Resource or waste? A perspective of plastics degradation in soil with a focus on end-of-life options. One step beyond

Abstract

Plastics have surpassed traditional materials across numerous industries due to their versatility, durability, and cost-effectiveness. However, their persistence in ecosystems, particularly in soil, presents serious environmental challenges. This narrative review builds on previous work by analysing over 300 studies on plastics in soil, with a focus on degradation and potential reuse. Special attention is given to research published since 2019. The review classifies plastics by resin type and examines their degradation processes under various soil conditions, covering both conventional and biodegradable polymers. Polyethylene emerges as the most extensively studied polymer, while interest in biodegradable alternatives like polylactic acid (PLA) and polybutylene adipate-co-terephthalate (PBAT) is increasing. Additionally, the review highlights advancements in microplastics research, particularly their interactions with co-contaminants and effects on soil organisms. Despite significant progress, challenges remain in standardizing methods for measuring plastic degradation in soil. The review emphasizes the need for further research to establish consistent methods and reliable indicators for degradation, while also exploring innovative recycling technologies for use in agricultural soil management. It stresses the importance of advancing a circular economy for plastics, integrating policy and practical solutions to reduce environmental impacts.

Keywords: Microplastics; Plastic degradation; Recycling technologies; Soil health.

Fig. 1

The life cycle of a plastic item, from production to recycling. This review discusses and enriches research in the field of plastics management by compiling current knowledge on plastic disposal and management from an environmental perspective, with a focus on degradation processes [©Elsevier].

Fig. 2

Microplastics are plastic particles smaller than 5 mm, arising from various sources, such as the breakdown of larger plastic debris or intentional production for industrial uses. This definition, derived from organizations like UNEP, NOAA, ECHA, and EEA, covers a broad range of plastics, including primary microplastics (manufactured to be small) and secondary microplastics (formed by the degradation of larger items). An analysis of multiple studies on microplastics in various soils, across different land uses and global regions [17], revealed the following qualitative-quantitative traits: median quantity 112 particles kg⁻¹ (Q1: 12 particles kg⁻¹, Q3: 863 particles kg⁻¹), median size 305 μm (Q1: 50 μm, Q3: 862 μm), and median quality PE, PP, PS (secondary polymers include PET, PVC, PA, PU, ABS). (DALL·E image).

Fig. 3

For long-term missions, astronauts need facilities for living, working, transportation, communication with Earth, and producing essential resources like oxygen and water. Transporting all this infrastructure from Earth would be extremely costly. To address this, the European Space Agency is exploring the possibility of 3D printing some of these facilities on Mars using the planet's soil. They are also exploring ways to recycle waste, such as repurposing unused plastic packaging, into new materials (Photo: FOTEC/©ESA).

Fig. 4

Polymers studied in the works reviewed from 2019. Various indicates that more than four polymers were considered in the individual study.

Fig. 5

The experimental conditions in the works reviewed from 2019: box and whiskers diagrams indicating (from the top) incubation days, maxima degradation rate of the polymer studied, and pH.

Fig. 6

Recycling plastic polymers faces challenges, particularly with blending different types due to compatibility issues, often resulting in final poor (mechanical) properties. Advances in using compatibilizers, nanofillers, and techniques like hydrothermal carbonization show promise in improving these properties, but practical application and environmental concerns, such as VOC emissions, remain significant hurdles. (DALL·E image).

Fig. 7

Separate collection of glass in Brussels. Glass, one of the oldest synthetic materials, dates to the third millennium bc, while plastic is a much more recent invention from the last century. Both glass and plastic share the ability to be recycled through cooling and reversible transitions, but glass can be recycled almost infinitely, whereas plastics often suffer from downcycling due to polymer diversity and immiscibility [1]. Plastics might ultimately be used for energy production, unlike glass. For both materials, the 3R strategy (reduce, reuse, recycle) would be the most effective waste management approach. Glass reuse is feasible for certain beverage containers but faces challenges in sanitation and durability. The reduction strategy applies similarly to glass and plastic, aiming to minimize material use in products [22]. Recycling for glass and plastic can involve either open-loop (where EoL materials are transformed into new products) or closed-loop processes (where materials are reused in their original form). Recycling all recyclable materials would be an excellent way forward. Glass is a good example, but its optimal (economically sustainable) recycling occurs if colours are separated. Analogy with plastics which, if separated by polymer, could be recycled very easily. Unrealistic scenario since, globally, only 19 % of municipal solid waste is recycled [97]. And, unfortunately, as nations become more affluent, they usually see greater industrial and urban development, alterations in housing and consumption habits, and a broader selection of products available, contributing to an increase in the average amount of waste produced per individual [98]. Recent research focuses on addressing plastic waste through innovative conversion methods into valuable chemicals [99,100] and, thinking positively, evaluating pathways toward achieving zero plastic pollution [101].