Synthetic nitrogen (N) fertilizers play a key role in human nutrition and crop production. The most widely used N source globally is urea; however, N loss via ammonia volatilization can be great in agricultural systems where urea is surface-applied. The objectives of the experiments reported in this dissertation were: 1) evaluate the performance of a new laboratory ammonia volatilization measurement system for measuring ammonia volatilization from coated granular urea; 2) determine if urease can be extracted from corn and soybean residues; 3) determine if differences in urease activity are present in corn and soybean residues; and 4) evaluate N content and yield of corn treated with surface-applied coated urea fertilizers. The laboratory ammonia volatilization system had a system recovery efficiency (SRE) of 97% of the applied N and the lowest variation in mg N captured in the acid traps when the air flow rate was 1.00 L min⁻¹, at 26°C, and an acid trap volume of 100 ml 0.02M phosphoric acid. Ammonia volatilization was greatest from 12-24 h after N application with a total of 17% of the applied N being lost during that period. The urease inhibitor N-(n-butyl) thiophosphoic triamide (NBPT) was the most effective ammonia volatilization control treatment and reduced ammonia losses 30-40% compared to urea in the laboratory trials. Urease was extracted from soybean residue and retained activity during extraction; however, urease from corn residue could not be identified in extracts. The agronomic field trials indicated that NBPT increased N concentration in corn ear leaves; however the effect on corn grain yield was masked by environmental conditions. The data from this study suggests that ammonia volatilization from granular urea can be effectively controlled using NBPT, and corn tissue N content in the field indicates that NBPT allows for more N to be utilized by the plant. The urease extraction showed that there may be differences in urease activity in different crop residues. Further research is needed to determine if varying levels of volatilization control are needed for urea applied to different crop residues in no-till systems. / Ph. D.

Tertiary butyl alcohol (TBA), methyl tertiary butyl ether (MTBE) and ethyl tertiary butyl ether (ETBE) are compounds with the potential for use as oxygenates in reformulated gasolines. Being relatively soluble in water, these organics, if accidentally discharged into the subsurface, may rapidly spread and pose threats to groundwater. The purpose of this work was to evaluate the biodegradation potential of these oxygenates in soils and to determine the influence of subsurface environments on their degradation. Biodegradation was evaluated in static soil/water microcosms. Aquifer material was collected from various depths at three sites with different soil characteristics. Potential electron acceptors including O₂ in the form of H₂O₂, nitrate or sulfate were added to induce the desired metabolism (aerobic respiration, denitrification, sulfate reduction, or methanogenesis). In each metabolic process, the influence of several subsurface environmental factors on biodegradation was investigated. The data show that biodegradation potential of MTBE, ETBE and TBA varied substantially with site and depth. TBA was the easiest compound to biodegrade, whereas MTBE was the most recalcitrant. Cleavage of the ether bond is the first and rate-limiting step in the degradation of ETBE and possibly MTBE. Addition of H₂O₂, caused chemical oxidation of MTBE and ETBE. The chemical oxidation was faster in the organically rich soils, but slower in the organic-poor soils. Soil microorganisms were able to catalyze the cleavage of the ether bond in ETBE but not MTBE. This biological reaction was not significant when chemical oxidation occurred. TBA, on the other hand, was aerobically biodegraded in all soils. Under denitrifying and anaerobic conditions TBA degradation occurred in all soils but the degradation of ETBE and MTBE was only observed at one of three sites. TBA degradation was enhanced by nutrient addition in the nutrient-poor soil but hindered by the presence of other easily-degraded organic compounds. Degradation of MTBE and ETBE occurred only in soils containing low organic matter with a pH around 5.5. No degradation of MTBE and ETBE was observed in the organic-rich soils and in the organically poor soils, the addition of ethanol inhibited MTBE and ETBE degradation. / Ph. D.

L'éthyl tert-butyl éther (ETBE) et le méthyl tert-butyl éther (MTBE) sont des éthers carburants utilisés comme additifs dans les essences sans plomb. Du fait de leur utilisation massive, de nombreux cas de pollutions d'aquifères ont été répertoriés, en particulier pour le MTBE, et ces composés représentent donc un risque sanitaire potentiel. Des travaux récents ont permis de mettre en évidence différents micro-organismes capables de dégrader ces composés malgré leur faible biodégradabilité dans l'environnement. Néanmoins, une meilleure compréhension de l'écologie et de la régulation de ces capacités de dégradation permettrait une meilleure gestion de la bioremédiation de sites contaminés par l'ETBE ou le MTBE.L'objectif de la thèse, réalisée dans le cadre d'un projet ANR Blanc (MiOxyFun), est de mieux comprendre l'écologie des communautés microbiennes impliquées dans la dégradation de ces éthers et leur relation avec la régulation ainsi qu'avec les cinétiques de dégradation de ces composés par des membres spécifiques de ces communautés. Ainsi, à partir de différents échantillons environnementaux venant de sites pollués par l'ETBE ou le MTBE, des enrichissements ont pu être réalisés en laboratoire afin d'étudier leurs microflores. Ces enrichissements ont été étudiés notamment pour leurs cinétiques de dégradation, la composition de leurs communautés bactériennes, et pour l'isolement de souches bactériennes directement impliquées dans la dégradation de ces composés. L'étude des cinétiques de dégradation de l'ETBE ou du MTBE par différents enrichissements obtenus sur ETBE (cinq) et sur MTBE (six) a permis de montrer des profils de dégradation très différents. La dégradation était généralement lente et s'accompagnait d'un faible rendement en biomasse avec parfois accumulation transitoire de tert-butanol (TBA). Les capacités de dégradation d'autres composés des essences (BTEXs et n-alcanes) étaient aussi différentes d'un enrichissement à l'autre, le benzène, entre autres, étant dégradé par 10/11 enrichissements. Des techniques d'empreinte moléculaire (RISA, DGGE) ont permis de constater que les communautés bactériennes présentes dans les cinq enrichissements sur ETBE étaient différentes de celles sur les enrichissements sur MTBE. Les enrichissements sur ETBE ont fait spécifiquement l'objet d'une étude par analyse de banques de clones réalisées à partir des gènes codant l'ARNr 16S de ces enrichissements. Cette étude a montré la prédominance des Proteobacteria dans trois enrichissements, la prédominance des Acidobacteria dans un autre ainsi qu'une composition plus héterogène dans le cinquième. De plus, des Actinobacteria ont été détectées dans les 5 enrichissements.En parallèle, plusieurs souches possédant des capacités de dégradation ont été isolées des enrichissements: Rhodococcus sp. IFP 2040, IFP 2041, IFP 2042, IFP 2043 (dégradant l'ETBE jusqu'au TBA), une Betaproteobacteria IFP 2047 (dégradant l'ETBE), Bradyrhizobium sp. IFP 2049 (dégradant le TBA), Pseudonocardia sp. IFP 2050 (dégradant l'ETBE et le MTBE), Pseudoxanthomonas sp. IFP 2051 et une Proteobacteria IFP 2052 (dégradant le MTBE). Une étude par qPCR sur les gènes codant l'ARNr 16S a montré la prédominance de certaines souches isolées dans les enrichissements ETBE. Enfin, plusieurs gènes connus comme étant impliqués dans la dégradation des éthers carburants ont pu être mis en évidence dans les enrichissements et dans certaines des souches isolées.

Polymer nanocomposites are promising materials for dielectrical use in high voltage applications and insulations. This work presents a study of nanocomposites based on poly(ethylene-co-butyl acrylate) with two different comonomer compositions and two different aluminium oxide nanoparticles. The nanoparticles were either untreated, or surface-treated with two different silanes, aminopropyl triethoxy silane and octyltriethoxy silane. The best level of dispersion was found for the polymer with 13 wt. % of butyl acrylate (EBA-13) whereas the low melt viscosity of the polymer with 28 wt. % of butyl acrylate (EBA-28) resulted in insufficient mixing with uneven dispersion as a result. Octyltriethoxy silane-treated particles were best dispersed in the polymer. The nanoparticles acted as nucleation agents in EBA-28, increasing the crystallization temperature by several degrees. Studies of the water uptake in the nanocomposite materials showed the effect of the enormous interfacial surfaces and great number of polar groups present on the nanoparticle surfaces. For the well-dispersed nanomaterials, the water sorption data could be modeled by a single Fickian equation, whereas materials that contained a sizeable fraction of large nanoparticle agglomerates showed a two stage sorption process, first a fast process associated with the saturation of the polymer phase and second, a slow diffusion process due to water sorption of large particle agglomerates. The long-term performance and interaction between the nanoparticles and the phenolic antioxidant (Irganox 1010) was investigated by differential scanning calorimetry in order to assess the oxidation induction time (OIT); the latter being proportional to the concentration of efficient antioxidant. It was found that the stabilizer was adsorbed to the untreated Al2O3 nanoparticles, resulting in a significant reduction in OIT. However, silanization of the nanoparticles resulted in an increase in OIT, compared to the materials containing untreated particles. Furthermore, it was shown that the stabilizer was not irreversibly adsorbed to the particles, allowing a gradual release of stabilizer with ageing time. / Polymera nanokompositer är lovande material för användning som dielektriskt material inom högspänningsområdet. I detta arbete studeras nanokompositer framställda av två olika sampolymerer av eten och butylakrylat (EBA-13 med 13 vikt% butylakrylat samt EBA-28 med 28 vikt% och två olika typer av nanopartiklar av Al2O3. Nanopartiklarna användes antingen som obehandlade eller efter silanisering med aminopropyltrietoxysilan- eller oktyltrietoxysilan. Den bästa partikeldispergeringen observerades för de material som baserats på EBA-13 medan den låga smältviskositeten hos EBA-28 resulterade i låga skjuvkrafter under kompounderingen och en observerat ojämn dispergering och förekomst av mikrometerstora agglomerat. Partiklar som silaniserats med oktyltrietoxysilan var lättast att dispergera. Nanopartiklarna fungerade som kärnbildare i EBA-28 vilket medförde en höjning av kristallisationstemperaturen. Vattensorptionsstudier demonstrerade dels effekten av den stora specifika gränsytan mellan partikel och matris och dels av förekomst av polära grupper lokaliserade till nämnda gränsyta. Kompositer med väldispergerade partiklar uppvisade en enkel Ficksk sorptionsprocess medan de material som innehöll en betydande mängd stora agglomerat påvisade en tvådelad process. Den första processen var kopplad till mättningen av polymermatrisen och den andra kunde länkas till vattenupptaget i de stora agglomeraten. Vidare undersöktes långtidsegenskaperna hos nanokompositerna, samt om det fanns någon växelverkan mellan nanopartiklar och en fenolbaserade antioxidant (Irganox 1010). DSC användes för att bestämma induktionstiden för oxidation (OIT) vilket är ett mått på koncentrationen av aktiv fenolisk antioxidant. Det framgick det att Irganox 1010 adsorberades på nanopartiklarna, vilket ledde till en minskning av OIT. Det framgick även att de material som innehöll silaniserade nanopartiklar hade högre OIT jämfört material med obehandlade partiklar. Antioxidanten var däremot inte irreversibelt bunden till nanopartiklarna, utan frigjordes från deras ytor och blev aktiv under åldringen. / QC 20110128

Les bactéries qui sont des sources prolifiques d'antibiotiques et des fournisseurs importants d’agents pharmaceutiques peuvent produire une grande variété de métabolites. Ainsi, la découverte de métabolites issus bactéries est un nouveau challenge pour les chimistes. Parmi ces sources, les lichens sont admis comme niches intéressantes de nouvelles bactéries et de nouveaux composés bactériens. Par conséquent, les communautés de micro-organismes associées aux lichens sont devenues des sources prometteuses pour la production de composés naturels actifs. Dans cette thèse, nous avons concentré notre travail sur l'isolement des bactéries de Rhizocarpon geogaphicum, l'un des lichens crustacés les plus populaires vivant sur la roche. Parmi les souches isolées, Paenibacillus odorifer a été sélectionnée pour poursuivre les travaux visant à produire des composés actifs. Après des étapes d’optimisation de culture, l’étude des extraits issus des cultures de P. odorifer soit par le bioréacteur soit en Erlenmeyer a permis l’isolement des métabolites : un polysaccharide antioxydant, deux dérivés tert-butylphénoliques cytotoxiques issus de la bioaccumulation et de la biotransformation de précurseurs, d'un nouvel alcaloïde cytotoxique, de deux diols, de deux dérivés de type furfural et quelques autres composés connus. Des hypothèses de biosynthèse ont pu être proposés pour certains composés. La diversité des métabolites isolés de P. odorifer indique que cette espèce possède un grand potentiel de production des composés actifs et est une bactérie utilisatrice de substrats tert-butyl phénoliques. / Bacteria which are prolific sources of antibiotics and important suppliers to the pharmaceutical agents can produce a wide variety of metabolites. Thus finding metabolites from the bacterial lineages represented new interests for chemists. Among that, lichens are admitted as a rich source of new bacterial lineages and novel bacterial compounds. Therefore, microorganism communities associated with lichens became significant subjects as great potential for the production of active natural compounds. In this thesis, we focus our work on the isolation of bacterial lineages from the lichen Rhizocarpon geographicum, one of the most popular crustose lichens dwelling on the rock. Among the strains isolated, Paenibacillus odorifer was selected for further work to produce active compounds. After the culture optimization steps, the study of extracts from the P. odorifer cultures either in the bioreactor or in Erlenmeyer flask led to the production of metabolites: an antioxidant polysaccharide, two cytotoxic tert-butylphenol derivatives which came from the bioaccumulation and biotransformation of precursors, a novel and cytotoxic alkaloid compound, two diol compounds, two furfural derivatives and some other known compounds. Putative biosynthetic pathways have been proposed for some compounds. The diversity of metabolites isolated from P. odorifer highlighted that this species possessed a great potential of the production active compounds and were a new case of tert-butyl phenol utilizing bacterium.

Materiais poliméricos (plásticos e borrachas) abrangem uma proporção continuamente crescente de resíduos urbanos e industriais descartados em aterros, seus impactos no meio ambiente são cada vez mais preocupantes. A implementação de novas tecnologias em prol da redução dos resíduos poliméricos, aceitáveis do ponto de vista ambiental e a um custo eficaz, provou ser um grande problema, face às complexidades inerentes para a reutilização dos polímeros. A radiação ionizante tem capacidade para alterar a estrutura e propriedades dos materiais poliméricos. As borrachas butílicas e halobutílicas têm sido usadas em larga escala, numa variedade de aplicações tais como partes de pneus e artefatos diversos. O principal efeito do fóton de alta energia, como raios gama nas borrachas butílicas e butílicas halogenadas é a geração de radicais livres, acompanhada por mudanças nas propriedades mecânicas. O objetivo deste trabalho é desenvolver processos de degradação controlada (desvulcanização) de borrachas butílicas e halobutílicas (cloro e bromo), de modo a caracterizar sua disponibilidade para transformação e alteração de suas propriedades. Os resultados experimentais obtidos mostraramm que as borrachas butílica e halobutílicas irradiadas a 25 kGy e posteriormente cisalhadas podem ser usadas como ponto de partida para misturas com borracha virgem. / Polymeric materials (plastics and rubbers) attain a continuous and raising proportion of urban and industrial scraps discarded in landfills; their impact on environment are more and more concerning. The implementation of new technologies in order to reduce impacts of plastic waste on the environment, at an effective cost, proved to be a great problem, due to inherent complexity for polymers re-using. Ionizing radiation is capable to modify structure and properties of polymeric material. Butyl and halobutyl rubbers have been used within a comprehensive scale, applications such as tires spare parts and diverse artifacts. The major high energy photon effect, as gamma-rays in butyl and halo butyl rubbers consists in free-radicals generation along changes in mechanical properties.This work aims to the development of controlled degradation processes (devulcanization) of butyl and halo butyl (chlorine and bromine) rubbers, in order to characterize their availability for transformation and modification of properties. Experimental results obtained showed that butyl and halobutyl rubbers,irradiated at 25 kGy and further sheared, are able to be used as an initial point for mixtures with pristine.

Materiais poliméricos (plásticos e borrachas) abrangem uma proporção continuamente crescente de resíduos urbanos e industriais descartados em aterros, seus impactos no meio ambiente são cada vez mais preocupantes. A implementação de novas tecnologias em prol da redução dos resíduos poliméricos, aceitáveis do ponto de vista ambiental e a um custo eficaz, provou ser um grande problema, face às complexidades inerentes para a reutilização dos polímeros. A radiação ionizante tem capacidade para alterar a estrutura e propriedades dos materiais poliméricos. As borrachas butílicas e halobutílicas têm sido usadas em larga escala, numa variedade de aplicações tais como partes de pneus e artefatos diversos. O principal efeito do fóton de alta energia, como raios gama nas borrachas butílicas e butílicas halogenadas é a geração de radicais livres, acompanhada por mudanças nas propriedades mecânicas. O objetivo deste trabalho é desenvolver processos de degradação controlada (desvulcanização) de borrachas butílicas e halobutílicas (cloro e bromo), de modo a caracterizar sua disponibilidade para transformação e alteração de suas propriedades. Os resultados experimentais obtidos mostraramm que as borrachas butílica e halobutílicas irradiadas a 25 kGy e posteriormente cisalhadas podem ser usadas como ponto de partida para misturas com borracha virgem. / Polymeric materials (plastics and rubbers) attain a continuous and raising proportion of urban and industrial scraps discarded in landfills; their impact on environment are more and more concerning. The implementation of new technologies in order to reduce impacts of plastic waste on the environment, at an effective cost, proved to be a great problem, due to inherent complexity for polymers re-using. Ionizing radiation is capable to modify structure and properties of polymeric material. Butyl and halobutyl rubbers have been used within a comprehensive scale, applications such as tires spare parts and diverse artifacts. The major high energy photon effect, as gamma-rays in butyl and halo butyl rubbers consists in free-radicals generation along changes in mechanical properties.This work aims to the development of controlled degradation processes (devulcanization) of butyl and halo butyl (chlorine and bromine) rubbers, in order to characterize their availability for transformation and modification of properties. Experimental results obtained showed that butyl and halobutyl rubbers,irradiated at 25 kGy and further sheared, are able to be used as an initial point for mixtures with pristine.

Third phase formation is an undesirable phenomenon during the PUREX process, which is a continuous liquid-liquid extraction approach for the reprocessing of uranium and plutonium from spent nuclear fuel. When third phase formation occurs, the organic extraction solution splits into two layers. The light upper layer, which is commonly named the light organic phase, contains a lower concentration of metal ions, tri-n-butyl phosphate (TBP) and nitric acids but is rich in the organic diluent. The heavy lower layer, which is commonly named the third phase, contains high concentrations of metal ions, TBP and nitric acids. As the third phase contains high concentrations of the uranium and plutonium complexes it can thus cause processing and safety concerns. Therefore, a comprehensive understanding of the mechanism of third phase formation is needed so as to improve the PUREX flowsheet. To investigate third phase formation through molecular simulations, one should first obtain reliable molecular models. A refined model for TBP, which uses a new set of partial charges generated from our density functional theory calculations, was proposed in this study. To compare its performance with other available TBP models, molecular dynamics simulations were conducted to calculate the thermodynamic properties, transport properties and the microscopic structures of liquid TBP, TBP/water mixtures and TBP/n-alkane mixtures. To our knowledge, it is only TBP model that has been validated to show a good prediction of the microscopic structure of systems that consist of both hydrophobic and hydrophilic species. This thesis also presents evidence that the light-organic/third phase transition in the TBP/n-dodecane/HNO3/H2O systems, which is relevant to the PUREX process, is an unusual transition between two isotropic, bi-continuous micro-emulsion phases. The light-organic /third phase coexistence was first observed using Gibbs Ensemble Monte Carlo (GEMC) simulations and then validated through Gibbs free energy calculations. Snapshots from the simulations as well as the cluster analysis of the light organic and third phases reveal structures akin to bi-continuous micro-emulsion phases, where the polar species reside within a mesh whose surface consists of amphiphilic TBP molecules. The non-polar n-dodecane molecules are outside this mesh. The large-scale structural differences between the two phases lie solely in the dimensions of the mesh. To our knowledge, the observation of the light-organic/third phase coexistence through simulation approaches and a phase transition of this nature have not previously been reported. Finally, this thesis presents evidence that the microscopic structure of the light organic phase of the Zr(IV)/TBP/n-octane/HNO3/H2O system, which is also related to the PUREX process, is different from that of the common hypothesis, where such system is consisted of large ellipsoidal reverse micelles. Snapshots from simulations, hydrogen bonding analysis and cluster analysis showed that the Zr4+, nitrate, TBP and H2O form extended aggregated networks. Thus, as above, we observe a bi-continuous structure but this time with embedded local clusters centred around the Zr4+ ions. The local clusters were found to consist primarily of Zr(NO3)4·3TBP complexes. This finding provides a new view of the structure of the Zr(IV)/TBP/n-octane/HNO3/H2O system.

Made available in DSpace on 2014-12-17T15:42:15Z (GMT). No. of bitstreams: 1 ChristianWL_DISSERT.pdf: 3506891 bytes, checksum: d575f05807a1548e862bbdb4ae0a6222 (MD5) Previous issue date: 2014-05-23 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / This work reports the synthesis of zeolites with different compositions (pure silica, Si/Ti and Si/Al), via hydroxide and fluoride medium using the cation 1-butyl-3- methylimidazolium as structure directing agent. Initially, the cation was synthesized in chloride form and used for the synthesis in hydroxide medium. An anion-exchange (Cl- for OH-) was required for the synthesis in fluoride medium. Different reactants were used for the formation of gels synthesis, resulting in the crystallization of MFI and TON phases, the latter predominant in many compositions. The cation and synthesized zeolites obtained were characterized by different techniques such as NMR, TG/DTG, XRD, SEM, N2 adsorption and desorption, DRS and EPMA. Besides characterizing the cation and zeolites, the mother liquor of hydroxide synthesis was characterized and it was possible to observe a modification of the cation in the synthesis conditions employed. The materials synthesized in this work can be applied in catalytic reactions and adsorption / Este trabalho relata a s?ntese de ze?litas de diferentes composi??es (puramente sil?cicas, Si/Ti e Si/Al), via meio b?sico e fluor?drico, utilizando o c?tion 1-butil-3- metilimidaz?lio como agente direcionador de estrutura. Inicialmente o c?tion foi sintetizado e utilizado na forma de cloreto para as s?nteses em meio b?sico e uma troca ani?nica (Cl- para OH-) foi necess?ria para as s?nteses em meio fluor?drico. Diferentes reagentes foram empregados para a forma??o dos g?is de s?ntese, resultando na cristaliza??o das fases MFI e TON, esta ?ltima predominante em muitas composi??es de gel testadas. O c?tion sintetizado e as ze?litas obtidas foram caracterizados por diferentes t?cnicas, como RMN, TG/DTG, DRX, MEV, adsor??o e dessor??o de N2, DRS e EPMA. Al?m de caracterizar o c?tion e as ze?litas, a ?gua-m?e das s?nteses em meio b?sico foi caracterizada e foi poss?vel verificar uma modifica??o do c?tion nas condi??es de s?ntese empregadas. Os materiais sintetizados neste trabalho podem ser aplicados em rea??es catal?ticas e de adsor??o

Methyl tert-butyl ether (MTBE) has been used as a gasoline additive to improve the combustion efficiency and to replace lead since 1978. It is the most commonly used oxygenate now due to its low cost, convenience of transfer, and ease of blending and production. MTBE has become a prevalent groundwater contaminant because it is widely used and it has been disposed inappropriately. MTBE has been demonstrated an animal carcinogen. The US Environmental Protection Agency (US EPA) has temporarily classified MTBE as a possible human carcinogen and has set its advisory level for drinking water at 20-40 µg/L based on taste and odor concerns. The Taiwan Environmental Protection Administration (TEPA) also classifies it as the Class IV toxic chemical substances. Currently, natural attenuation (NA) as well as natural bioremediation or enhanced bioremediation are attractive remediation options for contaminated sites due to their economic benefit and environmental friendly. In general, in situ microorganisms at the contaminated site play a very important role in site restoration. Although early studies suggested that the biodegradability of MTBE was not significant, recent laboratory and field reports reveal that MTBE can be biodegraded under aerobic and anaerobic conditions. In addition, evidences and some successful cases of MTBE attenuation have been reported that make natural attenuation a considerable remedial strategy. However, the biodegrading rate might decrease if the nutritional and physiological requirements are not met. Thus, it is important to assess the biodegradability of natural microorganisms under various site conditions to obtain optimal remedial conditions. Contributions of intrinsic biodegradation and other abiotic mechanisms to the removal and control of contaminants should also be evaluated to provide sufficient information for remedial option determination. Moreover, isolation and identification of the dominant native microorganisms will be helpful to following remediation tasks. In the first part of this study, microcosm study and microbial identification technologies (denaturing gradient gel electrophoresis, DGGE) were applied to assess the biodegradability of MTBE by indigenous microbial consortia and to identify the dominant microorganisms at a MTBE-contaminated site (Site A). In the second part of this study, thorough field investigations were performed to evaluate the occurrence of natural attenuation of MTBE at two MTBE-contaminated sites (Site A and Site B). In addition, a natural attenuation model, BIOSCREEN, was performed to assess the effectiveness of natural attenuation on MTBE containment. The main objectives of this study contained the following: (1)Evaluate MTBE biodegradability under different redox conditions by the indigenous microorganisms. (2)Determine the dominant native microorganisms in MTBE biodegradation for further application. (3)Assess the feasibility of using natural attenuation to control the MTBE plume. (4)Evaluate the contributions of intrinsic biodegradation patterns on natural attenuation processes by BIOSCREEN. Results from the microcosm study reveal that MTBE could be biodegraded by aquifer sediments without the addition of extra carbon sources under aerobic conditions. The production of tert-butyl alcohol (TBA), a degradation byproduct of MTBE, was detected. Complete removal of TBA was also observed by the end of the experiment. Results from aerobic microcosms study indicate that oxygen might be the major limiting factor of MTBE biodegradation at Site A. Thus, MTBE at this site could be removed via natural biodegradation processes with the supplement of sufficient oxygen. Microcosm study with extracted supernatant of aquifer sediments as the inocula show that the indigenous microorganisms were capable of using MTBE as the sole carbon and energy source. The calculated MTBE degradation rate was 0.597 mg/g cells/h or 0.194 nmole/mg cells/h. No MTBE removal was observed under various anaerobic conditions. Results suggest that aerobic biodegradation was the dominant degradation process and aerobic bioremediation might be a more appropriate option for the site remediation. According to the results of DGGE analysis, aerobic MTBE-biodegrading bacteria, Pseudomonas sp. and Xanthomonas sp., might exist at this site. Although results of microcosm study show that MTBE could not be degraded under anaerobic conditions, the microbial identification indicates that some novel anaerobic microbes, which could degraded MTBE, might be present at this site. In addition, anaerobic microbes caused the consumption of electron acceptors (e.g., nitrate, ferric iron) and removal of benzene, toluene, ethylbenzene, xylenes (BTEX), 1,2,4-trimethyl benzene (1,2,4-TMB), and 1,3,5-trimethyl benzene (1,3,5-TMB) (TMBs) in the anaerobic microcosms. These results also indicate that the potential of anaerobes activities was high at Site A. Based on the results from the field investigation, natural attenuation of MTBE was occurring at both sites. MTBE plume at Site B could be effectively controlled via natural attenuation processes. Nevertheless, MTBE plume at Site A has migrated to a farther downgradient area and passed the boundary of the site. Field investigation results indicate that the natural attenuation mechanisms of MTBE at both sites were occurring with the first-order attenuation rates of 0.0021 and 0.0048 1/day at Sites A and B, respectively. According to BIOSCREEN simulation, biodegradation was responsible for 78% and 59% of MTBE mass reduction at Sites A and B, respectively. The intrinsic biodegradation had significant contributions on the control of MTBE plumes. Moreover, the dilution and dispersion processes might be the major mechanisms for the attenuation of MTBE in the downgradient areas. However, results also reveal that intrinsic biological processes might still fail to contain the plume if the selected point of compliance is not appropriate. Results of this study suggest that natural attenuation might be feasible to be used as a remedial option for the remediation of MTBE-contaminated site on the premise that (1) detailed site characterization has been conducted, and (2) the occurrence and effectiveness of natural attenuation processes have been confirmed. Based on the results from the field investigation and laboratory microcosm studies, MTBE could be biodegraded by natural microbial populations at the studied sites under both aerobic and anaerobic conditions and natural attenuation would be applied as a remedial option at MTBE-contaminated sites. Results from this study would be useful in determining the favorable bioremediation conditions and designing an efficient and cost-effective bioremediation system such as monitored natural attenuation (MNA) or in situ or on-site MTBE bioremediation system for field application.