Hypothesis: Carbxylated graghene oxide nanosheets were synthesized and the nanosheets were applied to the surface modification of the polyethersulfonebased nanofiltration membranes for water treatment.Methods: Different concentrations of the synthesized carboxylated graphene oxide nanosheets were used as the surface modifiers to prepare the PES/PEI c-GO nanofiltration membranes. The prepared membranes were analyzed by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), 3D surface images (AFM) and X-ray diffraction (EDX) spectroscopy. The permeability and the separation performance of the constructed membranes were evalueted by the water contact angle, water content, the flow of pure water and the rejection of sodium sulfate salt and heavy metals.Findings: the FTIR analysis showed the formation of favorable bonds in the synthesized carboxylated graphene oxide nanosheets and the fabricated membranes. The membrane surface modification by c-GO nanosheets led to a decrease in membrane roughness and the contact angle decreased from 75° for the neat membrane to 36° for M1 at 0.001 g of carboxylated graphene oxide nanosheets. Moreover, the water content increased and M2 showed the highest water content. The highest pure water flux was obtained at 13.065 L/m2.h for the constructed M2 membrane containing 0.01 g of carboxylated graphene oxide nanosheets. In addition, the highest rejection of sodium sulfate salt (Na2SO4) was observed 67.5 % for the M3 membrane containing 0.1 g of c-GO nanosheets and the highest rejection of copper nitrate (Cu(NO3)2) was obtained 87.21% for the M1 membrane containing 0.001 g of c-GO nanosheets. Furthermore the obtained results indicated the improvement of the anti-fouling properties of the modified membranes containing carboxylated graphene oxide nanosheets compared to the base membrane
