Accelerated Passive Dosing of Hydrophobic Complex Mixtures–Controlling the Level and Composition in Aquatic Tests

Abstract

Petroleum products and essential oils are complex mixtures of hydrophobic and volatile chemicals and are categorized as substances of unknown or variable composition, complex reaction products, or biological materials (UVCBs). In aquatic testing and research of such mixtures, it is challenging to establish initial concentrations without the addition of cosolvents, to maintain constant concentrations during the test, and to keep a constant mixture composition in dilution series and throughout test duration. Passive dosing was here designed to meet these challenges by maximizing the surface area (A donor /V medium = 3.8 cm 2 /mL) and volume (V donor /V medium > 0.1 L/L) of the passive dosing donor in order to ensure rapid mass transfer and avoid donor depletion for all mixture constituents. Cracked gas oil, cedarwood Virginia oil, and lavender oil served as model mixtures. This study advances the field by (i) showing accelerated passive dosing kinetics for 68 cracked gas oil constituents with typical equilibration times of 5−10 min and for 21 cederwood Virginia oil constituents with typical equilibration times < 1 h, (ii) demonstrating how to control mixture concentration and composition in aquatic tests, and (iii) discussing the fundamental differences between solvent spiking, water-accommodated fractions, and passive dosing.

Assessing the Toxicity of Individual Aromatic Compounds and Mixtures to American Lobster (Homarus americanus) Larvae Using a Passive Dosing System

“…Our study found that PDMS O‐rings could repeatedly reconstitute AC mixtures, as confirmed by HPLC analysis. This is consistent with previous studies that have shown PDMS‐based passive dosing to be an effective tool to reconstitute and maintain well‐controlled whole‐oil exposures (Bera et al 2018; Hammershøj et al 2020). A limitation of our mixture tests is that we sent only one representative sample for chemical analysis, and it is assumed that the ratio at which the compounds are equilibrated into solution remains equal at different concentrations, which, though not demonstrated in our study, has been validated by previous studies (Rojo‐Nieto et al 2012).…”

Improving the Environmental Risk Assessment of Substances of Unknown or Variable Composition, Complex Reaction Products, or Biological Materials

“…Fate‐directed toxicity testing may provide the information necessary for these assessments as this research develops. Novel dosing methodologies such as passive dosing (Smith et al 2010) are promising methods to control the concentration of each mixture constituent by partitioning from a preloaded polymer donor (Jahnke et al 2016; Organisation for Economic Co‐operation and Development 2019a; Hammershøj et al 2020). Alternatively, WAF accuracy, precision, and relevance could be improved to make it more fit‐for‐purpose in environmental risk assessments.…”

Biodegradation kinetics testing of two hydrophobic UVCBs – potential for substrate toxicity supports testing at low concentrations

“…For the passive dosing setup, a translucent silicone rod (OD 3 mm, polydimethylsiloxane (PDMS)) without a whitening agent was ordered custom-made (Altec, UK). The rods were cleaned with ethyl acetate (purity $99.7%, Sigma-Aldrich, Denmark), ethanol (absolute, VWR International, Denmark) and ultrapure water as described in Hammershøj et al 4 Ultrapure water was produced with an Elga Purelab ex water system (Holm & Halby, Denmark). Silicone oil (CAS 63148-62-9, Sigma-Aldrich, Denmark) was used to make reference standards.…”

“…3 However, for hydrophobic UVCB substances such as petroleum products and many essential oils, it can be challenging to establish a well-dened and reproducible initial mixture concentration and composition with all constituents fully dissolved. 4 Furthermore, we recently reported delayed biodegradation kinetics of petroleum hydrocarbons tested individually at concentrations just below the solubility limit and attributed this to toxicity. 5 While the toxicity of mixtures is widely studied, there are some crucial data and knowledge gaps related to the effect of test concentration and mixture toxicity on the biodegradation kinetics of chemicals in mixtures.…”

“…Passive dosing was used to set initial concentrations in a surface water inoculum, thus avoiding undissolved compounds or co-solvents. 4 Automated headspace solid phase microextraction was applied directly on the test systems, which had the advantage of reduced test substance losses and an enhanced sensitivity for the most hydrophobic mixture constituents whereby the number of detectable mixture constituents was increased.…”