Electronic Competition Research Articles

Evaluating two concepts for the modelling of intermediates accumulation during biological denitrification in wastewater treatment

The accumulation of the denitrification intermediates in wastewater treatment systems is highly undesirable, since both nitrite and nitric oxide (NO) are known to be toxic to bacteria, and nitrous oxide (N2O) is a potent greenhouse gas and an ozone depleting substance. To date, two distinct concepts for the modelling of denitrification have been proposed, which are represented by the Activated Sludge Model for Nitrogen (ASMN) and the Activated Sludge Model with Indirect Coupling of Electrons (ASM-ICE), respectively. The two models are fundamentally different in describing the electron allocation among different steps of denitrification. In this study, the two models were examined and compared in their ability to predict the accumulation of denitrification intermediates reported in four different experimental datasets in literature. The N-oxide accumulation predicted by the ASM-ICE model was in good agreement with values measured in all four cases, while the ASMN model was only able to reproduce one of the four cases. The better performance of the ASM-ICE model is due to that it adopts an “indirect coupling” modelling concept through electron carriers to link the carbon oxidation and the nitrogen reduction processes, which describes the electron competition well. The ASMN model, on the other hand, is inherently limited by its structural deficiency in assuming that carbon oxidation is always able to meet the electron demand by all denitrification steps, therefore discounting electron competition among these steps. ASM-ICE therefore offers a better tool for predicting and understanding intermediates accumulation in biological denitrification.

Effect of carbon source and competition for electrons on nitrous oxide reduction in a mixed denitrifying microbial community

The competition for electrons has been recently demonstrated to affect the reduction rates of the nitrogen oxides in a methanol enriched denitrifying community. The aim of this study was to test if electron competition also occurred when other substrates were used for denitrification and if that could have an effect on the potential nitrous oxide (N2O) production and subsequent consumption. A denitrifying culture was developed in a sequencing batch reactor using nitrate as electron acceptor and a combination of acetate, ethanol and methanol as carbon sources. Four sets of batch tests were conducted using acetate, ethanol, methanol and a combination of the three carbon sources respectively. For each set the effect of nitrate, nitrite and nitrous oxide on each other reduction rates when present individually or in combination was assessed. Results show that reduction rates are affected by the type of substrate added, probably due to different microbial populations specialized with consuming a particular substrate. Also, N2O reduction rate is the most reduced under the different electron competition scenarios tested, which results in N2O accumulation in some cases. The effect of substrate limitation on N2O reduction was also assessed.

Processes controlling the fate of chloroethenes emanating from DNAPL aged sources in river–aquifer contexts

This work dealt with the physical and biogeochemical processes that favored the natural attenuation of chloroethene plumes of aged sources located close to influent rivers in the presence of co-contaminants, such as nitrate and sulfate. Two working hypotheses were proposed: i) Reductive dechlorination is increased in areas where the river–aquifer relationship results in the groundwater dilution of electron acceptors, the reduction potential of which exceeds that of specific chloroethenes; ii) zones where silts predominate or where textural changes occur are zones in which biodegradation preferentially takes place. A field site on a Quaternary alluvial aquifer at Torelló, Catalonia (Spain) was selected to validate these hypotheses. This aquifer is adjacent to an influent river, and its redox conditions favor reductive dechlorination. The main findings showed that the low concentrations of nitrate and sulfate due to dilution caused by the input of surface water diminish the competition for electrons between microorganisms that reduce co-contaminants and chloroethenes. Under these conditions, the most bioavailable electron acceptors were PCE and metabolites, which meant that their biodegradation was favored. This led to the possibility of devising remediation strategies based on bioenhancing natural attenuation. The artificial recharge with water that is low in nitrates and sulfates may favor dechlorinating microorganisms if the redox conditions in the mixing water are sufficiently maintained as reducing and if there are nutrients, electron donors and carbon sources necessary for these microorganisms.

The effect of alcohol concentration on the mass signal of CO2 detected by differential mass spectrometry

Differential electrochemical mass spectrometry (DEMS) is one of the powerful techniques for quantitative analysis of multiple products and their distribution from complex reactions occurred at the electrode/electrolyte interface, in which precise calibration of the mass signal is a prerequisite for accurate determination of product rate and current efficiencies of various products. In this work, we report the effect of alcohol concentration on the detected DEMS signal of CO2, a common final product from electrochemical oxidation of small organic molecules. Comparing to the solution with low alcohol concentration and the same CO2 concentration, we observe a significant reduction of mass signal of CO2 in a concentrated methanol or ethanol solution (>0.1M). Possible origins for this are: 1) alcohol molecules evaporate into the vacuum chamber and reduce the ionization probability for CO2 via competition for electrons; and 2) CO2 is trapped in the hydrophobic alcohol cages formed in the aqueous electrolyte. A proper way to eliminate alcohol concentration effect on the detected CO2 mass signal in DEMS studies on fuel cell related electrocatalysis is suggested.

Regeneration of spent NOx scrubber liquor using a dual-chamber microbial fuel cell

BACKGROUND Wet absorption via addition of Fe(II)EDTA combined with biological reduction for nitric oxide removal is a promising approach. Based on existing issues and related studies of NOx scrubber solution, a dual-chamber MFC system with a biocathode was utilized to perform simultaneous and continuous reduction of Fe(III)EDTA and Fe(II)EDTA-NO with energy recovery. RESULTS The experimental results showed that in this system almost all of the Fe(II)EDTA-NO was removed and the spent NOx scrubber liquor (consisting of Fe(II)EDTA-NO and Fe(III)EDTA) was regenerated 50%, combined with electricity generation of 15.3 Am-3 NCC at the same time. Microorganisms utilized Fe(II)EDTA and cathodic electrodes as electron donor for Fe(II)EDTA-NO reduction and, different from the complicated Fe(II)EDTA-NO reduction process, the electrode was the sole electron donor for Fe(III)EDTA reduction. Additionally, Fe(II)EDTA-NO and Fe(III)EDTA mutually inhibits each other, mostly due to the competition for electrons. The microbial community of the biocathode was dominated by members of the Betaproteobacteria class. CONCLUSION This study first proposed and confirmed that microbial fuel cells could be utilized for the regeneration of NOx removal liquor which provided a novel pathway for NO removal combined with energy recovery. © 2014 Society of Chemical Industry

The effect of poly-β-hydroxyalkanoates degradation rate on nitrous oxide production in a denitrifying phosphorus removal system

Poly-β-hydroxyalkanoates (PHAs) and free nitrous acid (FNA) have been revealed as significant factors causing nitrous oxide (N2O) production in denitrifying phosphorus removal systems. In this study, the effect of PHA degradation rate on N2O production was studied at low FNA levels. N2O production always maintained at approximately 40% of the amount of nitrite reduced independent of the PHA degradation rate. The electrons distributed to nitrite reduction were 1.6 times that to N2O reduction. This indicated that electron competition between these two steps was not affected by the PHA degradation rate. Continuous feed of nitrate was proposed, and demonstrated to reduce N2O accumulation by 75%. While being kept low, a possible compounding effect of a low-level FNA could not be ruled out. The sludge used likely contained both polyphosphate- and glycogen-accumulating organisms, and the results could not be simply attributed to either group of organisms.

Influence of Riboflavin on Nanoscale Zero-Valent Iron Reactivity during the Degradation of Carbon Tetrachloride

Experiments were conducted to investigate the effect of riboflavin on the reactivity of nanoscale zerovalent iron (NZVI) during three reaction cycles of carbon tetrachloride (CT) degradation. The degradation kinetics of CT by NZVI without riboflavin (0.556 ± 0.044 h(-1)) was 1.5 times higher than that with riboflavin (0.370 ± 0.012 h(-1)) in the first cycle. Riboflavin was rapidly reduced (65.0 ± 7.0 h(-1)) by NZVI during CT degradation, resulting in the slow degradation kinetics of CT in the first cycle due to competition for electrons from NZVI between riboflavin and CT. These results indicate that riboflavin is not effective as an electron shuttle for reduction of CT by NZVI. On the other hand, the degradation kinetics of CT by NZVI without riboflavin decreased to 0.122 ± 0.033 h(-1) in the third cycle, while that with riboflavin was significantly enhanced (0.663 ± 0.005 h(-1)). The results from X-ray analyses and transmission electron microscopy suggest that the decline in reactivity of NZVI without riboflavin in the third cycle resulted from continuous Fe(0) oxidation to iron oxides on the NZVI surface. In contrast, riboflavin enhanced the reactivity of NZVI by reductive dissolution of passive iron oxides on NZVI surface by reduced riboflavin. The experimental results suggest that riboflavin can play a pivotal role in the prolongation of NZVI reactivity in long-term in situ and ex situ applications of NZVI.

Greenhouse gas emissions from rice microcosms amended with a plant microbial fuel cell

Methane (CH4) release from wetlands is an important source of greenhouse gas emissions. Gas exchange occurs mainly through the aerenchyma of plants, and production of greenhouse gases is heavily dependent on rhizosphere biogeochemical conditions (i.e. substrate availability and redox potential). It is hypothesized that by introducing a biocatalyzed anode electrode in the rhizosphere of wetland plants, a competition for carbon and electrons can be invoked between electrical current-generating bacteria and methanogenic Archaea. The anode electrode is part of a bioelectrochemical system (BES) capable of harvesting electrical current from microbial metabolism. In this work, the anode of a BES was introduced in the rhizosphere of rice plants (Oryza sativa), and the impact on methane emissions was monitored. Microbial current generation was able to outcompete methanogenic processes when the bulk matrix contained low concentrations of organic carbon, provided that the electrical circuit with the effective electroactive microorganisms was in place. When interrupting the electrical circuit or supplying an excess of organic carbon, methanogenic metabolism was able to outcompete current generating metabolism. The qPCR results showed hydrogenotrophic methanogens were the most abundant methanogenic group present, while mixotrophic or acetoclastic methanogens were hardly detected in the bulk rhizosphere or on the electrodes. Competition for electron donor and acceptor were likely the main drivers to lower methane emissions. Overall, electrical current generation with BESs is an interesting option to control CH4 emissions from wetlands but needs to be applied in combination with other mitigation strategies to be successful and feasible in practice.

Application of Electromagnetic Shielding Technology in the Electronic System

With the wide application of electronic technology, electromagnetic interference has become one of the main factors to weaken electronic system performance. In communication technology and equipment, it is the development of light-duty design, low cost, high quality electronic components and electronic system that is the main object of this study. Radio frequency MEMS devices are low energy consumption, good stability. Therefore it has a strong competitiveness in the communications electronics. But EMI electromagnetic interference problem has increasingly become prominent, and based on this, this paper makes a further study on the main electromagnetic interference electromagnetic radiation of MEMS devices. In addition, this paper also establishes the mathematical model of electromagnetic radiation. Finally this paper makes an analysis and study on the shielding effect of MEMS antenna under certain magnetic materials. Through the simulation research, it is obtained that the electromagnetic effect of electronic system MEMS is good and its highest performance is more than 60 db. Whats more, shielding materials are within the range of 15 to 20 GHZ, whose shielding effects reach the maximum. The largest is close to 80 db.

Modeling Electron Competition among Nitrogen Oxides Reduction and N2O Accumulation in Denitrification

Competition for electrons among different steps of denitrification has previously been shown to occur, and to play an important role in the accumulation and emission of N2O in wastewater treatment. However, this electron competition is not recognized in the current denitrification models, limiting their ability to predict N2O accumulation during denitrification. In this work, a new denitrification model is developed for wastewater treatment processes. It describes electron competition among the four steps of denitrification, through modeling the carbon oxidation and nitrogen reduction processes separately, in contrast to the existing models that directly couple these two types of processes. Electron carriers are introduced to link carbon oxidation, which donates electrons to carriers, and nitrogen oxides reduction, which receives electrons from these carriers. The relative ability of each denitrification step to compete for electrons is modeled through the use of different affinity constants with reduced carriers. Model calibration and validation results demonstrate that the developed model is able to reasonably describe the nitrate, nitrite, and N2O reduction rates of a methanol-utilizing denitrifying culture under various carbon and nitrogen oxides supplying conditions. The model proposed, while subject to further validation, is expected to enhance our ability to predict N2O accumulation in denitrification.

Influence of hydronium, sulfate, chloride and other non-carbonate ions on hydrogen generation by anaerobic corrosion of granular cast iron

Permeable reactive barriers are successfully applied for the removal of various contaminants. The concomitant reduction of hydrogen ions and the subsequent formation of hydrogen gas by anaerobic corrosion lead to decreased pore volume filled with water and thus residence times, so called gas clogging. Long term column experiments were conducted to elucidate the impact of ubiquitous water constituents on the formation of hydrogen gas and potential passivation due to corrosion products. The collected gas volumes revealed a relation to the hydronium concentration (pH) but were only slightly increased in the presence of chloride and sulfate and not significantly influenced in the presence of phosphate, silicate, humic acid and ammonium compared to deionized water. Significant gas volumes within the reactive filling were verified by gravimetry. The presence of nitrate completely eliminated hydrogen formation by competition for electrons. Solid phase analyses revealed that neither chloride nor sulfate was incorporated in corrosion products in concentrations above 0.1 weight percent, and they did not alter the formation of mainly magnetite in comparison to deionized water.

UASB performance and electron competition between methane-producing archaea and sulfate-reducing bacteria in treating sulfate-rich wastewater containing ethanol and acetate

To find an appropriate method for sulfate-rich wastewater containing ethanol and acetate with COD/sulfate ratio of 1, a UASB reactor was operated for more than 180 days. The influences of HRT (hydraulic retention time) and OLR (organic loading rate) on organics and sulfate removal, gas production, and electrons utilization were investigated. The sludge activity and microorganism composition were also determined. The results indicated that this system removed more than 80% of COD and 30% of sulfate with HRT above 6h and OLR below 12.3 gCOD/L d. Further HRT decrease caused volatile fatty acids accumulation and performance deterioration. Except at HRT of 2h, COD and electron flow were mostly utilized by methane-producing archaea (MPA), and methane yield remained in the range of 0.18-0.24 LCH4/gCOD. Methane was mainly generated by Methanosaeta concilii GP6 with acetate as substrate, whereas sulfate was mainly reduced by incomplete-oxidizing Desulfovibrio species with ethanol as substrate.

Electron competition among nitrogen oxides reduction during methanol-utilizing denitrification in wastewater treatment

Limited availability of carbon sources has been regarded as an important factor leading to N2O accumulation during denitrification in wastewater treatment. By varying the carbon (methanol) loading rate to a methanol utilizing denitrifying culture in the presence of various electron acceptors (nitrate, nitrite, N2O and their combinations), this study quantitatively investigated the electron distribution among different nitrogen oxide reductases during denitrification. The results showed that electron competition occurs under not only carbon limiting but also carbon abundant conditions. The electron distribution among the nitrogen oxide reductases is affected by the carbon loading rate, with a lower fraction of electrons distributed to the N2O reductase with reduced carbon loading rate. N2O accumulation occurs when the electron flux going to nitrite reduction is higher than that going to N2O reduction. The study also showed that, for the culture investigated, the carbon to nitrogen ratio is not a key factor leading to N2O accumulation.

Simultaneous removal of selected oxidized contaminants in groundwater using a continuously stirred hydrogen-based membrane biofilm reactor

A laboratory trial was conducted for evaluating the capability of a continuously stirred hydrogen-based membrane biofilm reactor to simultaneously reduce nitrate (NO3−-N), sulfate (SO42-), bromate (BrO3−), hexavalent chromium (Cr(VI)) and para-chloronitrobenzene (p-CNB). The reactor contained two bundles of hollow fiber membranes functioning as an autotrophic biofilm carrier and hydrogen pipe as well. On the condition that hydrogen was supplied as electron donor and diffused into water through membrane pores, autohydrogenotrophic bacteria were capable of reducing contaminants to forms with lower toxicity. Reduction occurred within 1 day and removal fluxes for NO3−-N, SO42-, BrO3−, Cr(VI), and p-CNB reached 0.641, 2.396, 0.008, 0.016 and 0.031 g/(day·m2), respectively after 112 days of continuous operation. Except for the fact that sulfate was 37% removed under high surface loading, the other four contaminants were reduced by over 95%. The removal flux comparison between phases varying in surface loading and H2 pressure showed that decreasing surface loading or increasing H2 pressure would promote removal flux. Competition for electrons occurred among the five contaminants. Electron-equivalent flux analysis showed that the amount of utilized hydrogen was mainly controlled by NO3−-N and SO42- reduction, which accounted for over 99% of the electron flux altogether. It also indicated the electron acceptor order, showing that nitrate was the most prior electron acceptor while sulfate was the second of the five contaminants.

Aqueous phase photocatalytic nitrate destruction using titania based materials: routes to enhanced performance and prospects for visible light activation

Use of titania based photocatalysts for the selective reduction of nitrates to nitrogen through photocatalytic methods has attracted some interest in recent years and the need to operate this process remotely and without energy sources will ensure that a level of interest will remain in attempting to improve this process. There remain numerous avenues that have not yet been investigated in the effort to optimise a catalyst for this reaction. Work on metal containing systems offer the potential to; improve activity through offering a route to improved electron-hole separation, manipulate selectivity by controlling particle size which affects the flat band potential under Fermi equilibration conditions and for operation in the visible region by manipulating particle shape and using SPR properties. Opportunities to further manipulate all three of these situations by using bimetallic particles using core-shell, true alloy and other forms, remains open for exploration. The introduction of hetero-species into bulk titania in order to reduce the band gap and permit operation within the visible region. The key issues in the case of the reaction involved would be to retain the oxidation potential of the valance band but to sacrifice the reduction potential of the conduction band thus permitting nitrate reduction but avoiding competition for electrons by protons in solution. In a more general approach, the key challenge should be to obtain a route to uniform bulk doping without the need to use complex ion implantation routes. Composites offer the opportunity to manipulate adsorption capacity although as yet, attempts to improve nitrate adsorption with respect to other organic anions has not been attempted. They may also be used to improve electron transfer properties and to enhance charge-carrier lifetimes.

Removal of Co-Present Cr(VI) and Cu(II) by Fe<sup>0</sup>

Zero-valent iron (Fe0) as one kind of redox material has widely been used to remove heavy metals from aqueous environments due to its low cost, simplicity and reliability. In this paper, interaction of Cr(VI) with Cu(II) in Fe0-H2O systems was studied in order to understand better the performance of Fe0 in wastewater containing multiple heavy metals. Results showed that Cr(VI) had inhibitory impact on Cu(II) removal by Fe0, resulting from competition of electrons generated from Fe0, while Cu(II) obviously enhanced the removal of Cr(VI), which could result from redox of Cr(VI) with Fe2+ from reaction of Fe0 with Cu(II) and then formed weak acid environments which were favorable for the removal of Cr(VI).

Book Review: EU Electronic Communications Law, Competition and Regulation in the European Telecommunications Market, by Paul Nihoul & Peter Rodford. (2nd edn, Oxford University Press, 2011)

Book Review: <i>EU Electronic Communications Law, Competition and Regulation in the European Telecommunications Market</i>, by Paul Nihoul &amp; Peter Rodford. (2nd edn, Oxford University Press, 2011)

Effect of pH on N2O reduction and accumulation during denitrification by methanol utilizing denitrifiers

Acidic pH has previously been found to increase nitrous oxide (N2O) accumulation during heterotrophic denitrification in biological wastewater treatment. However, the mechanism of this phenomenon still needs to be clarified. By using an enriched methanol utilizing denitrifying culture as an example, this paper presents a comprehensive study on the effect of pH (6.0–9.0) on N2O reduction kinetics with N2O as the sole electron acceptor, as well as the effect of pH on N2O accumulation with N2O as an intermediate of nitrate reduction. The pH dependency of nitrate and nitrite reduction was also investigated. The maximum biomass-specific N2O reduction rate is higher than the corresponding maximum nitrate and nitrite reduction rates in the entire pH range studied. However, the maximum biomass-specific N2O reduction rate is much more sensitive to pH variation outside of the optimal range (pH 7.5 to pH 8.0) than the maximum biomass-specific nitrate and nitrite reduction rates. The half-saturation coefficient of the N2O reductase increased from 0.10 mg N2O-N/L to 0.92 mg N2O-N/L as pH increased from pH 6.0 to 9.0. At pH 6.0, approximately 20% and 40% of the denitrified nitrate accumulated as N2O in the presence and absence of methanol (as an exogenous carbon source), respectively. However, at pH 6.5, these fractions decreased to 0% and 30%, respectively. No N2O accumulation occurred at pH 7.0 to 9.0 independent of the availability of methanol. These results suggest that the competition for electrons among different nitrogen oxides reductases likely plays a role in N2O accumulation at low pH conditions.

Effect of groundwater geochemistry on pentachlorophenol remediation by smectite-templated nanosized Pd0/Fe0

Zero-valent iron holds great promise in treating groundwater, and its reactivity and efficacy depend on many surrounding factors. In the present work, the effects of solution chemistry such as pH, humic acid (HA), and inorganic ions on pentachlorophenol (PCP) dechlorination by smectite-templated Pd(0)/Fe(0) were systematically studied. Smectite-templated Pd(0)/Fe(0) was prepared by saturating the negatively charged sites of smectite clay with Fe(III) and a small amount of Pd(II), followed by borohydride reduction to convert Fe(III) and Pd(II) into zero-valent metal clusters. Batch experiments were conducted to investigate the effects of water chemistry on PCP remediation. The PCP dechlorination rate critically depends on the reaction pH over the range 6.0~10.0; the rate constant (k (obs)) increases with decreasing the reaction pH value. Also, the PCP remediation is inhibited by HA, which can be attributed to the electron competition of HA with H(+). In addition, the reduction of PCP can be accelerated by various anions, following the order: Cl(-) > HCO (3) (-) > SO (4) (2-) ~no anion. In the case of cations, Ca(2+) and Mg(2+) (10 mM) decrease the dechlorination rate to 0.7959 and 0.7798 from 1.315 h(-1), respectively. After introducing HA into the reaction systems with cations or/and anions, the dechlorination rates are similar to that containing HA alone. This study reveals that low pH and the presence of some anions such as Cl(-) facilitate the PCP dechlorination and induce the rapid consumption of nanosized zero-valent iron simultaneously. However, the dechlorination rate is no longer correlated to the inhibitory or accelerating effects by cations and anions in the presence of 10 mg/L HA.

Nitrite accumulation under constant temperature in anoxic denitrification process: The effects of carbon sources and COD/NO3-N

Effects of external carbon sources and COD/NO3-N on nitrite accumulation through denitrification were studied at a temperature of 28±2.0°C using mixed activated sludge. Nitrite accumulation was observed for each type of carbon source studied. Glucose resulted in the greatest nitrite accumulation and production rate, which were 14.51±2.41mg/L and 0.121±0.013gN/(gVSSd), respectively. Moreover, a higher COD/NO3-N ratio ranging from 1.0 to 15.0 increased accumulation to the maximum value of 0.34±0.03gN/(gVSSd). It was assumed that the competition for electrons between nitrite reductase and nitrate reductase led to different reduction rates and finally caused the accumulation. In addition, it was reasonable to use the pH and ORP as proxies for monitoring the real endpoint of the denitrification process with the addition of carbon sources.