Toxicity and carcinogenicity of t-butyl alcohol in rats and mice following chronic exposure in drinking water

Methyl-tert-butyl ether (MTBE): integration of rat and mouse carcinogenicity data with mode of action and human and rodent bioassay dosimetry and toxicokinetics indicates MTBE is not a plausible human carcinogen

Methyl-tert-butyl ether (MTBE) is a fuel oxygenate used in non-United States geographies. Multiple health reviews conclude that MTBE is not a human-relevant carcinogen, and this review provides updated mode of action (MOA), exposure, dosimetry and risk perspectives supporting those conclusions. MTBE is non-genotoxic and has large margins of exposure between blood concentrations at the overall rat 400 ppm inhalation NOAEL and blood concentrations in typical workplace or general population exposures. Non-cancer and threshold cancer hazard quotients range from a high of 0.046 for fuel-pump gasoline station attendants and are 100-1,000-fold lower for general population exposures. Cancer risks conservatively assuming genotoxicity for these same scenarios are all less than 1 × 10^-6. The onset of MTBE nonlinear toxicokinetics (TK) in rats at inhalation exposures less than 3,000 ppm, a dose that is also not practically achievable in fuel-use scenarios, indicates that high-dose specific male rat kidney and testes (3,000 and 8,000 ppm) and female mouse liver tumors (8000 ppm) are not quantitatively relevant to humans. Mode of action analyses also indicate MTBE male rat kidney tumors, and lesser so female mouse liver tumors, are not qualitatively relevant to humans. Thus, an integrated analysis of the toxicology, exposure/dosimetry, TK, and MOA data indicates that MTBE presents minimal human cancer and non-cancer risks.

Adverse outcome pathway (AOP): α2u-globulin nephropathy and kidney tumors in male rats

The National Research Council's vision of using adverse outcome pathways (AOPs) as a framework to assist with toxicity assessment for regulatory requirements of chemical assessment has continued to gain traction since its release in 2007. The need to expand the AOP knowledge base has gained urgency, with the U.S. Environmental Protection Agency's directive to eliminate reliance on animal toxicity testing by 2035. To meet these needs, our goal was to elucidate the AOP for male-rat-specific kidney cancer. Male-rat-specific kidney tumors occur through the ability of structurally diverse substances to induce α2u-globulin nephropathy (α2u-N), a well-studied mode of action (MoA) not relevant in humans that results in kidney tumor formation in male rats. An accepted AOP may help facilitate the differentiation from other kidney tumors MoAs. Following identification and review of relevant in vitro and in vivo literature, both the MIE and subsequent KEs were identified. Based on the weight of evidence from the various resources, the confidence in this AOP is high. Uses of this AOP include hazard identification, development of in vitro assays to determine if the MoA is through α2u-N and not relevant to humans resulting in decreased use of animals, and regulatory applications.

Immobilized laccase on zinc oxide nanoarray for catalytic degradation of tertiary butyl alcohol

Laccase is an effective biocatalyst in bioremediation process; however, the application of the enzyme is limited due to its cost, recovery, and stability. In this study, we developed, characterized and evaluated the efficiency of immobilized laccase on zinc oxide nanostructure to catalyze biodegradation of TBA in comparison to the suspended enzyme. The results showed that both immobilized and suspended laccase were capable of catalyzing TBA biodegradation; however, the efficiency of the immobilized laccase on TBA removal was higher than that of the suspended enzyme. The repeatability testing revealed the potential of the immobilized laccase for repeatedly catalyzing TBA biodegradation with storage capacity. While the V_max of immobilized enzyme was higher than suspended laccase (2.25 ± 0.542 mg TBA/h∙U vs. 1.47 ± 0.185 mg TBA/h∙U), the k_m of the immobilized enzyme was higher than the suspended laccase (67.9 ± 20.5 mg TBA/L vs. 33.5 ± 7.10 mg TBA/L). This suggests that the immobilized laccase is better in TBA removal, but has lower affinity with TBA than the suspended enzyme. Thus, immobilization of the enzyme can be applied to increase the efficiency and minimize the use of laccase for TBA remediation.

Aldehyde dehydrogenase 2 deficiency significantly exacerbates tert-butyl alcohol-induced toxicity in mice

Owing to the use of ethyl tert-butyl ether (ETBE) as a fuel additive, the possible adverse effects of ETBE exposure have become a public concern. Our previous study showed that ETBE-induced toxicity in aldehyde dehydrogenase 2 (Aldh2) gene knockout (KO) mice was caused by its primary metabolite acetaldehyde, which was toxic. However, it is unclear whether tert-butyl alcohol (TBA), another main metabolite of ETBE, plays a role in ETBE-induced toxicity. To investigate this relationship, we analyzed the changes of TBA concentrations in tissues after ETBE exposure, and then evaluated the toxicity after direct TBA treatment in both KO and wild-type (WT) mice. An exposure to 500 ppm ETBE via inhalation resulted in the formation of its three metabolites, TBA, 2-methyl-1,2-propanediol and ethanol, whose concentrations in the liver, brain, fat and testis of male KO mice were significantly higher than the corresponding concentrations observed in male WT mice. Direct treatment to TBA (20 mg/mL of drinking water) caused significant changes in relative organ weights and histopathology, and increased levels of genetic damages in both types of mice. These toxic effects were also seen in KO mice exposed to a lower concentration of TBA (5 mg/mL), which was associated with increased oxidative stress in serum (reduced glutathione and reduced glutathione/oxidized glutathione ratio decreased). Our findings indicate that ALDH2 is involved in the metabolism of ETBE and TBA, and ALDH2 deficiency could greatly increase the sensitivity to TBA-induced toxicity.

Physiologically based pharmacokinetic model for ethyl tertiary-butyl ether and tertiary-butyl alcohol in rats: Contribution of binding to α2u-globulin in male rats and high-exposure nonlinear kinetics to toxicity and cancer outcomes

In cancer bioassays, inhalation, but not drinking water exposure to ethyl tertiary-butyl ether (ETBE), caused liver tumors in male rats, while tertiary-butyl alcohol (TBA), an ETBE metabolite, caused kidney tumors in male rats following exposure via drinking water. To understand the contribution of ETBE and TBA kinetics under varying exposure scenarios to these tumor responses, a physiologically based pharmacokinetic model was developed based on a previously published model for methyl tertiary-butyl ether, a structurally similar chemical, and verified against the literature and study report data. The model included ETBE and TBA binding to the male rat-specific protein α2u-globulin, which plays a role in the ETBE and TBA kidney response observed in male rats. Metabolism of ETBE and TBA was described as a single, saturable pathway in the liver. The model predicted similar kidney AUC_0-∞ for TBA for various exposure scenarios from ETBE and TBA cancer bioassays, supporting a male-rat-specific mode of action for TBA-induced kidney tumors. The model also predicted nonlinear kinetics at ETBE inhalation exposure concentrations above ~2000 ppm, based on blood AUC_0-∞ for ETBE and TBA. The shift from linear to nonlinear kinetics at exposure concentrations below the concentration associated with liver tumors in rats (5000 ppm) suggests the mode of action for liver tumors operates under nonlinear kinetics following chronic exposure and is not relevant for assessing human risk. Copyright © 2016 The Authors Journal of Applied Toxicology Published by John Wiley & Sons Ltd.

Reassessment of MTBE cancer potency considering modes of action for MTBE and its metabolites

A 1999 California state agency cancer potency (CP) evaluation of methyl tert-butyl ether (MTBE) assumed linear risk extrapolations from tumor data were plausible because of limited evidence that MTBE or its metabolites could damage DNA, and based such extrapolations on data from rat gavage and rat and mouse inhalation studies indicating elevated tumor rates in male rat kidney, male rat Leydig interstitial cells, and female rat leukemia/lymphomas. More recent data bearing on MTBE cancer potency include a rodent cancer bioassay of MTBE in drinking water; several new studies of MTBE genotoxicity; several similar evaluations of MTBE metabolites, formaldehyde, and tert-butyl alcohol or TBA; and updated evaluations of carcinogenic mode(s) of action (MOAs) of MTBE and MTBE metabolite's. The lymphoma/leukemia data used in the California assessment were recently declared unreliable by the U.S. Environmental Protection Agency (EPA). Updated characterizations of MTBE CP, and its uncertainty, are currently needed to address a variety of decision goals concerning historical and current MTBE contamination. To this end, an extensive review of data sets bearing on MTBE and metabolite genotoxicity, cytotoxicity, and tumorigenicity was applied to reassess MTBE CP and related uncertainty in view of MOA considerations. Adopting the traditional approach that cytotoxicity-driven cancer MOAs are inoperative at very low, non-cytotoxic dose levels, it was determined that MTBE most likely does not increase cancer risk unless chronic exposures induce target-tissue toxicity, including in sensitive individuals. However, the corresponding expected (or plausible upper bound) CP for MTBE conditional on a hypothetical linear (e.g., genotoxic) MOA was estimated to be ∼2 × 10(-5) (or 0.003) per mg MTBE per kg body weight per day for adults exposed chronically over a lifetime. Based on this conservative estimate of CP, if MTBE is carcinogenic to humans, it is among the weakest 10% of chemical carcinogens evaluated by EPA.

Forensic analysis of tertiary-butyl alcohol (TBA) detections in a hydrocarbon-rich groundwater basin

Tertiary-butyl alcohol (TBA), a high-production volume (HPV) chemical, was sporadically detected in groundwater and coalbed methane (CBM) wells in southeastern Colorado's hydrocarbon-rich Raton Basin. TBA concentrations in shallow water wells averaged 75.1 μg/L, while detections in deeper CBM wells averaged 14.4 μg/L. The detection of TBA prompted a forensic investigation to try to identify potential sources. Historic and recent data were reviewed to determine if there was a discernable pattern of TBA occurrence. Supplemental samples from domestic water wells, monitor wells, CBM wells, surface waters, and hydraulic fracturing (HF) fluids were analyzed for TBA in conjunction with methyl tertiary-butyl ether (MTBE) and ethyl tertiary-butyl ether (ETBE), proxies for evidence of contamination from reformulated gasoline or associated oxygenates. Exploratory microbiological sampling was conducted to determine if methanotrophic organisms co-occurred with TBA in individual wells. Meaningful comparisons of historic TBA data were limited due to widely varying reporting limits. Mapping of TBA occurrence did not reveal any spatial patterns or physical associations with CBM operations or contamination plumes. Additionally, TBA was not detected in HF fluids or surface water samples. Given the widespread use of TBA in industrial and consumer products, including water well completion materials, it is likely that multiple diffuse sources exist. Exploratory data on stable isotopes, dissolved gases, and microbial profiling provide preliminary evidence that methanotrophic activity may be producing TBA from naturally occurring isobutane. Reported TBA concentrations were significantly below a conservative risk-based drinking water screening level of 8000 μg/L derived from animal toxicity data.

Combined toxicities of methyl tert-butyl ether and its metabolite tert-butyl alcohol on earthworms via different exposure routes

Methyl tert-butyl ether (MTBE) and tert-butyl alcohol (TBA) are among the major soil contaminants that threaten the health of soil ecosystems. Many MTBE-contaminated sites accumulate TBA, because TBA is the intermediate of MTBE biodegradation. To access the risk of MTBE and TBA in soil, we investigated the combined toxicities of MTBE and TBA using two earthworm species, Perionyx excavatus and Eisenia andrei, as well as the toxic effects via different exposure routes. The combined toxicity showed weak antagonistic effects (LC50mix values were slightly greater than 1.0), and sensitivity toward same pollutants differed in the two earthworm species. Moreover, the toxicity of MTBE and TBA was also affected by the exposure route; both filter paper and artificial soil tests showed that dermal-only exposure to MTBE had an even greater toxic effect than combined dermal and oral exposure. Thus, we suggest that diverse environmental factors including organic materials, the physicochemical properties of the contact media, and the exposure routes of the organism, should be taken into consideration when assessing the effects of pollutants on organisms in diverse environmental systems.

Ethyl tert-butyl ether (ETBE)-degrading microbial communities in enrichments from polluted environments

The ethyl tert-butyl ether (ETBE) degradation capacity and phylogenetic composition of five aerobic enrichment cultures with ETBE as the sole carbon and energy source were studied. In all cases, ETBE was entirely degraded to biomass and CO2. Clone libraries of the 16S rRNA gene were prepared from each enrichment. The analyses of the DNA sequences obtained showed different taxonomic compositions with a majority of Proteobacteria in three cases. The two other enrichments have different microbiota with an abundance of Acidobacteria in one case, whereas the microbiota in the second was more diverse (majority of Actinobacteria, Chlorobi and Gemmatimonadetes). Actinobacteria were detected in all five enrichments. Several bacterial strains were isolated from the enrichments and five were capable of degrading ETBE and/or tert-butyl alcohol (TBA), a degradation intermediate. The five included three Rhodococcus sp. (IFP 2040, IFP 2041, IFP 2043), one Betaproteobacteria (IFP 2047) belonging to the Rubrivivax/Leptothrix/Ideonella branch, and one Pseudonocardia sp. (IFP 2050). Quantification of these five strains and two other strains, Rhodococcus sp. IFP 2042 and Bradyrhizobium sp. IFP2049, which had been previously isolated from one of the enrichments was carried out on the different enrichments based on quantitative PCR with specific 16S rRNA gene primers and the results were consistent with the hypothesized role of Actinobacteria and Betaproteobacteria in the degradation of ETBE and the possible role of Bradyrhizobium strains in the degradation of TBA.

No Promoting Effect of Ethyl Tertiary-butyl Ether (ETBE) on Rat Urinary Bladder Carcinogenesis Initiated with N-Butyl-N-(4-hydroxybutyl)nitrosamine

The effects of ethyl tertiary-butyl ether (ETBE) on two-stage urinary bladder carcinogenesis in male F344 rats initiated with N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) were investigated at various dose levels with regard to possible promoting activity. Groups of 30 rats were given drinking water containing 500 ppm BBN, as an initiator, for 4 weeks and starting one week thereafter received ETBE by gavage (daily, 7 days/week) at dose levels of 0 (control), 100, 300, 500 or 1000 mg/kg/day until experimental week 36. No statistically significant differences in incidences of preneoplastic lesions, papillomas, and carcinomas of the urinary bladder were evident in rats treated with 100–1000 mg/kg/day ETBE as compared with control values. Furthermore, the average numbers of preneoplastic or neoplastic lesions per unit length of basement membrane in rats given 100–1000 mg/kg/day ETBE were also comparable to control values. However, papillomatosis of the urinary bladder was found in 4 out of 30 rats (13%) in the group given 1000 mg/kg/day ETBE, and soft stones in the urinary bladder were found in 3 out of these 4 rats. The results thus demonstrated that ETBE did not exert promotional activity on urinary bladder carcinogenesis. However, papillomatosis of the urinary bladder developed in small numbers of the rats given ETBE at 1000 mg/kg/day but not in rats given 500 mg/kg/day or lower doses.

Differential genotoxic effects of subchronic exposure to ethyl tertiary butyl ether in the livers of Aldh2 knockout and wild-type mice

Ethyl tertiary butyl ether (ETBE) is used as an additive to gasoline to reduce carbon monoxide emissions in some developed countries. So far, ETBE was not found with positive results in many genotoxic assays. This study is undertaken to investigate the modifying effects of deficiency of aldehyde dehydrogenase 2 (ALDH2) on the toxicity of ETBE in the livers of mice. Eight-week-old wild-type (WT) and Aldh2 knockout (KO) C57BL/6 mice of both sexes were exposed to 0, 500, 1,750, and 5,000 ppm ETBE for 6 h/day with 5 days per weeks for 13 weeks. Histopathology assessments and measurements of genetic effects in the livers were performed. Significantly increased accidences of centrilobular hypertrophy were observed in the livers of WT and KO mice of both sexes in 5,000 ppm group; there was a sex difference in centrilobular hypertrophy between male and female KO mice, with more severe damage in the males. In addition, DNA strand breaks, 8-hydroxyguanine DNA-glycosylase (hOGG1)-modified oxidative base modification, and 8-hydroxydeoxyguanosine as genetic damage endpoints were significantly increased in three exposure groups in KO male mice, while these genotoxic effects were only found in 5,000 ppm group of KO female mice. In WT mice, significant DNA damage was seen in 5,000 ppm group of male mice, but not in females. Thus, sex differences in DNA damage were found not only in KO mice, but also in WT mice. These results suggest that ALDH2 polymorphisms and sex should be taken into considerations in predicting human health effects of ETBE exposure.

Tertiary-Butanol: a toxicological review

Tert-Butanol is an important intermediate in industrial chemical synthesis, particularly of fuel oxygenates. Human exposure to tert-butanol may occur following fuel oxygenate metabolism or biodegradation. It is poorly absorbed through skin, but is rapidly absorbed upon inhalation or ingestion and distributed to tissues throughout the body. Elimination from blood is slower and the half-life increases with dose. It is largely metabolised by oxidation via 2-methyl-1,2-propanediol to 2-hydroxyisobutyrate, the dominant urinary metabolites. Conjugations also occur and acetone may be found in urine at high doses. The single-dose systemic toxicity of tert-butanol is low, but it is irritant to skin and eyes; high oral doses produce ataxia and hypoactivity and repeated exposure can induce dependence. Tert-Butanol is not definable as a genotoxin and has no effects specific for reproduction or development; developmental delay occurred only with marked maternal toxicity. Target organs for toxicity clearly identified are kidney in male rats and urinary bladder, particularly in males, of both rats and mice. Increased tumour incidences observed were renal tubule cell adenomas in male rats and thyroid follicular cell adenomas in female mice and, non-significantly, at an intermediate dose in male mice. The renal adenomas were associated with alpha(2u)-globulin nephropathy and, to a lesser extent, exacerbation of chronic progressive nephropathy. Neither of these modes of action can function in humans. The thyroid tumour response could be strain-specific. No thyroid toxicity was observed and a study of hepatic gene expression and enzyme induction and thyroid hormone status has suggested a possible mode of action.

Differential toxic effects of methyl tertiary butyl ether and tert-butanol on rat fibroblasts in vitro

Methyl tertiary butyl ether (MTBE) is the most widely used motor vehicle fuel oxygenate since it reduces harmful emissions due to gasoline combustion. However, the significant increase in its use in recent years has raised new questions related to its potential toxicity. In fact, although available data are somehow conflicting, there is evidence that MTBE is a toxic substance that may have harmful effects on both animals and humans and an unresolved problem is the role played by MTBE metabolites, especially tertiary butyl alcohol (TBA), in determining toxic effects due to MTBE exposure. In this study, the toxic effects of MTBE have been analyzed on a normal diploid rat fibroblast cell line (Rat-1) and compared to the effects of TBA. The results obtained suggest that both MTBE and TBA inhibit cell growth in vitro but with different mechanisms in terms of effects on the cell cycle progression and on the modulation of cell cycle regulatory proteins. In fact, MTBE caused an accumulation of cells in the S-phase of the cell cycle, whereas TBA caused an accumulation in the G0/G1-phase with different effects on the expression of cyclin D1, p27Kip1, and p53. Moreover, both MTBE and TBA were also shown to induce DNA damage, as assessed in terms of oxidative DNA damage and nuclear DNA fragmentation, that appeared to be susceptible of repair by the cell DNA-repair machinery. In conclusion, these findings suggest that both MTBE and TBA can exert, by acting through different molecular mechanisms, important biological effects on fibroblasts in vitro. Further studies are warranted to shed light on the mechanisms responsible for the observed effects and on their potential significance for the in-vivo exposure.

Physiologically based pharmacokinetic model of methyl tertiary butyl ether and tertiary butyl alcohol dosimetry in male rats based on binding to alpha2u-globulin

Current physiologically based pharmacokinetic (PBPK) models for the fuel additive methyl tertiary butyl ether (MTBE) and its metabolite tertiary butyl alcohol (TBA) have not included a mechanism for chemical binding to the male rat-specific protein alpha2u-globulin, which has been postulated to be responsible for renal effects in male rats observed in toxicity and carcinogenicity studies with MTBE. The objective of this work was to expand the previously published models for MTBE to include binding to alpha2u-globulin in the kidney of male rats. In the model, metabolism of MTBE was assumed to occur only in the liver via two saturable pathways. TBA metabolism was assumed to occur only in the liver via one saturable, low-affinity pathway and to be inducible following repeated exposures. The binding of MTBE and TBA to alpha2u-globulin was modeled as saturable and competitive and was assumed to only affect the rate of hydrolysis of alpha2u-globulin in the kidney. The developed model characterized the differences in kidney concentrations of MTBE and TBA in male versus female rats from inhalation exposures to MTBE, as well as the observed changes in blood and tissue concentrations from repeated exposure to TBA. The model-predicted binding affinity of MTBE to alpha2u-globulin was greater than TBA, and the hydrolysis rate of chemically bound alpha2u-globulin was approximately 30% of the unbound protein. This PBPK model supports the role of MTBE and TBA binding to the male rat-specific protein alpha2u-globulin as essential for predicting concentrations of these chemicals in the kidney following exposure.

Degradation of tert-butyl formate and its intermediates by an ozone/UV process

In this paper, the oxidation of tert-butyl formate (TBF) in aqueous solution by an ozone/UV process was described. The oxidation process was investigated experimentally in a semibatch reactor. The results of the study indicated that the ozone/UV process was very effective in oxidizing TBF. tert-Butyl alcohol (TBA), hydroxy-iso-butyraldehyde (HiBA), acetone, formaldehyde, and formic acid were identified as major primary intermediates during the oxidation of TBF. About 90% organic carbon balance was obtained indicating that most reaction intermediates have been identified and quantified. Some of the primary intermediates were also oxidized in the ozone/UV system. Accordingly, HiBA, acetone, formaldehyde, and formic acid were the primary intermediates of TBA oxidation. The oxidation of acetone in the ozone/UV system generated formaldehyde, pyruvaldehyde, acetic acid, formic acid as primary intermediates. It was also observed that the reaction intermediates formed during the oxidation of TBF react well in the ozone/UV system and complete mineralization could be achieved by the process.

Methyltertiary-Butyl Ether: Studies for Potential Human Health Hazards

When methyl tertiary-butyl ether (MTBE) in gasoline was first introduced to reduce vehicle exhaust emissions and comply with the Clean Air Act, in the United States, a pattern of complaints emerged characterised by seven "key symptoms." Later, carefully controlled volunteer studies did not confirm the existence of the specific key symptoms, although one study of self-reported sensitive (SRS) people did suggest that a threshold at about 11-15% MTBE in gasoline may exist for SRSs in total symptom scores. Neurobehavioral and psychophysiological studies on volunteers, including SRSs, found no adverse responses associated with MTBE at likely exposure levels. MTBE is well and rapidly absorbed following oral and inhalation exposures. Cmax values for MTBE are achieved almost immediately after oral dosing and within 2 h of continuous inhalation. It is rapidly eliminated, either by exhalation as unchanged MTBE or by urinary excretion of its less volatile metabolites. Metabolism is more rapid humans than in rats, for both MTBE and tert-butyl alcohol (TBA), its more persistent primary metabolite. The other primary metabolite, formaldehyde, is detoxified at a rate very much greater than its formation from MTBE. MTBE has no specific effects on reproduction or development, or on genetic material. Neurological effects were observed only at very high concentrations. In carcinogenicity studies of MTBE, TBA, and methanol (included as an endogenous precursor of formaldehyde, without the presence of TBA), some increases in tumor incidence have been observed, but consistency of outcome was lacking and even some degree of replication was observed in only three cases, none of which had human relevance: alpha(2u)-globulin nephropathy-related renal tubule cell adenoma in male rats; Leydig-cell adenoma in male rats, but not in mice, which provide the better model of the human disease; and B-cell-derived lymphoma/leukemia of doubtful pathogenesis that arose mainly in lungs of orally dosed female rats. In addition, hepatocellular adenomas were significantly higher in female CD-1 mice and thyroid follicular-cell adenomas were increased in female B6C3F1 mice treated with TBA, but these results lack any independent confirmation, which would have been possible from a number of other studies.

Mineralization of methyl tert-butyl ether and other gasoline oxygenates by Pseudomonads using short n-alkanes as growth source

Biodegradation of methyl tert-butyl ether (MTBE) by cometabolism has shown to produce recalcitrant metabolic intermediates that often accumulate. In this work, a consortium containing Pseudomonads was studied for its ability to fully degrade oxygenates by cometabolism. This consortium mineralized MTBE and TBA with C3-C7 n-alkanes. The highest degradation rates for MTBE (75 +/- 5 mg g(protein) (-1) h(-1)) and TBA (86.9 +/- 7.3 mg g(protein) (-1) h(-1)) were obtained with n-pentane and n-propane, respectively. When incubated with radiolabeled MTBE and n-pentane, it converted more than 96% of the added MTBE to (14)C-CO(2). Furthermore, the consortium degraded tert-amyl methyl ether, tert-butyl alcohol (TBA), tert-amyl alcohol, ethyl tert-butyl ether (ETBE) when n-pentane was used as growth source. Three Pseudomonads were isolated but only two showed independent MTBE degradation activity. The maximum degradation rates were 101 and 182 mg g(protein) (-1) h(-1) for Pseudomonas aeruginosa and Pseudomonas citronellolis, respectively. The highest specific affinity (a degrees (MTBE)) value of 4.39 l g(protein) (-1) h(-1) was obtained for Pseudomonas aeruginosa and complete mineralization was attained with a MTBE: n-pentane ratio (w/w) of 0.7. This is the first time that Pseudomonads have been reported to fully mineralize MTBE by cometabolic degradation.

Effects of tert-butyl acetate on maternal toxicity and embryo-fetal development in Sprague-Dawley rats

This study investigated the potential adverse effects of tert-butyl acetate (TBAc) on maternal toxicity and embryo-fetal development after maternal exposure of pregnant rats from gestational days 6 through 19. TBAc was administered to pregnant rats by gavage at 0, 400, 800, and 1,600 mg/kg/day. All dams were subjected to a Caesarean section on day 20 of gestation, and their fetuses were examined for any morphological abnormalities. At 1,600 mg/kg, maternal toxicity manifested as increases in the incidence of clinical signs and death, lower body weight gain and food intake, increases in the weights of adrenal glands and liver, and a decrease in thymus weight. Developmental toxicity included a decrease in fetal weight, an increase in the incidence of skeletal variation, and a delay in fetal ossification. At 800 mg/kg, only a minimal developmental toxicity, including an increase in the incidence of skeletal variation and a delay in fetal ossification, were observed. In contrast, no adverse maternal or developmental effects were observed at 400 mg/kg. These results show that a 14-day repeated oral dose of TBAc is embryotoxic at a maternally toxic dose (i.e., 1,600 mg/kg/day) and is minimally embryotoxic at a nonmaternally toxic dose (i.e., 800 mg/kg/day) in rats. However, no evidence for the teratogenicity of TBAc was noted in rats. It is concluded that the developmental findings observed in the present study are secondary effects to maternal toxicity. Under these experimental conditions, the no-observed-adverse-effect level of TBAc is considered to be 800 mg/kg/day for dams and 400 mg/kg/day for embryo-fetal development.

Comparative transcriptome analysis of Methylibium petroleiphilum PM1 exposed to the fuel oxygenates methyl tert-butyl ether and ethanol

High-density whole-genome cDNA microarrays were used to investigate substrate-dependent gene expression of Methylibium petroleiphilum PM1, one of the best-characterized aerobic methyl tert-butyl ether (MTBE)-degrading bacteria. Differential gene expression profiling was conducted with PM1 grown on MTBE and ethanol as sole carbon sources. Based on microarray high scores and protein similarity analysis, an MTBE regulon located on the megaplasmid was identified for further investigation. Putative functions for enzymes encoded in this regulon are described with relevance to the predicted MTBE degradation pathway. A new unique dioxygenase enzyme system that carries out the hydroxylation of tert-butyl alcohol to 2-methyl-2-hydroxy-1-propanol in M. petroleiphilum PM1 was discovered. Hypotheses regarding the acquisition and evolution of MTBE genes as well as the involvement of IS elements in these complex processes were formulated. The pathways for toluene, phenol, and alkane oxidation via toluene monooxygenase, phenol hydroxylase, and propane monooxygenase, respectively, were upregulated in MTBE-grown cells compared to ethanol-grown cells. Four out of nine putative cyclohexanone monooxygenases were also upregulated in MTBE-grown cells. The expression data allowed prediction of several hitherto-unknown enzymes of the upper MTBE degradation pathway in M. petroleiphilum PM1 and aided our understanding of the regulation of metabolic processes that may occur in response to pollutant mixtures and perturbations in the environment.

Temperature effect on tert-butyl alcohol (TBA) biodegradation kinetics in hyporheic zone soils

Background

Remediation of tert-butyl alcohol (TBA) in subsurface waters should be taken into consideration at reformulated gasoline contaminated sites since it is a biodegradation intermediate of methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and tert-butyl formate (TBF). The effect of temperature on TBA biodegradation has not been not been published in the literature.

Methods

Biodegradation of [U 14C] TBA was determined using hyporheic zone soil microcosms.

Results

First order mineralization rate constants of TBA at 5°C, 15°C and 25°C were 7.84 ± 0.14 × 10-3, 9.07 ± 0.09 × 10-3, and 15.3 ± 0.3 × 10-3 days-1, respectively (or 2.86 ± 0.05, 3.31 ± 0.03, 5.60 ± 0.14 years-1, respectively). Temperature had a statistically significant effect on the mineralization rates and was modelled using the Arrhenius equation with frequency factor (A) and activation energy (Ea) of 154 day-1 and 23,006 mol/J, respectively.

Conclusion

Results of this study are the first to determine mineralization rates of TBA for different temperatures. The kinetic rates determined in this study can be used in groundwater fate and transport modelling of TBA at the Ronan, MT site and provide an estimate for TBA removal at other similar shallow aquifer sites and hyporheic zones as a function of seasonal change in temperature.

Ethyl tertiary-butyl ether: a toxicological review

A number of oxygenated compounds (oxygenates) are available for use in gasoline to reduce vehicle exhaust emissions, reduce the aromatic compound content, and avoid the use of organo-lead compounds, while maintaining high octane numbers. Ethyl tertiary-butyl ether (ETBE) is one such compound. The current use of ETBE in gasoline or petrol is modest but increasing, with consequently similar trends in the potential for human exposure. Inhalation is the most likely mode of exposure, with about 30% of inhaled ETBE being retained by the lungs and distributed around the body. Following cessation of exposure, the blood concentration of ETBE falls rapidly, largely as a result of its metabolism to tertiary-butyl alcohol (TBA) and acetaldehyde. TBA may be further metabolized, first to 2-methyl-1,2-propanediol and then to 2-hydroxyisobutyrate, the two dominant metabolites found in urine of volunteers and rats. The rapid oxidation of acetaldehyde suggests that its blood concentration is unlikely to rise above normal as a result of human exposure to sources of ETBE. Single-dose toxicity tests show that ETBE has low toxicity and is essentially nonirritant to eyes and skin; it did not cause sensitization in a maximization test in guinea pigs. Neurological effects have been observed only at very high exposure concentrations. There is evidence for an effect of ETBE on the kidney of rats. Increases in kidney weight were seen in both sexes, but protein droplet accumulation (with alpha(2u)-globulin involvement) and sustained increases in cell proliferation occurred only in males. In liver, centrilobular necrosis was induced in mice, but not rats, after exposure by inhalation, although this lesion was reported in some rats exposed to very high oral doses of ETBE. The proportion of liver cells engaged in S-phase DNA synthesis was increased in mice of both sexes exposed by inhalation. ETBE has no specific effects on reproduction, development, or genetic material. Carcinogenicity studies have been conducted with ETBE, TBA, and ethanol (included in this review as an endogenous precursor of acetaldehyde in the absence of TBA). A single experiment with ETBE in rats and several experiments with ethanol in rats and mice were not considered adequate for an evaluation of ETBE carcinogenicity. In male rats only, TBA induced alpha(2u)-globulin nephropathy-related renal tubule adenomas. These are generally considered to have no human relevance. In addition, increases in thyroid follicular cell adenoma incidence were associated with TBA treatment in female mice. This result lacks independent confirmation and is not supported by experiments in which similar or higher internal doses of TBA were delivered.

Methyl tertiary-butyl ether mode of action for cancer endpoints in rodents

There are no reports of studies that evaluate if methyl tertiary-butyl ether (MTBE) exposure causes cancer in humans. This evaluation of MTBE carcinogenicity is based on the results of animal studies. A weak tumorigenic response was reported for both MTBE and TBA in one tumor type (kidney) in male rats, for MTBE in one other tumor type (testicular) in male rats, for MTBE in one tumor type (liver) in female mice, and for TBA in one tumor type (thyroid) in female mice. The weight of the evidence does not support a genotoxic mode of action (MOA). Non-genotoxic MOAs have been demonstrated or suggested that correspond to the weak tumorigenic responses. These MOAs either do not occur in humans or humans are much less susceptible to these effects. It is, therefore, unlikely that humans would be exposed to sufficient levels of MTBE to cause these tumorigenic responses.

Amended final report of the safety assessment of t-Butyl Alcohol as used in cosmetics

t-Butyl Alcohol (t-BuOH) is a tertiary aliphatic alcohol that is used as a solvent or an alcohol denaturant and as a perfume carrier in cosmetics. t-BuOH was reported as an ingredient in 32 formulations of eye makeup, fragrance, and shaving preparations, at concentrations ranging from 0.00001% and 0.3%. There is little acute oral toxicity in animals; e.g., the acute oral LD(50) in rats was 3.0 to 3.7 g/kg. In short-term oral studies in rats, t-BuOH at 2% (w/v) or less in drinking water did not cause gross organ or tissue damage in mice, although weight loss was reported and microscopic damage to livers and kidney and alterations such as centrilobular necrosis, vacuolation in hepatocytes, and loss of hepatic architecture were noted. Subchronic oral dosing with t-BuOH increased the mineralization of the kidney, nephropathy, and urinary bladder transitional cell epithelial hyperplasia in rats; and liver damage, chronic inflammation, hyperplasia of transitional cell epithelium urinary, and proliferative changes including hyperplasia and neoplasia in the thyroid in mice. Male rats exposed to t-BuOH were susceptible to alpha 2mu-globulin nephropathy. t-BuOH (99.9%) was a moderate to severe ocular irritant to rabbits and caused mild to moderate dermal irritation to rabbits. It was not considered to be a primary dermal irritant to rabbits. In animal studies, fetotoxicity generally increased with concentration, and fetal weights were slightly depressed at concentrations of 0.5% to 1% t-BuOH. t-BuOH produced a significant increase in the number of resorptions per litter. There was also a significant decrease in the number of live fetuses per litter. t-BuOH reduced maternal weight gain, litter sizes, birth weights, and weights at weaning, and increased perinatal and postnatal mortality. t-BuOH was not mutagenic in several bacterial and mammalian test systems. The principal effects from 2 years of exposure to t-BuOH in drinking water (up to 10 mg/ml for rats and 20 mg/ml for mice) were proliferative lesions (hyperplasia, adenoma, and carcinoma) in the kidneys of exposed male rats, and nephropathy in all exposed groups of female rats. There was some evidence of carcinogenic activity, but it was not consistent between species, sexes, or doses. A repeat-insult patch test (RIPT) test showed no potential for eliciting either dermal irritation or sensitization by 100% t-BuOH. Dermatitis can result from dermal exposure of humans to t-BuOH. In consideration of these data, it was concluded that t-BuOH was (at most) a weak carcinogen and unlikely to have significant carcinogenic potential as currently used in cosmetic formulations. In addition, the renal tubule effects found in male rats were likely an effect of alpha 2mu-globulin. In consideration of the reproductive and developmental toxicity data, the increased incidence of still births occurred at high exposure levels and was likely secondary to maternal toxicity. Based on the available animal and clinical data in this report, it was concluded that t-BuOH is safe as used in cosmetic products.

Acute toxicity and cancer risk assessment values for tert-butyl acetate

tert-Butyl acetate (TBAc) is an industrial chemical with potential uses as a degreaser and in architectural coatings. Limited chronic toxicity data exist for TBAc. However, acute inhalation exposure data are available for TBAc. Additionally, TBAc has been demonstrated to be substantially metabolized to tert-butanol (TBA) in rats, and a positive TBA genotoxicity study suggests that TBA may cause oxidative DNA damage. TBA has been shown to induce tumors in both rats and mice, and the Office of Environmental Health Hazard Assessment has calculated an oral cancer potency factor (CSF) for TBA of 3 x 10(-3)(mg/kg-day)(-1). Therefore, TBAc should be considered to pose a potential cancer risk to humans because of the metabolic conversion to TBA. An acute 1-h reference exposure level of 1 mg/m3 can be calculated from the extrapolated no observed adverse effect level of 50 mg/m3. A CSF of 0.002(mg/kg-day)(-1) can be derived for TBAc, assuming 100% metabolism of TBAc to TBA. An inhalation unit risk value for TBAc of 4 x 10(-7)(microg/m(3))(-1) can then be derived from the CSF value for TBAc by assuming a human breathing rate of 20 m3/day, 70% fractional absorption, and an average human body weight of 70 kg.

Evaluation of the ability of a battery of three in vitro genotoxicity tests to discriminate rodent carcinogens and non-carcinogens I. Sensitivity, specificity and relative predictivity

The performance of a battery of three of the most commonly used in vitro genotoxicity tests--Ames+mouse lymphoma assay (MLA)+in vitro micronucleus (MN) or chromosomal aberrations (CA) test--has been evaluated for its ability to discriminate rodent carcinogens and non-carcinogens, from a large database of over 700 chemicals compiled from the CPDB ("Gold"), NTP, IARC and other publications. We re-evaluated many (113 MLA and 30 CA) previously published genotoxicity results in order to categorise the performance of these assays using the response categories we established. The sensitivity of the three-test battery was high. Of the 553 carcinogens for which there were valid genotoxicity data, 93% of the rodent carcinogens evaluated in at least one assay gave positive results in at least one of the three tests. Combinations of two and three test systems had greater sensitivity than individual tests resulting in sensitivities of around 90% or more, depending on test combination. Only 19 carcinogens (out of 206 tested in all three tests, considering CA and MN as alternatives) gave consistently negative results in a full three-test battery. Most were either carcinogenic via a non-genotoxic mechanism (liver enzyme inducers, peroxisome proliferators, hormonal carcinogens) considered not necessarily relevant for humans, or were extremely weak (presumed) genotoxic carcinogens (e.g. N-nitrosodiphenylamine). Two carcinogens (5-chloro-o-toluidine, 1,1,2,2-tetrachloroethane) may have a genotoxic element to their carcinogenicity and may have been expected to produce positive results somewhere in the battery. We identified 183 chemicals that were non-carcinogenic after testing in both male and female rats and mice. There were genotoxicity data on 177 of these. The specificity of the Ames test was reasonable (73.9%), but all mammalian cell tests had very low specificity (i.e. below 45%), and this declined to extremely low levels in combinations of two and three test systems. When all three tests were performed, 75-95% of non-carcinogens gave positive (i.e. false positive) results in at least one test in the battery. The extremely low specificity highlights the importance of understanding the mechanism by which genotoxicity may be induced (whether it is relevant for the whole animal or human) and using weight of evidence approaches to assess the carcinogenic risk from a positive genotoxicity signal. It also highlights deficiencies in the current prediction from and understanding of such in vitro results for the in vivo situation. It may even signal the need for either a reassessment of the conditions and criteria for positive results (cytotoxicity, solubility, etc.) or the development and use of a completely new set of in vitro tests (e.g. mutation in transgenic cell lines, systems with inherent metabolic activity avoiding the use of S9, measurement of genetic changes in more cancer-relevant genes or hotspots of genes, etc.). It was very difficult to assess the performance of the in vitro MN test, particularly in combination with other assays, because the published database for this assay is relatively small at this time. The specificity values for the in vitro MN assay may improve if data from a larger proportion of the known non-carcinogens becomes available, and a larger published database of results with the MN assay is urgently needed if this test is to be appreciated for regulatory use. However, specificity levels of <50% will still be unacceptable. Despite these issues, by adopting a relative predictivity (RP) measure (ratio of real:false results), it was possible to establish that positive results in all three tests indicate the chemical is greater than three times more likely to be a rodent carcinogen than a non-carcinogen. Likewise, negative results in all three tests indicate the chemical is greater than two times more likely to be a rodent non-carcinogen than a carcinogen. This RP measure is considered a useful tool for industry to assess the likelihood of a chemical possessing carcinogenic potential from batteries of positive or negative results.

Start-up, microbial community analysis and formation of aerobic granules in a tert-butyl alcohol degrading sequencing batch reactor

We demonstrate that compact well-settling aerobic granules can be developed in a sequencing batch reactor (SBR) for the biological removal of tert-butyl alcohol (TBA) using a strategy involving step increases in TBA loading rate achieved through increasing TBA concentrations in the influent. A moderate selection pressure that included a cycle time of 24 h and a start-of-cycle TBA concentration of 100 mg/L was initially introduced to encourage the growth and retention of biomass and avoid biomass loss from hydraulic washout. Start-of-cycle TBA concentrations were increased to 150, 300, 450, and 600 mg/L on days 90, 100, 121, and 199, respectively. These increases were only introduced after complete TBA removal was accompanied by visible improvements in biomass concentration and biomass settling ability. This acclimation strategy produced incrementally higher biomass concentrations and better settling biomass with higher specific TBA biodegradation rates. Effluent TBA concentrations were consistently below the detection limit of 25 microg/L. Aerobic granules were first observed about 180 days after reactor start-up. The granules had a clearly defined shape and appearance, settled significantly faster than the suspended sludge in the reactor, and eventually became the dominant form of biomass in the reactor. The adapted granules were capable of complete TBA removal and contained a stable microbial population with a low diversity of sequences of community 16S rRNA gene fragments. This study indicates that it is possible to use aerobic granules for TBA remediation and will contribute to a better understanding of how microbial acclimation can be exploited in the SBR to biologically remove recalcitrant xenobiotics.

Microbial adaptation to biodegradation of tert-butyl alcohol in a sequencing batch reactor

This study demonstrates the utility of the sequencing batch reactor (SBR) to adapt microorganisms towards biological removal of tert-butyl alcohol (TBA). The reactor was inoculated with activated sludge and fed with TBA as the sole carbon source. Start-of-cycle TBA concentrations were initially set at 100 mgL(-1) with a cycle time of 24 h and a volumetric exchange ratio of 50% to maintain a TBA loading rate of not more than 100 mgL(-1)d(-1). Step increases in TBA loading rates up to 600 mgL(-1)d(-1) were achieved by first raising the start-of-cycle TBA concentration to 150 mgL(-1) on day 90 and subsequently by reducing the cycle time from 24 to 12, 8 and 6h on days 100, 121 and 199, respectively. This acclimation strategy favored the retention of increasingly higher densities of well-adapted microbial populations in the reactor. The increases in TBA loading produced better settling biomass and higher biomass concentrations with higher specific TBA biodegradation rates. Effluent TBA concentrations were consistently below the detection limit of 25 microgL(-1). The use of progressively shorter cycle times created selection pressures that fostered the self-immobilization of the reactor microorganisms into aerobic granules which first appeared on day 125. Specific TBA biodegradation rates in the granules followed the Haldane model for substrate inhibition, and peaked at 13.8 mgTBAgVSS(-1)h(-1) at a TBA concentration of 300 mgL(-1). Denaturing gradient gel electrophoresis (DGGE) analysis of PCR-amplified 16S rRNA genes from granules sampled between days 220 and 247 confirmed the existence of a highly stable microbial community with members belonging to the alpha, beta and delta subdivisions of Proteobacteria and the Cytophaga-Flavobacteria-Bacteroides (CFB) group.

Degradation of tert-butyl alcohol in dilute aqueous solution by an O3/UV process

This paper describes the degradation of tert-butyl alcohol (TBA) in dilute aqueous solution by an O3/UV process. The degradation process was investigated experimentally in a semi-batch reactor under various operational conditions, i.e., ozone gas (O3) dosage, UV light intensity, and water quality in terms of varying bicarbonate concentration. TBA was oxidized rapidly in the O3/UV system, and acetone, hydroxy-iso-butyraldehyde, and formaldehyde were identified as primary intermediates, whereas pyruvaldehyde and acetic, formic, pyruvic, and oxalic acids were generated as a result of further oxidation process. A good organic carbon balance was obtained, indicating that most reaction intermediates have been identified and quantified.

Chemically induced renal tubule tumors in the laboratory rat and mouse: review of the NCI/NTP database and categorization of renal carcinogens based on mechanistic information

The incidence of renal tubule carcinogenesis in male and female rats or mice with 69 chemicals from the 513 bioassays conducted to date by the NCI/NTP has been collated, the chemicals categorized, and the relationship between carcinogenesis and renal tubule hyperplasia and exacerbation of the spontaneous, age-related rodent disease chronic progressive nephropathy (CPN) examined. Where information on mechanism or mode of action exists, the chemicals have been categorized based on their ability to directly or indirectly interact with renal DNA, or on their activity via epigenetic pathways involving either direct or indirect cytotoxicity with regenerative hyperplasia, or exacerbation of CPN. Nine chemicals were identified as directly interacting with DNA, with six of these producing renal tubule tumors at high incidence in rats of both sexes, and in some cases also in mice. Ochratoxin A was the most potent compound in this group, producing a high tumor incidence at very low doses, often with metastasis. Three chemicals were discussed in the context of indirect DNA damage mediated by an oxidative free radical mechanism, one of these being from the NTP database. A third category included four chemicals that had the potential to cause DNA damage following conjugation with glutathione and subsequent enzymatic activation to a reactive species, usually a thiol-containing entity. Two chemicals were allocated into the category involving a direct cytotoxic action on the renal tubule followed by sustained compensatory cell proliferation, while nine were included in a group where the cell loss and sustained increase in renal tubule cell turnover were dependent on lysosomal accumulation of the male rat-specific protein, alpha2mu-globulin. In a sixth category, morphologic evidence on two chemicals indicated that the renal tumors were a consequence of exacerbated CPN. For the remaining chemicals, there were no pertinent data enabling assignment to a mechanistic category. Accordingly, these chemicals, acting through an as yet unknown mechanism, were grouped as either being associated with an enhancement of CPN (category 7, 16 chemicals), or not associated with enhanced CPN (category 8, 4 chemicals). A ninth category dealt with 11 chemicals that were regarded as producing increases in renal tubule tumors that did not reach statistical significance. A 10th category discussed 6 chemicals that induced renal tumors in mice but not in rats, plus 8 chemicals that produced a low incidence of renal tubule tumors in mice that did not reach statistical significance. As more mechanistic data are generated, some chemicals will inevitably be placed in different groups, particularly those from categories 7 and 8. A large number of chemicals in the series exacerbated CPN, but those in category 7 especially may be candidates for inclusion in category 6 when further information is gleaned from the relevant NTP studies. Also, new data on specific chemicals will probably expand category 5 as cytotoxicity and cell regeneration are identified as obligatory steps in renal carcinogenesis in more cases. Additional confirmatory outcomes arising from this review are that metastases from renal tubule tumors, while encountered with chemicals causing DNA damage, are rare with those acting through an epigenetic pathway, with the exception being fumonisin B1; that male rats and mice are generally more susceptible than female rats and mice to chemical induction of renal tubule tumors; and that a background of atypical tubule hyperplasia is a useful indicator reflecting a chemically associated renal tubule tumor response. With respect to renal tubule tumors and human risk assessment, chemicals in categories 1 and 2, and possibly 3, would currently be judged by linear default methods; chemicals in category 4 (and probably some in category 3) as exhibiting a threshold of activity warranting the benchmark approach; and those in categories 5 and 6 as representing mechanisms that have no relevance for extrapolation to humans.

Microbial degradation of methyl tert-butyl ether and tert-butyl alcohol in the subsurface

The fate of fuel oxygenates such as methyl tert-butyl ether (MTBE) in the subsurface is governed by their degradability under various redox conditions. The key intermediate in degradation of MTBE and ethyl tert-butyl ether (ETBE) is tert-butyl alcohol (TBA) which was often found as accumulating intermediate or dead-end product in lab studies using microcosms or isolated cell suspensions. This review discusses in detail the thermodynamics of the degradation processes utilizing various terminal electron acceptors, and the aerobic degradation pathways of MTBE and TBA. It summarizes the present knowledge on MTBE and TBA degradation gained from either microcosm or pure culture studies and emphasizes the potential of compound-specific isotope analysis (CSIA) for identification and quantification of degradation processes of slowly biodegradable pollutants such as MTBE and TBA. Microcosm studies demonstrated that MTBE and TBA may be biodegradable under oxic and nearly all anoxic conditions, although results of various studies are often contradictory, which suggests that site-specific conditions are important parameters. So far, TBA degradation has not been shown under methanogenic conditions and it is currently widely accepted that TBA is a recalcitrant dead-end product of MTBE under these conditions. Reliable in situ degradation rates for MTBE and TBA under various geochemical conditions are not yet available. Furthermore, degradation pathways under anoxic conditions have not yet been elucidated. All pure cultures capable of MTBE or TBA degradation isolated so far use oxygen as terminal electron acceptor. In general, compared with hydrocarbons present in gasoline, fuel oxygenates biodegrade much slower, if at all. The presence of MTBE and related compounds in groundwater therefore frequently limits the use of in situ biodegradation as remediation option at gasoline-contaminated sites. Though degradation of MTBE and TBA in field studies has been reported under oxic conditions, there is hardly any evidence of substantial degradation in the absence of oxygen. The increasing availability of field data from CSIA will foster our understanding and may even allow the quantification of degradation of these recalcitrant compounds. Such information will help to elucidate the crucial factors of site-specific biogeochemical conditions that govern the capability of intrinsic oxygenate degradation.

TBA biodegradation in surface-water sediments under aerobic and anaerobic conditions

The potential for [U-14C] TBA biodegradation was examined in laboratory microcosms under a range of terminal electron accepting conditions. TBA mineralization to CO2 was substantial in surface-water sediments under oxic, denitrifying, or Mn(IV)-reducing conditions and statistically significant but low under SO4-reducing conditions. Thus, anaerobic TBA biodegradation may be a significant natural attenuation mechanism for TBA in the environment, and stimulation of in situ TBA bioremediation by addition of suitable terminal electron acceptors may be feasible. No degradation of [U-14C] TBA was observed under methanogenic or Fe(III)-reducing conditions.

Effect of redox conditions on MTBE biodegradation in surface water sediments

Microbial degradation of methyl tert-butyl ether (MTBE) was observed in surface water-sediment microcosms under anaerobic conditions. The efficiency and products of anaerobic MTBE biodegradation were dependent on the predominant terminal electron-accepting conditions. In the presence of substantial methanogenic activity, MTBE biodegradation was nominal and involved reduction of MTBE to the toxic product, tert-butyl alcohol (TBA). In the absence of significant methanogenic activity, accumulation of [14C]TBA generally decreased, and mineralization of [U-14C]MTBE to 14CO2 generally increased as the oxidative potential of the predominant terminal electron acceptor increased in the order of SO4, Fe(III), Mn(IV) < NO3 < O2. Microbial mineralization of MTBE to CO2 under Mn(IV)-or SO4-reducing conditions has not been reported previously. The results of this study indicate that microorganisms inhabiting the sediments of streams and lakes can degrade MTBE effectively under a range of anaerobic terminal electron-accepting conditions. Thus, anaerobic bed sediment microbial processes may provide a significant environmental sink for MTBE in surface water systems throughout the United States.

Aerobic biodegradation of methyl tert-butyl ether by aquifer bacteria from leaking underground storage tank sites

The potential for aerobic methyl tert-butyl ether (MTBE) degradation was investigated with microcosms containing aquifer sediment and groundwater from four MTBE-contaminated sites characterized by oxygen-limited in situ conditions. MTBE depletion was observed for sediments from two sites (e.g., 4.5 mg/liter degraded in 15 days after a 4-day lag period), whereas no consumption of MTBE was observed for sediments from the other sites after 75 days. For sediments in which MTBE was consumed, 43 to 54% of added [U-(14)C]MTBE was mineralized to (14)CO(2). Molecular phylogenetic analyses of these sediments indicated the enrichment of species closely related to a known MTBE-degrading bacterium, strain PM1. At only one site, the presence of water-soluble gasoline components significantly inhibited MTBE degradation and led to a more pronounced accumulation of the metabolite tert-butyl alcohol. Overall, these results suggest that the effects of oxygen and water-soluble gasoline components on in situ MTBE degradation will vary from site to site and that phylogenetic analysis may be a promising predictor of MTBE biodegradation potential.

Toxicology and human health effects following exposure to oxygenated or reformulated gasoline

In order to replace antiknock leaded derivatives in gasoline, legislations were enacted in the United States and other countries to find safer additives and to reduce CO, O3, and volatile organic compounds (VOCs) in non-attainment areas. Oxygenates commonly used include various alcohols and aliphatic ethers. Methyl tert-butyl ether (MTBE) is the most widely used and studied ether oxygenate and is added to gasoline at concentrations up to 15% by volume. Inhalation of fumes while fueling automobiles is the main source of human exposure to MTBE. Humans are also exposed when drinking water contaminated with MTBE. Epidemiological, clinical, animal, metabolic and kinetic studies have been carried out to address human health risks resulting from exposure to MTBE. MTBE is an animal carcinogen, but its human carcinogenic potential remains unclear. Because MTBE functions as a non-traditional genotoxicant, several mechanisms were suggested to explain its mode of action, such as, functioning as a cytotoxic as opposed to a mitogenic agent; involvement of hormonal mechanisms; or operating as a promoter instead of being a complete carcinogen. Some studies suggested that carcinogenicity of MTBE might be due to its two main metabolites, formaldehyde or tributanol. A role for DNA repair in MTBE carcinogenesis was recently unveiled, which explains some, but not all effects. The totality of the evidence shows that, for the majority of the non-occupationally exposed human population, MTBE is unlikely to produce lasting adverse health effects, and may in some cases improve health by reducing the composition of emitted harmful VOCs and other substances. A small segment of the population (e.g. asthmatic children, the elderly, and those with immunodeficiency) may be at increased risk for toxicity. However, no studies have been conducted to investigate this hypothesis. Concern over ground and surface water contamination caused by persistent MTBE has lead the Environmental Protection Agency (EPA) to proposed reducing or eliminating its use as a gasoline additive. The major potential alternatives to MTBE are other forms of ethers such as ethyl tert-butyl ether (ETBE) or tert-amyl methyl ether (TAME), and alcohols such as ethanol. More definitive studies are needed to understand the mechanism(s) by which aliphatic ethers may pose health and environmental impacts. The switch from MTBE to ethanol is not without problems. Ethanol costs more to produce, poses challenges to the gasoline distribution system, extends the spread of hydrocarbons through ground water in gasoline plumes, and in the short-term is unlikely to be available in sufficient quantity. Moreover, its metabolite acetaldehyde is a possible carcinogen that undergoes a photochemical reaction in the atmosphere to produce the respiratory irritant peroxylacetate nitrate (PAN). Congress is addressing whether the Clean Air Act Amendments (CAA) provisions concerning reformulated gasoline (RFG) should be modified to allow refineries to discontinue or lessen the use of oxygenates.