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

Abstract

Virgin and environmentally aged polypropylene (PP) micropowders (V-PP and E-PP, respectively) were used as reference microplastics (MPs) in comparative photo- and thermo-oxidative ageing experiments performed on their mixtures with a natural ferrous sand (NS) and with a metal-free silica sand (QS). The ferrous NS was found to catalyze the photo-oxidative degradation of V-PP after both UV and simulated solar light irradiation. The catalytic activity in the V-PP/NS mixture was highlighted by the comparatively higher fraction of photo-oxidized PP extracted in dichloromethane, and the higher carbonyl index of the bulk polymer extracted with boiling xylene, when compared with the V-PP/QS mixture. Similarly, NS showed a catalytic effect on the thermal degradation (at T = 60 °C) of E-PP. The results obtained indicate that, under suitable environmental conditions (in this case, an iron-containing sediment or soil matrix, combined with simulated solar irradiation), the degradation of some types of MPs could be much faster than anticipated. Given the widespread presence of iron minerals (including the magnetite and iron-rich serpentine found in NS) in both coastal and mainland soils and sediments, a higher than expected resilience of the environment to the contamination by this class of pollutants is anticipated, and possible routes to remediation of polluted natural environments by eco-compatible iron-based minerals are envisaged.

Keywords: Hydrocarbon chain scission; Photodegradation; Polymer oxidation; Polyolefin; Thermal degradation; Transition metal.

Fig. 1

A PP photo-oxidative degradation pathways: H-abstraction (more likely on tertiary carbons), followed by oxygen pickup and further H-abstraction by the new peroxy-radical; the resulting thermo- and photo-labile hydroperoxide decomposes into oxy-radical, and eventually ketones upon various β-scissions mechanisms involving the main chain C–C bond; Norrish type reactions of ketones cause further main chain scissions, all the above generating new free radicals (highlighted in the figure), with cascade amplification of the degradation reactions. B Schematized photo-activated generation of ROS by hole (h⁺) or electron (e⁻) transfer from magnetite

Fig. 2

Experimental design (DCM and Xy extraction solvents, respectively; NS and QS natural ferrous sand and quartz sand, respectively; V-PP and E-PP micropowders of virgin and environmentally aged polypropylene, respectively)

Fig. 3

Carbonyl region in the FT-IR spectra of V-PP and E-PP; spectra normalized vs. the methylene bending absorption at 1462 cm.⁻¹

Fig. 4

TEM analysis of NS_Fe showing a composition of serpentine polymorphs and magnetite: a chrysotile fibers with 13.5% Fe; b electron diffraction pattern of polycrystalline area in a; c modulated antigorite with 3.84% Fe; d selected area electron diffraction (SAED) of the spot highlighted in c; e magnetite, Fe₃O₄ (76% Fe, close to stoichiometric 72%); f 3D electron diffraction of the cubic face-centered cell of magnetite, with a = 8.4 Å

Fig. 5

Time profile of DCM- extractable fraction from the V-PP/QS (blue bars) and V-PP/NS (red bars) mixtures after 20 h of UV irradiation, expressed as wt% of the total V-PP content; error bars are referred to triplicate experiments; dotted curves are the linear and exponential best fit for the DCM extracts over irradiation time, respectively

Fig. 6

Carbonyl region in the FT-IR spectra of DCM extracts from V-PP irradiated 35 days in solar box: (i) V-PP blank (not irradiated, black line); (ii) V-PP irradiated without NS_Fe (blue line); (iii) V-PP irradiated in the presence of NS_Fe (red line)