Nitrosylmetallochelates—III. Rotating ring disk electrode study of redox reaction of Fe(II)EDTA with NO−2

The oxidation process of the nitrosyliron(II)EDTA complex which was produced by the reaction of Fe(II)EDTA and NO^?₂ has been studied by the rotating disk electrode and the spectroscopic methods. The oxidation of Fe(II)(NO)_xEDTA proceeds via two steps occurring at different potentials. The first step is the oxidation of Fe(II)EDTA to Fe(III)EDTA which is limited by the rate of chemical reaction, and the second is oxidation of NO to NO^?₃ which corresponds to surface controlled process. This result suggests the possibility that the noxious species can be transferred to the innocuous ion.     

Reduction of Fe(II)EDTA‐NO using Paracoccus denitrificans and changes of Fe(II)EDTA in the system

BACKGROUND: In the BioDeNO_X technology for NO_X removal from flue gas, bioreduction of Fe(II)EDTA‐NO and Fe(III)EDTA are core processes. In this study, a newly isolated strain, Paracoccus denitrificans, was used to reduce Fe(II)EDTA‐NO with glucose and Fe(II)EDTA as donor electrons. To better understand the change law of Fe(II)EDTA, the process of Fe(II)EDTA‐NO reduction by P. denitrificans with glucose and Fe(II)EDTA as electron donors was investigated, and the factors that might affect Fe(II)EDTA concentration were studied. RESULTS: For the bioreduction process of Fe(II)EDTA‐NO, P. denitrificans could use glucose and Fe(II)EDTA as electron donors. At different stages, primary electron donors were different, thereby affecting the concentration of Fe(II)EDTA in the system. It was also proved that this strain not only reduced Fe(III)EDTA with glucose as the electron donor but also secreted several substances that reacted with Fe(III)EDTA, resulting in increased Fe(II)EDTA concentration in the solution. CONCLUSIONS: This work has shown that P. denitrificans can reduce Fe(II)EDTA‐NO and Fe(III)EDTA simultaneously to regenerate NO_X absorption solution. © 2012 Society of Chemical Industry     

Metal chelate absorption coupled with microbial reduction for the removal of NOx from flue gas

A novel process for the removal of NO_x from flue gas by a combined Fe(II)EDTA absorption and microbial reduction has been demonstrated. Fe(II)EDTA–NO and Fe(III)EDTA (EDTA: ethylenediaminetetraacetate) can be effectively reduced to the active Fe(II)EDTA in the reactor containing microorganisms. In a steady‐state absorption and regeneration process, the final removal efficiency of NO is up to 88%. The effects of four main parameters (i.e. NO, O₂ and SO₂ concentrations, and the amount of cyclic solution) on NO_x removal efficiency were experimentally investigated at 50 °C. The results provide some insight into conditions required for the successful removal of NO_x from flue gas using the approach of Fe(II)EDTA absorption combined with microbial reduction. Copyright © 2005 Society of Chemical Industry     

Electrocatalytic reduction of nitric oxide and other substrates on hydrogel triblock copolymer Pluronic films containing hemoglobin or myoglobin based on protein direct electrochemistry

Protein-Pluronic film modified electrodes were constructed by casting the mixture of hemoglobin (Hb) or myoglobin (Mb) and triblock copolymer poly(ethylene oxide)₁₀₀-poly(propylene oxide)₆₅-poly(ethylene oxide)₁₀₀ (Pluronic) solutions onto the surface of pyrolytic graphite (PG) electrodes. A pair of well-defined and nearly reversible CV peaks at about −0.35 V versus SCE in pH 7.0 buffers was observed for protein-Pluronic films, characteristic of the protein heme Fe(III)/Fe(II) redox couples. The films were characterized by scanning electron microscopy (SEM), UV-vis and IR spectroscopy, as well as electrochemistry. The heme proteins were quite stable and retained their near-native structure in Pluronic films. Oxygen, hydrogen peroxide, nitrite and nitric oxide were electrochemically catalyzed by protein-Pluronic films with significant lowering of reduction overpotential. Both nitrite and hydrogen peroxide underwent dismutation in electrocatalysis. The reduction of NO₂⁻ and H₂O₂ on protein-Pluronic films was actually the catalytic reduction of their disproportionation products, NO and O₂, respectively. The biocatalytic mechanism of these substrates at protein-Pluronic film electrodes were discussed and speculated.     

Nitric oxide enhanced reduction in a rotating drum biofilter coupled with absorption by FeII(EDTA)

BACKGROUND: The ongoing emission of nitric oxide (NO) is a serious persistent environmental problem, because it contributes to atmospheric ozone destruction and global warming. A novel and effective system was developed for the complete treatment of NO from flue gases. The system features NO absorption by Fe^II(EDTA) and biological denitrification in a rotating drum biofilter (RDB). RESULTS: After 100 mg L^?1 Fe^II(EDTA) was added to the nutrient solution, the results show that the NO removal efficiency was improved from 70.56% to 80.15%, the optimal temperature improved from 32.5 °C to 40.5 °C, and the pH improved from 7.5 to 8.0–8.3. A maximum NO removal efficiency of 96.5% was achieved when 500 mg L^?1 Fe^II(EDTA) was used in the nutrient solution. CONCLUSION: This experiment demonstrates that Fe^II(EDTA) could not only improve the mass transfer efficiency of NO from gas to liquid, but also serve as an electron donor for the biological reduction of NO to N₂. The new integrated treatment system seemed to be a promising alternative for the complete treatment of NO from flue gases. © 2012 Society of Chemical Industry     

Mode of action of ferric and ferrous iron salts in activated sludge

BACKGROUND: Both ferric iron Fe(III) and ferrous iron Fe(II) salts are commonly used for chemical phosphorus removal (CPR) in the activated sludge (AS) process but only a few studies have compared Fe(III) and Fe(II) salts regarding their effect on the physical and biological properties of AS. In this research, the units of a continuous flow laboratory‐scale AS plant were dosed with Fe(III) and Fe(II) salts at a concentration of 25 mg Fe dm^?3 feed and changes in the AS properties were measured as Fe accumulated or washed out during startup, normal operation and withdrawal of dosing. RESULTS: The morphological characteristics of the flocs showed marked differences depending on the type of ion used. Fe(II) dosed flocs were more compact, less filamentous and smaller in size than Fe(III) dosed flocs. The settleability index of the Fe(II) dosed sludge was lower than that of the Fe(III) dosed sludge. The activity of ammonium (NH₄⁺‐N) and nitrite (NO₂^?‐N) oxidizing bacteria was found to be affected by the accumulation of Fe relating products into the sludge. CONCLUSIONS: Fe(II) was a more effective flocculent than Fe(III) and this was attributed to its ability to form stronger ionic bonds with the flocs prior to its oxidation to Fe(III). A hypothesis explaining the mode of action of Fe(II) is proposed. Floc surface properties were enhanced, this being beneficial to the morphological characteristics and settleability with further implications for the operation of AS. However, the effect may be reversed at high Fe contents. Copyright © 2010 Society of Chemical Industry     

Nitrosymmetallochelates—I. Mechanism of FeNO—EDTA complex formation and its oxidation

A nitrosyliron(II) complex with EDTA was produced by reaction of the Fe(II)—EDTA complex and nitrous acid in citric acid—phosphate buffer solution. The experimental results pointed out that nitrous acid is reduced to nitrous oxide by Fe(II)—EDTA complex followed by the formation reaction of nitrosyliron(II) EDTA complex. The composition of this complex was determined to be Fe(II) (NO)₂EDTA by both electrochemical and spectroscopic methods.A mechanism of the oxidation of Fe(II) (NO)₂EDTA was proposed in which nitrous oxide breaks away from the iron ion. The photoeffect on this reaction is discussed.     

Reduction of [Fe(III)EDTA]− catalyzed by activated carbon modified with KOH solution

NO and SO₂ can be eliminated simultaneously by [Fe(II)EDTA]^2? solution with a pH range of 5.6–8.0 at 25–80 °C. Activated carbon is used to catalyze the regeneration of [Fe(II)EDTA]^2?. In this paper, KOH solution has been utilized to modify the carbon to improve its catalytic capability. Experimental results show that the optimal modification factors are as follow: KOH concentration 6.0 mol l^?1, impregnation time 9 h, activation temperature 700 °C and activation time 4 h. After KOH modification, the surface area of activated carbon decreases. But its basicity is enhanced, which plays an important role in improving the catalytic characteristics of activated carbon in the reduction of [Fe(III)EDTA]^?. The experimental results demonstrate that the activated carbon modified by concentrated KOH solution can get a higher NO removal efficiency than the original activated carbon.     

Denitrifying kinetics involving the distributed ratio of reductases

A suspended-growth batch reactor was used to denitrify synthetic wastewater containing various proportions of nitrate and nitrite. A competitive phenomenon between nitrate- and nitrite-reductase was studied utilizing various proportions of nitrate and nitrite in an anaerobic environment with a temperature of 30°C and methanol as carbon source. By using a non-linear regression technique, biokinetic constants of the maximum specific reduction rates of nitrate and nitrite (k₁, k₂) and the Monod half-saturation coefficients of nitrate and nitrite (K_s1, K_s2) for the proposed two-step denitrifying kinetics were 1·29 day^?1, 0·89 day^?1 and 14·3 mg NO-N dm^?3, 10.9 mg NO-N dm^?3, respectively. The result obtained from a series of chemostat studies indicated the Monod-type kinetic model was more accurate when the distributed ratio of nitrate- and nitrite-reductase in the proposed two-step denitrifying kinetics was taken into account.     

IR study on the reduction of NO and NO2 by hydrazine monohydrate over Fe-BEA zeolite

The adsorption of NO and NO₂ and their subsequent reduction by hydrazine monohydrate (HDM) over Fe-BEA zeolite were investigated using an FT-IR spectrophotometer equipped with an in-situ cell. Although NO and NO₂ molecules were adsorbed on Fe species in an unaltered state, some of them reacted with oxygen atoms, resulting in the adsorption of NO₂ and NO ₃ ^? , respectively. The reducing species that had originated from HDM on Fe-BEA selectively reduced these molecules to N₂, while a small amount of N₂O was formed in the reduction of NO by HDM. NO and NO₂ were rapidly reduced by HDM through their adsorbed state even at 150 °C, and Fe species were required for their adsorption and for the formation of reducing species from HDM.     

Improvement of the settling properties of Anammox sludge in an SBR

Biological systems for the treatment of wastewater have to provide optimum sludge retention to achieve high removal efficiencies. In the case of slow‐growing micro‐organisms, such as anaerobic ammonia‐oxidizing (Anammox) bacteria, episodes of flotation involving biomass wash‐out are especially critical. In this study a strategy based on the introduction of a mix period in the operational cycle of the Anammox Sequencing Batch Reactor (SBR) was tested for its effects on biomass retention and nitrite removal. Using this new cycle distribution the biomass retention inside the reactor improved as the solids concentration in the effluent of the SBR decreased from 20–45 to 5–10 mg VSS dm^?3 and the biomass concentration inside the reactor increased from 1.30 to 2.53 g VSS dm^?3 in a period of 25 days. A decrease of the sludge volume index (SVI) from 108 to 60 cm³ g VSS^?1 was also observed. Complete depletion of nitrite was achieved in the reactor only with the new cycle distribution treating nitrogen loading rates (g N‐NO₂^? + g N‐NH₄⁺ dm^?3 d^?1) up to 0.60 g N dm^?3 d^?1. Copyright © 2004 Society of Chemical Industry     

Reduction kinetics of Fe(III)NTA chelate by sulfite

Kinetics of the reduction of Fe(III)NTA (ferric ion coordinated to nitrilotriacetic acid) by sulfite were studied experimentally in aqueous solutions at 40-60°C and pH = 3.8-8.3. The reaction had been claimed to be a regeneration step of Fe(II) in the absorption of nitric oxide into aqueous solutions containing Fe(II)NTA and sulfites. The reaction rate could be expressed as first-order with respect to the concentration of Fe(III)NTA and of order minus one with respect to the concentration of Fe(II)NTA. The order of reaction with respect to HSO^?₃ was determined to be unity when the molar ratio of Na₂SO₃ to Fe(III)NTA was less than 5.     

Nitrate and nitrite reduction of a sulphide-rich environment

A sulphide-rich anaerobic sludge acclimated with a molasses wastewater was used to carry out studies on nitrate and nitrite reductions in continuously stirred batch reactors. It was shown that a COD/N-NO_x ratio as high as 65·6 mg mg⁻¹ did not promote dissimilatory reduction of nitrogen oxides to ammonia. Denitrification was characterized by a probable accumulation of gaseous intermediates, nitric oxide (NO) and nitrous oxide (N₂O), by sulphide consumption with concomitant elemental sulphur production and by an increase of the redox potential. In addition, sulphate reducers were completely inhibited by nitrogenous oxides. Cultures performed without any carbon source proved that denitrifiers were able to use sulphides as electron donors. Furthermore, while a lag phase preceded nitrate denitrification, nitrite was consumed immediately. Chemical reduction of nitrite by ferrous iron (Fe²⁺) was considered to be responsible for this difference. Evidence of such a chemodenitrification has been presented by using a sterilized sludge which kept its ability to reduce nitrite while it lost its capacity to use nitrate. Moreover, this chemical activity was favoured by Fe²⁺ addition. Finally, it has been suggested that during the cultures performed with non-sterilized sludge, a biological reduction of the ferric ions (Fe³⁺) would be coupled to nitrite chemodenitrification and would allow a regeneration of Fe²⁺. © 1998 SCI     

Evaluation of heterotrophic nitrite removal by a sulphide and acetate oxidizing mixed culture originated from an oil reservoir

BACKGROUND: Shortcut biological nitrogen removal (SBNR) has attracted much attention in recent years due to lower aeration and chemical oxygen demand (COD) requirements, shorter residence time and smaller biomass production. In this work an oil reservoir denitrifying culture, with the ability to function under autotrophic and heterotrophic conditions was used for heterotrophic denitritation. Using freely suspended cells, effects of nitrite concentration (10–50 mmol L^?1) and temperature (15–35 °C) on the kinetics of denitritation were investigated and a kinetic model was developed. Potential for enhancement of nitrite removal rate, and impacts of nitrite concentration and loading rate were investigated in a continuous biofilm reactor. RESULTS: Nitrite did not impose any inhibitory effect, even at the highest applied concentration of 50 mmol L^?1. Increase of temperature in the range 15–35 °C enhanced the reduction rate significantly. Fitting the experimental data into the model developed, values of biokinetic coefficients (µ_max?NO2, K_S?NO2, Y_X?NO2, Y_X?Ace?NO2 and Eµ‐_NO2) were determined. In the biofilm reactor increases in nitrite loading rate (through flow rate or feed nitrite concentration) led to a linear increase of nitrite removal rate, with the highest removal rate of 140.6 mmol L^?1 h^?1 achieved with a residence time of 0.19 h. CONCLUSION: The enrichment culture used in this study is not only a superior biocatalyst for simultaneous removal of sulphide, nitrate and BOD, it could also be used effectively in the denitritation step of an SBNR process. The kinetic model developed would certainly have beneficial applications in the design, operation and control of the SBNR process. Copyright © 2011 Society of Chemical Industry     

A Study of the Interaction of NO2 with Carbon Particles

The interaction NO₂ in air (0.5–35 ppm) with carbon particles led to three products: NO gas, and NO₂ ^? and NO₃ ^?, removed from the particles by water extraction. At 4 ppm or below, in dry or humid air, the product distribution, in relative molar amounts, was NO₃ ^? = 2NO₂ ^? = 2NO. At 20 ppm and above, the relative amounts of products depended on the presence of water vapor: in dry air NO = 3NO₃ ^? = 6NO₂ ^?; in humid air NO = NO₂ ^? = 2NO₃ ^?. For carbon slurries in water, [NO₂ ^?] = 6[NO₃ ^?] at an input concentration of NO₂ of 4 ppm. In comparison to carbon, alumina particles and glass beads removed NO₂ ineffectively. These results indicate that NO₂ oxidized the carbon particles while it was reduced to NO. NO₂ adsorbed at oxidized sites on the particles formed a surface species that was analyzed as nitrate. At high enough concentration of NO₂ (20 ppm and above), the interaction of NO and water vapor with the surface nitrate produced NO₂ ^?. In slurries NO, generated from interaction of NO₂ with carbon, reacted with surface nitrate or nitric acid in solution to form the relatively large quantities of nitrite. This work suggests that NO_x reactions with carbon in droplets or on wet surfaces could be important sources for the production of nitrous acid in the environment.     

Nitrosylmetallochelates—II. composition of FeNO—aminocarboxylic acid complexes in solution

Absorptive properties of nitric oxide by Fe(II)—aminocarboxylic acid complexes have been investigated, and the compositions of the Fe(II)NO—chelate complexes in solution determined by the electrochemical and spectroscopic methods. Fe(II)—EDTA forms the 1:2 complex (Fe(II) (NO)₂EDTA) with nitric oxide, —G and—EDDA the 1:1 complexes, and—NTA and—IDA both of the 1:1 and 1:2 complexes. The abilities of the Fe(II)—chelate complexes to absorb nitric oxide decreased in the order Fe(II)—EDTA > —NTA > —IDA > —G > —EDDA, which is in agreement with that of a decrease of its stability constant.     

Evaluation of biosorbents for Cu removal from wastewater in the presence of EDTA

BACKGROUND: This paper evaluates the use of several biosorbents for Cu removal from aqueous solutions in the absence and presence of ethylenediaminetetraacetic acid (EDTA). The objective was to determine the applicability of the sorption process after conventional physicochemical wastewater treatment, or as primary treatment, replacing the physicochemical process. RESULTS: Fixed‐bed experiments were performed at Cu influent concentrations of 2 and 20 mg dm^?3 and EDTA doses between 0 and 10 mg dm^?3. At low Cu concentration without EDTA, Cu uptake capacity followed the order Posidonia oceanica > chitosan > chitin > Scharlau AC > Darco AC, with a maximum, at C/C₀ = 0.2, of 23.2 mg g^?1. In the presence of EDTA, Cu was detected in the effluent from the beginning of the operation, except for the activated carbons and chitosan at low EDTA doses. At higher EDTA doses, the activated carbons showed the best performance. Uptakes at Cu concentration of 20 mg dm^?3 without EDTA were 51.6 (Posidonia oceanica) and 41.4 mg g^?1 (chitosan) at C/C₀ = 0.2. CONCLUSION: A sequence of one fixed bed with Posidonia oceanica followed by another with Scharlau AC should be an alternative to Cu precipitation, with Cu effluent concentration lower than 0.5 mg dm^?3 for more than 350 pore volumes. Copyright © 2007 Society of Chemical Industry     

Investigation of NO Adsorption and NO/O2 Co-adsorption on NOx-Storage-Components by DRIFT-Spectroscopy

NO adsorption and NO/O₂ co-adsorption on CeO₂ at different temperatures was studied by DRIFT-Spectroscopy. The results indicate that this oxide plays an important role in storing NO _x . FTIR studies show that NO adsorption is dominated by the formation of nitrite species. Furthermore, cis- and trans hyponitrite species are detected. Co-adsorption of NO/O₂ leads to the formation of nitrates. The experimental data show that the formation of nitrates is a consecutive reaction: adsorption of NO to form nitrite species (NO₂ ^–), followed by an oxidation to form nitrate species (NO₃ ^–).     

The interaction of water-soluble iron porphyrins with DNA films and the electrocatalytic properties for inorganic and organic nitro compounds

The electrochemistry of water-soluble iron porphyrins (Fe(n-TMPyP)) (where n=2 and 4) was studied as an electrochemically active film on DNA modified glassy carbon, gold, platinum, and transparent semiconductor tin oxide electrodes in solutions of various pH values. The two layers of the modified electrode containing the iron porphyrin and the DNA film were prepared by depositing the iron porphyrin on a DNA film modified electrode. The Fe(4-TMPyP)/DNA film was electrocatalytic reductive for p-nitrobenzoic acid in a weak acidic, or neutral aqueous solution through an Fe^II species, and the electrocatalytic reduction peak potential became more negative than the cathodic peak of the Fe^III/II redox couple. The electrocatalytic reduction properties by the Fe(2-TMPyP)/DNA film as catalysts for nitrite reduction have also been determined, and shown to be active through an Fe^I species and to be pH-dependent. The electrocatalytic oxidation properties of nitrite by Fe(n-TMPyP)/DNA (for n=2 and 4) film have also been determined and shown to be active through an Fe^IV species with the electrocatalytic oxidation efficiency of NO₂⁻ with Fe^IV(O)(n-TMPyP) being higher than with (HO)Fe^IV(O)(n-TMPyP). The electrocatalytic oxidation efficiency of NO₂⁻ by iron porphyrin is pH-dependent. The electrocatalytic reduction of p-nitrophenol by Fe(2-TMPyP)/DNA film are also discussed.     

Measurement and modeling of nitrous and nitric oxide emissions from a tea field in subtropical central China

Tea fields represent an important source of nitrous oxide (N₂O) and nitric oxide (NO) emissions due to high nitrogen (N) fertilizer applications and very low soil pH. To investigate the temporal characteristics of N₂O and NO emissions, daily emissions were measured over 2½ years period using static closed-chamber/gas chromatograph and chemiluminescent measurement system in a tea field of subtropical central China. Our results revealed that N₂O and NO fluxes showed similar temporal trends, which were generally driven by temporal variations in soil temperature and soil moisture content and were also affected by fertilization events. The measured average annual N₂O and NO emissions were 10.9 and 3.3 kg N ha^?1 year^?1, respectively, highlighting the high N₂O and NO emissions from tea fields. To improve our understanding of N-cycling processes in tea ecosystems, we developed a new nitrogenous gas emission module for the water and nitrogen management model (WNMM, V2) that simulated daily N₂O and NO fluxes, in which the NO was simulated as being emitted from both nitrification and nitrite chemical decomposition. The results demonstrated that the WNMM captured the general temporal dynamics of N₂O (NSE = 0.40; R² = 0.52, RMSE = 0.03 kg N ha^?1 day^?1, P < 0.001) and NO (NSE = 0.41; R² = 0.44, RMSE = 0.01 kg N ha^?1 day^?1, P < 0.001) emissions. According to the simulation, denitrification was identified as the dominant process contributing 76.5% of the total N₂O emissions, while nitrification and nitrite chemical decomposition accounted for 52.3 and 47.7% of the total NO emissions, respectively.