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. Polluted 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; and 2) address the challenges associated with in situ measurements of Cfree of strongly hydrophobic compounds by introducing periodic vibration to disrupt the 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'slaw and the absorption and desorption kinetics are identical in this polymer. However, the observed uptake profiles of pyrene in POM were not fully explained by Fickian diffusion 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 non-isotropic 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 mass transfer of 16 PAHs and also large molecular weight PCBs such as hexa-, hepta-, and octachloro-PCBs from sediment into PE. Periodic vibration also increased the dissipation rate of four performance reference compounds (PRCs) from passive samplers. Higher fractional loss of PRCs and closer approach to equilibrium in the vibrated deployment resulted in estimation of corrected porewater concentrations that were statistically indistinguishable from the true equilibrium values 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 passive sampler after the same deployment time.