| The space industry of China has made remarkable achievements and the ecological environment demand for launch site is higher and higher,and thus it is the key to carry out the research of space launch environmental pollution control technology for the construction of ecological space launch site.Launch site groundwater pollution caused by organic-inorganic contaminants is more and more serious and has become a common trend affecting the quality of groundwater and threatening people's drinking water safety.BTEX and nitrate are frequently detected organic and inorganic pollutants in groundwater whicn are the focus of environmental science in view of their carcinogenic,teratogenic and mutagenic effects to humans.Researches about the removal of single BTEX or nitrate contaminant show that,BTEX can be removed under denitrifying condition and the removal is enhanced by nitrate.However,the mixture pollution groundwater exhibits a typical characteristic of low carbon and nitrogen mass ratio(C/N),and poor nutrition generally influenced by BTEX or nitrate features such as solubility,leading to heterotrophic denitrification incomplete with nitrate or organic relative over dosage.So,it is very difficult to completely remove the mixture pollutants of BTEX and nitrate simultaneously by single heterotrophic denitrification as C/N imbalance.To our best knowledge,it is few studies about the simultaneous removal of BTEX and nitrate,and the mechanism of removing BTEX and nitrate simultaneous by the cooperation of heterotrophic denitrification and autotrophic denitrification is needed to further study.Thus,the present study is conducted to remove BTEX and nitrate simultaneously by the cooperation of heterotrophic and autotrophic denitrification with the fund of the National Natural Science Foundation of China(No.40872164,41502240).In this study,typical contaminated groundwater was collected from near Weihe river bank located in Guanzhong Plain with similar characteristics of launch site and the experiment methods of microcosm and dynamic soil column were adopted.Microcosm and soil column were built with collected groundwater,undisturbed aquifer medium and indigenous microbes sampled from Weihe river bank area,and then cultivated for denitrification habitats.The BTEX biodegradable,heterotrophic denitrification efficiency and impacts of easily degradable organics on heterotrophic denitrification were investigated by microcosm and dynamic soil column experiments.The composite(n ZVI/GAC)of nano-zero valent iron(n ZVI)supported on granular activated carbon(GAC)was prepared by adsorption-chemical reduction,and its structures such as dispersion and hydrogen gas production ability in water were analyzed using SEM,TEM,XRD,XPS,etc.On this basis,the efficiency and characteristics of autotrophic denitrification using hydrogen produced by n ZVI/GAC in water,and the feasibility,effectiveness and synergetic mechanism of combination heterotrophic and autotrophic denitrification to remove benzene and nitrate as well as their effect factors were investigated.The main results of this thesis are as follows:(1)Under denitrifying condition,BTEX can be used as organic carbon and electron donor of heterotrophic denitrifying microorganisms and removed accordingly.The degradation of benzene,toluene,ethylbenzene and xylene has a priority order.Under the experimental conditions,ethylbenzene degradation was the fastest followed by toluene and xylene,and benzene degradation was the slowest,and the degradation rates of benzene,toluene,ethylbenzene and xylene were 0.227,0.541,0.553 and 0.539 mg·L-1·d-1,respectively.(2)The mixture contaminants of BTEX and nitrate were able to be removed simultaneously by heterotrophic denitrification,although BTEX removal better,nitrate was over dosage mainly because of electron donors lack resulting in inadequate driving force for nitrogen removal in the late stage at low C/N under the experiment conditions.Easy degradation organics could enhance heterotrophic denitrification to remove benzene and nitrate,and the enhancement capacity was closely related to organic carbon species.Under experimental conditions,the strength of ethanol was the strongest,followed by glucose,and oxalic was the weakest,and the difference was caused by the nature of organics.Furthermore,the strengthening effect on heterotrophic denitrification was also closely related to the initial concentration of organics,and the strengthening was different even with the same organics at different concentrations.Under the experimental concentrations,a high concentration of ethanol(100.0~250.0 mg·L-1)showed an inhibition to benzene degradation,while a low concentration(10.0~100.0 mg·L-1)showed a promotion to benzene degradation.However,for nitrate,ethanol always showed promoting effects on nitrate degradation with all tested initial concentration.Both the degradation kinetics of benzene and nitrate exhibited linear to nonlinearity with ethanol concentration increase.(3)A composite(n ZVI/GAC)was prepared by adsorption-chemical reduction with GAC as carrier originated coconut shell.Characterization of SEM,TEM,XRD,XPS,etc.illustrated that n ZVI supported on GAC was "core-shell" structure with zero valence iron as core and oxidation iron as shell,and the load amount was 64.3 ?mol·g-1.Zeta potential for bare n ZVI was-10~-45 m V while n ZVI/GAC composite was-25~-70 m V,suggesting that aggregation tendency was significantly decreased,and moreover,the rate of decline of iron level in the air from 0.344 h-1 for n ZVI dropped to 0.159 h-1 for the composite.The amount of hydrogen generation and electron donor supply ability of n ZVI/GAC composite was significantly affected by the aqueous chemical composition like p H,and the lower of the p H value,the greater of the hydrogen yield,and the hydrogen generation rate was 0.244,0.137 and 0.097 ?mol·g-1·h-1 at p H 5.0,7.0 and 9.0 respectively.(4)The removal efficiency of autotrophic denitrification to nitrate was satisfied with the intensification of n ZVI/GAC.Chemical denitrification could remove nitrate more effective by n ZVI/GAC composite assistance,and the removal ratio increased from 35.9 % to 81.1 % at 90 min while removal rate(kobs)increased from 6.56×10-3 min-1 to 1.91×10-2 min-1.The involvement of GAC not only improved nitrate removal but also accelerated nitrate conversion to nitrogen mostly with minor byproducts of nitrite and ammonia,which was proved by the analysis of SEM,XRD and XPS.Autotrophic denitrification was also effective to remove nitrate using hydrogen as electron donor generated by n ZVI/GAC in solution,and the removal ratio increased from 12.7 % to 48.5 % with initial nitrate concentration 117.2 mg·L-1 at 90 d and the degradation was fit to first-order kinetic mode.(5)The experiments of microcosm and soil column indicated that it is not only possible but also feasible for the cooperation of heterotrophic denitrification and autotrophic denitrification supported with hydrogen generated by n ZVI/GAC composite.There was a stagnation period of nitrate degradation for single heterotrophic denitrification to remove benzene and nitrate simultaneously,however,the stagnation of nitrate degradation was disappeared when n ZVI/GAC was added.Soil column experiments showed that high removal of benzene and nitrate simultaneous were achieved and the removal ratio could reach to about 99.01 % and 91.10 % at 150 d indicating it is feasible to remove benzene and nitrate simultaneously by the cooperation of heterotrophic and autotrophic denitrification.(6)Microsom and soil column tests proved that the cooperation of heterotrophic and autotrophic denitrification had high removal efficiency to benzene and nitrate simultaneous with minor byproducts in the favour of synergistic effect of heterotrophic and autotrophic denitrification.The degradation reaction of benzene and nitrate could be well fitted by first-order kinetic model.The removal ratio of benzene and nitrate was over 90.0 % at 20 d while 99.8 % and 99.0 % at 60 d with NO2--N and NH4+-N concentration less than 1.0 mg·L-1 assisted by chemical,heterotrophic and autotrophic denitrification.Moreover,the concentration of intermediate NO2--N and NH4+-N increased at heterotrophic denitrification stage and declined at autotrophic denitrification stage,which could indicate the reaction process of heterotrophic and autotrophic denitrification combination.(7)The cooperation of heterotrophic and autotrophic denitrification to remove benzene and nitrate simultaneously was influenced by soil to water ratio,initial nitrate and benzene concentration,n ZVI/GAC dosage,organic species,and C/N ratio.Under experimental conditions,increasing soil to water ratio could promote the cooperation of heterotrophic and autotrophic denitrification,but the promotion ability was different for benzene and nitrate degradation.The promotion for nitrate was weak than for benzene as the tested system was low C/N ratio.When the initial concentration of nitrate increased from 95.0 to 350.0 mg·L-1,the degradation rate constants of benzene and nitrate increased from 0.15 and 0.52 mg·L-1·d-1 to 0.20 and 2.5 mg·L-1·d-1,and when n ZVI/GAC dosage increased from 2.5 to 10.0 mg·L-1,the removal rate increased form 0.16 and 0.79 mg·L-1·d-1 to 0.21 and 0.96 mg·L-1·d-1 in the synergy of heterotrophic and autotrophic denitrification.The promotion of addition ethanol on the cooperation of heterotrophic and autotrophic denitrification was very significant,but this promotion was closely related to C/N and the most obvious at the C/N of 1.0.Moreover,the maximum concentration and maximum time of accumulation for NO2--N and NH4+-N showed a positive correlation to the initial concentration of nitrate and n ZVI/GAC dosage in the test.In summary,it is feasible for the combination of heterotrophic denitrification and autotrophic denitrification assisted by n ZVI/GAC composite to remove BTEX and nitrate simultaneously from contaminated groundwater,which is benefit to explore remediation method for organic-inorganic polluted groundwater in a simple and easy way.Although the results are helpful to ecological environmental protection and construction of launch site,there are still many problems to be further research. |
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