In this study,transport behaviors of graphene oxide (GO) in saturated uncoated (i.e.,clean sand) and goethite-coated sand porous media were examined as a function of the phosphate.We found that phosphate enhanced the transport of GO over a wide range of solution chemistry (i.e.,pH 5.0-9.0 and the presence of 10 mmol/L Na+ or 0.5 mmol/L Ca2+).The results were mainly ascribed to the increase of electrostatic repulsion between nanoparticles and porous media.Meanwhile,deposition site competition induced by the retained phosphate was another important mechanism leading to promote GO transport.Interestingly,when the phosphate concentration increased from 0.1 to 1.0 mmol/L,the transportenhancement effect of phosphate in goethite-coated sand was to a much larger extent than that in clean sand.The observations were primarily related to the difference in the total mass of retained phosphate between the iron oxide-coated sand and clean sand columns,which resulted in different degrees of the electrostatic repulsion and competitive effect of phosphate.When the background solution contained 0.5 mmol/L Ca2+,phosphate could be bind to sand/goethite-coated sand surface by cation bridging;and consequently,promoted competition between phosphate and nanoparticles for deposition sites,which was an important mechanism for the enhanced effect of phosphate.Moreover,the DLVO theory was applicable to describe GO transport behaviors in porous media in the absence or presence of phosphate.Taken together,these findings highlight the important status and role of phosphate on the transport and fate of colloidal graphene oxide in the subsurface environment.