Fabrication of Composite Membranes for Filtration Applications

Abstract

Water is very essential for survival of all living things and is also critical in every aspect of economy. As the drinkable water resources are limited, removal of major contaminants from industrial effluents prior to disposal into water bodies has been enforced to reduce the pollution at source. Also, reuse of treated water in process industries helps in solving the water crisis. With the rapid industrialization and economic growth, the generation and discharge of wastewater from industrial/municipal sources also increased at the same pace, posing a serious concern on the environment. Among the different techniques for wastewater treatment, membrane separation is widely adopted because of its efficiency and reliability for continuous processing. Current membrane separation approaches emphasize the use of reverse osmosis, which is prohibitively expensive relative to microfiltration and ultrafiltration because of its high operating cost. Microfiltration and ultrafiltration membranes are composed of either expensive ceramic or low-durability polymeric materials. In the present work, low-cost ceramic membranes were prepared by using kaolin (KA), fly ash (FA) and dolomite. The effects of sintering temperature and raw materials composition (i.e., 0–100 wt% of KA in the mixture of KA + FA) on the membrane properties (strength, porosity, pore size, permeability, etc.) were studied. The raw material mixtures were subjected to TGA and the prepared membranes were characterized by SEM, XRD, liquid permeation, mechanical strength and chemical stability tests. The SEM analysis evidenced that the membranes were free of defects and had homogeneous surface structure with evenly distributed pores of similar sizes. A sintering temperature of 900 °C was found to be optimum for both kaolin-based and fly ash based membranes. Addition of 20% dolomite provided sufficient porosity to all membranes (28–51%). The porosity, strength and stability increased, while the pore diameter decreased with an increase in the kaolin content (M0–M100). The sintered mixture with a kaolin content of 75% (M75) was identified as the best membrane among others based on the pore characteristics (mean pore diameter 0.62 μm and porosity 46.3%). When applied to oil-water separation, M75 showed excellent filtration performance with good separation efficiency (up to 97.4% for 100 mg/L oil and up to 98.8% for 200 mg/L oil). The average permeate quality obtained at 1.03 bar pressure difference was found to be within the safe discharge limit (<10 mg/L). Although, the permeate flux declined steadily with time, it has been recovered through membrane regeneration. Model fitting to the flux data indicated the formation of a cake layer because of concentration polarization at the membrane surface during dead-end filtration. Then M75 ceramic membrane was coated with cellulose acetate (CA) using different polymer concentrations and dipping durations. The porosity of the membrane decreased considerably (from 58.4 % to 12.1 %) with an increase in polymer concentration from 2.5 to 10 wt. %. The values of the porosity increased and layer thickness decreased slightly with a decrease in dipping time (from 60 s to 15 s). The membrane prepared using 5 wt. % CA in acetone and 45 s dipping duration (T-3) had a polymer separation layer of thickness 14.2 µm and was found to be optimal with 51.8 % porosity, 35 nm pore diameter and a liquid (water) permeability of 7.39 × 10–10 m3/m2·s·Pa. This membrane was applied for micellar enhanced ultrafiltration of three heavy metal ions, namely, Cu(II), Cr(VI) and Ni(II). The prepared membrane (T-3) was highly effective in ultrafiltration applications as evidenced from complete rejection (≈ 100 %) of cetylpyridinium chloride (CPC) along with satisfactory removal (>99.7 %) of metal ions.