Evaluation of organic matter removal by H2O2 from microplastic surface by nano-physicochemical methods

Abstract

Microplastics (MPs) are often coated in natural organic matters (NOM) from environments, causing MP identification to become more complicated. Currently, the popular way to remove NOM from MPs is oxidative digestion using hydrogen peroxide (H2O2). However, removal efficiency of NOM on MP surface is still unclear. In this study, we evaluated NOM removal efficiency with digestion time (0, 6, 12 and 24 h) based on physicochemical imaging technologies including Infrared spectra (IR), Thermal Analysis (TA) and Lorentz Contact Resonance (LCR) combined with Atomic Force Microscope (AFM) at nanoscale. Results showed that the changes of C-O-C, -CH2 and -CH3 with digestion time were unreliable for evaluation due to irregular change trends and co-occurrence of these bands in both NOM and MPs. Although AFM-IR signal of C=O was getting weaker on MPs with digestion time, it was insufficient to indicate NOM removal since C=O also presents in weathered environmental MPs. Thermal and mechanical analysis based on TA and LCR at nanoscale showed that the distributions of softening temperature and stiffness tended to be of spatial homogeneity with digestion time, indicating gradual removal of NOM from MP surface. These indicated that a comprehensive IR-thermal-mechanical methods were better to evaluate the efficiency of H2O2 digestion for removal of NOM on MP surface from environment. Therefore, the combination of multiple technologies at nanoscale deserves to be developed for evaluating removal of NOM from MPs in the future. These results can help to provide methodological reference in monitoring and management of MP pollution.