Guest Editorial: Contaminant Source Zones: Remediation or Perpetual Stewardship?-0

<p><b>Copyright information:</b></p><p>Taken from "Guest Editorial: Contaminant Source Zones: Remediation or Perpetual Stewardship?"</p><p>Environmental Health Perspectives 2005;113(7):A438-A439.</p><p>Published online Jan 2005</p><p>PMCID:PMC1257656.</p><p>This is an Open Access article: verbatim copying and redistribution of this article are permitted in all media for any purpose.</p

Quantifying Impacts of Microcosm Mass Loss on Kinetic Constant Estimation

Microcosm experiments to assess microbial reductive dechlorination of chlorinated aliphatic hydrocarbons typically experience 5–50% mass loss due to frequent sampling events and diffusion through septa. A literature review, however, reveals that models fit to such experiments for kinetic constant estimation have generally failed to account for experimental mass loss. To investigate possible resultant bias in best-fit parameters, a series of numerical experiments was conducted in which Monod kinetic models with and without mass loss were fit to more than 1300 synthetic data sets, generated using published microcosm data. Models that failed to account for mass loss resulted in significant fitted parameter bias. Bias ranged from 5 to 45% of the parameter magnitude for Monte Carlo simulations with low (approximately 10%) mass loss to 20–120% for simulations with high (approximately 40%) mass loss. In addition, for high mass loss simulations, best-fit values consistently fell along the bounds of the optimization range. These results suggest that failure to properly account for mass loss in microcosms may lead to inaccurate estimation of kinetic constants and may explain some of the literature-reported variability in these parameters. A model is presented that provides a method for including sampling and diffusional mass losses to improve kinetic constant estimation accuracy

Accumulation of PFOA and PFOS at the Air–Water Interface

Knowledge of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) accumulation at the air–water interface is critical to understanding the fate and transport of these substances in subsurface environments. The surface tension of aqueous solutions containing PFOA and PFOS at concentrations ranging from 0.1 to >1000 mg/L and with dissolved solids (i.e., cations and anions) commonly found in groundwater was measured using the Wilhelmy plate method. The surface tensions of solutions containing dissolved solids were lower than those for ultrapure water, indicating an increase in the surface excess of PFOA and PFOS in the presence of dissolved solids. An equation for the surface excess of PFOA and PFOS with total dissolved solids was developed by fitting the measured surface tension values, which ranged from 72.0 to 16.7 mN/m, to the Szyszkowski equation. On the basis of mass distribution calculations for a representative unsaturated, fine-grained soil, up to 78% of the PFOA and PFOS mass will accumulate at the air–water interface, with the remaining mass dissolved in water and adsorbed on the solids

Investigation of the Transport and Deposition of Fullerene (C60) Nanoparticles in Quartz Sands under Varying Flow Conditions

A coupled experimental and mathematical modeling investigation was undertaken to explore nanoscale fullerene aggregate (nC60) transport and deposition in water-saturated porous media. Column experiments were conducted with four different size fractions of Ottawa sand at two pore-water velocities. A mathematical model that incorporates nonequilibrium attachment kinetics and a maximum retention capacity was used to simulate experimental nC60 effluent breakthrough curves and deposition profiles. Fitted maximum retention capacities (Smax), which ranged from 0.44 to 13.99 μg/g, are found to be correlated to normalized mass flux. The developed correlation provides a means to estimate Smax as a function of flow velocity, nanoparticle size, and mean grain size of the porous medium. Collision efficiency factors, estimated from fitted attachment rate coefficients, are relatively constant (∼0.14) over the range of conditions considered. These fitted values, however, are more than 1 order of magnitude larger than the theoretical collision efficiency factor computed from Derjaguin−Landau−Verwey−Overbeek (DLVO) theory (0.009). Data analyses suggest that neither physical straining nor attraction to the secondary minimum is responsible for this discrepancy. Patch-wise surface charge heterogeneity on the sand grains is shown to be the likely contributor to the observed deviations from classical DLVO theory. These findings indicate that modifications to clean-bed filtration theory and consideration of surface heterogeneity are necessary to accurately predict nC60 transport behavior in saturated porous media

Mathematical Modeling of the Transport and Dissolution of Citrate-Stabilized Silver Nanoparticles in Porous Media

A one-dimensional mathematical model is developed and implemented to describe the coupled transport of citrate-stabilized silver nanoparticles (nAg) and dissolved silver ions in porous media. This hybrid numerical simulator employs an Eulerian finite difference (FD) method to model the reactive transport of dissolved constituents and a Lagrangian (random-walk particle-tracking (RWPT)) approach to capture the transport and differential aging of nanoparticles. Model performance is demonstrated by comparison of simulations with data obtained from a series of nAg transport and dissolution column experiments. A three pore volume pulse of a citrate-stabilized nAg suspension (ca. 3 mg/L) was introduced into a 12 or 16 cm long column packed with water-saturated quartz sand at a pore-water velocity of ca. 7.6 m/day and pH 4 or 7. While low retention levels (ca.17%) and no dissolution were observed for the pH 7 column, analysis of column effluent samples for pH 4 conditions indicated that ca. 88% of the injected silver mass was retained in the column, while 6% was eluted as particles (nAg) and 6% as dissolved ions (Ag+). Hybrid model simulations, employing a lumped nAg dissolution coefficient of (3.45 ± 0.35) × 10–2/h, are shown to accurately capture measured nAg transport and Ag+ release behavior. A model sensitivity analysis explores the influence of flow velocity and particle size on nAg transport and fate, indicating that as velocity and particle size decrease, nAg dissolution and Ag+ transport processes increasingly dominate silver mobility

Experimental and Numerical Validation of the Total Trapping Number for Prediction of DNAPL Mobilization

The total trapping number (NT), quantifying the balance of gravitational, viscous, and capillary forces acting on an entrapped dense nonaqueous phase liquid (DNAPL) droplet, was originally developed as a criterion to predict the onset and extent of residual DNAPL mobilization in porous media. The ability of this approach to predict mobilization behavior, however, has not been rigorously validated in multidimensional systems. In this work, experimental observations of residual tetrachloroethene (PCE) mobilization in rectangular columns are compared to predictions obtained using a multiphase compositional finite-element simulator that was modified to incorporate the dependence of entrapped residual, flow, and transport parameters on the total trapping number. Consistent with calculated NT values (1.21 × 10−3–1.10 × 10−2), residual PCE-DNAPL was mobilized immediately upon contact with a low-interfacial tension (IFT) surfactant solution and rapidly migrated downward to form a bank of mobile DNAPL. The numerical model accurately captured the onset and extent of PCE-DNAPL mobilization, the angle and migration of the DNAPL bank, the swept path of the surfactant solution, and cumulative PCE recovery. These findings demonstrate the utility of the total trapping number for prediction of DNAPL mobilization behavior during low-IFT flushing

Development and Validation of a Two-Stage Kinetic Sorption Model for Polymer and Surfactant Transport in Porous Media

Understanding the sorption processes is critical to the successful design and implementation of a variety of technologies in subsurface application. Most transport models assume minimal interactions between adsorbed species and, thus, are unable to accurately describe the formation of adsorbed bilayers. To address this limitation, a two-stage kinetic sorption model is developed and incorporated into a one-dimensional advective–dispersive–reactive transport simulator. The model is evaluated using data obtained from column experiments conducted with a representative polymer [gum arabic (GA)] and a nonionic surfactant [Witconol 2722 (WT)] under a range of experimental conditions. Model simulations demonstrate that the first-stage polymer/surfactant-surface sorption rate is at least 1 order of magnitude greater than the second-stage rate, associated with bilayer formation, indicating that the first-stage reaction is more favorable. The reversibility of the second-stage sorption process is found to be compound-specific, with irreversible sorption observed for GA and prolonged tailing observed for WT. This study demonstrates that the developed two-stage kinetic model is superior to a two-stage equilibrium-based model in its replication of two-leg breakthrough curves observed in core flood experiments; the normalized root-mean-square error between measurement and regressed model simulations was reduced by an average of 41% with the kinetic approach

Experimental and Numerical Validation of the Total Trapping Number for Prediction of DNAPL Mobilization

The total trapping number (NT), quantifying the balance of gravitational, viscous, and capillary forces acting on an entrapped dense nonaqueous phase liquid (DNAPL) droplet, was originally developed as a criterion to predict the onset and extent of residual DNAPL mobilization in porous media. The ability of this approach to predict mobilization behavior, however, has not been rigorously validated in multidimensional systems. In this work, experimental observations of residual tetrachloroethene (PCE) mobilization in rectangular columns are compared to predictions obtained using a multiphase compositional finite-element simulator that was modified to incorporate the dependence of entrapped residual, flow, and transport parameters on the total trapping number. Consistent with calculated NT values (1.21 × 10−3–1.10 × 10−2), residual PCE-DNAPL was mobilized immediately upon contact with a low-interfacial tension (IFT) surfactant solution and rapidly migrated downward to form a bank of mobile DNAPL. The numerical model accurately captured the onset and extent of PCE-DNAPL mobilization, the angle and migration of the DNAPL bank, the swept path of the surfactant solution, and cumulative PCE recovery. These findings demonstrate the utility of the total trapping number for prediction of DNAPL mobilization behavior during low-IFT flushing

Coupling Aggressive Mass Removal with Microbial Reductive Dechlorination for Remediation of DNAPL Source Zones: A Review and Assessment-3

<p><b>Copyright information:</b></p><p>Taken from "Coupling Aggressive Mass Removal with Microbial Reductive Dechlorination for Remediation of DNAPL Source Zones: A Review and Assessment"</p><p>Environmental Health Perspectives 2004;113(4):465-477.</p><p>Published online 8 Dec 2004</p><p>PMCID:PMC1278488.</p><p>This is an Open Access article: verbatim copying and redistribution of this article are permitted in all media for any purpose, provided this notice is preserved along with the article's original DOI.</p

Coupling Aggressive Mass Removal with Microbial Reductive Dechlorination for Remediation of DNAPL Source Zones: A Review and Assessment-5

<p><b>Copyright information:</b></p><p>Taken from "Coupling Aggressive Mass Removal with Microbial Reductive Dechlorination for Remediation of DNAPL Source Zones: A Review and Assessment"</p><p>Environmental Health Perspectives 2004;113(4):465-477.</p><p>Published online 8 Dec 2004</p><p>PMCID:PMC1278488.</p><p>This is an Open Access article: verbatim copying and redistribution of this article are permitted in all media for any purpose, provided this notice is preserved along with the article's original DOI.</p