Combined temperature and salinity effects on the passive sampling of PAHs with an assessment of impacts to petroleum toxicity

Abstract

In equilibrium-based passive sampling applications, the accuracy of estimating freely dissolved concentration (C_free) of hydrophobic organic compounds (HOCs) relies on the passive sampler–water partition coefficient (K_PS–W) values applied. The vast majority of K_PS–W are generated under standard conditions: 20 °C in deionized or freshwater. Few empirically derived values are available for non-standard conditions. In this study, polyethylene (PE)–water partitioning coefficients (K_PE–W) were experimentally determined for 15 polycyclic aromatic hydrocarbons (PAHs, comprising 9 parent and 6 alkylated compounds) under three different temperature (10, 20, 30 °C) and salinity (0, 18 and 36‰) regimes, the K_PE–W values were found to correlate strongly with a variety of molecular parameters (e.g., octanol–water partition coefficients (K_OW), molecular weight (MW) and molecular volume (M_VOL)). The effects of temperature and salinity on the magnitude of K_PE–W were found to be substantial. For temperature, the values range between −0.005 and −0.023 log units per °C; these values indicate that every 10 °C rise in temperature would potentially decrease the K_PE–W by a factor of between 0.4 to 1.6. For salinity, the values range from 0.0028 to 0.0057 log units per unit ‰, indicating that an 18‰ increase in salinity would likely increase the K_PE–W by a factor of between 0.28 and 0.82. Moreover, temperature and salinity were shown to be independent of each other and non-interacting. Temperature effects were chemical-specific and moderately dependent on hydrophobicity (expressed as the K_OW), whereas salinity effects were independent of hydrophobicity. We also assessed the combined impact of temperature and salinity, which showed increasing effects with the hydrophobicity of the PAHs studied. Based on the results, K_PE–W values adjusted for site-specific temperature and salinity can be calculated. The impact of applying such site-specific values was demonstrated using a PE-based field monitoring dataset for PAHs from coastal waters of Grand Isle (LA, USA) collected during the 2010 Deepwater Horizon oil spill. When K_PE–W values were adjusted to 10 °C and 30 °C, the final freely dissolved concentrations (C_free) decreased or increased depending on the adjustment. Use of the results of this investigation allow for adjusting existing PE-based datasets to site-specific conditions resulting in more accurate C_free values for estimating exposure and adverse ecological effects.