Abstract  In order to characterize the effect of vegetation on performance of constructed wetlands (CWs) treating low and high chlorinated hydrocarbon, two pilot-scale horizontal subsurface flow (HSSF) CWs (planted with Phragmites australis and unplanted) treating sulphate rich groundwater contaminated with MCB (monochlorobenzene, as a low chlorinated hydrocarbon), (about 10 mg L(-1)), and PCE (perchloroethylene, as a high chlorinated hydrocarbon), (about 2 mg L(-1)), were examined. With mean MCB inflow load of 299 mg m(-2) d(-1), the removal rate was 58 and 208 mg m(-2) d(-1) in the unplanted and planted wetland, respectively, after 4 m from the inlet. PCE was almost completely removed in both wetlands with mean inflow load of 49 mg m(-2) d(-1). However, toxic metabolites cis-1,2-DCE (dichloroethene) and VC (vinyl chloride) accumulated in the unplanted wetland; up to 70% and 25% of PCE was dechlorinated to cis-1,2-DCE and VC after 4 m from the inlet, respectively. Because of high sulphate concentration (around 850 mg L(-1)) in the groundwater, the plant derived organic carbon caused sulphide formation (up to 15 mg L(-1)) in the planted wetland, which impaired the MCB removal but not statistically significant. The results showed significant enhancement of vegetation on the removal of the low chlorinated hydrocarbon MCB, which is probably due to the fact that aerobic MCB degraders are benefited from the oxygen released by plant roots. Vegetation also stimulated completely dechlorination of PCE due to plant derived organic carbon, which is potentially to provide electron donor for dechlorination process. The plant derived organic carbon also stimulated dissimilatory sulphate reduction, which subsequently have negative effect on MCB removal.

Abstract  This paper examines the potential for using laboratory synthesized nanoscale Pd/Fe bimetallic particles to reduce chlorinated ethenes. Rapid and complete dechlorination was achieved for six chlorinated ethenes: tetrachloroethene (PCE, C2Cl4), trichloroethene (TCE, C2HCl3), 1,1-dichloroethene (1,1-DCE, C2H2Cl2), cis- and trans-1,2-dichloroethene (c-DCE, t-DCE, C2H2Cl2), and vinyl chloride (VC, C2H3Cl). The chlorinated ethenes (20 mg 1(-1)) were completely reduced within 90 min at a metal loading of 5 g 1(-1). Ethane was the primary prod-act from these reactions, amount to 60-90% of the total carbon. Ethene (3-20%) was produced during the transformation of TCE, DCEs and VC. No chlorinated intermediates or final products were detected above the method detection limit ( < 5 mug 1(-1)). The remarkable performance of the nanoscale particles can be attributed to: (1) High specific surface area of the nanoscale metal particles, approximately 35 m(2) g(-1), tens to hundreds of times higher than commercial grade micro- or milli-scale iron particles; (2) Increased reactivity per unit metal surface area, largely due to the presence of the noble metal (Pd) on the surface. Values of the surface-area-normalized rate coefficients (k(SA)) were two orders of magnitude higher than those reported in the literature for larger iron particles. Due to their small particle size and high reactivity, the nanoscale bimetallic particles may be useful in a wide array of environmental applications including subsurface injection for groundwater treatment. (C) 2001 Elsevier Science B.V. All rights reserved.

Abstract  BIOSIS COPYRIGHT: BIOL ABS. Determination of the degradation potentials for a mixture of eight organic trace contaminants (benzene, toluene, o-xylene, naphthalene, tetrachloromethane, 1,1,1-trichloroethane, trichloroethene, tetrachloroethene) has been made by specially developed in situ microcosms under aerobic and anaerobic aquifer conditions. The developed in situ microcosms allowed for determination of the degradation potentials in the aquifer as represented by the combined groundwater and sediment and by the groundwater only. Six out of eight microcosms functioned hydraulically well as determined by means of a hydraulic tracer. Control experiments showed that the in situ microcosms were not as determined by means of a hydraulic tracer. Control experiments showed that the in situ microcosms were not subject to unaccounted losses of the contaminants except from sorption to sediment in the beginning of the experiments. All compounds were studied at initial concentrations of approximately 120 mug/l

Abstract  Canister sampling for the determination of atmospheric mixing ratios of nonmethane hydrocarbons (NMHCs), selected halocarbons, and methyl nitrate was conducted aboard the National Center for Atmospheric Research (NCAR) C-130 aircraft over the Pacific and Southern Oceans as part of the First Aerosol Characterization Experiment (ACE 1) during November and December 1995. A latitudinal profile, flown from 76 degrees N to 60 degrees S, revealed latitudinal gradients for most trace gases. NMHC and halocarbon gases with predominantly anthropogenic sources, including ethane, ethyne, and tetrachloroethene, exhibited significantly higher mixing ratios in the northern hemisphere at all altitudes. Methyl chloride exhibited its lowest mixing ratios at the highest northern hemisphere latitudes, and the distributions of methyl nitrate and methyl iodide were consistent with tropical and subtropical oceanic sources. Layers containing continental air characteristic of aged biomass burning emissions were observed above about 3 km over the remote southern Pacific and near New Zealand between approximately 19 degrees S and 43 degrees S. These plumes originated from the west, possibly from fires in southern Africa. The month-long intensive investigation of the clean marine southern midlatitude troposphere south of Australia revealed decreases in the mixing ratios of ethane, ethyne, propane, and tetrachloroethene, consistent with their seasonal mixing ratio cycle. By contrast, increases in the average marine boundary layer concentrations of methyl iodide, methyl nitrate, and dimethyl sulfide (DMS) were observed as the season progressed to summer conditions. These increases were most appreciable in the region south of 44 degrees S over Southern Ocean waters characterized as subantarctic and polar, indicating a seasonal increase in oceanic productivity for these gases.

Abstract  Coupling of methanogenic and methanotrophic catabolisms was performed in a single-stage technology equipped with a water electrolysis cell placed in the effluent recirculation loop. The electrolysis-generated hydrogen served as an electron donor for both bicarbonate reduction into CH4 and reductive dechlorination, while the O2 and CH4, supported the cometabolic oxidation of chlorinated intermediates left over by the tetrachloroethylene (PCE) transformation. The electrolytical methanogenic/methanotrophic coupled (eMaMoC) process was tested in a laboratory-scale setup at PCE loads ranging from 5 to 50 micromol/L(rx) x d (inlet concentrations from 4 to 11 mg/L), and at various hydraulic residence times (HRT). Degradation followed essentially a reductive dechlorination pathway from PCE to cis-1,2-dichloroethene (DCE), and an oxidative pathway from DCE to CO2. PCE reductive dechlorination to DCE was consistently over 98% while a maximum oxidative DCE mineralization of 89% was obtained at a load of 4.3 micromol PCE/ L(rx) x d and an HRT of 6 days. Controlling dissolved oxygen concentrations within a relatively low range (2-3 mg/L) seemed instrumental to sustain the overall degradation capacity. Degradation kinetics were further evaluated: the apparent half-saturation constant (K(s)) had to be set relatively high (29 microM) for the simulated data to best fit the experimental ones. In spite of such kinetic limitations, the eMaMoC system, while fueled by water electrolysis, was effective in building and sustaining a functional methanogenic/methanotrophic consortium capable of significant PCE mineralization in a single-stage process. Hence, degradation standards are within reach so long as the methanotrophic DCE-oxidizing potential, including substrate affinity, are optimized and HRT accordingly adjusted.

Abstract  Due to a greater understanding of the behaviour of the fuel oxygenate Methyl Tertiary Butyl Ether (MTBE) in groundwater, the United States Environmental Protection Agency (EPA) and the American Petroleum Institute (API) recently have acknowledged the the need for the development and application of additional remedial strategies to address the more extensive, longer lived, and faster moving dissolved MTBE fuel releases (API, 2000 and USEPA, 2000a).

The need for alternative methods for managing dissolved MTBE plumes Is particularly evident in the case of the Upper Glacial aquifer of Long Island, Now York. Hydrogeologic conditions In the this water table aquifer (i.e., high hydraulic conductivity, high average pore velocities, low organic carbon, and high rates of recharge) have been found to contribute to the formation of extensive, long, narrow and three-dimensional dissolved MTBE plumes that plunge Into the aquifer in response to recharge (Weaver et. al 1999). The characteristics of MTBE plumes in the Upper Glacial aquifer In combination with abundant sensitive receptors (mainly drinking water supply wells), often renders monitored natural attenuation (AMA) plume management strategies inappropriate, resulting In the need for plume control, frequently via pumping and treating (NYSDEC, 2000). In such cases, remedial costs can rise well beyond those associated with similar fuel releases that did not contain MTBE (USEPA, 1998a). Consequently, the application of remedial technologies for MTBE other than MNA, or pumping and treating, are Of great interest to those responsible for the management of dissolved MTBE plumes on Long Island or in similar hydrogeologic settings.

An alternative strategy for the remediation of dissolved MTBE plumes was recently A.-Id tested at an oxygenated fuel spill site on Long Island. The strategy was enhanced biodegradation via the application of Hydrogen Release Compound (HRC (TM)). HRC (TM) Is a form of polylactate ester that slowly releases biodegradation the aquifer and has been shown in other studies to foster methanogenic conditions that advance the reductive dechlorination Of perchloroethene (PCE) and trichloroethene (TCE) (Koeningsberg, 1998). Numerous reports have been written that discuss the biodegradation of MTBE under aerobic conditions, as well as microcosm studies in which which MTBE biodegradation was observed under anaerobic conditions. However, there are limited reports that document the natural anaerobic biodegradation of dissolved MTBE (McLoughlin, 2000). Despite the lack of documented natural anaerobic biodegradation of MTBE, It has been observed that MTBE, transport often occurs under anoxic conditions at oxygenated fuel releases as the result of the biodegradation of other fuel constituents, such RS benzene, toluene, ethylbenzene and xylene (BTEX), which deplete the available dissolved oxygen as well as other electron acceptors (nitrate, ferric iron, manganese, etc.) (USEPA, 2000c and API 1996), Therefore, an anaerobic biodegradation strategy is attractive due to Its synergy with the existing geochemical conditions. Consequently, the study was conceived and designed to test the ability of HRC (TM) to foster the anaerobic biodegradation of MTBE under methanogenic conditions (McLoughlin, 2000).

The application of HRC (TM) did result in the formation of a large area of enhanced reducing conditions in the vicinity and down gradient of the application zone. However, under these site conditions, the HRC (TM) application old not induce measurable methanogenic conditions with the associated elevated dissolved hydrogen concentrations required for significant MTBE anaerobic biodegradation. The high hydraulic conductivity and high average Core velocity at the site were likely responsible. Despite this, the Study can be Vle Wed as a Success since much was learned that can be used in future studies of anaerobic biodegradation of MTBE and the application of HRC (TM).

Abstract  Brominated aromatics are used in many different applications but occur also naturally. Here, we demonstrate organohalide respiration and growth of Dehalococcoides mccartyi strain CBDB1 with 1,2,4-tribromobenzene, all three dibrominated benzene congeners and monobromobenzene. All bromobenzenes were fully dehalogenated to benzene. Growth yields were between 1.8 × 10(14) and 2.8 × 10(14) cells per mol of bromide released. Furthermore, a newly designed high-throughput methyl viologen-based photometric microtiter plate assay was established to determine the activity of the reductive dehalogenases in resting cell assays of strain CBDB1 with brominated aromatics as electron acceptors. Activities of 2.8-13.2 nkat per mg total cell protein (0.16-0.8 units per mg total cell protein) were calculated after cultivation of strain CBDB1 on 1,2,4-tribromobenzene. Mass spectrometric analyses and activity assays with whole cell extracts of strain CBDB1 gave strong evidence that four to six reductive dehalogenases were involved in the dehalogenation of all tested brominated benzenes, including the reductive dehalogenases CbdbA80 and CbrA.

Abstract  Na/NH3 reductions have been used to dehalogenate polychlorinated biphenyls (PCBs), chlorinated aliphatic hydrocarbons (CAHs) and pesticides at diffusion controlled rates at room temperature in model compound studies in both dry NH3 and when water was added. The rate ratio of dechlorination (aliphatic and aromatic compounds) versus reaction of the solvated electron with water is very large, allowing wet soils or sludges to be remediated without an unreasonable consumption of sodium. Several soils, purposely contaminated with 1,1,1-trichloroethane, 1-chlorooctane and tetrachloroethylene, were remediated by slurring the soils in NH3 followed by addition of sodium. The consumption of sodium per mole of chlorine removed was examined as a function of both the hazardous substrate's concentration in the soil and the amount of water present. The Na consumption per Cl removed increases as the amount of water increases and as the substrate concentration in soil decreases. However, remediation was still readily accomplished from 5000 to 3000ppm to sub ppm levels of RCl in the presence of substantial amounts of water. PCB- and dioxin-contaminated oils were remediated with Na/NH3 as were PCB-contaminated soils and sludges from contaminated sites. Ca/NH3 treatments also successfully remediated PCB-contaminated clay, sandy and organic soils but laboratory studies demonstrated that Ca was less efficient than Na when substantial amounts of water were present. The advantages of solvated electron reductions using Na/NH3 include: (1) very rapid dehalogenation rates at ambient temperature, (2) soils (even clay soils) break down into particles and slurry nicely in NH3, (3) liquid ammonia handling technology is well known and (4) removal from soils, recovery and recycle of ammonia is easy due to its low boiling point. Finally, dechlorination is extremely fast even for the 'corner' chlorines in the substrate Mirex (structure in Eq. (5)).

Abstract  BIOSIS COPYRIGHT: BIOL ABS. The oxidation of several VOCs in the presence of humic acid and soil is described in this paper. It was found that while ozone does react appreciably with humic acid, as indicated by significant changes in the spectral characteristics of humic acid, the TOC levels in these solutions changed by only 3% or less. The oxidation of four olefinic VOCs occurred in solutions containing up to 120 mg humic acid, however, the extent to which each of the compounds reacted is very much compound specific. The effect of pH and ozone dosage on these reactions was considered. The effects of pH were weak for all compounds except trichloroethane. Ozone dosage had a significant effect on the extent to which each of the VOCs was oxidized, although no simple relationship between ozone dosage and the amount of VOC which reacted could be obtained. Complete oxidation of cis-dichloroethylene by ozone (22 mg/L) occurred in solutions containing 1.0 g of Eustis soil suspended in 10.0 mL water. Howev

Abstract  This study presents an experiment which characterizes reductive dechlorination of tetra chloroethylene (PCE) by green rusts (GRs) in the presence of Pt using a batch reactor system. Relative to GR alone, the rate of PCE reduction in GR suspensions was greatly enhanced with the addition of Pt(IV) (95% of PCE was removed in 30 h). PCE was mostly transformed to a nonchlorinated byproduct, acetylene rather than trichloroethylene, and the carbon mass recovery was 98% at the last sampling point. The reduction of PCE was four times faster for GR-F(Pt) than for GR-CO3(Pt), mainly due to the higher Fe(II) content of GR-F The estimated kinetic rate Constants of GR-CI(Pt) increased significantly (i.e., 0.17, 0.21, and 1.01 h(-1), respectively) with an incremental addition of Pt from 0.5 to 2 mM. X-ray diffraction analysis showed the transformation of GR to magnetite as an oxidation product. X-ray photoelectron spectroscopy analysis revealed that the oxidation was coupled to the reduction of Pt (IV to 0) on the GR surfaces. The scanning electron microscope with energy dispersive spectrometer measurement showed the formation of Pt particles on the surfaces of GRs modified with the Pt(IV).

Abstract  An important objective of the Pacific Exploratory Mission-West A (PEM-West A) was the chemical characterization of the outflow of tropospheric trace gases and aerosol particles from the Asian continent over the western Pacific Ocean. This paper summarizes the chemistry of this outflow during the period September - October 1991. The vertical distributions of CO, C2H6, and NOX showed regions of outflow at altitudes below 2 km and from 8 to 12 km. Mixing ratios of CO were approximate to 130 parts per billion by volume (ppbv), approximate to 1000 parts per trillion by volume (pptv) for C2H6, and approximate to 100 pptv for NOX in both of these regions. Direct outflow of Asian industrial materials was clearly evident at altitudes below 2 lan, where halocarbon tracer compounds such as CH3CCl3 and C2Cl4 were enhanced about threefold compared to aged Pacific air. The source attribution of species outflowing from Asia to the Pacific at 8-12 km altitude was not straightforward. Above 10 km altitude there were substantial enhancements of NOy, O-3, CO, CH4 SO2, C2H6, C3H8, C2H2, and aerosol Pb-210 but not halocarbon industrial tracers. These air masses were rich in nitrogen relative to sulfur and contained ratios of C2H2/CO and C3H8/C2H6 (approximate to 1.5 and 0.1 respectively) indicative of several-day-old combustion emissions. It is unclear if these emissions were of Asian origin, or if they were rapidly transported to this region from Europe by the high wind speeds in this tropospheric region (60 - 70 m s(-1)). The significant cyclonic activity over Asia at this time could have transported to the upper troposphere emissions from biomass burning in Southeast Asia or emissions from the extensive use of various biomass materials for cooking and space heating. Apparently, the emissions in the upper troposphere were brought there by wet convective systems since water-soluble gases and aerosols were depleted in these air masses, Near 9 km altitude there was a distinct regional outflow that appeared to be heavily influenced by biogenic processes on the Asian continent, especially from the southeastern area. These air masses contained CH4 in excess of 1800 ppbv, while CO2 and OCS were significantly depleted (349 - 352 ppmv and 450 - 500 pptv, respectively). This signature seemingly reflected CH4 emissions from wetlands and rice paddies with coincident biospheric uptake of tropospheric CO2 and OCS.

Abstract  Isotopic analysis and molecular-based bioassay methods were used in conjunction with geochemical data to assess intrinsic reductive dechlorination processes for a chlorinated solvent-contaminated site in Tucson, Arizona. Groundwater samples were obtained from monitoring wells within a contaminant plume comprising tetrachloroethene and its metabolites, trichloroethene, cis-1,2-dichloroethene, vinyl chloride, and ethene, as well as compounds associated with free phase diesel present at the site. Compound-specific isotope analysis was performed to characterize biotransformation processes influencing the transport and fate of the chlorinated contaminants. Polymerase chain reaction (PCR) analysis was used to assess the presence of indigenous reductive dechlorinators. The target regions employed were the 16s rRNA gene sequences of Dehalococcoides sp. and Desulfuromonas sp. and DNA sequences of genes pceA, tceA, bvcA, and vcrA, which encode reductive dehalogenases. The results of the analyses indicate that relevant microbial populations are present and that reductive dechlorination is presently occurring at the site. The results further show that potential degrader populations as well as biotransformation activity is nonuniformly distributed within the site. The results of laboratory microcosm studies conducted using groundwater collected from the field site confirmed the reductive dechlorination of tetrachloroethene to dichloroethene. This study illustrates the use of an integrated, multiple-method approach for assessing natural attenuation at a complex chlorinated solvent-contaminated site.

Abstract  Mixtures of chlorinated ethenes and ethanes are often found at contaminated sites. In this study, we undertook a systematic investigation of the inhibitory effects of 1,1,1-trichloroethane (1,1,1-TCA) and 1,1-dichloroethane (1,1-DCA) on chlorinated ethene dechlorination in three distinct Dehalococcoides-containing consortia. To focus on inhibition acting directly on the reductive dehalogenases, dechlorination assays used cell-free extracts prepared from cultures actively dechlorinating trichloroethene (TCE) to ethene. The dechlorination assays were initiated with TCE, cis-1,2-dichloroethene (cDCE), or vinyl chloride (VC) as substrates and either 1,1,1-TCA or 1,1-DCA as potential inhibitors. 1,1,1-TCA inhibited VC dechlorination similarly in cell suspension and cell-free extract assays, implicating an effect on the VC reductases associated with the dechlorination of VC to nontoxic ethene. Concentrations of 1,1,1-TCA in the range of 30-270 μg/L reduced VC dechlorination rates by approximately 50% relative to conditions without 1,1,1-TCA. 1,1,1-TCA also inhibited reductive dehalogenases involved in TCE and cDCE dechlorination. In contrast, 1,1-DCA had no pronounced inhibitory effects on chlorinated ethene reductive dehalogenases, indicating that removal of 1,1,1-TCA via reductive dechlorination to 1,1-DCA is a viable strategy to relieve inhibition.

Abstract  Iron-based degradative solidification/stabilization (DS/S-Fe(II)) is a modification of conventional solidification/stabilization (S/S) that incorporates degradative processes for organic contaminant destruction with immobilization. This study investigated the effectiveness of a binder mixture of Portland cement and slag in a DS/S-Fe(II) system to treat trichloroethylene (TCE), 1,1-dichloroethylene (1,1-DCE), vinyl chloride (VC), trichloromethane (CF), and dichloromethane (MC), which are major chlorinated hydrocarbons contained in waste oils and waste organic solvents. For TCE, 1,1-DCE, and VC, degradation experiments were conducted using three different binder combinations with Fe(II) (cement/Fe(II), slag/Fe(II), and cement/slag/Fe(II)). When cement and slag were mixed at a 1:1 ratio (% wt), the TCE and 1,1-DCE dechlorination rate was enhanced compared to that when cement or slag was used alone with Fe(II). Also, batch experiments were conducted in the solid phase consisting of cement, slag, sand, and Fe(II) to treat liquid wastes that contain chlorinated compounds at high concentrations. TCE was completely removed after 5 days in the cement/slag/sand/Fe(II) system, in which the initial TCE concentration was 11.8mM, with Fe(II) concentration of 565 mM. While the CF concentration was decreased by 95% after 5 days when the initial CF and Fe(II) concentration was 0.25 mM and 200 mM, respectively. However, MC was not degraded with the cement/slag/Fe(II) system.

Abstract  The objective of this study was to predict the inhalation toxicokinetics of chemicals in mixtures using an integrated QSAR-PBPK modelling approach. The approach involved: (1) the determination of partition coefficients as well as V(max) and K(m) based solely on chemical structure for 53 volatile organic compounds, according to the group contribution approach; and (2) using the QSAR-driven coefficients as input in interaction-based PBPK models in the rat to predict the pharmacokinetics of chemicals in mixtures of up to 10 components (benzene, toluene, m-xylene, o-xylene, p-xylene, ethylbenzene, dichloromethane, trichloroethylene, tetrachloroethylene, and styrene). QSAR-estimated values of V(max) varied compared with experimental results by a factor of three for 43 out of 53 studied volatile organic compounds (VOCs). K(m) values were within a factor of three compared with experimental values for 43 out of 53 VOCs. Cross-validation performed as a ratio of predicted residual sum of squares and sum of squares of the response value indicates a value of 0.108 for V(max) and 0.208 for K(m). The integration of QSARs for partition coefficients, V(max) and K(m), as well as setting the K(m) equal to K(i) (metabolic inhibition constant) within the mixture PBPK model allowed to generate simulations of the inhalation pharmacokinetics of benzene, toluene, m-xylene, o-xylene, p-xylene, ethylbenzene, dichloromethane, trichloroethylene, tetrachloroethylene and styrene in various mixtures. Overall, the present study indicates the potential usefulness of the QSAR-PBPK modelling approach to provide first-cut evaluations of the kinetics of chemicals in mixtures of increasing complexity, on the basis of chemical structure.

Abstract  Toxicity and exposure evaluations remain the two of the key components of human health assessment. While improvement in exposure assessment relies on a better understanding of human behavior patterns, toxicity assessment still relies to a great extent on animal toxicity testing and human epidemiological studies. Recent advances in computer modeling of the dose-response relationship and distribution of xenobiotics in humans to important target tissues have advanced our abilities to assess toxicity. In particular, physiologically based pharmacokinetic (PBPK) models are among the tools than can enhance toxicity assessment accuracy. Many PBPK models are available to the health assessor, but most are so difficult to use that health assessors rarely use them. To encourage their use these models need to have transparent and user-friendly formats. To this end the Agency for Toxic Substances and Disease Registry (ATSDR) is using translational research to increase PBPK model accessibility, understandability, and use in the site-specific health assessment arena. The agency has initiated development of a human PBPK tool-kit for certain high priority pollutants. The tool kit comprises a series of suitable models. The models are recoded in a single computer simulation language and evaluated for use by health assessors. While not necessarily being state-of-the-art code for each chemical, the models will be sufficiently accurate to use for screening purposes. This article presents a generic, seven-compartment PBPK model for six priority volatile organic compounds (VOCs): benzene (BEN), carbon tetrachloride (CCl(4)), dichloromethane (DCM), perchloroethylene (PCE), trichloroethylene (TCE), and vinyl chloride (VC). Limited comparisons of the generic and original model predictions to published kinetic data were conducted. A goodness of fit was determined by calculating the means of the sum of the squared differences (MSSDs) for simulation vs. experimental kinetic data using the generic and original models. Using simplified solvent exposure assumptions for oral ingestion and inhalation, steady-state blood concentrations of each solvent were simulated for exposures equivalent to the ATSDR Minimal Risk Levels (MRLs). The predicted blood levels were then compared to those reported in the National Health and Nutrition Examination Survey (NHANES). With the notable exception of BEN, simulations of combined oral and inhalation MRLs using our generic VOC model yielded blood concentrations well above those reported for the 95th percentile blood concentrations for the U.S. populations, suggesting no health concerns. When the PBPK tool kit is fully developed, risk assessors will have a readily accessible tool for evaluating human exposure to a variety of environmental pollutants.

Abstract  Formyl chloride has been generated in aqueous solution (i) by stopped-flow ozonation of vinyl chloride and (ii) by reacting dichloromethyl radicals with OH radicals using the pulse radiolysis technique. Vinyl chloride reacts in water with ozone (k=1.7x10(4) dm(3) mol(-1) s(-1), as determined by stopped-flow) yielding as final products (mel per mol of ozone) chloride ions (1.05), CO (1.01), and formate ions (0.06). Hydroxymethyl hydroperoxide (formaldehyde plus H2O2; 1.08) is also formed. HCl and formic acid are formed in less than 2 ms (the detection limit of the stopped-flow setup). At high pH the CO yield decreases (at pH 13.6 by 50%). It is concluded that the precursor of CO, HCl, and formic acid is formyl chloride. It predominantly decays into CO and HCl, and only at very high pH can hydrolysis to formic acid and HCl compete successfully. Using the pulse radiolysis technique dichloromethyl radicals are generated in Ar-saturated solutions from chloroform by reacting it with the solvated electron (originating from the radiolysis of water). The OH radicals (also from the radiolysis of water) partially react with the dichloromethyl radicals yielding dichloromethanol. Alternatively, dichloromethanol is generated in N2O-saturated solutions from dichloromethane, where some of the OH radicals are allowed to abstract an H atom from dichloromethane and another fraction to add to the dichloromethyl radicals. The observed conductivity changes are attributed to a very rapid decay (t(1/2)<20 mu s) of dichloromethanol into formyl chloride and HCl followed by the decay of formyl chloride into CO and HCl (k=10(4) s(-1)). From these data and the decrease of the CO yield at high pH (ozonation of vinyl chloride) it is estimated that the OH--induced hydrolysis of formyl chloride occurs with a rate constant of ca. 2.5x10(4) dm(3) mol(-1) s(-1).

Abstract  The widespread use of volatile chlorinated compounds like chloroform, trichloroethene and tetrachloroethene in industrialized societies causes a large annual release of these compounds into the environment. Due to their role as a source for halogen radicals involved in various catalytic atmospheric reaction cycles, including the regulation of the stratospheric and tropospheric ozone layers, these compounds also constitute a risk for drinking water resources as they can be transported to the groundwater from contaminated field sites or even from atmospheric deposition. Therefore, identification and investigation of sources and sinks of volatile chlorinated compounds are of particular interest. Chloroform, a major contributor to natural gaseous chlorine, was found to be emitted by several anthropogenic and natural sources including the oceans and terrestrial areas. The origin of chloroform in the terrestrial environment can be anthropogenic point sources, atmospheric deposition, release by vegetation and production directly in the soil. The calculated annual biogenic global chloroform emission is 700 Gg, and marine and terrestrial environments are nearly equal contributors. The estimated emissions from anthropogenic sources account for less than 10% of the estimated total emissions from all sources. Among terrestrial sources, forests have recently been identified as contributing to the release of chloroform into the environment. With the data available, annual emissions of chloroform to the atmosphere from forest sites were calculated and compared to other natural sources. At present knowledge, forests are only a minor source in the total biogenic flux of chloroform, contributing less than 1% to the annual global atmospheric input. However, it should be noted that data are available for Northern temperate forests only. The large tropical forest areas may provide a yet unknown input of chloroform.

Abstract  Efficient design of zero-valent metal permeable 'barriers' for the reduction of organohalides requires information regarding the pertinent reaction rates as well as an understanding of the resultant distribution of products. In this study, the pathways and kinetics for reaction of polychlorinated ethanes with Zn(0) have been examined in batch reactors. Reductive p-elimination was the only route through which hexachloroethane (HCA), 1,1,1,2-tetrachloroethane (1,1,1,2-TeCA), 1,1,2,2-tetrachloroethane (1,1,2,2-TeCA), 1,1,2-trichloroethane (1,1,2-TCA) and 1,2-dichloroethane (1,2-DCA) reacted. Pentachloroethane (PCA) reacted via concurrent reductive beta-elimination (93%) and hydrolysis (7%). As previously demonstrated, 1,1,1-trichloroethane (1,1,1-TCA) and 1,1-dichloroethane (1,1-DCA) reacted predominantly via reductive a-elimination. Attempts to correlate BET surface area-normalized rate constants (k(SA-BET)) with one-electron reduction potential (E-1) met with limited success, as HCA, PCA, 1,1,1,2-TeCA, and 1,1,1-TCA reacted at nearly identical rates despite substantial differences in E-1 values. Comparison of the pseudo-first-order rate constants (k(obs)) for these species with rate constants (k(L)a) obtained from a correlation for mass transfer to suspended particles revealed that the reaction of these species was mass transfer limited even though reaction rates were unaffected by mixing speed. Calculations suggest that mass transfer limitations may also play a role in the design of treatment systems for highly reactive species, with overall rate constants predicted to increase with flow velocity. (C) 1999 Elsevier Science B,V. All rights reserved.

Abstract  This paper presents the field evaluation results for an advanced chemical oxidation technology developed by Peroxidation Systems, Inc., of Tucson, Arizona. The technology, known as the perox-pure(TM) technology, was evaluated under the U.S. Environmental Protection Agency Superfund Innovative Technology Evaluation program at Lawrence Livermore National Laboratory (LLNL), Site 300 in Tracy, California, in September 1992. The perox-pure(TM) technology uses ultraviolet radiation and hydrogen peroxide to oxidize dissolved organic compounds in water. At the LLNL site, this technology was evaluated in treating groundwater contaminated with volatile organic compounds (VOC) including trichloroethene (TCE); tetrachloroethene (PCE); 1,1,1-trichloroethane (TCA); 1,1-dichloroethane (DCA); and chloroform. The perox-pure(TM) system generally produced an effluent that contained TCE, PCE, and DCA at levels below detection limits, and TCA and chloroform at levels slightly above detection limits. The system achieved maximum removal efficiencies of greater than 99.9, 98.7, and 95.8 percent for TCE, PCE, and DCA, respectively. The system also achieved removal efficiencies of up to 92.9 and 93.6 percent for TCA and chloroform, respectively. The treatment system effluent met California drinking water action levels and federal drinking water maximum contaminant levels for all VOCs at the 95 percent confidence level. Cost analysis indicated that the groundwater remediation cost for a 50-gallon per minute perox-pure(TM) system would range from $7 to $11 per 1,000 gallons, depending on contaminated groundwater characteristics. Of this total cost, the perox-pure(TM) system direct treatment cost would range from $3 to $5 per 1,000 gallons.

Abstract  Groundwater contaminated by dense, non-aqueous phase liquids (DNAPLs) such as chlorinated solvents has become a serious problem in some regions of Taiwan. The sources of these contaminants are due to industrial discharges. These chlorinated volatile organic compounds (VOCs) have been proven to be carcinogenic to humans. The groundwater is used for domestic drinking water supply in some cities of Taiwan and the severely contaminated groundwater has to be treated in order to meet the requirement of drinking water standards. This study covers two areas of work. In the first part, polluted groundwater samples were collected from the contaminated site and analytical results indicated measurable concentrations of 12 representative chlorinated VOCs in water samples. The primary VOCs detected included trichloroethene (TCE), tetrachloroethene (PCE), 1,1,2-trichloroethane (1,1,2-TCA), and I,l-dichloroethene (1,1-DCE). Second, to remove VOCs groundwater was treated using adsorption on activated carbon fiber (ACF). This involved pumping groundwater through vessels containing ACF. Most VOCs, including TCE, PCE, I,1,2-TCA, and DCE, were readily adsorbed onto ACF and are removed from the water stream. Our study showed that the technology was able to significantly reduce chlorinated VOCs concentrations in groundwater. (C) 2000 Elsevier Science Ltd. All rights reserved.

Abstract  The rates of attenuation of chlorinated solvents and their less chlorinated daughter products in ground water are slow as humans experience time. If concentrations of chlorinated organic compounds near the source are in the range of 10,000 to 100,000 micrograms/liter, then a residence time in the plume on the order of a decade or more will be required to bring initial concentrations to current MCLs for drinking water. Biodegradation as a component of natural attenuation can be protective of ground water quality in those circumstances where the time of travel of a plume to a receptor is long. In many cases, it will be necessary to supplement the benefit of natural attenuation with some sort of source control or plume management.

Abstract  BIOSIS COPYRIGHT: BIOL ABS. 1,1,2-Trichloroethylene (TCE), 1,1-dichloroethylene, cis and trans-1,2-dichloroethylene and tetrachloroethylene (PCE), at concentrations of 20 ppm in aqueous solutions were rapidly hydrodechlorinated to ethane (in a few minutes), on the surface of palladized iron in batch experiments that were performed in closed vials. No intermediate reaction products such as 1,1-dichloroethylene, 1,2-dichloroethylenes and vinyl chloride were detected at concentrations 1 ppm either in the headspace or in solution. The chloromethanes, CCl4, CHCl3 and CH2Cl2 were also dechlorinated to methane on palladized iron; the CCl4 was dechlorinated in a few minutes, the CHCl3, in less than an hour and the CH2Cl2, in 4-5 h. These results indicate that an above-ground treatment method can be designed for the treatment of groundwater contaminated with low molecular weight chlorinated hydrocarbons.

Abstract  Complete anaerobic dechlorination of chlorinated solvents such as trichloroethene (TCE) is essential for bioremediation of chloroethene-contaminated sites. We studied the influence of sulfate on microbial dechlorination of TCE to ethene both under transient and steady-state conditions, encompassing the range of hydrogen (H2) levels commonly found at contaminated sites. The results show that sulfate at a concentration of 2.5 mM limits microbial dechlorination by a mixed anaerobic culture by reducing the rate under steady-state hydrogen supply (a few nM H2), implying a H2 limited dechlorination. Conversely, sulfate did not affect dechlorination when rapid fermentation of lactate resulted in transient buildup of H2 to levels around two orders of magnitude higher compared to steady-state conditions. This has important implications both for optimizing culture conditions for dehalogenating microorganisms and for the efficiency of cleanup strategies. Our findings may contribute to the understanding and bioremediation of chloroethene contaminated environments containing sulfate.

Abstract  Kinetic studies with two different anaerobic mixed cultures (the PM and the EV cultures) were conducted to evaluate inhibition between chlorinated ethylenes. The more chlorinated ethylenes inhibited the reductive dechlorination of the less chlorinated ethylenes, while the less chlorinated ethylenes weakly inhibited the dechlorination of the more chlorinated ethylenes. Tetrachloroethylene (PCE) inhibited reductive trichloroethylene (TCE) dechlorination but not cis-dichloroethylene (c-DCE) dechlorination, while TCE strongly inhibited c-DCE and VC dechlorination. c-DCE also inhibited vinyl chloride (VC) transformation to ethylene (ETH). When a competitive inhibition model was applied, the inhibition constant (K(I)) for the more chlorinated ethylene was comparable to its respective Michaelis-Menten half-velocity coefficient, K(S). Model simulations using independently derived kinetic parameters matched the experimental results well. k(max) and K(S) values required for model simulations of anaerobic dechlorination reactions were obtained using a multiple equilibration method conducted in a single reactor. The method provided precise kinetic values for each step of the dechlorination process. The greatest difference in kinetic parameters was for the VC transformation step. VC was transformed more slowly by the PM culture (k(max) and K(S) values of 2.4+/-0.4 micromol/mg of protein/day and 602+/-7 microM, respectively) compared to the EV culture (8.1+/-0.9 micromol/mg of protein/day and 62.6+/-2.4 microM). Experimental results and model simulations both illustrate how low K(S) values corresponded to efficient reductive dechlorination for the more highly chlorinated ethylenes but caused strong inhibition of the transformation of the less chlorinated products. Thus, obtaining accurate K(S) values is important for modeling both transformation rates of parent compounds and their inhibition on daughter product transformation.