Sorption of cortisone on polypropylene, polystyrene, and high-density polyethylene microplastics

Microplastics are particular substances that have sizes between 1 μm and 5 mm. With their unique properties, microplastics represent one of the most discussed analytical challenges of recent years. Due to intensive anthropic activities around the world, microplastics can arrive in the aquatic environment and persist for many years in different conditions [S. D. Martinho, V. C. Fernandes, S. A. Figueiredo, and C. Delerue-Matos, ‘Study of the Potential Accumulation of the Pesticide Alpha- Endosulfan by Microplastics in Water Systems’, Polymers (Basel), vol. 14, no. 17, Sep. 2022]. Recent studies have shown that polypropylene, polystyrene and polyethylene are the most common microplastics revealed in the aquatic environment [J. Li, K. Zhang, and H. Zhang, ‘Adsorption of antibiotics on microplastics’, Environmental Pollution, vol. 237, pp. 460–467, Jun. 2018]. Microplastics have high specific surface area, high mobility and high hydrophobicity; for this reason, one of the main concerns of recent years about microplastics is that these substances can adsorb on their surface many dangerous contaminants and transport them over long distances, above all in aquatic environment. Sorption studies involving microplastics are essential to understand the mechanisms implicated in contaminant retention. In this research, a complete study of the sorption behaviour of a corticosteroid - cortisone - in polypropylene (PP), polystyrene (PS) and high-density polyethylene (HDPE) microplastics in two distinct matrices (ultrapure water and artificial seawater) was performed, using high-performance liquid chromatography coupled to a UV detector for the determination of cortisone. Kinetic and isotherm studies were performed using a batch design under controlled conditions: 1 g of microplastics pellets of 3-5 mm diameter, magnetic agitation, room temperature. The application of kinetic models (Pseudo-first order, Pseudo-second order, Intra-particle diffusion) [J. Wang and X. Guo, ‘Adsorption kinetic models: Physical meanings, applications, and solving methods’, Journal of Hazardous Materials, vol. 390. Elsevier B.V., May 15, 2020] and isotherm models (Henry, Langmuir, Freundlich, Redlich-Peterson, Temkin, Dubinin-Radushkevich) [J. Wang and X. Guo, ‘Adsorption isotherm models: Classification, physical meaning, application and solving method’, Chemosphere, vol. 258. Elsevier Ltd, Nov. 01, 2020] to the experimental data obtained, has allowed: to provide information on the efficiency of the adsorption process and its nature; to know the limits of the sorption process; to understand the structure properties of the selected microplastics; to evaluate the maximum capacity and the rate of the sorption process. The comparison of results in ultrapure water and artificial seawater revealed changes in sorption capacity and the predominant sorption mechanism involved. Overall, all studied microplastics showed sorption affinity towards cortisone, being polystyrene the one with the highest sorption capacity both in ultrapure water and in artificial seawater. In this case study, hydrophobic and electrostatic interaction played an important role in ultrapure water and artificial seawater, respectively. Electrostatic interactions are caused by the point-zero-charge pH (pHpzc) of the different microplastics (6.76, 6.69, 6.63 for PP, PS and HDPE, respectively [J. Li, K. Zhang, and H. Zhang, ‘Adsorption of antibiotics on microplastics’, Environmental Pollution, vol. 237, pp. 460–467, Jun. 2018]), analyte pKa (12.58 for cortisone) and matrix pH (6-7 for ultrapure water, 8-8.5 for artificial seawater). When both microplastics and the analyte exhibit a neutral charge, as it is the case of ultrapure water, hydrophobic interactions will be favoured, leading to aggregation of the apolar analyte in the apolar surface of the microplastics. On the other hand, in seawater, since matrix pH is higher than the pHpzc of the microplastics, a negative surface charge will be generated, whilst cortisone, due to its high pKa, will keep its neutral state. This situation, combined with the high ionic strength of seawater, will lead to a reduction in anchoring sites as cations are attracted to them, leading to a reduction in the overall sorption capacity of most microplastics.