Removal Of Nitric Oxide By Metal Chelate Absorption Coupled With Two Stage Bio-reduction Using Magnetically Immobilized Microorganisms

Nitrogen oxides （NO_x） are major by-products of combustion processes and itsemission from the flue gas of power plants has caused serious environmentalproblems e.g., acid rain, global warming, photochemical smog, depletion of the ozonelayer and health risk. The pollution of NO_xreceives increasing attention with theincreasing emission amount of NOxand the increasing requirement of people onenvironmental quality. The integrated approach of metal chelate absorption combinedwith microbial reduction （BioDeNO_x） has been applied to convert NO_xinto nitrogenin recent years. The approach which combines the advantages of both the chemicaland biological technologies is employed to achieve the removal of NO_xfrom thesimulated flue gas. However, suspended bacteria are employed in current researches,which may cause some problems especially in the BioDeNO_xsystem. Such as lowdensity of the bacteria makes the system unstable, and under unfavorable conditions,some others （for instance, sulfate reducing bacteria, etc.） may become predominatedbacteria. Immobilized bacteria can overcome these defects because of its excellentproperties such as high bacteria density and endurance. Magnetic microspheres, as anew immobilized supports, have been used in enzyme immobilization and cellseparation in recent years. Microorganisms immobilized by magnetic supports can beeasily recovered from a fluidized-bed reactor by applying an external magnetic field.Based on the above statement, a new approach by two stages of microbial reductionsystem with magnetic microspheres immobilized denitrifying bacteria andiron-reducing bacteria were proposed by our group. In this paper, magneticFe₃O₄-chitosan microspheres, prepared by co-precipitating of Fe²⁺and Fe³⁺ions withNaOH in the presence of chitosan, served as the magnetic carriers for immobilizationof iron-reducing bacteria and denitrifying bacteria. The magnetic Fe₃O₄-chitosanmicrospheres and the surface morphology of the immobilized bacteria werecharacterized by X-ray diffraction （XRD）, Fourier transform infrared （FT-IR）,thermogravimetric analysis （TGA）, magnetic property measurement system （MPMS）,scanning electron microscopy （SEM）, etc. The reduction properties of the immobilized bacteria and the effect of temperature, pH and the potential inhibitioncompounds (NO₂, NO₃, SO₃^2-and S^2-) in the scrubber solution were alsoinvestigated. Under the condition of continuous operation, the effects of operatingparameters on the final removal rate for NO were studied.Size analysis showed that the d（0.1）, d（0.5） and d（0.9） of the magnetic chitosanmicrospheres was4.242μm,79.654μm and113.811μm, respectively. The BETsurface area was79.47m²/g. The XRD results showed the existence of the iron oxideparticles with a cubic inverse spinel structure in the magnetic microparticles. Thesaturation magnetization of the microspheres was about20.0emu/g. The remnantmagnetization and coercivity of the microspheres were nearly to be0, whichdemonstrated it was classic superparamagnetic. The average mass content of Fe₃O₄inmagnetic microspheres by TGA was about45%. SEM showed that the bacteria wereadsorbed on the surface of microspheres.The reduction capacity of bacteria was identified through static experimentalinvestigation. The results showed that denitrifying bacteria immobilized by themagnetic Fe₃O₄-chitosan microspheres exhibited good performance onFe（II）EDTA-NO^2-reduction. The optimal temperature and pH of the bacteria wasunchanged, while the reduction activity of the bacteria decreased slightly afterimmobilization in the experimental conditions. Fe（II）EDTA-NO^2- reduction wasaffected by those potential competitive electron acceptors in the scrubber solutionsuch as SO₃^2-and S^2-, whether free or immobilized bacteria were used. Sulfitestimulated the reduction of Fe（II）EDTA-NO^2- and sulfide inhibited the reduction ofFe（II）EDTA-NO^2-. Under the dynamic experiment for reduction of Fe（II）EDTA-NO^2-by immobilized denitrifying bacteria, the microbial reduction process ofFe（II）EDTA-NO^2-fit the first order reaction dynamic model, and the reaction rateconstant was0.2123h^-1. Iron-reducing bacteria immobilized by the magneticFe₃O₄-chitosan microspheres exhibited good performance on Fe（III）EDTA-reduction.The optimal temperature and pH of the bacteria was unchanged, while the reductionactivity of the bacteria was improved obviously after immobilization in theexperimental conditions. Fe（III）EDTA-reduction was affected by those potentialcompetitive electron acceptors in the scrubber solution such as NO_<sup>2-, NO_<sup>3-and SO₃^2-, whether free or immobilized bacteria were used. The reduction of Fe（III）EDTA^-wasstimulated at low concentrations of NO₂^-or NO₃^-and inhibited at high concentrationsof NO₂^-or NO₃^-, whether free or immobilized bacteria was used. The reduction ofFe（III）EDTA-was inhibited strongly by sulfite. However, the immobilized bacteriaexhibited an improved tolerance against those compounds in the scrubbing solution.While for S2-, Fe（III）EDTA-reduction was most likely a result of both biologicalreduction and chemical oxidation of sulfide. Under the dynamic experiment forreduction of Fe（III）EDTA-by the free and immobilized bacteria, the microbialreduction process of Fe（III）EDTA-fit the first order reaction dynamic model, and thereaction rate constant was0.0905h-1and0.1494h^-1, respectively.The basical parameters of operation were identified through experimentalinvestigation. The results indicated that the rising speed of the liquid flow inmagnetically stabilized fluidized bed was156mL/min, the speed of simulation gasflow was1.0L/min. At inlet the concentration of NO was360mg/m³, the content ofO₂was5.0%（v/v）, the concentration of SO₂was2500mg/m3, the content of CO₂was15.0%（v/v）, the electric current was3A. The effect of certain gaseouscompounds （e.g. O₂, SO₂and CO₂） on the NO removal was also investigated. Theresults showed that the existence of O₂and SO₂had great negative effects on NOremoval, but the existence of CO₂didn’t have obvious influence on the removing ofNO. Under steady operation conditions, NO removal efficiency decreased with theincreasing of gas flow, NO concentration and O₂concentration. The NO removal ratecould keep at80%for ten days in the operation experiments.