Transitioning to Microplastic-Free Seed Coatings: Challenges and Solutions

Abstract

This review addresses the issue of replacing manufactured microplastics in seed coatings used in agriculture. Firstly, it focuses on the policy and regulatory actions taken on microplastics at a global level. There is no consensus within the scientific community on the definition of a microplastic and, more generally, on the classification of plastic debris. Nevertheless, several decision schemes have been proposed in an attempt to define the notion of microplastics. The different criteria relevant to this definition, such as the size, physical state, chemical structure, origin, and persistence of microplastics, are discussed, with a comparison being made between the REACH regulation and the scientific literature. Seed production and processing are also discussed, with the functions of seed coatings being explained in order to gain a better understanding of the properties to be considered in a substitution strategy for currently used microplastics. The main challenges are multiple; substitutes must provide the same performance as microplastics: (i) improving the adherence of the treatment to the seed, (ii) distributing the treatment more evenly over the seed, (iii) reducing the amount of dust-off when handling treated seed, and (iv) improving the seed flowability, which is particularly important during the sowing stage, all while preserving the physiological properties of the seed. Substitute polymers are proposed according to the desired performance and functional properties: two main chemical families of biopolymers were identified in the literature: polysaccharides and proteins. Among them, 13 and 6 polymers, respectively, complied with REACH regulation, demonstrating adhesion, dust reduction performances, and preservation of seed physiological quality in particular. This work aims to guide future studies on microplastic substitution in seed coatings, and to highlight research needs in this area. It is based on an analysis and discussion of the literature, identifying and listing potential substitutes.

Keywords: REACH; biopolymers; microplastics; regulation; seed coating.

Figure 1

The 5Rs principle for the prevention of plastic pollution, adapted from [39].

Figure 2

Planisphere depicting countries that have enacted bans or restrictions on the use of single-use plastic bags [43].

Figure 3

Planisphere listing countries that have enacted bans or are in the process of enacting legislation to prohibit the use of microbeads in rinse-off cosmetics, completed from [63,64].

Figure 4

Distribution of intentionally added microplastic discharges per year by sector, as reported by ECHA.

Figure 5

Diagram summarizing the various scenarios that may be employed to define primary and secondary microplastics.

Figure 6

Distribution of each type of microplastic according to central estimates by Boucher and Friot [40].

Figure 7

Comparison of the distribution of polymer types found in the marine environment obtained by Erni-Cassola et al. [5] (based on N = 40 studies) on the left, and the frequency of polymer identification in the marine environment obtained by Hidalgo-Ruiz et al. [139] (based on N = 42 studies) on the right.

Figure 8

Prevalence of polymers encountered according to sampling medium from Cassola et al. [5].

Figure 9

Assessment of the distribution of different sources of primary microplastics on a global scale: on the left, all primary microplastics (including non-manufactured MPs), and on the right, the share of manufactured MPs.

Figure 10

Uncoated (center), film-coated (furthest left), encrusted (left), and pelleted (2 different grades) onion seeds (right); photograph from The Encyclopedia of Seeds, Science Technology and Uses [175].

Figure 11

Representation of the solid, liquid, and vapor contact and corresponding components of the Young–Dupré equation.