Assessing Aquatic Baseline Toxicity of Plastic-Associated Chemicals: Development and Validation of the Target Plastic Model

We developed a Target
Plastic Model (TPM) to estimate the critical
plastic burden of organic toxicants in five types of plastics, namely,
polydimethylsiloxane (PDMS), polyoxymethylene (POM), polyacrylate
(PA), low-density polyethylene (LDPE), and polyurethane ester (PU),
following the Target Lipid Model (TLM) framework. By substituting
the lipid–water partition coefficient in the TLM with plastic–water
partition coefficients to create TPM, we demonstrated that the biomimetic
nature of these plastic phases allows for the calculation of critical
plastic burdens of toxicants, similar to the notion of critical lipid
burdens in TLM. Following this approach, the critical plastic burdens
of baseline (n = 115), less-inert (n = 73), and reactive (n = 75) toxicants ranged from
0.17 to 51.33, 0.04 to 26.62, and 1.00 × 10–6 to 6.78 × 10–4 mmol/kg of plastic, respectively.
Our study showed that PDMS, PA, POM, PE, and PU are similar to biomembranes
in mimicking the passive exchange of chemicals with the water phase.
Using the TPM, median lethal concentration (LC50) values
for fish exposed to baseline toxicants were predicted, and the results
agreed with experimental values, with RMSE ranging from 0.311 to 0.538
log unit. Similarly, for the same data set of baseline toxicants,
other widely used models, including the TLM (RMSE: 0.32–0.34),
ECOSAR (RMSE: 0.35), and the Abraham Solvation Model (ASM; RMSE: 0.31),
demonstrated comparable agreement between experimental and predicted
values. For less inert chemicals, predictions were within a factor
of 5 of experimental values. Comparatively, ASM and ECOSAR showed
predictions within a factor of 2 and 3, respectively. The TLM based
on phospholipid had predictions within a factor of 3 and octanol within
a factor of 4, indicating that the TPM’s performance for less
inert chemicals is comparable to these established models. Unlike
these methods, the TPM requires only the knowledge of plastic bound
concentration for a given plastic phase to calculate baseline toxic
units, bypassing the need for extensive LC50 and plastic–water
partition coefficient data, which are often limited for emerging chemicals.
Taken together, the TPM can provide valuable insights into the toxicities
of chemicals associated with environmental plastic phases, assisting
in selecting the best polymeric phase for passive sampling and designing
better passive dosing techniques for toxicity experiments.