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  • 1
    Language: English
    In: Environmental science & technology, 02 July 2013, Vol.47(13), pp.6900-7
    Description: Separating microbial- and physical-induced effects on the isotope signals of contaminants has been identified as a challenge in interpreting compound-specific isotope data. In contrast to simple analytical tools, such as the Rayleigh equation, reactive-transport models can account for complex interactions of different fractionating processes. The question arises how complex such models must be to reproduce the data while the model parameters remain identifiable. In this study, we reanalyze the high-resolution data set of toluene concentration and toluene-specific δ(13)C from the toluene-pulse experiment performed by Qiu et al. (this issue). We apply five reactive-transport models, differing in their degree of complexity. We uniquely quantify degradation and sorption properties of the system for each model, estimate the contributions of biodegradation-induced, sorption-induced, and transverse-dispersion-induced isotope fractionation to the overall isotope signal, and investigate the error introduced in the interpretation of the data when individual processes are neglected. Our results show that highly resolved data of both concentration and isotope ratios are needed for unique process identification facilitating reliable model calibration. Combined analysis of these highly resolved data demands reactive transport models accounting for nonlinear degradation kinetics and isotope fractionation by both reactive and physical processes such as sorption and transverse dispersion.
    Keywords: Models, Theoretical ; Toluene -- Chemistry ; Water Pollutants, Chemical -- Chemistry
    ISSN: 0013936X
    E-ISSN: 1520-5851
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  • 2
    Language: English
    In: Environmental science & technology, 07 November 2017, Vol.51(21), pp.12254-12263
    Description: In aqueous environments, hydrophobic organic contaminants are often associated with particles. Besides natural particles, microplastics have raised public concern. The release of pollutants from such particles depends on mass transfer, either in an aqueous boundary layer or by intraparticle diffusion. Which of these mechanisms controls the mass-transfer kinetics depends on partition coefficients, particle size, boundary conditions, and time. We have developed a semianalytical model accounting for both processes and performed batch experiments on the desorption kinetics of typical wastewater pollutants (phenanthrene, tonalide, and benzophenone) at different dissolved-organic-matter concentrations, which change the overall partitioning between microplastics and water. Initially, mass transfer is externally dominated, while finally, intraparticle diffusion controls release kinetics. Under boundary conditions typical for batch experiments (finite bath), desorption accelerates with increasing partition coefficients for intraparticle diffusion, while it becomes independent of partition coefficients if film diffusion prevails. On the contrary, under field conditions (infinite bath), the pollutant release controlled by intraparticle diffusion is not affected by partitioning of the compound while external mass transfer slows down with increasing sorption. Our results clearly demonstrate that sorption/desorption time scales observed in batch experiments may not be transferred to field conditions without an appropriate model accounting for both the mass-transfer mechanisms and the specific boundary conditions at hand.
    Keywords: Plastics ; Waste Water ; Water Pollutants, Chemical
    ISSN: 0013936X
    E-ISSN: 1520-5851
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  • 3
    Language: English
    In: Environmental science & technology, 03 January 2017, Vol.51(1), pp.280-290
    Description: N-Nitrosodimethylamine (NDMA) is a carcinogenic disinfection byproduct from water chloramination. Despite the identification of numerous NDMA precursors, essential parts of the reaction mechanism such as the incorporation of molecular O are poorly understood. In laboratory model systems for the chloramination of secondary and tertiary amines, we investigated the kinetics of precursor disappearance and NDMA formation, quantified the stoichiometries of monochloramine (NHCl) and aqueous O consumption, derived O-kinetic isotope effects (O-KIE) for the reactions of aqueous O, and studied the impact of radical scavengers on NDMA formation. Although the molar NDMA yields from five N,N-dimethylamine-containing precursors varied between 1.4% and 90%, we observed the stoichiometric removal of one O per N,N-dimethylamine group of the precursor indicating that the oxygenation of N atoms did not determine the molar NDMA yield. Small O-KIEs between 1.0026 ± 0.0003 and 1.0092 ± 0.0009 found for all precursors as well as completely inhibited NDMA formation in the presence of radical scavengers (ABTS and trolox) imply that O reacted with radical species. Our study suggests that aminyl radicals from the oxidation of organic amines by NHCl and N-peroxyl radicals from the reaction of aminyl radicals with aqueous O are part of the NDMA formation mechanism.
    Keywords: Dimethylnitrosamine ; Oxygen
    ISSN: 0013936X
    E-ISSN: 1520-5851
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  • 4
    Language: English
    In: Environmental science & technology, 16 June 2015, Vol.49(12), pp.7161-9
    Description: The microbially mediated reactions, that are responsible for field-scale natural attenuation of organic pollutants, are governed by the concurrent presence of a degrading microbial community, suitable energy and carbon sources, electron acceptors, as well as nutrients. The temporal lack of one of these essential components for microbial activity, arising from transient environmental conditions, might potentially impair in situ biodegradation. This study presents results of small scale flow-through experiments aimed at ascertaining the effects of substrate-starvation periods on the aerobic degradation of toluene by Pseudomonas putida F1. During the course of the experiments, concentrations of attached and mobile bacteria, as well as toluene and oxygen were monitored. Results from a fitted reactive-transport model, along with the observed profiles, show the ability of attached cells to survive substrate-starvation periods of up to four months and suggest a highly dynamic exchange between attached and mobile cells under growth conditions and negligible cell detachment under substrate-starvation conditions. Upon reinstatement of toluene, it was readily degraded without a significant lag period, even after a starvation period of 130 days. Our experimental and modeling results strongly suggest that aerobic biodegradation of BTEX-hydrocarbons at contaminated field sites is not hampered by intermittent starvation periods of up to four months.
    Keywords: Bacterial Adhesion ; Geologic Sediments -- Microbiology ; Pseudomonas Putida -- Growth & Development ; Toluene -- Metabolism
    ISSN: 0013936X
    E-ISSN: 1520-5851
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  • 5
    Language: English
    In: Environmental science & technology, 20 November 2018, Vol.52(22), pp.13511-13522
    Description: Sorption to the polystyrene (PS) of multiwell plates can affect the exposure to organic chemicals over time in in vitro and in vivo bioassays. Experimentally determined diffusion coefficients in PS ( D) were in a narrow range of 1.25 to 8.0 · 10 m s and PS-water partition constants ( K) ranged from 0.04 to 5.10 log-units for 22 neutral organic chemicals. A kinetic model, which explicitly accounts for diffusion in the plastic, was applied to predict the depletion of neutral organic chemicals from different bioassay media by sorption to various multiwell plate formats. For chemicals with log K 〉 3, the medium concentrations decreased rapidly and considerably in the fish embryo toxicity assay but medium concentrations remained relatively constant in the cell-based bioassays with medium containing 10% fetal bovine serum (FBS), emphasizing the ability of the protein- and lipid-rich medium to compensate for losses by multiwell plate sorption. The PS sorption data may serve not only for exposure assessment in bioassays but also to model the contaminant uptake by and release from plastic packaging material and the chemical transport by PS particles in the environment.
    Keywords: Organic Chemicals ; Polystyrenes
    ISSN: 0013936X
    E-ISSN: 1520-5851
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  • 6
    Language: English
    In: Environmental science & technology, 02 July 2013, Vol.47(13), pp.6892-9
    Description: The injection of a mixed toluene and D2O (conservative tracer) pulse into a pristine mesoscale aquifer enabled a first direct experimental comparison of contaminant-specific isotopic fractionation from sorption versus biodegradation and transverse dispersion on a relevant scale. Water samples were taken from two vertically resolved sampling ports at 4.2 m distance. Analysis of deuterium and toluene concentrations allowed quantifying the extent of sorption (R = 1.25) and biodegradation (37% and 44% of initial toluene at the two sampling ports). Sorption and biodegradation were found to directly affect toluene (13)C/(12)C breakthrough curves. In particular, isotope trends demonstrated that biodegradation underwent Michaelis-Menten kinetics rather than first-order kinetics. Carbon isotope enrichment factors obtained from an optimized reactive transport model (Eckert et al., this issue) including a possible isotope fractionation of transverse dispersion were ε(equ)(sorption) = -0.31 ‰, ε(kin)(transverse-dispersion) = -0.82 ‰, and ε(kin)(biodegradation) = -2.15 ‰. Extrapolation of our results to the scenario of a continuous injection predicted that (i) the bias in isotope fractionation from sorption, but not transverse dispersion, may be avoided when the plume reaches steady-state; and (ii) the relevance from both processes is expected to decrease at longer flow distances when isotope fractionation of degradation increasingly dominates.
    Keywords: Toluene -- Chemistry ; Water Pollutants, Chemical -- Chemistry
    ISSN: 0013936X
    E-ISSN: 1520-5851
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  • 7
    Language: English
    In: Environmental science & technology, 15 January 2010, Vol.44(2), pp.688-93
    Description: Mass transfer, mixing, and therefore reaction rates during transport of solutes in porous media strongly depend on dispersion and diffusion. In particular, transverse mixing is a significant mechanism controlling natural attenuation of contaminant plumes in groundwater. The aim of the present study is to gain a deeper understanding of vertical transverse dispersive mixing of reaction partners in saturated porous media. Multitracer laboratory experiments in a quasi two-dimensional tank filled with glass beads were conducted and transverse dispersion coefficients were determined from high-resolution vertical concentration profiles. We investigated the behavior of conservative tracers (i.e., fluorescein, dissolved oxygen, and bromide), with different aqueous diffusion coefficients, in a range of grain-related Peclet numbers between 1 and 562. The experimental results do not agree with the classical linear parametric model of hydrodynamic dispersion, in which the transverse component is approximated as the sum of pore diffusion and a compound-independent mechanical dispersion term. The outcome of the multitracer experiments clearly indicates a nonlinear relation between the dispersion coefficient and the average linear velocity. More importantly, we show that transverse mechanical dispersion depends on the diffusion coefficient of the compound, at least at the experimental bench-scale. This result has to be considered in reactive-transport models, because the typical assumption that two reactants with different aqueous diffusive properties are characterized by the same dispersive behavior does not hold anymore.
    Keywords: Chemical Phenomena ; Bromides -- Chemistry ; Chemistry Techniques, Analytical -- Methods ; Fluorescein -- Chemistry ; Oxygen -- Chemistry
    ISSN: 0013-936X
    E-ISSN: 15205851
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  • 8
    Language: English
    In: Environmental science & technology, 07 June 2016, Vol.50(11), pp.5729-39
    Description: Compound-specific isotope analysis (CSIA) receives increasing interest for its ability to detect natural degradation of pesticides and pharmaceuticals. Despite recent laboratory studies, CSIA investigations of such micropollutants in the environment are still rare. To explore the certainty of information obtainable by CSIA in a near-environmental setting, a pulse of the pesticide bentazone, the pesticide metabolite 2,6-dichlorobenzamide (BAM), and the pharmaceuticals diclofenac and ibuprofen was released into a mesoscale aquifer with quasi-two-dimensional flow. Concentration breakthrough curves (BTC) of BAM and ibuprofen demonstrated neither degradation nor sorption. Bentazone was transformed but did not sorb significantly, whereas diclofenac showed both degradation and sorption. Carbon and nitrogen CSIA could be accomplished in similar concentrations as for "traditional" priority pollutants (low μg/L range), however, at the cost of uncertainties (0.4-0.5‰ (carbon), 1‰ (nitrogen)). Nonetheless, invariant carbon and nitrogen isotope values confirmed that BAM was neither degraded nor sorbed, while significant enrichment of (13)C and in particular (15)N corroborated transformation of diclofenac and bentazone. Retardation of diclofenac was reflected in additional (15)N sorption isotope effects, whereas isotope fractionation of transverse dispersion could not be identified. These results provide a benchmark on the performance of CSIA to monitor the reactivity of micropollutants in aquifers and may guide future efforts to accomplish CSIA at even lower concentrations (ng/L range).
    Keywords: Carbon Isotopes ; Groundwater
    ISSN: 0013936X
    E-ISSN: 1520-5851
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  • 9
    In: Environmental Science & Technology, Oct, 1993, Vol.27(10), p.2086(12)
    Description: A gas-exchange model that contains variables for direct gas exchange over the surface of water and in the bubbles trapped within the water during a cascade in a river was developed. Results from field experiments indicate that the importance of each variable is dependent on the physicochemical properties of the contaminant compounds. This model points out the limitations of two film models of gas exchange.
    Keywords: Hydrostatics -- Research ; Rivers -- Environmental Aspects ; Cascades (Fluid dynamics) -- Research
    ISSN: 0013-936X
    E-ISSN: 15205851
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  • 10
    Language: English
    In: Environmental science & technology, 01 April 2004, Vol.38(7), pp.2089-96
    Description: Numerical and theoretical studies have indicated that pore-scale mixing can be the limiting process for reactions among dissolved compounds in porous media. It has been claimed that multicomponent reactions in porous media could be accurately estimated using mixing coefficients obtained from point-like measurements of conservative tracer concentrations. In this study, we verify these concepts experimentally by tracer tests in a homogeneously packed saturated sand column. Fiber-optic fluorometry was applied to detect point-related concentrations of fluorescein, which was used as both the conservative and the reactive tracer. In the reactive tracer experiment, an acidic solution containing the tracer was displaced by an alkaline solution without tracer. Since the fluorescence of fluorescein is quenched at low pH, the fluorescence intensity measured in the reactive breakthrough curve indicated the mixing of the two solutions. The measured reactive breakthrough curves were compared to predictions based on the conservative breakthrough curves. Predictions and measurements agreed well. Our results imply that incomplete mixing on the pore scale is of minor significance for field-scale applications. On this scale, however, even weak sorption might influence mixing significantly.
    Keywords: Models, Theoretical ; Environmental Pollutants -- Analysis
    ISSN: 0013-936X
    E-ISSN: 15205851
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