Sara Exojo-Trujillo

In freshwater environments, microplastics (MPs) have emerged as important organic contaminants vectors. This study investigates the individual and combined toxicity of microparticles of polystyrene (PS) and polylactic acid (PLA) with two contaminants of major concern in Spanish waters─pyrene (PYR) and methyl chlorpyrifos (CPF). Adsorption behavior was assessed using adsorption isotherms fitted to mathematical models such as Henry, Langmuir, Freundlich, and Temkin. PYR adsorption on both polymers followed monolayer behavior (Langmuir model), while CPF adsorption was best described by the Freundlich model, indicating heterogeneous multilayer adsorption. Toxicity of these particles was assessed in Daphnia magna using acute and chronic toxicity assays. Acute toxicity tests showed that coexposure to MPs and contaminants increased immobilization, suggesting that adsorption onto MPs may influence contaminant partitioning and exposure. When organisms were exposed only to the contaminant remaining in the aqueous phase of CPF and PYR (after adsorption), EC50 values increased due to this reduced concentration. Regarding chronic exposure, significant effects were observed on Daphnia survival and reproduction, particularly when PS contained adsorbed contaminants on its surface. Therefore, this study indicates that MPs modify contaminant behavior and provides mechanistic insight relevant for understanding contaminant exposure and hazard processes in aquatic systems.

Microplastics (MPs) are recognised as emerging vectors for hydrophobic organic contaminants in aquatic environments due to their relatively large surface area and the diversity of their polymer chemistries compositions. This study investigates the sorption behaviour of two priority polycyclic aromatic hydrocarbons (PAHs), pyrene (PYR) and fluoranthene (FLU), onto six common MPs: poly(m-xylene adipamide) (PA-MXD6), high- and low-density polyethylene (HDPE, LDPE), polypropylene (PP), polyethylene terephthalate (PET), and polylactic acid (PLA). Sorption isotherms and kinetics were evaluated under simulated freshwater conditions at environmentally relevant concentrations (1–50 µg·L⁻¹). Despite the low MP concentration used (0.2 g·L⁻¹), over 80% of the initial PAH content was removed by polyolefins, and more than 50% by all other MPs. Sorption capacity was strongly dependent on particle surface area. Langmuir, Henry, and Freundlich isotherms models were fitted, with linear behaviour prevailing at low concentrations. Analysis using the Dubini–-Radushkevich model confirmed that sorption involves chemisorption contributions, mainly through π–π interactions and hydrophobic interactions (polyolefins). Mechanistically, molecular diffusion within the MP matrix was not governing the sorption process, as diffusion coefficients varied with particle size instead of polymer chemistry. Instead, sorption appears to be governed by PAH diffusion through the hydrodynamic boundary layer and subsequent retention on the MP surface. Empirically, kinetic data fitted the pseudo-second-order model, further supporting that the sorption process involves chemisorption. These findings highlight the role of MPs as vectors for PAHs in freshwater systems and their potential application in contaminant removal. Expressing sorption per unit surface area is recommended for accurate assessment. This work contributes to understanding the environmental behaviour of MPs and their implications for pollutant transport and toxicity.

This study presents a theoretical framework for modeling sorption and release kinetics of substances in polymeric materials with planar, cylindrical, and spherical geometries. Fick’s second law was expressed in dimensionless variables and solved numerically using a finite-difference approach to generate universal profiles for mass transfer. These profiles were fitted with double-exponential equations, yielding explicit expressions that allow for straightforward estimation of diffusion coefficients from experimental data. The method was validated using literature data for films, fibers, and microspheres, showing excellent agreement with reported values. Unlike classical analytical solutions, which are limited to planar systems under ideal conditions, the proposed approach is applicable to diverse geometries commonly employed in packaging, biomedical devices, controlled-release formulations, and environmental technologies.