Establishing and controlling dissolved concentrations of hydrophobic organics by partitioning from a solid phase

Bioavailable concentrations of hydrophobic organic substances in aqueous tests often decrease as a result of (bio)sorption, volatilization, or degradation. In the present study, partitioning driven administering (PDA) is introduced as an approach to maintain constant exposure levels of hydrophobic organics in aqueous tests. The working principle of PDA is to control dissolved concentrations by partitioning from a dominating solid phase. For this purpose, test compounds are loaded on an octadecyl Empore disk, which subsequently is placed in an aqueous test system. Stable aqueous concentrations for a range of halogenated aromatic substances (log K-OW 3.9-7.1) were established within minutes to hours, and the resulting concentrations indicate that equilibrium partitioning governs the release from the disk. Furthermore, PDA's capability to maintain stable concentrations against a depletion process was demonstrated, and PDA was successfully applied to dose 1,3,5-tribromobenzene and hexachlorobenzene in algal toxicity tests. Finally, the feasibility to administer concentrations up to aqueous solubility, to administer mixtures, and to dominate the fugacity of multicompartment systems is discussed.