Natural iron-containing minerals catalyze the degradation of polypropylene microplastics: a route to self-remediation learnt from the environment

The detection of significantly different concentrations of highly degraded polyolefin microplastics (MPs) in two nearby marine sandy beaches in northern Tuscany, Italy, prompted the present investigation on the possible role of the sandy matrix in accelerating polymer degradation, already occurring as a result of photo-oxidation. The concentrations of polypropylene (PP) and polyethylene (PE) MPs in the sand sediments was based on the P.I.S.A. (Polymer Identification and Specific Analysis) procedure, allowing determination of the total mass of all particles below a given threshold (here 2 mm, set by the preliminary sieving of the sampled beach sand). The cause for the much lower total polyolefin concentration in one of the two beaches (Lecciona, within a natural park stretching along the coast) was successfully correlated with the peculiar sand composition, characterized by a relatively high content of iron-containing minerals such as magnetite and ferrous serpentine (a hydrous magnesium iron phyllosilicate). The ferrous sand significantly accelerated the photo-oxidative degradation of PP, as measured on model virgin MPs after both UV and Xe lamp irradiation (the latter simulating solar light). Similarly, the ferrous sand showed a catalytic effect on the thermal (at T=60 °C) degradation of environmentally aged PP MPs kept in the dark during the thermal treatment. These results clearly indicate that the natural environment may possess, under appropriate conditions (here the presence of iron in the natural matrix in which the MPs are dispersed, combined with intensive sun exposure), effective self- cleaning ability, and thus a better than expected resilience to the ever increasing pollution in the so-called plastisphere.