A biofilm electrode reactor (BER) is proposed to effectively regenerate Fe(II)EDTA, a solvent for NOx removal from flue gas, from Fe(II)EDTA–NO, a spent solution. In this study, the performance, mechanism, and kinetics of the bioelectrochemical reduction of Fe(II)EDTA–NO were investigated. The pathways of Fe(II)EDTA–NO reduction were investigated via determination of nitrogen element balance in the BER and an abiotic electrode reactor. The experimental results indicate that the chelated NO (Fe(II)EDTA–NO) is reduced to N2 with N2O as an intermediate. However, the oxidation of NO occurred in the absence of Fe(II)EDTA in abiotic reactors. Furthermore, the accumulation of N2O was suppressed with the help of electricity. The preponderant electron donor for reduction of Fe(II)EDTA–NO was also confirmed via analysis of the electron conservation. About 87% of Fe(II)EDTA–NO was reduced using Fe(II)EDTA as the electron donor in the presence of both glucose and cathode electrons while the cathode electrons were utilized for the reduction of Fe(III)EDTA to Fe(II)EDTA. Michaelis–Menten kinetic constants of bioelectrochemical reduction of Fe(II)EDTA–NO were also calculated. The maximum reduction rate of Fe(II)EDTA–NO was 13.04 mol m–3 h–1, which is 50% higher than that in a conventional biofilter.

The chemical absorption-biological reduction (CABR) integrated process is regarded as a promising technology for NOx removal from flue gas. To advance the scale-up of the CABR process, a mathematic model based on mass transfer with reaction in the gas, liquid, and biofilm was developed to simulate and predict the NOx removal by the CABR system in a biotrickling filter. The developed model was validated by the experimental results and subsequently was used to predict the system performance under different operating conditions, such as NO and O2 concentration and gas and liquid flow rate. NO distribution in the gas phase along the biotrickling filter was also modeled and predicted. On the basis of the modeling results, the liquid flow rate and total iron concentration were optimized to achieve >90% NO removal efficiency. Furthermore, sensitivity analysis of the model revealed that the performance of the CABR process was controlled by the bioreduction activity of Fe(III)EDTA. This work will provide the guideline for the design and operation of the CABR process in the industrial application.

In this work, a novel hydrophilic ionic liquid (IL) 1-hydroxyethyl-3-methylimidazolium lysine ([C2OHmim][Lys]) with a considerable CO2 absorption capacity of 1.68 mol CO2/mol IL was synthesized. Three reaction products between CO2 and neat [C2OHmim][Lys] were successfully separated and detected by 13C NMR. Meanwhile, the absorption capacity of [C2OHmim][Lys] aqueous solutions with an initial conentration of 1mol/L was 1.2604 mol CO2/mol IL. The reaction mechanism and kinetic study of CO2 adsorption into an aqueous solution of [C2OHmim][Lys] were also investigated by using 13C NMR and wetted wall column. In the early stage of absorption, the dominant chemical reaction was the formation of carbamate and belonged to the fast reaction. The reaction order was found to be an average value of 1.862 with respect to [C2OHmim][Lys]. At high CO2 loading (more than 0.95 mol CO2/mol IL), the hydrolysis of carbamate was the main reaction, and it was between the fast reaction and the medium-speed reaction. Also, the two amino groups on the lysine anion have different reaction behaviors in the hydrolysis stage, suggesting that the carbamate linked to carboxyl in lysine anion participates in the hydration reaction while the other carbamate does not.

Biological reduction of nitric oxide (NO) with ferrous chelate is the main step for the chemical absorption–biological reduction (CABR) integrated method to remove nitrogen oxide (NOx) from flue gas. Heterotrophic bacteria play a dominant role in the CABR process, and their reactivity is seriously affected by carbon source and electron donor. Therefore, the consumption and utilization pathways of glucose were investigated. The study on glucose metabolites shows that the accumulation of acetate should be alleviated, which make it possible to keep running the bioreactor normally, although the volatile fatty acids (VFAs) may be beneficial as an electron donor for the reactions in CABR. The reduction of complex NO mainly depends upon the concentration of Fe(II) and acetate. The main utilization pathway of glucose can be expressed as glucose → pentanoic acid → butyric acid → propionic acid → acetic acid → CO2. Under experimental conditions of 670 mg m–3 NO inlet concentration, 0–10% O2 concentration, and 8 h of hydraulic retention time (HRT), more than half of inlet elemental carbon (glucose) was released in the form of gas after 240 h of operation. VFAs, especially acetic acid, mainly existed in the liquid phase, and CO2 was mainly observed in the gas phase.

A chemical absorption–biofilm electrode reactor (CABER) integrated system was used for removal of nitrogen monoxide (NO) from flue gas. Effects of the electric current on NO removal efficiency, concentration of Fe(II)EDTA, and consumption rate of glucose in the stabilization phase were investigated. Results indicate that the optimum impressed current was 0.04 A [i.e., 66.7 A m–3 net cathodic compartment (NCC) of the current density]. Under this condition, the consumption rate of glucose was 0.462 g h–1. Performance evaluation of this new approach was investigated under optimum conditions as well. It is noted that minimum residence time was only 20 s, maximum oxygen tolerability was 10%, and maximum elimination capacity of NO was 104.2 g of NO m–3 h–1. The contribution of H2 and glucose in reduction of Fe(III)EDTA was also studied. The results indicated that increasing the H2 supply appropriately could reduce the consumption of glucose. This new approach showed a better performance on NO removal and a larger processing load than those of the chemical absorption–biological reduction (CABR) integrated system.

The rapid growth of NOx emissions in China is mainly due to intensive fossil fuel consumption. In order to control NOx emissions, a multiyear NOx emission inventory was established by a bottom-up approach for the period 2000–2010. The results showed that NOx emissions increased by 2.1 times from 11.81 million tons (Mt) in 2000 to 24.33 Mt in 2010. We found that NOx emissions had exceeded SO2 emissions in 2009 by comparison with their emission trends. We also found that the unbalanced NOx emissions in Eastern China and Western China are mainly due to the different gross regional product and industrial structure. Accounting for 70% of total energy consumption in China, coal is the largest NOx emission source among all the fossil fuels. In addition, the increased use of diesel and gasoline has spurred the increase of NOx emissions from the transportation sector. Manufacturing, electricity production, and transportation together composed about 90% of the national NOx emissions. Meanwhile, energy consumption and NOx emissions in China are predicted to be 3908.5 Mt standard coal equivalent (SCE) and 19.7 Mt in 2020 with this scenario analysis, respectively. To achieve a desired NOx reduction target, China should take strict measures to control NOx emissions, such as improvement in reduction technology, promulgation of new emission standards, and joint control by various Chinese provinces.

Anthropogenic nitrogen oxides (NOx) emitted from the fossil-fuel-fired power plants cause adverse environmental issues such as acid rain, urban ozone smoke, and photochemical smog. A novel chemical absorption–biological reduction (CABR) integrated process under development is regarded as a promising alternative to the conventional selective catalytic reduction processes for NOx removal from the flue gas because it is economic and environmentally friendly. CABR process employs ferrous ethylenediaminetetraacetate [Fe(II)EDTA] as a solvent to absorb the NOx following microbial denitrification of NOx to harmless nitrogen gas. Meanwhile, the absorbent Fe(II)EDTA is biologically regenerated to sustain the adequate NOx removal. Compared with conventional denitrification process, CABR not only enhances the mass transfer of NO from gas to liquid phase but also minimize the impact of oxygen on the microorganisms. This review provides the current advances of the development of the CABR process for NOx removal from the flue gas.

An aqueous blend of monoethanolamine (MEA) and 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4) for CO2 absorption from simulated flue gas was investigated using a double stirred cell at a CO2 partial pressure of 15 kPa. It was found that the values of the enhancement factor (E) and the second-order reaction rate constant (k2,mix) for CO2 absorption into mixed solution were higher than those into a single MEA aqueous solution with the same MEA concentration. k2,mix and k2, IL were found to be 3487.6 m3·kmol–1·s–1 and 1936.7 m3·kmol–1·s–1 at 303.15 K, respectively. The Arrhenius equation of CO2 absorption was also estimated. The results proved the assumption that [Bmim]BF4 had an active effect on the CO2 hydration. The diffusion and solubility of the absorbent in the solution were the limiting factors of the reaction.

A promising technique called chemical absorption–biological reduction (CABR) integrated approach has been developed recently for the nitrogen oxides (NOx) removal from flue gases. The major challenge for this approach is how to enhance the rate of the biological reduction step. To tackle the challenge, a three-dimensional biofilm-electrode reactor (3D-BER) was utilized. This reactor provides not only considerable amount of sites for biofilm, but also many electron donors for bioreduction. Factors affecting the performance of 3D-BER were optimized, including material of the third electrode (graphite), glucose concentration (1000 mg·L–1), and volume current density (30.53 A·m–3 NCC). Experimental results clearly demonstrated that this method significantly promotes the bioreduction rate of Fe(II)EDTA-NO (0.313 mmol·L–1·h–1) and Fe(III)EDTA (0.564 mmol·L–1·h–1) simultaneously. Experiments on the mechanism showed that Fe(II)EDTA serves as the primary electron donor in the reduction of Fe(II)EDTA-NO, whereas the reduction of Fe(III)EDTA took advantage of both glucose and electrolysis-generated H2 as electron donors. High concentration of Fe(II)EDTA-NO or Fe(III)EDTA interferes the bioreduction of the other one. The proposed methodology shows a promising prospect for NOx removal from flue gas.

Fe(II)EDTA is an effective absorbent for the integrated chemical absorption–biological reduction system in removing nitric oxide (NO) from flue gas. However, this absorbent is subject to some defects, such as oxidation by oxygen. In order to overcome this drawback, instead of using Fe(II)EDTA solely, a mixed absorbent containing both Fe(II)EDTA and Fe(II)Cit (Cit = citrate) is employed to absorb NO from simulated flue gas. The mixed absorbent not only shows a high NO absorption capacity similar to a Fe(II)EDTA absorbent, but also exhibits high NO absorption rate and good resistance to oxidation in simulated flue gas. This mixed absorbent can maintain 80% NO removal efficiency at 323 K for more than two hours at an inlet NO concentration of 670 mg·m–3, which is almost as effective as the Fe(II)EDTA absorbent at the same Fe(II) concentration. The oxidation rate constant of Fe(II) in the mixed absorbent is also slower than that in the Fe(II)EDTA absorbent. The optimal molar ratio of Fe(II)Cit to Fe(II)EDTA in the mixed absorbent was found to be 3:1. The effects of several key factors for NO removal, such as the inlet concentrations of NO (200–670 mg·m–3), O2 (1–6.5%), and SO32– (0–43 mg·L–1) and the pH of the mixed absorbent, have been studied. Interestingly, results seem to suggest that SO32– is beneficial for the removal of NO in the system. These findings provide fundamental data for the design of NO removal system for industrial applications with mixed Fe(II)EDTA/Fe(II)Cit absorbent.

The biological reduction of Fe(III) ethylenediaminetetraacetic acid (EDTA) is a key step for NO removal in a chemical absorption–biological reduction integrated process. Since typical flue gas contain oxygen, NO2− and NO3− would be present in the absorption solution after NO absorption. In this paper, the interaction of NO2−, NO3−, and Fe(III)EDTA reduction was investigated. The experimental results indicate that the Fe(III)EDTA reduction rate decrease with the increase of NO2− or NO3− addition. In the presence of 10 mM NO2− or NO3−, the average reduction rate of Fe(III)EDTA during the first 6-h reaction was 0.076 and 0.17 mM h−1, respectively, compared with 1.07 mM h−1 in the absence of NO2− and NO3−. Fe(III)EDTA and either NO2− or NO3− reduction occurred simultaneously. Interestingly, the reduction rate of NO2− or NO3− was enhanced in presence of Fe(III)EDTA. The inhibition patterns observed during the effect of NO2− and NO3− on the Fe(III)EDTA reduction experiments suggest that Escherichia coli can utilize NO2−, NO3−, and Fe(III)EDTA as terminal electron acceptors.

A chemical absorption−biological reduction integrated approach, which combines the advantages of both the chemical and biological technologies, is employed to achieve the removal of nitrogen monoxide (NO) from the simulated flue gas. The biological reduction of NO to nitrogen gas (N2) and regeneration of the absorbent Fe(II)EDTA (EDTA:ethylenediaminetetraacetate) take place under thermophilic conditions (50 ± 0.5 °C). The performance of a laboratory-scale biofilter was investigated for treating NOx gas in this study. Shock loading studies were performed to ascertain the response of the biofilter to fluctuations of inlet loading rates (0.48∼28.68 g NO m−3 h−1). A maximum elimination capacity (18.78 g NO m−3 h−1) was achieved at a loading rate of 28.68 g NO m−3 h−1 and maintained 5 h operation at the steady state. Additionally, the effect of certain gaseous compounds (e.g., O2 and SO2) on the NO removal was also investigated. A mathematical model was developed to describe the system performance. The model has been able to predict experimental results for different inlet NO concentrations. In summary, both theoretical prediction and experimental investigation confirm that biofilter can achieve high removal rate for NO in high inlet concentrations under both steady and transient states.

Biological reduction of nitric oxide (NO) from Fe(II) ethylenediaminetetraacetic acid (EDTA)-NO to dinitrogen (N2) is a core process for the continual nitrogen oxides (NOx) removal in the chemical absorption–biological reduction integrated approach. To explore the biological reduction of Fe(II)EDTA-NO, the stoichiometry and mechanism of Fe(II)EDTA-NO reduction with glucose or Fe(II)EDTA as electron donor were investigated. The experimental results indicate that the main product of complexed NO reduction is N2, as there was no accumulation of nitrous oxide, ammonia, nitrite, or nitrate after the complete depletion of Fe(II)EDTA-NO. A transient accumulation of nitrous oxide (N2O) suggests reduction of complexed NO proceeds with N2O as an intermediate. Some quantitative data on the stoichiometry of the reaction are experimental support that reduction of complexed NO to N2 actually works. In addition, glucose is the preferred and primary electron donor for complexed NO reduction. Fe(II)EDTA served as electron donor for the reduction of Fe(II)EDTA-NO even in the glucose excessive condition. A maximum reduction capacity as measured by NO (0.818 mM h−1) is obtained at 4 mM of Fe(II)EDTA-NO using 5.6 mM of glucose as primary electron donor. These findings impact on the understanding of the mechanism of bacterial anaerobic Fe(II)EDTA-NO reduction and have implication for improving treatment methods of this integrated approach.

•A Pandoraea sp. strain WL1 could efficiently degrade sole p-xylene.•A complete pathway for p-xylene degradation via p-toluic acid was proposed.•Growth and degradation kinetics for p-xylene removal by strain WL1 were studied.•p-Xylene removal using a biotrickling filter inoculated with strain WL1 was evaluated.In this study, a novel Pandoraea sp. strain WL1 capable of mineralizing p-xylene as sole carbon and energy source was isolated from the activated sludge of a pharmaceutical wastewater treatment plant. A nearly complete degradation of 16.6 ∼ 99.4 mg L−1 p-xylene in the liquid-phase was achieved within 6 ∼ 18 h accompanied by 15.9 ∼ 56.3 mg dry cell weight (DCW) L−1 for bacterial growth. A complete pathway for p-xylene degradation by strain WL1 was presented through identification of a major intermediate (p-toluic acid) and final products (2.193 gCO2 gp-xylene−1 of CO2 production and 0.215 gDCW gp-xylene−1 of bacterial yield). Kinetics of bacterial growth and p-xylene degradation were evaluated using Haldane–Andrews model and pseudo first-order model, respectively. Furthermore, a biotrickling filter (BTF) was employed to evaluate the application of strain WL1 on the removal of gas-phase p-xylene under gas flow rates of 0.41 ∼ 1.98 m3 h−1 for inlet loading rates of 5 ∼ 248 g m−3 h−1. The BTF inoculated with strain WL1 proved to be robust against fluctuations of gas flow rates and inlet p-xylene concentrations. All the results obtained highlight the potential of strain WL1 for the treatment of p-xylene.

Sargassum based activated carbon (SAC) doped with various transition metals was developed via impregnation as a new catalyst for selective catalytic reduction of NO with NH3 (NH3-SCR) in the temperature range of 50–250 °C with gas hourly space velocity (GHSV) of 80,000 h−1. The samples were characterized by means of Brunauer–Emmett–Teller method (BET), scanning electron microscope (SEM), NH3 temperature-programmed desorption (NH3-TPD), H2 temperature-programmed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS). The impacts of water (H2O) and sulfur dioxide (SO2) on the SCR activity of the CrO3/SAC catalyst were also discussed. The experimental results showed that the introduction of Cr increased the acid sites formed on the surface of catalysts and enhanced the SCR reaction rate by the valence changing between Cr6+ and its lower oxidized states (Cr5+, Cr3+ and Cr2+). The catalyst with a Cr/SAC mass ratio of 2%:1 exhibited the best NOx-removing performance, with NOx conversion greater than 90% at the temperature of 125–150 °C. Moreover, it had excellent water and sulfur tolerance, making the Cr-doped SAC catalyst as a good candidate for reducing the NOx emission from fired power plants.