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Berkeley ETHSTD 196 - Evaluating MTBE Biodegradation with Hydrocarbon Metabolizing Cultures

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Evaluating MTBE Biodegradation with Hydrocarbon Metabolizing Cultures Kou-San Ju Abstract Methyl tert-butyl ether (MTBE) is added to reformulated gasoline to meet the 1990 Clean Air Act directives. Widespread use of MTBE in gasoline has resulted in groundwater contamination throughout the United States, increasingly impacting public drinking water supplies. Biodegradation of MTBE can offer an efficient and low-cost method of treating MTBE contaminated groundwater. It has been proposed that alkane-degrading bacterial cultures can co-metabolize MTBE. To evaluate the hypothesis that all alkane-degrading bacteria can co-metabolize MTBE, mixed cultures were enriched on iso-pentane, octane, decane, dodecane, tetradecane, pentadecane, and hexadecane alkanes as the sole carbon sources and then tested for their ability to co-metabolize MTBE. A toluene enrichment culture was also screened to see if aromatic-degrading cultures could co-metabolically degrade MTBE. Vials were sealed using Mininert caps and MTBE concentrations monitored by GC/FID headspace analysis. Microcosms were supplemented with enrichment substrate on day seven to test MTBE degradation stimulation. Degradation of MTBE to below detection limits of 0.1 mg/L in less than three days was observed by the iso-pentane enrichment culture. Partial but incomplete co-metabolism was observed in octane and decane cultures. Dodecane, tetradecane, pentadecane, and hexadecane cultures did not degrade MTBE. The toluene enrichment degraded MTBE to near below detection limits during the duration of the assay. Positive stimulation of MTBE degradation only occurred with the toluene culture, resulting in removal of MTBE below detection limits. The toluene enrichment was found capable of metabolizing iso-pentane. These experiments showed that of the alkanes tested, only bacteria able to grow on iso-pentane were efficient at MTBE co-metabolism. This suggests that MTBE degradation is limited to a specific subgroup of alkane degrading bacteria and possibly to bacteria able to degrade branched aromatic hydrocarbons. Continued studies with pure and mixed cultures will be performed to clarify interactions between alkane and aromatic metabolism, and MTBE biodegradation.Introduction Methyl tert-butyl ether (MTBE) is a synthetic additive originally introduced to replace lead as an octane-enhancer and anti-knocking agent in gasoline (API 1998). MTBE is now used as an oxygenate, to reduce automobile carbon monoxide emissions in fall and winter months, under programs mandated by the 1990 Clean Air Act Amendments. It is blended into gasoline at levels of 15% by volume to meet the federal standards of 2.0% oxygen by weight for reformulated gasoline. While alternative additives such as ethanol are available for increasing fuel oxygenation, they are not economically favorable to due to higher costs of synthesizing the alcohol. In addition, costs of transporting fuel additives are low for MTBE, which is produced at fuel refineries, while ethanol needs to be imported over greater distances (US EPA 1998). Increased use of MTBE in reformulated gasoline has resulted in groundwater contamination throughout the United States, mainly from leaking underground storage tanks. High MTBE groundwater contamination has led to closures of drinking water well fields (Hitzig 1998). Even the very low concentrations of 35 µg/L of MTBE in water easily detectable and renders it unfit for human use (US EPA 1998). MTBE persists in groundwater due to the reduced exposure to air for volatilization and resistance to biodegradation (Yeh 1991). Bacterial degradation of MTBE could provide an attractive solution to groundwater contamination. Such treatment can be performed in-situ at the site of contamination or ex-situ with biological water treatment reactors (Salanitro 2000, Stringfellow 2000). With in-situ bioremediation bacterial cultures naturally existing are metabolically stimulated with substrate addition to degrade the contaminant to non-toxic compounds. Alternatively selected or engineered bacterial cultures with the desired degradation ability are injected into the contaminated site and provided with the required metabolic stimulus to degrade the recalcitrant contaminants. In ex-situ bioremediation, the contaminated water is extracted from the ground and treated by selected bacteria. The effluent exiting the bioreactor is usually contaminant free, and is cycled can be pumped into the water table or treated further with downstream systems. Although MTBE-degrading bacteria have been isolated, there are still unanswered questions about which specific members of the microbial community are capable of degrading MTBE, the enzymatic pathways and metabolic pathways involved, and most fundamentally, the differences between degradation enzymes that explains the restrictions in substrate utilization in bacteria. Understanding might suggest ways for stimulating faster growth of MTBE-degrading organismsfor either in-situ or ex-situ treatment systems (Prince 2000). Increased understanding of degradation processes can also potentially lead to engineered substrate specificity for bioremediation of other recalcitrant compounds and improved stability and efficiency of such processes, as well as other microbial biotechnology processes. Cultures able to metabolize MTBE have been found in activated sludge, air and ground water treatment systems, soils, sediments, and even Gingko fruit (Solano-Serena 2000, Stringfellow 2000, Bradley 1999, Garnier 1999, Hanson 1999, Mo 1997, Steffan 1997, Salanitro 1994). Several pure strains of bacteria able to degrade MTBE have been described (Solano-Serena 2000, Garnier 1999, Hanson 1999, Steffan 1997, Mo 1997, Salinitro 1994). In such strains as PM1 that are able to utilize MTBE as the sole carbon source, studies show that the bacteria demonstrated a very slow growth rate of only 19% of 14C-MTBE at 20 µg/ml to 14C-labeled cells in 120 hours. Only 0.18 milligrams of cells were yielded per milligram of MTBE removed. Toxic activity of MTBE and high energetic requirements in breaking down MTBE has been to explain its low occurrence within the microbial community and the slow rates of metabolism (Prince 2000). Many studies have demonstrated that alkane-degrading cultures can co-metabolize MTBE (Stringfellow 2000, Solano-Serena 2000, Bradley 1999, Garnier, 1999, Hyman 1999, Hyman 1998, Steffan 1997). With co-metabolism, the


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Berkeley ETHSTD 196 - Evaluating MTBE Biodegradation with Hydrocarbon Metabolizing Cultures

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