Transformation of graphene oxide by ferrous iron: Environmental implications

Abiotic transformation of graphene oxide (GO) in aquatic environments can markedly affect the fate, transport, and effects of GO. The authors observed that ferrous iron (Fe[II])an environmentally abundant, mild reductantcan significantly affect the physicochemical properties of GO (examined by treating aqueous GO suspensions with Fe2+ at room temperature, with doses of 0.032mM Fe(2+)permg/L, 0.08mM Fe(2+)permg/L, and 0.32mM Fe(2+)permg/L GO). Microscopy data showed stacking of GO nanosheets on Fe2+ treatment. Spectroscopy evidence (X-ray diffraction, Fourier transform infrared transmission, Raman and X-ray photoelectron spectroscopy) showed significant changes in GO surface O-functionalities, in terms of loss of epoxy and carbonyl groups but increase of carboxyl group. The reduction mechanisms were verified by treating model organic molecules (styrene oxide, p-benzoquinone, and benzoic acid) resembling O-containing fragments of GO macromolecules with Fe2+. With sedimentation and adsorption experiments (using bisphenol A as a model contaminant), the authors demonstrated that Fe2+ reduced GOs still maintained relatively high colloidal stability, whereas their adsorption affinities were significantly enhanced. Thus, reduction of GO by mild reductants might be of greater environmental concerns than by stronger reducing agents (e.g., N2H4 and S2-), because the latter can result in too significant losses of surface O-functionalities and colloidal stability of GO. This interesting aspect should be given consideration in the risk assessment of GO. Environ Toxicol Chem 2015;34:1975-1982. (c) 2015 SETAC