Data for: Assessment of cryogenic pretreatment for simulating environmental weathering in the formation of surrogate micro- and nanoplastics from agricultural mulch film

Microplastics (MPs) and nanoplastics (NPs) from mulch films and other plastic materials employed in vegetable and small fruit production pose a major threat to agricultural ecosystems. For conducting controlled studies on MPs' and NPs' (MNPs') ecotoxicity to soil organisms and plants and fate and transport in soil, surrogate MNPs are required that mimic MNPs that form in agricultural fields. We have developed a procedure to prepare MPs from plastic films or pellets using mechanical milling and sieving, and conversion of the resultant MPs into NPs through wet grinding, both steps of which mimic the degradation and fragmentation of plastics in nature. The major goal of this study was to determine if cryogenic exposure of two biodegradable mulch films effectively mimics the embrittlement caused by environmental weathering in terms of the dimensional, thermal, chemical, and biodegradability properties of the formed MNPs. We found differences in size, surface charge, thermal and chemical properties, and biodegradability in soil between MNPs' prepared from cryogenically treated vs. environmentally weathered films, related to the photochemical reactions occurring in the environment that were not mimicked by cryogenic treatment, such as depolymerization and cross-link formation. We also investigated the size reduction process for NPs and found that the size distribution was bimodal, with populations centered at 50 nm and 150–300 nm, and as the size reduction process progressed, the former subpopulation's proportion increased. The biodegradability of MPs in soil was greater than for NPs, a counter-intuitive trend since greater surface area exposure for NPs would increase biodegradability. The result is associated with differences in surface and chemical properties and to minor components that are readily leached out during the formation of NPs. In summary, the use of weathered plastics as feedstock would likely produce MNPs that are more realistic than cryogenically-treated unweathered films for use in experimental studies.

微塑料（Microplastics, MPs）与纳米塑料（Nanoplastics, NPs，二者统称MNPs）源自蔬菜及小浆果种植中使用的覆盖地膜与其他塑料材料，对农业生态系统构成重大威胁。为开展关于MNPs对土壤生物及植物的生态毒性、以及其在土壤中的归趋与运移的可控性研究，需要能够模拟农田中天然形成的MNPs的替代MNPs。
本研究开发了一套制备流程：先通过机械研磨与筛分从塑料薄膜或颗粒中制备MPs，再通过湿磨将所得MPs转化为NPs，上述两个步骤均模拟了自然界中塑料的降解与破碎过程。本研究的核心目标是，探究两种可降解地膜经低温处理后，其所生成的MNPs在尺寸、热学、化学及生物降解性能方面，能否有效模拟自然风化引发的塑料脆化效应。
研究发现，经低温处理地膜制备的MNPs与经自然风化地膜制备的MNPs，在土壤中的尺寸、表面电荷、热学与化学性质以及生物降解性方面均存在差异；这种差异与环境中发生的、低温处理无法模拟的光化学反应有关，例如解聚与交联形成过程。
本研究同时探究了NPs的尺寸缩减过程，发现其粒径分布呈双峰特征，两个峰值分别位于50 nm与150–300 nm区间；随着尺寸缩减过程推进，位于50 nm的子群体占比逐渐升高。
土壤中MPs的生物降解性高于NPs，这一趋势与直觉相悖——因为NPs的暴露比表面积更大，理论上应具有更高的生物降解性。该结果与两者表面及化学性质的差异，以及NPs形成过程中易于浸出的微量组分有关。
综上，以自然风化塑料为原料制备的MNPs，相较于以低温处理的未风化地膜制备的MNPs，更适用于实验研究，其模拟效果更为真实。