Modelling the uptake of suspended materials and salts in nearshore waters by plastics using nuclear microscopy and depth profiling analytical tools

Plastic debris becomes a persistent contaminant in aquatic systems. The adherence of abiotic and biotic materials to plastics may potentiate structural changes and their degradation. How the major and minor elements present in sea water interact with polymers is largely unknown. Nuclear microprobe clusters PIXE and FIBS techniques, which offer unique possibilities to characterize the materials deposited on the surface of plastics rejected to the aquatic environment. In particular, these techniques enable to examine the penetration of elements in deposits into the plastic structure. Food packaging polymers, high-density polyethylene (HDPE) and polyethylene terephthalate (PET) exposed to turbid water of the Tagus estuary were studied. The deposition mosaic contained clastic, biota and saline components. A major finding was the diffusion of the ion Cl- in the polymer matrix. This was possible by examining elemental profiles taken in transversal sections of the polymers. The depth structure of deposits was estimated using RBS and PIXE data applying the programs OMDAQ2007 and WiNDF, which illustrate the capabilities of these analytical tools in the micro-scale analysis of deposits. The results obtained pointed out for a multilayer depth structure which can decode the complex arrangement of cellular and sediment materials deposited on the polymer's surface. In addition, it was possible to identify sources of uncertainties in simulating the interface and the changeable polymer matrix in sub-surface layers.