In-Tube Passive Dosing of Hydrophobic Organic Chemicals: Controlling Freely Dissolved Concentrations in Flow-Through and Large-Volume Experiments

Passive dosing is increasingly used to control exposure concentrations of hydrophobic organic chemicals (HOCs) in small-sized aquatic tests. This study introduces in-tube passive dosing to control HOC exposure in larger test volumes and flow-through experiments. Water was dosed by passing it through a tube holding HOC-loaded silicone rods. We equipped a 6 m PTFE tube (ID 10 mm) with four parallel silicone rods (OD 3 mm) to accommodate water flows from 0.2 to 120 L/h and developed a mass transfer kinetic model to capture the chemodynamics in the system. The first experiment was conducted with fluoranthene to determine the dosing kinetics, reproducibility, and long-term performance of the system. Aqueous concentrations were measured by molecular fluorescence at different flows and positions within the tube. The second experiment was conducted with a complex petroleum mixture (cracked gas oil). Concentrations of 22 mixture constituents were measured by solid phase microextraction coupled to GC-MS. Overall, the system provided stable and reproducible water concentrations, which were at equilibrium for flows less than 10 mL/min and at steady state for higher flows. Fluoranthene concentrations declined less than 20% when dosing 1 m³ during 1 week. The mass transfer kinetic model can now be used to scale the system for various applications.