A Review of Bioremediation and Natural Attenuation of MTBE Lawrence C Davis and Larry E Erickson Departments of Biochemistry and Chemical Engineering Kansas State University Manhattan KS 66506 ldavis ksu edu for correspondence Published online in Wiley InterScience www interscience wiley com DOI 10 1002 ep 10028 Methyl tert butyl ether MTBE has been the focus of much attention because it is used in large amounts and was reportedly relatively recalcitrant to bioremediation or natural attenuation Beginning with a few papers a decade ago evidence has been presented that in fact under suitable conditions it is amenable to bioremediation Many species from widely disparate microbial genera are able to consume it either as a sole carbon source or as a cometabolite Optimal conditions differ from site to site Both aerobic and anaerobic conditions may permit MTBE degradation with a range of electron acceptors from oxygen through Fe III Mn IV sulfate nitrate and methanogenesis MTBE metabolism in the vadose zone can be highly active The published literature suggests that natural populations are adapting to MTBE and reported rates of biodegradation appear to be larger in the more recent literature Plants may serve as ef cient conduits to withdraw MTBE from the wet subsurface releasing it to the atmosphere or the vadose zone where it may be metabolized or diffuse into the atmosphere where it is quickly photodegraded The major remaining issues are the time required to attain speci ed criteria of cleanup or whether augmentation is necessary for effective remediation 2004 American Institute of Chemical Engineers Environ Prog 23 243 252 2004 Keywords tion MTBE bioremediation phytoremedia INTRODUCTION MTBE more formally methyl tert butyl ether has been extensively used as an octane booster and oxygenate additive in reformulated gasoline in the United States With MTBE being distributed nationwide the potential for contamination of soil and groundwater is substantial Efforts to develop strategies for cleanup have produced a large number of publications a selec 2004 American Institute of Chemical Engineers Environmental Progress Vol 23 No 3 tion of which are discussed below 1 60 MTBE has certain advantages over ethanol in the behavior of gasoline blends to which it is added and it is ef ciently produced from iso butylene Current production 2003 of MTBE is over 200 000 barrels per day 8 106 L in the United States and U S usage is about 1 3 greater 12 Physical and organoleptic properties of MTBE make it a challenge to deal with effectively Unlike most gasoline constituents MTBE is highly soluble in water At room temperature its solubility is about 50 g L 20 times greater than that of BTEX the most soluble gasoline constituents benzene toluene xylenes and ethyl benzene As discussed by Kinner 30 it also partitions strongly from air to water The dimensionless Henry s Law constant is in the range of 0 01 to 0 04 depending on temperature 10 whereas that of benzene is about 0 2 Thus MTBE is more likely to dissolve in water and less likely to volatilize from gasoline or water to air than is the case for other gasoline constituents Some of the important physical and chemical properties of MTBE and BTEX compounds are shown in Tables 1 and 2 Because of its small Henry s constant MTBE is expensive to remove from water by aeration Removal using adsorption is expensive because the quantity that adsorbs to activated carbon is relatively small based on the small values of the octanol water partition coef cient and sorption coef cient Keller et al 27 evaluated four physicochemical treatment technologies and reported estimated costs associated with each technology When no air treatment is required air stripping is the lowest cost technology for high ow rates whereas hollow ber membranes are cost effective for low ow rates Granular activated carbon is most cost effective if air treatment costs are included Advanced oxidation processes are the most expensive of the four options Although health effects of low level MTBE contamOctober 2004 243 Table 1 Physical and chemical properties of MTBE from 54 Physical state Molecular formula Molecular weight Melting point Boiling point Water solubility Density Koc log Kow Vapor pressure Henry s law constant Odor threshold Taste threshold Volume in gasoline Water solubility when in gasoline Colorless liquid C5H12O 88 15 109 C 55 2 C 51 g L 25 C 0 74 g mL 25 C 12 3 11 0 estimated 1 24 245 mmHg 25 C 0 02 at 25 C 15 40 ppb 40 140 ppb Up to 15 Up to 5100 ppm Similar values are presented by Keller et al 27 Jacobs et al 24 and Seagren and Becker 45 ination are uncertain and disputed see 13 it has a potent taste impact in water at levels of 10 30 g L ppb Thus even small spills of MTBE are detectable and the USEPA 49 issued a health advisory recommending that drinking water levels be kept below 20 40 g L This would provide a very large safety margin compared to known biological effects Many states have implemented regulations on MTBE contamination of water and a number have banned its use in gasoline Through the year 2000 38 states had action levels cleanup levels or drinking water standards for MTBE 37 The drinking water levels varied but were all below 250 g L As of March 2003 restrictions or outright bans were pending in 16 states Five states proposing bans or severe restriction depend on MTBE for oxygenates and account for 45 of MTBE use nationwide 12 Thus if implemented these restrictions could markedly reduce new incidents of MTBE contamination of groundwater One liter of MTBE ha could contaminate all the yearly rainfall on that area to a level of about 100 ppb assuming 1 m yr rain Avoiding such contamination in areas where there is a lot of reformulated gasoline use has proven challenging When MTBE constitutes 10 15 of the gasoline containment systems have to maintain nearly perfect ef cacy to avoid some contamination of groundwater This has been a particular issue in California which consumes a large fraction of all U S MTBE although the expected magnitude of the problem is debated 26 33 Given the widespread nature of MTBE contamination and the intense efforts to remediate it a large number of studies have been published that detail an extensive amount of research Prince 40 provided an excellent critical summary of what was known about microbial degradation of MTBE More recently Fayolle et al 14 Fiorenza et al 16 and Seagren and Becker 45 provided reviews that describe some of the