Biomedical subjects

Shannon L Bartelt-Hunt

Publications and source records attributed to Shannon L Bartelt-Hunt.

5 recordsLinked to original sources

PubMed2006

Application of heuristic optimization techniques and algorithm tuning to multilayered sorptive barrier design.

Although heuristic optimization techniques are increasingly applied in environmental engineering applications, algorithm selection and configuration are often approached in an ad hoc fashion. In this study, the design of a multilayer sorptive barrier system served as a benchmark problem for evaluating several algorithm-tuning procedures, as applied to three global optimization techniques (genetic algorithms, simulated annealing, and particle swarm optimization). Each design problem was configured as a combinatorial optimization in which sorptive materials were selected for inclusion in a landfill liner to minimize the transport of three common organic contaminants. Relative to multilayer sorptive barrier design, study results indicate (i) the binary-coded genetic algorithm is highly efficient and requires minimal tuning, (ii) constraint violations must be carefully integrated to avoid poor algorithm convergence, and (iii) search algorithm performance is strongly influenced by the physical-chemical properties of the organic contaminants of concern. More generally, the results suggest that formal algorithm tuning, which has not been widely applied to environmental engineering optimization, can significantly improve algorithm performance and provide insight into the physical processes that control environmental systems.

Algorithms↗

PubMed2006

Fate of chemical warfare agents and toxic industrial chemicals in landfills.

One component of preparedness for a chemical attack is planning for the disposal of contaminated debris. To assess the feasibility of contaminated debris disposal in municipal solid waste (MSW) landfills, the fate of selected chemical warfare agents (CWAs) and toxic industrial chemicals (TICs) in MSW landfills was predicted with a mathematical model. Five blister agents [sulfur mustard (HD), nitrogen mustard (HN-2), lewisite (L), ethyldichloroarsine (ED), and phosgene oxime (CX)], eight nerve agents [tabun (GA), sarin (GB), soman (GD), GE, GF, VX, VG, and VM], one riot-control agent [CS], and two TICs [furan and carbon disulfide] were studied. The effects of both infiltration (climate) and contaminant biodegradability on fate predictions were assessed. Model results showed that hydrolysis and gas-phase advection were the principal fate pathways for CWAs and TICs, respectively. Apart from CX and the TICs, none of the investigated compounds was predicted to persist in a landfill for more than 5 years. Climate had little impact on CWA/TIC fate, and biodegradability was only important for compounds with long hydrolysis half-lives. Monte Carlo simulations were performed to assess the influence of uncertainty in model input parameters on CWA/TIC fate predictions. Correlation analyses showed that uncertainty in hydrolysis rate constants was the primary contributor to variance of CWA fate predictions, while uncertainty in the Henry's Law constant and landfill gas-production rate accounted for most of the variance of TIC fate predictions. CWA hydrolysates were more persistent than the parent CWAs, but limited information is available on abiotic or biotic transformation rates for these chemicals.

Biodegradation, Environmental↗

PubMed2003

Nonionic organic solute sorption onto two organobentonites as a function of organic-carbon content.

Sorption of three nonionic organic solutes (benzene, trichloroethene, and 1,2-dichlorobenzene) to hexadecyltrimethylammonium bentonite (HDTMA bentonite) and benzyltriethylammonium bentonite (BTEA bentonite) was measured as a function of total organic-carbon content at quaternary ammonium cation loadings ranging from 30 to 100% of the clay's cation-exchange capacity. Sorption of all three solutes to HDTMA bentonite was linear and sorption of all three solutes by the HDTMA bentonite increased as the organic-carbon content of the clay increased. 1,2-Dichlorobenzene sorbed most strongly to HDTMA bentonite, followed by benzene and TCE. The stronger sorption of benzene to HDTMA bentonite compared to TCE was unexpected based on a partition mechanism of sorption and consideration of solute solubility. LogK(oc) values for all three solutes increased with organic-carbon content. This suggests that the increased organic-carbon content alone may not explain the observed increase in sorption capacity. Sorption of the three solutes to BTEA bentonite was nonlinear and solute sorption increased with decreasing organic-carbon content, with a peak in the magnitude of solute sorption occurring at an organic-carbon content corresponding to 50% of CEC. Below 50% of CEC, sorption of all three solutes to BTEA bentonite decreased with decreasing organic-carbon content. Surface area measurements indicate that the surface area of both organobentonites generally decreased with increasing organic-carbon content. Since nonionic organic solute sorption to BTEA bentonite occurs by adsorption, the reduced sorption is likely caused by the reduction in surface area corresponding to increased organic-cation loading.

Journal Article↗

PubMed2003

Sorption and permeability of gasoline hydrocarbons in organobentonite porous media.

We investigate the use of organobentonites as liners for underground gasoline storage tanks to reduce the risk of subsurface contamination. A series of permeability measurements were conducted on two types of organobentonites: benzyltriethylammonium-bentonite (BTEA-bentonite) and hexadecyltrimethylammonium-bentonite (HDTMA-bentonite). Both water and commercial unleaded gasoline were used as the permeant liquids. Results of these measurements indicate that the intrinsic permeability of the organobentonite decreases by one to two orders of magnitude when the permeant liquid is changed from water to gasoline. Results of batch sorption measurements reveal that benzene sorption to both organobentonites from water is greater than benzene sorption to conventional bentonite. The magnitude of benzene sorption is related to the loading of the organic quaternary ammonium cation on the clay. As the HDTMA cation loading increases from 25% of cation exchange capacity (CEC) to 120% of CEC, benzene sorption increases. However, as the BTEA cation loading increases from 40 to 120% of CEC, benzene sorption decreases. Collectively, these results suggest that organobentonites can be used effectively to reduce hydrocarbon migration rates beneath leaking underground gasoline storage tanks, and that the optimal organic cation loading with respect to pollutant sorption may be less than 50% of cation exchange capacity for some organobentonite-solute combinations.

Adsorption↗

PubMed2002

Measurement of effective air diffusion coefficients for trichloroethene in undisturbed soil cores.

In this study, we measure effective diffusion coefficients for trichloroethene in undisturbed soil samples taken from Picatinny Arsenal, New Jersey. The measured effective diffusion coefficients ranged from 0.0053 to 0.0609 cm2/s over a range of air-filled porosity of 0.23-0.49. The experimental data were compared to several previously published relations that predict diffusion coefficients as a function of air-filled porosity and porosity. A multiple linear regression analysis was developed to determine if a modification of the exponents in Millington's [Science 130 (1959) 100] relation would better fit the experimental data. The literature relations appeared to generally underpredict the effective diffusion coefficient for the soil cores studied in this work. Inclusion of a particle-size distribution parameter, d10, did not significantly improve the fit of the linear regression equation. The effective diffusion coefficient and porosity data were used to recalculate estimates of diffusive flux through the subsurface made in a previous study performed at the field site. It was determined that the method of calculation used in the previous study resulted in an underprediction of diffusive flux from the subsurface. We conclude that although Millington's [Science 130 (1959) 100] relation works well to predict effective diffusion coefficients in homogeneous soils with relatively uniform particle-size distributions, it may be inaccurate for many natural soils with heterogeneous structure and/or non-uniform particle-size distributions.

Air Pollutants↗