Journal of Hazardous Materials B137 (2006) 1417–1423 Uptake, metabolism, and toxicity of methyl tert-butyl ether (MTBE) in weeping willows Xiao-Zhang Yu, Ji-Dong Gu ∗ Laboratory of Environmental Toxicology, Department of Ecology & Biodiversity, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, People’s Republic of China Received 2 February 2006; received in revised form 7 April 2006; accepted 7 April 2006 Available online 25 April 2006 Abstract Methyl tert-butyl ether (MTBE) is a high volume production chemical and the most commonly used gasoline oxygenate. Uptake, metabolism and toxicity of MTBE in trees were investigated in this study. Pre-rooted weeping willows (Salix babylonica L.) were exposed to hydroponic solution spiked with MTBE and incubated at 25.0 ± 1 ◦ C for 168h. The normalized relative transpiration (NRT) rate of weeping willows was used to determine toxicity. MTBE and possible intermediate tert-butyl alcohol (TBA) in solution, tissues of aerial parts of plants, and air were analyzed. Results from the toxicity test showed that severe signs of toxicity (the reduction of the NRT ≥35%) were only found at the treatment group with high doses of MTBE 400 mg L -1 . Neither chlorosis of leaves nor large reduction in the NRT was observed at MTBE exposure to weeping willows ≤200 mg L -1 . Almost all applied MTBE was removed from the hydroponic solution by plants in all treatment groups. Small amounts of MTBE were detected in the plant tissues, but a large fraction of the applied MTBE was found in the air through plant transpiration. Mass balance studies showed that MTBE was assimilated into the plants from hydroponic solution but was not metabolized during transport in the plant. Phytovolatilization was the only relevant removal process for MTBE. Transpiration stream concentration factor (TSCF), an important parameter for design of engineered MTBE phytoremediation systems, was estimated to be 1.12. In conclusion, although this compound is persistent to the attack by plant enzymes, atmospheric MTBE is much more susceptible to photo-oxidation for decomposition. Phytoremediation of MTBE polluted soils and groundwater is an alternative to presently available remediation technologies. © 2006 Elsevier B.V. All rights reserved. Keywords: Metabolism; Methyl tert-butyl ether (MTBE); Phytoremediation; Toxicity; Willows 1. Introduction The large volume and commercial use of methyl tert-butyl ether (MTBE) as a fuel oxygenate, replacing alkyl lead addi- tives, can be traced back to the late 1970s in the United States to deal with air pollution. Over the past several decades MTBE as additives to gasoline intended to either boost ratings of fuel or to reduce air pollution has been accepted worldwide. The annual production of MTBE increased from 1.38 billion lb in 1984 to 24.1 billion lb in 1993, making it the second on the list of organic chemicals being manufactured in the U.S. [1]. In 1998, approximately 24% of all gasoline contained oxy- genates in the U.S. [2]. The cause of MTBE leaking into the environment is mainly attributed to gasoline spills and leaks ∗ Corresponding author. Tel.: +852 2299 0605; fax: +852 2517 6082. E-mail address: jdgu@hkucc.hku.hk (J.-D. Gu). from pipelines, underground- and aboveground-storage tanks, and transport accidents [3]. Due to its high solubility in water and low sorption tendency in soils, MTBE can rapidly penetrate the soil layer and enter the groundwater shortly after the spill. It has become one of the most problematic pollutants in urban soils and groundwater worldwide [4]. A draft lifetime health advisory limit of 20–35 gL -1 has been issued in the United States [5]. The toxicity of MTBE to animals and humans is well doc- umented. It has been established that MTBE is carcinogenic to animals [6]. Inhalation of MTBE vapors may cause headaches and nausea at higher concentrations (ppm) for people [7]. Respi- ratory, neurological, cardiac, and allergic symptoms associated with chronic and low level exposure of MTBE were also reported [8]. The U.S. EPA has classified MTBE as a possible human car- cinogen [5]. Toxicity of MTBE to algae, invertebrates, and fish has been intensively studied [9–12]. No work, however, has been conducted on MTBE toxicity to terrestrial plants. In the literature review by Nellessen and Fletche [13], the response of vascular 0304-3894/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2006.04.024