PubMed Health⌕ Search

Biomedical subjects

David Werner

Publications and source records attributed to David Werner.

11 recordsLinked to original sources
PubMed2006

Modeling polychlorinated biphenyl mass transfer after amendment of contaminated sediment with activated carbon.

The sorption kinetics and concentration of polychlorinated biphenyls (PCBs) in historically polluted sediment is modeled to assess a remediation strategy based on in situ PCB sequestration by mixing with activated carbon (AC). We extend our evaluation of a model based on intraparticle diffusion by including a biomimetic semipermeable membrane device (SPMD) and a first-order degradation rate for the aqueous phase. The model predictions are compared with the previously reported experimental PCB concentrations in the bulk water phase and in SPMDs. The simulated scenarios comprise a marine and a freshwater sediment, four PCB congeners, two AC grain sizes, four doses of AC, and comparison with laboratory experiments for up to 540 days of AC amendment slowly mixed with sediment. The model qualitatively reproduces the observed shifts in the PCB distribution during repartitioning after AC amendment but systematically overestimates the overall effect of the treatment in reducing aqueous and SPMD concentrations of PCBs by a factor of 2-6. For our AC application in sediment, competitive sorption of the various solutes apparently requires a reduction by a factor of 16 of the literature values for the AC-water partitioning coefficient measured in pure aqueous systems. With this correction, model results and measurements agree within a factor of 3. We also discuss the impact of the nonlinearity of the AC sorption isotherm and first-order degradation in the aqueous phase. Regular mixing of the sediment accelerates the benefit of the proposed amendment substantially. But according to our scenario, after AC amendment is homogeneously mixed into the sediment and then left undisturbed, aqueous PCB concentrations tend toward the same reduction after approximately 5 or more years.

Absorption↗
PubMed2006

Clearspeech: a display reader for the visually handicapped.

Many domestic appliances and much office equipment is controlled using a keypad and a small digital display. Programming such devices is problematical for the blind and visually handicapped. In this paper, we describe a device that may be used to read the displays on these devices. The device is designed to accept a description of the display being read, which specifies the types and locations of elements of the display. Images are captured using a handheld webcam. Images are processed to remove the distortions due to the camera orientation. The elements of the screen are interpreted and a suitable audio output is generated. In suitably illuminated scenes, the display data is interpreted correctly in approximately 90% of the cases investigated.

Communication Devices for People with Disabilities↗
PubMed2005

Phenanthrene and pyrene sorption and intraparticle diffusion in polyoxymethylene, coke, and activated carbon.

We report sorption isotherms and uptake kinetics for phenanthrene and pyrene with three organic model sorbents: polyoxymethylene (POM), coke, and activated carbon. We combine batch equilibration and kinetic experiments with the direct observation of the long-term diffusion of phenanthrene and pyrene as measured within cross-sectioned particles using microprobe laser-desorption laser-ionization mass spectroscopy (muL2MS). For POM pellets, the intraparticle concentration profiles predicted from kinetic batch experiments and a polymer diffusion model with spherical geometry are in agreement with the independent muL2MS measurements. For coke particles, the apparent diffusivities decreased with smaller particle size. These trends in diffusivities were described by a sorption-retarded pore diffusion model with a particle-size-dependent solid-water partitioning coefficient obtained from apparent equilibrium observed in the kinetic batch studies. For activated carbon, the muL2MS measurements showed faster radial diffusion of phenanthrene and pyrene into the particle interior than predicted from diffusion models based on a single sorption domain and diffusivity. A branched pore kinetic model, comprising polycyclic aromatic hydrocarbon (PAH) macropore diffusion with kinetic exchange of PAH between macroporous and microporous domains, fits the experimental observations better. Because of parallel macro- and microdiffusion processes, nonlinear sorption isotherms, and a concentration-dependent diffusivity, it is not possible to make independent parameter estimations for intraparticle diffusion in activated carbon using our present procedures.

Adsorption↗
PubMed2005

The sequestration of PCBs in Lake Hartwell sediment with activated carbon.

Recent laboratory studies with a marine sediment from a tidal mudflat in San Francisco Bay demonstrated that a low dose of activated carbon enhances the sequestration of PCBs and PAHs under well-mixed conditions. Here we compare activated carbon treatment for a freshwater sediment from a rural site, using PCB-contaminated sediment from Lake Hartwell, SC. An activated carbon dose of 2% of the dry sediment mass lowered total aqueous PCB concentrations by more than 95% after 1 month of treatment and more than 98% after 6 months. Aqueous PCB concentrations remained below detection limits following 18 months of sediment-carbon contact in slowly mixed systems. Uptake of PCBs into semipermeable membrane devices was reduced by 78%, 91% and 97% over 1, 6 and 18 months, respectively. These tests show that PCB sequestration with activated carbon improves with contact time and is not diminished by prolonged mixing with sediment. Desorption studies confirmed the sequestration, in which 74% of the total PCB mass could be desorbed from untreated Lake Hartwell sediment within 30 days as compared to only 7% after activated carbon treatment for 6 months. We compare these observations with marine sediment from San Francisco Bay and propose a conceptual model to rationalize how sediment properties relate to the reduction in aqueous PCB concentrations during activated carbon treatment.

Carbon↗
PubMed2005

Effects of dose and particle size on activated carbon treatment to sequester polychlorinated biphenyls and polycyclic aromatic hydrocarbons in marine sediments.

Recent laboratory studies show that mixing activated carbon with contaminated sediment reduces the chemical and biological availability of hydrophobic organic contaminants. In this study, we test the effects of varying the activated carbon dose and particle size in reducing the aqueous availability of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) and the uptake of PCBs by two benthic organisms. We mixed PCB- and PAH-contaminated sediment from Hunters Point Naval Shipyard, San Francisco Bay (CA, USA), for one month with activated carbon, at doses of 0.34, 1.7, and 3.4% dry mass basis. We found that increasing the carbon dose increased the effectiveness in reducing PCB bioaccumulation. In 56-d uptake tests with the benthic organisms Neanthes arenaceodentata and Leptocheirus plumulosus, PCB bioaccumulation was reduced by 93 and 90%, respectively, with 3.4% carbon. Increasing the dose also increased the effectiveness in reducing PCB and PAH aqueous concentrations and uptake by semipermeable membrane devices and quiescent flux of PCBs to overlying water. Decreasing activated carbon particle size increased treatment effectiveness in reducing PCB aqueous concentration, and larger-sized activated carbon (400-1,700 microm) was ineffective with a contact period of one month. We invoke a numerical model based on intraparticle diffusion in sediment and activated carbon particles to help interpret our experimental results. This model was useful in explaining the trends for the effect of activated carbon dose and particle size on PCB aqueous concentrations in well-mixed systems.

Carbon↗
PubMed2005

Physicochemical characterization of coke-plant soil for the assessment of polycyclic aromatic hydrocarbon availability and the feasibility of phytoremediation.

Coke oven site soil was characterized to assess the particle association and availability of polycyclic aromatic hydrocarbons (PAHs). We identified various carbonaceous materials including coal, coke, pitch, and tar decanter sludge. Most of the PAHs were associated with the polymeric matrix of tar sludge or hard pitch as discrete particles, coatings on soil mineral particles, or complex aggregates. The PAH availability from these particles was very low due to hindered diffusive release from solid tar or pitch with apparent diffusivities of 6 x 10(-15) for phenanthrene, 3 x 10(-15) for pyrene, and 1 x 10(-15) cm2/s for benzo[a]pyrene. Significant concentrations of PAHs were observed in the interior of solid tar aggregates with up to 40,000 mg/kg total PAHs. The release of PAHs from the interior of such particles requires diffusion over a substantial distance, and semipermeable membrane device tests confirmed a very limited availability of PAHs. These findings explain the results from three years of phytoremediation of the site soil, for which no significant changes in the total PAH concentrations were observed in the test plot samples. The observed low bioavailability of PAHs probably inhibited PAH phytoremediation, as diffusion-limited mass transfer would limit the release of PAHs to the aqueous phase.

Acetone↗
PubMed2003

In situ method to measure effective and sorption-affected gas-phase diffusion coefficients in soils.

Transport modeling, risk assessment, and the evaluation of remediation strategies at contaminated sites require the knowledge of gas diffusivities in soil. A field method is presented, which determines the tortuosity factor and the mass fraction in the air phase of a volatile compound in situ. The compound is injected into the unsaturated zone together with a conservative gaseous tracer to form a point source. Concentrations are monitored at the injection point during 8 h and evaluated with an analytical equation for reactive transport. The air-filled porosity is determined independently. From these data, both the effective and the sorption-affected diffusion coefficients are obtained. Results are reported for volatile organic pollutants in both a lysimeter and a sandy soil. The measurements show good reproducibility. Batch experiments suggest that tracers were not truly conservative at subsurface temperatures. This may lead to a systematic underestimation of the effective diffusion coefficient by less than 10%, but the sorption-affected diffusion coefficients were probably overestimated by 15-20%. Nevertheless, the in situ method can avoid considerable uncertainties associated with choosing appropriate empirical relationships for the tortuosity factor or deviations from natural soil conditions in laboratory experiments.

Adsorption↗
PubMed2003

Small-volume releases of gasoline in the vadose zone: impact of the additives MTBE and ethanol on groundwater quality.

A controlled gasoline spill experiment was performed under outdoor conditions typical for winter in temperate regions to study the fate of methyl tert-butyl ether (MTBE), ethanol, benzene, and selected other petroleum hydrocarbons. Artificial gasoline containing MTBE and ethanol (5% w/w of each) was placed at a defined depth into a 2.3 m thick unsaturated zone of alluvial sand overlying a gravel aquifer in a lysimeter. During an initial period of 41 days without recharge, MTBE and hydrocarbon vapors migrated by vapor-phase diffusion to groundwater, while ethanol vapors were naturally attenuated. In a subsequent period of 30 days with 5-mm daily recharge, all soluble compounds including ethanol were transported to the groundwater. Ethanol disappeared concomitantly with benzene and all other petroleum hydrocarbons except isooctane from the aerobic groundwater due to biodegradation. MTBE persisted for longer than 6 months at concentrations larger than 125000 microg L(-1). No evidence for MTBE biodegradation was found, whereas > 99.6% of ethanol removal from the lysimeter was due to biodegradation. It is concluded that MTBE-free gasoline would be less harmful for groundwater resources and that ethanol is an acceptable substitute.

Biodegradation, Environmental↗
PubMed2002

Diffusive partitioning tracer test for nonaqueous phase liquid (NAPL) detection in the vadose zone.

This paper proposes the theory and practical application of a new partitioning tracer test for nonaqueous phase liquid (NAPL) detection in the vadose zone, which is based on diffusion. A mixture of chlorofluorocarbons as gaseous tracers is injected into the vadose zone to form a point source at the injection point. While the tracers diffuse away, small volumes of gas are withdrawn from the injection point. The quantitative determination of the NAPL saturation is based on a comparison of the concentration decline of tracers with different air-NAPL partitioning coefficients. The test has been evaluated in laboratory sand columns contaminated with dodecane. NAPL in saturations of 0.8-4% of the total porosity have been quantified in a wide range of different water contents. Actual and measured NAPL saturations calculated as an average from four different tracer pairs agreed within +/-30%. The new method was successfully used for repeated NAPL quantification in a large-scale field lysimeter contaminated with artificial kerosene. This rapid and inexpensive test is potentially of value for site investigations especially in combination with soil gas measurements, because it requires similar equipment. Possible applications are source delineation and repeated NAPL quantification in situ during a remediation.

Air↗
PubMed2002

Vapor phase transport and biodegradation of volatile fuel compounds in the unsaturated zone: a large scale lysimeter experiment.

The vapor phase transport and biodegradation of typical fuel compounds including volatile petroleum hydrocarbons and methyl tert-butyl ether (MTBE) was studied in a large scale field lysimeter representing a 2.3 m thick sandy unsaturated zone over a gravel aquifer. A mixture of 13 fuel compounds with MTBE (5%) was placed at a defined depth in the unsaturated zone to obtain a homogeneous source zone with a residual NAPL saturation. The upward and downward transport of fuel vapors and the biodegradation by indigenous microorganisms were monitored during 70 days. Using tracers in water and NAPL, it was shown that fuel compounds were transported by vapor phase diffusion only. All fuel compounds except MTBE disappeared from the lysimeter below the analytical detection limits within 70 days. MTBE accumulated in groundwater but volatilized from the unsaturated zone. First-order biodegradation rates were estimated in the unsaturated zone to range from <0.05 d(-1) for MTBE up to 8.7 d(-1) for octane. Aerobic biodegradation of degradable fuel compounds to CO2 started without any lag phase and removed about 3 times more fuel mass than volatilization. The study illustrates the recalcitrance of MTBE vapors compared to other fuel vapors, leading to a significant groundwater pollution with MTBE.

Air Pollutants↗