Chemical Oxidation of MTBE and TBA

Methyl tertiary butyl ether (MTBE) is a synthetic chemical that was historically used as an octane booster additive to gasoline. Increasingly there have been concerns about its toxicity and potential carcinogenicity. Because of these concerns, 18 states regulate MTBE levels in groundwater and 17 have banned its use in gasoline. Because of its chemical characteristics, MTBE contaminated sites are difficult to remediate. MTBE readily dissolves and spreads in water. Additionally, MTBE resists biodegradation, does not sorb to soil, and has a low Henry’s Law constant. As a result, the extent of MTBE contamination is usually much greater than that of the other common gasoline components. Because of these factors, remediation of MTBE-impacted groundwater can be very difficult and costly. MTBE can breakdown in groundwater to form tert-butyl alcohol, TBA. TBA is also an impurity in or is formulated with MTBE. Many states are also beginning to regulate TBA in groundwater. The problem of remediating groundwater contaminated with MTBE is complicated by the presence or formation of TBA. There is a considerable interest in finding an efficient technology that can be used for remediation of MTBE. The utilization of in-situ chemical oxidation (ISCO) is becoming a more common method for treatment of MTBE. Previous case studies and research have shown that a variety of oxidants will reduce concentrations of MTBE. However, the production of tertiary butyl alcohol (TBA) with many of these oxidants poses a significant problem. Fenton’s reagent, permanganate, ozone and uncatalyzed persulfate all produce TBA. The oxidation of MTBE by activated persulfate does not generate much TBA. Additionally, activated persulfate will also oxidize TBA if it is present. INTRODUCTION. MTBE has been used as a gasoline additive since 1979. It helps fuel to burn cleaner and boosts the octane value. The Clean Air Act requires states with nonattainment for CO (carbon monoxide) to have 2.7% oxygen content in the gasoline used during winter months. This equates to a 15% MTBE content. Federal regulations requiring reformulated gasoline to reduce emissions require a 2% oxygen level (~11% MTBE). It should be noted that the oxygen requirement for gasoline does not specify MTBE; however, MTBE has been the most common oxygenate. Beginning in the 1990s, there has been increasing public concerns about the effect of MTBE on human health. Much of the public’s concern has been driven by the fact that MTBE has a low odor and taste threshold; as low as 5 -20 μg/L with some subjects. The Oxygenated Fuels Association in 1998 recommended a secondary contaminant level for taste and odor of 15 μg/L as being protective of 95% of the population. There is little human health data for MTBE. However, in some animal studies, drinking water with MTBE caused gastrointestinal irritation, liver and kidney damage, and nervous system effects in rats and mice. Inhalation of MTBE for long periods in one study with rats caused kidney cancer; another study with mice resulted in liver cancer (ASTDR 1997).