Granite dust derived silica embedded molybdenum disulfide-polyethersulfone mixed matrix membranes for oil-water separation / Nurina Adriana Abdul Razak

Granite dust contains very fine particles that can cause respiratory health issues and pollute air and water sources. The amount of granite dust (GD) waste generated is in millions of tonnes annually. This amount is increasing rapidly, leading to numerous disposals, governance, and environmental issues. Therefore, finding a feasible application of GD-derived materials could significantly resolve the waste problem. In this work, a simple, low-cost sol-gel method was used to synthesise silica nanoparticles from GD. In order to improve the sol-gel process efficiency, an acid leaching pre treatment process was added prior to the extraction of silica in the form of sodium silicate and the formation of silica gel through neutralisation processes. Various types of acids such as citric acid, acetic acid and hydrochloric acid were used to remove metallic impurities in the GD. Besides acid types, the acid concentrations (0.5 M, 1 M, 2 M, and 3 M) and reaction temperatures (25 °C, 60 °C, 80 °C, and 90 °C) were also varied to investigate their role during the acid leaching pre-treatment. The results revealed that the use of acid leaching pre-treatment can effectively eliminate most contaminants such as Al2O3, K2O, Fe2O3 and Ag2O in the GD. By using 1 M citric acid at 80°C, the purity of GD was significantly improved, and it was used as a precursor to synthesis silica. It was observed that if the acid concentration was less than 1 M, the impurities could not be completely removed as the acid strength was insufficient to break the metal oxide bonds in the GD. In addition, if the pre-treatment temperature was too high, the acid might be vaporise. The XRD pattern of the silica confirms the amorphous nature with BET specific surface area and particle size of 90.39 m2 /g and 228.4 nm, respectively. The silica derived from GD was then hydrothermally synthesised with molybdenum disulfide (MoS2) and used as a filler in the fabrication of polyether sulfone mixed matrix membranes (PES MMMs) for oil-water separation. The amount of MoS2-silica added used was varied from 0.25 wt% to 2.0 wt%, and their effects on mechanical, chemical, and surface morphology of the PES MMMs were evaluated. The addition of silica into MoS2 leads to significant enhancement in porosity, hydrophilicity and water flux as compared to bare PES and MoS2-PES membranes. The performances of MoS2-silica PES membranes were then investigated for oil-water separation using different oil concentrations (500 ppm, 1000 ppm, and 1500 ppm) at 1 bar and room temperature. The membrane was first compacted for 2 hours at 2 bar before the oil-water separation test to maintain a stable flux. It was found that the oil water flux of the composite membranes increased significantly from 5.85 LMH to 75.79 LMH when silica was added. This was due to the enhancement in membrane properties and morphology structure. The highest permeation flux of 75.79 LMH was obtained using 2M-S-PES. Generally, the oil rejection of the MoS2-silica PES MMMS fabricated in this work was maintained above 95%. This study indicates that the environmentally acid leaching pre-treatment used in this work can improve the quality and yield of silica produced. It also confirms the potential of using GD-derived silica as one of filler for MMMs fabrication.