Modeling the Kinetics of Hydrogen Formation by Zerovalent Iron: Effects of Sulfidation on Micro- and Nano-Scale Particles

Characteristic study of biological CaCO3-supported nanoscale zero-valent iron: stability and migration performance

nZVI has attracted much attention in the remediation of contaminated soil and groundwater, but the application is limited due to its aggregation, poor stability, and weak migration performance. The biological CaCO3 was used as the carrier material to support nZVI and solved the nZVI agglomeration, which had the advantages of biological carbon fixation and green environmental protection. Meanwhile, the distribution of nZVI was characterised by SEM-EDS and TEM carefully. Subsequently, the dispersion stability of bare nZVI and CaCO3@nZVI composite was studied by the settlement experiment and Zeta potential. Sand column and elution experiments were conducted to study the migration performance of different materials in porous media, and the adhesion coefficient and maximum migration distances of different materials in sand columns were explored. SEM-EDS and TEM results showed that nZVI could be uniformly distributed on the surface of biological CaCO3. Compared with bare nZVI, CaCO3@nZVI composite suspension had better stability and higher absolute value of Zeta potential. The migration performance of nZVI was poor, while CaCO3@nZVI composite could penetrate the sand column and have good migration performance. What's more, the elution rates of bare nZVI and CaCO3@nZVI composite in quartz sand columns were 5.8% and 51.6%, and the maximum migration distances were 0.193 and 0.885 m, respectively. In summary, this paper studies the stability and migration performance of bare nZVI and CaCO3@nZVI composite, providing the experimental and theoretical support for the application of CaCO3@nZVI composite, which is conducive to promoting the development of green remediation functional materials.

Modeling the Role in pH on Contaminant Sequestration by Zerovalent Metals: Chromate Reduction by Zerovalent Magnesium

The role of pH in sequestration of Cr(VI) by zerovalent magnesium (ZVMg) was characterized by global fitting of a kinetic model to time-series data from unbuffered batch experiments with varying initial pH values. At initial pH values ranging from 2.0 to 6.8, ZVMg (0.5 g/L) completely reduced Cr(VI) (18.1 μM) within 24 h, during which time pH rapidly increased to a plateau value of ∼10. Time-series correlation analysis of the pH and aqueous Cr(VI), Cr(III), and Mg(II) concentration data suggested that these conditions are controlled by combinations of reactions (involving Mg0 oxidative dissolution and Cr(VI) sequestration) that evolve over the time course of each experiment. Since this is also likely to occur during any engineering applications of ZVMg for remediation, we developed a kinetic model for dynamic pH changes coupled with ZVMg corrosion processes. Using this model, the synchronous changes in Cr(VI) and Mg(II) concentrations were fully predicted based on the Langmuir-Hinshelwood kinetics and transition-state theory, respectively. The reactivity of ZVMg was different in two pH regimes that were pH-dependent at pH < 4 and pH-independent at the higher pH. This contrasting pH effect could be ascribed to the shift of the primary oxidant of ZVMg from H+ to H2O at the lower and higher pH regimes, respectively.

Corrosion behaviors and kinetics of nanoscale zero-valent iron in water: A review

Knowledge on corrosion behaviors and kinetics of nanoscale zero-valent iron (nZVI) in aquatic environment is particularly significant for understanding the reactivity, longevity and stability of nZVI, as well as providing theoretical guidance for developing a cost-effective nZVI-based technology and designing large-scale applications. Herein, this review gives a holistic overview on the corrosion behaviors and kinetics of nZVI in water. Firstly, Eh-pH diagram is introduced to predict the thermodynamics trend of iron corrosion. The morphological, structural, and compositional evolution of (modified-) nZVI under different environmental conditions, assisted with microscopic and spectroscopic evidence, is then summarized. Afterwards, common analytical methods and characterization technologies are categorized to establish time-resolved corrosion kinetics of nZVI in water. Specifically, stable models for calculating the corrosion rate constant of nZVI as well as electrochemical methods for monitoring the redox reaction are discussed, emphasizing their capabilities in studying the dynamic iron corrosion processes. Finally, in the future, more efforts are encouraged to study the corrosion behaviors of nZVI in long-term practical application and further build nanoparticles with precisely tailored properties. We expect that our work can deepen the understanding of the nZVI chemistry in aquatic environment.

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.

Degradation of Chloroform by Zerovalent Iron: Effects of Mechanochemical Sulfidation and Nitridation on the Kinetics and Mechanism

Chloroform (CF) is a widely used chemical reagent and disinfectant and a probable human carcinogen. The extensive literature on halocarbon reduction with zerovalent iron (ZVI) shows that transformation of CF is slow, even with nano, bimetallic, sulfidated, and other modified forms of ZVI. In this study, an alternative method of ZVI modification─involving simultaneous sulfidation and nitridation through mechanochemical ball milling─was developed and shown to give improved degradation of CF (i.e., higher degradation rate and inhibited H₂ evolution reaction). The composite material (denoted as S-N(C)-ZVI) gave synergistic effects of nitridation and sulfidation on CF degradation. A complete chemical reaction network (CRN) analysis of CF degradation suggests that O-nucleophile-mediated transformation pathways may be the main route for the formation of the terminal nonchlorinated products (formate, CO, and glycolic polymers) that have been used to explain the undetected products needed for mass balance. Material characterizations of the ZVI recovered after batch experiments showed that sulfidation and nitridation promoted the formation of Fe₃O₄ on the S-N(C)-ZVI particles, and the effect of aging on CF degradation rates was minor for S-N(C)-ZVI. The synergistic benefits of sulfidation and nitridation on CF degradation were also observed in experiments performed with groundwater.

Graphene oxide supported sulfidated nano zero-valent iron (S-nZVI@GO) for antimony removal: The role of active oxygen species and reaction mechanism

Sulfidated nano zero-valent iron (S-nZVI) was used to remove various pollutants from wastewater. However, the instability, poor dispersibility, and low electron transfer efficiency of S-nZVI limit its application. Herein, graphene oxide supported sulfidated nano zero-valent iron (S-nZVI@GO) was successfully synthesized using graphene oxide (GO) as a carrier. The properties of S-nZVI@GO were characterized by scanning electron microscopy coupled to X-ray photoelectron spectroscopy (SEM-EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) concerning the surface morphology, crystalline structure, and elemental components. S-nZVI@GO displayed an excellent capacity for antimony (Sb) removal under aerobic conditions (96.7%), with a high adsorption capacity (Q_max = 311.75 mg/g). It maintained a high removal rate (over 90%) during a wide pH range (3-9). More importantly, S-nZVI@GO activated the molecular oxygen in water via a single-electron pathway to produce •O₂－ and H₂O₂, and then oxidized trivalent antimony (Sb(III)) to pentavalent antimony (Sb(V)) and further separated it by synergistic adsorption and co-precipitation. Therefore, S-nZVI@GO shows excellent potential for Sb contamination remediation.

Synthesis, characterization and performances of green rusts for water decontamination: A review

In recent years, the application of green rusts (GRs) for water purification has received significant attention, but its full understanding has not been well achieved. Then, the comprehension about the synthesis and characteristics of GRs can highly favor their decontamination performances for the site-specific conditions. This review comprehensively summarized the synthesis, characteristics and performances of GRs including the GR (Cl^-), GR (CO₃^2-) and GR (SO₄^2-) for sequestration of various aqueous pollutants (e.g., tetrachloride, Cr(VI), Se(VI), and U(VI), etc.). Generally, the different reactivity of GRs toward contaminants is strongly dependent on the GRs' characteristics (e.g., interlayer distance, specific surface area, and Fe(II) content) and solution chemistry (e.g., pH, background electrolytes, dissolved oxygen, and contaminant concentration, etc.). In addition, the reaction mechanisms of GRs with the contaminants involve the redox reactions, adsorption, catalytic oxidation, interlayer and octahedral incorporation, which can mutually or singly contribute to the decontamination to varying degrees. Particularly, this review addressed the transformation pathways of GRs under various solution chemistry conditions and clarified that the stability of GRs should be the key challenge for the real application. Finally, how to effectively use the GRs for water decontamination was proposed, which will significantly benefit the rational control of environmental pollution.

Enhanced biogas biological upgrading from kitchen wastewater by in-situ hydrogen supply through nano zero-valent iron corrosion

The in-situ hydrogen supply by nano zero-valent iron (nZVI, nFe⁰) corrosion provided a feasible way to improve the efficiency of biogas biological upgrading. This work studied the effects of nZVI at different dosages (0, 2, 4, 6, 8 and 10 g/L) on anaerobic digestion of kitchen wastewater by two buffer systems 2-[4-(2-hydroxyethyl) piperazin-1-yl] ethanesulfonic acid (HEPES) and sodium hydrogen carbonate (NaHCO₃). The addition of nZVI improved the content of methane (CH₄) and stability of anaerobic digestion process. In HEPES buffer system, the CH₄ was all increased and the maximum reached 90.51% with 10 g/L nZVI, higher than 32.25% compared to the control. The maximum hydrogen enrichment (HE) was 113 ppb after nZVI addition, indicating the mass transfer efficiency of hydrogen (H₂) was improved. Microbial community analysis showed that the total relative abundance of Methanobacterium and Methanolinea at 10 g/L nZVI was 53.72%, which was 1.62 times of the control group. However, in the NaHCO₃ buffer system with 10 g/L nZVI addition, the content of CH₄ and the loosely bound extracellular polymeric substances (LB-EPS) was lower than the control. The results indicated that the addition of nZVI was feasible for biogas upgrading, and the bidirectional effect of nZVI on the promotion or inhibition of bio-methanation might be related to the buffer system of the anaerobic process.

Even Incorporation of Nitrogen into Fe0 Nanoparticles as Crystalline Fe4N for Efficient and Selective Trichloroethylene Degradation

Surface modification of microscale Fe powder with nitrogen has emerged recently to improve the reactivity of Fe⁰ for dechlorination. However, it is unclear how an even incorporation of a crystalline iron nitride phase into Fe⁰ nanoparticles affects their physicochemical properties and performance, or if Fe⁰ nanoparticles with a varied nitridation degree will act differently. Here, we synthesized nitridated Fe⁰ nanoparticles with an even distribution of N via a sol-gel and pyrolysis method. Nitridation expanded the Fe⁰ lattice and provided the Fe₄N species, making the materials more hydrophobic and accelerating the electron transfer, compared to un-nitridated Fe⁰. These properties well explain their reactivity and selectivity toward trichloroethylene (TCE). The TCE degradation rate by nitridated Fe⁰ (up to 4.8 × 10^-2 L m^-2 h^-1) was much higher (up to 27-fold) than that by un-nitridated Fe⁰, depending on the nitridation degree. The materials maintained a high electron efficiency (87-95%) due to the greatly suppressed water reactivity (109-127 times lower than un-nitridated Fe⁰). Acetylene was accumulated as the major product of TCE dechlorination via β-elimination. These findings suggest that the nitridation of Fe⁰ nanoparticles can change the materials' physicochemical properties, providing high reactivity and selectivity toward chlorinated contaminants for in situ groundwater remediation.

Simultaneous Sequestration of Humic Acid-Complexed Pb(II), Zn(II), Cd(II), and As(V) by Sulfidated Zero-Valent Iron: Performance and Stability of Sequestration Products

Heavy metal(loid)s (HMs) such as Pb(II), Zn(II), Cd(II), and As(V) are ubiquitously present in co-contaminated soil and shallow groundwater, where the humic acid (HA)-rich environments can significantly influence their sequestration. In this study, sulfidated zero-valent iron (S-ZVI) was found to be able to simultaneously sequestrate these HA-complexed HMs. Specially, the HA-complexed Pb(II), Zn(II), Cd(II), and As(V) could be completely removed by S-ZVI within 60 min, while only 35-50% of them could be sequestrated within 72 h by unsulfidated ZVI. Interestingly, different from the S-ZVI corrosion behavior, the kinetics of HM sequestration by S-ZVI consisted of an initial slow reaction stage (or a lag phase) and then a fairly rapid reaction process. Characterization results indicated that forming metal sulfides controlled the HM sequestration at the first stage, whereas the enhanced ZVI corrosion and thus-improved adsorption and/or coprecipitation by iron hydroxides governed the second stage. Both metal-oxygen and metal-sulfur bonds in the solid phase could be confirmed by X-ray photoelectron spectroscopy and extended X-ray absorption fine structure analysis. Moreover, the transformation of S species from SO₄^2-, SO₃^2-, and S₂^2- to S^2- under reducing conditions could allow the sequestrated HMs to remain stable over a long period.

Enhanced dechlorination of trichloroethene by sulfidated microscale zero-valent iron under low-frequency AC electromagnetic field

In this study an electromagnetic heating strategy is proposed for remediation of trichloroethene (TCE) by ball milled, sulfidated microscale zero valent iron (S-mZVI^bm) particles. S-mZVI^bm is ferromagnetic, which generates heat under the application of a low-frequency alternating current electromagnetic field (AC EMF). We found that the temperature reached up to ~120 ℃ during 30-min electromagnetic induction heating of 10 g/L S-mZVI^bm (with S/Fe molar ratio of 0.1), compared with ~55 ℃ and ~80 ℃ for ZVI and ball milled mZVI^bm, respectively. The application of AC EMF accelerated the TCE degradation rate (k_TCE = 5.5 × 10^-1 h^-1) by up to 4-fold without compromising or even enhancing electron efficiency of S-mZVI^bm compared to no-heating. Furthermore, this process halved the generation of chlorinated intermediate, cis-DCE. In contrast, water-bath heating only increased the dechlorination rate 2-fold with unchanged cis-DCE generation and lowered electron efficiency. This is attributed to both rising temperature by induction heating and accelerated ZVI corrosion and surface Fe⁰ exposure caused by AC EMF. In real groundwater, the AC EMF maintained the same promoting effects for TCE dechlorination by S-mZVI^bm. This study shows that combination of filed-scale available AC EMF with S-mZVI^bm provides a promising approach for remediation of chlorinated hydrocarbons in contaminated groundwater.

Coincorporation of N and S into Zero-Valent Iron to Enhance TCE Dechlorination: Kinetics, Electron Efficiency, and Dechlorination Capacity

Sulfidated zero-valent iron (S-ZVI) enhances the degradation of chlorinated hydrocarbon (CHC) in contaminated groundwater. Despite numerous studies of S-ZVI, a versatile strategy to improve its dechlorination kinetics, electron efficiency (ε_e), and dechlorination capacity is still needed. Here, we used heteroatom incorporation of N(C) and S by ball-milling of microscale ZVI with melamine and sulfur via nitridation and sulfidation to synthesize S-N(C)-mZVI^bm particles that contain reactive Fe-N_X(C) and FeS species. Sulfidation and nitridation synergistically increased the trichloroethene (TCE) dechlorination rate, with reaction constants k_SA of 2.98 × 10^-2 L·h^-1·m^-2 by S-N(C)-mZVI^bm, compared to 1.77 × 10^-3 and 8.15 × 10^-5 L·h^-1·m^-2 by S-mZVI^bm and N(C)-mZVI^bm, respectively. Data show that sulfidation suppressed the reductive dissociation of N(C) from S-N(C)-mZVI^bm, which stabilized the reactive Fe-N_X(C) and reserved electrons for TCE dechlorination. In addition to lowering H₂ production, S-N(C)-mZVI^bm dechlorinated TCE to less reduced products (e.g., acetylene), contributing to the material's higher ε_e and dechlorination capacity. This synergistic effect on TCE degradation can be extended to other recalcitrant CHCs (e.g., chloroform) in both deionized and groundwater. This multiheteroatom incorporation approach to optimize ZVI for groundwater remediation provides a basis for further advances in reactive material synthesis.

Improved performance and applicability of copper-iron bimetal by sulfidation for Cr(VI) removal

The reactivity of zero-valent iron (ZVI) for the Cr(VI) removal in groundwater is mainly limited by the formation of a passivation layer during its application in permeable reactive barrier (PRB). A kind of sulfidated copper-iron bimetal (S-ZVICu) with high reactivity for Cr(VI) removal was prepared by depositing FeS_x onto copper modified ZVI via a one-pot method. The surface characteristic, reactivity and Cr(VI) removal performance of S-ZVICu were investigated. It was found that S-ZVICu had a Cr(VI) removal capacity as high as 67.5 mg/g and little risk of secondary contaminant of Cu(II). The optimal Cu/Fe mass ratio and S/Fe molar ratio were 0.0125 and 0.084, respectively. The S-ZVICu exhibited great superiority of Cr(VI) removal compared with ZVI, sulfidated ZVI (SZVI) and coper-iron bimetal (ZVICu). Mineralogy and morphology analysis showed that S-ZVICu had a hierarchical structure of Fe⁰/Cu⁰/FeS_x, which could effectively reduce the risk of secondary contaminant of copper ions. The mechanism analysis suggested that the copper and FeS_x successively plated on the surface of ZVI played a dual role in promoting the corrosion of zero-valent iron, and was facilitated to electron transfer between Fe⁰, Cu⁰, FeS_x and Cr(VI). In addition, the loose FeS_x layer had a positive effect on alleviating the oxidation of ZVI in air, which was helpful in maintaining the reactivity of S-ZVICu in the air. S-ZVICu is an environmentally friendly material for sustainable and effective removal of Cr(VI) in groundwater.

Triton X-100 improves the reactivity and selectivity of sulfidized nanoscale zerovalent iron toward tetrabromobisphenol A: Implications for groundwater and soil remediation

Sulfidized nanoscale zerovalent iron (SNZVI) with improved reactivity and selectivity has shown great potential for environmental remediation. However, it is unclear if SNZVI could be applied for the remediation of soil washing solution, and how a soil-washing surfactant affects the reactivity and selectivity of SNZVI. Here, we assess the impact of Triton X-100 (TX-100) on the reactivity and selectivity of a sulfidized commercial NZVI toward tetrabromobisphenol A (TBBPA). While sulfidation of NZVI improved its reactivity and electron efficiency toward TBBPA, TX-100 could further improve these promoting effects, which was 8-21 and 4-7 times higher than those without TX-100, respectively, depending on TX-100 concentration. Because TX-100 could induce the solubilization of TBBPA, sorb onto the SNZVI surface, and favor the subsequent sorption and degradation of TBBPA. SNZVI performance for successive treatments of TBBPA contaminated water was also greatly improved by TX-100. Moreover, washing the TBBPA-contaminated soil with TX-100 could efficiently extract the TBBPA, and almost all of the TBBPA in the soil washing solution could be efficiently degraded by SNZVI. These results suggest that TX-100 is a good additive to SNZVI for improving its performance, and SNZVI coupled with TX-100 can be a promising technology for the remediation of TBBPA-contaminated soil.

Origin of the hydrophobicity of sulfur-containing iron surfaces

Sulfur-containing iron materials such as sulfidized nanoscale zerovalent iron (SNZVI) have shown outstanding water remediation performance in many recent studies, which is largely attributed to its high hydrophobicity compared to that of NZVI. However, the role of sulfur in the reactions, and the origin of the hydrophobicity of SNZVI, were still unclear. In this paper, for the first time, we conducted ab initio molecular dynamics simulation using an explicitly solvated model on both Fe and S-containing Fe surfaces, to explore the hydrophobicity of S-containing Fe materials. We found that the high hydrophobicity of these S-containing Fe surfaces originates from the hydrophobic nature of S: both doping S on top of the Fe surface and inserting S onto an Fe surface can significantly improve the surface hydrophobicity by increasing the distance between the water layer and the Fe surface. This exposes empty Fe sites which do not interact with water and in turn reduces hydrogen evolution. To compare with the theoretical analysis, we experimentally analyzed the hydrophobicity of both NZVI and SNZVI surfaces, leading to a good agreement with our theoretical analysis. We then theoretically show that the doping of other p-block elements (e.g., N and P) to iron surfaces can also create a hydrophobic phenomenon. Most importantly, this study points out that the potential contribution of hydrophobicity to the reactivity on liquid-phase reaction materials should not be ignored in the mechanistic analysis.

nZVI Impacts Substrate Conversion and Microbiome Composition in Chain Elongation From D- and L-Lactate Substrates

Medium-chain carboxylates (MCC) derived from biomass biorefining are attractive biochemicals to uncouple the production of a wide array of products from the use of non-renewable sources. Biological conversion of biomass-derived lactate during secondary fermentation can be steered to produce a variety of MCC through chain elongation. We explored the effects of zero-valent iron nanoparticles (nZVI) and lactate enantiomers on substrate consumption, product formation and microbiome composition in batch lactate-based chain elongation. In abiotic tests, nZVI supported chemical hydrolysis of lactate oligomers present in concentrated lactic acid. In fermentation experiments, nZVI created favorable conditions for either chain-elongating or propionate-producing microbiomes in a dose-dependent manner. Improved lactate conversion rates and n-caproate production were promoted at 0.5-2 g nZVI⋅L^-1 while propionate formation became relevant at ≥ 3.5 g nZVI⋅L^-1. Even-chain carboxylates (n-butyrate) were produced when using enantiopure and racemic lactate with lactate conversion rates increased in nZVI presence (1 g⋅L^-1). Consumption of hydrogen and carbon dioxide was observed late in the incubations and correlated with acetate formation or substrate conversion to elongated products in the presence of nZVI. Lactate racemization was observed during chain elongation while isomerization to D-lactate was detected during propionate formation. Clostridium luticellarii, Caproiciproducens, and Ruminococcaceae related species were associated with n-valerate and n-caproate production while propionate was likely produced through the acrylate pathway by Clostridium novyi. The enrichment of different potential n-butyrate producers (Clostridium tyrobutyricum, Lachnospiraceae, Oscillibacter, Sedimentibacter) was affected by nZVI presence and concentrations. Possible theories and mechanisms underlying the effects of nZVI on substrate conversion and microbiome composition are discussed. An outlook is provided to integrate (bio)electrochemical systems to recycle (n)ZVI and provide an alternative reducing power agent as durable control method.

Anaerobic Corrosion of Zero-Valent Iron at Elevated Temperatures

Increasing groundwater temperatures caused by global warming, subsurface infrastructure, or heat storage projects may interfere with groundwater remediation techniques using zero-valent iron (ZVI) technology by accelerating anaerobic corrosion. The corrosion behavior of three ZVIs widely used in permeable reactive barriers (PRBs), Peerless cast iron (PL), Gotthart-Maier cast iron (GM), and an ISPAT iron sponge (IS), was investigated at temperatures between 25 and 70 °C in half-open batch reactors by measuring the volume of hydrogen gas generated. Initially, the corrosion rates of all tested ZVIs increased with temperature; at temperatures ≤40 °C, a material-specific steady state is reached, and at temperatures >40 °C, passivation causes a decrease in long-term corrosion rates. The observed corrosion behavior was therefore assumed to be superimposed by accelerating and inhibiting effects, caused by surface precipitates where the fitting of measured corrosion rates by a modeling approach, using the corroded amount of Fe⁰ to account for passivating minerals, yields intrinsic activation energies (E_a, _ZVI) of 81, 90, and 107 kJ mol^-1 for IS, GM, and PL, respectively. An increase in H₂ production might not be directly transferable to an increase in general ZVI reactivity; however, the results suggest that an increase in chlorinated hydrocarbon degradation rates can be expected for ZVI-PRBs in the immediate vicinity of low-temperature underground thermal energy storages (UTESs) or in the impact areas of high-temperature UTES with temperatures of ≤40 °C.

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.

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.

Enhanced tetrabromobisphenol A debromination by nanoscale zero valent iron particles sulfidated with S0 dissolved in ethanol

Modification of nanoscale zero-valent iron (nZVI) with reducing sulfur compounds has proven to improve the reactivity of nZVI towards recalcitrant halogenated organic contaminants. In this study, we develop a novel method for the preparation of sulfidated nZVI (S-nZVI) with S0 (a low cost and available reducing sulfur agent) dissolved in ethanol under mild conditions and apply it for the transformation of tetrabromobisphenol A (TBBPA), a potential persistent organic pollutant. Surface analysis shows that S0 dissolved in ethanol has been successfully doped into nZVI via a reaction with Fe0 to form a relatively homogeneous layer of FeS/FeS2 on the nZVI surface. The H2 production test and the electrochemical analysis show that the FeS/FeS2 layer not only slows the H2 evolution reaction but also enhances the electron transfer. Debromination kinetics indicate that the resulting S-nZVI with a S/Fe ratio of 0.015-0.05 possesses higher debromination activity for TBBPA and its debromination products (i.e., tri-BBPA, di-BBPA, mono-BBPA and BPA) in comparison with nZVI. Among them, S-nZVI at a S/Fe of 0.025 (S-nZVIS-0.025) has the greatest debromination rate constant (kobs) of 1.19 ± 0.071 h-1 for TBBPA. It debrominates TBBPA at a faster rate than other conventional S-nZVI made from Na2S and Na2S2O4 and has been successfully applied in the treatment of TBBPA-spiked environmental water samples (including river water, groundwater, and tap water). The results suggest that the modification of nZVI with S0 dissolved in ethanol is a simple, safe, inexpensive, and effective sulfidation technique, which can be applied for the large-scale production of S-nZVI for treating contaminated water.

Oxidative degradation of phenol by sulfidated zero valent iron under aerobic conditions: The effect of oxalate and tripolyphosphate ligands

After adding either organic or inorganic ligands, sulfidated nano-zero-valent iron (SnZVI) was used for aerobic degradation of phenol, and the effect of the ligand species on oxidation performance was investigated. We found that SnZVI hardly degraded phenol in the absence of ligand addition. Ligands initiated and promoted the degradation of pollutants by SnZVI. The data herein show that a characteristic inorganic ligand, tripolyphosphate (TPP), is more effective in enhancing oxidation than a characteristic organic ligand oxalate. In addition to the scavenging of reactive oxidants by the organic ligand, more ferrous ion (Fe(II)) dissolution from SnZVI in the TPP system is another cause for the superior enhancement by the inorganic ligand. In the oxalate system, as the sulfur content of SnZVI increased, the oxidation efficiency increased because FeS shell promoted the transfer of electrons to produce more reactive oxygen species (ROS). In TPP system, the effect of sulfur content on oxidation performance is more complex. The SnZVI with low sulfur content showed poor oxidation performance compared with that of nZVI. Further experiments proved that sulfidation might weaken the complexation of TPP with surface bound Fe, which would slow down the ionic Fe(II) dissolution rate. Therefore, sulfidation has the dual effects of enhancing electron transfer and inhibiting the complexation of inorganic ligands. In addition, the mechanisms of ROS generation in different ligand systems were investigated herein. Results showed that the critical ROS in both the oxalate and TPP systems are hydroxyl radicals, and that they are produced via one-electron activation of O₂.

Sulfidation of Zero-Valent Iron by Direct Reaction with Elemental Sulfur in Water: Efficiencies, Mechanism, and Dechlorination of Trichloroethylene

Sulfidation can enhance both the reactivity and selectivity (i.e., electron efficiency, ε_e) of zero-valent iron (ZVI) in contaminant removal, which may make this technology cost-effective for a wider range of water treatment applications. However, current sulfidation methods involve either hazardous or unstable sulfidation agents (e.g., Na₂S, Na₂S₂O₃, and Na₂S₂O₄) or energy-intensive preparations (e.g., mechanochemical sulfidation with elemental sulfur). In this study, we demonstrate that very efficient sulfidation of microscale ZVI (mZVI) can be achieved at all S/Fe molar ratios (∼100% sulfidation efficiency, ε_s) simply by direct reaction between elemental sulfur (S⁰) and ZVI in an aqueous suspension at ambient temperature. In comparison, the ε_s values obtained using Na₂S, Na₂S₂O₃, or Na₂S₂O₄ as the sulfidation agents were only ∼23, ∼75, and ∼38%, respectively. The sulfidated mZVI produced using the new method reacts with trichloroethylene (TCE) with very high rates and electron efficiencies: rate constants and electron efficiencies were 800- and 79-fold higher than those of the unsulfidated mZVI. The enhanced performance of this material, together with the operational advantages of S⁰ for sulfidation (including safety, stability, and cost), may make it a desirable product for full-scale engineering applications.

Al-Kahtani A, Khan JA, Iqbal J, Boczkaj G, Gul I, Bushra M. Bismuth-Doped Nano Zerovalent Iron: A Novel Catalyst for Chloramphenicol Degradation and Hydrogen Production

In this study, we showed that doping bismuth (Bi) at the surface of Fe⁰ (Bi/Fe⁰, bimetallic iron system)-synthesized by a simple borohydride reduction method-can considerably accelerate the reductive degradation of chloramphenicol (CHP). At a reaction time of 12 min, 62, 68, 74, 95, and 82% degradation of CHP was achieved with Fe⁰, Bi/Fe⁰-1 [1% (w/w) of Bi], Bi/Fe⁰-3 [3% (w/w) of Bi], Bi/Fe⁰-5 [5% (w/w) of Bi], and Bi/Fe⁰-8 [8% (w/w) of Bi], respectively. Further improvements in the degradation efficiency of CHP were observed by combining the peroxymonosulfate (HSO₅ ^-) with Bi/Fe⁰-5 (i.e., 81% by Bi/Fe⁰-5 and 98% by the Bi/Fe⁰-5/HSO₅ ^- system at 8 min of treatment). Interestingly, both Fe⁰ and Bi/Fe⁰-5 showed effective H₂ production under dark conditions that reached 544 and 712 μM by Fe⁰ and Bi/Fe⁰-5, respectively, in 70 mL of aqueous solution containing 0.07 g (i.e., at 1 g L^-1 concentration) of the catalyst at ambient temperature.

Iron and Sulfur Precursors Affect Crystalline Structure, Speciation, and Reactivity of Sulfidized Nanoscale Zerovalent Iron

The reactivity of sulfidized nanoscale zerovalent iron (SNZVI) is affected by the amount and species of sulfur in the materials. Here, we assess the impact of the Fe (Fe²⁺ and Fe³⁺) and S (S₂O₄^2-, S^2-, and S₆^2-) precursors used to synthesize both NZVI and SNZVI on the resulting physicochemical properties and reactivity and selectivity with water and trichloroethene (TCE). X-ray diffraction indicated that the Fe precursors altered the crystalline structure of both NZVI and SNZVI. The materials made from the Fe³⁺ precursor had an expanded lattice in the Fe⁰ body-centered-cubic (BCC) structure and lower electron-transfer resistance, providing higher reactivity with water (∼2-3 fold) and TCE (∼5-13 fold) than those made from an Fe²⁺ precursor. The choice of the S precursor controlled the S speciation in the SNZVI particles, as indicated by X-ray absorption spectroscopy. Iron disulfide (FeS₂) was the main S species of SNZVI made from S₂O₄^2-, whereas iron sulfide (FeS) was the main S species of SNZVI made from S^2-/S₆^2-. The former SNZVI was more hydrophobic, reactive with, and selective for TCE compared to the latter SNZVI. These results suggest that the Fe and S precursors can be used to select the conditions of the synthesis process and provide selected physicochemical properties (e.g., S speciation, hydrophobicity, and crystalline structure), reactivity, and selectivity of the SNZVI materials.

Electron competition and electron selectivity in abiotic, biotic, and coupled systems for dechlorinating chlorinated aliphatic hydrocarbons in groundwater: A review

Chlorinated aliphatic hydrocarbons (CAHs) have been frequently detected in aquifers in recent years. Owing to the bioaccumulation and toxicity of CAHs, it is essential to explore high-efficiency technologies for their complete dechlorination in groundwater. At present, the most widely used abiotic and biotic remediation technologies are based on zero-valent iron (ZVI) and functional anaerobic bacteria (FAB), respectively. However, the main obstacles to the full potential of both technologies in the field include their lowered efficiencies and increased economic costs due to the co-existence of a variety of natural electron acceptors in the environment, such as dissolved oxygen (DO), nitrate (NO₃^-), sulfate (SO₄^2-), ferric iron (Fe (III)), bicarbonate (HCO₃^-), and even water, which compete for electrons with the target contaminants. Therefore, a clear understanding of the mechanisms governing electron competition and electron selectivity is significant for the accurate evaluation of the effectiveness of both technologies under natural hydrochemical conditions. We collected data from both abiotic and biotic CAH-remediation systems, summarized the dechlorination and undesired reactions in groundwater, discussed the characterization methods and general principles of electron competition, and described strategies to improve electron selectivity in both systems. Furthermore, we reviewed the emerging ZVI-FAB coupled system, which integrates abiotic and biotic processes to enhance dechlorination performance and electron utilization efficiency. Lastly, we propose future research needs to quantitatively understand the electron competition in abiotic, biotic, and coupled systems in more detail and to promote improved electron selectivity in groundwater remediation.

Effects of non-reducible dissolved solutes on reductive dechlorination of trichloroethylene by ball milled zero valent irons

Non-reducible solution anions have been well recognized to affect reactivity of ZVI in dechlorinating chlorinated hydrocarbons. However, their effects and corresponding functional mechanisms on electron efficiency (ε_e) of ZVI remain unclear. In this study, mechanochemically modified microscale sulfidated and unsulfidated ZVI particles (i.e., S-mZVI^bm and mZVI^bm) and trichloroethylene (TCE) were used as model particles and contaminant to explore such effects. PO₄^3- as a corrosion promoter enhanced initial dechlorination rate by both particles. However, its passivating role as a surface complex agent became significant at the later stage of dechlorination by mZVI^bm, while sulfidation alleviated this effect without inhibition of dechlorination. Compared with enhancing dechlorination, PO₄^3- promoted hydrogen evolution reaction (HER) to a higher extent, decreasing ε_e for both particles by 17-73 %. HCO₃- negligibly affected dechlorination by both particles, while elevated HER. Thus, HCO₃- [5 mM] decreased ε_e for S-mZVI^bm and mZVI^bm by 1.9 % and 22 %. Different from PO₄^3- and HCO₃-, Cl- and SO₄^2- showed no significant effects on dechlorination, HER, and therefore ε_e for both particles. These results imply that even though some co-existing anions (i.e., PO₄^3- and HCO₃-) acting as corrosion promoters could improve the dechlorination by ZVIs, they would lead to decreased ε_e and shortened particle reactive lifetime.

Sulfidated nanoscale zero-valent iron is an efficient material for the removal and regrowth inhibition of antibiotic resistance genes

Antibiotic resistance genes (ARGs) and mobile gene elements (MGEs), the emerging genetic contaminants, are regarded as severe risks to public health for impairing the inactivation efficacy of antibiotics. Secondary effluents from wastewater treatment plants are the hotspots for spreading these menaces. Herein, sulfidated nanoscale zero-valent iron (S-nZVI) was occupied to remove ARGs and MGEs in secondary effluents and weaken the regrowth capacity of their bacterial carriers. The effects of S/Fe molar ratios (S/Fe), initial pH and dosages on 16S rRNA and ARGs removal were also investigated. Characterization, mass balance and scavenging experiments were conducted to explore the mechanisms of the gene removal. Quantitative PCR (qPCR) and high throughput fluorescence qPCR showed more than 3 log unit of 16S rRNA and seven out of 10 ARGs existed in secondary effluent could be removed after S-nZVI treatment. The mechanisms might be that DNA accepted the electron provided by the Fe⁰ core of S-nZVI after being adsorbed onto S-nZVI surface, causing the decrease of 16S rRNA, ARGs and lost their regrowth capacity, especially for typical MGE (intI1) and further inhibiting the vertical gene transfer (VGT) and intI1-induced horizontal gene transfer (HGT). Fe⁰ core was oxidized to iron oxides and hydroxides at the same time. High throughput sequencing, network analysis and variation partitioning analysis revealed the complex correlations between bacteria and ARGs in secondary effluent, S/Fe could directly influence ARGs variations, and bacterial genera made the greatest contribution to ARGs variations, followed by MGEs and operational parameters. As a result, S-nZVI could be an available reductive approach to deal with bacteria and ARGs.

Quantifying the efficiency and selectivity of organohalide dechlorination by zerovalent iron

The efficiency and selectivity of zerovalent iron-based treatments for organohalide contaminated groundwater can be quantified by accounting for redistribution of electrons derived from oxidation of Fe⁰. Several types of efficiency are reviewed, including (i) the efficiency of Fe(0) utilization, ε_Fe(0), (ii) the electron efficiency of target contaminant reduction, ε_e, and (iii) the electron efficiency of natural reductant demand (NRD) involving H₂O, O₂, and co-contaminants such as nitrate, ε_NRD. Selectivity can then be calculated by using ε_e/ε_NRD. Of particular interest is ε_e and the key to its determination is measuring the total quantity of electrons provided by Fe⁰ oxidation, which can be based on either the loss of Fe(0), the formation of Fe(ii)/Fe(iii), or the composition of the total reaction products. Recently, many data have accumulated on ε_e for the treatment of various chlorinated solvents (esp. trichloroethylene, TCE) by zerovalent iron (ZVI), and analysis of these data shows that ZVI particle properties (e.g., stabilization with polymers, bimetallic modification, sulfidation, etc.) and other operational factors have variable effects on ε_e. Of particular interest is that pre-exposure of ZVI to reduced sulfur species (i.e., sulfidation) consistently improves the ε_e of contaminant reduction, mainly by suppressing the reduction of water.

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.

Abiotic Degradation of Chlorinated Solvents by Clay Minerals and Fe(II): Evidence for Reactive Mineral Intermediates

For decades, there has been evidence that Fe-containing minerals might contribute to abiotic degradation of chlorinated ethene (CE) plumes. Here, we evaluated whether Fe(II) in clay minerals reduces tetrachloroethene (PCE) and trichloroethene (TCE). We found that structural Fe(II) in both low (SWy-2) and high (NAu-1) Fe clay minerals did not reduce PCE or TCE under anoxic conditions. There was also no reduction of PCE or TCE after adding 5 mM dissolved Fe(II) to the clay mineral suspensions. In the presence of high Fe(II) concentrations (20 mM), however, PCE and TCE reduction products were observed in the presence of low Fe-content clay mineral SWy-2. Mössbauer spectroscopy results indicate that a mixed-valent Fe(II)-Fe(III) precipitate formed in the reactive SWy-2 suspensions. In contrast, in suspensions containing 20 mM Fe(II) alone or Fe-free clay mineral (Syn-1), we observed a purely Fe(II)-containing precipitate (Fe(OH)₂) and also PCE and TCE reduction products. Interestingly, the amount of CE products decreased in the order of Fe-free clay mineral Syn-1 > Fe(OH)₂ > low Fe-content clay mineral SWy-2, suggesting that clay mineral Fe controlled the formation of the reactive mineral phase. Additional experiments with hexachloroethane (HCA) revealed that faster HCA reduction occurred with decreasing clay mineral Fe content. Kinetic modeling yielded invariable second-order rate constants and increasing concentrations of reactive Fe(II) as the Fe(II)/Fe(total) content of the precipitates increased. Our data suggest that clay mineral Fe(III) is a sink for electrons from added Fe(II) that otherwise might have reduced the CEs. Furthermore, our findings are consistent with the hypothesis that active precipitation of Fe(II)-containing reactive mineral intermediates (RMI) may be important to CE reduction and suggest that RMI formation depends on clay mineral presence and Fe content.

Water Treatment Using Metallic Iron: A Tutorial Review

Researchers and engineers using metallic iron (Fe0) for water treatment need a tutorial review on the operating mode of the Fe0/H2O system. There are few review articles attempting to present systematic information to guide proper material selection and application conditions. However, they are full of conflicting reports. This review seeks to: (i) Summarize the state-of-the-art knowledge on the remediation Fe0/H2O system, (ii) discuss relevant contaminant removal mechanisms, and (iii) provide solutions for practical engineering application of Fe0-based systems for water treatment. Specifically, the following aspects are summarized and discussed in detail: (i) Fe0 intrinsic reactivity and material selection, (ii) main abiotic contaminant removal mechanisms, and (iii) relevance of biological and bio-chemical processes in the Fe0/H2O system. In addition, challenges for the design of the next generation Fe0/H2O systems are discussed. This paper serves as a handout to enable better practical engineering applications for environmental remediation using Fe0.

Effects of Sulfidation and Nitrate on the Reduction of N-Nitrosodimethylamine by Zerovalent Iron

Competition among oxidizing species in groundwater and wastewater for the reductive capacity of zerovalent iron (ZVI) makes the selectivity of ZVI for target contaminant degradation over other reduction pathways a major determinant of the feasibility of ZVI-based water treatment processes. The selectivity for reduction of contaminants over water is improved by sulfidation, but the effect of sulfidation on other competing reactions is not known. The interaction between these competing reactions was investigated using N-nitrosodimethylamine (NDMA) as the target contaminant, nitrate as a co-contaminant, and micrometer-sized ZVI with and without sulfidation. Unsulfidated ZVI reduced NDMA to dimethylamine via N,N-dimethylhydrazine, but the addition of nitrate decreased the rate of NDMA reduction and increased the quantity of intermediate observed. With sulfidated ZVI, the kinetics and products of NDMA reduction were similar to those with unsulfidated ZVI, but no inhibitory effect of nitrate was observed. Conversely, the reduction of nitrate-which dominated NDMA reduction in unsulfidated ZVI systems-was strongly inhibited by sulfidation. H₂ and Fe²⁺ generation by sulfidated ZVI was almost independent of nitrate concentration. Therefore, sulfidation improved the efficiency of NDMA reduction by ZVI in the presence of nitrate mainly by inhibiting nitrate reduction. The shift in selectivity of ZVI for NDMA over nitrate upon sulfidation was due to replacement of Fe⁰/Fe_xO_y surface sites with FeS.

Sulfidation of ZVI/AC composite leads to highly corrosion-resistant nanoremediation particles with extended life-time

Nanoscale zero-valent iron (nZVI) is a powerful reductant for many water pollutants. The lifetime of nZVI in aqueous environments is one of its limitations. Sulfidation of the nZVI surface by reduced sulfur species is known to significantly modify the particle properties. In the present study we examined various post-synthesis sulfidation methods applied on Carbo-Iron, a composite material where iron nanostructures are embedded in colloidal activated carbon (AC) particles. In such cases, where ZVI is surrounded by carbon, sulfidation largely inhibits the anaerobic corrosion of ZVI in water whereas its dechlorination activity was slightly increased. Even at a very low molar S/Fe ratio of 0.004 a strong decrease of the corrosion rate by a factor of 65 was achieved, while concurrently dechlorination of tetrachloroethene (PCE) was accelerated by a factor of three compared to the untreated particles. As a consequence, over 98% of the reduction equivalents of the sulfidated ZVI were utilized for the reduction of the target contaminant (33 mg L^-1 PCE) under simulated groundwater conditions. In a long-term experiment over 160 days the extended life-time and the preservation of the reduction capacity of the embedded ZVI were confirmed. Reasons for the modified reaction behavior of Carbo-Iron after sulfidation compared to previously studied nZVI are discussed. We hypothesize that the structure of the carbon-embedded iron is decisive for the observed reaction behavior. In addition to reaction rates, the product pattern is vastly different compared to that of sulfidated nZVI. The triple combination of ZVI, AC and sulfur makes the composite particle very suitable for practical in-situ applications.

Fe0/H2O Filtration Systems for Decentralized Safe Drinking Water: Where to from Here?

Inadequate access to safe drinking water is one of the most pervasive problems currently afflicting the developing world. Scientists and engineers are called to present affordable but efficient solutions, particularly applicable to small communities. Filtration systems based on metallic iron (Fe0) are discussed in the literature as one such viable solution, whether as a stand-alone system or as a complement to slow sand filters (SSFs). Fe0 filters can also be improved by incorporating biochar to form Fe0-biochar filtration systems with potentially higher contaminant removal efficiencies than those based on Fe0 or biochar alone. These three low-cost and chemical-free systems (Fe0, biochar, SSFs) have the potential to provide universal access to safe drinking water. However, a well-structured systematic research is needed to design robust and efficient water treatment systems based on these affordable filter materials. This communication highlights the technology being developed to use Fe0-based systems for decentralized safe drinking water provision. Future research directions for the design of the next generation Fe0-based systems are highlighted. It is shown that Fe0 enhances the efficiency of SSFs, while biochar has the potential to alleviate the loss of porosity and uncertainties arising from the non-linear kinetics of iron corrosion. Fe0-based systems are an affordable and applicable technology for small communities in low-income countries, which could contribute to attaining self-reliance in clean water supply and universal public health.

The Impact of Selected Pretreatment Procedures on Iron Dissolution from Metallic Iron Specimens Used in Water Treatment

Studies were undertaken to determine the reasons why published information regarding the efficiency of metallic iron (Fe0) for water treatment is conflicting and even confusing. The reactivity of eight Fe0 materials was characterized by Fe dissolution in a dilute solution of ethylenediaminetetraacetate (Na2–EDTA; 2 mM). Both batch (4 days) and column (100 days) experiments were used. A total of 30 different systems were characterized for the extent of Fe release in EDTA. The effects of Fe0 type (granular iron, iron nails and steel wool) and pretreatment procedure (socking in acetone, EDTA, H2O, HCl and NaCl for 17 h) were assessed. The results roughly show an increased iron dissolution with increasing reactive sites (decreasing particle size: wool > filings > nails), but there were large differences between materials from the same group. The main output of this work is that available results are hardly comparable as they were achieved under very different experimental conditions. A conceptual framework is presented for future research directed towards a more processed understanding.