Michael L. Shelley

A model of the uptake, translocation, and accumulation of lead (Pb) by maize for the purpose of phytoextraction

Abstract Phytoextraction is a remediation technology that uses plants to remove heavy metals from soil. Effective implementation requires an understanding of plant processes that control uptake and translocation of metals from the soil. These processes are poorly understood. The purpose of this work is to gain insights on the plant mechanisms that control uptake and translocation of lead (Pb) from the soil and how these mechanisms interact to control levels of Pb accumulation in the plant. A mechanistic system dynamics modeling approach was used, simulating extraction and translocation of Pb by a maize plant. Confidence in the model structure was achieved through application of classical system dynamics validation procedures. Results of model simulations suggest that precipitation of Pb as a Pb-phosphate is one of the most important mechanisms in this system. Maximum saturable uptake rate of Pb and effective root mass may also be key plant parameters. The model may also be used to test phytoextraction management scenarios, two of which were tested in this study.

A dynamic model of bioavailability of metals in constructed wetland sediments

Abstract Constructed wetlands used for stormwater treatment accumulate metals primarily in their sediment. This sediment has the potential to produce toxic effects in benthic or aquatic organisms. Bioavailability of metals in sediments is directly linked to pore water metal activity. The mechanisms that influence pore water metal activity include physical, chemical, and biological processes. A system dynamics model was developed to represent these processes and the major influences affecting pore water metal activity in a treatment wetland receiving stormwater influent. The model structure and behavior were tested and validated using several system dynamics validation techniques. The model was run using metal specific parameter values typical of metals commonly found in stormwater runoff. Simulation results demonstrate that chemical processes of acid volatile sulfide (AVS) and organic carbon in binding metal in reduced sediments are the greatest influences in controlling metal bioavailability. The effect of bioturbation, as represented in the model, was negligible. The amount of organic carbon in the sediment plays the biggest role in controlling metal bioavailability in the long run.

An inhalation distribution model for the lactating mother and nursing child

A rule-of-thumb methodology is presented to assist in assessing risk to a nursing child due to the mothers occupational inhalation exposure. The method represents an example of the use of physiologically based pharmacokinetic modeling using state-of-the-art computational techniques. A computer model is developed to describe distribution of non-metabolized, inhaled contaminants into a mother/child system as a function of the contaminants blood:air and octanol:water partition coefficients. Risk is assessed in terms of the area under the blood concentration vs. time curve of the exposure chemical. Since low partition values yield low risk for the nursing child and high values yield high risk, the model is exercised over a range of intermediate values (blood:air = [2,25]; octanol:water = [100, 1500]). Results are thus applicable to chemicals for which the mothers dose is a strong factor in estimating the childs risk. The most notable observation is that, for the range of partition values used, this model never predicts a risk for the child greater than 25% of that of the mother. An equation is provided (based on model results) that expresses the childs risk as a fraction of the mothers risk.

Modeling the In Vivo Case with In Vitro Nanotoxicity Data

As more in vitro nanotoxicity data appear in the literature, these findings must be translated to in vivo effects to define nanoparticle exposure risk. Physiologically based pharmacokinetic (PBPK) modeling has played a significant role in guiding and validating in vivo studies for molecular chemical exposure and can develop as a significant tool in guiding similar nanotoxicity studies. This study models the population dynamics of a single cell type within a specific tissue. It is the first attempt to model the in vitro effects of a nanoparticle exposure, in this case aluminum (80 nm) and its impact on a population of rat alveolar macrophages (Wagner et al. 2007, J. Phys. Chem. B 111:7353–7359). The model demonstrates how in vitro data can be used within a simulation setting of in vivo cell dynamics and suggests that PBPK models should be developed quickly to interpret nanotoxicity data, guide in vivo study design, and accelerate nanoparticle risk assessment.

A Risk Assessment Approach for Nursing Infants Exposed to Volatile Organics through the Mother's Occupational Inhalation Exposure

Abstract A “rule-of-thumb” methodology is presented to assist in assessing risk to a nursing child due to the mothers occupational inhalation exposure. The method represents an example of the use of physiologically-based pharmacokinetic modeling using state-of-the-art computational techniques. A computer model is developed to describe distribution of nonmetabolized, inhaled contaminants into a mother/child system as a function of the contaminants blood: air and octanol: water partition coefficients. Effective dose is calculated in terms of the area under the blood concentration vs. time curve of the exposure chemical. Since low partition values yield low risk for the nursing child and high values yield high risk, the model is exercised over a range of intermediate values (blood: air = [2, 25]; octanol: water = [100, 1500]). Results are thus applicable to chemicals for which the mothers dose is a strong factor in estimating the childs risk. The most notable observation is that, for the range of partiti...

Natural attenuation potential of tricholoroethene in wetland plant roots: role of native ammonium-oxidizing microorganisms.

Bench-scale microcosms with wetland plant roots were investigated to characterize the microbial contributions to contaminant degradation of trichloroethene (TCE) with ammonium. The batch system microcosms consisted of a known mass of wetland plant roots in aerobic growth media where the roots provided both an inoculum of root-associated ammonium-oxidizing microorganisms and a microbial habitat. Aqueous growth media, ammonium, and TCE were replaced weekly in batch microcosms while retaining roots and root-associated biomass. Molecular biology results indicated that ammonium-oxidizing bacteria (AOB) were enriched from wetland plant roots while analysis of contaminant and oxygen concentrations showed that those microorganisms can degrade TCE by aerobic cometabolism. Cometabolism of TCE, at 29 and 46 μg L(-1), was sustainable over the course of 9 weeks, with 20-30 mg L(-1) ammonium-N. However, at 69 μg L(-1) of TCE, ammonium oxidation and TCE cometabolism were completely deactivated in two weeks. This indicated that between 46 and 69 μg L(-1) TCE with 30 mg L(-1) ammonium-N there is a threshold [TCE] below which sustainable cometabolism can be maintained with ammonium as the primary substrate. However, cometabolism-induced microbial deactivation of ammonium oxidation and TCE degradation at 69 μg L(-1) TCE did not result in a lower abundance of the amoA gene in the microcosms, suggesting that the capacity to recover from TCE inhibition was still intact, given time and removal of stress. Our study indicates that microorganisms associated with wetland plant roots can assist in the natural attenuation of TCE in contaminated aquatic environments, such as urban or treatment wetlands, and wetlands impacted by industrial solvents.