Microplastics as Contaminant Vectors: Influencing the Bioaccessibility of Naphthalene from Freshwater

Abstract

Microplastics (MPs) are found throughout the environment and pose risks to human health as particulate pollutants. In addition, they have the potential to transfer environmental contaminants to humans and other sensitive receptors. However, there is a paucity of information on the vector effects of MPs for toxic persistent organic pollutants (POPs) in freshwater systems. The purpose of this research was to investigate the capacity of MPs to transfer naphthalene, a carcinogenic polycyclic aromatic hydrocarbon (PAHs), from freshwater systems to the human gastrointestinal tract (GIT). The focus was on characterizing the sorption mechanisms of naphthalene and its bioaccessibility (i.e., an estimate of the fraction of an ingested dose that becomes soluble in gastro-intestinal fluids). Three types of MPs, medium-density polyethylene (MDPE), polypropylene (PP), and polystyrene (PS), were selected as model compounds. The sorption behaviour of naphthalene onto these MPs was investigated through the kinetic and isotherm experiments in freshwater, and the bioaccessible fraction of naphthalene in the gastric and intestinal digestive phases was measured using a modified physiologically based extraction test (PBET). The effects of aging of MPs on the sorption of naphthalene was also investigated. The sorption kinetics of naphthalene onto MPs followed a pseudo second order model, with external mass transport as the rate-controlling step. Sorption was dominated by a hydrophobic interaction mechanism, as indicated by Langmuir and linear isotherm models. The sorption capacity of naphthalene followed the decreasing order of MDPE > PP > PS. The bioaccessibility of naphthalene was higher in the intestinal phase (28 to 60%) than in the gastric phase (24 to 40%). Artificial aging altered MP physico-chemical characteristics, with the sorption capacity of naphthalene decreasing by 25% for MDPE and increasing by 35% and 26% for PP and PS, respectively. These findings demonstrate that MPs can act as potential carriers for the transfer of POPs from freshwater environments to the human body. Consequently, MPs may provide a pathway (i.e., a vector effect) for human exposure to contaminants and pose a risk to human health. Further studies are necessary to assess the vector effect and risks of MPs for transferring other POPs in freshwater.