Effects of natural minerals on the adsorption of 17β-estradiol and bisphenol A on graphene oxide and reduced graphene oxide

Extensive applications of graphene oxide (GO) and reduced GO (rGO) have led to their release into aquatic environments, where a wide variety of pollutants and natural minerals co-exist. In this study, we have investigated the effects of montmorillonite, kaolin, and goethite on the adsorption of 17 beta-estradiol (E2) and bisphenol A (BPA) on GO and rGO. Results have shown that these minerals inhibited the adsorption of E2 and BPA on GO and rGO in the order goethite > kaolin > montmorillonite. The largest inhibitory effect of goethite was ascribed to its strong electrostatic attraction with oppositely charged GO and rGO. In comparison to kaolin, the adsorption of E2 and BPA on montmorillonite had a weaker inhibitory effect on their adsorption on GO and rGO. The minerals showed greater inhibitory effects on the adsorption of E2 and BPA on GO than on rGO because of differences in the geometrical structures and properties of these nanomaterials. As confirmed by TEM, acid-base titration, FTIR, and XRD, GO sheets interacted more strongly with the minerals through wrapping their particles, cross-linking them through electrostatic attraction and hydrogen bonding, and/or intercalating into the interlayers of montmorillonite. These face-to-face or face-to-edge interactions occupied more adsorption sites on the surface of GO and thus resulted in a greater inhibitory effect than the spherical particle-particle interactions between rGO and minerals. To the best of our knowledge, this is the first study on the effects of natural minerals on the adsorption of pollutants on GO or rGO, which is important to assess the fate and potential risks of the released nanomaterials and co-existing pollutants in aquatic environments.