Ph.D. Dissertation Defense: Mehregan Jalalizadeh
Ph.D. Dissertation Defense Announcement
Candidate: Mehregan Jalalizadeh
Friday, June 16th at 9:00 am in TRC Room 206
Dissertation title: Understanding the science and improving the application of passive sampling
Abstract: Hydrophobic organic compounds (HOCs) such as polychlorinated biphenyls (PCBs) and polyaromatic hydrocarbons (PAHs) are bioaccumulative and toxic pollutants found in the environment. Contaminated aquatic sediments often serve as a source of HOCs by discharging the legacy pollutants into the aqueous phase (e.g., sediment porewater and surface water). To regulate and manage contaminated sediments, adequate models are needed that can accurately relate bioaccumulation and toxicity to concentration in environmental media. The freely dissolved concentration in aqueous phase (Cfree) is a useful indicator of chemical activity and can be accurately related to the amount of contaminant that accumulates in aquatic organisms or transfers diffusively between phases. To address the challenges associated with directly measuring ultra-low levels of Cfree, passive sampling using sorptive polymers has emerged as a promising technique for accurate measurement of Cfree.
The primary objectives of this research were to: 1) understand the diffusive process of HOCs in polyethylene (PE) and polyoxymethylene (POM) passive samplers by direct observation of the fluorescence signal at different stages of pyrene absorption from water into the polymers, and simulation of the kinetics using numerical integration of Fickian diffusion. 2) address the challenges associated with in situ measurements of Cfree of strongly hydrophobic compounds by introducing periodic vibration to disrupt external boundary layer around the passive samplers during deployment time and enhance the mass transport into the samplers.
The results indicate that the uptake process in PE is governed by Fick’s law and the absorption and desorption kinetics are identical in this polymer. However, the observed uptake profiles of pyrene in POM were non-Fickian and release kinetics out of POM was slower compared to uptake into the polymer. We showed that POM passive samplers are inappropriate for water deployments due to anisotropic exchange kinetics in the polymer. But they can be used for static sediment deployments, as the overall kinetics of exchange for pyrene is controlled by slow transport through sediment. Periodic vibration of PE passive samplers during deployment time enhanced the mass transfer of 16 PAHs and also large molecular weight PCBs such as hexa-, hepta-, and octachloro-PCBs from sediment into PE. Vibrated deployment resulted in estimation of porewater concentrations that were statistically indistinguishable from the measured values in the well-mixed system even after a short 7-day deployment. Porewater concentrations of the strongly hydrophobic PCB congeners were overestimated by up to an order of magnitude in the static system after the same deployment time.
