Tailoring high performance pervaporation membranes for brine treatment

Abstract

A 95 million m3 of freshwater is produced using daily desalination, which results in 141.5 million m3 (roughly 1.5 times the permeate water) of brine production as a byproduct. This doctoral thesis is focused on presenting a sustainable solution using pervaporation by synthesizing high performance pervaporation membranes for brine treatment, considering the scalability and long-term operational stability. Pervaporation desaaes based on cutting edge 2D materials (sulfonated graphene oxide and MXenes) for brine treatment. Firstly, mixed matrix asymmetric membranes based on polyacrylonitrile (PAN) were synthesized using different doses of kaolin (2, 5, 7, and 10 wt%) via non-solvent induced phase separation and were hydrolyzed later with an alkaline solution. The synergistic effect of hydrophilic kaolin addition and alkaline hydrolysis improved the mechanical strength, while smaller wet zones because of partial hydrolysis resulted in reduced salt penetration during pervaporation without compromising permeation flux. A 2% kaolin dosage and 1 h hydrolysis with 2 M NaOH were selected as optimized conditions to synthesize hydrolyzed polyacrylonitrile (HPAN-2) membranes specifically for brine treatment. The HPAN-2 membranes managed to generate 58.7 Kg m-2 h-1 of permeation flux and 99.91% salt rejection while treating 10 wt% NaCl feed solution operating at 65 °C temperature. Moreover, long-term pervaporation results revealed that most of the salt was crystallized on the membrane surface because of the presence of a thinner wet zone caused by shorter hydrolysis time which guarantees long-term filtration ability without significant salt penetration through the membranes. In 2nd chapter, the optimized hydrolysis of polyacrylonitrile (PAN) used during the first task was utilized to synthesize ideal PAN support membranes to host a polyamide (PA) layer on top after interfacial polymerization (IP). A novelty in this task was the replacement of tap water coagulation bath directly with an alkaline solution which helped in providing coactive delayed phase inversion and alkaline hydrolysis at the same time. The membrane synthesized using co- and post-hydrolysis were compared to form a PA layer after IP and were named as PA-HPAN-Post and PA-HPAN-Co. Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and hydrophilicity tests showed that PA-HPAN-Co has significantly less thickness, better hydrophilicity, and stronger hydrolysis reaction as compared to PA-HPAN-Post. Due to these reasons, the PA-HPAN-Co managed to perform 74.2 Kg m-2 h-1 of flux (1.33 times higher than PA-HPAN-Post) and salt rejection of 99.97% (7 times less permeate conductivity than HPAN-Post) with 10 wt% NaCl feed at 70 °C. The third task of this thesis was a synthesis of sulfonated graphene oxide (SGO) membrane on mixed cellulose ester (MCE) microfiltration support for brine desalination by pervaporation. The SGO-based thin film composite (TFN) membrane was blended with polyvinyl alcohol as an intercalant and, later on, crosslinked with the help of glutaraldehyde (GA). The final SGO-PVA-GA-based TFN membrane showed excellent coating layer stability, which managed to survive strong sonication tests. Also, due to uneven interlayer spacing of SGO, the TFN membrane formed a tortuous brick (SGO) and mortar (PVA) structure which managed to offer absolute salt rejection 99.99% with permeation flux of 62 Kg m-2 h-1 at 65 °C feed temperature and 10 wt% NaCl concentration. Moreover, the presence of sulfonic acid functional groups enhanced the hydrophilicity of the membrane and negative surface charge density, which led to added resistance against silica scaling and organic fouling as well. Lastly, the MXene-based thin film composite (TFC) membrane was synthesized using a vacuum filtration technique. The complex and long crosslinking process used in task 3 was replaced with additive-free self-crosslinking of MXene membrane using inherent hydroxyl functional groups by thermal annealing at 80, 140, and 180 °C in a vacuum oven. A 140 °C (M140) was found to be an optimized crosslinking temperature which showed a decent decrease in d-spacing and exhibited reduced membrane swelling due to the formation of Ti-O-Ti bond as a result of self-crosslinking reaction without a significant drop in permeation flux. Moreover, the M-140 membrane managed to generate a permeation flux of 70 Kg m-2 h-1 with 99.9% salt rejection using 10wt% NaCl feed at 70 °C. A continuous 48 h pervaporation test was performed with simulated brine, and no NaCl was found on the permeate side, which was examined through SEM along with no significant decline in permeation flux. Overall, the problems associated with low flux pervaporation membranes were pragmatically solved by developing selectively tailored membranes for the required purpose. The commonly observed drawbacks like salt penetration through the membrane, silica scaling, organic fouling, membrane swelling, and lower hydrophilicity were improved through proposed membrane synthesis and functionalization of 2D materials and polymers used for synthesis.