Abstract (Go to Presentation)
By: Ellen Moyer, Ph.D., P.E., Principal,
Greenvironment, LLC, Montgomery, MA
Chickadee Remediation Company, Houston, TX
This seminar reviews physical, chemical, and biodegradation characteristics of the fuel oxygenates tert butyl alcohol (TBA), methyl tert butyl ether (MTBE), and ethanol. Consideration of these characteristics leading to the optimization of remedial technologies employed at affected groundwater sites is evaluated and illustrated through a number of case studies. Optimal remedial strategies take advantage of the characteristics of TBA, MTBE, ethanol, and gasoline hydrocarbons such as benzene, toluene, ethylbenzene, and xylenes that may be present. Remedial strategies often consist of a sequence of actions starting with receptor protection and source control, followed by remediation of residual and dissolved contamination, and ending with natural attenuation.
TBA is often found in association with MTBE. Three potential sources of TBA are: 1) as a fuel oxygenate in its own right (although currently not in wide use); 2) as co-product present at low percent levels in commercial MTBE; and 3) as an intermediate product of MTBE biodegradation and chemical oxidation. In addition, it can be an artifact created during certain water sample preservation and analytical procedures.
TBA and MTBE differ from gasoline hydrocarbons in several physical characteristics, being relatively more soluble and less adsorptive. MTBE also has a higher vapor pressure, and TBA has a lower Henry’s Law constant, than gasoline hydrocarbons. These physical attributes influence the selection and optimization of remedial options for residual and dissolved constituents.
Like TBA and MTBE, ethanol is more soluble and less adsorptive than gasoline hydrocarbons. In addition, it is more rapidly biodegraded than TBA, MTBE, and gasoline hydrocarbons, under both aerobic and anaerobic conditions. Preferential ethanol biodegradation can deplete electron acceptors in the subsurface, resulting in longer plumes of other gasoline constituents than would be the case if ethanol were not present. Biodegradation of TBA and MTBE by naturally occurring bacteria has been demonstrated at many sites. Several pure cultures of microorganisms have been demonstrated to degrade these constituents aerobically to carbon dioxide and water. There is also evidence of TBA and MTBE mineralization by mixed cultures in aerobic/anaerobic environments. In the field, increasing the dissolved oxygen concentration increases aerobic biodegradation rates in a strong dose-response relationship.
Biodegradation has also been demonstrated with other electron acceptors including nitrate, iron, sulfate and carbon dioxide. Biodegradation is an important mechanism for natural attenuation of TBA, MTBE, and ethanol.
The seminar will explore the selection and execution of current and emerging technologies for the remediation of gasoline components, including residual and dissolved TBA, MTBE, and ethanol. Commonly used technologies include: soil vapor extraction; bioventing; air sparging; in situ ground water bioremediation; ex situ groundwater bioremediation; pump and treat; in situ chemical oxidation; and monitored natural attenuation.
TBA and ethanol have moderate vapor pressures typical of other gasoline constituents, while MTBE has a higher vapor pressure. Consequently, soil vapor extraction is very effective for removing these chemicals from the unsaturated zone (soil conditions permitting), where catalytic oxidation or thermal oxidation can be used to treat higher concentrations of these constituents aboveground (along with other gasoline constituents). Granular activated carbon (GAC) can be used for aboveground treatment of lower concentrations of MTBE. GAC is not typically the optimal technology for treating TBA or ethanol because of their low tendency to adsorb. Biofilters can be used to treat MTBE, TBA, and ethanol.
Air sparging is less effective at removing TBA, MTBE, and ethanol dissolved in water than it is for higher Henry's Law constant constituents, like benzene. However, when higher air to water ratios are used, sparging can be effective. TBA and ethanol are treated by biodegradation enhanced by the oxygenation that occurs as a result of air sparging, whereas MTBE is treated by both biodegradation and stripping processes.
High water solubility and low tendency to adsorb make these three constituents very amenable to groundwater extraction. Recovered groundwater can be treated aboveground by ex situ bioremediation or advanced oxidation processes. Additional options in the case of MTBE include carbon or resin adsorption, or air stripping followed by carbon adsorption or catalytic oxidation of the vapor phase. Air stripping of MTBE requires higher air to water ratios than for gasoline hydrocarbons.
A number of case studies will be presented that illustrate the effect of physical and biodegradation characteristics on remediation technology selection. Every site is different, with its own set of characteristics and challenges. Regardless of the composition of gasoline, rapid source control is critical to minimize environmental impact and to reduce overall remediation cost Optimal strategies take advantage of site characteristics as well as the specific characteristics of TBA, MTBE, and ethanol for cost-effective, timely, and environmentally sound remediation of these chemicals.
Biographies of Presenters
Ellen Moyer, Ph.D., P.E, Principal of Greenvironment,
LLC, is a recognized expert in the assessment and remediation of VOC
contamination. She has an M.S. in Environmental Engineering and a Ph.D. in Civil
Engineering. She has presented numerous seminars on assessment and remediation
of VOCs and was the lead editor of an MTBE Remediation Handbook, now in its
second printing. Dr. Moyer has managed all phases of assessment and remediation
work, and her numerous projects have employed a wide range of in situ and ex
situ remediation technologies at diverse sites with organic and inorganic
contaminants. Her Ph.D. research investigated soil vapor extraction, air
sparging, and bioventing of gasoline VOCs.
Richard Sloan is President of Chickadee Remediation
Co., whose primary business is to remediate contaminated soil and groundwater to
the extent necessary to protect public health and the environment and acquire
the long-term site environmental liability. Sloan has developed and implemented
timely, cost-effective and environmentally-sound remediation plans for numerous
Superfund, RCRA, and other sites with affected soils and groundwater. He has
successfully established community/agency/company/contractor partnerships to
focus the project efforts on common goals and apply a broad-based technical
approach for each site. Sloan is also President of Chickadee Mining Co., which
uses environmentally-sensitive procedures and equipment for precious metals
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