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Reduction of Nitro Aromatic Compounds by Zero-Valent Iron Metal
Abstract
The properties of iron metal that make it useful in remediation of chlorinated solvents may also lead to reduction of other groundwater contaminants such as nitro aromatic compounds (NACs). Nitrobenzene is reduced by iron under anaerobic conditions to aniline with nitrosobenzene as an intermediate product. Coupling products such as azobenzene and azoxybenzene were not detected. First-order reduction rates are similar for nitrobenzene and nitrosobenzene, but aniline appearance occurs more slowly (typical pseudo-first-order rate constants 3.5 × 10-2, 3.4 × 10-2, and 8.8 × 10-3 min-1, respectively, in the presence of 33 g/L acid-washed, 18−20 mesh Fluka iron turnings). The nitro reduction rate increased linearly with concentration of iron surface area, giving a specific reaction rate constant (3.9 ± 0.2 × 10-2 min-1 m-2 L). The minimal effects of solution pH or ring substitution on nitro reduction rates, and the linear correlation between nitrobenzene reduction rate constants and the square-root of mixing rate (rpm), suggest that the observed reaction rates were controlled by mass transfer of the NAC to the metal surface. The decrease in reduction rate for nitrobenzene with increased concentration of dissolved carbonate and with extended exposure of the metal to a particular carbonate buffer indicate that the precipitation of siderite on the metal inhibits nitro reduction.
... For recalcitrant substances, however, pure adsorption barriers are often not sufficient; an additional pollutant degradation function is required [12,13]. Alternative methods for NAC removal exist, such as chemical reduction, e.g. with zero-valent iron, but this produces reduction products that have comparable or even higher toxicities than the initial NAC contaminants [14,15]. However, some of the partially reduced NACs, like diaminonitrotoluenes (DANTs), are known to be removed from the aqueous phase due to reactions with soil organic matter [16]. ...
... With total NAC concentrations of 10 mg L − 1 and a zeolite concentration of 5 g L − 1 , an experimental setup was chosen where the major amount of the NACs is present in the adsorbed state (q 0 ≈ 0.2 wt%, c free << 1 mg L − 1 , compare Figs. SI- [6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Due to the large extent of adsorption of the NACs, educt depletion cannot be directly measured by analysis of the liquid phase. ...
Nitroaromatic compounds (NACs) are prominent explosives. In this context, these toxic substances were released into the environment and cause long-lasting groundwater contamination. In preparation of a possible in-situ remediation, colloidal Fe-zeolites were investigated for their capabilities as adsorbents and oxidation catalysts. It was shown that the Fe-zeolites FeBEA35 and FeFAU55 are potent inorganic adsorbents for NACs and simultaneously capable of activating H2O2 as Fenton-like oxidation catalysts. Adsorption isotherms of 15 NACs on both zeolites were measured to evaluate the option of coupling adsorptive contaminant enrichment with oxidative degradation. The faujasite-type zeolite FeFAU55 showed a distinct S-type adsorption behaviour and reached significantly higher NAC loadings of > 20 wt%. For FeBEA35, L-type adsorption isotherms and maximum loadings qmax of about 4 wt% were obtained. Degradation of all NACs, monitored by nitrate formation, was observed. Apparent rate constants of the NACs with hydroxyl radicals in a homogeneous, stoichiometric Fenton reaction were related to the heterogeneous system to examine the role of adsorption on the oxidative degradation. Beneficial influence of the adsorption on the oxidation rates was identified. The results of this work open up promising prospects for future application of Fe-zeolites for the in-situ remediation of NAC-contaminated groundwater.
... Nitrobenzene is widely used in the industrial production of different products including pesticides, fertilizers, dyes, drugs, and rubber [1]. However, improper discharge of nitrobenzene from industries can cause serious environmental issues [2]. ...
... The Langmuir−Hinshelwood kinetics are characteristic of surface-mediated processes. 4 The data series for pH 0 = 3 and 4 ( Figure 3D) show both behaviors: with the initial drop more strongly evident in the pH 3 data and the Langmuir−Hinshelwood behavior more strongly evident in the pH 4 data. ...
... [15] Even though most of the studies have been performed from the electrosynthetic point of view, [16][17] the chemical reduction of nitrobenzene to phenylhydroxylamine has been also performed using reductants or catalysts. [18][19][20] In this regard, the electrochemical reduction of nitrosobenzene (PhNO) has been also studied due to its importance as intermediate precursor of most of the final products obtained by reduction of nitrobenzene, either in aqueous media [21][22][23] or in solvents like dimethylformamide, [24,25] acetonitrile, [26][27][28][29] DMSO, [30][31] mixtures of solvents [32,33] and liquid ammonia. [34] In presence of proton donors, the electrochemical reduction of nitrosobenzene is simpler with respect to that of nitrobenzene in the sense that its reduction involves the half of the electron and proton transfer steps (Eq. ...
The electrochemical reduction mechanism of nitrosobenzene in acetonitrile was reconsidered in this work. The commonly accepted mechanism assumes that the nitrosobenzene radical anion dimerize to yield a dianion, which decompose into azoxybenzene as the reduction product. Even though this proposal is consistent with preparative experiments, it does not reproduce fully the experimental voltammetric behaviour when it is simulated. Also, It does not explain why the apparent electron number is different in the time scale of cyclic voltammetry and preparative electrolysis. To understand these discrepancies, it was proposed the intervention of reactions consuming part of the starting nitrosobenzene, which allowed to reconsider the role of alternative chemical reactions like that occurring between the anion of phenylhydroxylamine with nitrosobenzene and the hydroxyl ion with nitrosobenzene. In this framework, these concomitant reactions were additionally included in the mechanism to simulate the voltammetric behaviour at different scan rates. Thermodynamic and kinetic parameters of the extended mechanism are reported. Since in the hydroxyl ion is either product or reactive in some reaction steps, the global mechanism was deemed to occurs as a kind of loop.
... These results show that though wet chemical extraction can give some information on the extent of Al substitution, it is unable to differentiate the Al substitution amounts in different types of Fe (hydr) oxides. This may be probably caused by dissolution of gibbsite or other poorly crystalline Al (hydr)oxides during the wet chemical extraction process [65,66], the ineffectiveness of DCB to completely dissolve certain Fe (hydr)oxides such as magnetite and hematite [67][68][69], and the capability of these extracts to reduce Fe(III) in specific clay minerals and thus to extract Fe from these minerals [70,71]. XRD analysis allows for the quantification of Al substitution levels in different types of crystalline Fe (hydr)oxides, but not for poorly crystalline Fe phases such as ferrihydrite. ...
... The minor balance gap could be attributed to the slight adsorption of ArNH 2 by FeS mineral ( Figure S3) and the potential production of other nondetected intermediates (e.g., azoxybenzene and azobenzene) with very low transient concentrations. 38 It is noteworthy that ArNH 2 was the dominant nitrogen-containing reduction product of ArNO 2 , exhibiting a well-maintained nitrogen balance (Figure 1c). ...
... To mediate this problem, many methods have been developed to disrupt or modulate the distribution of iron (hydr)oxides on the surface of ZVI , including acid washing (Agrawal and Tratnyek, 1995), H 2 pretreatment (Liou et al., 2005), ultrasonication (Geiger et al., 2002), dosing chemicals (e.g., Fe 2+ or oxidants) (Qin et al., 2017, Fan et al., 2019aUllah et al., 2020), and externalization of weak magnetic field (WMF) (Liang et al., 2014a), etc. Although these strategies can enhance the initial reactivity of ZVI toward the target contaminant, they are also able to accelerate the side-reaction of ZVI with the natural substances (e.g., O 2 , and H 2 O/H + ), which results in the waste of Fe 0 content and thus reducing the reductive durability of ZVI against the target contaminants. ...
Sulfated zero-valent iron (SZVI) has shown promising applications in wastewater treatment. However, the rapid decline in the reactivity of SZVI with time limits its real practice. To mediate this problem, partial aging was proposed to improve the reactive durability of SZVI. Taking Cr(VI) as the target contaminant, we found that the aged ZVI (AZVI) gradually lost reactivity as aging time increased from 0.5 to 2 d. Counter-intuitively, the partially aged SZVI (ASZVI) showed greater reactivity than SZVI when exposed to oxygenated water for a period ranging from 0.5 to 14 d. In addition, the ASZVI with 0.5 d of aging time (ASZVI-0.5) not only maintained reactivity in successive runs but also increased the Cr(VI) removal capacity from 9.1 mg/g by SZVI to 19.1 mg/g by ASZVI-0.5. Correlation analysis further revealed that the electron transfer from the Fe ⁰ core to the shell was mediated by the conductive FeS and FeS 2 in the subshell of ASZVI. Meanwhile, the lepidocrocite and magnetite on the surface of ASZVI facilitated Cr(VI) adsorption and subsequent electron transfer for Cr(VI) reduction. Moreover, the iron (hydr)oxide shell could retain the conductive FeS and FeS 2 in the subshell, allowing ASZVI to reduce Cr(VI) efficiently and sustainably. In general, partial aging can enhance the reactive durability of ZVI when coupled with sulfidation and this synergistic effect will be beneficial to the application of SZVI-based technology for wastewater treatment.
... For the initial screening, trisubstituted pyrroles 4a−c were synthesized with a previously reported methodology that used FCCs as the platform. 19,20 Subsequently, 1,3dinucleophilic system 6a−c was synthesized by amination of the corresponding pyrrole through a nitration/nitro reduction process based on a published methodology, 21 slightly modified to afford compounds 6a−c in high yields (Scheme 2). ...
A novel synthetic methodology is reported for the synthesis of fluorescent pyrrolo[1,2-a]pyrimidines. Fischer carbene complexes served as the synthetic platform for (3+3) cyclization to form the heterocyclic moiety. The reaction process furnished two products, their ratio being modulated by the metal, base, and solvent. The selectivity exhibited was studied by analyzing the potential energy surface with density functional theory tools. The photophysical properties of absorption and emission were also evaluated. The dyes absorbed at wavelengths of 240-440 nm, depending on the substituents. The maximum emission wavelength was in the range of 470-513 nm, with quantum yields of 0.36-1.0 and a high Stokes shift range of 75-226 nm.
... Also, industrial filing iron waste can be used to add two electrons to a variety of environmental pollutants by converting it into zero-valent iron [10]. It has been demonstrated to be effective in reducing nitro-aromatic chemicals, insecticides, nitrate, and ions of metal as Cr (VI) [11][12][13][14], among other chlorinated solvents [15] studied the capability of zero-valent iron to remove arsenic compounds for the groundwater. Their results showed that more than 98% of arsenate could be removed steadily with a hydraulic resident time of two hours at last, and the effluent meets the drinking water standard. ...
... ZVI reductively transformed NTO into its amine daughter product, ATO, indicating that the mechanism of NTO removal by ZVI involves reduction of nitro groups. It is well established that nitro-containing compounds such as nitroguanidine and nitrobenzene are reductively transformed by ZVI into their amine daughter product, with occasional detection of nitroso or hydroxylamine intermittent products (Agrawal and Tratnyek 1995;Rios-Valenciana et al. 2022). In our experiments, nitroso or hydroxylamine products were not detected during the reductive transformation of (1) NTO by ZVI. ...
The need for effective technologies to remediate the insensitive munitions compound 3-nitro-1,2,4-triazol-5-one (NTO) is emerging due to the increasing use by the US Army and environmental concerns about the toxicity and aqueous mobility of NTO. Reductive treatment is essential for the complete degradation of NTO to environmentally safe products. The objective of this study is to investigate the feasibility of applying zero-valent iron (ZVI) in a continuous-flow packed bed reactor as an effective NTO remediation technology. The ZVI-packed columns treated an acidic influent (pH 3.0) or a circumneutral influent (pH 6.0) for 6 months (ca. 11,000 pore volumes, PVs). Both columns effectively reduced NTO to the amine product, 3-amino-1,2,4-triazol-5-one (ATO). The column treating the pH-3.0 influent exhibited prolonged longevity in reducing NTO, treating 11-fold more PVs than the column treating pH-6.0 influent until the breakthrough point (defined as when 85% of NTO was removed). The exhausted columns (defined as when only 10% of NTO was removed) regained the NTO reducing capacity by reactivation using 1 M HCl, fully removing NTO. After the experiment, solid-phase analysis of the packed-bed material showed that ZVI was oxidized to iron (oxyhydr)oxide minerals such as magnetite, lepidocrocite, and goethite during NTO treatment. This is the first report on the reduction of NTO and the concomitant oxidation of ZVI in continuous-flow column experiments. The evidence indicates that treatment in a ZVI-packed bed reactor is an effective approach for the removal of NTO.
Graphical abstract
... Catalytic transfer hydrogenation has traditionally been carried out in the presence of Ru, 19 Rh, 20 Pd, 21 or Ni 22 metal catalysts. Only a few researchers have used low-cost Co, 23 Fe, 24 or Zn. 25 Several other reducing agents, in addition to gaseous hydrogen, have been utilized to efficiently reduce nitrobenzene when employed in combination with metals, including boranes, 26 NaBH 4 , 27 silanes, 28 and hydrazine. 29,30 Some of the heterogeneous cobalt catalyst reports for nitro reduction are shown in Scheme 1. [31][32][33] All of these reports use hazardous H 2 gas as a source of hydrogen. ...
A heterogeneous CoOCN composite, synthesized via a one-pot reaction of [Co(NO3)2] and urea at 500 °C in muffle furnace. The composite was fully characterized by the FTIR, Raman, powder XRD and XPS techniques. The catalyst was found to be efficient for the hydrations of aryl alkynes and nitriles under aerobic conditions. Apart of that, the catalyst exhibits high catalytic performance for the reduction of nitroarenes under the inert gas free conditions. This multitasking CoOCN was found to be highly significant for the all derivatives of nitrobenzene, alkynes and nitriles as good to excellent yields were obtained. The catalyst was recovered quantitatively from the reaction mixture by simple filtration and consequently reused for seven consecutive cycles in all reactions without significant loss of catalytic activity. Hence, the synthesized CN doped CoOCN composite worked as multitasking catalyst for various value added organic transformations, and it is highly economical and reusable upto seven catalytic cycles without any activation, even the last cycle was producing a reasonable yields up to 48-50%.
... Fenton oxidation with hydrogen peroxide activation produces sludge with high levels of Fe ions, which is expensive to treat later (Ramirez et al. 2007;Melgoza et al. 2009). Zero-valent metal powder is prone to aggregation and oxidation, which is difficult to separate from solutions, resulting in low degradation reactivity (Fu et al. 2014;Ruiz et al. 2000;Agrawal and Tratnyek 1995). Therefore, a new method to efficiently degrade azo dye wastewater should be explored. ...
The Fe75B12.5Si12.5 and Fe75B12.5C12.5 amorphous alloy ribbons were prepared by the melt spinning method. The decolorization performances of these ribbons were investigated in details. It is found that the Fe75B12.5C12.5 amorphous ribbons and Fe75B12.5Si12.5 annealed ribbons only adsorbed the azo dye molecules, with no chemical degradation process. However, the Fe75B12.5Si12.5 amorphous ribbons can reduce -N = N- to -NH2 because of their high reactivity and the local galvanic effect that occurred during the reaction to accelerate electron transfer. The reaction rate constant kobs is 0.0872 min⁻¹, 0.0474 min⁻¹, and 0.0064 min⁻¹ for Fe75B12.5Si12.5 amorphous ribbons, Fe75B12.5C12.5 amorphous ribbons, and Fe75B12.5Si12.5 annealed ribbons in the same condition, respectively. Fe75B12.5Si12.5 amorphous ribbons can effectively degrade Acid Orange II (AO II) azo dyes and achieve decolorization by breaking azo bonds in the dye in a short time, indicating the prominent capacity of Fe75B12.5Si12.5 ribbons on the degradation of AO II. Furthermore, the influence of chemical factors such as ribbons thickness, reaction temperature, initial pH, and AO II concentration of the solution on the reaction rate constant kobs of Fe75B12.5Si12.5 amorphous ribbons had also been studied. The kobs can reach 0.177 min⁻¹ under optimal conditions. In addition, all the degradation processes in this work were fitted well with the pseudo-first-order kinetic model. The results are guidance for the practical applications, and they have important implications in developing Fe-based amorphous alloys for functional application materials in the field of wastewater treatment.
... It is considered as a highly erratic and explosive organic solvent [1]. So, exposure of living beings to an environment having NB concentration exceeding 0.11 mgL -1 is highly threatening [2]. More importantly, the United States Environmental Protection Agency (USEPA) has recognized the NB as Group 2B carcinogen. ...
This work reports a first-principles density functional theory (DFT) investigation of nitrobenzene (NB) sensing in pristine and metal-decorated biphenylene (p-BP and m-BP, m=Sc, Cu, Li, and Pd) 2D monolayer. Though the proposed p-BP monolayer displayed excellent adsorption of NB molecule compared to other reported 2D materials, it gets physisorbed on the p-BP surface with -0.621 eV adsorption energy. The metal decoration drastically reformed the NB adsorption capacity of BP and induced remarkable changes to the electronic properties. The ab-initio molecular dynamics simulations were employed to analyze the room temperature structural integrity of the m-BP-based NB sensor. Among all the systems considered, the Pd decorated BP monolayer came out as the potential NB sensor with a reasonable adsorption energy of -1.42 eV, charge transfer of 0.38 e, the recovery time of 0.92 s at 598 K, and structural solidity at ambient temperature. The higher diffusion barrier of 0.99 eV confirmed that the Pd dopant does not show the tendency to form clusters. The study demonstrates that the Pd decorated BP is highly apposite for NB sensing and is beneficial for practical application. Our study opens a new avenue for designing and developing efficient biomolecule sensors.
... In the past few decades, zero-valent iron (ZVI) has been widely applied in groundwater remediation and wastewater treatment, owing to its relatively strong ability of contaminant treatment, environmental friendliness, and low cost [1,2]. However, the surface passivation of ZVI extremely decreases its reactivity and longevity, thereby limiting its practical application [3,4]. ...
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.
... A number of chemical and biological methods have been developed that are used to reduce nitrobenzene to aniline, such as microelectrolysis [102], electrochemical reduction [103], and the biological anaerobic process [104]. Typically, chemical methods can reduce nitrobenzene faster than biological processes, but they require more or significant amounts of chemicals and relatively expensive noble and transition metal catalysts, which add cost and can cause secondary pollution. ...
Methods for creating various 3D morphologies of composites based on chitosan and copper nanoparticles stabilized by it in carbonic acid solutions formed under high pressure of saturating CO2 were developed. This work includes a comprehensive analysis of the regularities of copper nanoparticles stabilization and reduction with chitosan, studied by IR and UV-vis spectroscopies, XPS, TEM and rheology. Chitosan can partially reduce Cu2+ ions in aqueous solutions to small-sized, spherical copper nanoparticles with a low degree of polydispersity; the process is accompanied by the formation of an elastic polymer hydrogel. The resulting composites demonstrate antimicrobial activity against both fungi and bacteria. Exposing the hydrogels to the mixture of He or H2 gases and CO2 fluid under high pressure makes it possible to increase the porosity of hydrogels significantly, as well as decrease their pore size. Composite capsules show sufficient resistance to various conditions and reusable catalytic activity in the reduction of nitrobenzene to aniline reaction. The relative simplicity of the proposed method and at the same time its profound advantages (such as environmental friendliness, extra purity) indicate an interesting role of this study for various applications of materials based on chitosan and metals.
... According to the World Health Organization (WHO) and American Public Health Association (APHA), all type of (waste) water is considered to be toxic to the environment and humans when reaching the limit of 2 mg/L and above [7][8][9]. Moreover, it causes serious health problems, such as methemoglobinemia, shortness of breath, nausea, vomiting, and liver cancer [10][11][12]. The NB industrial waste in drinking water is harmful to the environment because this is stable in water and the electron-withdrawing nature of the nitro group on the aromatic ring [11]. ...
Nanoscale materials are of significant interest and excellent electrocatalysts for feasible and rapid electrochemical sensing of toxic nitro compounds and are a crucial need for human health and safety applications. This work designed a sphere-like MoS2 nanosheet array decorated on reduced graphene oxide (MoS2 NSAs/rGO) by a simple hydrothermal method for accurate electrochemical detection of nitrobenzene (NB) in water samples. The physicochemical characterizations are well characterized and confirm the successful MoS2 NSAs/rGO formation. Further, the MoS2 NSAs/rGO modified GCE investigated for NB determination using the voltammetry technique, which shows a wide linear range of 0.005-849.505 µM with a low limit of detection (LOD) of 0.0072 µM and excellent sensitivity of 1.8985 µA µM⁻¹.cm⁻². Moreover, the developed sensor exhibited good cycle stability (80.35%), selectivity, repeatability, and reproducibility. Finally, the real-sample analysis for the developed sensor accomplished in tap and river water samples with appreciable recoveries thus provides an effective platform for trace level detection of hazardous NB determination in environmental samples.
... It is considered as a highly erratic and explosive organic solvent [1]. So, exposure of living beings to an environment having NB concentration exceeding 0.11 mgL -1 is highly threatening [2]. More importantly, the United States Environmental Protection Agency (USEPA) has recognized the NB as Group 2B carcinogen. ...
... It can also be supported by the increased surface area from 37.1 to 40.9 m 2 g -1 (Table 1) with the addition of cerium from x = 0.0 to x = 0.2 compositions. The rate of nitrobenzene reduction shows a linear dependency on the surface area of the catalyst [72]. Afterward, in the case of x = 0.3, the isolated yield (%) has a minor drop with the enhanced surface area value (42.3 m 2 g -1 ). ...
This paper describes the synthesis of Ce-substituted Y-type barium hexaferrites (Co2–Y) {Ba2Co2Fe12-xCexO22 (0 ≤ x ≥ 0.30)} through the sol-gel auto-combustion route and its suitability as a heterogeneous catalyst for the reduction of nitroaromatic-compounds, one of the significant water contaminants. The synthesized nanocrystalline hexaferrite powders were characterized using XRD, FTIR, BET, SEM, and VSM. Furthermore, the catalytic functioning of the Co2–Y hexaferrite powder was evaluated based on the isolated yield of aniline in the reaction product through column chromatography. It was found that 100 mg pure Co2–Y hexaferrite samples could provide complete conversion of nitrobenzene and 65% selectivity of aniline within the water as a solvent and two mmol of reducing agent, hydrated hydrazine (N2H2.H2O), at 100 °C for 12 h. The influence of Ce substitution at the iron site for the aniline selectivity was also investigated in optimized situations. In comparison with pure Co2–Y hexaferrite, the catalytic efficiency was considerably enriched in all substituted samples and obtained its extreme performance (73% selectivity of aniline) with Ba2Co2Fe11.80Ce0.20O22 ferrite. The enriched performance was explained with the help of surface chemistry using the XPS & BET analysis for these samples. The as-prepared catalysts were magnetically-separable and recycled for five subsequent trials without any significant losses of catalytic performance. This catalyst may be convenient to be adopted for industrial applications due to its ease of synthesis, extraordinary stability, superior catalytic competence, and cost-effectiveness.
There has been a growing interest in the design and development of graphene based composite materials with superior performances for environmental catalytic applications. But in most of the studies the synthesis conditions require elevated temperatures and expensive working setups (high temperature furnaces, autoclaves, inert atmosphere conditions etc.). In this reported work, the nitrogen doped reduced graphene oxide supported CuCo2O4 (NG/CuCo2O4) composites were prepared through a simple one pot synthesis method under mild conditions (∼95 °C and air atmosphere) and successfully employed as catalysts for the reduction of toxic 4-nitrophenol (4NP). The characterization results revealed the successful formation of NG/CuCo2O4 composites with a possible charge transfer interaction between nitrogen doped reduced graphene oxide support of CuCo2O4. The NG/CuCo2O4 hybrids exhibited robust catalytic activity in 4NP reduction with an activity factor of 261.5 min−1 g−1. A 4NP conversion percentage which is as high as 99.5% was achieved within 11 min using the NG/CuCo2O4 catalyst. The detailed kinetic analysis confirmed the Langmuir-Hinshelwood model for the NG/CuCo2O4 catalysed 4NP reduction. The nitrogen doped reduced graphene oxide support modified the electronic levels of CuCo2O4 nanoparticles through electron transfer interactions and enhanced the catalytic activity of CuCo2O4 in NG/CuCo2O4 through improved adsorption of reactant ions and effective generation of active hydrogen species. The good reusability and stability along with profound activity of NG/CuCo2O4 catalyst makes it a promising material for wide scale catalytic applications.
An efficient visible-light-induced iron-catalyzed reduction of nitroarenes to anilines by using N-ethylmorpholine (NEM) as a reductant under mild conditions has been developed. The reaction proceeds with photosensitizer-free conditions and features...
In this study Nanoscale Zero-Valent Iron Fe 0 (NZVI) and Nano Zero Valent-Iron supported on pillared clay(NZVI/PILC) have been prepared and characterizations by physical method such as Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The degradation of acidic aqueous solutions of the Acid red 315 (AR 315) azo dye has been studied by NZVI, pillared clay (PILC) and NZVI-B. The effect of different process parameters, such as solution pH, amount of dosage (NZVI, PILC and NZVI/PILC), time reaction effect and other experimental variable, such as (Azo dye concentration and inorganic salts effect) has been investigated to determined optimization method for removal. The concentration of azo dye measured before and after treatment by using UV-Vis Spectrophotometry method. The experimental results showed that AR 315 azo dye solution (100 mg/L, 1.6×10-4 M) was completely removed by NZVI at optimum conditions (amount of NZVI = 1.0 g, 120 min and pH = 3). While the removal efficiency with NZVI/PILC and PILC were 80% and 0% respectively.
Gold nanoparticles supported on hydroxyapatite functions as a very efficient catalyst for the reduction of nitroarenes as well as for the degradation of azo dyes. The reaction takes place in aqueous medium at room temperature, using sodium borohydride as the source of hydrogen. The catalyst was prepared by a deposition–precipitation process using gold (III) chloride trihydrate solution containing hydroxyapatite as the support. The catalyst was thoroughly characterized by a pltehora of analytical techniques viz., TEM, HRTEM, FESEM, powder XRD, EDX and FTIR. The catalyst was then employed after optimization of reaction conditions. No additives or inert atmosphere was required and a very low loading of gold was sufficient enough to promote the reaction. Reaction kinetics studies were performed on the reduction of 4-nitrophenol to 4-aminophenol and a very high apparent rate constant of 1.63 × 10–2 s⁻¹ was obtained. Reaction kinetics studies have also been demonstrated for the degradation of methyl orange and congo red dyes. Appreciable apparent rate constants namely 8.678 × 10⁻³ and 3.464 × 10⁻³ s⁻¹ were obtained for the degradation of methyl orange and congo red dyes respectively. The catalyst was recoverable by simple centrifugation and can be reused for at least five reaction cycles.
The military is switching over to insensitive munitions compounds (IMCs) to avoid unintentional detonations during handling and use of explosives. 3-nitro-1,2,4-triazol-5-one (NTO) is an important component of IMCs. NTO may contaminate the subsurface due to its high aqueous solubility. Thus, there is a need to develop remediation technologies for the treatment of NTO-containing (waste) water. This study demonstrated that zero-valent iron (ZVI) reductively transformed NTO to its daughter product, 3-amino-1,2,4-triazol-5-one. The pseudo first-rate constant (k1) of NTO reduction by micron-sized ZVI at pH 3 was 192.6 h−1. Kinetic degradation experiments performed at different pH values showed that ZVI did not effectively reduce NTO at pH 6 (k1 = 0.6 h−1) or higher. The rapid NTO reduction in acidic conditions may be due to dissolution of iron precipitates on the ZVI surface. Additional experiments were conducted to assess the effectiveness of various depassivating pretreatments with deionized water, acetic acid, hydrochloric acid, or bicarbonate. Treatment with 1 M HCl for 15 min was the most effective depassivation method for a ZVI material containing a thick passivating layer (ca. 880 nm), achieving 84.0% NTO removal after 10 min of reaction. On the other hand, a milder treatment involving washing with a diluted bicarbonate solution (60 mM) was sufficient for a ZVI material that was less passivated (estimated thickness of the passivating layer ≈ 300 nm). This study demonstrates that ZVI treatment is a promising approach for the remediation of NTO-contaminated sites or wastewater and provides critical information to optimize this process.
Soils contaminated with multiple heavy metal(loid)s (HMs) such as lead (Pb), cadmium (Cd), and arsenic (As) are of great concern in many countries. In this study, taking three lead-zinc smelter soils, the performance of sulfidated zero-valent iron (S-ZVI) toward Pb, Cd, and As immobilization was systemically investigated. Results showed that more than 88% of water-extractable Pb and Cd could be immobilized and transformed into reducible, oxidizable, and/or reducible forms by S-ZVI within 3 h, whereas only 3-56% of them could be immobilized by unsulfidated ZVI even after 72 h. Meanwhile, the phytoavailability of the tested HMs could be effectively reduced by 79% after S-ZVI amendment. More importantly, anoxic/oxic incubation tests revealed that the dissolved concentrations of HMs were much lower in S-ZVI-treated soils than in the untreated or unmodified ZVI-treated soils. Speciation analysis further suggested that unmodified ZVI seemed to reduce the long-term soil stability by changing the residual HMs species to mild-acid soluble and/or reducible ones. In contrast, S-ZVI could effectively alleviate the remobilization of HMs under the changeover of soil redox environments. All these findings indicate that S-ZVI may be a promising amendment for the immobilization of Pb, Cd, and As in smelter-contaminated soil.
Metallic glasses (MGs) as effective catalysts have been extensively studied due to essentially disordered atomic configurations and widely adjustable micro-morphologies. The catalysis performance could be greatly promoted by introducing additional crystalline phases in the amorphous matrix due to the synergistic advantages of the crystalline and amorphous phases. However, the conventional casting and annealing approaches induced amorphous-crystalline (a/c) composites restrict the synergistic and galvanic cells effects because the generated crystalline phases are easily coarsened with meager a/c interfaces. Here, the artificial ultra-fine a/c Fe76Si8B13Nb3 catalyst with spinodal decomposition morphology and extremely high dense a/c interfaces of 2 × 10¹⁶ m⁻² are achieved from MG film precursor with nanoscale phase separation by controllable surface diffusion during deposition and suppressive crystalline coarsening procedures. The designed ultra-fine a/c catalyst exhibits admirable cycling degradation property and extraordinary dye degradation efficiency of 300 times than that of the commercial Fe powder. Especially, the outstanding catalytic performances of a/c composite are achieved without the additional involvement of hydrogen peroxide assistance, which provides an environmental-friendly neutral catalytic condition and avoids the corrosive damage during commercial sewage-treatment. This work provides a distinct perspective to design and regulate catalytic performances by amorphous precursor with pre-existent ultra-fine structures.
Hydrogenation of chloronitrobenzenes (CNB) emerges as a prominent topic of interest in catalysis research owing to its enormous industrial importance. However, hydrogenation of CNB is difficult due to the undesired side reaction of hydrodechlorination. As a result, selective nitro group hydrogenation in the presence of other reducing groups (C-Cl) continues to be challenging. Recent research has focused on the development of more selective catalysts. A huge variety of metal-based nanocatalysts and significant research outcomes have been reported in recent years. However, nanoparticle agglomeration and the cost-effectiveness of the catalyst are considered the critical controlling parameter for hydrogenation reactions. Carbon materials have become particularly promising carriers for such hydrogenation processes to overcome this challenge due to their unique chemical and structural properties. More recently, the widespread usage of novel carbons such as carbon nanotubes and graphene, as well as the fine-tuning of surface chemistry with heteroatom doping, have recently opened up new avenues for their applications in catalysis, particularly in CNB hydrogenation. Hence, this review aims at the selective hydrogenation of CNB on carbon-supported metal-based catalysts and summarizes the latest improvements in the use of different carbonaceous materials in this regard, and investigates the role of carbon support through the mechanistic pathway.
The application of iron—carbon (Fe—C) micro-electrolysis to wastewater treatment is limited by the passivation potential of the Fe—C packing. In order to address this problem, high-gravity intensified Fe—C micro-electrolysis was proposed in this study for degradation of dinitrotoluene wastewater in a rotating packed bed (RPB) using commercial Fe—C particles as the packing. The effects of reaction time, high-gravity factor, liquid flow rate and initial solution pH were investigated. The degradation intermediates were determined by gas chromatography-mass spectrometry, and the possible degradation pathways of nitro compounds by Fe—C micro-electrolysis in RPB were also proposed. It is found that under optimal conditions, the removal rate of nitro compounds reaches 68.4% at 100 min. The removal rate is maintained at approximately 68% after 4 cycles in RPB, but it is decreased substantially from 57.9% to 36.8% in a stirred tank reactor. This is because RPB can increase the specific surface area and the renewal of the liquid—solid interface, and as a result the degradation efficiency of Fe—C micro-electrolysis is improved and the active sites on the Fe—C surface can be regenerated for continuous use. In conclusion, high-gravity intensified Fe—C micro-electrolysis can weaken the passivation of Fe—C particles and extend their service life.
Nitrophenols are identified as the priority organic pollutants due to the chemical stability, water solubility, persistence, and toxicity to human health and the environment. Hence, removal of nitrophenols from waste water is vitally essential. In this study, amino-rich coordination polymer Cu2I2(MA)2 (MA = melamine) has been applied for efficient adsorption and catalytic reduction of nitrophenols, like 4-nitrophenol (4-NP), 2, 4-dinitrophenol (DNP) and 2, 4, 6-trinitrophenol (TNP). The effect of various parameters like contact time, initial concentrations, pH, and temperature on adsorption were investigated. The adsorption of nitrophenols fitted the pseudo-second-order kinetic model and Langmuir isotherms model well. The maximum adsorption capacities were 285.71, 232.02, and 131.57 mg g⁻¹ for 4-NP, DNP, and TNP when initial concentrations were 50 mg L⁻¹ at 293.15 K, respectively. The adsorption of nitrophenols is a spontaneous, endothermic, and entropy-driven process. The reduction reaction followed the pseudo-first-order kinetics, and the kinetic rate constants were 0.4413, 0.3167, and 0.17538 min⁻¹ for 4-NP, DNP, and TNP, respectively. The effect of initial nitrophenols concentration, anions, and temperature on reduction process was investigated. The mechanism of adsorption and catalytic reduction of Cu2I2(MA)2 was studied. The results demonstrated that Cu2I2(MA)2 exhibits excellent adsorption and catalytic activity to remove nitrophenols.
We present a novel pincer ligand based on the pyridine scaffold for functionalizing magnetic GO (graphene oxide). Pincer ligands are known for their great control of coordination spheres in catalysts. GO is used as solid support for immobilizing gold nanoparticles to synthesize a heterogeneous catalyst for reducing nitroarenes. The reduction of nitroarenes to aromatic amines is not only of environmental importance but can be used as a model reaction too. The yields of the reduction reactions are promising. Additionally, the reaction conditions are mild and the catalyst has good recyclability and great stability. The nanocatalyst was monitored and characterized by spectroscopic methods.
While it has been recognized that sulfidation can effectively improve the reactivity of microscale zero valent iron (mZVI), there is limited understanding of nitrobenzene (ArNO2) removal by sulfidated mZVI. To understand the reduction capacity and pathway of ArNO2 by sulfidated mZVI, ball-milling sulfidated mZVI (S-mZVIbm) with different S/Fe molar ratios (0-0.2) was used to conduct this experiment. The results showed that sulfidation could efficiently enhance ArNO2 removal under iron-limited and iron excess conditions, which was attributed to the presence of FeSx sites that could provide higher Fe(0) utilization efficiency and stronger passivation resisting for S-mZVIbm. The optimum ArNO2 reduction could be obtained by S-mZVIbm with S/Fe molar ratio at 0.1, which could completely transform ArNO2 to aniline (ArNH2) with a rate constant of 4.36 × 10-2 min-1 during 120-min reaction. FeSx phase could act as electron transfer sites for ArNO2 reduction and it could still be reserved in S-mZVIbm after reduction reaction. The product distribution indicated that sulfidation did not change the types of reduction products, while the removal of ArNO2 by S-mZVIbm was a step-by-step reduction progress along with the adsorption of ArNH2. In addition, a faster reduction of ArNO2 in groundwater/soil system further demonstrated the feasibility of S-mZVIbm in the real field remediation.
Chemosensors are the molecular probes which are frequently used for analyte (metal ions, anions, explosives etc.) detection via photophysical changes. Analyte detection constitutes an active topic of research because the imbalanced consumption of these chemical species produces harmful effects to the ecosystem as well as to living beings. In recent years, several molecular probes of different frameworks are known for analyte detection. Among these, Schiff bases are more proficient and effective as compared to other chemosensors. Chromones itself possessing various physiological and photochemical properties but introduction of azomethine bond in chromones makes it useful probe for sensing applications. Colorimetric and fluorimetric sensing are promising techniques used to detect different chemical species in solvent system. They provide high sensitivity, reliability and quick response in comparison to the other reported techniques. The motive of this review is to provide better insight of the sensing mechanism of chromone derived Schiff bases and their practical utility in analyte detection to the researchers.
As a clean, fast, and efficient strategy for chemical synthesis, mechanochemical ball-milling approach has been developed promisingly to prepare functional materials for water treatment. Based on our previous findings that both the accelerated corrosion and facilitated electron-transfer are crucial for contaminants removal by zerovalent iron (ZVI), ball-milled FeCl2 and ZVI (Fe²⁺-ZVIbm) was synthesized for metal(loid)s sequestration in this study. The results showed the complete removal of Se(VI) was obtained by Fe²⁺-ZVIbm and there was no residual Fe²⁺ in the solution after Se(VI) removal. Compared to other enhanced-ZVI techniques, Fe²⁺-ZVIbm showed evident superiorities for Se(VI) removal in many aspects, including removal rate, removal capacity, utilization ratio of reductants, electron efficiency of reductants, specific removal capacity, adaptability of initial pH, and operation cost. The influence of storage condition on the performance of Fe²⁺-ZVIbm was examined and the performance of Fe²⁺-ZVIbm for Se(VI) removal could be ensured when it was sealed-stored in air. The rates and capacities of various metal(loid)s (V(V), Cr(VI), Cu(II)-EDTA, As(III), As(V), Se(IV), Sb(III)-tartrate, Sb(V), and U(VI)) removal by Fe²⁺-ZVIbm were increased by 2.2–32.4 times and 2.8–11.6 times, respectively, compared to those by pristine ZVI. In the semi-continuous flow reactor, Fe²⁺-ZVIbm synthesized with industrial-grade ZVI powders displayed advantages on the removal of target contaminants (V, P, Se, As) in two practical wastewaters, further revealing its great potential for application.
Ammonia is a common contaminant in municipalities where human waste causes nitrification of local water bodies. In mining, ammonia contamination occurs as a byproduct of biological water treatment, from the use of ammonium nitrate fuel oil in explosives, and from the exposure of ammonia rich soils during the excavation process. In particular, gold mine effluent represents a significant source of ammonia and nitrogen-based contaminants. Current biological and abiotic treatment processes are difficult to employ at the scale required at mine sites due to the high operating costs, or are limited in effectiveness due to a lack of natural resources required to facilitate the treatment. This article evaluates the use of electrooxidation as a cost effective alternative to treating ammonia-laden wastewater in mining applications. Two mixed metal oxide electrodes are assessed in this article: IrO
$_{2}$
/Ti and RuO
$_{2}$
/Ti anodes. A Monte Carlo simulation is performed to determine a probabilistic range of capital and operating expenditure for a mining operation deploying an electrooxidation wastewater treatment system. The lowest capital cost of operating the electrochemical treatment occurs at a current density of 200 A/m
$^{2}$
, where the number of cells required for treatment is minimized.
Uranyl MOF {H3O·[(UO2)(H3TTHA)]·2H2O} (1) with a 2D network structure was successfully constructed by flexible 1,3,5-triazine-2,4,6-triamine hexaacetic acid (H6TTHA) as ligand and UO2(CH3COO)2·2H2O as metal source. The compound 1 was basically characterized by elemental analysis, IR spectra, UV-vis spectra, PXRD pattern and thermogravimetric analysis. Based on the fluorescence characteristics of 1, the fluorescence response to nitro compounds such as p-nitrophenol (PNP), p-dinitrobenzene (pDNB) and p-nitrotoluene (PNT) were also explored at room temperature. The experimental results showed that the compound 1 had a good potential for detecting nitro compounds due to its low detection limit and high quenching constant.
The effect of several sulphur compounds: sodium sulphate, sodium sulphide, ferrous sulphide,pyrite and an organosulphonic acid on the kinetics of the iron (Fe °) induced degradation of carbon tetrachloride was examined under aerobic conditions. It was observed that all of the sulphur compounds investigated significantly accelerated the reaction. The mechanisms of the processes studied as well as their possible influence on the efficiency of the iron-induced dehalogenation of pollutants, both in situ and in above-ground treatment are discussed.
Numerous synthetic chemicals contain one or several nitro groups that are bound to an aromatic ring. Figure 1 shows the structures of some prominent representatives of such nitroaromatic compounds (NACs). The high toxicity of some NACs, particularly the mutagenic and carcinogenic potential of some nitrated polycyclic aromatic hydrocarbons (PAHs), has led to considerable interest in the fate of such compounds in the environment. Due to their widespread use, NACs are ubiquitous contaminants, especially in aqueous environments. In addition to contamination originating from agricultural use, from production facilities, and waste disposal sites, diffuse input into the pedosphere via the atmosphere has been documented (21, 27, 36, 37, 59, 65, 69, 80, 81). Atmospheric production of significant quantities of NACs by photochemical processes has been reported (19, 29, 42, 80). Table 1 lists some typical concentrations of NACs that have been measured in various compartments of the environment. Very high concentrations of nitroaromatic explosives (2,4,6-trinitrotoluene (TNT) and by-products) have been found especially in soil and subsurface systems. At those sites, significant concentrations of substituted aromatic amines that may have been formed from the reduction of NACs are frequently encountered.
Electrochemical CO2 reduction was investigated on 32 metal electrodes in aqueous KHCO3 medium. The current efficiency of CO2 reduction on Ni, Ag, Pb, and Pd increases significantly with lowering the temperature. The ratio of reduction products are also changed by lowering temperature. Potential dependence of HCOOH and H2on an Hg electrode supports the electron transfer mechanism for HCOOH production. Formation of methane and ethylene is observed on almost all metal electrodes used, although the efficiency is mainly very low except for Cu. A periodic table for CO2 reduction, which is drawn based on the dependence of reduction products on various metals, suggests the existence of a systematic rule for the electrocatalytic reduction of CO2 on metal surfaces.
The reduction of nitrobenzene by dihydroflavins (or dihydroisoalloxazines) in organic solvents leads to N-phenylhydroxylamine and flavins (or isoalloxazines). Nitrosobenzene is very rapidly reduced to N-phenylhydroxylamine, and azobenzene leads to hydrazobenzene. Azoxybenzene is sluggishly reduced to hydrazobenzene and aniline. N-phenylhydroxylamine also slowly oxidizes reduced flavins, likely via disproportionation (to nitrosobenzene and aniline) followed by reaction of the product with dihydroflavin. The reactions of nitrobenzene and six para-substituted nitrobenzenes with dihydro-3-methyllumiflavin in DMF over a range of concentrations follow good second-order kinetics (first order in each reactant). The second-order rate constants fit a Hammett relationship using σ- substituent constants, ρ- = +3.6. On the basis of these data along with their relationship to electrochemical and other aromatic nitro reduction methods, a tentative initial step involving electron transfer is proposed. The azobenzene reaction also displays first-order behavior in each reactant (second order overall). No intermediates were observed spectrophotometrically in any of these systems. Aliphatic nitro compounds are unreactive to dihydroflavins.
Xenobiotics are manmade compounds often introduced into the environment at concentrations that cause undesirable effects. They can be transformed as a result of biotic and abiotic processes, leading to changes in their chemical state and ultimately in their toxicity and reactivity. Ideally, xenobiotics are transformed into carbon dioxide, water, and mineral elements. However, many of them are converted to intermediate products that can be as toxic as or more so than their parent compounds. This article focuses on the complexation of organic xenobiotics with humic material in aquatic or terrestrial environments. The information presented here focuses on the mechanism by which halogenated phenols and anilines are bound to humic material and the chemical nature of these bound residues. Data on the toxicity and ultimate stability of bound chemicals is also presented. Studies indicate that enzymatic coupling is a promising method for detoxification of environments contaminated with xenobiotics. 33 refs., 2 figs., 2 tabs.
Generally, aromatic amines are regarded as effective inhibitors for pickling and acid cleaning of mild steel in hydrochloric acid. Most previous studies advocate the view that the inhibition is a result of adsorption of the π-electron cloud of the aromatic ring on the iron and steel surface through vacant dπ orbital of iron. Later studies indicate a possible electrostatic adsorption or chemisorption due to synergistic action between halide ion and amine molecules as responsible for inhibition. In this paper, an attempt is made to study the mechanism of adsorption by using the concept of φ-scale of potential as proposed by Antropov. The potential of zero charge (PZC) of mild steel is studied by two different techniques, and the mechanism of adsorption thus predicted has been found to corroborate the results of in situ Raman scattering study.
The adsorption of a large number of nitroaromatic compounds (NACs) to mineral surfaces, particularly to homoionic kaolinites, has been investigated. The results demonstrate that NACs may adsorb specifically and reversibly to the negatively charged siloxane surface of kaolinite. The strength of adsorption depends on the structure of the compound (i.e., type of substituent) and on the type of cation adsorbed to the siloxane surface. In the presence of strongly hydrated cations (e.g., Li+, Na+, Mg2+, Ca2+, Al3+), no significant specific adsorption of NACs is observed, while for more weakly hydrated cations (e.g., NH4+, K+, Rb+, Cs+), the distribution coefficient, K(d), of a given NAC, increases with decreasing free energy of hydration of the cation. We propose that electron donor-acceptor (EDA) complexes between surface oxygens of the siloxane surface and a given NAC are responsible for the observed specific adsorption. Some simple model calculations indicate that such EDA complexes may have a significant impact on the transport and the fate of NACs (and, possibly, of other organic pollutants exhibiting electron acceptor properties) in the subsurface environment.
In this research, aqueous ferrous carbonate, FeCO3, was studied in a batch reactor designed to maintain rigorously anoxic conditions in the absence of reducing agents. The precipitation kinetics were measured from 27 to 80°C and were found to be approximately 100 times slower than any measured precipitation rate previously reported for a 2:2 sparingly soluble salt. Dissolution kinetics results are reported at 26 and 60°C. Solubility product constant measurements are reported from 25 to 94°C. Statistical analyses and application of the Nielsen theory of surface reaction rates suggest that the precipitation of ferrous carbonate is surface reaction rate limited with an Arrhenius activation energy of 108.3 kJ/mol. The dissolution of ferrous carbonate is probably also surface reaction rate limited.
Dosage de six nitro- et de trois dinitrophenols dans les eaux de pluies recueillies sur site urbain en Suisse, par chromatographie en phase gazeuse couplee a une spectrometrie de masse. Des etudes recentes montrent que ces composes sont des polluants secondaires formes apres transformation tropospherique de composes aromatiques tels que le toluene et les methylphenols
The stability criteria for pitting dissolution of metals are examined on the basis of recent results. Pitting dissolution is stable if the local ion concentration buildup at pit sites exceeds a certain critical value, which for pits at noble potentials appears to correspond to the lowest ion concentration required for the transition from etching dissolution to brightening dissolution. The smallest pit size for stable pitting decreases with anodic potential, and hence the larger defect on the metal surface gives rise to the less noble pit initiation potential. The local pH at pit sites seems to contribute to the initiation of etching pits at less noble potentials, whereas the stability of brightening pits at noble potentials is determined by the local concentration of the aggressive anion.
The properties of iron electrode in CO2-saturated sodium chloride supporting electrolyte were investigated by cyclic voltammetry of static and rde-Ferrovac electrode in the potential region from −1.1 to −0.3 V (nhe). In addition to the reduction of H3O+ ions formed due to the protolysis of carbonic acid, the direct (activated) reduction of both H2CO3 molecules and HCO−3 ions was observed. It is proposed that catalytic, ErC′i mechanism accounts for the occurrence of the latter processes.
The polarographic behaviour of parathion, its major metabolites (paraoxon and p-nitrophenol), and of methylparathion, EPN and pentachloronitrobenzene has been studied over a wide pH range. Differential pulse polarography is used to differentiate between parathion, p-nitrophenol and pentachloronitrobenzene. An indirect determination of parathion in the presence of paraoxon can be based on their respective rates of hydrolysis in 0.5 M sodium hydroxide solution. The electrochemical behaviour of these compounds has also been investigated in solutions containing tetraalkylammonium salts as the supporting electrolyte.
The electrochemical reduction of nitrosobenzene to phenylhydroxylamine has been examined in aqueous medium between pH 0.4 and 13, by polarography and by cyclic voltammetry. The results are analyzed using the theory of the 9-member square scheme with protonations at equilibrium (E. Laviron, J. Electroanal. Chem., 146 (1983) 15; R. Meunier-Prest and E. Laviron, J. Electroanal. Chem., 328 (1992) 33). A study of the variations of the apparent heterogeneous and surface rate constants shows that the sequences of addition of the electrons and protons are successively H+e−H+e−, e− H+H+e− and e−H+e−H+. The values of the elementary surface electrochemical rate constants deduced from our results are of the order of 109 s−1, i.e. of the order of magnitude predicted by Brown and Anson (J. Electroanal. Chem., 92 (1978) 133). The elementary heterogeneous rate constants are much higher than predicted by the theory of the square scheme, which can be attributed to an increase in the apparent reversibility, owing to the occurrence of the ‘surface’ path, parallel to the heterogeneous path (cf. E. Laviron, J. Electroanal. Chem., 124 (1981) 19). The global 4e− reduction of an aromatic nitro compound to the corresponding hydroxylamine takes place via two successive 9-member square schemes linked by the dehydration of the intermediate dihydroxylamine; the reaction paths are determined.
The electrochemical reduction of 4-nitrobenzophenone RNO2 has been examined in aqueous medium on a Hg electrode between H0 = −5 and pH 14, by polarography and cyclic voltammetry. It occurs in three main steps: 2e− reduction to dihydroxylamine, which dehydrates to give the nitroso compound, itself reducible (2e−) to the hydroxylamino form. The results were analyzed using the theory of the nine-member square scheme with fast protonations [E. Laviron, J. Electroanal. Chem., 146 (1983) 15]. The phenomena observed constitute an epitome of those occurring during the reduction of organic compounds in aqueous medium, as far as two main factors, adsorption and protonations, are concerned.In acidic medium (pH 1), the first 2e− reduction appears as heterogeneous, although it occurs via the adsorbed species. The apparent heterogeneous electrochemical rate constant is about 105 larger than the elementary rate constants.For pH & > 1, adsorption is much stronger, and the reactions are of a purely surface nature, in polarography and a fortiori for any value of the sweep rate in cyclic voltammetry. The calculated values of the elementary surface electrochemical constants, 7.8 × 108 s−1, are of the order of magnitude predicted by Brown and Anson's formula.The first 2e− stage is controlled by the electrochemical reaction up to H0 ≈ −2, and by dehydration for H0(pH) larger than about 0. The order of addition of the electrons and protons at each pH is determined. The global 4e− reduction is of the ECE type up to pH ≈ 10. A novel feature of the reduction of aromatic nitro compounds in alkaline medium has been brought out; basically a 1e− reversible wave, followed by a 3e− irreversible wave, should be observed. However, the influence of the dehydration causes the two waves to merge, producing a single 4e− wave (EECE process).
The general aspect of mixed electrolysis of nitro compounds and alkyl halides in very weakly acidic organic media is estimated as a function of the nature of organic nitro derivatives and that of the halide which can no longer be considered in all cases as electrophiles. When the transient nitroso compound has the property of a radical scavenger, the ESR spectrum may be observed. However, the intensity of the signal depends strongly on the nature of the organic halide: the reasons why no signal is obtained with primary organic halides are estimated both from the chemical and electrochemical points of view.
Chemical reactions at surfaces, important in heterogeneous catalysis, may also play a role—besides that of charge transfer—during electrode processes; heterogeneous proton transfer is of particular significance. Investigations with aromatic carbonyl and nitro compounds, as well as with oxygen, show that proton transfer —like electron transfer—takes place even at electrodes almost completely covered with inhibitor molecules. The rate of proton transfer, like that of electron transfer, under these conditions depends on the surface concentration of the reacting species present in the adsorption layer besides the inhibitor. The dependence of this surface concentration on the bulk concentration of the inhibitor is given by a previously derived relation for processes taking place at a homogeneous surface (mercury). Polarographic investigations with nitro compounds show that the product of the primary electrode reaction, the anion radical, can arrange itself in two different ways within the adsorption layer. One of the two orientations—with the ring plane parallel to the electrode surface—favours electron transfer from the electrode, while the other —perpendicular to the electrode with the −NO2− group towards the solution— favours proton transfer from the solution.
An a.c. impedance characterization of the mechanical damages caused in situ to the passive film on iron has been performed. Straining at constant rate and abrasion by projecting a suspension of particles are compared. The results are discussed in terms of the classical model involving a sequence of depassivation-repassivation events induced by local breakdown of the film. This model is able to account for the increase of d.c. passive current in both cases. From a.c. data it is concluded that breakdown occurs only under abrasion. This conclusion is corroborated by SEM of the abraded surface and the observation of individual peaks of current on the oscillographic trace. In the case of straining the results are consistent with a large plasticity of the film and no breakdown. A model accounting for the dependence of the d.c. current on the straining rate and for the main impedance features is proposed.
Azoxybenzene formation from nitrosobenzene and N-phenylhydroxylamine has been studied in the pH range 1–11 with different buffers in 10% methanol at 25 °C and ionic strength 0.5 (KCl). The reaction exhibits general-acid (α= 0.29), general-base (β= 0.15), and specific-base catalysis by hydroxide ion. The plot of log k2(k2= second-order rate constant)vs. pH, the specific-base catalysis by hydroxide ion, and the substituent effects demonstrate that in the pH range studied only one step, i.e. the dehydration of a N,N′-dihydroxy intermediate, rapidly formed from the reagents, is rate-determining. The mechanism suggested in this work is also supported by the general acid- and base-catalysis and by the effect of the solvent polarity on the rate of the reaction. Other mechanisms and intermediates are considered, and shown to be inadequate in explaining all the experimental results.
The radiotracer method described previously was applied for investigations of adsorption processes occuring in the system: iron electrodeposited electrode and CO2-saturated (14C-labelled) neutral electrolyte. In addition to adsorption of 14C-containing species in the electrodeposited film, both reversible and irreversible adsorption of those species was identified. The irreversible adsorption was interpreted to result from incorporation of 14C-species (most probably HCO−3 ions) to the passive layer formed on the iron electrode. The reversible adsorption was concluded to occur due to the weak interactions of carbonic acid with the oxidized iron surface. Lewis acid and base concept of adsorption was used to account for the results. The role of the reversible adsorption process in the accelerated corrosion of steel due to the presence of carbon dioxide in aqueous solutions is discussed.
Abstract not Available.
An examination has been made of the mechanism of breakdown of passive films on iron in borate buffer solution Formula caused by chloride ions. Various electrochemical kinetic criteria were measured. XPS, SIMS, and ISS studies were made of the systems used in the electrochemical work. The rate of breakdown was found to be proportional to Formula and Formula and exponentially dependent on the electrode breakdown potential and field drop in the oxide film. XPS data showed that when chloride ions caused breakdown, the Formula and Formula ratios changed from 2 to 1.5 and 0.5 to 0.1, respectively. SIMS data revealed that heating passive films up to 200°C drove out water from the films and that chloride ions penetrated the whole film thickness on breakdown. ISS data indicated that on changing from a passive to a depassivated film, the Formula ratio changed from 2.07 to 1.5. Discussion of the electrochemical kinetic data shows that it is inconsistent with adsorption‐displacement models, pore models, and chemico‐mechanical models, but is not inconsistent with ion‐exchange processes, point‐defect models, and hydrated polymeric oxide models. Confrontation of the spectroscopic data with the expectations of the latter three models shows some points of agreement with all these models, but the data taken together is most consistent with the hydrated polymeric oxide model.
A summary is presented of the chemical reactivity of several chloromethanes and chloroethanes as compared with the unsaturated species, trichloroethylene and perchloroethylene. A total of eleven chlorinated solvents were investigated. Metal reactivity-corrosion was accelerated in all the solvents when a water phase was present. Although solvent attack on aluminum, iron, or zinc is negligible in dry trichloroethylene or perchloroethylene, aluminum and zinc corrosion generally occurs in dry systems containing the saturated chlorinated solvents. Iron corrosion is no great problem in these dry solvents. The reactivity of trichloroethylene and perchloroethylene with amines is minor as contrasted to the C//1 and saturated C//2 solvents. The presence of the olefinic bond in trichloro or perchloroethylene renders the chlorine atoms less labile in metal or amine reactions, but this point of unsaturation affords easy oxidative degradation. 1,1,2-Trichloroethane is the only saturated solvent that shows appreciable oxidative breakdown.
The reduction of aqueous CO2 by zero-valent iron was studied in batch and column experiments. Ten hydrocarbons up to C5 were identified as products of the reduction process and were shown to have Anderson−Schulz−Flory (ASF) product distributions. A direct consequence of the ASF product distribution is that a significant mass of hydrophobic hydrocarbons may remain sorbed to the iron surface. Based on a reaction mechanism proposed for the electroreduction of aqueous CO2 with nickel electrodes, iron acts as both a reactant by corroding to supply electrons and as a catalyst by promoting the formation and growth of hydrocarbon chains. Water is also a reactant in the system. When iron is used to enhance the dechlorination of chlorinated organic compounds, the slow desorption of the hydrocarbon products may become the rate-limiting step in the reaction.
The reaction of several ring-substituted anilines with humate has been studied in aqueous solution. The primary amines bind to humate in two phases. Initially, a rapid, reversible equilibrium is established, which may represent formation of imine linkages with the humate carbonyls. Subsequently, there is a slow reaction that is not readily reversed. The slow reaction is thought to represent 1,4 addition to quinone rings followed by tautomerization and oxidation to give an amino-substituted quinone. The slow reaction is not likely to be imine formation or other processes that require primary amines because N-methylaniline, a secondary amine, also reacts with humate. These processes represent ways in which aromatic amines may be converted to latent forms with undetermined effects on the biosphere.
The kinetics of reductive transformations of a series of monosubstituted nitrobenzenes and nitrophenols have been investigated in aqueous solution containing reduced sulfur species and small concentrations of either a naphthoquinone or an iron porphyrin. Boith the two naphthoquinones and the iron porphyrin used in this study mediated the reduction of the nitro group. In all three cases, the rate of reduction of the nitrobenzenes and of the nitrophenols was strongly pH dependent. Dissociated nitrophenols were reduced ~ 3-4 times more slowly as compared to the nondissociated species. For the substituted nitrobenzenes, the effect of substitution on the reaction rate could be described by a linear free energy relationship (LFER) of the general form log k = aE(h)(1') + b, where k is the second-order rate constant for reaction with the hydroquinone monophenolate or the iron porphyrin, respectively, and E(h)(1') is the one-electron reduction potential of the nitroaromatic compound. Competition between different nitrobenzenes was observed in the case of the iron porphyrin, while no effects were found for the reaction with the hydroquinones. The results of this study form an important base for the evaluation and interpretation of reductive transformation processes of nitroaromatic compounds in the environment.
Natural organic matter (NOM) from a variety of sources has been shown to mediate the reduction of substituted nitrobenzenes in aqueous solution containing hydrogen sulfide. Pseudo-first-order rate constants were proportional to NOM concentrations and increased with increasing pH and decreasing reduction potential (E(h)) of the solution. At fixed pH and E(h), the carbon-normalized rate constants (k(NOM)) of a given compound varied less than 1 order of magnitude among NOMs derived from various natural waters. The effect of substituents on the reaction rate could be described by a linear free energy relationship (LFER) of the general form log k(NOM) = aE(h)1'(ArNO2) + b, where E(h)1'(ArNO2) is the one-electron reduction potential of the nitroaromatic compound. Such LFERs were applied successfully to predict the k(NOM) values of previously untested compounds. The results of this study suggest that hydroquinone moieties within the NOM may play a pivotal role in the mediation of electron-transfer reactions involving organic pollutants.
The reaction of nitrosobenzene with aniline, to give azobenzene, in basic conditions was studied. It was shown that the reaction exhibits general base catalysis by different buffers giving a Brønsted coefficient β = 0·318. As in previous studies, a two-step process with a first step of attack of aniline on nitrosobenzene to give an addition intermediate and a second step of dehydration of this intermediate is proposed to interpret the mechanism of the reaction. The analysis of the Brønsted relationship and of the intermediate of the reaction led to the suggestion that hydoxide ion catalyses the reaction by a mechanism of general base catalysis in the dehydration step.
Comparing reduction potentials of aromatic nitro compounds with those of corresponding nitroso compounds, by studying shifts of half-wave and peak potentials with pH and by following the decrease of limiting current in buffered 35% DMF-water mixtures, the sequence of protons and electrons in the four-electron reduction step was proved to be: H+, e, H+, e, 2e, 2H+. Arylhydroxylamines formed in the four-electron reduction of nitro compounds can be protonated and further reduced, they can undergo dehydration (provided that the dehydration product is stable), they can undergo chemical reactions with intermediates of the reduction of the nitro group, like ArNO, they can be electrooxidized to nitrosobenzenes (which can undergo chemical reactions) or they can react with carbonyl groups when present. Stabilization of the dehydrated form can occur in quinonemethide (nifedipine) or in some quinone-like heterocyclic derivatives. Side reactions of ArNHOH and ArNO are demonstrated by smaller peaks in CV and by controlled potential electrolysis. Reactions of ArNHOH with carbonyl groups were observed by formation of new waves of nitrones in the reduction of nitrobenzenaldehydes.
The reductive transformation of azobenzene and selected derivatives was investigated in anaerobic sediment/water systems. The azo compounds exhibited pseudo-first-order disappearance kinetics through at least three half-lives. The reduction kinetics of these compounds was studied as a function of their reduction potential and sediment/water distribution coefficient. There was no apparent correlation between the observed disappearance rate constant and reduction potential. In general, as the distribution coefficient increased, the rate of reduction decreased. Values for the pseudo-first-order rate constant for disappearance ranged from 5.11 × 10−3 min−1 for methyl red to 6.03 × 10−6 min−1 for 4,4′-dichloroazobenzene. Removal of the solid phase from the sediment/water samples gave a filtrate with little or no reactivity. Chemical sterilization of the sediment/water sample with formaldehyde and treatment with m-cresol, a dehydrogenase inhibitor, or sodium azide, a metabolic inhibitor, had little effect on the observed reduction rate constants for azobenzene, indicating an abiotic reduction process. Heat sterilization indicated that the reducing agent was heat labile. In studies with 4,4′-dimethoxyazobenzene, the observed rate constant for reduction increased with increasing sediment concentration. Based on the results of these studies, a model for the reduction process was developed that incorporates a nonreactive sorptive sink and a reactive site, both of which are associated with the sediment.
Laboratory tests were conducted to examine zero-valent iron as an enhancing agent in the dehalogenation of 14 chlorinated methanes, ethanes, and ethenes. All compounds were tested by batch procedures in which 10 g of 100-mesh electrolytic iron was added to 40 ml hypovials. Aqueous solutions of the respective compounds were added to the hypovials, and the decline in concentration was monitored over time. Substantial rates of degradation were observed for all compounds tested with the exception of dichloromethane. The degradation process appeared to be pseudo first-order with respect to the organic compound, with the rate constant appearing to be directly proportional to the surface area to volume ratio and increasing with increasing degree of chlorination. Column tests showed the process to proceed under flow conditions with degradation rates indpendent of velocity and consistent with those measured in the batch tests. When normalized to 1 m2/ml, the t50 values ranged from 0.013 to 20 hr, and were about 5 to 15 orders of magnitude lower than values reported for natural rates of abiotic degradation. The results indicate abiotic reductive dechlorination, with iron serving as the source of electrons; the mechanism is, however, uncertain. Based on the rapid rates of degradation, both in situ and aboveground applications for remediation of contaminated ground water are proposed.
Nitrobenzene was completely degraded by mixed cultures using a sequential anaerobic-aerobic treatment process. Under anaerobic conditions in a fixed-bed column aniline was formed from nitrobenzene through gratuitous reduction by cells of sewage sludge. This reaction was accelerated by the addition of glucose. Complete mineralization of aniline was accomplished by subsequent aerobic treatment using activated sludge as inoculum. The maximum degradation rate of nitrobenzene (4.5 mM) in the two-stage system was 552 mg l–1d–1, referring to 154 mg of nitrobenzene per gram of glucose. In a second experimental phase glucose as cosubstrate and H-donor was replaced by synthetic waste containing ethanol, methanol, isopropanol and acetone. Again, nitrobenzene (1.9 mM) was completely degraded (maximum degradation rate of 237 mg l–d–1, referring to 251 mg per gram of solvents). The major advantage of the described two-stage process is that the reduction of nitrobenzene by anaerobic pretreatment drastically reduces emission by stripping during aerobic treatment.
A review is made of some essential subjects on the passivtion process, the passive film, and the film breakdown. Passivation of metals results from the formation of a condensed phase of continuous oxide layer on the metal surface. The electrochemical stability of passivated metals depends not only on the chemical property but also on the electronic property of the passive films. The electronic avalanche breakdown of passive film occurs when the film is thick, while the ionic breakdown emerges with thin oxide films in the presence of aggressive anions. The film breakdown is followed either by repassivation or by pitting corrosion depending on the degree of metal salt enrichment at the film breakdown sites.
The solubility of siderite in acid, neutral, and alkaline medium was investigated. The experiments were performed as potentiometric titrations under two constant partial pressures of CO2 (g). This yielded the following equilibrium constants at 25°C in a 1.0 M NaClO4 medium (quoted errors through this work are given at 2σ confidence level). 1.1) The solubility constant of FeCO3(s) , with 2.2) The formation constants for the two determined Fe(II) carbonate complexes: , with , with .By extrapolation to infinite dilution, the following reaction equilibria are estimated for the infinite dilution standard state: , with , with , with .The determined solubility constant is in fair agreement with previously derived solubility data. The identified complexes and their formation constants emphasize their importance in various anoxic waters. However, carbonate complex formation does not account for the supersaturation conditions with regard to siderite equilibrium observed in many other present-day environments.
Tetrachloroethylene was transformed by iron powder (4.1g/L) in oxygen-free, HEPES-buffered (pH 7) water at 50°C with a half-life of 20 days. The only products observed were the reactive intermediate, trichloroethylene, and ethene and ethane. 1,1,1-Trichloroethane, 1,1-dichloroethylene, and tetrachloroethylene were transformed by iron at room temperature in both autoclaved buffered water and in two non-autoclaved landfill leachates. The pattern and degree of removal were similar in all cases. Dichloromethane, 1,1-dichloroethane, and 1,4-dichlorobenzene were also tested, but were not removed from any of the systems. If manganese rather than iron was used, the substrates transformed depended upon the aqueous phase. Some biological transformations were seen in Leachate 2, but the activity was reduced by manganese and completely suppressed by iron.
Polycyclic aromatic hydrocarbons substituted with nitro groups (nitro-PAHs) are important environmental pollutants. They comprise many direct-acting mutagens and are easily transformed into other derivatives by metabolism or irradiation. A new property of nitro-PAHs is documented: they are phototoxic. Irradiation of human eythrocytes with 1-nitropyrene, 2-nitropyrene, 1,3-dinitropyrene and 1,6-dinitropyrene led to hemolysis. Erythrocytes treated in the dark with the sensitizers, or with previously irradiated sensitizers, did not hemolyze. 1-Nitropyrene was also studied with a series of Escherichia coli strains. The phototoxicity was largely oxygen dependent, and the photodynamic effect combined Type I and Type II mechanisms. Light-dependent inactivation of Haemophilus influenzae DNA transforming activity by 1-nitropyrene was partially oxygen dependent, but the photosensitized nicking of pBR322 supercoiled DNA occurred more readily anaerobically.
Elemental iron and zinc reduced part-per-thousand levels of aqueous-phase carbon tetrachloride to chloroform in a few hours. Free metal ions, chloride ion and hydrogen gas were produced in the reaction; protons were consumed. Process kinetics were dependent on solution pH, surface area of the elemental metal, carbon tetrachloride concentration, buffer selection and solvent composition (volume fraction 2-propanol). Reaction rate was first-order with respect to carbon tetrachloride at concentrations less than 7.5 mM. This class of reactions offers promise as a means for initiating the destruction of heavily halogenated organic compounds.
The precipitation of iron from an iron sulphate containing wastewater by aragonite, calcite and a variety of limestones and limesands was examined at pH values near 6 in an atmosphere of carbon dioxide. Siderite (ferrous carbonate) and a calcium siderite containing 10 mol% calcium were the only iron-containing products detected by XRD, and accounted for substantially all the iron removed from solution. Calcium siderite was the major product and constituted between 50 and 100% of the iron containing product.The rate of precipitation of iron was proportional to the square of the supersaturation of the solution with respect to siderite, and increased with increase in pH and alkalinity. Precipitation occurred at different rates with different carbonate materials, tending to increase with increasing proportion of aragonite in the material. Precipitation did not occur in suspensions of calcite and was slow even in suspensions of aragonite where equilibrium was not reached within 40 days.These data suggest that the frequent occurrence of groundwaters apparently supersaturated with respect to siderite may be due either to slow precipitation of siderite, or to equilibrium of the groundwater with respect to a more soluble calcium siderite. The minerals present in an aquifer thus need to be identified before the saturation state of the associated groundwater with respect to siderite can be ascertained.
Reduction of chlorinated solvents by fine-grained iron metal was studied in well-mixed anaerobic batch systems in order to help assess the utility of this reaction in remediation of contaminated groundwater. Iron sequentially dehalogenates carbon tetrachloride via chloroform to methylene chloride. The initial rate of each reaction step was pseudo-first-order in substrate and became substantially slower with each dehalogenation step. Thus, carbon tetrachloride degradation typically occurred in several hours, but no significant reduction of methylene chloride was observed over 1 month. Trichloroethene (TCE) was also dechlorinated by iron, although more slowly than carbon tetrachloride. Increasing the clean surface area of iron greatly increased the rate of carbon tetrachloride dehalogenation, whereas increasing pH decreased the reduction rate slightly. The reduction of chlorinated methanes in batch model systems appears to be coupled with oxidative dissolution (corrosion) of the iron through a largely diffusion-limited surface reaction.
Using 10 monosubstituted nitrobenzenes as model compounds, the interdependence between the reduction of organic pollutants and microbial iron reduction in anaerobic aquifers has been studied in laboratory column systems. All nitroaromatic compounds (NACs) investigated were stoichiometrically reduced to the corresponding amino compounds. It is proposed that NAC reduction occurred primarily by a reaction with surface-bound iron species, which served as mediators for the transfer of electrons originating from microbial oxidation of organic material by iron-reducing bacteria. Although the different NACs studied exhibited very different one-electron reduction potentials, they were reduced at very similar rates under all conditions investigated, indicating that the regeneration of reactive sites and not the electron transfer to the NAC was the rate-limiting process. It is also proposed that the presence of reducible organic pollutants such as NACs may significantly enhance the activity of iron-reducing bacteria in aquifers, in that reduction of such compounds continuously regenerates easily available Fe(III) species.
Ph.D. Environmental Science and Engineering Reductive dehalogenation is an important reaction that generally leads to detoxification of many halogenated methanes. Halogenated methanes are widely used in industrial and commercial applications and the inadvertent or deliberate release of these chemicals has caused contamination of the atmosphere, soil and groundwater. The research presented here details the study of several systems for reductive dehalogenation of chlorinated methanes. The first system described in this dissertation involves reductive dechlorination of chlorinated methanes by laboratory cultures of methanogens. A vessel was constructed that allowed maintenance of anaerobic conditions and minimized losses of the volatile chlorocarbons. Methylene chloride was not dechlorinated in the presence of pure cultures of methanogens. Similarly, dechlorination did not occur in enrichments made with samples from several different anaerobic digesters. Abiotic dehalogenation studies showed that cobalamins, cobalt-centered macrocyclic compounds, catalyzed the reductive dechlorination of several halomethanes in anaerobic, closed batch systems. These studies focused on immobilization of cobalamins to several types of supports for use in pollution remediation strategies. Cyanocobalamin bound to Epoxy-Activated Sepharose 6B and talc catalyzed the rapid reduction of carbon tetrachloride and methylene chloride to sequentially reduced products. Corroding iron metal was also studied as a reductant for halogenated methanes. Several chlorinated methanes were reductively dechlorinated in closed, anaerobic, laboratory-scale model systems containing granular iron. Carbon tetrachloride was sequentially dehalogenated, via chloroform, to methylene chloride. The initial rate of each reaction was pseudo-first order in substrate and declined substantially with each dehalogenation step. Trichloroethene was also dechlorinated by iron, although more slowly than carbon tetrachloride. The reaction of chlorinated methanes appears to involve a direct interaction between the substrate and the iron surface. When surface condition is constant, the rate of reaction is roughly first-order in iron surface area and the rate increases markedly with increasing iron surface area. Studies were also performed to determine the effects of microbial and geochemical processes that developed during a field demonstration in which iron metal had been buried in the path of a chlorinated-solvent contaminated plume. Two sets of cores were obtained and examined for microbial and geochemical developments one, and two, years after burial of the iron.