No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Effects of dissolved oxygen on formation of corrosion products and concomitant oxygen and nitrate reduction in zero-valent iron systems with or without aqueous Fe2+
Abstract
Batch tests were conducted in zero-valent iron (ZVI or Fe0) systems to investigate oxygen consumption and the effect of dissolved oxygen (DO) on formation of iron corrosion products, nitrate reduction, the reactivity of Fe0, the role Fe2+ (aq) played, and the fate of Fe2+. The study indicates that without augmenting Fe2+ (aq), neither nitrate nor DO could be removed efficiently by Fe0. In the presence of Fe2+ (aq), nitrate and DO could be reduced concomitantly with limited interference with each other. Unlike nitrate reduction, DO removal by Fe0 did not consume Fe2+ (aq). A two-layer structure, with an inner layer of magnetite and an outer layer of lepidocrocite, may be formed in the presence of DO. When DO depleted, the outer lepidocrocite layer was transformed to magnetite. The inner layer of magnetite, even in a substantial thickness, might not impede the Fe0 reactivity as much as the thin interfacial layer between the oxide coating and liquid. Surface-bound Fe2+ may greatly enhance the electron transfer from the Fe0 core to the solid-liquid interface, and thus improve the performance of the Fe0 process.
... Over the past decades, metallic iron (Fe(0)), referred to as zero-valent iron (ZVI), has attracted significant attention for the remediation and treatment of water polluted with a wide range of contaminants [12]. ZVI has proven to be an effective material for removing multiple contaminants, including arsenic species [13][14][15], NO 3 − [16][17], NO 2 − , hexavalent chromium [18], heavy metals [19,20], as well as halogenated and nitrated organic compounds [21]. In the last 20 years, numerous studies have been conducted to assess the dominant processes of contaminant removal in ZVI/H 2 O systems [14,17,[22][23][24][25]. ...
... ZVI has proven to be an effective material for removing multiple contaminants, including arsenic species [13][14][15], NO 3 − [16][17], NO 2 − , hexavalent chromium [18], heavy metals [19,20], as well as halogenated and nitrated organic compounds [21]. In the last 20 years, numerous studies have been conducted to assess the dominant processes of contaminant removal in ZVI/H 2 O systems [14,17,[22][23][24][25]. The investigated experimental conditions include iron source, particle size, presence of dissolved oxygen, contaminant concentration, solution chemistry, chemical modification of the particle surface, the use of inert materials mixed with ZVI in different proportions, and the material Abbreviations: ANN, artificial neural network; CA, cluster analysis; CL, corrosion layer; D, input data matrix; E, square error; f, number of neurons in the input layer; HIPR, holdback input parameter randomization method; HRT, hydraulic retention time; LS, learning set; MSE, mean square error; m ZVI , ZVI loading; NH, number of neurons in the hidden layer; P, loadings matrix in PCA; PC, principal component; PCA, principal component analysis; r, number of neurons in the output layer; RSSCT, rapid small-scale column tests; T, scores matrix in PCA; TS, test set; VS, validation set; X 1 , X 2 , X 3 , controlled factors; Y, response variables; ZVI, zerovalent iron; δ ZVI , ZVI packing density. ...
... Ferrous species are relatively soluble for the typical pH ranges of natural waters. In contrast, Fe(III) species are rather insoluble above pH 4, and their precipitation onto the ZVI surface leads to the formation of a corrosion layer (CL) [17,37]. The precise structure of the CL depends on the aqueous phase composition, the hydrodynamic conditions, and the contribution of aging processes [17,19,35]. ...
Multivariate statistical techniques and artificial neural networks (ANNs) were used for the analysis, interpretation, and modeling of the results obtained in the study of zero-valent iron (ZVI) reactive beds designed for contaminant removal. A wide range of operating conditions was evaluated through more than 120 rapid small-scale column tests (RSSCT). The production of Fe(II) and Fe(III) species, dissolved oxygen consumption, and pH variation along the reactive bed were used as response variables for evaluating the process performance. Due to the complexity of the system, and the difficulty in defining and fitting kinetic parameters, ANN models were used to simulate the system without the need for kinetic expressions. Therefore the latter were used for assessing the system behavior within the investigated experimental domain and for evaluating the relative importance of the operating factors. In addition, the application of the multivariate techniques cluster analysis (CA) and principal component analysis (PCA) revealed underlying relationships among the response variables. Moreover, although multiple physicochemical processes are involved, the results obtained through PCA indicate that the main trends can be rationalized by considering a few key reactions only. The strategy of analyzing RSSCT results with different numerical techniques provides valuable knowledge for designing real-scale ZVI-based treatments aimed at the efficient elimination of a wide range of contaminants in the aqueous phase.
... To avoid the inhibition of denitrification, several studies have been conducted under anoxic conditions, by purging the solution with nitrogen gas [16,47,57] or argon gas [49,52,53]. On the other hand, some works were performed under oxic conditions to assess the effect of the DO on the denitrification process performance [58][59][60][61][62][63][64]. ...
... However, Huang and Zhang suggest that oxygen did not influence denitrification by Fe 0 when used together with dissolved Fe 2+ [59]. Indeed, when utilizing both magnetitecoated Fe 0 and uncoated Fe 0 to treat a 100 mg/L nitrate solution, in the presence of Fe 2+ (aq), no differences were noticed in the gradual reduction in the Fe 2+ , in the stable increase in the pH, or in the formation of a large quantity of black precipitate, with or without O2. ...
... This brownish precipitate was presumably lepidocrocite (γ-FeOOH), while the black precipitate was magnetite (Fe3O4). On the basis of the two-layer semiconductor model, the coating of oxide formed on the iron surface is composed of an inner layer of magnetite and an outer layer of maghemite or lepidocrocite [59]. It was observed that electrons can migrate more easily in the magnetite layer than in the maghemite or lepidocrocite coating [59]. ...
Nitrate is a widespread water contaminant that can pose environmental and health risks. Various conventional techniques can be applied for the removal of nitrate from water and wastewater, such as biological denitrification, ion exchange, nanofiltration, and reverse osmosis. Compared to traditional methods, the chemical denitrification through zero-valent metals offers various advantages, such as lower costs, simplicity of management, and high efficiencies. The most utilized material for chemical denitrification is zero-valent iron (ZVI). Aluminium (ZVA), magnesium (ZVM), copper (ZVC), and zinc (ZVZ) are alternative zero-valent metals that are studied for the removal of nitrate from water as well as from aqueous solutions. To the best of our knowledge, a comprehensive work on the use of the various zero-valent materials that are employed for the removal of nitrate is still missing. Therefore, in the present review, the most recent papers concerning the use of zero-valent materials for chemical denitrification were analysed. The studies that dealt with zero-valent iron were discussed by considering microscopic (mZVI) and nanoscopic (nZVI) forms. For each Fe 0 form, the effects of the initial pH, the presence or absence of dissolved oxygen, the initial nitrate concentration, the temperature, and the dissolved ions on the nitrate removal process were separately evaluated. Finally, the different materials that were employed as support for the nanoparticles were examined. For the other zero-valent metals tested, a detailed description of the works present in the literature was carried out. A comparison of the various features that are related to each considered material was also made.
... Finally, by calculating the sum of the MA-SHAP values of each Given this condition, this finding confirms that the availability of active sites plays the most critical role in enhancing the reduction process, followed by the presence of enough H + to accelerate iron corrosion and to create new active sites. Most importantly, this finding is able to explain the conflicting results regarding the impact of dissolved oxygen on nitrate reduction rate [39,[84][85][86][87][88]. The preferable oxygen conditions are aerobic conditions, which encourage the production of additional reducers, and anoxic conditions, which prevent the inhibition of denitrification [3]. ...
... Notably, the discussion of mechanism interpretation based on data-driven modeling is limited to the variables included in the modeling process. An additional feature with the exact value of the dissolved oxygen concentration might be more representative to emphasize the effect of the oxygen present in the nitrate reduction process by ZVI [86,99]. Furthermore, the anion concentration feature may also improve the model performance since it has been reported to affect the nitrate reduction process [100]. ...
... Chemical denitrification of nitrates by means of zero-valent metals has attracted a great deal of interest, with a special focus on zero-valent iron (ZVI) [142,[166][167][168]. Relevant investigations on the use of bimetallic catalysts, using H2 as reductant agent, are also significant [169][170][171][172]. ...
... The presence of lepidocrocite can slow down the nitrate reduction because the transition of electrons is easier in magnetite. [168]. ...
Nitrate (NO3−) pollution of surface and groundwater bodies is a global problem of increasing concern, which has stimulated significant research interest. Nitrogen is crucial for life as a macronutrient for living organisms on Earth, but the global nitrogen cycle has been seriously altered by intensification of human activities, leading to eutrophication and hypoxic conditions of aquatic ecosystems. Due to nitrogen overfertilization, intensive agricultural practices generate huge nitrate fluxes that inadvertently deteriorate water quality. Different industrial processes also contribute to NO3− pollution in the environment. There are multiple technologies capable of achieving effective denitrification of waterbodies to ensure safe NO3− levels. Either separation-based or transformation-based denitrification technologies must address the challenges of by-product generation, increased energy demand, and reduced environmental footprint. This paper highlights the most used approaches, along with some promising alternatives for remediation of nitrate-polluted waters.
... Nitrate nondestructive treatment methods, such as electrodialysis, ion exchange, and reverse osmosis, produce residual streams with high nitrate concentrations that require proper disposal [3][4][5][6][7]. The commonly used nitrate-destructive treatment method, biological denitrification, produces water that necessitates deep postdisinfection and waste sludge that needs to be dealt with [8][9][10]. ...
... The low conversion of nitrate by the AC-supported catalyst could be attributed to the absence of transition metal Cu as seen in ICP results. It is well known that the presence of transition metal is mandatory for the conversion of nitrate [6]. As we can see, 4Pd-1Cu/FNTC2 and 4Pd-1Cu/SBA-15 show the lowest catalytic activity although they have advantages in terms of smooth mass transfer of reactants and products during the reaction due to its mesopores structure. ...
As the population grows and the demand for water rises, the development of efficient and sustainable water purification techniques is becoming increasingly important to ensure access to clean and safe water in the future. The pollution of surface and groundwater by nitrate ( NO 3 − ) is a growing global concern due to the rise in nitrogen-rich waste released from agriculture and industry. The removal of nitrate ions from aqueous media using bimetallic catalysts loaded on several supports was studied. Multiwalled carbon nanotubes, activated carbon, titanium dioxide, titanium dioxide/multiwalled carbon nanotubes, and Santa Barbara Amorphous-15 were used as supports to synthesize these bimetallic catalysts. The effects of the support type, supported metal, and catalyst reduction method on the nitrate reduction activity in water were investigated. The catalysts were characterized by X-ray diffraction, fourier-transform infrared spectroscopy, Brunauer-Emmett-Teller isotherm, inductively coupled plasma spectroscopy, and field emission gun scanning transmission electron microscope. In terms of nitrate conversion, high-temperature hydrogen reduction of the catalysts was a more effective method of catalyst preparation than NaBH4 reduction. Except for the carbon nanotube-TiO2 composite, pH fixation using CO2 flow improved the efficiency of supported catalysts. The catalysts 1Pd–1Cu/TiO2 and 1Pd–Cu/SBA-15 presented the highest catalytic activity, but the latter was the most selective to nitrogen.
... Similar results were also observed with Fe (OH) 2 unless Fe +2 was added. These results are consistent with Huang and Zhang (2005), who reported that Fe +2 sorbed onto corroded ZVI, which developed a lepidocrocite-magnetite surface coating, promoted nitrate reduction; a lack of sorbed Fe 2+ decreased reactivity and contaminant removal. The literature collectively reported that free Fe +2 reacts slowly with some contaminants having reduction potentials above that of the transition of Fe +3 to Fe +2 and plays a critical role in co-precipitation and some redox reactions. ...
... Dissolved oxygen, when available, also accepts electrons to facilitate iron corrosion. Moreover, contaminants and other redox-active species in groundwater also accept electrons, cathodically promoting the corrosion of ZVI when thermodynamically feasible (Gillham and O'Hannesin, 1994;Puls et al., 1999;Huang and Zhang, 2005). ...
Permeable reactive barriers (PRBs) are used for groundwater remediation at contaminated sites worldwide. This technology has been efficient at appropriate sites for treating organic and inorganic contaminants using zero-valent iron (ZVI) as a reductant and as a reactive material. Continued development of the technology over the years suggests that a robust understanding of PRB performance and the mechanisms involved is still lacking. Conflicting information in the scientific literature downplays the critical role of ZVI corrosion in the remediation of various organic and inorganic pollutants. Additionally, there is a lack of information on how different mechanisms act in tandem to affect ZVI-groundwater systems through time. In this review paper, we describe the underlying mechanisms of PRB performance and remove isolated misconceptions. We discuss the primary mechanisms of ZVI transformation and aging in PRBs and the role of iron corrosion products. We review numerous sites to reinforce our understanding of the interactions between groundwater contaminants and ZVI and the authigenic minerals that form within PRBs. Our findings show that ZVI corrosion products and mineral precipitates play critical roles in the long-term performance of PRBs by influencing the reactivity of ZVI. Pore occlusion by mineral precipitates occurs at the influent side of PRBs and is enhanced by dissolved oxygen and groundwater rich in dissolved solids and high alkalinity, which negatively impacts hydraulic conductivity, allowing contaminants to potentially bypass the treatment zone. Further development of site characterization tools and models is needed to support effective PRB designs for groundwater remediation.
... With NO 3 -, the decline of dissolved Fe(II) and p-HBA concentration in mZVI bm /O 2 system reached 33.26 % and 28.59 %, respectively, which was apparently larger than that in S-mZVI bm /O 2 system (5.89 % and 5.71 %, respectively). Commonly, NO 3 is a well-documented corrosion inhibitor that cause the passivation of ZVI surface, and its coexistence with O 2 can result in a thicker layer of iron oxides to prevent the iron dissolution, decrease the electron transfer and reactivity of Fe(0) [11,39,40], whereas the presence of FeS x sites can weaken the interaction with NO 3 to reduce the passivation influence [41]. Additionally, no significant differences of dissolved Fe(II) and p-HBA formation were observed in S-mZVI bm /O 2 system with different NO 3 concentration (Fig. S6), which further verified the corrosion resistance of S-mZVI bm towards NO 3 -. ...
Under oxic environment, the corrosion of zero valent iron (ZVI) connects with its oxidation and adsorption capacity, but little is known about the effects of sulfidation on those processes of ZVI. To assess the oxidant production from ball-milling unsulfidated and sulfidated microscale ZVI (mZVIbm and S-mZVIbm), the transformation of benzoic acid (BA) to p-hydroxybenzoic acid (p-HBA) was used as the oxidation probe reaction. During the oxidation reaction, the concentration of p-HBA in S-mZVIbm/O2 systems rapidly reached equilibrium (< 12 μM) within 40 min regardless of sulfur loading content, whereas it could gradually reach 26 μM in mZVIbm/O2 system within 240 min despite the slower generation of p-HBA at initial stage. Combining with the characteristic analysis, the lower oxidation capacity of S-mZVIbm/O2 system might be ascribed to the enhanced Fe(II) dissolution that caused a larger solution pH rise along with the loss of Fe(0) and FeSx sites and the formation of lepidocrocite, which was not beneficial for oxygen activation. Under the identical conditions such as material dosage and co-existing anions, the oxidation capacity of mZVIbm/O2 system also exceeded S-mZVIbm/O2 system. In addition, the comparative experiment of Zn²⁺ immobilization by S-mZVIbm and S-mZVIbm with or without BA was investigated. mZVIbm showed insignificant Zn²⁺ immobilization, whereas BA could enhance Zn²⁺ immobilization by S-mZVIbm due to the enhanced corrosion that could provide more adsorption sites. Overall, this work is valuable for assessing the oxidation capacity of S-mZVIbm/O2 system and BA effect on heavy metal immobilization of S-mZVI in wastewater treatment.
... DO impacts the NO 3 − -N reductase activity and metabolic pathways, thus affecting the transfer of electron donors during denitrification (Ji et al., 2015). Therefore, an appropriate amount of DO is essential for NO 3 − -N biodegradation (Chen et al., 2018a;Huang and Zhang, 2005). Generally, denitrifying microorganisms are anaerobic or parthenogenic, and the optimal DO amount is 0.02-2 mg/L. ...
In recent years, iron mediated autotrophic denitrification has been a concern because it overcomes the absence of organic carbon and has been successfully used in denitrification for low C/N ratio wastewater. However, there is currently a lack of a more systematic summary of iron-based materials that can be used for denitrification, and no detailed overview about the mechanism of iron mediated autotrophic denitrification has been reported. In this study, the iron materials with different valence states that can be used for denitrification were summarized, and emphasized, as well as the mechanism in different interaction systems were emphasize. In addition, the contribution of various microorganisms in nitrate reduction were analyzed and the effects of operating conditions and water quality were evaluated. Finally, the challenges and shortcomings of the denitrification process were discussed aiming to find better practical engineering applications of iron-based denitrification.
... When the oxygen in soil is insufficient, Fe(OH) 2 reacts with oxygen to generate magnetite (Fe 3 O 4 ): 38,44 3Fe(OH) 2 ...
In this paper, the electrochemical corrosion behaviour of Q235, X65, X70, and X80 low-carbon steel was systematically studied by a variety of test techniques using natural saline soil containing 1.1% salt under laboratory conditions. The electrochemical corrosion behaviour, macro-micro corrosion morphology, and corrosion product composition of these four low-carbon steels in saline soil were studied to explore their salt corrosion resistance and reveal their corrosion mechanisms. The research results showed that oxygen absorption corrosion occurred in all four low-carbon steels in the saline soil, and the corrosion types were all localised corrosion. The corrosion process of Q235 steel was controlled by mass transfer, while the corrosion processes of X65, X70, and X80 steel were controlled by charge transfer. The corrosion rates of these four low-carbon steels in saline soil followed the order Q235 > X65 ≈ X70 > X80. Variation in elemental composition was the main reason for this difference in corrosion behaviour. Finally, microscopic test results showed that local corrosion pits were present on the surface of the steel sheet specimens, and the uniformity and compactness of the corrosion product accumulation were poor.
... The latter, which is prevalent in nature, has a faster kinetics. 33,34 An electron transfer pathway originates by local areas of the surface acting as cathode and anode, leading to the spontaneous formation of ionic Fe species ready to react with the available foreign ions in the local environments. 35 More local anodic and cathodic sites at different regions of Fe foam form with optimizing the corrosion engineering with guest ion concentration and time. ...
... Fe(II) abundance increases from 11.7% to 18.9% after the reaction, suggesting the conversion of some ≡Fe(III) to ≡Fe(II). Previous studies reported that the Fe 0 core could contribute to the conversion of ≡Fe(III) to ≡Fe(II) (Shi et al., 2014;Huang and Zhang, 2005). Further, Fig. 4d shows that the intensity of Fe-O bonds (530.1 eV) in O 1 s spectra increases after the reaction. ...
Direct catalytic decomposition of methane (CDM) has been studied as a possible emission-free hydrogen production route for over 100 years. However, the high cost of catalyst regeneration limits its practical applications. Here, we demonstrate that the solid by-product from CDM using Fe ore catalysts compromising carbon nano onions encapsulated with magnetic Fe cores ([email protected]) can serve as efficient and recyclable Fenton catalysts for pollutant degradation. [email protected] /H2O2 has better performance than FeSO4/H2O2 at similar Fe concentrations and can be used to decompose various pollutants. Mechanistic studies reveal that graphitic carbon layers and encapsulated Fe⁰ contribute to their high catalytic activity. Further, [email protected] can be easily recovered from an aqueous solution and reused due to the encapsulated magnetic Fe particles. Over three reused cycles, [email protected] /H2O2 only yields 1/8 of Fe sludges compared to FeSO4/H2O2, significantly reducing Fe sludge treatment costs. Overall, [email protected] demonstrates excellent application potentials in water and wastewater treatment, making H2 production via CDM economically more viable.
... During nitrification, the increased nitrate level may react with metallic pipes, causing a continuous supply of ammonium at the metal surface that can support the nitrifying bacterial growth. 4,118 As shown in reaction (56), ammonium can be recycled from nitrate in iron pipes. McIntyre and Mercer 119 detected a complete conversion of nitrate to ammonium in GI pipes during stagnation. ...
Major pathways of monochloramine disinfectant decay, kinetics involved, various influencing factors and the existing models to determine the chloramine decay in drinking water distribution systems are reviewed.
... In addition, Fe(II) may have facilitated the autoreduction of lepidocrocite to magnetite (Yoon et al., 2011). From the viewpoint of electron transfer, magnetite can be considered a semiconductor (10 2 -10 3 Ω -1 cm -1 ) and is believed to facilitate the electron transfer from the ZVI core to the solid-liquid interface (Huang and Zhang, 2005). At the same time, lepidocrocite has poor conductivity (bandgap of 2.3 eV) and is mainly responsible for the passivation of ZVI (Mu et al., 2017). ...
Selenium (Se) is an essential element with application in manufacturing from food to medical industries. Water contamination by Se is of concern due to anthropogenic activities. Recently, Se remediation has received increasing attention. Hence, different types of remediation techniques are listed in this work, and their potential for Se recovery is evaluated. Sorption, co-precipitation, coagulation and precipitation are effective for low-cost Se removal. In photocatalytic, zero-valent iron and electrochemical systems, the above mechanisms occur with reduction as an immobilization and detoxification process. In combination with magnetic separation, the above techniques are promising for Se recovery. Biological Se oxyanions reduction has been widely recognized as a cost-effective method for Se remediation, simultaneously generating biosynthetic Se nanoparticles (BioSeNPs). Increasing the extracellular production of BioSeNPs and controlling their morphology will benefit its recovery. However, the mechanism of the microbial production of BioSeNPs is not well understood. Se containing products from both microbial reduction and abiotic methods need to be refined to obtain pure Se. Eco-friendly and cost-effective Se refinery methods need to be developed. Overall, this review offers insight into the necessity of shifting attention from Se remediation to Se recovery.
... When the aging time exceeds 24 h, γ-FeOOH could continue to transfer to Fe 3 O 4 phase through the following reaction pathways (Eqs. 5 and 6) (Huang and Zhang, 2005;Liu et al., 2015;Zhang and Huang, ...
The naturally-formed iron (hydr)oxides on the surface of zero valent iron (ZVI) have long been considered as passivation layer and inert phases which significantly reduce the reaction activities when they are employed in environmental remediation. Although it seems there are no direct benefits to keep these passivation layers, here, we show that such phases are necessary intermediates for the transformation to iron sulfides through an anion exchange pathway during sulfidation of ZVI. The pre-formed (hydr)oxides undergo a phase evolution upon aging and specific phases can be effectively trapped, which can be confirmed by a combination of different characterization techniques including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRPD), and X-ray absorption near edge structure (XANES) spectroscopy (XANES). Interestingly, after sulfidation, the resultant samples originated from different (hydr)oxides demonstrate different activities in the Cr(VI) sequestration. The XANES investigation of Fe K edge and Fe L2,3 edge indicates Fe remains the same after sulfidation, suggesting a non-redox, anion exchange reaction pathway for the production of iron sulfides, where O²⁻ anions are directly replaced with S²⁻. Consequently, the structural characteristics of the parent (hydr)oxides are inherited by the as-formed iron sulfides, which make them behave differently because of their different structural natures.
... In fact, commercial Fe 0 specimens used in water treatment are covered by a pre-existing oxide scale consisting of an inner Fe3O4 layer and an outer Fe2O3 layer. The inner layer is electronically conductive by virtue of the semi-conductive nature of Fe3O4 (band gap: 0.11 eV) Simpraga 2004, Huang andZhang 2005). However, electron transport is hindered by the outer non-conductive Fe2O3 layer. ...
The suitability of remediation systems using metallic iron (Fe0) has been extensively discussed during the past 3 decades. It has been established that aqueous Fe0 oxidative dissolution is not caused by the presence of any contaminant. Instead, the reductive transformation of contaminants is a consequence of Fe0 oxidation. Yet researchers are still maintaining that electrons from the metal body are involved in the process of contaminant reduction. According to the electron efficiency concept, electrons from Fe0 should be redistributed to: i) contaminants of concern (COCs), ii) natural reducing agents (e.g., H2O, O2), and/or iii) reducible co-contaminants (e.g. NO3-). The electron efficiency is defined as the fraction of electrons from Fe0 oxidation which is utilized for the reductive transformations of COCs. This concept is in frontal contradiction with the view that Fe0 is not directly involved in the process of contaminant reduction. This communication recalls the universality of the concept that reductive processes observed in remediation Fe0/H2O systems are mediated by primary (e.g., FeII, H/H2) and secondary (e.g., Fe3O4, green rusts) products of aqueous iron corrosion. The critical evaluation of the electron efficiency concept suggests that it should be abandoned. Instead, research efforts should be directed towards tackling the real challenges for the design of sustainable Fe0-based water treatment systems based on fundamental mechanisms of iron corrosion.
... Ryu also observed nitrate reduction to ammonium using zero-valent iron [32]. Higher dissolved oxygen concentrations have been shown to increase reactions between nitrates in water and zero-valent iron particles, and dissolved oxygen levels were reduced after the introduction of ZVI particles [33], as demonstrated also in this study. ...
Groundwater depletion is a critical agricultural irrigation issue, which can be mitigated by supplementation with water of higher microbiological risk, including surface and reclaimed waters, to support irrigation needs in the United States. Zero-valent iron (ZVI) filtration may be an affordable and effective treatment for reducing pathogen contamination during crop irrigation. This study was performed to determine the effects of ZVI filtration on the removal and persistence of Escherichia coli, and pepper mild mottle virus (PMMoV) in irrigation water. Water was inoculated with E. coli TVS 353, filtered through a ZVI filtration unit, and used to irrigate cucurbit and cruciferous crops. Water (n = 168), leaf (n = 40), and soil (n = 24) samples were collected, the E. coli were enumerated, and die-off intervals were calculated for bacteria in irrigation water. Variable reduction of PMMoV was observed, however E. coli levels were consistently and significantly (p < 0.05) reduced in the filtered (9.59 lnMPN/mL), compared to unfiltered (13.13 lnMPN/mL) water. The die-off intervals of the remaining bacteria were significantly shorter in the filtered (−1.50 lnMPN/day), as compared to the unfiltered (−0.48 lnMPN/day) water. E. coli transfer to crop leaves and soils was significantly reduced (p < 0.05), as expected. The reduction of E. coli in irrigation water and its transfer to crops, by ZVI filtration is indicative of its potential to reduce pathogens in produce pre-harvest environments.
... One possible hypothesis is that the low concentrations of strongly adsorbing phosphate ions in the groundwater throughout the field trial (Non-Detect ± 0.1 mg/L) could have prevented the growth of surface layers on the electrodes in low current density reactor (van Genuchten et al., 2014a). Another hypothesis is the presence of high levels of dissolved oxygen (post electrolysis, 7.4 ± 0.3 mg/L), which could have prevented the formation of a magnetite surface layer, as seen in other Fe-EC systems (Huang and Zhang, 2005;van Genuchten et al., 2016). ...
Small, low-income, and rural communities across the United States are disproportionately exposed to arsenic contaminated drinking water because existing treatment solutions are too expensive and difficult to operate. This paper describes efforts to overcome some barriers and limitations of conventional iron electrocoagulation (Fe-EC) to enable its use in the rural Californian (U.S.) context. Barriers and limitations of Fe-EC's application in rural California considered in this work include: 1) Frequent labor intensive electrode cleaning is required to overcome rust accumulation, 2) Electrolysis durations are long, reducing throughput for a given system size, and 3) Waste needs compliance with California standards. We report results from an investigation for overcoming these limitations via a field trial on a farm in Allensworth, a small, low-income, rural community in California. Our strategies to overcome each of the above barriers and limitations are respectively, 1) operating the Fe-EC reactor at high current density to result in sustained Fe production, 2) operating at high charge dosage rate with external H2O2, and 3) characterization of the arsenic-laden waste, and are discussed further in the paper. Main findings are: (1) Fe-EC removed arsenic consistently below the federal (and state) standard of 10 µg/L, (2) high current density failed to sustain Fe production whereas low current density did not, (3) electrolysis time decreased from >1 hour to <2 min with H2O2 dosing of 5 mg/L at higher charge dosage rates, and (4) dilution of As-sludge is required to comply with State's non-hazardous waste status, (5) discrepancies were observed between lab and field results in using current density to overcome labor-intensive electrode cleanings. Finally, implications of overcoming limitations to scale-up of Fe-EC in relevant California communities are discussed.
... Characterization results suggested that with the increase of the initial Fe(II) concentration, the fractions of Fe 0 and γ-Fe 2 O 3 in the Se(VI)-treated ZVI under aerobic conditions dropped, while that of lepidocrocite (γ-FeOOH) and magnetite (Fe 3 O 4 ) increased , which enhanced Se(VI) adsorption and the subsequent electron transfer between the underlying Fe 0 and the surface-decorated Se(VI). As shown in Figure 5.6, the Fe(II)-induced improvement in the rate constants of Se(VI) sequestration by Fe 0 and the electron selectivity of Fe 0 towards Se(VI) under aerobic conditions were attributed to the weak acidity arising from the Fe(II) addition (Huang and Zhang, 2005;Liu et al., 2013) and the facilitated Se VI) enrichment by the γ-FeOOH, along with the favored electron conductivity by Fe 3 O 4 generated on the surface of the ZVI particle. ...
Selenium contamination of air, aquatic environments, soils and sediments is a serious environmental concern of increasing importance. Selenium has a paradoxical feature in bringing about health benefits under the prescribed level, but only a few fold increase in its concentration causes deleterious effects to flora and fauna, humans and the environment.
This book Environmental Technologies to Treat Selenium Pollution: Principles and Engineering: presents the fundamentals of the biogeochemical selenium cycle and which imbalances in this cycle result in pollution.overviews chemical and biological technologies for successful treatment of selenium contaminated water, air, soils and sediments.explores the recovery of value-added products from selenium laden waste streams, including biofortication and selenium-based nanoparticles and quantum dots.
This book may serve both as an advanced textbook for undergraduate and graduate students majoring in environmental sciences, technology or engineering as well as as a handbook for tertiary educators, researchers, professionals and policy makers who conduct research and practices in selenium related fields. It is essential reading for consulting companies when dealing with selenium related environmental (bio)technologies.
ISBN: 9781789061048 (Paperback)
ISBN: 9781789061055 (eBook)
... Water characteristics may affect the distribution system and vice versa. Very soft water [3], high concentrations of soluble sulfates, chlorides, and natural organic matter [4], as well as dissolved oxygen [5] can dramatically affect pipe corrosion. On the other hand, the presence of metals in water can be due to the quality and material of pipelines, as well as their operating conditions. ...
Conventional high throughput methods assaying the chemical state of water and the risk of heavy metal accumulation share common constraints of long and expensive analytical procedures and dedicated laboratories due to the typical bulky instrumentation. To overcome these limitations, a miniaturized optical system for the detection and quantification of inorganic mercury (Hg2+) in water was developed. Combining the bioactivity of a light-emitting mercury-specific engineered Escherichia coli—used as sensing element—with the optical performance of small size and inexpensive Silicon Photomultiplier (SiPM)—used as detector—the system is able to detect mercury in low volumes of water down to the concentration of 1 µg L−1, which is the tolerance value indicated by the World Health Organization (WHO), providing a highly sensitive and miniaturized tool for in situ water quality analysis.
Because of the unstable wastewater quantity and quality, the biological treatment efficiency of digested effluent was not as expected. A convenient and effective way was eagerly required to improve the efficiency of biological treatment. By sheet iron addition (R1), the COD and TN removal efficiencies under continuous flow condition increased by 59% and 37% respectively. The bulk pH maintained at around 7.5 which benefited most bacteria, while in the control (R0, without sheet iron addition) the pH decreased to 5.0. Both chemical and bio-removal of COD existed in R1, but the chemical removal dominated (63.71%). The enhanced COD removal efficiency came from the chemical oxidation by Fe3+ (47.43%) and Fe0 (10.86%). For the TN removal, the enhancement mainly came from the improvement of anammox activity by Fe3+ (14.87%), the bio-oxidation of ammonium with Fe3+ as electron acceptor (8.78%), and the bio-reduction of nitrate/nitrite with Fe2+ and H2 as electron donor (35.76%). By the first-order kinetic fitting analysis, the COD and TN removal rate in R1 was higher than that in R0. Thus, for a quick and high COD and TN removal from digested effluent, the addition of Fe0/Fe2+/Fe3+ was suggested, and the best form should be Fe0 (e.g., sheet iron). The addition of sheet iron reduces the cost of nitrogen removal and improves the efficiency of COD and TN removal. Comparing with the combined processes, this novel approach has potential advantages with simple operation and high efficiency. It endows the biological process much broader application in digested effluent treatment.
Ferrous ions (Fe²⁺) can effectively promote the removal of pollutants by sulfidated zero-valent iron (S-ZVI), but the role of anions coexisting with Fe²⁺ was often ignored. This study systematically compared the performances of S-ZVI/FeCl2 and S-ZVI/FeSO4 systems for chloramphenicol (CAP) and nitrobenzene (NB) removal. The results showed that FeCl2 and sulfidation had a synergistic promoting effect on the reaction, but FeSO4 could not promote the removal of nitro compounds by S-ZVI. Liquid chromatography/mass spectrometry (LC/MS) and UV-vis analysis showed that only the nitro group was reduced during the removal of CAP and NB. The results of batch experiments showed that both NaCl and Na2SO4 could effectively promote the reaction, but the promotion effect of Na2SO4 weakened with the decrease of pH. X-ray photoelectron spectroscopy (XPS) analysis showed that Fe²⁺ could promote the adsorption of SO4²⁻ on the surface of S-ZVI. A possible mechanism was proposed: Fe²⁺ could maintain a low pH level because of hydrolysis, which promoted the adsorption of Cl⁻ and SO4²⁻ on the surface of S-ZVI. Cl⁻ promoted the reaction because of its pitting corrosion, while SO4²⁻ competed with nitro groups for the adsorption sites. Our findings suggest that the coexisting anions are quite essential in the removal of nitro compounds by S-ZVI/Fe²⁺ systems.
Pre-magnetization can boost the activity of zero-valent iron (ZVI), facilitating the removal of contaminants by pre-magnetized ZVI (Pre-ZVI) activated peroxides. However, Pre-ZVI’s persistence and stability in the activation of peroxides are little known. In this work, we investigated the ability of dry and wet Pre-ZVI activated peroxymonosulfate (Pre-ZVI/PMS) to enhance the removal of ofloxacin (OFX), the influence of environmental parameters as well as the evaluation of the long-term performance of Pre-ZVI/PMS. The results showed that the dry Pre-ZVI/PMS system had better removal performance in the wide pH range (4–9), with OFX removal efficiencies of over 90% and high PMS utilization within 120 min. The presence of different environmental factors significantly affected the degradation process. Remarkably, the ability of dry and wet Pre-ZVI to activate PMS decreased over time, with all systems experiencing a decrease of reaction rate constant (k) of approximately 20–47% from day 1 to day 30. The catalytic efficacy could be considerably recovered by re-magnetization, but it dropped again with time. The vulnerable sites and degradation mechanism of OFX were determined by liquid chromatography–tandem mass spectrometry analysis and density functional theory calculation, and the toxicities of the transformation products in the system were predicted. This work is a guide for clarifying the long-term viability of the Pre-ZVI-activated peroxides used in environmental remediation.
Corrosion of metals in the tidal zone shortens the service life of facilities considerably and causes extensive economic losses each year. However, the contribution of microbiologically influenced corrosion (MIC) to this progress is usually ignored, and consequently the research on the mechanism of MIC in the tidal zone is highly desirable. In this study, the impact of the typical marine strain Pseudomonas aeruginosa on EH40 steel corrosion in the simulated tidal zone was evaluated. P. aeruginosa accelerated the corrosion of EH40 steel in the simulated tidal zone and its corrosion promotion efficiency rose over time. The environmental stress promoted the metabolism, energy production, and secretion of phenazines of P. aeruginosa, which promoted extracellular electron transfer between bacteria and steel, and accelerated MIC. The study proposes a possible mechanism of MIC in the tidal zone at the molecular biological level, which is of theoretical significance for evaluating the corrosion risks of marine equipment.
Sulfidated nanoscale zero-valent iron (S-nZVI) was applied in the treatment of pentachlorophenol (PCP)-polluted water to explore the reaction pathways and the effects of water chemistry on PCP adsorption and transformation.
Aqueous solutions of MnCl 2 ·4H 2 O and Tiron (disodium 4,5-dihydroxy-1,3-benzenedisulfonate) rapidly remove dioxygen (O 2 ) from aqueous solution at a rate of ~20 mg∙ L ⁻¹ min ⁻¹ with turnover frequencies (TOFs) of up...
Little is known about the microbiologically influenced corrosion of metals exposed to the marine tidal zone alternately immersed by seawater. Corrosion of EH40 steel affected by Halomonas titanicae was investigated under alternate and full immersion in the present work. It was found that the corrosion was accelerated by the transition from full immersion to alternate pattern in abiotic media, but the corrosion inhibition efficiency of H. titanicae towards steel was enhanced by this transition. These differences were associated with thin electrolyte layers, the increase in the number of sessile cells, and their probably enhanced aerobic respiration activity by alternate immersion.
In order to treat heavy metal pollutants in acid mine drainage, zero-valent iron/phosphoric titanium dioxide (PTO-3nZVI and PTO-nZVI) was developed with phosphoric acid treatment and loading of zero valent iron and showed excellent adsorption of heavy metals. PTO-3nZVI and PTO-nZVI greatly improved the adsorption capacity for the targeted heavy metal Cd(II) (308 mg·g⁻¹ for PTO-3nZVI and 206 mg·g⁻¹ for PTO-nZVI) through complexation and coprecipitation. Additionally, competitive adsorption occurred between Cd(II) and coexisting heavy metal ions (Pb(II) and Cu(II)), resulting in different adsorption effects for PTO-3nZVI and PTO-nZVI, and their adsorption efficiencies decreased in the order Cu(II)>Pb(II)>Cd(II) for PTO-3nZVI and Pb(II)>Cu(II)>Cd(II) for PTO-nZVI. Based on these two aspects, X-ray photoelectron spectrometer (XPS) analyses and Density Functional Theory (DFT) calculations were used to characterize the adsorption behaviors of PTO-3nZVI and PTO-nZVI. In particular, competitive relationships for different heavy metals were clearly revealed by binding energies, Frontier Molecular Orbitals (FMOs) analyses, electrostatic potentials, and total potential and differential charge analyses of adsorption sites, binding sites and binding strengths.
To find a method for efficient removal and conversion of inorganic nitrogen compounds in three reduction systems, which are the sponge iron (s-Fe⁰)，the sponge iron + biochar (s-Fe⁰/BC) micro-electrolysis system and the sponge iron + biochar + manganese sand (s-Fe⁰/BC/MS)-enhanced micro-electrolysis system, the studies have been completed on the conditions of different material ratios and initial pH. When the initial pH is 7, the nitrate removals reached to 7.6%, 61.5% and 80.3%, the nitrogen ratio of products reached to 5.8%, 22.4% and 38.3% respectively in s-Fe⁰, s-Fe⁰/BC (3:1) and s-Fe⁰/BC/MS (6:2:1) systems. The s-Fe⁰/BC/MS system was the best at nitrate removal over a wide pH range (2-12).At the same time, the nitrogen selectivity in the s-Fe⁰/BC/MS system was always better than that in other two systems. The X-ray photoelectron spectroscopy (XPS) results showed that the addition of manganese sand could enhance the ratio of Fe3O4 in the sponge iron surface oxide, reduce the passivation of the sponge iron surface and accelerate the electron transfer rate. As a catalyst for the corrosion of sponge iron, manganese sand can increase the corrosion of sponge iron and enhance the micro-electrolysis role between iron and carbon.
The activity of micro-sized zero-valent iron (MZVI) material for nitrate removal in neutral pH and low C/N ratios water needs to be improved. In this study, micro-sized zero-valent iron@chitosan (MZVI@CS) material was synthesized through embedding MZVI particles into chitosan (CS) gel by sol-gel method, and was used for deep removal of NO3--N in the absence of organic carbon sources and neutral pH. The NO3--N removal rate of MZVI@CS was 0.37 mg-N·L-1·d-1 (dosage of 1%, initial pH = 7, 25 °C, initial nitrate concentration = 15 mg-N·L-1), which was 11.33 times higher than that of MZVI. The apparent activation energy (Ea) of MZVI@CS with nitrate was 38.23 kJ·mol-1. MZVI@CS can remove nitrate effectively at a low concentration (15 mg-N·L-1). A stable denitration rate (0.37-2.28 mg-N·L-1·d-1) could be maintained under weak acidic, neutral and alkaline conditions (pH = 5-9). More than 80% of reduced nitrate was converted to N2, and only a small amount was converted to NH4+ or NO2-. The gel structure of MZVI@CS eliminated the agglomeration between MZVI particles while the forming of Fe-CS chelates reduced the formation of iron oxide and solved the problems of passivation, hence successfully strengthened the NO3--N removal efficiency of MZVI. Therefore MZVI@CS has great application potential in NO3--N deep removal of water bodies with neutral pH and low C/N ratios.
Ammonium as the major reduction intermediate has always been the limitation of nitrate reduction by cathodic reduction or nano zero-valent iron (nZVI). In this work, we report the electrochemical nitrate removal by magnetically immobilized nZVI anode on RuO2–IrO2/Ti plate with ammonia-oxidizing function. This system shows maximum nitrate removal efficiency of 94.6% and nitrogen selectivity up to 72.8% at pH of 3.0, and it has also high nitrate removal efficiency (90.2%) and nitrogen selectivity (70.6%) near neutral medium (pH = 6). As the increase of the applied anodic potentials, both nitrate removal efficiency (from 27.2% to 94.6%) and nitrogen selectivity (70.4%–72.8%) increase. The incorpration of RuO2–IrO2/Ti plate with ammonia-oxidizing function on the nZVI anode enhances the nitrate reduction. The dosage of nZVI on RuO2–IrO2/Ti plate (from 0.2 g to 0.6 g) has a slight effect (the variance is no more than 10.0%) on the removal performance. Cyclic voltammetry, Tafel analysis and electrochemical impedance spectroscopy (EIS) were further used to investigate the reaction mechanisms occurring on the nZVI surfaces in terms of CV curve area, corrosion voltage, corrosion current density and charge-transfer resistance. In conclusion, high nitrate removal performance of magnetically immobilized nZVI anode coupled with RuO2–IrO2/Ti plate may guide the design of improved electrochemical reduction by nZVI-based anode for practical nitrate remediation.
The oxygen reduction reaction (ORR) activated by Fe⁰ in the presence of three aminopolycarboxylic acids (CAs), i.e. nitrilotriacetic acid (NTA), ethylenediamine-N,N'-disuccinic acid (EDDS) and ethylenediaminetetraacetic acid (EDTA), for the degradation of sulfamethazine (SMT) was investigated. At optimum conditions, Fe⁰/EDDS/O2, Fe⁰/EDTA/O2 and Fe⁰/NTA/O2 systems presented SMT removal of 58.2%, 75.3% and 93.8%, respectively, being much higher than that in Fe⁰/O2 system (1.36%). The generation of surface-bound Fe²⁺ (Fe²⁺) and dissolved iron ion was enhanced by CAs. ORR through a two-electron transfer pathway was mainly responsible for H2O2 generation in NTA and EDTA systems, while a single-electron ORR was the major source for producing H2O2 in EDDS system. •OH produced by the homogeneous reaction of Fe²⁺ and H2O2 was the main species for SMT degradation. Fe⁰/EDDS/O2 produced more ¹O2 than Fe⁰/EDTA/O2 and Fe⁰/NTA/O2, however, the radical contributed negligibly to SMT removal. The caging effect of CAs might be a major factor influencing the reaction rate of Fe²⁺ and O2. CAs provided protons to accelerate the electron transfer, the production of Fe²⁺ and thus the contaminant removal. This study is of great significance for revealing ORR mechanisms in the Fe-chelate system.
In this study, hydrogen-autotrophic microorganisms and zero-valent iron (Fe⁰) were filled into columns to investigate hydrogenotrophic denitrification effect on cadmium (Cd(II)) removal and column life-span with sand, microorganisms, Fe⁰ and bio-Fe⁰ columns as controls. In terms of the experiment results, the nitrate-mediated bio-Fe⁰ column showed a slow Cd(II) migration rate of 0.04 cm/PV, while the values in the bio-Fe⁰ and Fe⁰ columns were 0.06 cm/PV and 0.14 cm/PV respectively, indicating much higher Cd(II) removal efficiency and longer service life of the nitrate-mediated bio-Fe⁰ column. The XRD and SEM-EDX results implied that this improvement was attributed to hydrogenotrophic denitrification that caused more serious iron corrosion and larger amount of secondary mineral generation (e.g., green rust, lepidocrocite and goethite). These active minerals provided more reaction sites for Cd(II) adsorption and further immobilization. In addition, the decrease of Cd(II) migration front and the increase of removal capacity along the bio-Fe⁰ column mediated by nitrate presented an uneven distribution in reactive zone. The latter half part was identified to be a more active region for Cd(II) immobilization. The above results indicate that the introduction of nitrate and microorganisms will improve the performance of iron-based permeable reactive barriers for the remediation of Cd(II)-containing groundwater.
The pollution of nitrate (NO3⁻) in groundwater has become an environmental problem of general concern and requires immediate remediation because of adverse human and ecological impacts. NO3⁻ removal from groundwater is conducted mainly by chemical, biological, and coupled methods, with the removal efficiency of NO3⁻ considered the sole performance indicator. However, in addition to the harmless form of N2, the reduced NO3⁻ could be transformed into other intermediates, such as nitrite (NO2⁻), nitrous oxide (N2O), and ammonia (NH4⁺), which may have direct or indirect negative impacts on the environment. Therefore, increasing N2 selectivity is a significant challenge in reducing NO3⁻ in groundwater, which seriously impedes the large-scale implementation of available remediation technologies. In this work, we comprehensively overview the most recent advances in N2 selectivity regarding the understanding of emerging groundwater NO3⁻ removal technologies. Mechanisms of by-product production and strategies to enhance the selective reduction of NO3⁻ to N2 are discussed in detail. Furthermore, we proposed topics for further research and hope that the total environmental impacts of remediation schemes should be evaluated comprehensively by quantifying all potential intermediate products, and promising strategies should be further developed to enhance N2 selectivity, to improve the feasibility of related technologies in actual remediation.
Boron-doped graphene dots (BGDs) were synthesized by a hydrothermal method using the disaccharide maltose as the carbon precursor and borax as the boron dopant. The precursors were heated to 500 °C for 5 h to obtain the carbonized BGDs (CBGDs). B doping was carried out to increase the surface area and improve the electron transfer reaction for quick and effective reactive oxygen species generation. BGDs and CBDGs showed different oxygen contents and functionalities, surface areas, morphologies, and zeta-potentials. Zero-valent iron (ZVI) was activated mechanochemically by surface passivation using BGDs and CBGDs under atmospheric conditions, and their dye removal performance via sonochemical degradation was evaluated. Owing to the greater oxygen functionality of BGDs, the [email protected] were fully decorated with ZVI, and less activation occurred that inhibited the sonochemical dye degradation properties of [email protected]. In contrast, a sheet-like structure and activation of ZVI on [email protected] make it an active catalyst. The maximum surface area (413.82 m²/g) and high negative zeta potential (−38.25 mV) of CBGDs make it a good adsorbent. BGDs have a low surface area (20.78 m²/g), self-assembling structure, non-planer arrangement, and a twisted structure with interlocking spheres that disfavor π–π stacking between the dye and BGDs and make it non-adsorbent. The rapid generation of reactive oxygen species, the existence of iron with mixed-valence states, and the increase in surface area due to boron doping facilitate the electron transfer process to make the catalyst active and reusable even after 10 cycles.
The damage in the form of the leakage occurred at the end piping transporting industrial water in the press hall of the automotive plant in the specific position of 12 o'clock after twelve years of operation. The piping diameter 80 mm, wall thickness 4 mm, is made of low carbon steel produced according to the Slovak standard STN 41 1353 equivalent to the S235 according to the EU standards or ASTM A519-82 grade 1020. In order to determine the cause and course of corrosion damage to the piping there were performed the analyses of the transported industrial water (composition, properties, pH), ultrasonic measurements of piping wall thickness, piping steel material (microstructure, chemical composition) corrosion products from the entire inner surface of the piping and from the point of the leakage and pinholes (visual, EDS and X-ray). The UT measurements of wall thickness changes revealed the massive corrosion damage manifested as pinholes at the 12 o'clock position with the max. depth of the pinholes 3.6mm. The analysis of the industrial water confirmed the quality corresponding to drinking water. The X-ray analysis of the corrosion products from the pinholes and leakage confirmed the corrosion process in the humid atmosphere with the dominance of Goethite, Akaganeite and the low CaCO3 content. The presence of Akaganeite and the direct confirmation of the Cl⁻ presence in the corrosion products by the pinholes EDS analyses designates Cl⁻ as the accelerator of pinholes corrosion in the humid atmosphere above the stagnant water level, propagating against the direction of gravity (12 o'clock) in the air bubble created by improper venting of the piping.
Nitrate (NO3⁻) removal on nanoscale zero-valent iron-based nanoparticles (nFe⁰ particles) represents one efficient and green technology for nitrate pollution abatement, but its development is hindered by the low product selectivity towards harmless N2. Herein we demonstrate the inferior performance of nFe⁰ in N2 production originates from its over-strong affinity with both the H* and the N-intermediates (denoted as N*), leading to a low N/H molar ratio and a poor mobility of N* on Fe surface that facilitates the formation of NH3 rather than N2. Increasing NO3⁻ feeding concentration or lowing nFe⁰ dose can uplift the N2 selectivity up to 41.9% (vs. 19.3% by single nFe⁰ in removing 20 mg L⁻¹ NO3⁻-N), but has to sacrifice NO3⁻ removal efficiency and yield more toxic NO2⁻. The nFe⁰ surface modification by 12.0 wt% palladium (Pd) gives rise to a N2 selectivity of 46.0% and a NO3⁻ removal efficiency above 90% (vs. 100% by single nFe⁰). Theoretical calculations reveal the critical role of Pd in weakening the binding strengths of H* and N* on catalyst, which enables to reduce the H* adsorption and promote the migration of N* that increases the N*-N* encountering possibility for N2 formation. This correlation between surface chemistry and NO3⁻ conversion may guide the design of improved Fe⁰-based materials for practical nitrate remediation.
Decomplexation is an effective approach and key step for heavy metal-organic complex removal because conventional chemical precipitation is ineffective. In this study, a novel strategy of coupling discharge plasma and iron internal microelectrolysis (“plasma+iron”) was developed for decomplexation of Cu-ethylenediaminetetraacetic acid (Cu-EDTA). There were distinct synergistic effects on Cu-EDTA decomplexation in the “plasma+iron” system, resulting from Fenton reaction, Fe(III) displacement, and flocculation. Cu-EDTA decomplexation efficiency and energy efficiency were 1.53 and 2.31 times as those in single plasma system. Iron powder accelerated H2O2 decomposition and ·OH formation, especially ·OH production on the surface of iron, which mitigated the negative effects of radical quenchers. Moreover, co-precipitation with polymeric iron further promoted the removal of Cu ions released. Some small organic acids, amides, amines, alcohols, and NO3⁻ were identified as a result of the effective Cu-EDTA decomplexation by the “plasma+iron” strategy. The synergetic decomplexation pathways of Cu-EDTA were then further proposed.
Permeable reactive barriers (PRBs) are receiving a great deal of attention as an innovative, cost-effective technology for in situ clean up of groundwater contamination. A wide variety of materials are being proposed for use in PRBs, including zero-valent metals (e.g., iron metal), humic materials, oxides, surfactant-modified zeolites (SMZs), and oxygen- and nitrate- releasing compounds. PRB materials remove dissolved groundwater contaminants by immobilization within the barrier or transformation to less harmful products. The primary removal processes include: (1) sorption and precipitation, (2) chemical reaction, and (3) biologically mediated reactions. This article presents an overview of the mechanisms and factors controlling these individual processes and discusses the implications for the feasibility and long-term effectiveness of PRB technologies.
Knowledge concerning the chemical reduction of NO 3 ⁻ to gaseous products, a process of potential practical significance as an antipollution device, is sparse. The influence of pH on chemical reduction of NO 3 ⁻ ‐N (approximate concentration 25 ppm) by Fe ²⁺ in the presence and absence of Cu ²⁺ was studied over a pH range from 6 to 10. After 24‐hours of controlled pH incubations under a helium atmosphere NO 3 ⁻ , NO 2 ⁻ , N 2 O, NO, N 2 , and NH 4 ⁺ were determined. The initial Fe ²⁺ /NO 3 ⁻ mole ratio was 8. Reduction of NO 3 ⁻ was negligible in the absence of Cu ²⁺ , but was pronounced above pH 7 in the presence of approximately 5 ppm Cu ²⁺ . Formation of NH 4 ⁺ increased with pH and was the dominant process at pH 9 and 10. Nitrous oxide and N 2 accumulations were greatest in the pH range from 8 to 8.5 and negligible at pH 6 and 10. Nitrite formation was small except at pH 9 and 10. Trace quantities of NO accumulated during incubation if the pH was allowed to drop below 6.
Levels of Cu ²⁺ and Fe ²⁺ influenced the extent and nature of NO 3 ⁻ reduction at pH 8. Maximum reduction of NO 3 ⁻ (93%) and maximum gas production (equivalent to 61% of the original NO 3 ⁻ ) occurred when the Fe ²⁺ /NO 3 ⁻ mole ratio was 12 and the Cu ²⁺ level was approximately 10 ppm. The N 2 O/N 2 mole ratio in the evolved gases decreased as the Cu ²⁺ level was increased from approximately 1 to 10 ppm and as the Fe ²⁺ /NO 3 ⁻ mole ratio was increased from 8 to 12. Nitrate was relatively stable at a Cu ²⁺ content of 0.1 ppm irrespective of the Fe ²⁺ /NO 3 ⁻ ratio.
There is a limited amount of information about the effects of mineral precipitates and corrosion on the lifespan and long-term performance of in situ Fe° reactive barriers. The objectives of this paper are (1) to investigate mineral precipitates through an in situ permeable Fe° reactive barrier and (2) to examine the cementation and corrosion of Fe° filings in order to estimate the lifespan of this barrier. This field scale barrier (225-ft long x 2-ft wide x 31-ft deep) has been installed in order to remove uranium from contaminated groundwater at the Y-12 plant site, Oak Ridge, TN. According to XRD and SEM-EDX analysis of core samples recovered from the Fe° portion of the barrier, iron oxyhydroxides were found throughout, while aragonite, siderite, and FeS occurred predominantly in the shallow portion. Additionally, aragonite and FeS were present in up-gradient deeper zone where groundwater first enters the Fe° section of the barrier. After 15 months in the barrier, most of the Fe° filings in the core samples were loose, and a little corrosion of Fe° filings was observed in most of the barrier. However, larger amounts of corrosion (10-150 m thick corrosion rinds) occurred on cemented iron particles where groundwater first enters the barrier. Bicarbonate/carbonate concentrations were high in this section of the barrier. Byproducts of this corrosion, iron oxyhydroxides, were the primary binding material in the cementation. Also, aragonite acted as a binding material to a lesser extent, while amorphous FeS occurred as coatings and infilings. Thin corrosion rinds (2-50 m thick) were also found on the uncemented individual Fe° filings in the same area of the cementation. If corrosion continues, the estimated lifespan of Fe° filings in the more corroded sections is 5 to 10 years, while the Fe° filings in the rest of the barrier perhaps would last longer than 15 years. The mineral precipitates on the Fe° filing surfaces may hinder this corrosion but they may also decrease reactive surfaces. This research shows that precipitation will vary across a single reactive barrier and that greater corrosion and subsequent cementation of the filings may occur where groundwater first enters the Fe° section of the barrier.
We have used x-ray scattering to measure the structure of the passive oxide film formed at high anodic potentials on Fe(110) and Fe(001). The crystalline film has a small crystallite size (â50 â«) and is oriented with the substrate. The film structure is based on FeâOâ, but with cation vacancies on octahedral and tetrahedral sites (80% and 66% occupancies, respectively) and with cations occupying octahedral interstitial sites (12% occupancy). These results resolve the long-standing controversy surrounding the film structure and provide a basis for understanding and modeling film properties important for corrosion resistance. {copyright} {ital 1997 } {ital The American Physical Society}
Although considerable research has been conducted on nitrate reduction by zerovalent iron (ZVI), the process kinetics at near neutral pH has not been studied thoroughly. In this study, a kinetic model with a double-Langmuir-adsorption formulation was developed to represent site saturation effects of aqueous Fe2+ and NO3 on nitrate reduction in a ZVI system at near neutral pH. Both an analytical solution and a numerical solution of the proposed model were developed. The kinetic parameters were evaluated based on batch experiments with two types of ZVI. Sensitivity analysis indicates that it is rather difficult and unreliable to estimate the parameters of the proposed multivariable nonlinear kinetic model from a single test curve. A better strategy is to obtain reliable parameters by designing specific experiments to target one parameter each time. The results indicate that, although the values of the kinetic parameters might change with different types of iron powder, the kinetic model can fit the experimental data very well, and therefore is consistent with the proposed mechanism.
The effects of three selected Good's pH buffers on the performance of an Fe-0/nitrate/H2O system were evaluated. The Good's pH buffer itself did not reduce nitrate directly. Nitrate reduction by iron powder at near-neutral pH was negligible in an unbuffered system, but it was greatly enhanced with the presence of the buffer. A significant amount of aqueous Fe2+ (or Fe3+) was released after adding the Good's pH buffer into the Fe-0/H2O system with or without nitrate. In general, the pH of the buffered solution increased from the initial pH (= similar to4.6-5.3, depending on buffer's pK(a)) to near-neutral pH. After the initial pH hiking, the pH in the system was more or less stable for a period of time (similar to5-10 h, usually concurrent with a fairly stable aqueous Fe2+). The pH then drifted to similar to7.1 to 8.6, depending on the buffer's initial concentration, the buffer's pK(a) and the consumption of Fe2+ concurrent with nitrate reduction. While a common assumption made by researchers is that Good's pH buffers do not directly participate in reaction processes involved in contaminant remediation, this study shows that as side effects, the Good's pH buffer may react with iron powder.
When the pH of a reaction mixture of γ-FeO(OH) and the iron(II) ion is raised from 5 to 9, the γ-FeO(OH) is transformed to stoicheiometric Fe3O4 at 25 °C. The transformation reaction is triggered by the adsorption of the iron(II) ion on γ-FeO(OH) at a pH above 7.3, and the adsorbed FeII–γ-FeO(OH) is subsequently transformed to Fe3O4. In each step, one proton is released, a total of two protons being released in the reaction. The reaction of the transformation step is written as follows: [γ-FeO(OH)]2FeOH+→ Fe3O4+ H2O + H+ and includes a dissolution–precipitation process. The adsorption of iron(II) ion on Fe3O4 seems to take place along with the formation of the intermediate by the adsorption of iron(II) ion on γ-FeO(OH).
The oxide layer that lies at the iron-water interface under environmental conditions can influence the redn. of solutes by acting as a passive film, semiconductor, or coordinating surface. As a passive film, oxides may inhibit reaction by providing a phys. barrier between the underlying metal and dissolved oxidants. Sustained redn. of solutes requires localized defects in the passive film (e.g., pits), or some mechanism for transferring electrons through the oxide. In the semiconductor model, conduction band electrons from the oxide may contribute to solute redn., but electron hopping (resonance tunneling) appears to be more important due to the high population of localized states in oxides formed under environmental conditions. Ultimately, electron transfer to the solute must occur via a precursor complex at the oxide-water interface. For dehalogenation of chlorinated aliph. compds. on an iron oxide surface, a surface complexation model suggests that the outer-sphere precursor complex is weak and
The passive film formed on iron in pH 8.4, 0.136 M borate buffer over a broad potential range was characterized by in situ x‐ray absorption near edge structure. On stepping the potential to a value between −0.6 and +0.4 V relative to a mercurous
sulfate reference electrode (MSE), a passive film forms without detectable dissolution. Formation of a passive film at potentials
between −0.8 and −0.65 V is accompanied by dissolution during the early stages of passivation. At −0.9 V, the iron did not
passivate, but continued to dissolve. The valence state of iron in the film is predominantly
with 4 to 10%
at high potentials (+0.4 V), and 14 to 20%
at the lower potentials. The behavior on changing the solution concentration (pH 8.4, 0.01 M) and pH (pH 7.4, 0.1 M) was compared with passivation in the “classical” borate buffer (pH 8.4, 0.136 M). Passivation at +0.4 V in the modified borate buffers is associated with dissolution during the early stages of passivation,
but the films that form have average oxidation states similar to those observed in pH 8.4, 0.136 M borate buffer. This indicates that the susceptibility to dissolution during passivation does not influence the valence state
of the final film.
Iron was anodically oxidized in a borate‐boric acid buffer solution of pH 8.4 in the potential range extending from the active region to oxygen evolution. In the active region there appeared to
be some
on the surface. In the passive region the iron was covered with an oxide film 10–30Aå thick, depending on the potential.
The structure of the film was studied by its cathodic behavior and electron diffraction and was found to consist of an inner
“
” and an outer “
” layer. The outermost part of the film had a defect structure of the general formula
. The thicknesses of the layers and the number of defects were found to be functions of the anodic potential. The chemical
reactions involved in the anodic passivation and the subsequent cathodic reduction processes are discussed.
To elucidate the reduction mechanism of N-nitrosodimethylamine (NDMA) by granular iron, various electrochemical experiments using a mercury electrode were conducted. The studies included differential pulse voltammetry and exhaustive potentiostatic electrolysis. The results of the NDMA electroreduction experiments were compared with the results obtained in the column and batch experiments of Part 1 of this study. The results show that (1) electroreduction of NDMA occurs at potentials more negative than −1.3 V and this potential cannot be achieved under the conditions of the column and batch experiments and (2) different reduction products of NDMA were observed in the electrochemical tests relative to the column and batch tests. That is, dimethylamine (DMA) and nitrous oxide were formed in the electrochemical reduction experiments, whereas ammonia and DMA were produced in the column and batch experiments. The difference in product formation and more importantly the fact that the iron cannot reach the potentials required for electroreduction indicate that the reduction of NDMA on iron cannot take place by direct electron transfer. The process of catalytic hydrogenation was found to be consistent with all experimental observations and is proposed as the alternative mechanism.
This research investigated the long-term performance of zero-valent iron for mediating the reductive dechlorination of trichloroethylene (TCE). Over a 2-year period, rates of TCE dechlorination in columns packed with iron filings were measured in simulated groundwaters containing either 3 mM CaSO4, 5 mM CaCl2, or 5 mM Ca(NO3)2. At early elapsed times, TCE reaction rates were pseudo-first-order in TCE concentration and were independent of the solution pH. With increasing elapsed time, reaction rates deviated from pseudo-first-order behavior due to reactive site saturation and increased iron surface passivation toward the influent end of each column. The extent of passivation was dependent on both the TCE concentration and the background electrolyte solution. For most of the investigation, TCE reaction rates in 3 mM CaSO4 and 5 mM CaCl2 solutions were statistically identical at the 0.05 confidence level. However, TCE reaction rates in 5 mM Ca(NO3)2 were slower. In columns operated using chloride- and sulfate-containing waters, the effective half-life for TCE dechlorination increased from approximately 400 min after 10 days elapsed to approximately 2500 min after 667 days. The effective TCE half-life in the nitrate-containing water increased from approximately 1500 min after 10 days to approximately 3500 min after 667 days. Measurements of iron corrosion rates in nitrate and chloride solutions showed that nitrate contributed to increased iron surface passivation and decreased rates of iron corrosion. Corrosion current measurements indicated that halocarbon reduction on fresh iron surfaces was cathodically controlled, whereas on aged iron surfaces, iron corrosion was anodically controlled. Anodic control of iron corrosion contributed to the development of reactive site saturation with time and to similar reaction rates for TCE and perchloroethylene. Passivation of the iron surfaces was found to be dependent on the adhering tendency of the corrosion products and not on the overall mass of corrosion products in the columns. The decrease in TCE reaction rates over time can be attributed to anodic control of iron corrosion and not to increasing reactant mass transfer limitations associated with diffusion through porous corrosion products.
The impact of microbiological and geochemical processes has been a major concern for the long-term performance of permeable reactive barriers containing zero-valent iron (Fe0). To evaluate potential biogeochemical impacts, laboratory studies were performed over a 5-month period using columns containing a diverse microbial community. The conditions chosen for these experiments were designed to simulate high concentrations of bicarbonate (17−33 mM HCO3-) and sulfate (7−20 mM SO42-) containing groundwater regimes. Groundwater chemistry was found to significantly affect corrosion rates of Fe0 filings and resulted in the formation of a suite of mineral precipitates. HCO3- ions in SO42--containing water were particularly corrosive to Fe0, resulting in the formation of ferrous carbonate and enhanced H2 gas generation that stimulated the growth of microbial populations and increased SO42- reduction. Major mineral precipitates identified included lepidocrocite, akaganeite, mackinawite, magnetite/maghemite, goethite, siderite, and amorphous ferrous sulfide. Sulfide was formed as a result of microbial reduction of SO42- that became significant after about 2 months of column operations. This study demonstrates that biogeochemical influences on the performance and reaction of Fe0 may be minimal in the short term (e.g., a few weeks or months), necessitating longer-term operations to observe the effects of biogeochemical reactions on the performance of Fe0 barriers. Although major failures of in-ground treatment barriers have not been problematic to date, the accumulation of iron oxyhydroxides, carbonates, and sulfides from biogeochemical processes could reduce the reactivity and permeability of Fe0 beds, thereby decreasing treatment efficiency.
A combination of new and previously reported data on the kinetics of dehalogenation by zero-valent iron (Fe0) has been subjected to an analysis of factors effecting contaminant degradation rates. First-order rate constants (kobs) from both batch and column studies vary widely and without meaningful correlation. However, normalization of these data to iron surface area concentration yields a specific rate constant (kSA) that varies by only 1 order of magnitude for individual halocarbons. Correlation analysis using kSA reveals that dechlorination is generally more rapid at saturated carbon centers than unsaturated carbons and that high degrees of halogenation favor rapid reduction. However, new data and additional analysis will be necessary to obtain reliable quantitative structure−activity relationships. Further generalization of our kinetic model has been obtained by accounting for the concentration and saturation of reactive surface sites, but kSA is still the most appropriate starting point for design calculations. Representative values of kSA have been provided for the common chlorinated solvents.
Leaching of nitrate from soils and sediments can be reduced in anoxic environments due to denitrification to N2O/N2 or reduction of nitrate to ammonium. While microbial dissimilatory reduction of nitrate to ammonia is well known, it is shown here that this conversion can also proceed at appreciable rates in abiotic systems in the presence of green rust compounds [FeII4FeIII2(OH)12SO4·yH2O]. In the reaction nitrate is stoichiometrically reduced to ammonium, and magnetite (Fe3O4) is the sole Fe-containing product. At a constant pH of approximately 8.25 and 25 °C, the rate expression is given as: d[NH4+]/dt = k[Fe(II)]GR[NO3-],where k = 4.93 × 10-5 ± 0.39 × 10-5 L mol-1 s-1. In anoxic soils and sediments, this reaction may also lead to a nitrate to ammonium reduction, at rates of similar magnitude or even higher than microbial reduction rates. Hence green rust should be considered a possible important reductant for nitrate reduction to ammonium in subsoils, sediments, or aquifers where microbially mediated reduction rates are small.
The structures of films formed on iron in borate buffer (pH = 8.4) were investigated by in situ surface enhanced Raman spectroscopy (SERS). The identities of the films were shown to be a function of potential and history. When the films were removed from solution their identities changed. All films were amorphous. In the active range, the film consisted of species resembling FeO and Fe(OH)2. One constituent was common to all films, regardless of the potential at which they were formed. This species had a vibrational spectrum that was similar to that of Fe(OH)2 and yet it appeared to consist of a mixture of Fe(II) and Fe(III). This component is referred to as Fe(II)(III)(OH)∗x. In addition, films formed in the active region also contained a species that resembled FeO. In the potential range between the passivation potential and the Flade poential, the film consisted of a mixture of Fe(II)(III)(OH)∗x and a species that resembled Fe3O4. Films formed above the Flade potential were composed of an inner region of Fe(II)(III)(OH)∗x and Fe3O∗4, and an outer, porous layer that resembled γ-FeOOH.
The abilities of zero-valent iron powder and hydrogen with a palladium catalyst (H2/Pd-alumina) to hydrodehalogenate 1,2-dibromo-3-chloropropane (DBCP) to propane under water treatment conditions (ambient temperature and circumneutral pH) were compared. DBCP reacted with iron powder (100–200 mesh, 36 g/l) in HEPES-buffered water (pH = 7.0) with a of 2.5 min and in different groundwaters (pH = 8.2–8.7) with a ranging from 41–77 min. Dissolved O2 and NO−3 slowed the transformation, whereas sulfate and nitrite had little effect. Iron removed 60 mg/l of nitrate within 14 min with nitrite as an intermediate. In 75 ml groundwater containing 22.5 mg 1% Pd-alumina catalyst under 10% H2 partial pressure, DBCP was transformed to propane within minutes. The rate in groundwater was slower by about 30% compared to Milli-Qtm water. SO2−4, NO−3, Cl− or O2 added to Milli-Q water only slightly inhibited DBCP transformation by H2/Pd-alumina, while SO2−3 had a much stronger inhibitory affect.
The reduction of nitrate to ammonia occurs with nearly complete conversion at room temperature and pressure under aerobic conditions in the presence of iron and either HCl or a pH buffer. A 50.0 mL solution of 12.5 millimolar nitrate is rapidly reduced to ammonia when exposed to 4.00 g of 325 mesh iron at pH 5.0, 0.05 M sodium acetate/acetic acid. The pseudo-first order rate constant was 0.053 min−1, Under conditions of pH 6.0 buffer, (i.e. 0.1 M 4-morpholineethanesulfonic acid adjusted to pH 6.0) and pH 7.0 buffer (0.1 M 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid adjusted to pH 7.0), the rate constants were 0.0408 min−1 and 0.0143 mint, respectively. In unbuffered solutions there was no loss in nitrate and no production of ammonia. A more concentrated nitrate solution (100 mL of 1.0 M sodium nitrate) was also reduced to ammonia in the presence of 2.5 M HCl with the slow addition of 50.0 g of 325 mesh iron.
Chemical reduction of nitrate by metallic iron (Fe0) was studied as a potential technology to remove nitrate from water. The effects of pH and the iron-to-nitrate ratio on both nitrate reduction rate and percent removal were investigated. Rate constants and the apparent reaction order with respect to nitrate were determined and a mass balance was obtained. Rapid nitrate reduction by iron powder was observed only at pH≤4. pH control with sulfuric acid significantly prolonged nitrate reduction and increased the percent removal. At high nitrate loadings, both the rate and the percent removal increased with decreasing pH. An iron-to-nitrate ratio of 120 m2 Fe0/mol NO3 or higher was required to completely remove nitrate within an hour. An apparent reaction order of 1.7 with respect to nitrate was observed, which may be partly due to the inhibitory effect of sulfate. Ammonia was the end product of nitrate reduction and accounted for all nitrate transformed under our experimental conditions. Acidity is the principal factor which controls the rate and the extent of nitrate removal by Fe0. The rapid reduction of nitrate at low pH was most likely due to either direct reduction by Fe0 or indirect reduction by surface hydrogen derived from proton. Ferrous species, Fe2+ and Fe(OH)2, were probably not involved in this reaction.
Permeable reactive barriers (PRBs) are receiving a great deal of attention as an innovative, cost-effective technology for in situ clean up of groundwater contamination. A wide variety of materials are being proposed for use in PRBs, including zero-valent metals (e.g., iron metal), humic materials, oxides, surfactant-modified zeolites (SMZs), and oxygen- and nitrate-releasing compounds. PRB materials remove dissolved groundwater contaminants by immobilization within the barrier or transformation to less harmful products. The primary removal processes include: (1) sorption and precipitation, (2) chemical reaction, and (3) biologically mediated reactions. This article presents an overview of the mechanisms and factors controlling these individual processes and discusses the implications for the feasibility and long-term effectiveness of PRB technologies.
The effect of nitrate on the reduction of TCE by commercial granular iron was investigated in column experiments designed to allow for the in situ monitoring of the iron surface film with Raman spectroscopy. Three column experiments were conducted; one with an influent solution of 100 mg/l nitrate+1.5 mg/l TCE, and two control columns, one saturated directly with 100 mg/l nitrate solution, the other pre-treated with Millipore water prior to the introduction of a 100 mg/l nitrate solution. In the presence of nitrate, TCE adsorbed onto the iron, but there was little TCE reduction to end-products ethene and ethane. The iron used (Connelly, GPM, Chicago) is a product typical of those used in permeable granular iron walls. The material is covered by an air-formed high-temperature oxidation film, consisting of an inner layer of Fe(3)O(4), and an outer, passive layer of Fe(2)O(3). In the control column pre-treated with Millipore water, the passive Fe(2)O(3) layer was removed upon contact with the water in a manner consistent with an autoreduction reaction. In the TCE+nitrate column and the direct nitrate saturation column, nitrate interfered with the removal of the passive layer and maintained conditions such that high valency protective corrosion species, including Fe(2)O(3) and FeOOH, were stable at the iron surface. The lack of TCE reduction is explained by the presence of these species, as they inhibit both mechanisms proposed for TCE reduction by iron, including catalytic hydrogenation, and direct electron transfer.
Under anoxic conditions, zerovalent iron (Fe(0)) reduces nitrate to ammonium and magnetite (Fe3O4) is produced at near-neutral pH. Nitrate removal was most rapid at low pH (2-4); however, the formation of a black oxide film at pH 5 to 8 temporarily halted or slowed the reaction unless the system was augmented with Fe(2+), Cu(2+), or Al(3+). Bathing the corroding Fe(0) in a Fe(2+) solution greatly enhanced nitrate reduction at near-neutral pH and coincided with the formation of a black precipitate. X-ray diffractometry and scanning electron microscopy confirmed that both the black precipitate and black oxide coating on the iron surface were magnetite. In this system, ferrous iron was determined to be a partial contributor to nitrate removal, but nitrate reduction was not observed in the absence of Fe(0). Nitrate removal was also enhanced by augmenting the Fe(0)-H2O system with Fe(3+), Cu(2+), or Al(3+) but not Ca(2+), Mg(2+), or Zn(2+). Our research indicates that a magnetite coating is not a hindrance to nitrate reduction by Fe(0), provided sufficient aqueous Fe(2+) is present in the system.
The effect of low pH (2-4.5) on nitrate reduction in an iron/nitrate/water system was investigated through batch experiments conducted in a pH-stat. The results showed that nitrate could be rapidly reduced to ammonium at pH 2-4.5. A black coating, consisted of both Fe(II) and Fe(III), was formed on the surface of iron grains as an iron corrosion product. X-ray diffractometry indicated that the black coating was poorly crystalline, and its spectrum could not be matched with commonly known iron oxides/hydroxides/oxide hydroxides or green rust I/II. The black coating does not inhibit the reactivity of Fe0 (at least at pH < 3). The black coating was unstable and evolved with time into other oxides under certain conditions. A kinetic model incorporating the effects of pH on nitrate reduction and Langmuir adsorption of nitrate was proposed, and the parameters were estimated by nonlinear curve fitting. Based on this model, the two major effects of pH on the kinetics of nitrate reduction are that: (a) H+ ions directly participate in the redox reaction of nitrate reduction following first-order kinetics; and (b) H+ ions affect the nitrate adsorption onto reactive sites.
This study investigated the reaction mechanisms of nitrate (NO3-) with zerovalent iron (ZVI) media under conditions relevantto groundwatertreatment using permeable reactive barriers (PRB). Reaction rates of NO3- with freely corroding and with cathodically or anodically polarized iron wires were measured in batch reactors. Tafel analysis and electrochemical impedance spectroscopy (EIS) were used to investigate the reactions occurring on the iron surfaces. Reduction of NO3- by corroding iron resulted in near stoichiometric production of NO2-, which did not measurably react in the absence of added Fe(II). Increasing NO3- concentrations resulted in increasing corrosion currents. However, EIS and Tafel analyses indicated that there was little direct reduction of NO3- at the ZVI surface, despite the presence of water reduction. This behavior can be attributed to formation of a microporous oxide on the iron surfaces that blocked reduction of NO3- and NO2- but did not block water reduction. This finding is consistent with previous observations that NO3- impedes reduction of organic compounds by ZVI. Nitrite concentrations greater than 4 mM resulted in anodic passivation of the iron, but passivation was not observed with NO3- concentrations as high as 96 mM. This indicates that the passivating oxide preventing NO3- reduction was permeable toward cation migration. Since reaction with Fe(0) can be excluded asthe mechanism for NO3- and NO2- reduction, reaction with Fe(II)-containing oxides coating the iron surface is the most likely reaction mechanism. This suggests that short-term batch tests requiring little turnover of reactive sites on the iron surface may overestimate long-term rates of NO3- removal because the effects of passivation are not apparent in batch tests conducted with high initial Fe(II) to NO3- ratios.
Transformation of atrazine and nitrate in contaminated water by iron-promoted processes
- J Singh
- T C Zhang
- P J Shea
- S D Comfort
- L S Hundal
- D S Hage
Singh, J., Zhang, T.C., Shea, P.J., Comfort, S.D., Hundal, L.S., Hage, D.S., 1996. Transformation of atrazine and nitrate in contaminated water by iron-promoted processes. Proceed-ings of the Water Environment Federation 69th Annual Conference and Exposition, vol. 3, Dallas, TX, pp.143–150.
Transformation of g-FeOOH to Fe 3 O 4 by adsorption of iron(II) ion on g-FeOOH
- Y Tamaura
- K Ito
- T Katsura
Tamaura, Y., Ito, K., Katsura, T., 1983. Transformation of g-FeOOH to Fe 3 O 4 by adsorption of iron(II) ion on g-FeOOH. J. Chem. Soc. Dalton Trans. 1983, 189–194.
Mineral-water interfacial reactions: kinetics and mechanisms
- Scherer