Adsorption of diclofenac and metronidazole on real microplastics of different nature and size

, Munoz Macarena, Ortiz David, Nieto-Sandova l Julia, Gomez-Herrero Esther, De Pedro Zahara M., Casas Jose A..

Microplastics (MPs) are recognized as micropollutants carriers in the aquatic environment. Accordingly, the evaluation of their sorption capacity has received major attention in the last few years. Nevertheless, in most works, pure polymers have been used as surrogates for real MPs, which can lead to significant underestimation for their actual values. On the other hand, one kind of plastic has been usually tested, making difficult comparison purposes. In this work, MPs were obtained by cutting up commercial plastic products like bottles, containers and trays. The particles were further grinded using a cryogenic mill in order to achieve different sizes (25 – 1000 mm). Four representative plastic polymers were evaluated: polystyrene (PS), polypropylene (PP), high density polyethylene (HDPE) and polyethylene terephthalate (PET). Diclofenac (DCF) and metronidazole (MNZ) were selected as target pollutants given their widespread occurrence in the environment but also their significantly different properties (structure, hydrophobicity and size). The equilibrium adsorption experiments (5 days) were performed under ambient conditions using different pollutant concentrations (0.5 – 15 mgL-1). The reactor volume and MP mass were established at 1.5 mL and 10 mg, respectively. The experimental data were well-described by the Langmuir model, which suggests that monolayer adsorption took place in all cases. Nevertheless, important differences were found in the MPs adsorption capacity considering both their nature but also the kind of micropollutant tested. The MPs adsorption capacity followed the order: PS¿HDPE¿PP¿PET. On the other hand, regardless of the MP nature, DCF adsorption was significantly higher than that achieved for MNZ ( 450 vs. 20 ngg-1). The increase of hydrophobicity of the MP and/or the micropollutant led to an increase on the adsorption capacity, which suggests that hydrophobic interactions govern the adsorption mechanism. Finally, the MP adsorption capacity increased by decreasing the particle size, consistent with the increase of exposed surface area.

View online