Weathering and degradation of polylactic acid masks in a simulated environment in the context of the COVID-19 pandemic and their effects on the growth of winter grazing ryegrass

Abstract

The COVID-19 pandemic has led to explosive growth in the production and consumption of disposable medical masks, which has caused new global environmental problems due to the improper disposal of these masks and lack of effective mask recycling methods. To reduce the environmental load caused by the inability of synthetic plastics to degrade, polylactic acid (PLA) masks, as a biodegradable environmentally friendly plastic, may become a solution. This study simulated the actual degradation process of new PLA masks in different environments by soaking them in various solutions for 4 weeks and explored the influence of the treated PLA fabric fibers on the growth of winter ryegrass. The results show that the weathering degradation of PLA fibers in water mainly occurs through the hydrolysis of ester bonds, and weathering leads to cheese-like and gully-like erosion on the surface of the PLA fiber fabric layer and finally to fiber fracture and the release of microplastics (MPs). The average number of MPs released within 4 weeks is 149.5 items/piece, the particle size is 20-500 µm (44%), and 63.57% of the MPs are transparent fibers. The outer, middle, and inner layers of weathered PLA masks tend to be hydrophilic and have lower mechanical strength. PLA fibers after different treatment methods affect the growth of winter ryegrass. PLA masks are undoubtedly a greener choice than ordinary commercial masks, but in order to confirm this, the entire degradation process, the final products, and the impact on the environment need to be further studied. In the future, masks may be developed to be made from more environmentally friendly biodegradable materials that can have good protecting effects and also solve the problem of end-of-life recycling. A SYNOPSIS: Simulation of the actual degradation process of PLA masks and exploration of the influence of mask degradation on the growth of winter ryegrass.

Keywords: Degradation; Growth inhibition; Hydrophilicity; Microplastics; Polylactic acid mask.

Graphical abstract

Fig. 1

SEM imaging of the inner, middle, and outer layers of the masks after 4 weeks of immersion and degradation under different conditions. (a, f, k) The three-ply fabric of the original masks. (b, g, l) Three-layer fiber fabric soaked in a weak alkaline environment for four weeks. (c, h, m) Image of the seawater group. (d, i, n) Image of the pond aquaculture water group. (e, j, o) Image of the Fenton group.

Fig. 2

Infrared spectra of the three layers of the fiber fabrics in each group after 4 weeks and infrared spectrogram of the PLA mask hydrolysis process. A) Infrared spectrum of the outer layer. B) Infrared spectrum of the middle layer. C) Infrared spectrum of the inner layer. D) Infrared spectrum of detached mask fragments and original masks. E) Infrared spectrum of the soaking solution and original masks. F) Visual degradation process of PLA masks.

Fig. 3

Water contact angles of the inner, middle, and outer layers of the masks after 4 weeks of immersion and degradation under different conditions. (a, f, k) The three-ply fabric of the original mask group. (b, g, l) Weakly alkaline water environment. (c, h, m) Seawater group. (d, i, n) Pond aquaculture water group. (e, j, o) Fenton Group.

Fig. 4

Changes in the mechanical tensile strength and surface conditions of the woven PLA fiber layer after soaking and aging. A) Mechanical strength of the samples in different groups. B) Surface condition of the fiber layer observed under a stereoscopic microscope.

Fig. 5

Morphology and types of fibrous microplastics under a microscope. A) Infrared spectrogram of fibrous microplastics. B) Morphology of microplastics with a body microscope under different magnifications. C) Colored microplastics mixed in a new PLA mask fabric layer during processing.

Fig. 6

Particle size, number, and color percentage of microplastics in each group. A, B, C, D) Characteristics of microplastics attached to masks in each group. E, F G H) Characteristics of microplastics detaching in solution in each group.

Fig. 7

Life activities and metabolic capacity of winter ryegrass. A) Seed germination rate and harvest rate of each population. B) Bud length height of each population over 14 recording cycles. C) Number of dead seeds with data records. D) Average chlorophyll a and chlorophyll b contents of each population at harvest time. E) Average moisture content at harvest of each population. F) pH of soil at harvest for each population.