to remove the heavy metals from polluted water.

Environmental implications and applications of engineered nanoscale magnetite and its hybrid nanocomposites: A review of recent literature

This review focuses on environmental implications and applications of engineered magnetite (Fe3O4) nanoparticles (MNPs) as a single phase or a component of a hybrid nanocomposite that exhibits superparamagnetism and high surface area. MNPs are synthesized via co-precipitation, thermal decomposition and combustion, hydrothermal process, emulsion, microbial process, and green approaches. Aggregation/sedimentation and transport of MNPs depend on surface charge of MNPs and geochemical parameters such as pH, ionic strength, and organic matter. MNPs generally have low toxicity to humans and ecosystem. MNPs are used for constructing chemical/biosensors and for catalyzing a variety of chemical reactions. MNPs are used for air cleanup and carbon sequestration. MNP nanocomposites are designed as antimicrobial agents for water disinfection and flocculants for water treatment. Conjugated MNPs are widely used for adsorptive/separative removal of organics, dyes, oil, arsenic, phosphate, molybdate, fluoride, selenium, Cr(VI), heavy metal cations, radionuclides, and rare earth elements. MNPs can degrade organic/inorganic contaminants via chemical reduction or catalyze chemical oxidation in water, sediment, and soil. Future studies should further explore mechanisms of MNP interactions with other nanomaterials and contaminants, economic and green approaches of MNP synthesis, and field scale demonstration of MNP utilization.

The influences of iron characteristics, operating conditions and solution chemistry on contaminants removal by zero-valent iron: A review

For successful application of a zero-valent iron (ZVI) system, of particular interest is the performance of ZVI under various conditions. The current review comprehensively summarizes the potential effects of the major influencing factors, such as iron intrinsic characteristics (e.g., surface area, iron impurities and oxide films), operating conditions (e.g., pH, dissolved oxygen, iron dosage, iron pretreatment, mixing conditions and temperature) and solution chemistry (e.g., anions, cations and natural organic matter) on the performance of ZVI reported in literature. It was demonstrated that all of the factors could exert significant effects on the ZVI performance toward contaminants removal, negatively or positively. Depending on the removal mechanisms of the respective contaminants and other environmental conditions, an individual variable may exhibit different effects. On the other hand, many of these influences have not been well understood or cannot be individually isolated in experimental or natural systems. Thus, more research is required in order to elucidate the exact roles and mechanisms of each factor in affecting the performance of ZVI. Furthermore, based on these understandings, future research may attempt to establish some feasible strategies to minimize the deteriorating effects and utilize the positive effects so as to improve the performance of ZVI.

Aging of Zerovalent Iron in Synthetic Groundwater: X-ray Photoelectron Spectroscopy Depth Profiling Characterization and Depassivation with Uniform Magnetic Field

Scanning electron microscopy (SEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) depth profiling were employed to characterize the aged zerovalent iron (AZVI) samples incubated in synthetic groundwater. The AZVI samples prepared under different conditions exhibited the passive layers of different morphologies, amounts, and constituents. Owing to the accumulation of iron oxides on their surface, all the prepared AZVI samples were much less reactive than the pristine ZVI for Se(IV) removal. However, the reactivity of all AZVI samples toward Se(IV) sequestration could be significantly enhanced by applying a uniform magnetic field (UMF). Moreover, the flux intensity of UMF necessary to depassivate an AZVI sample was strongly dependent on the properties of its passive layer. The UMF of 1 mT was strong enough to restore the reactivity of the AZVI samples with Fe3O4 as the major constituent of the passive film or with a thin layer of α-Fe2O3 and γ-FeOOH in the external passive film. The flux intensity of UMF necessary to depassivate the AZVI samples would increase to 2 mT or even 5 mT if the AZVI samples were covered with passive films being thicker, denser, and contained more γ-FeOOH and α-Fe2O3. Furthermore, increasing the flux intensity of UMF facilitated the reduction of Se(IV) to Se(0) by AZVI samples.

An insight in magnetic field enhanced zero-valent iron/H2O2 Fenton-like systems: Critical role and evolution of the pristine iron oxides layer

This study demonstrated the synergistic degradation of 4-chlorophenol (4-CP) achieved in a magnetic field (MF) enhanced zero-valent iron (ZVI)/H2O2 Fenton-like (FL) system and revealed an interesting correlative dependence relationship between MF and the pristine iron oxides layer (FexOy) on ZVI particles. First, a comparative investigation between the FL and MF-FL systems was conducted under different experimental conditions. The MF-FL system could suppress the duration of initial lag degradation phase one order of magnitude in addition of the significant enhancement in overall 4-CP degradation. Monitoring of intermediates/products indicated that MF would just accelerate the Fenton reactions to produce hydroxyl radical more rapidly. Evolutions of simultaneously released dissolved iron species suggested that MF would not only improve mass-transfer of the initial heterogeneous reactions, but also modify the pristine ZVI surface. Characterizations of the specific prepared ZVI samples evidenced that MF would induce a special evolution mechanism of the ZVI particles surface depending on the existence of FexOy layer. It comprised of an initial rapid point dissolution of FexOy and a following pitting corrosion of the exposed Fe(0) reactive sites, finally leading to appearance of a particular rugged surface topography with numerous adjacent Fe(0) pits and FexOy tubercles.

Considerable Fenton and photo-Fenton reactivity of passivated zero-valent iron

Passivated zero-valent iron has no longer reductive reactivity, but it can still be used as an effective Fenton catalyst.