Circular economy in mussel farms: polypropylene recovery
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Clarissa Ciarlantini1*, Antonella Piozzi1, Iolanda Francolini1, Stefania Di Vito2, Fabrizio Soddu2, Elisa Scocchera2, Marzia Mattioli2, Loris Pietrelli2
1Chemistry Department, Sapienza, Rome, Italy 2Legambiente, Rome, Italy
*e-mail address of the presenting author
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Mussels represent an important source of low-cost protein for many people [1]. Mussel production is an environmentally friendly and sustainable activity; as mussels feed by filtering nutrients from sea water (0.252 kg CO2 eq. per 1 kg of mussel produced against over 20 kg CO2 eq. per 1 kg of beef produced). Nevertheless, the accidental dispersion on the seabed and along beaches of plastic “socks” used for their farming represents a significant environmental problem. Mussel farming can be really considered as a simple bio-based strategy to provide an efficient method to remove nutrients from eutrophic waters. Basically, for each kg of mussels produced, about 1 linear meter (20 g/m) of polypropylene (PP) net is used. Therefore, in face of the Mediterranean Sea production (320 ktonnes), about 6400 tonnes of plastic waste could be produced (European Waste Code, CER 020104). Considering the landfill disposal cost (0.25–0.30 €/kg), it is plausible to hypothesize that a certain number of mussel farmers usually abandon used socks at sea contributing significantly to marine litter. The recovery and recycling of the polymer material can represent a proper solution for the management of the plastic waste produced by mussel farming. However, the presence of organic materials, algae and biofouling on the surface of mussel socks makes PP not easily recyclable with traditional systems. The LIFE MUSCLES project, funded by the EU, was established to develop a newly treatment method for recovering polypropylene from mussel nets [2]. Through experiments performed on a laboratory scale, it was verified that it is possible to effectively remove biofilm (93%) from the polymeric material using the Fenton reaction (30% H2O2 solution, t=40 min, pH=2, T=25°C and FeSO4 as catalyst). However, to simplify the process and reduce costs, further experiments were carried out with pressurized water (25-30 bar) and in the absence of oxidizing agent (H2O2). The results showed excellent biofilm removal (98%) after 30 minutes. Moreover, process costs could be further reduced by recycling the washing water after filtration. The spectroscopic, thermal and mechanical characterization of the socks shredded and washed for 30 minutes demonstrated that the treatments undergone by the polymer had a non-significant impact on the PP physical properties, thus allowing its reuse in the same supply chain. In particular, the regenerated PP had an elastic modulus value (0.697±0.058 GPa) comparable to that of pristine PP (0.758±0.069 Gpa). Therefore, as the Life MUSCLES project fully embraces the circular economy as a production model, prioritizing material recycling, to demonstrate the feasibility of this treatment process, a mobile recycling pilot plant capable of processing 300 kg of mussel nets per day was constructed. This approach will effectively lower expenses associated with new mussel nets while mitigating environmental damage.
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References
[1] Matarazzo Suplicy F., Reviews in Aquaculture 2020, 12, 204–223
[2] Pietrelli L., Polymers, 2022, 14, 3469.