Effects and Impacts of Different Oxidative Digestion Treatments on Virgin and Aged Microplastic Particles

Abstract

Although several sample preparation methods for analyzing microplastics (MPs) in environmental matrices have been implemented in recent years, important uncertainties and criticalities in the approaches adopted still persist. Preliminary purification of samples, based on oxidative digestion, is an important phase to isolate microplastics from the environmental matrix; it should guarantee both efficacy and minimal damage to the particles. In this context, our study aims to evaluate Fenton's reaction digestion pre-treatment used to isolate and extract microplastics from environmental matrices. We evaluated the particle recovery efficiency and the impact of the oxidation method on the integrity of the MPs subjected to digestion considering different particles' polymeric composition, size, and morphology. For this purpose, two laboratory experiments were set up: the first one to evaluate the efficacy of various digestion protocols in the MPs extraction from a complex matrix, and the second one to assess the possible harm of different treatments, differing in temperatures and volume reagents used, on virgin and aged MPs. Morphological, physicochemical, and dimensional changes were verified by Scanning Electron Microscope (SEM) and Fourier Transformed Infrared (FTIR) spectroscopy. The findings of the first experiment showed the greatest difference in recovery rates especially for polyvinyl chloride and polyethylene terephthalate particles, indicating the role of temperature and the kind of polymer as the major factors influencing MPs extraction. In the second experiment, the SEM analysis revealed morphological and particle size alterations of various entities, in particular for the particles treated at 75 °C and with major evident alterations of aged MPs to virgin ones. In conclusion, this study highlights how several factors, including temperature and polymer, influence the integrity of the particles altering the quality of the final data.

Keywords: FTIR; Fenton’s reagent; SEM; aged; microplastics; oxidative digestion; virgin; weathering.

Figure 1

(a) Virgin MPs products by cutting common plastic items. (b) Pre-production pellets added in experiment two. Images produced by Carl Zeiss Tessovar Microscope.

Figure 2

The morphological aspect of some polymers (PVC, PET, PP, PE, PS) before and after the ageing process. These polymers, together with PA fibre and pellets (PP, PE), were exposed to UVA (photo-oxidation) in the climatic chamber for 20 days and then at a temperature of 45 °C for a further 20 days in dry conditions.

Figure 3

Recovery rates of PVC, PS, PE, PP, and PET, of three-dimensional sizes, after six different treatments varying for H₂O₂ volumes and temperatures. Values are expressed as a percentage mean value of two replicates ± the standard deviation.

Figure 4

Particulars of PVC fractures (yellow boxes on the left) due to the abrupt oxidation reaction at 75 °C that led to the fragmentation of PVC in tiny particles trapped in the soil matrix (red circles on the right).

Figure 5

Comparison between the IR spectra of different polymers before (black lines, t₀) and after 40 days of ageing (red lines, t₄₀). New peaks formed after ageing are indicated in the spectra of each polymer. Absorption areas related to ageing from 3100 to 3700 cm⁻¹ (hydroxyl groups) are evident in all polymers. (a) PA fiber; (b) PE fragment (c) PET fragment; (d) PP fragment; (e) PS fragment; (f) PVC fragment; (g) PE pellet; (h) PP pellet.

Figure 6

Alterations caused by treatment at 30 °C on virgin PET and PVC: (a) size reduction and corrosion of virgin PET margins, (from 853 to 708 µm); (b) PVC dimensional expansion from 627 µm to 722 µm.

Figure 7

Effects of treatment at 50 °C on virgin PS and PVC: (a) formation of a hole on the surface of the PS; (b) PVC particle surface with small holes.

Figure 8

Effects of oxidative digestion treatment at 75 °C on the virgin PVC: (a) morphological acquisition of the entire PVC particle after treatment; (b) detail of large holes inside the PVC particle.

Figure 9

Corrosive treatment effects on PET and PP particles: comparison of size measurements, before and after treatment, emphasizes the corrosion of virgin PET (a) and PP (b).

Figure 10

Morphological aspects of pellets before and after different treatments. Virgin PE and PP pellets highlight high resistance to oxidative digestion.

Figure 11

Focus on the effects of treatment at 50 °C on aged particles: (a) detail of PE curling; (b) more cracks in the PET; (c) small holes on the PVC surface; (d) fraying of PA fibre.

Figure 12

Focus on the effects of treatment at 75 °C on aged particles: (a) small holes on the PVC surface and loss of the polymer material; (b) corrosion and loss of PP polymer material; (c) formation of large cracks in PET; (d) corrosion and loss of PE polymer material; (e) breaking and fraying of the fibre.

Figure 12

Focus on the effects of treatment at 75 °C on aged particles: (a) small holes on the PVC surface and loss of the polymer material; (b) corrosion and loss of PP polymer material; (c) formation of large cracks in PET; (d) corrosion and loss of PE polymer material; (e) breaking and fraying of the fibre.

Figure 13

Morphological aspects of aged pellets before and after different treatments: abraded areas are highlighted as the temperature increases.

Figure 13

Morphological aspects of aged pellets before and after different treatments: abraded areas are highlighted as the temperature increases.