Application of X-ray fluorescence spectrometry-based techniques in studies dealing with metal adsorption onto microplastics

At present, water pollution by microplastics (MPs) is a significant environmental issue, affecting aquatic ecosystems and human health. One of the main concerns about MPs is the possible adsorption of pollutants due to their large specific surface area and chemical properties. In particular, the role of MPs as vector and carrier for trace metals in the environment is still poorly understood. In this contribution, a novel analytical approach based on the combined use of X-ray fluorescence spectrometry techniques is proposed for investigating metal adsorption onto MPs. Quantitative analysis of supernatants before and after batch adsorption experiments was performed by total reflection X-ray fluorescence spectrometry (TXRF). Comparable results were obtained using inductively coupled plasma optical emission spectrometry (ICP-OES), a technique widely used for trace metal analysis in this type of microplastic studies. In parallel, MPs collected on filters after the adsorption process were directly qualitatively analysed, without any sample pretreatment, by energy-dispersive X-ray fluorescence spectrometry (EDXRF), enabling direct verification of metal adsorption and providing complementary information to TXRF results. The applicability of the methodology was assessed through the evaluation of Zn adsorption onto unaged and UV aged “true-to-life” polyethylene (PE), polystyrene (PS), and polypropylene (PP) MPs with different particle sizes (<63 μm and 63–80 μm). Surface modifications induced by the UV aging process were further characterized by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectrometry. Results showed that metal adsorption depends on polymer type and is strongly influenced by aging processes, with aged MPs exhibiting higher adsorption capacity.