Evaluation of graphene oxide-coated ultrafiltration membranes for humic acid removal at different pH and conductivity conditions

Graphene oxides (GO) were coated on three different commercially available polyethersulfone (PES) ultrafiltration membranes (nominal molecular weight cutoffs; 5, 10, and 30 kDa) via a simple vacuum filtration process in our laboratory. The physicochemical properties of resultant membranes were analyzed by Fourier-transform infrared spectroscopy, Raman spectroscopy, field-emission scanning electron microscopy, confocal laser scanning microscopy, and water contact angle measurements. The GO-coated membranes appeared to be more hydrophilic, resulting in approximately 20% higher pure water flux than pristine PES membranes. It was also found that GO nanosheets were not easily damaged or detached from the PES membrane support by filtration or water rinsing due to strong hydrogen bonding interactions between the sulfone and carboxylic groups. Moreover, the humic acid rejection of GO-coated membranes (85.3–93.9%) was significantly higher by approximately 3.4 times depending on various pH and conductivity conditions than that of the pristine PES membranes (25.2–34.8%). Along with the increase in pH and conductivity, HA rejection was slightly increased for both pristine PES and GO-coated membranes: nevertheless, GO-coated membranes (5.3%) were less affected by high pH/conductivity conditions for the humic acid rejection than the pristine PES membranes (19.0%). In particular, it was relatively easier to remove foulant from the GO-coated membrane surface by water rinsing than from the pristine membranes, resulting in high membrane recovery and antifouling capabilities at various pH and conductivity conditions.