Risultati e discussione
3.3 Quantificazione mediante Py-GC/MS
Nel tentativo di quantificare il rapporto tra componente riconducibile a polistirene rispetto a quella riconducibile alla parte poliolefinica è stata tentata una quantificazione del contenuto polimerico mediante Py-GC/MS.
A questo proposito sono state preparate tre capsule portacampione contenenti quantità crescenti di polistirene vergine e, dopo pirolisi, è stato integrato il segnale dello stirene (tabella 3.17). Riportando l’area in funzione della quantità pirolizzata è stata ottenuta la curva di figura 3.61.
Tabella 3.17: quantità di standard pesato e aree stirene.
Standard PS Peso (mg) Area
1 0,020 651144816
2 0,034 1221784166
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Figura 3.61: curva di calibrazione del PS.
La stessa procedura è stata utilizzata con la frazione ad alto peso molecolare dell’estratto del campione G3040012 (peso 0,044 mg, area stirene 1692554422), che ha fornito il valore indicato dal punto arancione della curva in figura 3.62.
Figura 3.62: curva di calibrazione del PS (punti in blu) con aggiunta della frazione ad alto peso molecolare
del campione G3040012 (punto arancione).
Ricalcolando la quantità di stirene trattata in Py-GC/MS è stato stimato un recupero del 108%.
Pertanto, in base all’equazione della curva di calibrazione costruita a partire dallo standard di PS, è stata fatta una stima della percentuale della frazione riconducibile a PS presente negli estratti dei campioni analizzati mediante Py-GC/MS e, per differenza, è stata calcolata anche la percentuale di poliolefine (tabella 3.18).
y = 4E+10x - 2E+08 R² = 1 0,00E+00 5,00E+08 1,00E+09 1,50E+09 2,00E+09 2,50E+09 0 0,01 0,02 0,03 0,04 0,05 0,06 0,07 A re a Massa (mg)
Calibrazione PS + G3040012
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Tabella 3.18: percentuale di materiale riconducibile a PS e poliolefine nei campioni analizzati in Py-GC/MS.
Campione Quantità PS (%) Altro (%)
G3040001 9,3% 90,7%
G3040004 8,7% 91,3%
G3040005 11,1% 88,9%
G3040012
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Conclusioni
L’approccio analitico sviluppato per l’analisi della sabbia del litorale pisano ha chiaramente messo in evidenza la presenza di quantità ben misurabili di materiale organico di origine non naturale, riconducibile a processi di degradazione dei detriti di materiale plastico spiaggiato. Il lavoro sperimentale svolto ha permesso di mettere a punto un metodo di estrazione tipo “Kumagawa” che opera una filtrazione a caldo della fase organica contemporaneamente all’estrazione. Inoltre è stato possibile quantificare per pesata il materiale recuperato per estrazione con diclorometano.
La caratterizzazione chimica del materiale estratto dalle sabbie è stata effettuata tramite Py- GC/MS e spettroscopia FT-IR. I dati hanno mostrato che il materiale estratto è costituito da una miscela di composti idrocarburici non naturali sia di natura alifatica che aromatica riconducibili ai processi di degradazione di materiale plastico quale polistirene, polietilene e polipropilene, ampiamente presente nella sabbia come detriti con granulometria superiore ai 2 mm. L’analisi mediante GPC ha messo in evidenza che i composti estratti sono ripartiti su due distribuzioni di pesi molecolari a cui corrispondono anche una differente composizione chimica in termini di natura alifatica e aromatica. In particolare è stato osservato un intervallo di pesi molecolari compreso tra 85,5 e 173 kDa, relativo alla frazione prevalentemente aromatica riconducibile a prodotti di degradazione di polistirene e un intervallo di pesi molecolari compreso tra 0,7 e 1,4 kDa relativo alla frazione prevalentemente alifatica riconducibile con buona probabilità a prodotti di degradazione di polietilene e polipropilene.
I valori dell’indice di polidispersità, maggiori dell’unità, indicano che i polimeri analizzati sono caratterizzati da un’alta variabilità del peso molecolare in accordo con i meccanismi di fotodegradazione dei polimeri.
I risultati relativi a 16 campioni di sabbia raccolta presso la spiaggia di Marina di Vecchiano hanno mostrato una concentrazione media minima di microplastiche nella zona della battigia (5,9±0,4 mg/Kg) e una massima nella zona delle dune (139±130 mg/Kg).
È stato anche tentato un primo approccio di quantificazione mediante Py-GC/MS, costruendo una curva di calibrazione con uno standard di PS. I risultati sono sembrati promettenti ed è stato possibile fare una stima non solo del PS presente nel campione, ma anche, per differenza, delle poliolefine.
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