Gli impianti municipali di trattamento dei rifiuti organici contribuiscono alla contaminazione dell'ambiente con residui di plastica biodegradabile con potenziale di persistenza presumibilmente più elevato
Dec 23, 2023Rapporti scientifici volume 12, numero articolo: 9021 (2022) Citare questo articolo
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Si prevede che la plastica biodegradabile (BDP) si mineralizzi facilmente, in particolare in condizioni di compostaggio tecnico. Tuttavia, la complessità della matrice del campione ha in gran parte impedito studi di degradazione in condizioni realistiche. Qui sono stati studiati i residui di BDP di compost e fertilizzanti provenienti da impianti municipali combinati di trattamento dei rifiuti organici anaerobici/aerobici all'avanguardia. Abbiamo trovato frammenti di BDP > 1 mm in numero significativo nei compost finali destinati come fertilizzante per l'agricoltura e il giardinaggio. Rispetto ai sacchetti compostabili originali, i frammenti di BDP recuperati hanno mostrato differenze nelle proprietà dei materiali, che li rendono potenzialmente meno inclini a un’ulteriore biodegradazione. I frammenti di BDP < 1 mm sono stati estratti alla rinfusa e hanno raggiunto lo 0,43% in peso del peso secco del compost. Infine, il fertilizzante liquido prodotto durante il trattamento anaerobico conteneva diverse migliaia di frammenti BDP < 500 µm per litro. Pertanto, le domande del nostro studio, se il BDP attualmente disponibile sono compatibili con le applicazioni in aree di rilevanza ambientale, come la produzione di fertilizzanti.
Le plastiche biodegradabili (BDP) sono sempre più proposte come alternative ecologiche alla plastica di base per fogli, involucri e sacchetti. Un’area in cui l’utilizzo del BDP potrebbe apportare benefici significativi è la raccolta dei rifiuti organici domestici. Attualmente la maggior parte dei rifiuti organici domestici raccolti sono contaminati da sacchetti di plastica convenzionali, presumibilmente perché una parte significativa della popolazione preferisce, se non proprio farlo, raccogliere i propri rifiuti organici in tali sacchetti. Tuttavia, la plastica convenzionale non dovrebbe entrare in un impianto di trattamento dei rifiuti organici, poiché non si degraderà. Di conseguenza devono essere rimossi il più completamente possibile dai rifiuti organici in entrata mediante elaborati processi di selezione, che tra l'altro portano anche a notevoli perdite di materiale organico degradabile. Poiché il biogas (elettricità, calore) e i fertilizzanti prodotti da questo materiale creano i ricavi, mentre i rifiuti devono essere smaltiti a costi considerevoli, qualsiasi perdita di questo tipo non è nell'interesse dei gestori dell'impianto. Nonostante l'elaborata preparazione, l'ingresso di plastica negli impianti di trattamento dei rifiuti organici non può essere completamente impedito e sono state introdotte norme rigorose, tra l'altro, per quanto riguarda la quantità massima di plastica consentita, ad esempio nel compost certificato di alta qualità, come < 0,1 % secondo §3, 4b DüMV e §3, 4c DüMV. Per ragioni di praticabilità, per la quantificazione della contaminazione vengono conteggiati solo i frammenti di plastica > 2 mm, limite che si prevede verrà abbassato a frammenti > 1 mm nel prossimo futuro. In questa situazione, i sacchetti di plastica compostabili sono visti come un’opzione interessante, soprattutto perché le condizioni durante il trattamento dei rifiuti organici tecnici tramite compostaggio dovrebbero essere ideali per la loro scomposizione e nei supermercati sono apparsi sacchetti dedicati alla raccolta dei rifiuti organici domestici. Certo, non tutti gli effetti negativi delle pellicole e dei sacchetti negli impianti di trattamento dei rifiuti organici verrebbero automaticamente risolti attraverso l’introduzione di sacchetti biodegradabili. È noto che gli operatori temono per i loro macchinari, in particolare durante la digestione anaerobica, dove non si prevede che i materiali biodegradabili si disintegrino in misura significativa. Tuttavia, molto dipende dalle condizioni operative effettive. Le piante con miscelazione attiva possono incontrare maggiori difficoltà rispetto alle piante in scatola.
Una tipica definizione di biodegradabilità è fornita nella norma europea EN 13432 (Requisiti per imballaggi recuperabili mediante compostaggio e biodegradazione – Schema di prova e criteri di valutazione per l'accettazione finale degli imballaggi1), la quale afferma che un materiale è biodegradabile, se viene convertito ("mineralizzato ') dall'attività microbica in presenza di ossigeno in CO2, acqua, sali minerali e biomassa o in assenza di ossigeno in metano, CO2, acqua, sali minerali e biomassa. Sebbene la definizione sia chiara, la biodegradazione effettiva viene generalmente stimata in modo non specifico attraverso il confronto della CO2 prodotta da una coltura standard aerobica in presenza del materiale di prova rispetto a una coltura senza e con una coltura contenente quantità simili di un materiale naturale biodegradabile come la cellulosa. In queste circostanze non si apprende nulla sul meccanismo di degradazione del materiale biodegradabile, in particolare se una parte significativa di esso rimane sotto forma di micro e nanoplastiche, cioè particelle, che si ritiene abbiano un notevole impatto sulla salute ambientale e umana2. Inoltre, gli attuali materiali biodegradabili/compostabili non sono certificati per la disintegrazione in condizioni anaerobiche. Inoltre, il termine compostabile viene utilizzato nel contesto della plastica biodegradabile. La norma EN 13432 definisce un materiale come compostabile, se il 90% in peso del materiale è frammentato (disintegrato) in particelle < 2 mm, cioè al di sotto del limite al quale le particelle “contano”, dopo dodici settimane di compostaggio standardizzato e completamente mineralizzato per il 90% in peso entro 6 mesi. Il restante 10% in peso può essere trasformato in biomassa o semplicemente frammentato in microplastica. Inoltre un materiale compostabile non può apportare metalli pesanti né introdurre effetti ecotossici nel compost finale.
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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