Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 81:306–311 (2006)DOI: 10.1002/jctb.1394

Metal chelate absorption coupled withmicrobial reduction for the removal of NOxfrom flue gasWei Li,∗ Cheng-Zhi Wu and Yao ShiDepartment of Environmental Engineering, Zhejiang University (Yuquan Campus), Hangzhou 310027, China

Abstract: A novel process for the removal of NOx from flue gas by a combined Fe(II)EDTAabsorption 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 containingmicroorganisms. In a steady-state absorption and regeneration process, the final removal efficiency of NOis up to 88%. The effects of four main parameters (i.e. NO, O2 and SO2 concentrations, and the amount ofcyclic solution) on NOx removal efficiency were experimentally investigated at 50 ◦C. The results providesome insight into conditions required for the successful removal of NOx from flue gas using the approachof Fe(II)EDTA absorption combined with microbial reduction. 2005 Society of Chemical Industry

Keywords: NOx removal; Fe(II)EDTA; microorganisms; absorption; flue gas

INTRODUCTIONThe emission of nitrogen oxides (NOx) is one of themajor causes of acid rain. One way of preventing NOx

pollution is to reduce the emissions of NOx from theflue gas of power plants, one of the major contributorsto NOx emission. However, nitric oxide (NO), themain component of NOx, has a very low solubilityin aqueous solution, and is difficult to remove byconventional absorption methods. The use of metalchelate additives in wet flue gas desulfurization (FGD)systems for combined removal of NOx and SO2 hasbeen reported by several authors.1–5 The additionof Fe(II)EDTA (EDTA: ethylenediaminetetraacetate)in scrubbing liquor is able to promote the solubilityof NO by the formation of Fe(II)EDTA–NO. Thebound NO reacts with sulfite/bisulfite ions (fromthe dissolution of SO2) and is then converted toN2O and nitrogen–sulfur compounds. Subsequently,Fe(II)EDTA is regenerated.1,6,7 However, there aremany drawbacks associated with this approach.One problem is the difficulty in the removal ofnitrogen–sulfur compounds from the liquor due totheir high solubility in water. Another problem is theease of oxidation of Fe(II)EDTA by oxygen to formFe(III)EDTA, which is not capable of binding NO.8,9

While Fe(III)EDTA can be reduced to Fe(II)EDTAby sulfite/bisulfite ions, the regeneration rate is slowbecause of low rate constants and low concentrationsof sulfite/bisulfite ions in limestone slurries. As a

result, the concentration of the active Fe(II)EDTAin solutions is low, and thus adequate NO removalcannot be sustained.

A promising approach can be applied to regen-erate Fe(II)EDTA–NO and Fe(III)EDTA usingmicroorganisms.10–12 The complex of Fe(II)EDTA–NO formed in the scrubbing solution can be reducedby denitrifying bacteria, which are similar to theones used for denitrification in the treatment ofmunicipal wastewater.13,14 On the other hand, sev-eral microorganisms are able to couple oxidation ofhydrogen or organic compounds to the reduction ofFe(III) and gain energy for growth. Dissimilatory iron-reducing bacteria have been isolated from a varietyof anoxic environments, and are widely distributedamong bacteria, as evidenced by 16S rRNA gene(rDNA) sequences.15,16 Kieft et al.17 reported that athermophilic bacteria, designated SA-01, can use O2,NO3

−, Fe(III) and S0 as terminal electron acceptorsfor growth.

Our previous study has shown that the formation ofFe(II)EDTA–NO and Fe(III)EDTA in the scrubbingsolution can undergo effective bio-reduction by twoheterotrophic bacterial strains.11 The reduction forFe(II)EDTA–NO and Fe(III)EDTA with microor-ganisms can be expressed by the following equations:

Fe(II)EDTA–NO + electron donormicroorganism−−−−−−−−−−→N2 + Fe(II)EDTA

∗ Correspondence to: Wei Li, Department of Environmental Engineering, Zhejiang, University (Yuquan Campus), Hangzhou 310027, ChinaE-mail: w li@zju.edu.cnContract/grant sponsor: National Natural Science Foundation of China; contract/grant number: 20176052Contract/grant sponsor: Scientific Research Foundation for Returned Overseas Chinese Scholars, Ministry of Education(Received 21 January 2005; revised version received 22 May 2005; accepted 29 July 2005)Published online 24 October 2005

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Fe(III)EDTA + electron donormicroorganism−−−−−−−−−−→Fe(II)EDTA

The aim of the present study was to investigate,by laboratory-scale experiments, the possibilities ofremoval of NOx from simulated flue gas combinedwith metal chelate absorption and microbial regenera-tion.

MATERIALS AND METHODSChemicalsTrisodium ethylenediaminetetraacetate (Na3EDTA,99%), FeSO4.7H2O (99.5%), and D-glucose (99.5%)were from Shanghai Chemical Reagent Co., ShanghaiChina; 1% NO in N2, 10% SO2 in N2, O2,CO2, and N2 were obtained from Zhejiang GasCo. All other chemicals were analytical gradereagents.

Cultivation of microorganismsThe microorganisms used in this study were derivedfrom activated sludge of a municipal wastewatertreatment plant and enrichment-cultivated usingNO3

− and Fe(III) as terminal electron acceptors.The enrichment cultivation was conducted in a250 mL flask with a liquid volume of 150 mL at50 ◦C on a rotary shaker (150 rpm). After completionof enrichment, the microorganisms were fed intothe reactor for adaptation to Fe(II)EDTA–NOand Fe(III)EDTA reduction. The concentration ofFe(II)EDTA–NO was gradually increased over aperiod of several days from 2 to 10 mmol L−1. Detaileddescription about the microorganisms’ cultivation andadaptation can be found in our previous reports.10,11

The bio-reduction of Fe(II)EDTA–NO and Fe(III)using microorganisms was performed well after a longperiod of adaptation. Experimental results showedthat NO can be simply removed by Fe(II)EDTAin aqueous solution containing microorganisms.Furthermore, a simulated flue gas containing NO,SO2, CO2, O2 with N2 balance was fed into thereactor and a steady-state operation for NO removalcould be performed.

Analytical techniquesThe inlet and outlet gas concentration of NO and SO2

were measured with a chemiluminescent NOx analyzerand a fluorescent SO2 analyzer. A cold trap was usedfor removal of moisture before the gas entered theanalyzers. The chemiluminescent NOx analyzer hasindependent outputs for NO, NO2, and NOx. Duringthe experiments, NO, NO2, and NOx in the simulatedflue gas were examined. Results indicated that therewas scarcely any NO2 produced, and the concentrationof NO almost equaled that of NOx. It was also foundthat only 0.87% of the NO was removed when the testgas passed though the cold trap. The produced biasdata are so small that they can be ignored.

The concentration of ferrous ions and totaliron in solution were determined by the 1,10-phenanthroline colorimetric method at 510 nm. Fe(II)was measured by filtering a 0.05 mL sample througha 0.25 µm nylon syringe filter directly into ferrozineand reading the A510 immediately. In order todetermine the total amount of iron in solution,hydroxylamine hydrochloride (NH2OH.HCl) wasused to reduce ferric into ferrous ions at pH < 2.Fe(III) concentrations were calculated from thedifference between total Fe and Fe(II). Preliminaryexperiments showed a difference between Fe2SO4 andFe(II)EDTA during the Fe(II) ferrozine colorimetricassay in the presence of EDTA, most likely due tocompetition between EDTA and ferrozine for Fe(II).Therefore, standard curves were prepared with Fe(II)standard solutions having EDTA concentrations equalto those used in the reduction experiments.

Experimental set-up for NO removal processThe schematic diagram of experimental set-up for theremoval of NO is shown in Fig. 1. Two glass column(50 mm in diameter ×300 mm in height) and a holdingtank with a maximum capacity of 5000 mL were usedfor NO removal. One column with a sintered glasswas used as absorber. The other one packed with glassfiber was used for regeneration. The simulated fluegas for the most part containing 15% CO2, 3% O2,and 250 mL m−3 NO passed through a sintered glassat the bottom of the absorber for NO absorption. Thesolution coming from the bioreactor flowed into thetop of the absorber. After absorbing NO, the solutionsflowed into the holding tank and were then regeneratedin the bioreactor. A total of 4000 mL aqueous solutioncontaining 25 mmol L−1 Fe(II)EDTA was preparedin the bioreactor and the initial pH of the solutionwas adjusted by adding some K2HPO4 and KH2PO4

at 6.5–7.0. After the microorganisms in the solutionhad been cultured and adapted, the simulated flue gaswas bubbled into the absorber, making contact withthe above solution containing the microorganisms.During the absorption and bio-regeneration process,the concentration of Fe(II) in the bioreactor wasanalyzed. The flow rate of the simulated flue gas wasfixed at 1000 mL min−1 and the pH of the solution wasmonitored during the whole process. All experimentswere conducted at a temperature of 50 ◦C.

RESULTS AND DISCUSSIONA comparison of NO removal using Fe(II)EDTAsolution with or without microorganismsAfter a period of 36 days of adaptation for themicroorganisms by absorption and regeneration, themicroorganisms grew well. These adapted bacteriahad a good ability to regenerate Fe(II)EDTA–NOand Fe(III)EDTA even though the simulated flue gascontained little oxygen. In order to prove the abilityof these microorganisms for bio-reduction, a simplecomparison test was done in this system. The results

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AbsorberpH monitor

NOx analyzer

Cold trap

Sampling

Holding tank

Bioreactor

Pump

Regeneration zone

Vent

O2 NOCO2 SO2

Mixer

Mixer

N2

Vent

Sintered glass

Figure 1. A schematic of the experimental set-up for NO removal.

0 50 100 150 200 250 3000

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NO

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oval

, %

t, min

without microorganismswith microorganisms

Figure 2. A comparison of NO removal with or withoutmicroorganisms at 50 ◦C. Experimental conditions: amount of cyclicsolution: 4000 mL, liquid-flow rate: 300 mL min−1, gas-flow rate:1000 mL min−1, 15% CO2, 3% O2, 250 mL m−3 NO,25 mmol L−1 Fe(II)EDTA.

are shown in Fig. 2. When only the Fe(II)EDTAsolution was used to absorb NO, Fe(II)EDTA–NOand Fe(III)EDTA in the solution could not beregenerated. Thus, the concentration of Fe(II)EDTAdecreased quickly (data not shown), and the NOremoval efficiency also decreased. After about 3.5 h ofabsorption, all the active Fe(II)EDTA was convertedto Fe(II)EDTA–NO and Fe(III)EDTA and the NO

removal efficiency decreased to zero. However, if thesolution contained a large number of microorganisms,Fe(II)EDTA–NO and Fe(III)EDTA in the solutioncould be quickly and effectively reduced to theactive Fe(II)EDTA. Thus a constant concentrationof Fe(II)EDTA in the cyclic solution was maintainedduring the process, and a steady-state absorption andregeneration process could be obtained. The finalremoval efficiency of NO was up to 88% under thisoperational condition.

Effect of NO concentration on NO removalefficiencyThe experimental results for various NO concentra-tions are shown in Fig. 3. The removal efficiency ofNO was about 90%, and did not change significantlywhen the inlet NO concentration was in the range of100–350 mL m−3; however a further increase in theNO concentration caused a decrease in the efficiencyof NO removal. When the inlet NO concentrationincreased up to 500 mL m−3, the removal efficiencydecreased to 68%. This was dependent on the bio-reduction rate. As the reduction of Fe(II)EDTA–NOby the microorganisms has a limited capacity, theamount of Fe(II)EDTA regenerated by the microor-ganisms should be kept constant, and the concentra-tion of active Fe(II)EDTA in the solution also remainsconstant. Thus, a further increase of inlet NO concen-tration will decrease the final NO removal efficiency.

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100 200 300 400 50050

60

70

80

90

100N

O r

emov

al, %

NO, mL m-3

Figure 3. Effect of inlet NO concentration on NO removal efficiency.Experimental conditions: amount of cyclic solution: 4000 mL,liquid-flow rate: 300 mL min−1, gas-flow rate: 1000 mL min−1, 15%CO2, 3% O2, 25 mmol L−1 Fe(II)EDTA.

Effect of oxygen concentration on NO removalefficiencyFlue gas from power plants usually contains asmall percentage (1–8%) of oxygen depending oncombustion conditions. Therefore, experiments onthe effect of oxygen on NO removal were performed.Figure 4(A) shows the results when the total amountof cyclic solution was 4000 mL. Many studies haveproven that Fe(II)EDTA in solution was easilyoxidized to Fe(III)EDTA when oxygen was presentin the flue gas.8,9 The higher the amount ofoxygen in the flue gas, the faster the reactionof Fe(II)EDTA with oxygen takes place. Thus,the concentration of Fe(II)EDTA in the solutiondecreased at the beginning of absorption. The amountof NO absorbed by Fe(II)EDTA also decreased, andthe NO removal efficiency decreased. At the sametime, Fe(III)EDTA could be reduced to Fe(II)EDTAby microorganisms. After a period of operation (about3 h, depending on oxygen concentration), a constantFe(II) concentration could be reached if the bio-regeneration rate of Fe(II) was equal to the oxidationrate of Fe(II), and a steady-state process was obtained.Thus, the NO removal efficiency can be kept constant.

In order to further investigate the effect of inletoxygen concentrations on NO removal efficiency, theamount of cyclic solution was decreased to 1500 mL.The results of the NO removal from simulated fluegas with various oxygen concentrations are shown inFig. 4(B). The NO removal efficiency decreased withan increase of the concentration of oxygen. However, itshould be pointed out that the NO removal efficiencydecreased sharply (from 93.7% to 70.2%) with theincrease in operation time (from 913 to 1208 min)when the simulated flue gas contained 8% O2, and theprocess was at an unsteady state. As the bio-reductionof Fe(III)EDTA to Fe(II)EDTA was rate limiting,12

(A)

0 100 200 300 400 500 600 70050

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0%

Figure 4. Effect of oxygen concentration on NO removal efficiency.(A) Amount of cyclic solution: 4000 mL; (B) amount of cyclic solution:1500 mL. Experimental conditions: liquid-flow rate: 300 mL min−1,gas-flow rate: 1000 mL min−1, 15% CO2, 250 mL m−3 NO,25 mmol L−1 Fe(II)EDTA.

the Fe(III) reduction rate by microorganisms wasslower than the oxidation rate of Fe(II) with oxygenwhen the simulated flue gas contained very highconcentrations of oxygen. Thus, the concentrationof Fe(II)EDTA in the solution gradually decreased,and the amount of NO removal by Fe(II)EDTAalso decreased. In addition, the microorganisms usedto reduce Fe(II)EDTA–NO to Fe(II)EDTA mightbe inhibited when the solution contained too muchdissolved oxygen. All the above factors cause adecrease in the NO removal efficiency.

Effect of SO2 concentration on NO removalefficiencyA simulated flue gas containing various SO2 concen-trations was supplied to the absorber. The outlet NOand SO2 concentrations were measured and recordedduring the absorption process. Figure 5 shows theexperimental results under those conditions. Almost99% of SO2 was removed during the operation(data not shown). Comparing these results with thoseobtained without using SO2 in the simulated flue gas,the removal efficiency of NO had decreased a little.

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500 1000 1500 2000 2500 3000 350050

60

70

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90

100N

O r

emov

al, %

SO2, mL m-3

Figure 5. Effect of SO2 concentration on NO removal efficiency.Experimental conditions: amount of cyclic solution: 4000 mL,liquid-flow rate: 300 mL min−1, gas-flow rate: 1000 mL min−1, 15%CO2, 3% O2, 250 mL m−3 NO, 25 mmol L−1 Fe(II)EDTA.

However, it can be seen in Fig. 5 that the SO2 concen-tration had no significant influence on the NO removalefficiency. This implies that this approach combinedwith the traditional FGD process has the potential forsimultaneous removal of NO and SO2 from flue gas.

Effect of the amount of cyclic solution on NOremoval efficiencyFigure 6 shows the result of the effect of the amountof cyclic solution on NO removal efficiency with8% O2 in the flue gas. It can be seen that withthe decreased amount of cyclic solution, the NOremoval efficiency decreased significantly. A steady-state process could be reached when the amountof cyclic solution was over 2500 mL, and the NOremoval efficiency could be kept constant after 2 hof operation. However, the NO removal efficiencygradually decreased with an increase in operation time

0 50 100 150 200 250 300 35050

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NO

rem

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t, min

4000 mL2500 mL1500 mL

Figure 6. Effect of the amount of cyclic solution on NO removalefficiency. Experimental conditions: liquid-flow rate: 300 mL min−1,gas-flow rate: 1000 mL min−1, 15% CO2, 8% O2, 250 mL m−3 NO,25 mmol L−1 Fe(II)EDTA.

when the amount of cyclic solution was 1500 mL.With the current experimental set-up, we found thatmost microorganisms stayed in the solution. Thus,when the amount of cyclic solution decreased, thenumber of microorganisms also decreased. Therefore,the amount of the active Fe(II)EDTA regeneratedby the microorganisms decreased under the samebio-reduction rate. This caused a decrease in theNO removal efficiency. These results indicate thata sufficient amount of cyclic solution must be usedto obtain a high and constant NO removal efficiencywhen the oxygen concentration in the flue gas is high.

Consumption of ethanol and glucose during thebio-regeneration processIn our previous study,10,12 using glucose as carbonsource allowed the bacteria to reduce more Fe(III)to Fe(II) than when ethanol was used. Thus, glu-cose was used as the carbon source for reductionof Fe(III)EDTA during the absorption and bio-regeneration cycle. However, many studies on denitri-fication by heterotrophic denitrifying microorganismsusing different carbon sources in wastewater treat-ment indicted that ethanol should be better thanglucose.13,14 Therefore, a mixed carbon source withglucose and ethanol was simultaneously added for thebio-reduction of Fe(III)EDTA and Fe(II)EDTA–NOin the current study. The stoichiometric relationshipsfor the reduction of Fe(II)EDTA–NO with ethanoland Fe(III)EDTA with glucose in the system can beexpressed as:

6Fe(II)EDTA–NO + C2H5OH −−−→ 3N2 + 2CO2

+ 3H2O + 6Fe(II)EDTA

24Fe(III)EDTA + C6H12O6 + 24OH−

−−−→ 24Fe(II)EDTA + 6CO2 + 18H2O

According to the above stoichiometric relationships,the theoretical consumption of ethanol is 1/6 molC2H5OH per mol Fe(II)EDTA, and that of glucoseis 1/24 mol C6H12O6 per mol Fe(III)EDTA. Theabove equations do not include the consumption ofcarbon source for cell mass synthesis and maintenance.Moreover, considering that the flue gas usuallycontains 1–8% O2 in practice, consumption ofethanol and glucose will increase the values foundduring laboratory experiments. In our experiments,the approximate ratio of ethanol to NO was0.5 mol/mol and glucose to Fe(III) was 0.125mol/mol,which applies to the bio-regeneration process. Theexperimental results indicate that bio-regeneration canbe completed under these conditions. An optimumratio of ethanol to NO and glucose to Fe(III)EDTAshould be determined in future research.

CONCLUSIONSThe results have shown that it was possible toremove NO by Fe(II)EDTA absorption combined

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with microbial regeneration. A steady-state absorptionand regeneration process and high removal efficiencyof NO can be obtained when the absorption solutioncontains microorganisms.

About 90% of NO is removed when the inlet NOconcentration is in the range of 100–350 mL m−3.Further increase in inlet NO concentration causeda decrease in the NO removal efficiency. The NOremoval efficiency decreased with an increase inthe concentration of oxygen. Although the removalefficiency of NO has decreased little in the presenceof SO2 in the simulated flue gas, this approachcombined with the FGD process has the potentialfor simultaneous removal of NO and SO2. In orderto obtain high bio-reduction capacity for Fe(II)EDTAand constant NO removal efficiency, sufficient amountof cyclic solution needs to be used when high oxygenconcentrations are present in the flue gas. Theconsumptions of ethanol and glucose during the bio-regeneration process have been analyzed but furtherstudies need to be done.

Based on the current results, this study may providea basis for developing a new approach for removal ofNOx from flue gas.

ACKNOWLEDGEMENTSThe project was supported by the National NaturalScience Foundation of China (No. 20176052) and theScientific Research Foundation for Returned OverseasChinese Scholars, Ministry of Education.

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