MTBE ambient water quality criteria development: a public/private partnership

Adsorption of methyl tert-butyl ether (MTBE) onto ZSM-5 zeolite: Fixed-bed column tests, breakthrough curve modelling and regeneration

ZSM-5, as a hydrophobic zeolite, has a good adsorption capacity for methyl tert-butyl ether (MTBE) in batch adsorption studies. This study explores the applicability of ZSM-5 as a reactive material in permeable reactive barriers (PRBs) to decontaminate the MTBE-containing groundwater. A series of laboratory scale fixed-bed column tests were carried out to determine the breakthrough curves and evaluate the adsorption performance of ZSM-5 towards MTBE under different operational conditions, including bed length, flow rate, initial MTBE concentration and ZSM-5 dosage, and regeneration tests were carried out at 80, 150 and 300 °C for 24 h. Dose-Response model was found to best describe the breakthrough curves. MTBE was effectively removed by the fixed-bed column packed with a ZSM-5/sand mixture with an adsorption capacity of 31.85 mg g^-1 at 6 cm bed length, 1 mL min^-1 flow rate, 300 mg L^-1 initial MTBE concentration and 5% ZSM-5 dosage. The maximum adsorption capacity increased with the increase of bed length and the decrease of flow rate and MTBE concentration. The estimated kinetic parameters can be used to predict the dynamic behaviour of column systems. In addition, regeneration study shows that the adsorption capacity of ZSM-5 remains satisfactory (>85%) after up to four regeneration cycles.

Kinetic and equilibrium modelling of MTBE (methyl tert-butyl ether) adsorption on ZSM-5 zeolite: Batch and column studies

The intensive use of methyl tert-butyl ether (MTBE) as a gasoline additive has resulted in serious environmental problems due to its high solubility, volatility and recalcitrance. The feasibility of permeable reactive barriers (PRBs) with ZSM-5 type zeolite as a reactive medium was explored for MTBE contaminated groundwater remediation. Batch adsorption studies showed that the MTBE adsorption onto ZSM-5 follows the Langmuir model and obeys the pseudo-second-order model with an adsorption capacity of 53.55 mg g^-1. The adsorption process reached equilibrium within 24 h, and MTBE was barely desorbed with initial MTBE concentration of 300 mg L^-1. The mass transfer process is found to be primarily controlled by pore diffusion for MTBE concentrations from 100 to 600 mg L^-1. pH has little effect on the maximum adsorption capacity in the pH range of 2-10, while the presence of nickel reduces the capacity with Ni concentrations of 2.5-25 mg L^-1. In fixed-bed column tests, the Dose-Response model fits the breakthrough curve well, showing a saturation time of ∼320 min and a removal capacity of ∼18.71 mg g^-1 under the conditions of this study. Therefore, ZSM-5 is an extremely effective adsorbent for MTBE removal and has a huge potential to be used as a reactive medium in PRBs.

Gasoline toxicology: overview of regulatory and product stewardship programs

Significant efforts have been made to characterize the toxicological properties of gasoline. There have been both mandatory and voluntary toxicology testing programs to generate hazard characterization data for gasoline, the refinery process streams used to blend gasoline, and individual chemical constituents found in gasoline. The Clean Air Act (CAA) (Clean Air Act, 2012: § 7401, et seq.) is the primary tool for the U.S. Environmental Protection Agency (EPA) to regulate gasoline and this supplement presents the results of the Section 211(b) Alternative Tier 2 studies required for CAA Fuel and Fuel Additive registration. Gasoline blending streams have also been evaluated by EPA under the voluntary High Production Volume (HPV) Challenge Program through which the petroleum industry provide data on over 80 refinery streams used in gasoline. Product stewardship efforts by companies and associations such as the American Petroleum Institute (API), Conservation of Clean Air and Water Europe (CONCAWE), and the Petroleum Product Stewardship Council (PPSC) have contributed a significant amount of hazard characterization data on gasoline and related substances. The hazard of gasoline and anticipated exposure to gasoline vapor has been well characterized for risk assessment purposes.

Using Monte Carlo analysis to characterize the uncertainty in final acute values derived from aquatic toxicity data

Many ambient water quality criteria established to protect aquatic life from acute toxicity are calculated using a procedure described in the US Environmental Protection Agency's "1985 Guidelines" (USEPA 1985). The procedure yields a final acute value (FAV) from acceptable median lethal or effective concentrations (LC50 or EC50, respectively) that is a single-point, deterministic estimate of the concentration of a chemical substance that will protect 95% of aquatic species from >50% mortality or other acute toxic effects. However, because of variation and uncertainty associated with toxicity test results, uncertainty in the estimated FAV exists that is not accounted for by the 1985 Guidelines procedure. Here, Monte Carlo analysis is used to characterize this uncertainty. The analysis uses Cu EC50 values adjusted for differences in test water chemistry obtained from USEPA's final freshwater Cu criteria guidance published in 2007. Additional Monte Carlo simulations illustrate Cu FAV distributions obtained using a subset of tested species and assuming fewer replicate tests. The deterministic procedure yields an FAV of 4.68 µg/L for the complete data set. By comparison, 3 replicate Monte Carlo simulations yielded mean FAVs of 4.66 µg/L. The 5th and 95th percentiles of the distribution of calculated FAVs were 4.14 µg/L and 5.20 µg/L, respectively. Reducing the number of tested genera from 27 to 8 (the minimum recommended by the 1985 Guidelines) and setting the number of tests per species equal to 3 yielded 5th and 95th percentiles of 1.22 µg/L and 6.18 µg/L, respectively, compared to a deterministic estimate of 2.80 µg/L. Results of this study indicate that Monte Carlo analysis can be used to improve the understanding and communication of uncertainty associated with water quality criteria derived from acute toxicity data using the 1985 Guidelines. This may benefit the development, revision, and application of these criteria in the future.

Perchlorate reduction by a novel chemolithoautotrophic, hydrogen-oxidizing bacterium

Water treatment technologies are needed that can remove perchlorate from drinking water without introducing organic chemicals that stimulate bacterial growth in water distribution systems. Hydrogen is an ideal energy source for bacterial degradation of perchlorate as it leaves no organic residue and is sparingly soluble. We describe here the isolation of a perchlorate-respiring, hydrogen-oxidizing bacterium (Dechloromonas sp. strain HZ) that grows with carbon dioxide as sole carbon source. Strain HZ is a Gram-negative, rod-shaped facultative anaerobe that was isolated from a gas-phase anaerobic packed-bed biofilm reactor treating perchlorate-contaminated groundwater. The ability of strain HZ to grow autotrophically with carbon dioxide as the sole carbon source was confirmed by demonstrating that biomass carbon (100.9%) was derived from CO2. Chemolithotrophic growth with hydrogen was coupled with complete reduction of perchlorate (10 mM) to chloride with a maximum doubling time of 8.9 h. Strain HZ also grew using acetate as the electron donor and chlorate, nitrate, or oxygen (but not sulphate) as an electron acceptor. Phylogenetic analysis of the 16S rRNA sequence placed strain HZ in the genus Dechloromonas within the beta subgroup of the Proteobacteria. The study of this and other novel perchlorate-reducing bacteria may lead to new, safe technologies for removing perchlorate and other chemical pollutants from drinking water.

Toxicity of methyl tert-butyl ether to marine organisms: ambient water quality criteria calculation

In response to increasing concerns over the detection of methyl tert-butyl ether (MTBE) in groundwater and surface water and its potential effects in aquatic ecosystems, industry and the United States Environmental Protection Agency (USEPA) began to collaborate in 1997 to develop aquatic toxicity databases sufficient to derive ambient water quality criteria for MTBE consistent with USEPA requirements. Acute toxicity data for seven marine species, chronic toxicity data for an invertebrate, and plant toxicity data were developed to complete the saltwater database. The species tested were Cyprinodon variegatus, Gasterosteus aculeatus, Callinectes sapidus, Mytilus galloprovincialis, Palaemonetes pugio, Rhepoxynius abronius, Americamysis bahia, and Skeletonema costatum. The toxicity tests were conducted in accordance with USEPA and American Society for Testing and Materials testing procedures and Good Laboratory Practice guidelines. Data developed from this study were consistent with existing data and showed that MTBE has low acute and chronic toxicity to the marine species tested. Based upon measured MTBE concentrations, acute effects were found to range from 166 mg MTBE/l for the grass shrimp to 1950 mg MTBE/l for marine mussel. The no-observed effect concentration for the reproduction and growth of mysids was 26 mg MTBE/l during the life cycle test. The toxicity of MTBE to saltwater organisms is comparable to its toxicity to the freshwater species tested. Reported MTBE concentrations in coastal waters are several orders of magnitude lower than concentrations observed to cause effects in marine organisms.