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

Weak magnetic field and coexisting ions accelerate phenol removal by ZVI/H2O2 system: Their efficiency and mechanism

Human activity and industrial production have led to phenol becoming a significant risk factor. The proper treatment of phenol in wastewater is essential. In this study, the utilization of weak magnetic field (WMF) and zero-valent iron (ZVI) was proposed to activate H2O2 to degrade phenol contaminant. The results show that the weak magnetic field has greatly enhanced the reaction rate of ZVI/H2O2 removal of phenol. The removal rates of phenol by ZVI/H2O2/WMF generally decreased with increasing initial pH and phenol concentrations, and firstly increase and then decrease with increasing Fe0 or H2O2 dosage. When the initial pH is 5.0, ZVI concentration of 0.2 g L-1, H2O2 concentration of 6 mM, and phenol concentration of 100 mg L-1 were used, complete removal of phenol can be achieved within 180 min at 25 °C. The degradation process was consistent with the pseudo-first-order kinetic model when the experimental data was fitted. The ZVI/H2O2/WMF process exhibited a 1.05-2.66-fold enhancement in the removal rate of phenol under various conditions, surpassing its counterpart lacking WMF. It was noticed that the presence of 1-5 mM of Ca2+, Mg2+, Cl-, SO42- ions can significantly enhance the kinetics of phenol removal by ZVI/H2O2 system with or without WMF to 2.22-10.40-fold, but NO3-, CO32-, PO43- inhibited the reaction significantly in the following order: PO43- > CO32- > NO3-. Moreover, pre-magnetization for 3 min could enhance the ZVI/H2O2 process which was valuable in treatment of real wastewater. The hydroxyl radical has been identified as the primary radical species responsible for phenol degradation. The presence of WMF accelerates the corrosion rate of ZVI, thereby promoting the release of Fe2+ ions, which in turn induces an increased production of hydroxyl radicals and facilitates phenol degradation. The compounds hydroquinone, benzoquinone, catechol, maleic acid, and CO2 were identified using GC-MS, and degradation pathways were proposed. Employing WMF in combination with various ions like Ca2+, Mg2+, Cl-, SO42- is a novel method, which can enhance oxidation capacity of ZVI/H2O2 and may lead to economic benefit.

Insights into the contrasting effects of sulfidation on dechlorination of chlorinated aliphatic hydrocarbons by zero-valent iron

Contrasting effects of sulfidation on contaminants reduction by zero-valent iron (ZVI) has been reported in literature but the underlying mechanisms remain unclear. Here, under well-controlled conditions, we compared the performance of ZVI and sulfidated ZVI (S-ZVI) toward a series of chlorinated compounds. Results revealed that, although S-ZVI was more reactive than ZVI toward hexachloroethane, pentachloroethane, tetrachloroethylene, and trichloroethene, sulfidation hindered the dechlorination of the other ten tested chlorinated aliphatics by a factor of 1.5-125. Moreover, S-ZVI may lead to an accumulation of toxic partially-dechlorinated products. Analogous to its effects on ZVI reactivity, sulfidation also exerted positive, negligible, or negative effects on the electron efficiency of ZVI. Solvent kinetic isotope effect analysis suggested that direct electron transfer rather than reaction with atomic hydrogen was the dominant reduction mechanism in S-ZVI system. Hence, the sulfidation enhancing effects could be expected only when direct electron transfer is the preferred reduction route for target contaminants. Furthermore, linear free energy relationships analysis indicated one-electron reduction potential could be used to predict the transformation of chlorinated ethanes by S-ZVI, whereas for chlorinated ethenes, their adsorption properties on S-ZVI determined the dechlorination process. All these findings may offer guidance for the decision-making regarding the application of S-ZVI.

Hydrogen Nanobubbles Generated In Situ from Nanoscale Zerovalent Iron with Water to Further Enhance Selenite Sequestration

Gas nanobubbles used for water treatment and recovery give rise to great concern for their unique advantages of less byproducts, higher efficiency, and environmental friendliness. Nanoscale zerovalent iron (nZVI), which has also been widely explored in the field of environmental remediation, can generate gas hydrogen by direct reaction with water. Whether nanoscale hydrogen bubbles can be produced to enhance the pollution removal of the nZVI system is one significant concern involved. Herein, we report direct observations of in situ generation of hydrogen nanobubbles (HNBs) from nZVI in water. More importantly, the formed HNBs can enhance indeed the reduction of Se(IV) beyond the chemical reduction ascribed to Fe(0), especially in the anaerobic environment. The possible mechanism is that HNBs enhance the reducibility of the system and promote electron transport in the solution. This study demonstrates a unique function of HNBs combined with nZVI for the pollutant removal and a new approach for in situ HNB generation for potential applications in the fields of in situ remediation agriculture, biotechnology, medical treatment, health, etc.

Antimony(V) behavior during the Fe(II)-induced transformation of Sb(V)-bearing natural multicomponent secondary iron mineral under acidic conditions

Fe(II)-induced phase transformations of secondary iron minerals have attracted considerable attention due to their influence on antimony (Sb) mobility. However, Fe(II)-induced natural multicomponent secondary iron mineral (nmSIM) transformations and the corresponding repartitioning of Sb on nmSIM under acidic conditions upon Fe(II) exposure have not been systematically examined. Herein, we investigated the effect of Fe(II) on nmSIM mineralogy and Sb mobility in Sb(V)-bearing nmSIM at pH 3.8 and 5.6 at various Fe(II) concentrations over 15 d. The Sb(V)-bearing nmSIM phase transformation occurred under both strongly and weakly acidic conditions without Fe(II) exposure, while the presence of Fe(II) significantly intensified the transformation, and substantial amounts of intermediary minerals, including jarosite, ferrihydrite, lepidocrocite and fougerite, formed during the initial reaction stage, especially at pH 5.6. X-ray diffraction (XRD) analyses confirmed that goethite and hematite were the primary final-stage transformation products of Sb(V)-bearing nmSIM, regardless of Fe(II) exposure. Throughout the Sb(V)-bearing nmSIM transformation at pH 3.8, Sb release was inversely related to the Fe(II) concentration in the initial stage, and after maximum release was achieved, Sb was gradually repartitioned onto the nmSIM. No Sb repartitioning occurred in the absence of Fe(II) at pH 5.6, but the introduction of Fe(II) suppressed Sb release and improved Sb repartitioning on nmSIM. This transformation was conducive to Sb reimmobilization on Sb(V)-bearing nmSIM due to the structural incorporation of Sb into the highly crystalline goethite and hematite generated by the Sb(V)-bearing nmSIM transformation, and no reduction of Sb(V) occurred. These results imply that Fe(II) can trigger mineralogical changes in Sb(V)-bearing nmSIM and have important impacts on Sb partitioning under acidic conditions. These new insights are essential for assessing the mobility and availability of Sb in acid mine drainage areas.

Reductive sequestration of Cr (VI) by phosphorylated nanoscale zerovalent iron

The cracked phosphorylated nanoscale zerovalent iron (p-nZVI) has a strong electron selectivity towards the reductive removal of many heavy metal ions in waters. However, the unintended environmental risk after interactions with impurities or wastewater are not involved. Therefore, in this study, the phosphate group was successfully adsorbed into p-nZVI, and the cracked p-nZVI was successfully prepared with an optimal P/Fe ratio of 0.5%. The dosages of p-nZVI and temperatures were positively correlated with the removal rates. The removal process of Cr(VI) was more suitable by the Langmuir isothermal model（R2 > 0.99). The process of Cr (VI) (10, 20 and 40 mg/L) removal more fitted the pseudo first-order reaction model, while the process of Cr (VI) (60, 80 mg/L) removal more fitted the pseudo second-order reaction model. The Cr (VI) removal rates gradually decreased when the pH was increased. Dissolved oxygen slowed nanoiron reaction rates. The order of inhibition on the reactivity towards Cr(VI) was SiO32- > SO42- > PO43- > NO3- > HCO3-.The facilitation followed the order of Cd2+>Cu2+>Mg2+>Mn2+>Ca2+. Ca2+ showed an inhibitory effect, but all other cations showed different degrees of facilitation. The promotion effect is relatively similar in presence of Mn2+ or Mg2+. HA had a significant inhibitory effect. Environmental friendly p-nZVI had a good effect in simulated groundwater, seawater, river water and secondary effluent of the urban sewage treatment plant. The main pathway to remove Cr (VI) was in situ reduction by p-nZVI. The improved adsorption and reduction effect of p-nZVI on heavy metal ions in water was due to the structural change and the phosphate group.

Minor chromium passivation of S-ZVI enhanced the long-term dechlorination performance of trichlorethylene: Effects of corrosion and passivation on the reactivity and selectivity

The corrosion and surface passivation of sulfidized zero-valent iron (S-ZVI) by common groundwater ions and contaminants are considered to be the most challenging aspects in the application of S-ZVI for remediation of chlorinated contaminants. This study investigated the impacts of corrosive chloride (Cl-) and passivation of hexavalent chromium (Cr(VI)) on the long-term reactivity, selectivity, corrosion behavior, and physicochemical properties during the 60-day aging process of S-ZVI. Although the co-existing of Cl- promoted the initial reactivity of S-ZVI, the rapid consumption of Fe° content shortened the reactive lifetime owing to the insufficient electron capacity. Severe passivation by Cr(VI) (30 mg L-1) preserved the Fe° content but significantly interfered with the reductive sulfur species, resulting in an increase in electron transfer resistance. In comparison, minor passivated S-ZVI (5.0 mg L-1 Cr(VI)) inhibited the hydrogen evolution while concurrently mitigating the further oxidation of the reductive iron and sulfur species, which significantly enhanced the long-term reactivity and selectivity of S-ZVI. Furthermore, the enhancement effect of minor passivation could be detected in the aging processes of one-step, two-step, and mechanochemically synthesized S-ZVI particles with different S/Fe ratios and precursors, which further verified the advantages of minor passivation. This observation is inspirable for the development of innovative strategies for environmental remediation by S-ZVI-based materials.

A critical analysis of sources, pollution, and remediation of selenium, an emerging contaminant

Selenium (Se) is an essential metalloid and is categorized as emerging anthropogenic contaminant released to the environment. The rise of Se release into the environment has raised concern about its bioaccumulation, toxicity, and potential to cause serious damages to aquatic and terrestrial ecosystem. Therefore, it is extremely important to monitor Se level in environment on a regular basis. Understanding Se release, anthropogenic sources, and environmental behavior is critical for developing an effective Se containment strategy. The ongoing efforts of Se remediation have mostly emphasized monitoring and remediation as an independent topics of research. However, our paper has integrated both by explaining the attributes of monitoring on effective scale followed by a candid review of widespread technological options available with specific focus on Se removal from environmental media. Another novel approach demonstrated in the article is the presentation of an overwhelming evidence of limitations that various researchers are confronted with to overcome achieving effective remediation. Furthermore, we followed a holistic approach to discuss ways to remediate Se for cleaner environment especially related to introducing weak magnetic field for ZVI reactivity enhancement. We linked this phenomenal process to electrokinetics and presented convincing facts in support of Se remediation, which has led to emerge 'membrane technology', as another viable option for remediation. Hence, an interesting, innovative and future oriented review is presented, which will undoubtedly seek attention from global researchers.

Enhancement of iron-based nitrogen removal with an electric-magnetic field in an upflow microaerobic sludge reactor (UMSR)

Traditional denitrification often produces high operating costs and excessive sludge disposal expenses due to conventional carbon sources. A novel electric-magnetic field (MF) 48 mT with Fe⁰ and C-Fe⁰ powder in an upflow microaerobic sludge reactor (UMSR) improved nitrogen removal from wastewater without organic carbon resources and gave richness to the heterotrophic bacterial community. In the current study, the reactor was operated for 78 ± 2 days, divided into five stages (without Fe⁰, with Fe⁰, coupling with MF, without coupling with MF, and coupling with MF again), at a hydraulic retention time (HRT) of 2.5 h, with an influent loading of ammonium (NH₄⁺-N) 50 ± 2 mg/L, at 25-27 °C, and less than 1.0 mg/L dissolved oxygen (DO). The results demonstrated nitrogen removal efficiency enhanced after coupling with MF on the levels of NO₃^--N by 76% with an effluent concentration of 8.7 mg/L, NH₄⁺-N by 72% with an effluent concentration of 13.6 mg/L, and total nitrogen removal (TN) by 76%, respectively. After coupling the MF with the reactor, the microbial community data analysis showed the dominant abundance of ammonia-oxidizing bacteria, heterotrophic nitrifying bacteria, and denitrifying bacteria on the level of Anaerolineaceae_uncultured 2%, which is capable of denitrification that uses Fe²⁺ as an electron source, Gemmatimonadaceae_uncultured 4%, Hydrogenophaga 4% which is capable of catalyzing hydrogenotrophic denitrification and correlating to nitrate removal, denitrification and desulfurization bacteria SBR1031_norank 18%, anammox-bacteria Saccharimonadales_norank 2%, and (AOM) Limnobacter 3% in the sludge.

Insight into catalytic activation of bisulfite for lomefloxacin degradation by simple composite of calcinated red mud

Antibiotic was detected in many environments, and it had posed a serious threat to human health. The advanced oxidation process has been considered an effective way to treat antibiotics. In this work, using industrial waste red mud (RM) as raw material, a series of modified RM (MRM-T; T donates the calcination temperature) was obtained via a facile calcination method and applied to activate sodium bisulfite (NaHSO₃) for the lomefloxacin (LOM) degradation. Among all MRM-T, MRM-700 exhibited superior catalytic activity, and approximately 89% of LOM (10 mg/L) was degraded at 30 min through the activation of NaHSO₃ ([NaHSO₃] = 0.5 g/L) by MRM-700 ([MRM-700] = 0.9 g/L). Moreover, the kinetic constant of LOM removal in the MRM-700/NaHSO₃ system (0.082 min^-1) was 16.4 times higher than that of the RM-raw/NaHSO₃ system (0.005 min^-1). The as-synthesized product of MRM-700 was characterized by N₂ adsorption-desorption isotherms, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectra. The result indicated that the catalyst possessed excellent pore structure, high specific area, and abundant Fe³⁺ sites, and the lattice of Fe₂O₃ was doped after calcination, both of which were favorable for the activation of NaHSO₃. The quenching experiment proved that •SO₄^- and •OH^- active species were produced in MRM-700/NaHSO₃ system, and •SO₄^- played a dominant role in LOM removal. In addition, the potential LOM degradation pathway was analyzed via UPLC-MS technology and density functional theory (DFT) calculation, and the toxicity of the treated LOM solution was tested by the culture of mung bean sprouts. This study not only provided a feasible strategy for the valuable use of RM to activate NaHSO₃ but also offered a cost-effective catalyst for the efficient removal of pollutants in wastewater.

Heterogeneous Fenton oxidation of 2,4-dichlorophenol catalyzed by PEGylated nanoscale zero-valent iron supported by biochar

The excessive use of herbicides and fungicides containing 2,4-dichlorophenol (2,4-DCP) has led to serious environmental water pollution; 2,4-DCP is chemically stable and difficult to be degraded effectively by biological and physical methods. And the degradation of 2,4-DCP using advanced oxidation techniques has been a hot topic. Biochar, polyethylene glycol, ferrous sulfate, and sodium borohydride were used to synthesize the heterogeneous catalyst PEGylated nanoscale zero-valent iron supported by biochar (PEG-nZVI@BC). The catalyst was characterized using scanning electron microscope (SEM) and other means to determine its physicochemical properties. Catalytic performance and mechanism of this catalyst with hydrogen peroxide for the oxidation of 2,4-DCP were investigated. The results showed that PEG-nZVI@BC had good dispersibility, stability, and inoxidizability; the degradation efficiency of 50 mg/L 2,4-DCP by PEG-nZVI@BC/H₂O₂ system 92.94%, 1.68 times higher than that of nZVI/H₂O₂ system; there are both free radical and non-free radical pathways in PEG-nZVI@BC/H₂O₂ system; the degradation process of 2,4-DCP includes hydroxylation, dechlorination, and ring-opening. Overall, PEG-nZVI@BC is a promising heterogeneous catalyst for the degradation of 2,4-DCP.

Iron boride boosted Fenton oxidation: Boron species induced sustainable FeIII/FeII redox couple

The regeneration of Fe(II) is the rate-limiting step in the Fenton/Fenton-like chain reactions that seriously hinder their scientific progress towards practical application. In this study, we proposed iron boride (FeB) for the first time as a new material to sustainably decompose H₂O₂ to generate hydroxyl radicals, which can non-selectively degrade a wide array of refractory organic pollutants. Fe(II) can be steadily released by the stepwise oxidation of FeB to stimulate Fenton reaction, meanwhile, B-B bonds as electron donors on the surface of FeB effectively promote the regeneration of Fe(II) from Fe(III) species and significantly accelerate the production of hydroxyl radicals. The low generation of toxic by-products and the high utilization rate of iron species validly avoid the secondary organic/metal pollution in the FeB/H₂O₂ system. Therefore, FeB mediated Fenton oxidation provides a novel strategy to realize a green and long-lasting environmental remediation.

Effect of soil pH on thermally enhanced desorption of m-xylene by zero-valent iron particles under an electromagnetic field

This study, for the first time, evaluates a novel method for the desorption of contaminants from soil that uses the heat generated by zero-valent iron (ZVI) under low-frequency electromagnetic fields (EMF), and elucidates the specific effects of soil pH upon the process. It was found that the temperature of soil mixed with ZVI could reach up to ∼60 °C within 20 min under the applied EMF, and after 60 min the residual fraction of m-xylene in soil decreased by 86.4% compared to no-ZVI soil. The most efficient desorption of m-xylene occurred at a soil of pH 5. Desorption was related to the net heating capacity of the ZVI particles, which was defined by pH-dependent formation of surface corrosion products. The preservation of metal iron and formation of Fe(II) species was favored for heat generation. Soil pH also affected m-xylene retention and the local thermal conduction from ZVI to m-xylene by regulating the surface properties of fulvic acid and ZVI. This study provides valuable information regarding the impact of pH on the thermal desorption of soil contaminants by ZVI coupled with EMF and illustrates the potential of the method in the remediation of contaminated sites.

Assessment of sulfidated nanoscale zerovalent iron for in-situ remediation of cadmium-contaminated acidic groundwater at a zinc smelter

Unprecedented high concentrations of heavy metals have been detected in the groundwater at a zinc smelter in Seokpo, South Korea. The outflow of the contaminated groundwater into the nearby Nakdong River must be prevented by some means such as permeable reactive barrier (PRB). As a reactive material for injection-type PRB, we have tested sulfidated nanoscale zerovalent iron (S-nZVI) to assess its efficacy in remediating the groundwater from the smelter. The S-nZVI efficiently removed Zn, Ni, and Al in the groundwater, and neutralized the groundwater to pH > 6. Sulfidation of nZVI greatly increased the removal of Cd (99.8%) compared to that by nZVI (7.2%). MINEQL+ modeling and particle characterization were performed to elucidate the forms of heavy metals in the solution and on the surface of S-nZVI. Raman and XPS results suggested that FeS on the surface of S-nZVI reacted with Cd(II) and Zn(II), forming more-stable CdS and ZnS. Sequential application of NaHCO₃ after S-nZVI treatment in a column setup was suited for the removal of remaining Zn and Fe as well as the reduction of microbial toxicity. This study guides to use of S-nZVI for in-situ remediation of cadmium-contaminated groundwater with other coexisting heavy metals from a zinc smelter.

Investigate the multipath erasure of nitrobenzene over nanoscale zero-valent-iron/N-doped biochar hybrid with extraordinary reduction performance

In this study, the facile carbothermal reduction method was enforced using urea as dopant to modify the structure and chemical composition of nanoscale zero-valent-iron/biochar hybrid thereby boosting its reduction performance. Through fine-tuning the N-doped amount, the optimal nZVI/N-doped BC was obtained, which exhibited more active sites (nZVI, persistent free radicals (PFRs), pyrrolic-N) and superior electrochemical conductivity. With these blessings, the electrons originating from galvanic cell reaction could zip along the highway within the hybrid. Taking nitrobenzene (NB) as the target pollutant, the quantitative analysis revealed that the NB reduction and adsorption removal efficiency were dramatically improved by 2.42 and 2.78 times, respectively. What's more, combining the in-situ experimental detection and theoretical calculations, unexpected NB reductive multipath with respect to PFRs and pyrrolic-N accelerating the Fe³⁺/Fe²⁺ cycle within the nZVI/N-doped BC system was decoded. The enhancement of Fe³⁺/Fe²⁺ cycle improved the electron utilization efficiency and maintained the reduction reactivity of the hybrid. This work raised awareness of the mechanisms regarding the reduction performance of nZVI/N-doped BC elevated by N-doped and the pollutant reductive pathway within the system, uncovered the dusty roles of PFRs and N-species during the reduction process.

Valorization of ball-milled waste red mud into heterogeneous catalyst as effective peroxymonosulfate activator for tetracycline hydrochloride degradation

Red mud (RM), a kind of iron-rich industrial waste produced in the alumina production process, can be utilized as a potential iron-based material for the removal of refractory organic pollutants from wastewater in advanced oxidation processes (AOPs). In this work, high-iron RM (rich in iron) was activated in a ball mill and applied as an effective activator of peroxymonosulfate (PMS) for tetracycline hydrochloride (TC-HCl) degradation. Compared with that of unmilled RM (69.7%), the TC-HCl decomposition ratios of ball-milled RM (BM-RM) (72.2%-92.0%) were all improved in the presence of PMS. Systematic characterization suggested that ball milling could optimize the physicochemical properties of RM, such as increased surface area, increased oxygen vacancies, enhanced electrical conductivity, and increased exposure of Fe(II) sites, all of which could effectively improve RM for PMS activation to degrade TC-HCl. The quenching experiments and electron paramagnetic resonance technique revealed that ¹O₂ and SO₄·^- contributed dominantly to the TC-HCl degradation. Ultra performance liquid chromatography mass spectrometry analysis combined with density functional theory calculation revealed that the degradation pathways of TC-HCl were driven by hydroxylation, N-demethylation and dehydration in BM-RM/PMS system. Based on quantitative structure-activity relationship prediction using the Toxicity Estimation Software Tool software, the toxicity of almost all intermediates was significantly reduced. An obvious inhibition effect on TC-HCl was occurred in the presence of Cl^-, whereas the presences of NO₃^- and SO₄^2- had little effect. However, HCO₃^- improved TC-HCl removal efficiency. BM-RM had a wide working pH range (pH = 3-11) and showed good stability and reusability in use. Overall, this work not only offers a simple and promising approach to improve the catalytic activity of RM, but also opens new insights into the ball-milled RM as an effective PMS activator for wastewater treatment.

Fast photo-Fenton-like oxidation in bismuth catalysis: A novel Fe(III) self-doped sodium bismuthate nanosheet

Sodium bismuthate dihydrate (NaBiO₃.2 H₂O, NBH) nanosheets were successfully prepared in this study using the persulfate oil bath oxidation method. Benefited from the unique layered structure of NBH, the Fe(III) as a variable valence metal ion was explored for enhancing NBH activation of peroxymonosulfate (PMS) to degrade levofloxacin (LVF) in the visible-light catalytic system. Based on results of the reactive oxygen species (ROS) quenching experiments and electron paramagnetic resonance (EPR) analysis, singlet oxygen (¹O₂) and superoxide radical (O₂^·-) were identified as the main ROS. The morphology, chemical structure, and optical properties of NBH were analyzed using various characterization methods. It was confirmed that Fe(III) embedded in the NBH via the ion exchange with Na, resulting in lattice oxygen vacancies on the surface of the NBH, after the formation of oxygen defect sites, reacts with PMS in the solution to produce active oxygen species with oxidizing efficiency. This study expands the technological application of NBH in the catalytic oxidation of variable valence metals, which are essential for the removal of fluoroquinolone antibiotics.

New insights into iron/nickel-carbon ternary micro-electrolysis toward 4-nitrochlorobenzene removal: Enhancing reduction and unveiling removal mechanisms

The ternary micro-electrolysis material iron/nickel-carbon (Fe/Ni-AC) with enhanced reducibility was constructed by introducing the trace transition metal Ni based on the iron/carbon (Fe/AC) system and used for the removal of 4-nitrochlorobenzene (4-NCB) in solution. The composition and structures of the Fe/Ni-AC were analyzed by various characterizations to estimate its feasibility as reductants for pollutants. The removal efficiency of 4-NCB by Fe/Ni-AC was considerably greater than that of Fe/AC and iron/nickel (Fe/Ni) binary systems. This was mainly due to the enhanced reducibility of 4-NCB by the synergism between anode and double-cathode in the ternary micro-electrolysis system (MES). In the Fe/Ni-AC ternary MES, zero-iron (Fe⁰) served as anode involved in the formation of galvanic couples with activated carbon (AC) and zero-nickel (Ni⁰), respectively, where AC and Ni⁰ functioned as double-cathode, thereby promoting the electron transfer and the corrosion of Fe⁰. The cathodic and catalytic effects of Ni⁰ that existed simultaneously could not only facilitate the corrosion of Fe⁰ but also catalyze H₂ to form active hydrogen (H*), which was responsible for 4-NCB transformation. Besides, AC acted as a supporter which could offer the reaction interface for in-situ reduction, and at the same time provide interconnection space for electrons and H₂ to transfer from Fe⁰ to the surface of Ni⁰. The results suggest that a double-cathode of Ni⁰ and AC could drive much more electrons, Fe²⁺ and H*, thus serving as effective reductants for 4-NCB reduction.

Effect of Spatial Distribution of nZVI on the Corrosion of nZVI Composites and Its Subsequent Cr(VI) Removal from Water

There have been many studies on contaminant removal by fresh and aged nanoscale zero-valent iron (nZVI), but the effect of spatial distribution of nZVI on the corrosion behavior of the composite materials and its subsequent Cr(VI) removal remains unclear. In this study, four types of D201-nZVI composites with different nZVI distributions (named D1, D2, D3, and D4) were fabricated and pre-corroded in varying coexisting solutions. Their effectiveness in the removal of Cr(VI) were systematically investigated. The results showed acidic or alkaline conditions, and all coexisting ions studied except for H₂PO₄⁻ and SiO₃²⁻ enhanced the corrosion of nZVI. Additionally, the Cr(VI) removal efficiency was observed to decrease with increasing nZVI distribution uniformity. The corrosion products derived from nZVI, including magnetite, hematite, lepidocrcite, and goethite, were identified by XRD. The XPS results suggested that the Cr(VI) and Cr(III) species coexisted and the Cr(III) species gradually increased on the surface of the pre-corroded D201-nZVI with increasing iron distribution uniformity, proving Cr(VI) removal via a comprehensive process including adsorption/coprecipitation and reduction. The results will help to guide the selection for nZVI nanocomposites aged under different conditions for environmental decontamination.

Agar-stabilized sulfidated microscale zero-valent iron: Its stability and performance in chromate reduction

Sulfidated microscale zero-valent iron (SmZVI) attracts much attention recently in remediation of contaminated groundwater, but whether polymer coating on SmZVI would impact on its reactivity and capacity is yet to be understood. In this work, SmZVI was prepared by milling mZVI with elemental sulfur, and its stability in agar solution was evaluated. The impact of polymer coating on SmZVI grains' capacity and reactivity for chromate reduction was then examined. Experimental results indicated that SmZVI having the best overall performance was attained by grinding mZVI with elemental sulfur at 0.05 S/Fe molar ratio for 10 h. SmZVI's stability can be substantially improved if dispersed in 2.0 g/L agar solution. Existence of agar films on the SmZVI grain (A-SmZVI) lowered the material's capacity for chromate reduction by 56%, and the associated reaction kinetics by 70.4%, as estimated by pseudo first-order reaction model using the early-stage experimental data. Analysis of XPS spectra of A-SmZVI post reaction with chromate indicated that multiple reductive species including Fe⁰, Fe(II), FeS, and S(-II) may have jointly participated in the redox reaction taking place on the A-SmZVI-water interface. Fitting of XPS data supported that S(-II) was oxidized to SO₄^2-, S₂O₃^2-, and S⁰, in order of decreasing surface concentration.

Magnetically attracted iron scrap anode based electrocoagulation for phosphate removal

This study shows the effectiveness of a novel electrocoagulation process using magnetically attracted iron scrap anodes for phosphate removal from aqueous solution. The effect of contact time, reaction temperature, dose of iron scrap, initial phosphate concentration, applied voltage, pH, magnetic force, and the species of competing anions on the efficiency of phosphate removal and the reaction products has been investigated. The techniques of XRD, XPS, and VSM were used to characterize the elemental composition and the types of the reaction products in order to clarify the interaction between novel anode and phosphate ions. The removal of phosphate was fitted by a pseudo first-order reaction kinetic model. The results showed that magnetically attracted iron scrap anodes were electrodissoluted under an applied potential and reacted with phosphate into Fe-hydroxo-phosphate complexes. The work suggested that electrocoagulation using magnetically attracted iron scrap anodes had the potential to become a promising technique for phosphate precipitation.

Hollow-structured amorphous prussian blue decorated on graphitic carbon nitride for photo-assisted activation of peroxymonosulfate

Heterogeneous activation of peroxymonosulfate (PMS) is one of the most promising techniques for wastewater treatment. Herein, an ingenious system by coupling of photocatalysis and PMS activation was developed, using hollow-structured amorphous prussian blue (A-PB) decorated on graphitic carbon nitride (g-C₃N₄) as the catalyst. Degradation of bisphenol A (BPA) via the A-PB-g-C₃N₄ mediated PMS activation under visible light (Vis) was systematically investigated. Astonishingly, it was found that ~ 82.0%, 92.6%, 98.2% and 99.3% of BPA (40 mg/L) were removed within 2, 4, 6 and 7 min, respectively, suggesting the extremely strong oxidizing capacity of the A-PB-g-C₃N₄/PMS/Vis system. Synergistic effect between the decorated A-PB and the g-C₃N₄ substrate promoted the Fe(III)/Fe(II) redox cycling and facilitated the charge transfer at the A-PB/g-C₃N₄ heterojunction interface. As a result, both photocatalysis and heterogeneous activation of PMS were boosted in the A-PB-g-C₃N₄/PMS/Vis system, leading to the production of large amount of reactive oxygen species (ROS). The various ROS (SO₄^•-, HO•, •O₂^- and ¹O₂) was responsible for the ultrafast degradation of BPA. Moreover, the A-PB-g-C₃N₄ catalyst also exhibited outstanding reusability and stability, retaining 98.9% of the removal percentage for BPA after five consecutive reaction cycles. This study suggests that the A-PB-g-C₃N₄ can be an all-rounder to bridge photocatalysis and PMS activation, and shed a new light on the application of multiple ROS for the ultrafast elimination of micropollutants from wastewater.

Remediation of trichloroethylene by microscale zero-valent iron aged under various groundwater conditions: Removal mechanism and physicochemical transformation

Microscale zero-valent iron (mZVI) has been widely used for the in-situ groundwater remediation of various pollutants. However, the aging behavior of injected mZVI particles limits the widespread application in groundwater remediation projects. To assess the long-term reactivity of mZVI particles, the mechanism of trichloroethylene (TCE) degradation by various aged mZVI particles (A-mZVI) was determined by quantitatively evaluating the contributions of chemical reduction and adsorption. Further, this study investigated the physicochemical transformation of mZVI particles aged under various hydraulic conditions (static and dynamic), redox conditions (anoxic and aerobic) and aging durations (152 d and 455 d). The results show that the removal of TCE by different A-mZVI particles increased the sorption capacity in the initial period (0-6 h). However, in the long term, a significant inhibition of TCE removal was observed because of the decreased TCE reduction capacity caused by the hindrance of electron transfer, which was generated by corrosion precipitates. Furthermore, the characterization results demonstrated that despite the significant differences in the apparent morphology of the A-mZVI particles in various groundwater conditions, the final crystal corrosion products were mainly Fe₃O₄. Thus, the aging and inactivation of mZVI particles on TCE removal were promoted under the aerobic conditions. In addition, the structure of mZVI particles collapsed from the micro- to nanoscale under anaerobic dynamic over 455 d. No substantial impact on the final TCE removal was observed for the A-mZVI particles prepared under various hydraulic conditions and aging times. These findings provide insights regarding the impact mechanisms of corrosion precipitates on the removal of target contaminant and provide implications for long-term mZVI application under various target aquifer conditions.

FeNX(C)-Coated Microscale Zero-Valent Iron for Fast and Stable Trichloroethylene Dechlorination in both Acidic and Basic pH Conditions

FeN_X in Fe single-atom catalysts can be the active site for adsorption and activation of reactants. In addition, FeN_X species have been shown to facilitate electron transfer between Fe and the carbon supports used in newly developed metal-air batteries. We hypothesized that the combination of FeN_X species with granular zero-valent iron (ZVI) might result in catalyzed reductive decontamination of groundwater contaminants such as trichloroethylene (TCE). Here, such materials synthesized by ball milling microscale ZVI with melamine and the resulting N species were mainly in the form of pyridinic, pyrrolic, and graphitic N. This new material (abbreviated as N-C-mZVI^bm) dechlorinated TCE at higher rates than bare mZVI^bm (about 3.5-fold) due to facilitated electron transfer through (or around) the surface layer of iron oxides by the newly formed Fe-N_X(C). N-C-mZVI^bm gave higher k_TCE (0.4-1.14 day^-1) than mZVI^bm (0-0.4 day^-1) over a wide range of pH values (4-11). Unlike most ZVI systems, k_TCE for N-C-mZVI^bm increased with increasing pH values. This is because the oxide layer that passivates Fe⁰ at a high pH is disrupted by Fe-N_X(C) formed on N-C-mZVI^bm, thereby allowing TCE dechlorination and HER under basic conditions. Serial respike experiments gave no evidence of decreased performance of N-C-mZVI^bm, showing that the advantages of this material might remain under field applications.

Simultaneously enhanced degradation of N, N-dimethylacetamide and reduced formation of iron sludge by an efficient electrolysis catalyzed ozone process in the presence of dissolved silicate

The generation of sludge is the main issue in iron-based electrochemical techniques. Interestingly, in this study, the effluent was totally limpid and iron sludge did not generate when dissolved silicate (Na₂SiO₃) was used as the electrolyte in an electrolysis catalyzed ozone (ECO-Na₂SiO₃) system. More importantly, the pseudo-first-order rate constants (0.112 min^-1) for DMAC degradation in ECO-Na₂SiO₃ process was much higher than those of ECO systems using other electrolytes. An inhibition film formed on the iron electrode surface was identified to inhibit excess corrosion of iron electrodes and efficiently catalyze decomposition of ozone simultaneously. It was confirmed that hydroxyl radical (^•OH) played a dominant role for the degradation of DMAC, and O₂^•- and H₂O₂ were also contained in ECO-Na₂SiO₃ system. The contributions of contained oxidative reactions in ECO-Na₂SiO₃ system were quantitatively evaluated. Finally, the degradation pathway of DMAC was proposed. This work provides an effective way for protecting electrode from corrosion in electrochemical process.

Unveiling the Role of Sulfur in Rapid Defluorination of Florfenicol by Sulfidized Nanoscale Zero-Valent Iron in Water under Ambient Conditions

Groundwater contamination by halogenated organic compounds, especially fluorinated ones, threatens freshwater sources globally. Sulfidized nanoscale zero-valent iron (SNZVI), which is demonstrably effective for dechlorination of groundwater contaminants, has not been well explored for defluorination. Here, we show that SNZVI nanoparticles synthesized via a modified post-sulfidation method provide rapid dechlorination (∼1100 μmol m^-2 day^-1) and relatively fast defluorination (∼6 μmol m^-2 day^-1) of a halogenated emerging contaminant (florfenicol) under ambient conditions, the fastest rates that have ever been reported for Fe⁰-based technologies. Batch reactivity experiments, material characterizations, and theoretical calculations indicate that coating S onto the metallic Fe surface provides a highly chemically reactive surface and changes the primary dechlorination pathway from atomic H for nanoscale zero-valent iron (NZVI) to electron transfer for SNZVI. S and Fe sites are responsible for the direct electron transfer and atomic H-mediated reaction, respectively, and β-elimination is the primary defluorination pathway. Notably, the Cl atoms in florfenicol make the surface more chemically reactive for defluorination, either by increasing florfenicol adsorption or by electronic effects. The defluorination rate by SNZVI is ∼132-222 times higher with chlorine attached compared to the absence of chlorine in the molecule. These mechanistic insights could lead to new SNZVI materials for in situ groundwater remediation of fluorinated contaminants.

Countereffect of glutathione on divalent mercury removal by nanoscale zero-valent iron in the presence of natural organic matter

Although there have been multiple studies on the effects of natural organic matter (NOM) on zero-valent iron (ZVI) removal of several regulated heavy metal ions from contaminated water, the role of NOM on Hg(II) removal by nanoscale ZVI (nZVI) has not yet been studied. The experimental results showed that in the presence of 100 mg L^-1 of Suwannee River NOM (SRNOM), the Hg(II) removal ratio by nZVI decreased from 89% to 36% after 80 min of reaction. Similar trends were observed in the long-term test maintained for 15 days, attributable to the surface passivation of nZVI by SRNOM. In contrast, addition of 100 μM glutathione (GSH) to the nZVI suspensions increased the Hg(II) removal ratio from 85% to 96% after 15 days of reaction. Furthermore, adding 100 μM of GSH to the nZVI and SRNOM suspensions largely improved the removal efficiency of Hg(II) to be > 99% after 9 days of reaction, related to the enhanced dissolution of Fe(II) and consequent formation of lepidocrocite and maghemite on the nZVI surface. The addition of thiolic compounds is suggested as a promising step in overcoming the inhibitory effect of SRNOM for the remediation of Hg(II) using nZVI technology.

Enhanced reactivity and electron selectivity of GAC-Fe-Cu ternary micro-electrolysis system toward p-chloronitrobenzene under oxic conditions

A novel GAC-Fe-Cu ternary micro-electrolysis system was synthesized for the removal of p-chloronitrobenzene (p-CNB) under oxic conditions. p-CNB could be efficiently removed by GAC-Fe-Cu at a wide initial pH range of 1.0-9.0. In particular, the p-CNB removal efficiency of 96.96 % was obtained at initial pH of 7.2, and the degradation (44.96 %) was the major removal pathway. Additionally, reduction and oxidation simultaneously contributed to the degradation of p-CNB. The results indicated that OH was the prime reactive species under acidic conditions while O₂^- dominated the degradation of p-CNB under neutral conditions. Reduction reaction was remarkably enhanced in the presence of dissolved oxygen and the iron corrosion could be accelerated by in-situ generated H₂O₂. Furthermore, XPS analysis of GAC-Fe-Cu revealed the surface-mediated electron transfer and oxidant generation process. The excellent degradation efficiency of p-CNB at initial pH of 7.2 was attributed to the enhanced electron selectivity of GAC-Fe-Cu as well as the high selectivity of near-surface generated O₂^- toward p-CNB and its intermediate products.

Removal of reactive brilliant red X-3B by a weak magnetic field enhanced Fenton-like system with zero-valent iron

The effect of a weak magnetic field (WMF) on the removal of reactive brilliant red X-3B (X-3B) by zero-valent iron (ZVI)/H₂O₂ was studied. The optimum conditions for the removal of X-3B by the ZVI/H₂O₂/WMF system were as follows: pH = 4.0, X-3B was 50 mg L⁻¹, H₂O₂ was 8 mM, and ZVI with particle size of 20 μm was 0.5 g L⁻¹. The X-3B decolorization rate could reach 99.41% in 10 minutes. The superposed WMF increased the working pH of ZVI from 3.0 to 4.0. The main part of ZVI/H₂O₂ removal kinetics of X-3B followed the zero order rate law. In this study, the removal effect of X-3B by pre-magnetization ZVI was not as good as that of real-time magnetization, but it was better than the removal of X-3B by the ZVI/H₂O₂ system. The ZVI/H₂O₂/WMF system still had the ability to remove X-3B after 4 consecutive cycles. The use of WMF improved the removal of X-3B by ZVI/H₂O₂ mainly due to the corrosion of ZVI. Under acidic conditions, WMF enhanced the activity of ZVI, which promoted the efficiency of the Fenton reaction. The use of WMF to enhance the ZVI/H₂O₂ removal X-3B was a promising and environmental friendly process because it did not require additional energy and expensive reagents and did not cause secondary pollution.

The effect of a weak magnetic field (WMF) on the removal of reactive brilliant red X-3B (X-3B) by zero-valent iron (ZVI)/H₂O₂ was studied.

Synergistic degradation of sulfamethoxazole in an oxalate-enhanced Fered-Fenton system: The critical heterogeneous solid-liquid interfacial mechanism and an insight in practical application

It was demonstrated in this study that appropriate concentrations of oxalate (Ox) would lead to greatly accelerated electro-generation of Fe²⁺ but obviously lower power consumption in the Fered-Fenton system. Depending on the Ti electrode with pristine TiO₂ layer, effects of important parameters on the SMX degradation were investigated in the Fered-Fenton-Ox system. It was found that the heterogeneous interfacial electrochemically reduction of Fe^III was critical in the Fered-Fenton-Ox system relying on the surface hydroxyl bonding Fe^III-Ox and formation of FeOTi bonds. A heterogeneous-homogeneous reaction mechanism was therefore proposed. It included the heterogeneous interfacial electrochemical generation of Fe^II-Ox and the heterogeneous-homogenous Fenton oxidation of pollutants. The promotional role of Ox would be also homogenous and heterogeneous, i.e. maintaining ferric at higher pH and forming specific Fe^III-Ox complex as well as accelerating the solid-liquid interfacial heterogeneous iron cycle. Furthermore, a continuous-flow pilot study was conducted in treating a printing and dyeing industrial wastewater. As compared to conventional Fenton and Fered-Fenton systems, the Fered-Fenton-Ox system could achieve more efficient COD removal with a relative low cost/△COD, suggesting great advantages in its practical applications for treating real industrial complex wastewaters.

Phosphorus recovery from the liquid phase of anaerobic digestate using biochar derived from iron-rich sludge: A potential phosphorus fertilizer

A novel technique for phosphorus recovery from the liquid phase of anaerobic digestate was developed using biochar derived from iron-rich sludge (dewatered sludge conditioned with Fenton's reagent). The biochar pyrolyzed from iron-rich sludge at a low temperature of 300 °C (referred to as Fe-300 biochar) showed a better phosphorus (P) adsorption capacity (most of orthophosphate and pyrophosphate) than biochars pyrolyzed at other higher temperatures of 500-900 °C, with the maximum P adsorption capacity of up to 1.843 mg g^-1 for the liquid phase of anaerobic digestate. Adsorption isotherms study indicated that 70% P was precipitated through chemical reaction with Fe elements, i.e., Fe(II) and Fe(III) existed on the surface of the Fe-300 biochar, and other 30% was through surface physical adsorption as simulated by a dual Langmuir-Langmuir model using the potassium dihydrogen orthophosphate (KH₂PO₄) as a model solution. The seed germination rate was increased up to 92% with the addition of Fe-300 biochar after adsorbing most of P, compared with 66% without the addition of biochar. Moreover, P adsorbed by the chemical reaction in form of iron hydrogen phosphate can be solubilized by a phosphate-solubilizing microorganism of Pseudomonas aeruginosa, with the total solubilized P amount of 3.045 mg g^-1 at the end of an incubation of 20 days. This study indicated that the iron-rich sludge-derived biochar could be used as a novel and beneficial functional material for P recovery from the liquid phase of anaerobic digestate. The recovered P with biochar can be re-utilized in garden soil as an efficient P-fertilizer, thus increasing the added values of both the liquid phase of anaerobic digestate and the iron-rich sludge.

Fast degradation, large capacity, and high electron efficiency of chloramphenicol removal by different carbon-supported nanoscale zerovalent iron

It remains unclear that which kind of carbon support is better for improving the reactivity of nanoscale zerovalent iron (nZVI) without the adsorption effects of carbon. Finding appropriate contaminants that could be degraded by nZVI with high capacity and electron utilization is crucial for exploring the applications of nZVI. High degradation rate (up to 3.70 min^-1) and high capacity (up to 3000 mg g^-1) of antibiotic chloramphenicol (C₁₁H₁₂Cl₂N₂O₅, CAP) removal with high electron utilization (>97%) was achieved by different carbon supported nZVI in this study. Carbon powder (CP) was found to be the best support, possessing good distribution and reactivity of nZVI. 99% of CAP was removed by CP-nZVI after 3 min, without the electron consumption via the side reaction between nZVI and water, suggesting that CAP could outcompete with water for the electrons from nZVI. The entire pathway of CAP removal was elucidated based on UPLC-MS/MS analysis. Partial degradation of CAP (denitration and dechlorination) was enough to take away the antimicrobial properties. These results suggest a promising application scenario of carbon supported nZVI for the remediation of CAP-contaminated water to reduce the antibiotic selection pressure of the environment.

Coupled Effect of Sulfidation and Ferrous Dosing on Selenate Removal by Zerovalent Iron Under Aerobic Conditions

Both the reactivity and the removal capacity of zerovalent iron (ZVI) for the target contaminant are important for applying ZVI in wastewater treatment. In this study, the feasibility of combining sulfidation treatment and Fe²⁺ dosing (S-ZVI/Fe²⁺) to enhance the performance of ZVI for Se(VI) removal was comprehensively investigated under aerobic conditions. Se(VI) was first adsorbed on the surface of ZVI particles and then reduced to Se(IV) and Se(0) with Se(0) being the final product in S-ZVI/Fe²⁺ system. This system bore the advantages of both sulfidation treatment (S-ZVI) and Fe²⁺ dosing (ZVI/Fe²⁺) for Se(VI) removal. The amounts and rate constants of Se(VI) removal in S-ZVI/Fe²⁺ system were increased by 1.8-32.8 times and 11.7-194.0 times, respectively, compared to those in pristine ZVI system. Sulfidation significantly accelerated the corrosion of Fe⁰ thus improved the removal rate of Se(VI). The promoting effect of Fe²⁺ on Se(VI) sequestration by S-ZVI should be mainly associated with the following facts: Fe²⁺ could maintain a relatively low pH level during Se(VI) removal by S-ZVI; Compared to S-ZVI alone, the consumption of Fe⁰ in S-ZVI/Fe²⁺ by O₂/H⁺ was slower, and thus the electron efficiency of S-ZVI was elevated; Fe²⁺ dosing facilitated electron transfer by forming semiconductive Fe₃O₄.

Structural transformation and potential toxicity of iron-based deposits in drinking water distribution systems

Discoloration events in drinking water distribution systems (DWDSs) are usually considered an aesthetic issue rather than a health concern, and the potential toxicity of the iron-based particles resuspended from deposits in DWDSs has not been a focus. More importantly, it has not been recognized that the iron-based particles may have structural transformation under the complex condition in DWDSs which would further increase their adverse effects. In the present study, iron particle-dominated loose deposits, which were collected from a real DWDSs through pipe flushing, were firstly found to possess obvious toxicity to human liver cells. To further evaluate the potential harms of the deposits, FeOOH crystals (which is one of the most representative components in the deposits of DWDSs) were grown with different types of coexisting matters which may emerge in DWDSs. Results showed that the FeOOH had obvious structure transformation with coexisting matters which further influenced their toxicity: the samples with sharp surfaces had higher toxicity than those with smooth surfaces. Interestingly, although the FeOOH particles formed with perfluorooctanoic acid (FeOOH-PFOA) did not have the sharpest surface or smallest particle size among all the samples, they demonstrated the highest toxicity with strong generation of reactive oxygen species. Experimental and theoretical studies verified that PFOA induced the electron migration around Fe in FeOOH-PFOA particles. The FeOOH-PFOA not only was able to capture electrons directly from DNA, but also generated ROS from O₂ using DNA as an electron donor which might greatly enhance the oxidative damage to cells. This study would broaden the understanding of the potential harms of deposits in DWDSs.

Liquid Nitrogen Activation of Zero-Valent Iron and Its Enhanced Cr(VI) Removal Performance

In this study, we report that liquid nitrogen treatment is a promising zero-valent iron activation method that does not remove the iron oxide shell; this can improve the apparent Cr(VI) removal rate constant of zero-valent iron by about 4-120 times, depending on the particle sizes and the suppliers of zero-valent iron. It was found that liquid nitrogen, with its low temperature of 77 K, could crack the iron oxide shell of zero-valent iron to produce abundant fractures because of the different thermal expansion coefficients of iron oxide and iron. These fractures provided suitable mass transfer channels for the inward transfer of water/oxygen molecules to the iron core and the subsequent in situ generation of Fe(II) for the reduction of Cr(VI) to Cr(III). More importantly, systematic characterizations confirmed the generation of an Fe(III)/Cr(III)/Cr(VI) composite on the surface of zero-valent iron during the removal, suggesting its environmental benignancy. This study provides a novel physical zero-valent iron activation method, sheds light on the importance of the iron oxide shell of zero-valent iron on Cr(VI) removal, and clarifies the intrinsic Cr(VI) removal mechanism of zero-valent iron.

Intercalation of Nanosized Fe3C in Iron/Carbon To Construct Multifunctional Interface with Reduction, Catalysis, Corrosion Resistance, and Immobilization Capabilities

As a robust reducing system in industrial wastewater treatment, iron/carbon (Fe/C) microelectrolysis suffers from surface passivation and low utilization efficiency. Herein, we introduced Fe₃C into the Fe/C system to develop a core-shell Fe⁰/Fe₃C/C nanorod with a multifunctional interface (Fe₃C/C) providing reduction, catalysis, adsorption, and corrosion resistance. The results proved that the fabricated Fe⁰/Fe₃C/C possesses 5.6 times higher reduction capacity (220 mg/g) for Cr(VI) reduction but a relatively lower Fe leakage (2.7 mg/L) than Fe/C. On the basis of the results of electrochemical characterization (Tafel polarization curves and electrochemical impedance spectroscopy), the corrosion-resistant Fe₃C/C shell can significantly prevent surface passivation of the Fe⁰ core, whereas Fe₃C efficiently catalyzes electron transfer from the inner Fe⁰ to the external carbon surface. Moreover, the reductive species involved in Cr(VI) removal were identified as hydrogen atoms, adsorbed Fe(II) ions, and electrons tunneling from Fe⁰. STEM, XPS, and Mössbauer spectroscopies were further adopted to characterize the interface reaction of Fe⁰/Fe₃C/C during the Cr(VI) removal process. Finally, the reaction mechanism for Cr(VI) reduction over Fe⁰/Fe₃C/C was proposed, and the distribution of active sites was inferred.

Effect of phosphate on amorphous iron mineral generation and arsenic behavior in paddy soils

Arsenic (As) contamination was detected in paddy soils of Guangdong, China due to mining and weathering processes. Furthermore, As may be released into the soil and irrigation water during the application of phosphate (P). In this study, As behavior was assessed in three paddy soils (S6, S8 and TR) along the Hengshi river using batch and circular flow experiments with different phosphate application doses (0, 1, 5, 10, 50, 100 mg/L). The results indicate that pH variation (3-7) and higher phosphate concentrations in solution, can induce the release of As, with total As release ranked in the order: S6 > S8 > TR. In addition, As^V was the main state affected by phosphate in the circular soil solution. In particular, after 7 days of P₁₀ application, the highest As concentration in S6, S8 and TR soil solutions reached 2298.4, 829.9 and 153.9 μg/L respectively, with the As^V state accounting for 93%, 97% and 18% of As. Some minerals were found to be generated in the middle container, most of which were amorphous iron or aluminum oxides and hydroxides, as confirmed by XRD. With mineral generation, the As concentration in soil solutions decreased to 314.2, 98.1 and 34.1 μg/L. The SEM results indicate that the minerals became more fine (<100 nm) when more P was applied. In addition, XPS, SEM-eds and elemental analysis results also revealed that As distribution was closely associated with iron minerals. Along with soil depth, P influenced the state and distribution of iron minerals and As in the topsoil, while phosphate increased the available As and reduced the amorphous iron mineral content in the soil. Therefore, it is essential to evaluate As behavior in paddy soils, to monitor and avoid potential food security risks.

Characterization methods of zerovalent iron for water treatment and remediation

Appropriately selecting methods for characterizing the reaction system of zerovalent iron (ZVI) favors its application for water treatment and remediation. Hence, a survey of the available ZVI characterization techniques used in laboratory and field studies are presented in this review for clarifying the characteristic properties, (in-situ) corrosion processes, and corrosion products of ZVI system. The methods are generally classified into four broad categories: morphology characterization techniques, (sub-)surface and bulk analysis mainly via the spectral protocols, along with the (physio)electrochemical alternatives. Moreover, this paper provides a critical review on the scopes and applications of ZVI characterization methodologies from several perspectives including their suitable occasions, availability, (semi-)quantitative/qualitative evaluations, in/ex-situ reaction information, advantages, limitations and challenges, as well as economic and technical remarks. In particular, the characteristic spectroscopic peak locations of typical iron (oxyhydr)oxides are also systematically summarized. In view of the complexity and variety of ZVI system, this review further addresses that different characterization methods should be employed together for better assessing the performance and mechanisms of ZVI-involved systems and thereby facilitating the deployment of ZVI-based installations in real practice.

Enhanced removal of heavy metals by zerovalent iron in designed magnetic reactors

A magnetic propeller agitator and a magnetic reactor were designed to enhance the removal of heavy metals by zerovalent iron (ZVI) in comparison with the non-magnetic reactor. The weak magnetic field (WMF) applied significantly improved the CuII-EDTA removal by ZVI from 10% without WMF to 98% with WMF within 2.5 h at pHini 6.0. The pseudo-first-order rate constants of Cu(II) and As(V) removal by ZVI in the magnetic reactor were increased by 1.51-5.17 and 2.97-5.91 fold, respectively, compared to those obtained in the non-magnetic reactor. The performance of ZVI for treating practical industrial wastewater in the designed magnetic reactor was tested, and the removal of total Cu, P and Zn by ZVI was greatly accelerated. After precipitation of the practical wastewater samples, the concentrations of total Cu, P, Zn decreased to the industrial drainage standard values in 20, 3, 25 min, respectively, in the magnetic reactor, whereas the reaction time needed to eliminate total P and Zn was 10 and 60 min, and the residual total Cu still exceeded the drainage standard values in 2 h in the non-magnetic reactor. The application of magnetic reactor for industrial wastewater treatment is expected to improve the sustainability of ZVI technology.

Effect of field site hydrogeochemical conditions on the corrosion of milled zerovalent iron particles and their dechlorination efficiency

Milled zerovalent iron (milled ZVI) particles have been recognized as a promising agent for groundwater remediation because of (1) their high reactivity with chlorinated aliphatic hydrocarbons, organochlorine pesticides, organic dyes, and a number of inorganic contaminants, and (2) a possible greater persistance than the more extensively investigated nanoscale zerovalent iron. We have used laboratory-scale batch degradation experiments to investigate the effect that hydrogeochemical conditions have on the corrosion of milled ZVI and on its ability to degrade trichloroethene (TCE). The observed pseudo first-order degradation rate constants indicated that the degradation of TCE by milled ZVI is affected by groundwater chemistry. The apparent corrosion rates of milled ZVI particles were of the same order of magnitude for hydrogeochemical conditions representative for two contaminated field sites (133-140mmolkg^-1day^-1, indicating a milled ZVI life-time of 128-135days). Sulfate enhances milled ZVI reactivity by removing passivating iron oxides and hydroxides from the Fe⁰ surface, thus increasing the number of reactive sites available. The organic matter content of 1.69% in the aquifer material tends to suppress the formation of iron corrosion precipitates. Results from scanning electron microscopy, X-ray diffraction, and iron K-edge X-ray adsorption spectroscopy suggest that the corrosion mechanisms involve the partial dissolution of particles followed by the formation and surface precipitation of magnetite and/or maghemite. Numerical corrosion modeling revealed that fitting iron corrosion rates and hydrogen inhibitory terms to hydrogen and pH measurements in batch reactors can reduce the life-time of milled ZVI particles by a factor of 1.2 to 1.7.

Enhancing the efficiency of zero valent iron by electrolysis: Performance and reaction mechanism

Electrolysis was applied to enhance the efficiency of micron-size zero valent iron (mFe⁰) and thereby promote p-nitrophenol (PNP) removal. The rate of PNP removal by mFe⁰ with electrolysis was determined in cylindrical electrolysis reactor that employed annular aluminum plate cathode as a function of experimental factors, including initial pH, mFe⁰ dosage and current density. The rate constants of PNP removal by Ele-mFe⁰ were 1.72-144.50-fold greater than those by pristine mFe⁰ under various tested conditions. The electrolysis-induced improvement could be primarily ascribed to stimulated mFe⁰ corrosion, as evidenced by Fe²⁺ release. The application of electrolysis could extend the working pH range of mFe⁰ from 3.0 to 6.0 to 3.0-10.0 for PNP removal. Additionally, intermediates analysis and scavengers experiments unraveled the reduction capacity of mFe⁰ was accelerated in the presence of electrolysis instead of oxidation. Moreover, the electrolysis effect could also delay passivation of mFe⁰ under acidic condition, as evidenced by SEM-EDS, XRD, and XPS analysis after long-term operation. This is mainly due to increased electromigration meaning that iron corrosion products (iron hydroxides and oxides) are not primarily formed in the vicinity of the mFe⁰ or at its surface. In the presence of electrolysis, the effect of electric field significantly promoted the efficiency of electromigration, thereby enhanced mFe⁰ corrosion and eventually accelerated the PNP removal rates.