Insights into the degradation of microplastics by Fenton oxidation: from surface modification to mineralization

This work aims at evaluating the fate of microplastics (MPs) along Fenton oxidation. For such goal, realistic MPs (150–250 μm) of five representative polymer types (PET, PE, PVC, PP and EPS) were obtained from commercial plastic products by cryogenic milling. Experiments (7.5 h) were performed under relatively severe operating conditions: T = 80 °C; pH0 = 3; [H2O2]0 = 1000 mgL−1 (15 doses, 1 every 0.5 h); [Fe3+]0 = 10 mgL−1 (5 doses, 1 every 1.5 h). Slight MPs weight losses (∼10%) were achieved after Fenton oxidation regardless the MP nature. Nevertheless, oxidation yield clearly increased with decreasing the particle size given their higher exposed surface area (up to 20% weight loss with 20–50 μm EPS MPs). Clearly, MPs suffered important changes in their surface due to the introduction of oxygenated groups, which made them more acidic and hydrophilic. Furthermore, MPs progressively reduced their size. In fact, they can be completely oxidized to CO2, as demonstrated in the oxidation of PS nanoplastics (140 nm), where 70% mineralization was achieved. The nature of the plastic particles had a relevant impact on its overall oxidation, being more prone to be oxidized those polymers which contain aromatic rings in their structures (EPS and PET) compared to those formed by alkane chains (PE, PP and PVC). In the latter, the presence of substituents also reduced their oxidation potential. Remarkably, possible leachates released along reaction were more quickly oxidized than the MPs/NPs, so it can be assumed that these dissolved compounds would be completely removed once the solid particles are eliminated. Notably, the leachates obtained upon MPs oxidation were more biodegradable than the released from the fresh solids. All this knowledge is crucial for the understanding of MPs oxidation by the Fenton process and opens the door for the design and optimization of this technology either for water treatment or for analytical purposes (MPs isolation)This research has been supported by the Autonoma University of Madrid and Community of Madrid through the project SI1-PJI-2019- 00006, and by the Spanish MINECO through the project PID2019- 105079RB-I00. D. Ortiz thanks the Spanish MIU for the FPU predoctoral grant (FPU19/04816). M. Munoz and J. Nieto-Sandoval thank the Spanish MINECO for the Ramon ´ y Cajal postdoctoral contract (RYC2016-20648) and the FPI predoctoral grant (BES-2017-081346), respectively. C. Romera-Castillo was supported by JIN-2019 project (PID2019-109889RJ-I00) from the Spanish Ministry of Science and Innovation and the Agencia Estatal de Investigacion and acknowledges the “Severo Ochoa Centre of Excellence” accreditation (CEX2019- 000928-S