Les usines municipales de traitement des biodéchets contribuent à la contamination de l’environnement par des résidus de plastiques biodégradables au potentiel de persistance potentiellement plus élevé.
Dec 23, 2023Rapports scientifiques volume 12, Numéro d'article : 9021 (2022) Citer cet article
3602 Accès
9 citations
76 Altmétrique
Détails des métriques
Les plastiques biodégradables (BDP) devraient se minéraliser facilement, notamment dans des conditions de compostage technique. Cependant, la complexité de la matrice d’échantillons a largement empêché les études de dégradation dans des conditions réalistes. Ici, les composts et les engrais provenant d'usines municipales de traitement combiné anaérobie/aérobie de pointe des biodéchets ont été étudiés pour détecter les résidus de BDP. Nous avons retrouvé des fragments de BDP > 1 mm en nombre significatif dans les composts finaux destinés à servir d'engrais pour l'agriculture et le jardinage. Par rapport aux sacs compostables vierges, les fragments de BDP récupérés présentaient des différences dans leurs propriétés matérielles, ce qui les rend potentiellement moins sujets à une biodégradation ultérieure. Les fragments de BDP < 1 mm ont été extraits en vrac et représentaient 0,43 % en poids du poids sec du compost. Enfin, l'engrais liquide produit lors du traitement anaérobie contenait plusieurs milliers de fragments de BDP < 500 µm par litre. Par conséquent, notre étude se demande si les BDP actuellement disponibles sont compatibles avec des applications dans des domaines pertinents pour l'environnement, tels que la production d'engrais.
Les plastiques biodégradables (BDP) sont de plus en plus proposés comme alternatives écologiques aux plastiques de base pour les films, les emballages et les sacs. Un domaine dans lequel l’utilisation du BDP pourrait s’avérer très utile est la collecte des déchets ménagers organiques. Actuellement, la plupart des biodéchets ménagers collectés sont contaminés par des sacs en plastique conventionnels, probablement parce qu'une fraction importante de la population préfère, voire pas du tout, collecter ses biodéchets dans de tels sacs. Toutefois, les plastiques conventionnels ne sont pas censés entrer dans une usine de traitement des biodéchets, car ils ne se dégraderont pas. Ils doivent donc être éliminés le plus complètement possible des biodéchets entrants au moyen de procédures de tri élaborées, ce qui entraîne également d'importantes pertes de matières organiques dégradables. Étant donné que le biogaz (électricité, chaleur) et les engrais produits à partir de ces matériaux génèrent des revenus, alors que les déchets doivent être éliminés à des coûts considérables, une telle perte n'est pas dans l'intérêt des exploitants de l'usine. Malgré une préparation complexe, l'entrée de plastiques dans les usines de traitement des biodéchets ne peut être complètement empêchée et des réglementations strictes ont été introduites, entre autres, concernant la quantité maximale de plastique autorisée, par exemple dans un compost certifié de haute qualité, par exemple < 0,1 en poids. % selon §3, 4b, DüMV et §3, 4c, DüMV. Pour des raisons de praticité, seuls les fragments de plastique > 2 mm sont comptés pour la quantification de la contamination, limite qui devrait être abaissée aux fragments > 1 mm dans un avenir proche. Dans cette situation, les sacs en plastique compostables sont considérés comme une option intéressante, d'autant plus que les conditions lors du traitement des biodéchets techniques par compostage devraient être idéales pour leur décomposition et que des sacs dédiés à la collecte des biodéchets ménagers sont apparus dans les supermarchés. Certes, tous les effets néfastes des films et des sacs dans les installations de traitement des biodéchets ne seraient pas automatiquement résolus par l'introduction de sacs biodégradables. Les opérateurs craignent pour leurs machines, en particulier lors de la digestion anaérobie, où les matériaux biodégradables ne devraient pas se désintégrer de manière significative. Cependant, cela dépend en grande partie des conditions d’exploitation réelles. Les plantes à mélange actif peuvent rencontrer plus de difficultés que les buis.
Une définition typique de la biodégradabilité est donnée dans la norme européenne EN 13432 (Exigences relatives aux emballages récupérables par compostage et biodégradation – Schéma de tests et critères d'évaluation pour l'acceptation finale des emballages1), qui stipule qu'un matériau est biodégradable s'il est transformé (« minéralisé »). ') par activité microbienne en présence d'oxygène en CO2, eau, sels minéraux et biomasse ou en l'absence d'oxygène en méthane, CO2, eau, sels minéraux et biomasse. Bien que la définition soit claire, la biodégradation réelle est généralement estimée de manière non spécifique en comparant le CO2 produit par une culture standard aérobie en présence du matériel d'essai par rapport à une culture sans ainsi qu'à une culture contenant des quantités similaires de un matériau naturel biodégradable tel que la cellulose. Dans ces circonstances, on ne sait rien du mécanisme de dégradation du matériau biodégradable, en particulier si une partie importante de celui-ci reste sous forme de micro et nanoplastiques, c'est-à-dire de particules considérées comme ayant un impact considérable sur l'environnement et la santé humaine2. De plus, les matériaux biodégradables/compostables actuels ne sont pas certifiés pour une désintégration dans des conditions anaérobies. De plus, le terme compostable est utilisé dans le contexte des plastiques biodégradables. La norme EN 13432 définit un matériau comme compostable si 90 % en poids du matériau est fragmenté (désintégré) en particules < 2 mm, c'est-à-dire en dessous de la limite à laquelle les particules « comptent », après douze semaines de compostage standardisé et entièrement minéralisé à 90 % en poids. dans les 6 mois. Les 10 % poids restants pourront être transformés en biomasse ou simplement fragmentés en microplastique. De plus, un matériau compostable ne doit pas apporter de métaux lourds ni introduire d'effets écotoxiques dans le compost 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>
Envoyer une demande
Envoyer
English
Deutsch
Español
Italiano
Português
日本語
한국어
Русский
