polymers Article Degradation of Plastics in Simulated Landfill Conditions Quecholac-Piña Xochitl 1, Hernández-Berriel María del Consuelo 1, Mañón-Salas María del Consuelo 2, Espinosa-Valdemar Rosa María 3 and Vázquez-Morillas Alethia 3,* 1 Tecnológico Nacional de México/Instituto Tecnológico de Toluca, Av. Tecnológico S/N Col. Agrícola Bellavista, Metepec, Edo de México C.P. 52149, Mexico; [email protected] (Q.-P.X.); [email protected] (H.-B.M.d.C.) 2 Sociedad Mexicana de Ciencia y Tecnología Aplicada a Residuos Sólidos, A. C., Priv. Molcajete 44 Fracc. Hacienda de las Fuentes, Calimaya C.P. 52227, Mexico; [email protected] 3 Departamento de Energía, Unidad Azcapotzalco, Universidad Autónoma Metropolitana, Av. San Pablo 180, Col. Reynosa Tamaulipas, Azcapotzalco, Ciudad de México C.P. 02200, Mexico; [email protected] * Correspondence: [email protected]; Tel.: +52-55-53198-9057 Abstract: Different degradable plastics have been promoted as a solution for the accumulation of waste in landfills and the natural environment; in Mexico, the most popular options are oxo- degradable, which degrade in a sequential abiotic–biotic process, and compostable plastics. In this research, high-density polyethylene, oxo-degradable high-density polyethylene, and certified compostable plastic were exposed to simulated landfill conditions in an 854-day-long experiment to assess their degradation. High-density polyethylene showed limited degradation, due mainly to surface erosion, evidenced by a 13% decrease in elongation at break. The pro-oxidant additive in the oxo-degradable plastic increased this loss of mechanical properties to 27%. However, both plastic films kept their physical integrity and high molecular weight by the end of the experiment, evidencing degradation but no biodegradation. While the compostable film fragmented, had a Citation: Xochitl, Q.-P.; María del Consuelo, H.-B.; María del Consuelo, lower molecular weight at the end of the experiment, and decreased the presence of C=O bonds, M.-S.; Rosa María, E.-V.; Alethia, V.-M. this degradation took place remarkably slower than expected from a composting process. Results Degradation of Plastics in Simulated show that oxo-degradable and compostable plastics will not biodegrade readily in landfills. This fact Landfill Conditions. Polymers 2021, should be known and understood for decision-makers to match the characteristics of the materials to 13, 1014. https://doi.org/10.3390/ the features of the waste management systems. polym13071014 Keywords: oxo-degradable; polyethylene; biodegradation Academic Editor: Magdalena Krystyjan Received: 24 February 2021 1. Introduction Accepted: 22 March 2021 Published: 25 March 2021 Plastics emerged as a basic material for the functioning of the global society in the 20th century. The plastic production has increased continuously since the 1950s, reaching 359 Mt Publisher’s Note: MDPI stays neutral in 2018 [1]. Although they are used in different sectors, such as communication, medicine, with regard to jurisdictional claims in agriculture, and transportation, 36% of the global plastic production in 2015 was related to published maps and institutional affil- packaging [2], mainly in products with short useful lives, such as bags and bottles. As a iations. result of the growing production and discard, plastic waste has become an issue of global concern, accounting for 12% of the waste produced worldwide [3]. Conventional plastics do not biodegrade quickly, as microbial species that can metabolize polymers are rare in nature [4]. Different strategies have been implemented in order to decrease the environmental Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. impact of plastic waste. While some of them focus on reducing the consumption and use of This article is an open access article plastics, such as plastic bag bans, other interventions tackle the end-of-life impacts through distributed under the terms and recycling and energy recovery [5]. However, the recovery of the material or energetic conditions of the Creative Commons value of plastics requires complementary actions, such as separate collection or sorting and Attribution (CC BY) license (https:// infrastructures [5–7]. The spread of the recycling and recovery of the heat capacity has been creativecommons.org/licenses/by/ limited by a lack of technical or economic resources, especially in developing countries. As 4.0/). a result, a significant proportion of plastics end up buried in landfills and on dumpsites. Polymers 2021, 13, 1014. https://doi.org/10.3390/polym13071014 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 1014 2 of 13 Besides, improper management leads to losses of plastic waste to the environment [8]; it was calculated that 6.2 Mt of macroplastics were lost to the environment in 2015 [9], and it was estimated that 50–90% of marine debris is plastic [10]. For this trend to change, the current infrastructure for waste management would need to increase by fourfold [11]. In this scenario, biodegradation of plastics has been promoted as an alternative to decreasing these materials’ impact at the end of their useful life. This attribute is expected to be helpful for the plastic products which are most likely to be mismanaged or whose recycling is not feasible [12], such as packaging and medical products [13]. Alternatives include biobased and biodegradable plastics, such as polyhydroxyalkanoates (PHA) [14] and novel materials produced from proteins and polysaccharides [15]. They are considered a promising option to decrease the plastics’ environmental impact by lowering the use of fossil resources and promoting these materials’ integration into natural cycles. Different microorganisms capable of degrading conventional plastics in laboratory conditions have been isolated. The enzymatic process usually happens in two stages: surface adsorption of enzymes and hydroperoxidation or hydrolysis of chemical bonds [16], and is highly dependent on the chemical structure of the plastics [17]. However, the specific process of polymer cleavage by bacteria remains largely unknown [18]. The idea of biodegradable plastics appeals to consumers [19]. However, their potential to be a solution to plastic pollution is uncertain, given that the conditions required for biodegradation are not always fulfilled in the environment [20] or in waste management scenarios. Two main types of plastics have been promoted as biodegradable in developing countries: compostable, which degrade aerobically by the action of microorganisms under the conditions of an industrial composting plant [21–23], and oxo-degradable, which degrade in a sequential abiotic-biotic process triggered by the oxidation promoted by UV radiation [24–26]. The consumption of plastics in Mexico reached 6221 kTon in 2019 (50 kg/person/year); 3499 kTon were covered by local production, and the rest was imported [27]. As a result of the increase in plastic waste, oxo-degradable and compostable plastics have been promoted by local authorities to substitute conventional plastics, especially for carrying bags. How- ever, in Mexico, as well as in other Latin American countries, compost facilities are scarce. This lack of infrastructure and improper waste management practices by consumers [28] provoked that the most likely end-of-life scenario for plastics sold as biodegradable are landfills and dumpsites, where aeration, microorganism population, and humidity will be remarkably different from those of composting. The need to develop research that realisti- cally predicts the degradability of plastics has been signaled before. Although there were studies focused on the analysis of biodegradation of plastics in anaerobic environments [29], most of them aimed to assess digestion processes [30], which do not represent the condi- tions found in landfills and dumpsites: long retention times (in the order of decades), lack of aeration and mixing, heterogeneity, varying temperature, and moisture [31]. The goal of our research was to contribute to filling this gap of knowledge. In this context, this re- search aimed to assess the in-laboratory degradation of compostable, oxo-degradable, and conventional plastics under the burial conditions that could be found in local landfills in order to determine the fate of these materials in the current waste management conditions. Besides comparing the different materials, we tried to assess if they would degrade in the current waste management conditions, as well as the consequences that could be inferred from their performance. 2. Materials and Methods In this research, three different plastic films available in the Mexican market were used to assess their behavior in landfill conditions. Ecovio®, a certified compostable plastic produced by BASF, is produced by combining the compostable fossil-based plastic Ecoflex® (polybutylene adipate terephthalate, PBAT) and biobased polylactic acid [32]. High-density polyethylene (HDPE) containing the prooxidant additive d2w® and a conventional HDPE with the same gauge were used as examples of oxo-degradable and conventional plastic Polymers 2021, 13, 1014 3 of 13 films used in the production of plastic bags. Conventional HDPE and oxodegradable HDPE were provided by Plásticos Degradables (Cuernavaca, Mexico), while ECOVIO® was provided by the Mexican filial of BASF. 2.1. Preparation of the Solid Waste Mixture A mixture resembling the composition of the urban solid waste of Mexico City, as reported previously [33], was prepared (Table1) to guarantee