As estações de tratamento de biorresíduos municipais contribuem para a contaminação do meio ambiente com resíduos de plásticos biodegradáveis com suposto maior potencial de persistência
Dec 23, 2023Scientific Reports volume 12, Artigo número: 9021 (2022) Citar este artigo
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Espera-se que os plásticos biodegradáveis (BDP) mineralizem facilmente, especialmente sob condições de compostagem técnica. No entanto, a complexidade da matriz da amostra impediu em grande parte estudos de degradação em condições realistas. Aqui, compostos e fertilizantes de estações municipais de tratamento combinado anaeróbico/aeróbico de biorresíduos de última geração foram investigados quanto a resíduos de BDP. Encontramos fragmentos de BDP > 1 mm em número significativo nos compostos finais destinados a fertilizantes para agricultura e jardinagem. Em comparação com os sacos compostáveis imaculados, os fragmentos de BDP recuperados mostraram diferenças nas suas propriedades materiais, o que potencialmente os torna menos propensos a uma maior biodegradação. Fragmentos de BDP < 1 mm foram extraídos a granel e chegaram a 0,43% em peso do peso seco do composto. Finalmente, o fertilizante líquido produzido durante o tratamento anaeróbico continha vários milhares de fragmentos de BDP <500 µm por litro. Portanto, nosso estudo questiona se os BDP atualmente disponíveis são compatíveis com aplicações em áreas de relevância ambiental, como a produção de fertilizantes.
Os plásticos biodegradáveis (BDP) são cada vez mais propostos como alternativas ecológicas aos plásticos básicos para folhas, embalagens e sacos. Uma área onde a utilização do BDP poderia trazer benefícios significativos é a recolha de resíduos domésticos orgânicos. Actualmente, a maior parte dos resíduos biológicos domésticos recolhidos está contaminada por sacos de plástico convencionais, presumivelmente porque uma fracção significativa da população prefere, se é que prefere, recolher os seus resíduos biológicos nesses sacos. No entanto, os plásticos convencionais não devem entrar numa estação de tratamento de resíduos biológicos, uma vez que não se degradarão. Consequentemente, têm de ser removidos tão completamente quanto possível dos resíduos biológicos recebidos através de procedimentos de triagem elaborados, o que, incidentalmente, também conduz a perdas significativas de material orgânico degradável. Uma vez que o biogás (eletricidade, calor) e os fertilizantes produzidos a partir desse material geram as receitas, embora os resíduos tenham de ser eliminados a custos consideráveis, qualquer perda desse tipo não é do interesse dos operadores das instalações. Apesar da preparação elaborada, a entrada de plásticos em estações de tratamento de resíduos biológicos não pode ser completamente evitada e foram introduzidas regulamentações rigorosas, entre outras coisas, no que diz respeito à quantidade máxima de plástico permitida, por exemplo, em composto certificado de alta qualidade, como < 0,1 peso % de acordo com §3, 4b, DüMV e §3, 4c, DüMV. Por razões de praticabilidade, apenas os fragmentos de plástico > 2 mm são contados para a quantificação da contaminação, limite que se espera que seja reduzido para fragmentos > 1 mm num futuro próximo. Nesta situação, os sacos de plástico compostáveis são vistos como uma opção atractiva, nomeadamente porque as condições durante o tratamento técnico dos bio-resíduos por compostagem devem ser ideais para a sua decomposição e surgiram nos supermercados sacos dedicados para efeitos de recolha de bio-resíduos domésticos. É certo que nem todos os efeitos adversos das películas e sacos nas estações de tratamento de resíduos biológicos seriam automaticamente resolvidos através da introdução de sacos biodegradáveis. Sabe-se que os operadores temem pelas suas máquinas, especialmente durante a digestão anaeróbica, onde não se espera que os materiais biodegradáveis se desintegrem de forma significativa. No entanto, muito neste aspecto depende das condições reais de operação. As plantas com mistura ativa podem enfrentar mais dificuldades do que as plantas de caixa.
Uma definição típica de biodegradabilidade é dada na Norma Europeia EN 13432 (Requisitos para embalagens recuperáveis através de compostagem e biodegradação - Esquema de testes e critérios de avaliação para a aceitação final de embalagens1), que afirma que um material é biodegradável, se for convertido ('mineralizado ') pela atividade microbiana na presença de oxigênio em CO2, água, sais minerais e biomassa ou na ausência de oxigênio em metano, CO2, água, sais minerais e biomassa. Embora a definição seja clara, a biodegradação real é normalmente estimada de uma forma não específica através de uma comparação do CO2 produzido por uma cultura aeróbia padrão na presença do material de teste em comparação com uma cultura sem, bem como uma cultura contendo quantidades semelhantes de um material biodegradável natural, como a celulose. Nestas circunstâncias, nada se sabe sobre o mecanismo de decomposição do material biodegradável, em particular, se uma parte significativa do mesmo permanece como micro e nanoplásticos, ou seja, partículas, que são consideradas como tendo um impacto considerável no ambiente e na saúde humana2. Além disso, os atuais materiais biodegradáveis/compostáveis não são certificados para desintegração em condições anaeróbicas. Além disso, o termo compostável é utilizado no contexto de plásticos biodegradáveis. A EN 13432 define um material como compostável, se 90% em peso do material estiver fragmentado (desintegrado) em partículas <2 mm, ou seja, abaixo do limite em que as partículas “contam”, após doze semanas de compostagem padronizada e totalmente mineralizado em 90% em peso dentro de 6 meses. Os 10% restantes em peso podem ser transformados em biomassa ou simplesmente fragmentados em microplástico. Além disso, um material compostável não pode trazer metais pesados nem introduzir efeitos ecotóxicos no composto final.
2 mm, which, according to these studies, were no longer in evidence after the composts had been conditioned by the customary sieving steps. In one case, foils certified as biodegradable were purposely introduced in controlled amounts into the digestion/composting process, and again no plastic fragments were visible in the finished—sieved—compost6. The size fraction < 2 mm was not considered in any of these studies./p> 5 mm fraction corresponding to the contamination by residual “macroplastic” (5 mm is a commonly used upper size limit for “microplastic”, anything larger is macroplastic) and a 1–5 mm fraction corresponding to the regulatory relevant residual contamination by microplastic. The lower limit of 1 mm rather than 2 mm was chosen in anticipation of the expected changes in regulation, where the replacement of the 2 mm limit by a 1 mm limit is imminent./p> 5 mm and/or the 1–5 mm sieving fractions using FTIR analysis3 (Fig. 1; Table 1). All recovered fragments appeared to stem from foils, bags or packaging, since they were thin compared to their length and width (see Suppl Figure S1 for typical examples). Fragments with overlapping signatures, most likely PBAT/PLA mixtures or blends, were also found (see Suppl Figure S2 for the interpretation of the spectra). In addition, the recorded BDP fragment spectra (Fig. 1A) showed high similarity to the FTIR spectra of commercial compostable bags sold in the vicinity of the biowaste treatment plants (Fig. 1B), which together with the geometry of the recovered fragments led us to assuming that the majority of the BDP entered the biowaste in the form of such bags./p> 5 mm or 1–5 mm) in which the fragment was found, and finally, the fragment number. Fragment F#1_5mm_4 therefore represents the 4th fragment collected in the > 5 mm size fraction from the finished compost of plant number 1. Bags were arbitrarily numbered 1–10, see Suppl Table S1 for supplier information. The spectra (in grey) of the reference materials for PLA and PBAT are given as basis for the interpretation. Spectra in red refer to test samples consisting only of PBAT, while those in blue indicate samples composed of PBAT/PLA mixtures./p> 5 mm size fraction (Table 1) and for that reason has become state-of-the-art in preparing quality composts (contamination by plastic fragments > 2 mm of less than 0.1 wt%). Given that the size of the fragments is a crucial factor regarding ecological risk, we analyzed the sizes (length Î width) of the BDP fragments in comparison to that of the plastic fragments with signatures of commodity plastics such as PE (Fig. 2). BDP fragments found in a given compost sample tended to be smaller than the fragments stemming from non-BDP materials, which may indicate that BDPs degrade faster or tend to disintegrate into tinier particles than commodity plastics. This may also explain why in the compost from plant #2, no BDP fragments were found in the particle fraction retained by the 5 mm sieve (> 5 mm fraction), while 19 such particles were found in the fraction then retained by the 1 mm sieve (1–5 mm fraction). Interestingly, plant #2 is the only one included in our study that uses no mechanical breakdown of the incoming biowaste. This reduces the mechanical stress on the incoming material. Mechanical stress can alter the properties of plastic foils such as the crystallinity whereby crystallinity has been shown to influence the biological degradation of BDP such as PLA7./p> 1 mm. (A) Fragments found in the finished compost from plant #1, (B) in the finished compost from plant #2, and (C) in the pre-compost from plant #3. For reasons of statistical relevance, only samples containing more than 20 BDP fragments per kg of compost were included in the analysis./p> 5 mm or 1–5 mm) in which the fragment was found, and finally, the fragment number. Bags were arbitrarily numbered 1–10, see Suppl Table S1 for supplier information. The spectra (in grey) of the reference materials for PLA and PBAT are given as basis for the interpretation. Spectra in red refer to test samples consisting only of PBAT, while those in blue indicate samples composed of PBAT/PLA mixtures. (C) Chemical structures of PLA and PBAT, chemical shifts of the protons are assigned as indicated in the reference spectra in (B)./p> 5 mm or 1–5 mm) in which the fragment was found, and finally, the fragment number./p> 5 mm or 1–5 mm) in which the fragment was found, and finally, the fragment number./p> 1 mm were found in the collected LF samples. This is hardly surprising, given that the LF is produced by press filtration of the digestate after the anaerobic stage. Such a filtration step can be expected to retain fragments > 1 mm in the produced filter cake, which goes into the composting step, leaving the filtrate, i.e. the LF, essentially free of such particles. Anaerobic digestion is currently not assumed to contribute significantly to the degradation of BDP17,22, but the process conditions (mixing, pumping) may promote breakdown of larger fragments, particularly when additives such as plasticizers23 leach out of the material./p> 20,000 BDP microparticles of a size ranging from 10 µm to 500 µm enter each m2 of agricultural soil whenever LF is applied on agricultural surfaces./p> 1 mm. Six compost samples representing the more contaminated ones based on the content of fragments > 1 mm, namely, f#1, f#2, p#3, f#3, p#4 and f#4 (nomenclature: f or p for finished or pre-compost, followed by plant number), were extracted with a 90/10 vol% chloroform/methanol mixture. The amounts of PBAT and PLA in the obtained extracts were then quantified via 1H-NMR (Table 4). Briefly, the intensity of characteristic signals in the extract spectra of the compost samples (see Suppl Figure S4) were compared to peak intensities produced by calibration standards of the pure polymer dissolved at a known concentration in the chloroform/methanol. All samples and standards were normalized using the 1,2-dichloroethan signal at 3.73 ppm as internal standard. See also Suppl Figure S5 for an exemplification of the quantification of the PBAT/PLA ratios. Based on the amounts of PBAT and PLA extracted from a known amount of compost, the total mass concentration (wt% dry weight) of these polymers in the composts was calculated./p> 2 mm. Moreover, residues of PBAT and PLA were found in all investigated compost samples, including the finished compost from plant #4, which had shown no contamination by larger BPD fragments (Table 1). The pre-compost from that plant had shown a few contaminating BDP fragments in the > 5 mm fraction. However, in regard to the fragments < 1 mm, the composts from plant #4 showed a similar incidence, at least for PLA, as the finished compost samples from the other plants (Table 4)./p> 1.100 U mL−1), Pektinase L-40 (activity: > 900 U mL−1, Exo PGA, > 300 U mL−1 Endo PGA, > 300 U mL−1 Pektinesterase), and Cellulase TXL (activity: > 30 U mL−1) were from ASA Spezialenzyme GmbH (Wolfenbüttel, Germany), Viscozyme L (activity: > 100 FBG U g−1) was from Novozymes A/S (Bagsværd, Denmark)./p> 1 mm, approximately 3 L of the compost sample was weighed and evenly distributed into 6 glass vessels (capacity 3 L each). The material was suspended in 2.5 L of water and first sieved with a mesh size of 5 mm (yielding fraction > 5 mm). All particles retained by the sieve were collected with tweezers and transferred to the system for ATR-FTIR analysis, see below, while the material passing the sieve was sieved again at 1 mm, followed again by collection of the retained particles (yielding fraction 1–5 mm), which were subsequently also analyzed by ATR-FTIR. Sieves were from Retsch GmbH (Haan, Germany; test sieve, IS 3310-1; body/mesh, S-Steel; body, 200 mm × 50 mm. For the analysis of the chemical nature of the collected particles Attenuated total reflection—Fourier transform infrared (ATR-FTIR) spectrometry (spectrometer: Alpha ATR unit, Bruker 27; equipped with a diamond crystal for measurements) was used. Spectra were taken from 4000 to 400 cm−1 (resolution 8 cm−1, 16 accumulated scans, Software OPUS 7.5) and compared with entries from an in-house database described previously24 or the database provided by the manufacturer of the instrument (Bruker Optik GmbH, Leipzig, Germany). This comparison of the IR-spectra allowed to distinguish biodegradable from conventional plastic fragments, but also from residues of other materials including unknowns. An incident light microscope (microscope, Nikon SMZ 754T; digital camera, DS-Fi2; camera control unit, DS-U3; software, NIS Elements D) was used for visual documentation of all particles identified by ATR-FTIR as synthetic plastics (biodegradable or otherwise)./p> 1 mm. For the preparation of the plastic fragments < 1 mm (down to 10 µm) an adjusted enzymatic-oxidative digestion method based on a method suggested by Löder et al. 2017 was adapted25. For this, the liquid fertilizer sample was mixed well with a metal rod and 50 mL were quickly poured into a 300 mL glass beaker (Schott-Duran). The metal rod and the glass beakers were washed in advance with Millipore water. Then 50 mL of a 10 wt% sodium dodecyl sulfate (SDS) solution (≥ 95 % SDS; Karl Roth) was added and the mixture incubated at 50 °C for 72 h under gentle agitation (Universal Shaker SM 30 B, Edmund Bühler GmbH, Bodelshausen, Germany). Subsequently, 2 × 25 mL of 30% hydrogen peroxide was slowly added under a fume hood. Since the reaction of hydrogen peroxide with organic matter is highly exothermic, an ice bath was used to keep the reaction temperature below 40 °C. Once the reaction had subsided and the mixture had again reached room temperature, the solution was filtered over a 10 µm stainless-steel-mesh filter (47 mm diameter, Rolf Körner GmbH, Niederzier, Germany) with a vacuum filtration unit (3-branch stainless-steel vacuum manifold with 500 mL funnels and lids, Sartorius AG, Göttingen, Germany). All filtrations were conducted under a laminar flow hood to minimize contamination with microplastics from the surrounding air. All matter retained by the filter was rinsed with filtered (0.2 µm) deionized water to remove residual chemicals. Afterwards, the retained matter was rinsed into a fresh 300 mL glass beaker with approximately 50 mL of 0.1 M Tris-HCl buffer (pH 9.0). As particles tended to adhere to the stainless-steel filter, the filter was also placed into the beaker. Ten milliliters of Protease A-01 solution were added and the beaker was incubated at 50 °C for 12 h with gentle agitation. Afterwards, the filter was thoroughly rinsed off into the beaker with filtered deionized water to recover any adhering particles and then used to filter the incubated solution. The retained matter was rinsed into a fresh glass beaker with 25 mL of 0.1 M NaAc buffer (pH 5). The filter was again placed in the jar as well, 5 mL of the Pektinase L-40 solution was added, and the beaker was incubated for 72 h at 50 °C. The filter was rinsed and used to filter the sample as before. Any matter retained by this filtration step was again rinsed into a fresh glass beaker with 25 mL of 0.1 M NaAc buffer (pH 5). The filter was again placed in the beaker, 1 mL of a Viscozyme L solution was added, and the jar was incubated at 50 °C for 48 h. The sample was filtered and the retained matter was transferred into 25 mL of a 0.1 M NaAc buffer (pH 5). Five mL of Cellulase TXL solution was added and the jar was incubated at 40 °C for 24 h./p>3.0.CO;2-3" data-track-action="article reference" href="https://doi.org/10.1002%2F%28SICI%291099-1581%28199704%298%3A4%3C203%3A%3AAID-PAT627%3E3.0.CO%3B2-3" aria-label="Article reference 8" data-doi="10.1002/(SICI)1099-1581(199704)8:43.0.CO;2-3"Article CAS Google Scholar /p>
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