Abstract
Abstract : A series of ultrafiltration (UF) membranes based on the hydrophobic polyvinylidene fluoride (PVDF) and hydrophilic polyethersulfone (PES) containing new nanofillers based on graphene oxide (GO) were fabricated and their performance assessed against both organic dye and metal ion rejection. The incorporated nanofillers were GO nanosheets coated with hyperbranched polyethyleneimine (HPEI, HPEI@GO) in the first instance and its composite with silver nanoparticles (AgNPs, AgNPs/HPEI@GO). Different weight concentrations (wt%) of these modifiers were incorporated into PES or PVDF polymers matrix followed by fabrication of composite membranes through phase inversion method. The prepared materials, i.e. GO nanosheets, the various GO composite and the fabricated membranes were characterised using various physicochemical techniques such as Fourier Transform Infrared (FTIR), Transmission Electron Microscope (TEM), Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS) Thermal Gravimetric Analysis (TGA), Braunuer Emmet Teller (BET), X-ray diffraction (XRD), Raman, Atomic Force Microscopy (AFM), water uptake, contact angle (CA) as well as membranes performance through pure water flux, flux recovery and rejection capacities. Raman spectroscopy was primarily used to confirm the nature of GO while FTIR was utilised to confirm the presence of the HPEI polymer deposited on GO nanosheets. For instance, the coating of the HPEI on the surface of GO nanosheets via grafting of HPEI on the surface of GO nanosheets was confirmed by the presence of the CN groups and cationic NH groups on the vibration pattern of HPEI@GO composite. On the other hand, the SEM-EDs analyses revealed the elemental composition of the synthesised GO nanosheets and GO composite to be Carbon, Oxygen, Nitrogen, and Silver affirming the FTIR results. The pure water flux of the unmodified PES membranes was improved from 35±0.52 L/m2h to the range of 77±0.92 to120±1.44 L/m2h (which is 114% to 243%) for the series of GO@PES membranes compared to the baseline membranes. Thus, in the series of HPEI/GO@PES containing membranes, the flux improved to the range of 55±0.82 to vi 89±1.33 L/m2h (which is 57% to 171% increase) and the series of AgNPs/HPEI/GO@PES membranes were improved to the range of 40±0.60 to 50±0.75 L/m2h (which is 14% to 66% increase). This observed improve could be attributed to the incorporated charge and oxygen functionalities. The Flux recovery ratio (FRR) of the synthesised membranes were also improved from 45% for the unmodified PES membranes to the range of 80% to 93% for the series of GO@PES membranes, 74% to 78% for the series of HPEI/GO@PES and 74% to 75% for the series of AgNPs/HPEI/GO@PES membranes due to less attachment of the bovine serum albumin (BSA) solution on the membranes surface and pores. The fouling propensity of PES membranes was further evaluated using organic pollutants such as MO, MB, AR, CR, and BPA after a prolong filtration and washing. The FRR of the unmodified PES membranes was 42%. This was improved to the range of 80% to 92% for the series of GO@PES membranes, 76% to 78% for the series of HPEI/GO@PES membranes and 71% to 73% for the series of AgNPs/HPEI/GO@PES membranes due to less attachment of dye molecules on the membranes surface and pores, indicating that the modified PES membranes had improved antifouling properties. The cleaned membranes were further subjected to SEM-EDS surface morphology and elemental composition analysis to ascertain the degree of foulants attachment on the membrane’s surfaces i.e. a post-mortem of the membranes was undertaken. This confirmed that the unmodified PES membranes were more fouled. The observed improvement in the antifouling properties of the modified membranes could be attributed to the presence of hydrophilic functionalities from the incorporated nanofillers. The series of the positively charged HPEI/GO@PES membranes at pH 6.8 and 8.0 showed a progressive increase in rejection capacity with increase in weight concentration of the GO composite in the PES matrix. At pH 6.8, the rejection capacity was enhanced from 90% to 95% for Pb2+, 77% to 85% of Cr6+ and 69% to 81% of Cd2+ which were higher compared to the rejection capacity of the unmodified PES membranes. Similar trend in rejection pattern was observed at pH 8.0 for the same series of membranes. For the series of GO@PES, and AgNPs/HPEI/GO@PES membranes, the ability of these membranes to reject these metal ions at pH 6.0, 6.8, vii and 8.0 declines as the weight concentration of the GO nanosheets and GO composite increased in the PES matrix. This could be due to increased membranes micropores and macro-voids (SEM images). The positively charged PES membranes displayed high removal capacity of above 90% for small positively charged methylene blue dye and less rejection capacity of 73% and less for a bigger negatively charged amaranth dye. The difference in the rejection capacity of the synthesised membranes could be attributed to the chemical nature of the membranes surface in contact with the pollutants, molecular weight cut off of the membranes films as well as the surface charge of the pollutants to be rejected. As a result to which the positively charged membranes had high removal efficiency for the methylene blue dye. A similarly improve in rejection pattern of pollutants were also observed for the graphene-based nanomaterials modified PVDF membranes. The observed significant improvement in the modified membranes can be attributed to the presence of oxygenated and charged functionalities on the surfaces of the synthesised graphenebased nanofillers compared to unmodified membranes...
M.Sc. (Chemistry)