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The environmental implications of carbon nanomaterials have received much attention. Nonetheless, little is known about how carbon nanomaterials might affect the abiotic transformation of organic contaminants in aquatic environments. In this study, we observed that three functionalized multiwalled carbon nanotubes (MWCNTs)-including a hydroxylated MWCNT (OH-MWCNT), a carboxylated MWCNT (COOH-MWCNT), and an aminated MWCNT (NH2MWCNT)-all had strong catalytic effects on the dehydrochlorination of 1,1,2,2-tetrachloroethane (TeCA) at three different pH (7, 8, and 9); notably, the most significant effects (up to 130% increase in reaction rate) were observed at pH 7, at which reaction kinetics was very slow in the absence of MWCNT. The primary mechanism was that the -NH2 group and the deprotonated -COOH and -OH groups serve as bases to catalyze the reaction. Modeling results indicate that at any given pH the transformation kinetic constants of MWCNT-adsorbed TeCA were up to 2 orders of magnitude greater than the respective kinetic constant of dissolved TeCA. The overall catalytic effects of the MWCNTs depended both on the basicity of the surface functionalities of MWCNT and on the adsorption affinities of MWCNT for TeCA. Interestingly, Suwannee River humic add-selected as a model dissolved organic matter-had negligible effects on the dehydrochlorination kinetics, even though it is rich in surface O-functionalities. An important environmental implication is that carbon nanotubes released into the environment might significantly affect the fate of chlorinated solvents. [ABSTRACT FROM AUTHOR]

Palladized nanoscale zerovalent iron (Pd/NZVI) has been utilized for source zone control, yet the reductant responsible for pollutant transformation and the optimal conditions for subsurface application remain poorly understood. Here, trends in Pd/Fe reactivity toward 1,1,1,2-tetrachloroethane (1,1,1,2-TeCA) and cis-dichloroethene (cis-DCE) were compared in H2O and D2O batch systems as a function of pH, chlorinated solvent concentration, Pd surface loading, Pd/Fe mass loading, Pd/Fe aging time, and zerovalent iron [Fe(0)] particle size. For Pd/NZVI, the solvent kinetic isotope effect [i.e., kobs(H2O)/kobs(D2O) or SKIE] for 1,1,1,2-TeCA and cis-DCE reduction increased substantially with Pd loading and Pd/NZVI concentration, evidence that multiple pathways exist for chlorinated solvent reduction. At low Pd loadings and Pd/NZVI concentrations with relatively small SKIEs (less than 5), we propose that modest reactivity enhancements (≤10-fold) reflect more efficient electron transfer to 1,1,1,2-TeCA from Fe(0) facilitated by Pd nanodeposits. Much larger SKIEs (e.g., exceeding 100 for cis-DCE) imply the involvement of atomic hydrogen in more reactive systems with high Pd loadings and Pd/NZVI concentrations. Generally, evidence of SKIEs supporting a dominant role for atomic hydrogen was not observed for Pd/Fe prepared from micrometer-sized Fe(0), or for any size of nonpalladized Fe(0). During anaerobic aging of Pd/NZVI, decreases in the SKIE for 1,1,1,2-TeCA reduction suggest that atomic hydrogen will contribute to reactivity for only approximately 1 week after application. [ABSTRACT FROM AUTHOR]

Nanoscale zerovalent iron (NZVI) was aged over 30 days in suspension (2 g/L) with different anions (chloride, perchlorate, sulfate, carbonate, nitrate), anion concentrations (5, 25, 100 mN), and pH (7, 8). During aging, suspension samples were reacted periodically with 1,1,1,2-tetrachloroethane (1,1,1,2-TeCA) and Cr(VI) to determine the time scales and primary mode of NZVI reactivity loss. Rate constants for 1,1,1,2-TeCA reduction in Cl-, SO42-, and ClO4- suspensions decreased by 95% over 1 month but were generally equivalent to one another, invariant of concentration and independent of pH. In contrast, longevity toward 1,1,1,2-TeCA depended upon NO3- and HCO3- concentration, with complete reactivity loss over 1 and 14 days, respectively, in 25 mN suspensions. X-ray diffraction suggests that reactivity loss toward 1,1,1,2-TeCA in most systems results from Fe(0) conversion into magnetite, whereas iron carbonate hydroxide formation limits reactivity in HCO3- suspensions. Markedly different trends in Cr(VI) removal capacity (mg Cr/g NZVI) were observed during aging, typically exhibiting greater longevity and a pronounced pH-dependence. Notably, a strong linear correlation exists between Cr(VI) removal capacities and rates of Fe(II) production measured in the absence of Cr(VI). While Fe(0) availability dictates longevity toward 1,1,1,2-TeCA, this correlation suggests surface-associated Fe(II) species are primarily responsible for Cr(VI) reduction. [ABSTRACT FROM AUTHOR]

The real significance and engineering potential for bioenhanced dissolution of chlorinated ethene dense nonaqueous phase liquid (DNAPL) contaminants are currently not well understood, in part because they can be influenced by a complex set of factors, including microbial competition for growth substrates. Mathematical simulations were performed to evaluate the effects of competition between Dehalococcoides etheno genes and Desulfuromonas michiganensis for the electron acceptor tetrachloroethene (PCE) on the distribution of dehalorespirers, PCE dissolution, and the extent of PCE detoxification. The modeling results demonstrate that the outcome of competition between populations for growth substrates can have a significant impact on bioenhancement and, thus, on DNAPL source zone longevity and identify the key factors in determining the outcome of competition and its effects on DNAPL dissolution. The potential for bioenhancement is greatest at lower groundwater velocities. At higher velocities, kinetic properties play a key role in determining which population dominates and where, and the amount of bioenhancement that is realized. Engineered bioremediation techniques that maintain multiple dehalorespiring populations may offer the best approach for optimizing the twin cleanup goals of reduced source zone longevity and complete detoxification while maximizing the utilization of added electron donors. [ABSTRACT FROM AUTHOR]

In this work, four surfactant-like humic acids (HAs) obtained from garden lignocellulose wastes and kitchen food wastes mixed with garden-lignocellulose wastes, both before and after composting, were tested for surfactant properties and the ability to solubilize tetrachloroethene (PCE). The waste-derived HAs showed good surfactant properties, lowering the water surface tension from 74 mN m-1 to 45.4 ± 4.4 mN m-1, with a critical micelle concentration (CMC) of 1.54 ± 1.68 g L-1, which is lower than many synthetic ionic surfactants. CMC was affected by both waste origin and composting processes. The addition of food waste and composting reduced CMC by adding alkyl-C (measured by CP MAS -13C NMR) and N- and S-HA contents (amide molecules), so that a multistep regression was found [CMC = 24.6 - 0.189 alkyl C - 2.64 (N + 5); R² = 0.77, P < 0.10, n = 6]. The four HAs solubilized PCE at the rate of 0.18-0.47 g PCE/g aqueous biosurfactant. These results were much higher than those reported in the literature for a commercial HA (0.026 g/g), but they were in line with those measured in this work for nonionic surfactants such as Tween-80 (0.69 g/g) and Triton X-100 (1.08 g/g). [ABSTRACT FROM AUTHOR]

Despite widespread implementation of zero-valent iron remediation schemes, the manner and order of chemical bond cleavage in iron-mediated organohalide transformations remains imperfectly understood. We present insights from carbon isotope fractionation for the dehalogenation of 1,1,2,2-tetrachloroethane (1,1,2,2-TeCA) and 1,1,1- trichloroethane (1,1,1-TCA) by various reactants. Elimination of HCI by OH- gave isotope fractionation in 1,1,2,2-TeCA of ϵ = -25.6‰, KIEc = 1.02 to 1.03 per carbon center, consistentwith a concerted (E2) mechanism. In contrast, 1,1,1-TCA reduction by Cr(ll), Fe(0), and Cu-plated iron (Cu/Fe) resulted in ϵ -13.6‰ to -15.8‰ indicating the initial involvement of a single C-Cl bond (KlEc ≈ 1.03). 1,1,2,2-TeCA reduction by Cr(II), Fe(0), and Cu/Fe yielded ϵ = 19.3‰, and -17.0‰, respectively. In the two latter cases, depletion of the minor product TCE by 26‰ indicated its formation via nonreductive dehydrohalogenation. The major 1,1,2,2-TeCA reduction products, cis- and trans-DCE, differed by 2.3‰ ± 1.0‰ in Cr(ll) systems, but were equivalent in Fe(O) and Cu/Fe systems. In contrast, the ratio of cis-DCE to trans-DCE concentration was 2.5 for reduction with Cr(II) and Fe(0), but ~3.8 with Cu/Fe. Complementary isotope and concentration data therefore suggest differences in the transition state geometry and/or reaction intermediates in each reductant system. [ABSTRACT FROM AUTHOR]

The ability of modified Fenton reactions to promote simultaneous chemical and biological oxidation in an artificially contaminated soil was studied in batch laboratory slurry reactors. Tetrachloroethene (PCE) and oxalate (OA) were used to distinguish chemical oxidation from aerobic heterotrophic metabolism. PCE was mineralized by Fenton reactions, but OA was not oxidized. Indigenous soil microorganisms did not degrade added PCE aerobically but readily assimilated OA. Fenton reactions were promoted at the natural soil pH (7.6) by adding H2O2 and Fe(III), with nitrilotriacetic acid (NTA) as a chelator, at a constant molar ratio of H2O2/Fe(III)/NTA of 50:1:1. The •OH-mediated mineralization of PCE was demonstrated by adding 2-propanol (an •OH scavenger), which inhibited PCE oxidation. In subsequent dosing studies, PCE oxidation served as an indicator of Fenton reactions, while OA assimilation, dissolved oxygen (DO) concentration, and heterotrophic plate counts were indicators of aerobic microbial activity. Increasing Fenton doses to 20 times that required to achieve 95% PCE oxidation only delayed OA assimilation by 500 min and reduced plate counts by 1.5 log units g-1 soil. Results show that aerobic metabolism can coexist with Fenton oxidation in soils. [ABSTRACT FROM AUTHOR]

While most sites and all characterized PCE and TCE dechlorinating anaerobic bacteria produce cis-DCE as the major DCE isomer, significant amounts of trans-DCE are found in the environment We have obtained microcosms from some sites and enrichment cultures that produce more trans-DCE than cis-DCE. These cultures reductively dechlorinated PCE and ICE to trans-DCE and cis-DCE simultaneously and in a ratio of 3(±0.5):1 that was stable through serial transfers with a variety of electron donors and occurred in both methanogenic and nonmethanogenic enrichments. Two sediment-free, nonmethanogenic enrichment cultures produced trans-DCE at rates of up to 2.5 μmol L-1 day-1. Dehalococcoides populations were detected in both trans-DCE producing cultures by their 16S rRNA gene sequences, and trans-DCE was produced in the presence of ampicillin. Because trans-DCE can be the major product from PCE and ICE microbial dechlorination, high fractions of trans-DCE at chloroethene-contaminated sites are not necessarily from source contamination. [ABSTRACT FROM AUTHOR]

Flux in non-aqueous-phase liquid (NAPL)-contaminated systems containing active microbial populations (including Dehalococcoides sp.) was investigated using a quantitative mass balance and phase distribution approach. Batch systems containing mixed NAPL with an initial tetrachloroethene (PCE) mole fraction ranging from 0.1 to 0.4 provided a means for comparing systems where mass transfer and aqueous concentration were controlled by the initial NAPL composition. Although the use of mixed NARL with increasing PCE mole fractions introduced a mass-transfer variable on the abiotic dissolution rate, it was determined that biological systems produced flux rates that were similar to each other regardless of the initial PCE mole fraction. Thus, organisms appeared to be dechlorinating near their maximum conversion rates, and the result was the accumulation of cis-1,2-dichloroethene (cDCE) followed by slow conversion to vinyl chloride (VC). Increases in the initial PCE mole fractions in the NAPL had a negative impact on product distribution due to the presence of a larger concentration of a more favorable electron acceptor. Because the mass converted to cDCE was present largely in the dissolved phase in all systems, the production of this metabolite was a favorable outcome in terms of NAPL dissolution. The pH dropped as low as 4.9 in active systems, indicating that the amount of MCI released during the reductive dechlorination process was large enough to overwhelm the buffering capacity. This pH effect was more pronounced in systems that exhibited extensive dechlo- rination to VC, further suggesting that rapid dechlorination of PCE NAPL can alter chemical characteristics in source zone regions. [ABSTRACT FROM AUTHOR]

The limitations associated with conventional pump and treat technology have generated interest in using enhanced in-situ flushing as an alternative for remediating source zones contaminated with immiscible liquid. This research investigates the effectiveness of cyclodextrin as a solubility-enhancement agent to enhance the removal of tetrachloro-ethene (PCE) from a physically isolated section of an aquifer. An important component of this project was the implementation of reagent recovery and reuse. This field experiment presented the rare opportunity, under strict regulatory guidance, to inject PCE into the surficial aquifer cell created with two sets of sheet piles driven into an underlying clay unit. The well-controlled conditions specific to this experiment allowed quantification of mass balances, which is problematic for many contaminated field sites. The fact that mass balances can be obtained provides the ability to determine remediation effectiveness with unusual accuracy for a field project The saturated zone within the test cell was flushed with a 15 wt % cyclodextrin solution. The cyclodextrin solution increased the aqueous concentration of PCE in the extraction-well effluent to as much as 22 times the concentrations obtained during the water flush conducted prior to the complexing sugar flush (CSF). The seven pore-volume CSF removed the equivalent of approximately 33 L of PCE from the subsurface. This equates to 48% of the total initial mass, based on the volume of PCE present prior to the CSF (68.6 L). Conversely, the seven pore-volume water flush conducted prior to the CSF removed the equivalent of 2.7 L of PCE. The use of cyclodextrin as a flushing agent, especially in a recycling configuration, appears to hold promise for successful remediation of chlorinated-solvent-Contaminated source zones. [ABSTRACT FROM AUTHOR]

The anaerobic halorespiring microorganism, Sulfurospirillum, multivorans, was observed in the pore structure of an etched silicon wafer to determine how flow hydrodynamics and mass transfer limitations along a transverse mixing zone affect biomass growth. Tetrachloroethene (PCE, an electron acceptor, 0.2 mM) and lactate (an electron donor 2 mM) were introduced as two separate and parallel streams that mixed along a reaction line in the pore structure. The first visible biomass occupied a single line of pores in the direction of flow, a few pore bodies from the micromodel centerline. This growth was initially present as small aggregates; over time, these grew and fused to form finger-like structures with one end attached to downgradient ends of the silicon posts and the other end extending into pore bodies in the direction of flow. Biomass did not grow in pore throats as expected, presumably because shear forces were not favorable. Over the next few weeks the line of growth migrated upward into the PCE zone and extended over a width of up to five pore spaces. When the PCE concentration was increased to 0.5 mM, the microbial biomass increased and growth migrated down toward the lactate side of the micromodel. A new analytical model was developed and used to demonstrate that transverse hydrodynamic dispersion likely caused the biomass to move in the direction observed when the PCE concentration was changed. The model was unable, however, to explain why growth migrated upward when the PCE concentration was initially constant. We postulate that this occurred because PCE, not lactate, sorbed to biofilm components and that biomass on the lactate side of the micromodel was limited in PCE. [ABSTRACT FROM AUTHOR]

A laboratory microcosm study and a pilot scale field test were conducted to evaluate biostimulation and bioaugmentation to dechlorinate tetrachloroethene (PCE) to ethene at Kelly Air Force Base. The site groundwater contained about 1 mg/L of PCE and lower amounts of trichloroethene (TCE) and cis-1,2-dichloroethene (cDCE). Laboratory microcosms inoculated with soil and groundwater from the site exhibited partial dechlorination of TCE to cDCE when amended with lactate or methanol. Following the addition of a dechlorinating enrichment culture, KB-1, the chlorinated ethenes in the microcosms were completely converted to ethene. The KB-1 culture is a natural dechlorinating microbial consortium that contains phylogenetic relatives of Dehalococcoides ethenogenes. The ability of KB-1 to stimulate biodegradation of chlorinated ethenes in situ was explored using a closed loop recirculation cell with a pore volume of approximately 64 000 L. The pilot test area (PTA) groundwater was first amended with methanol and acetate to establish reducing conditions. Under these conditions, dechlorination of PCE to cDCE was observed. Thirteen liters of the KB-1 culture were then injected into the subsurface. Within 200 days, the concentrations of PCE, TCE, and cis-1,2-DCE within the PTA were all below 5 µg/L, and ethene production accounted for the observed mass loss. The maximum rates of dechlorination estimated from field data were rapid (halflives of a few hours). Throughout the pilot test period, groundwater samples were assayed for the presence of Dehalococcoides using both a Dehalococcoides-specific PCR assay and 16S rDNA sequence information. The sequences detected in the PTA after bioaugmentation were specific to the Dehalococcoides species in the KB-1 culture. These sequences were observed to progressively increase in abundance and spread downgradient within the PTA. These results confirm that organisms in the KB-1 culture populated the PTA aquifer and contributed to... [ABSTRACT FROM AUTHOR]

Investigates the influence of dechlorinating microorganisms on tetrachloroethene (PCE) and its reduction products in a residual nonaqueous phase liquid (NAPL) source zone. Biologically-enhanced removal of PCE from NAPL source zones; Column systems.

Investigates the competitive sorption of trichloroethene (TCE) and tetrachloroethene in natural solids, polymers and zeolites. Sorption mechanisms of the volatile organic compounds; Factors controlling competition between volatile organic contaminants in the subsurface.

Determines the potential for biological reductive tetrachloroethane (PCE) and trichloroethane (TCE) dehalogenation at high concentration. Effect on competing microorganisms, including methanogens and homoacetogens, on the dehalogenation process; Effects of ethene compounds on the bacterial community; Rate of dissolution of dense, nonaqueous-phase liquids.

Examines the heterogeneous photodegradation of tetrachloroethene in air on the two clay particles kaolinite and halloysite under photoillumination with an induction period. Insensitivity of induction period and tetrachloroethene removal rate to the tetrachloroethene partial pressure or the relative humidity; Acceleration of the photodegradation of tetrachloroethene.

Studies the abiotic and biotic transformations of 1,1,2,2-tetrachloroethane under methanogenic conditions. Transformations involved in the degradation of polychlorinated alkanes; Reductive dechlorination; Dihaloelimination as a two-electron transfer reaction.

Presents the results of a study which investigated degradation reactions controlling the fate of 1,1,2,2-tetrachloroethane (PCA) in a freshwater tidal wetland that is a discharge area for a contaminated aquifer. Field and laboratory evidence of PCA degradation; Implications for natural attenuation.

Focuses on a model used to describe the fermentation of electron donors and competition for the evolution of hydrogen between hydrogenotrophic tetrachloroethene dechlorinators (PCE) and methanogens. Details on the composition of PCE dechlorinators; Competitive advantages of dechlorinators; Information on organic substrates.

Presents a comparative study of butyric acid, ethanol, lactic acid and propionic acid as hydrogen donors for the reductive dechlorination of tetrachloroethene. Suggestion of a strategy for selective enhancement of dechlorination; Butyric acid enrichment culture development; Long-term operation of ethanol-fed dechlorinating culture.

The article presents a response to the comment on the paper by Elsner, M., Cwiertny, D. M., Roberts A. L., and Sherwood Lollar, B. titled "1,1,2,2-Tetrachloroethane Reactions with OH-, Cr(II), Granular Iron, and a Copper-Iron Bimetal: Insights from Product Formation and Associated Carbon Isotope Fractionation." The authors maintain that science is best served by bringing as wide an array as possible of different tools bearing on complex problems. Moreover, they pointed out that it is only through harnessing the mixed data afforded by a variety of various experimental approaches, that they hope to better understanding the multiple roles that granular iron may play in affecting contaminant attenuation.

The article presents a comment on 1,1,2,2-Tetrachloroethane Reactions with OH-, Cr(II), Granular Iron, and a Copper-Iron Bimetal: Insights from Product Formation and Associated Carbon Isotope Fractionation. It is stated that the paper of Elsner, M., Cwiertny, D., Roberts, A. L., and Lollar, B.S. reiterates the importance of isotope fractionation for the clarification of reaction mechanisms. However, because used hydrodynamic regimes were not appropriate to environmental conditions, it is difficult to see how these results can enable a better prediction of the formation of harmful versus benign organohalide reaction products. It is suggested that a unified procedure for the investigation of processes in Fe0-H2O systems is needed.

Presents a comment on the field evidence on the degradation of 1,1,2,2-tetrachloroethane in a freshwater tidal wetland. Details on the hydrogenolysis and dichloroelimination mechanisms; Information on microbial ecosystems reducing equivalents; Discussion on the effect of a short supply of reducing equivalents.

Responds to the comments by Bhaskar S. Ballapragada, et al. on the article entitled `Comparative Kinetics of Hydrogen Utilization for Reductive Dechlorination of Tetrachloroethene and Methanogenesis in an Anaerobic Enrichment Culture,' featured in a 1996 issue of `Environmental Science & Technology.'

Opinion. Comments on the article `Comparative Kinetics of Hydrogen Utilization for Reductive Dechlorination of Tetrachloroethene and Methanogenesis in an Anaerobic Enrichment Culture,' featured in a 1996 issue of `Environmental Science & Technology'. Apparent discrepancy in stoichiometry between hydrogen gas use, methane production and PCE dehalogenation.