Polyamide thin film nanocomposite membrane incorporated with carbon nitride for forward osmosis desalination

Abstract

Forward osmosis (FO) is an emerging desalination process. It has been extensively studied to enhance the production of fresh water owing to its lower energy consumption and fouling tendency compared to the conventionally used reverse osmosis (RO). The design of a desired membrane structure has been recognized as one of the most crucial factors to counter some drawbacks of FO processes, such as internal concentration polarization effect and reverse passage of the draw solute. Hence, the main objective of this study is to develop a thin film nanocomposite (TFN) FO membrane for desalination application. The polyamide (PA) TFN FO membranes incorporated with protonated and unprotonated carbon nitride (CN) were prepared through interfacial polymerization of m-phenylenediamine and trimesoyl chloride. CN was synthesized through a thermal condensation method using melamine as the precursor. The protonated carbon nitride (pCN) was obtained by treating the as-synthesized CN with inorganic acid. pCN morphology observed less agglomeration nanosheet compared to CN and the size was shown to be approximately 28.95 nm based on the transmission electron microscopy images. Besides that, the acid treatment towards CN had changed the surface charge from -34.6 to 8.3 mV due to positive charged hydrogen absorption on the CN structure. Also, pCN peak on x-ray diffraction analysis pattern representing planar graphitic interlayer was shifted from 28° to 27.2° that makes the distance to become 0.325 from 0.318 nm. Meanwhile, on attenuated total reflectance Fourier transform infrared spectra, broader peak was observed on N-H stretching of CN instead of pCN. Performance evaluation of the TFN membrane was conducted in RO and FO modes. In RO mode, the water permeability and salt rejection were determined, while in FO system, the structural parameter and the reverse salt flux were determined in both active layers facing feed solution (AL-FS) and active layer facing draw solution (AL-DS). With the addition of pCN within the substrate, the pore and leaf-like structure became larger, as observed in the field emission scanning electron microscopy cross-sectional images. The presence of pCN had also increased the average surface roughness of the substrate. The formation of PA through IP was performed with 0.05, 0.1, and 0.15 w/v% loadings of pCN and CN dispersed in TMC monomer solution. Based on atomic force microscope images, the increasing loading of pCN and CN within the PA layer increased the surface roughness of the resultant TFN membrane as compared to that of TFC membrane. The decrease in water contact angle observed through goniometry analysis suggested the increase in the surface hydrophilicity of the TFN membrane. Other than that, the membrane surface charge was also changed. TFC membrane showed high negativity of -47.3 mV. However, the presence of pCN decreased the surface negativity to -5.76 mV and with the increasing loadings of CN, the negativity was further reduced to -10.2 mV compared to TFC membrane. The effect of the loading of nanomaterials in the range of 0.05 to 0.15 % on the performance of the membranes was also studied. Among the membranes prepared, 0.05 CN-pCN-TFN membranes which contained 0.05 w/v% CN in PA layer and 0.5 w/v% pCN within the support membrane was identified as the best performing membrane. The water flux achieved was 6.20 and 9.23 Lm-2h-1 in AL-FS and AL-DS mode, respectively. The reverse salt flux was recorded as 0.08 and 0.03 gm-2h-1 for AL-FS mode and AL-DS mode, respectively. With this optimal membrane, fouling behaviour was studied and compared with TFC membrane by using sodium alginate and bovine serum albumin (BSA) as model foulants. 0.05 CN-pCN-TFN membrane outperformed the TFC membrane in both tests with water flux reduced to 96 % after 9 h operation compared to TFC membrane which had reduced to 91.5 % for sodium alginate test and maintained at 100 % of water flux after 9 h operation for BSA compared to 97.5 % water flux for TFC membrane. This work evidenced the potential of using both CN and pCN in the design and fabrication of TFN to simultaneously achieve improved water flux, salt rejection and antifouling properties.