Seeing as we are unable or unwilling to deal with the present micro and nano-particles resulting from decades of plastic use and disposal, I worry about the environmental effects of graphene.
A few things I've found:
https://en.wikipedia.org/wiki/Graphene
"10 µm few-layered graphene flakes were able to pierce cell membranes in solution"
http://www.kurzweilai.net/graphenes-negative-environmental-i...
"In groundwater, which typically has a higher degree of hardness and a lower concentration of natural organic matter, the graphene oxide nanoparticles tended to become less stable and eventually settle out or be removed in subsurface environments. In surface waters, where there is more organic material and less hardness, the nanoparticles remained stable and moved farther, especially in the subsurface layers of the water bodies."
(also http://online.liebertpub.com/doi/abs/10.1089/ees.2013.0392)
Funny enough, its not actually banned in the USA, just a liability nightmare. I ocasionally still see brake pads with the stuff. I suspect graphine, and other 'nano' materials will be similar in practice.