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Effects of polymer ageing on formation and fate of microplastics in sea water
Summary
Researchers modeled the chemical and hydrolytic aging of polyamide 6 in seawater using a new kinetic model that tracks chain scission and embrittlement, identifying the ductile-to-brittle transition as the physical threshold for microplastic fragment formation, while DNA metabarcoding revealed that bacterial communities colonizing the polymer surface shift distinctly depending on polymer type and weathering state.
The problem of microplastics (MP) has become a global major concern in recent years. These particles (< 5 mm) are found in all oceans and ecosystem compartments. Polyamide 6 (PA6) is a polymer widely used in the maritime domain however it is found in sparse quantity in the environment. The fate of PA6 in the ocean was studied by a new approach in order to separate the different physicochemical phenomena involved in degradation (chemical ageing and hydrodynamic stress).Chemical ageing was carried out, considering oxidative and hydrolysis reactions. Changes in physico-chemical properties were characterized, in particular to follow the chain scission process and the interaction between water and PA6 during degradation. These data were used to design a new kinetic model with the aim of predicting chain scission processes, considering the water content, at ocean temperature. Changes in tensile properties have highlighted an embrittlement of the polymer. This phenomenon reflects a decrease in stress transmitters (tie molecules and entanglements).During embrittlement, the PA6 behavior, ductile initially, becomes brittle. This ductile/brittle transition was assumed to be the criterion to formation of microplastics. The kinetic model was then used to define the time required to form PA6 microplastics in the ocean. The influence of hydrodynamic stress was considered through a single experimental device and has leaded to polymer fragmentation in the brittle state. Interaction between polymers (unaged and aged) and prokaryotic organisms of the marine environment was also considered by a DNA metabarcoding approach. A high specificity of bacteria was observed depending on the kind of polymer and ageing conditions. The diversity of bacterial communities depends on the polarity of surfaces.