Graphene oxide (GO) holds great promise for a broad array of applications in many fields,but also poses serious potential risks to human health and the environment.In this study,the adsorptive properties of GO toward Ca2+ and Na+ were investigated using batch adsorption experents,zeta potential measurements,and spectroscopic analysis.When pH increased from 4 to 9,Ca2+ adsorption by GO and the zeta potential of GO increased significantly.Raman spectra suggest that Ca2+ was strongly adsorbed on the GO via-COOCa+ formation.On the other hand,Na+ was adsorbed into the electrical diffuse layer as an inert counterion to increase the diffuse layer zeta potential.While the GO suspension became unstable with increasing pH from 4 to 10 in the presence of Ca2+,it was more stable at higher pH in the NaCl solution.The findings of this research provide insights in the adsorption of Ca2+ on GO and fundamental basis for prediction of its effect on the colloidal stability of GO in the environment.