in situ remediation of mtbe utilizing ozone
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REMEDIATION Winter 2002
There has been a great deal of focus on methyl tertiary butyl ether (MTBE) over the past few
years by local, state, and federal government, industry, public stakeholders, the environmental
services market, and educational institutions. This focus is, in large part, the result of the
widespread detection of MTBE in groundwater and surface waters across the United States. The
presence of MTBE in groundwater has been attributed primarily to the release from
underground storage tank (UST) systems at gasoline service stations. MTBEs physical and
chemical properties are different than other constituents of gasoline that have traditionally been
cause for concern [benzene, toluene, ethylbenzene, and xylenes (BTEX)]. This difference in
properties is why MTBE migrates differently in the subsurface environment and exhibits different
constraints relative to mitigation and remediation of MTBE once it has been released to
subsurface soils and groundwater.
Resource Control Corporation (RCC) has accomplished the remediation of MTBE from
subsurface soil and groundwater at multiple sites using ozone. RCC has successfully applied
ozone at several sites with different lithologies, geochemistry, and concentrations of constituents
of concern. This article presents results from several projects utilizing in situ chemical oxidation
with ozone. On these projects MTBE concentrations in groundwater were reduced to remedial
objectives usually sooner than anticipated. 2002 Wiley Periodicals, Inc.
Methyl tertiary butyl ether (MTBE) is a fuel additive that has been added to gasolineas an octane enhancing replacement for lead and more recently as an oxygenate, addedto reformulated gasoline to reduce air emissions.The more prevalent use of MTBE hasresulted in an increased presence of it in our environment.Various studies have beenand are being conducted to define the health effects of MTBE in the environment.
MTBE has received a great deal of attention and is being evaluated as anotherchemical in the environment reportedly causing public health problems.The United StatesEnvironmental Protection Agency (EPA) considers MTBE a possible human carcinogen,and it produces a disagreeable taste and odor at very low concentrations in drinking water.Based on these factors, the EPA has issued a drinking water advisory concentration of 20to 40 parts per billion (ppb) as a guideline for permissible levels in potable water.
MTBE was first associated with gasoline beginning in 1979, when the AtlanticRichfield Company was authorized by the EPA to add it to their gasolineformulation. MTBE was first used as an octane booster, to replace lead in gasoline.Initially, MTBE was used at relatively low concentrations (1 percent to 3 percent);gradually increasing to around 7 percent by the late 1980s.
2002 Wiley Periodicals, Inc.Published online in Wiley Interscience (www.interscience.wiley.com). DOI: 10.1002/rem.10055
Jeffrey C. Dey
In Situ Remediation of MTBE UtilizingOzone
The relatively highsolubility of MTBE, whichallows it to mix well withgasoline, also causes it todissolve in groundwater athigh concentrations.
MTBE is in a family of gasoline additives known as oxygenates, which improve thecombustion of gasoline in engines, and thus burn cleaner. As a consequence, MTBEconcentrations in unleaded gasoline increased, up to 15 percent, during the early 1990sin response to the requirements of the EPAs Clean Air Act Amendments. However,while MTBE has been instrumental in improving the nations air quality, it has beendetrimental to groundwater and surface water quality.
Numerous reports and investigations presented over the past five years haveidentified widespread detection of MTBE in groundwater and surface water acrossthe country. According to the U.S. Geological Surveys (USGS) National Water-Quality Assessment program in 1993 and 1994, MTBE was the second mostfrequently detected chemical in shallow groundwater from urban areas. Of the 210wells and springs sampled by the USGS, 27 percent contained MTBE. This widedistribution and occurrence of MTBE is due in large part to the same chemicalcharacteristics that make it well suited as a gasoline additive. The relatively highsolubility of MTBE, which allows it to mix well with gasoline, also causes it todissolve in groundwater at high concentrations. MTBE has a low adsorptioncoefficient that prevents it from significantly adsorbing to soil, which causes it totravel farther and faster through subsurface soils. The chemical stability of MTBE,which is a beneficial property for a fuel additive, causes it to persist for long periodsof time in the subsurface without degrading.
Overall, MTBE is undesirable in groundwater, even at low concentrations, becauseit spreads rapidly in water and is extremely persistent.Thus the question remains: whatis the best way to remediate MTBE?
The effectiveness of remediation techniques is directly affected by the physical andchemical nature of the constituent to be remediated. In general, MTBE is much lesslikely to adsorb to soil or organic carbon than other components of gasoline. MTBE isabout 30 times more soluble than benzene in water. MTBEs Henrys Law constant isabout one tenth that of benzene, and therefore, is much more difficult to remove fromthe dissolved phase.
MTBE behaves differently than other gasoline constituents of concern, and sitecharacterization plans and remedial designs should consider the compounds specificbehaviors. Conventional groundwater remediation techniques including pump and treat,air sparging, bioremediation, and point-of-use treatment have been used to remediateMTBE in groundwater. Each of these remedial methods has shown some promise andhas site-specific cases where they have been quite effective for removing or remediatingMTBE. However, the consensus of the regulated community is that these traditionalmethods generally used to remediate BTEX components are typically more expensiveand lead to longer remediation projects. MTBEs high solubility in water, low rate ofadsorption to soil, and low rate of biodegradation are some of the key characteristicsthat impact remediation.
Ozone is a molecule comprised of three oxygen atoms. Ozone is a relatively unstablemolecule with a very short half-life.The half-life of ozone in air is approximately two
In Situ Remediation of MTBE Utilizing Ozone
2002 Wiley Periodicals, Inc.78
minutes, as it degrades back to oxygen. In water, dissolved ozone has a half-life ofapproximately 20 minutes, as it reacts with water to form free-radical ions, ordegrades back to oxygen.This short life span makes ozone ideal for environmentalremediation of contaminants, as it reacts quickly in the subsurface and rapidly revertsto harmless oxygen.
During its short life span, ozone is a very reactive oxidizer. Ozone has the thirdhighest oxidation potential in nature (behind fluorine and hydroxyl-radicals).Thehigher oxidation potential of ozone allows it to destroy a wide range of compounds,including MTBE.
Ozone can be introduced to the subsurface as either a gas (ozone sparging) or aliquid (dissolved ozone injection). In gas phase, ozone is typically introduced to thesubsurface (either alone or with air or oxygen) through specifically designed ozonesparge wells.The gas sparge rate is typically lower than that of air sparging, because theaim is to maximize mass transfer to the dissolved phase and/or maximize contact timewith contaminants. In aqueous phase, ozone dissolved in water is typically injected tothe subsurface through injection wells, trenches, or infiltration galleries.The result ofusing ozone for environmental cleanup is rapid remediation of some of the most arduouscontaminants, including MTBE.
When ozone is dissolved in groundwater, the groundwater itself becomes aremediating agent, which can destroy contaminants. Dissolved ozone distribution in thesaturated zone is typically better than gas distribution, as it is less affected bypreferential flow pathways, upward migration, and has the benefit of aqueous diffusion.
In groundwater, dissolved ozone can directly destroy MTBE, it can degrade to free-radical ions (which in turn oxidize MTBE), or it can degrade to dissolved oxygen, whichcan enhance natural biodegradation of residual contaminants. Ozone is up to 12.5 timesmore soluble in water than oxygen, which allows ozone to dissolve into water at morethan 500 ppm and saturate the water with dissolved oxygen following degradation.
The oxidation of MTBE occurs through the following degradation path, eventuallyyielding carbon dioxide and water.
MTBE TBA Acetone Formaldehyde Carbon DioxideC5H12O C4H10O C3H6O CH2O CO2
This article presents two detailed case studies and a summary of three other sites whereozone was used to effectively remove MTBE.
Case Study 1: Ozone Sparge Remediation-Former Service Station
The subject site is an out-of-service, former retail petroleum service station located insoutheastern Pennsylvania. Environmental assessment activities at the site, followingunderground storage tank removal activities, revealed the presence of gasoline floatingon the water table beneath the site and high concentrations of petroleum hydrocarbonsin soil and groundwater.Target compounds exceeding the Pennsylvania Department ofEnvironmental Protection (PADEP) cleanup standards included benzene, toluene,ethylbenzene, xylenes, naphthalene, and MTBE.
REMEDIATION Winter 2002
2002 Wiley Periodicals, Inc. 79
When ozone is dissolvedin groundwater, the
groundwater itselfbecomes a remediating