Biodegradation of graphene oxide-polymer nanocomposite films in wastewater
The synthesis of polymer nanocomposites has been extensively investigated by many researchers, however, the end of life fate of polymer nanocomposites is still largely unknown. It is expected that at the end of their service life, these polymer nanocomposites will most likely end up in soil and water systems where microorganisms will interact and, perhaps, even biodegrade them. In this study, we investigate the ability of wastewater microorganisms to biodegrade nanocomposite films containing different graphene oxide (GO) loads (0% to 0.6%, (w/w%)) embedded in a model biopolymer (i.e. chitosan). The ability of wastewater microorganisms to grow and form biofilms on the surface of the nanocomposite films was determined by live and dead staining assisted with confocal laser scanning microscopy. The capability of wastewater biofilms to biodegrade nanocomposites was assessed through nanocomposite film weight losses, Fourier transformed infrared (FTIR) and scanning electron microscopy (SEM) analyses. Results showed that microorganisms present in the activated sludge can grow on the surface of the nanocomposites and biodegrade the polymer surrounding the graphene oxide nanoplatelets. As the biopolymer gets degraded, there is increasing exposure of GO on the surface, which yields microbial inactivation and biofilm growth inhibition. To determine the evolution of the toxicity of the nanocomposite during biodegradation. We determined the emergence of the sharp edges of GO on the surface of the nanocomposite through atomic force microscopy (AFM), as well as the production of reactive oxygen species (ROS) with the Ellman's assay before and after biodegradation of the nanocomposites. The results show that as GO surfaces the nanocomposite film during biodegradation, there is increasing production of ROS, which explains the increasing inactivation of the microorganisms.