Chemical Absorption-biological Reduction Research Articles

Mass transfer vectors for nitric oxide removal through biological treatments

The reduction of nitric oxide (NO) emissions to atmosphere has been recently addressed using biological technologies. However, NO removal through bioprocesses is quite challenging due to the low solubility of NO in water. Therefore, the abatement of NO emissions might be improved by adding a chelating agent or a mass transfer vector (MTV) to increase the solubility of this pollutant into the aqueous phase where the bioprocess takes place. This research seeks to assess the performance of different non-aqueous phase liquids (NAPs): n-hexadecane (HEX), diethyl sebacate (DSE), 1,1,1,3,5,5,5-heptamethyl-trisiloxane (HTX), 2,2,4,4,6,8,8-heptamethylnonane (HNO), and high temperature silicone oil (SO) in chemical absorption–biological reduction (CABR) integrated systems. The results showed that HNO and HTX had the maximum gas-liquid mass transfer capacity, being 0.32 mol NO/kmol NAP and 0.29 mol NO/kmol NAP, respectively. When an aqueous phase was added to the system, the mass transfer gas–liquid of NO was increased, with HTX reaching a removal efficiency of 82 ± 3% NO with water, and 88 ± 6% with a phosphate buffer solution. All NAPs were tested for short-term toxicity assessment and resulted neither toxic nor inhibitory for the biological activity (denitrification). DSE was found to be biodegradable, which could limit its applicability in biological processes for gas treatment. Finally, in the CABR system tests, it was shown that NO elimination improved in a short time (30 min) when the three mass transfer vectors (HEX, HTX, HNO) were added to enriched denitrifying bacteria.

Simultaneous removal of NOX and SO2 from flue gas in an integrated FGD-CABR system by sulfur cycling-mediated Fe(II)EDTA regeneration

Chemical absorption-biological reduction (CABR) process is an attractive method for NOX removal and Fe(II)EDTA regeneration is important to sustain high NOX removal. In this study a sustainable and eco-friendly sulfur cycling-mediated Fe(II)EDTA regeneration method was incorporated in the integrated biological flue gas desulfurization (FGD)-CABR system. Here, we investigated the NOX and SO2 removal efficiency of the system under three different flue gas flows (100 mL/min, 500 mL/min, and 1000 mL/min) and evaluated the feasibility of chemical Fe(III)EDTA reduction by sulfide in series of batch tests. Our results showed that complete SO2 removal was achieved at all the tested scenarios with sulfide, thiosulfate and S0 accumulation in the solution. Meanwhile, the total removal efficiency of NOX achieved ∼100% in the system, of which 3.2%–23.3% was removed in spray scrubber and 76.7%–96.5% in EGSB reactor along with no N2O emission. The optimal pH and S2−/Fe(III)EDTA for Fe(II)EDTA regeneration and S0 recovery was 8.0 and 1:2. The microbial community analysis results showed that the cooperation of heterotrophic denitrifier (Saprospiraceae_uncultured and Dechloromonas) and iron-reducing bacteria (Klebsiella and Petrimonas) in EGSB reactor and sulfide-oxidizing, nitrate-reducing bacteria (Azoarcus and Pseudarcobacter) in spray scrubber contributed to the efficient removal of NOX in flue gas.

A Critical Review of the Biological Processes in the Chemical Absorption- Biological Reduction Integrated System for NO Removal

Exploring low-cost, green, and safe technologies to provide an alternative to the conventionally selective catalytic reduction process is key to control the NOx emission from small-scale boilers and other industrial processes. For meeting this demand, the chemical absorption-biological reduction integrated system has been developed recently. This system applies Fe(II)EDTA for NO absorption and for the absorbent regeneration by iron-reducing and denitrifying bacteria . Many studies have focused on the enhancements of mass transfer and biological reaction, among which the biological processes were the rate-limiting steps. This review summarizes the current researches on the biological processes in the CABR system, which focuses on the mechanism and enhancement of biochemical reactions, and it also provides the possible directions for future research.

Performance and Microbial Community Analysis of an Electrobiofilm Reactor Enhanced by Ferrous-EDTA.

The biological reduction of ferrous ethylenediaminetetraacetic acid (EDTA-FeII-NO and EDTA-FeIII) is an important process in the integrated electrobiofilm reduction method, and it has been regarded as a promising alternative method for removing NOx from industrial boiler flue gas. EDTA-FeII-NO and EDTA-FeIII are crucial substrates that should be biologically reduced at a high rate. However, they inhibit the reduction processes of one another when these two substrates are presented together, which might limit further promotion of the integrated method. In this study, an integrated electrobiofilm reduction system with high reduction rates of EDTA-FeII-NO and EDTA-FeIII was developed. The dynamic changes of microbial communities in the electrobiofilms were mainly investigated to analyze the changes during the reduction of these two substrates under different conditions. The results showed that compared to the conventional chemical absorption-biological reduction system, the reduction system exhibited better performance in terms of resistance to substrate shock loading and high microbial diversities. High-throughput sequencing analysis showed that Alicycliphilus, Enterobacteriaceae, and Raoultella were the dominant genera (>25% each) during the process of EDTA-FeII-NO reduction. Chryseobacterium had the ability to endure the shock loading of EDTA-FeIII, and the relative abundance of Chryseobacterium under abnormal operation conditions was up to 30.82%. Ochrobactrum was the main bacteria for reducing nitrate by electrons and the relative abundance still exhibited 16.11% under shock loading. Furthermore, higher microbial diversity and stable reactor operation were achieved when the concentrations of EDTA-FeII-NO and EDTA-FeIII approached the same value (9 mmol·L−1).

Biological Reduction of Nitric Oxide for Efficient Recovery of Nitrous Oxide as an Energy Source.

Chemical absorption-biological reduction based on Fe(II)EDTA is a promising technology to remove nitric oxide (NO) from flue gases. However, limited effort has been made to enable direct energy recovery from NO through production of nitrous oxide (N2O) as a potential renewable energy rather than greenhouse gas. In this work, the enhanced energy recovery in the form of N2O via biological NO reduction was investigated by conducting short-term and long-term experiments at different Fe(II)EDTA-NO and organic carbon levels. The results showed both NO reductase and N2O reductase were inhibited at Fe(II)EDTA-NO concentration up to 20 mM, with the latter being inhibited more significantly, thus facilitating N2O accumulation. Furthermore, N2O accumulation was enhanced under carbon-limiting conditions because of electron competition during short-term experiments. Up to 47.5% of NO-N could be converted to gaseous N2O-N, representing efficient N2O recovery. Fe(II)EDTA-NO reduced microbial diversity and altered the community structure toward Fe(II)EDTA-NO-reducing bacteria-dominated culture during long-term experiments. The most abundant bacterial genus Pseudomonas, which was able to resist the toxicity of Fe(II)EDTA-NO, was significantly enriched, with its relative abundance increased from 1.0 to 70.3%, suggesting Pseudomonas could be the typical microbe for the energy recovery technology in NO-based denitrification.

Simultaneous cobalt(III)-histidine reduction and aerobic denitrification by Paracoccus versutus LYM

Cobalt(II)-histidine [Co(II)His] is potentially a better alternative to ferrous complexes in the chemical absorption-biological reduction (CABR) flue gas denitrification process in view of its higher oxygenation reversibility. Though with excellent O2-resistant ability, Co(II)His was still gradually oxidized into Co(III)His, losing NO binding capacity. Thus, Co(III)His biological reduction is an indispensable step in CABR process. Co(III)His reduction by Paracoccus versutus LYM under aerobic condition in the presence of nitrate or nitrite was investigated. Results indicated that simultaneous Co(III)His reduction and aerobic denitrification were achieved by strain LYM. Co(III)His reduction was significantly promoted by denitrification process, but dramatically inhibited by 5–15 mM sulfite. Co(II)His absorbent regeneration could be facilitated by adjusting O2 supply properly or adding nitrogen and carbon source regularly. These findings provide a basis for the application of Co(II)His as the absorbent in the CABR process and qualify P. versutus LYM as an applicable and competitive strain for this process.

Kinetics of biocathodic electron transfer in a bioelectrochemical system coupled with chemical absorption for NO removal

A microbial electrolysis cell (MEC) has been developing for enhanced absorbent regeneration in a chemical absorption-biological reduction integrated process for NO removal. In this work, the kinetics of electron transfer involved in the biocathodes along Fe(III)EDTA and Fe(II)EDTA-NO reduction was analyzed simultaneously. A modified Nernst-Monod kinetics considering the Faraday efficiency was applied to describe the electron transfer kinetics of Fe(III)EDTA reduction. The effects of substrate concentration, biocathodic potential on current density predicted by the model have been validated by the experimental results. Furthermore, extended from the kinetics of Fe(III)EDTA reduction, the electron transfer kinetics of Fe(II)EDTA-NO reduction was developed with a semi-experimental method, while both direct electrochemical and bioelectrochemical processes were taken into consideration at the same time. It was revealed that the developed model could simulate the electron transfer kinetics well. This work could not only help advance the biocathodic reduction ability and the utilization efficiency of electric power, but also provide insights into the industrial scale-up and application of the system.

Effects of cobalt-histidine absorbent on aerobic denitrification by Paracoccus versutus LYM

To overcome the problem that ferrous complexes are easily oxidized by O2 and then lose NO binding ability in the chemical absorption-biological reduction (CABR) process, cobalt(II)-histidine [Co(II)His] was proposed as an alternative. To evaluate the applicability of Co(II)His, the effects of CoHis absorbent on the aerobic denitrification by Paracoccus versutus LYM were investigated. Results indicated that His significantly promoted nitrite reduction. The inhibition effects of CoHis absorbent could be substantially alleviated by increasing the initial His/Co2+ to 4 or higher. CoHis with concentrations of 4, 8, 12, 16 and 20 mM presented no distinct effect on nitrite reduction, but slightly inhibited the reduction of nitrate, resulting in longer lag of nitrate reduction, and obviously promoted the growth of strain LYM. In the presence of 5, 10, 15 and 20 mM CoHis absorbent, the main denitrification product was N2 (not less than 95.0%). This study is of significance in verifying the applicability of Co(II)His in the CABR process, and provides a referable CoHis absorbent concentration as 20 mM with an initial His/Co2+ of 4 for the future experiments.

Evaluation of polypyrrole-modified bioelectrodes in a chemical absorption-bioelectrochemical reduction integrated system for NO removal

A Chemical absorption-bioelectrochemical reduction (CABER) system is based on Chemical absorption-biological reduction (CABR) system, which aims at NO removal and has been studied in many of our previous works. In this paper, we applied polypyrrole (PPy) on the electrode of bioelectrochemical reactor (BER) of CABER system, which induced a much higher current density in the cyclic voltammetry (CV) curve for the electrode itself and better NO removal rate in the system. In addition, a Microbial Electrolysis Cell (MEC) is constructed to study its strengthening mechanism. Results shows that PPy-MEC has a greater Faraday efficiency and higher reduction rate of Fe(III)EDTA and Fe(II)EDTA-NO in the solution when compared to original Carbon MEC, which confirms the advantage of PPy-modified electrode(s) in the CABER system. The results of this study are reported for illustration of potential of CABER technology and design of low-cost high-efficiency NOx control equipment in the future.

Two-Stage Chemical Absorption–Biological Reduction System for NO Removal: System Start-up and Optimal Operation Mode

A novel chemical absorption–biological reduction (CABR) integrated process, employing Fe(II)EDTA as an enhanced absorbent, is a promising technology for nitrogen oxides removal. In this work, we developed a new two-stage CABR system applying a mixed cultivation model of denitrifying bacteria and iron-reducing bacteria, which consists of a sieve-plate tower and a bioreduction tower to separate the absorption and reduction processes. The start-up period of the two-stage system was shortened to 19 days, while that of the one-stage CABR system was 46 days. In addition, the two-stage CABR system featured a better oxygen-resistance ability and a higher NO removal loading. In effort to optimize system operation, we compared different modes of system operation and found that (1) continuous addition of glucose was better than the batch-type addition and that (2) the NO removal efficiency could be maintained at >90% while the FeEDTA concentration was higher than 4 mmol/L; however, reducing the initial concentration...

Two-Stage Chemical Absorption-Biological Reduction System for NO Removal: Model Development and Footprint Estimation

The two-stage chemical absorption-biological reduction (CABR) system, comprised of an absorption column and a bioreactor, is regarded as a promising option for NO removal from the middle- and small-sized boilers. In this work, a steady-state rate-based model was developed for the two-stage CABR system. The developed model was validated by the data obtained from a laboratory two-stage CABR setup and then used for the estimation of the footprints for treating a 5 × 104 m3 h–1 flue gas from a 14 MW coal-fired steam boiler. For a baseline case (L/G = 10 L m–3, Cin,NO = 350 ppm, Cin,O2 = 6% (v/v)), the designed absorption column size was 3.60 × 8.75 m (d × h), while the bioreduction column was set at 3.60 × 8.50 m (d × h). Furthermore, sensitive analysis including the influence of gas concentrations, packing properties, and microorganism activities was investigated to optimize the design and operation of two-stage CABR process. Under optimized conditions, the footprint of the bioreactor can be downsized by 22.74%. It is believed that this work can provide fundamental data for the industrial application of the two-stage CABR system in the middle- and small-sized boilers.

A Biophysicochemical Model for NO Removal by the Chemical Absorption-Biological Reduction Integrated Process.

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.

Enhanced NOx removal performance and microbial community shifts in an oxygen-resistance chemical absorption–biological reduction integrated system

The chemical absorption–biological reduction (CABR) integrated process, which employed ferrous ethylenediaminetetraacetate as a solvent, is promising for NOx removal. However, oxygen in the flue gas has a great potential to retard the NOx removal. In this work, an oxygen-resistance CABR system inoculated with enriched culture was established. It features a shorter start-up period and a better performance of NOx removal compared with the conventional CABR system. The microbial community shifts in taxa and diversity in response to oxygen were also investigated. High-throughput sequencing analysis showed that Enterococcus, Clostridium sensu stricto, Chelatococcus, Petrimonas, Escherichia/Shigella, Cupriavidus were the major groups of bacteria, and denitrifying bacteria (Petrimonas, Cupriavidus, Chelatococcus, Enterococcus), accounting for 63.29% of the total bacteria, were dominant in the absence of oxygen. Variation in oxygen concentration resulted in the change of the dominant bacteria. For example, Enterococcus, Petrimonas, and Cupriavidus were dominant in the presence of 0vol%, 6vol%, and 10vol% oxygen, respectively. The diversity of the microbial community also varied along the gas flow path at the different levels of oxygen. Although the increase in oxygen concentration significantly impacted the taxa and distribution of the microbial community, the NOx removal efficiency of the enhanced CABR system was not noticeably changed because of the high species richness under the high oxygen concentration case.

Pathway of FeEDTA transformation and its impact on performance of NOx removal in a chemical absorption-biological reduction integrated process.

A novel chemical absorption-biological reduction (CABR) integrated process, employing ferrous ethylenediaminetetraacetate (Fe(II)EDTA) as a solvent, is deemed as a potential option for NOx removal from the flue gas. Previous work showed that the Fe(II)EDTA concentration was critical for the NOx removal in the CABR process. In this work, the pathway of FeEDTA (Fe(III)/Fe(II)-EDTA) transformation was investigated to assess its impact on the NOx removal in a biofilter. Experimental results revealed that the FeEDTA transformation involved iron precipitation and EDTA degradation. X-ray photoelectron spectroscopy analysis confirmed the iron was precipitated in the form of Fe(OH)3. The iron mass balance analysis showed 44.2% of the added iron was precipitated. The EDTA degradation facilitated the iron precipitation. Besides chemical oxidation, EDTA biodegradation occurred in the biofilter. The addition of extra EDTA helped recover the iron from the precipitation. The transformation of FeEDTA did not retard the NO removal. In addition, EDTA rather than the iron concentration determined the NO removal efficiency.

Re-acclimation performance and microbial characteristics of a thermophilic biofilter for NOx removal from flue gas.

Currently, a novel chemical absorption-biological reduction (CABR) integrated process, employing Fe(II)EDTA as a solvent, is being under development to reduce the cost of NOx removal from flue gas. In this work, the NO removal profile, re-acclimation performance, and microbial characteristics in a thermophilic biofilter were investigated at the conditions typical to CABR process. The biofilter comprised of four layers of packing material with a surface area of 1200 m(2) m(-3). Experimental results revealed that the biofilter could remove 95 % of the fed NO at typical flue gas conditions. As the gas residence time varied from 90 to 15 s, the NO removal efficiency decreased from 100 to 56.5 % due to the NO mass transfer limitation. The longer period of the biofilter shutdown required more time for its re-acclimation. For example, after 8-day shutdown, the biofilter was re-acclimated in 32 h. Denaturing gradient gel electrophoresis analysis of PCR-amplified product showed that Pseudomonas, a group of denitrifier, was dominant in the biofilter. Because the Pseudomonas was abundant at the bottom layer of packed-bed, the bottom layer contributed to 60-70 % of the total NO removal. In addition, Pseudomonas gradually faded away along the gas flow path from the bottom to the top of biofilter, resulting in a significant decrease in NO removal at the other three packed-bed layers. These observed results will provide the process engineering and scale-up data with respect to the biofilter operations to help advance the CABR process to pilot-scale testing.

Generation, utilization, and transformation of cathode electrons for bioreduction of Fe(III)EDTA in a biofilm electrode reactor related to NOx removal from flue gas.

A chemical absorption-biological reduction (CABR) integrated system, which employs iron chelate as a solvent, is under development for NOx removal from flue gas. Biofilm electrode reactor (BER) is deemed as a promising bioreactor to regenerate the iron chelate. Although it has been proved that BER can significantly enhance the bioreduction of Fe(III)EDTA, the bioelectrochemistry mechanism involved in the bioreduction of Fe(III)EDTA remains unknown. This work aims to explore this mechanism via the analysis of the generation, utilization, and transformation of cathode electrons in the BER. The results indicate that the generation of cathode electrons follows Faraday's law. The generated cathode electrons were used to produce H2 and directly reduce Fe(III)EDTA in the BER. Meanwhile, the produced H2 served as an electron donor for bioreduction of Fe(III)EDTA. The excess H2 product was transformed to simple organics, e.g., methanol by the hydrogen autotrophy of Pseudomonas under the inorganic and anaerobic conditions. Overall, this work revealed that the reduction of Fe(III)EDTA in the BER was enhanced by both direct electrochemical reduction and indirect bioreduction using H2 as an intermediate. It is also interesting that the excess H2 product was transformed to methanol for microbial metabolism and energy storage in the BER.

N2O production in the FeII(EDTA)-NO reduction process: the effects of carbon source and pH

Chemical absorption-biological reduction (BioDeNOx), which uses Fe(II)(EDTA) as a complexing agent for promoting the mass transfer efficiency of NO from gas to water, is a promising technology for removing nitric oxide (NO) from flue gases. The carbon source and pH are important parameters for Fe(II)(EDTA)-NO (the production of absorption) reduction and N2O emissions from BioDeNOx systems. Batch tests were performed to evaluate the effects of four different carbon sources (i.e., methanol, ethanol, sodium acetate, and glucose) on Fe(II)(EDTA)-NO reduction and N2O emissions at an initial pH of 7.2 ± 0.2. The removal efficiency of Fe(II)(EDTA)-NO was 93.9%, with a theoretical rate of 0.77 mmol L(-1) h(-1) after 24 h of operation. The highest N2O production was 0.025 mmol L(-1) after 3 h when glucose was used as the carbon source. The capacities of the carbon sources to enhance the activity of the Fe(II)(EDTA)-NO reductase enzyme decreased in the following order based on the C/N ratio: glucose > ethanol > sodium acetate > methanol. Over the investigated pH range of 5.5-8.5, the Fe(II)(EDTA)-NO removal efficiency was highest at a pH of 7.5, with a theoretical rate of 0.88 mmol L(-1) h(-1). However, the N2O production was lowest at a pH of 8.5. The primary effect of pH on denitrification resulted from the inhibition of nosZ in acidic conditions.

Evaluation of NOx Removal from Flue Gas by a Chemical Absorption–Biological Reduction Integrated System: Glucose Consumption and Utilization Pathways

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 ga...

Current advances of integrated processes combining chemical absorption and biological reduction for NO x removal from flue gas.

Anthropogenic nitrogen oxides (NO x ) 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 NO x 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 NO x following microbial denitrification of NO x to harmless nitrogen gas. Meanwhile, the absorbent Fe(II)EDTA is biologically regenerated to sustain the adequate NO x 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 NO x removal from the flue gas.

Evaluation of NO Removal from Flue Gas by a Chemical Absorption–Biological Reduction Integrated System: Complexed NO Conversion Pathways and Nitrogen Equilibrium Analysis

The conversion of complexed nitrogen oxide [Fe(II)EDTA–NO] is the core process in the chemical absorption–biological reduction (CABR) integrated system. Additionally, flue gas of power plants in China contains 3–8% oxygen (O2), which plays a dominant role in the system. However, previous studies on the reduction of complexed NO were conducted in strictly anaerobic vials. Nitrogen utilization for microorganism growth was not taken into consideration. In this study, the mechanism and conversion pathways of complexed NO were investigated with 0–10% O2. Results demonstrated that, with O2, except for being complexed as Fe(II)EDTA–NO, a part of NO was oxidized to NO2. Then, Fe(II)EDTA–NO and NO2 were oxidized to NO2–/NO3– via nitrosification/nitrification or direct chemical reaction, and NO2–/NO3– were reduced to N2 by complexed NO-reducing bacteria. Besides, 75% of nitrogen was discharged by gas, mainly as N2. There was 18 and 3% of nitrogen accumulated in biological and liquid phases, respectively.