Shahrooz, Mahdi (2024) Stimuli-Responsive Membranes for Membrane Distillation of Oily Waters. PhD thesis, Victoria University.

Abstract

Membrane distillation (MD) is a thermally-driven process for water/wastewater treatment which can produce high-quality distillates from highly-saline solutions. However, the presence of different types of oils in the wastewaters can pose serious challenges in their treatment using conventional hydrophobic membranes. These oils are abundant in emulsified or unemusified forms in the wastewaters encountered in a wide variety of industries including municipal wastewater, food, textile, and oil and gas industries. A key challenge in the treatment oily wastewaters using MD is the tendency of the oil droplets to adhere to the membranes leading to pore blockage (fouling) and liquid intrusion (wetting), thus compromising the membrane performance. Janus membranes are among the many new membranes that have been developed previously, but these membranes face practical limitations due to the weak adhesion between membrane and modification layers. One of the approaches to address this problem is the application of surface grafting methods to form polymer brushes on the membranes but has only been considered in MD to a limited extent. Of the surface grafted brushes, polyelectrolytes brushes offer salinity-responsive anti-oil-fouling membrane properties for practical applications of MD. Grafting of polymer brushes is usually performed using atom-transfer radical polymerization (ATRP) that requires an oxygen-free reaction medium. This further limits the applicability of this method for membranes due to the need for special equipment and tedious de-oxygenation procedure. Therefore, the main aim of the current thesis is to address the oil adhesion and wetting in MD by preparing a salinity-responsive membrane using polyelectrolyte brushes grafted on the surface of a commercial hydrophobic membrane. Further, a new approach to address the oxygen-free ATRP challenge was proposed using UV-assisted oxygen tolerant ATRP performed under open atmosphere for grafting a negatively charged polyelectrolyte (poly(acrylic acid)) (PAA) onto commercial hydrophobic PVDF membranes (mean pore size = 0.2 ). The success of the grafting reaction was confirmed by attenuated total-reflection Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) analyses. Surface morphology of the PVDF-PAA membrane was found to be unaffected by the PAA chains and the membrane surface became more hydrophilic as determined by water contact angle. Salinity response of the prepared membranes was evaluated using underwater oil adhesion tests in aqueous solutions with different salinities (from Milli-Q water to 3 M NaCl). It was found that the PVDF-PAA membrane became underwater superoloephobic when NaCl concentration was above 0.01 M inhibiting the oil drop adhesion; while lower salinities allowed surface adhesion of oil. In contrast, the pristine PVDF membrane was wetted by the oil droplet regardless of the salinity of the test medium. New models were proposed in an attempt to explain the surface structure of the PVDF-PAA membrane and its underwater oleophobicity. Analyses of the proposed models showed that the underwater oleophobicity of the PVDF-PAA membrane can be attributed to the combined effect of the PAA patches on the top of the surface roughness features as well as the air-water interfaces located on the roughness features and pore entrances. MD testing in “direct contact” (DCMD) mode over 20 hours using a 0.1 % (v/v) dodecane dispersion containing 0.1 M NaCl showed a high oil adhesion resistance (anti-fouling) for the PVDF-PAA membrane indicated by its stable permeate flux (~17 kg/(m2.h)) and no increase in permeate conductivity. Meanwhile, the unmodified PVDF membrane rapidly lost flux, dropping by 75% within the first five hours of the test. This test gave the first two key conclusions and scientific findings of this study, the first confirming salinity responsive polyelectrolyte brushes are effective at resisting oil adhesion on MD membranes, and the second confirming the effectiveness of the newly proposed scalable oxygen tolerant ATRP as a method to graft these brushes to readily available membrane substrates. Another part of the study aimed to explore the mechanisms of oil adhesion and resistance by the PAA brushes. The oil adhesion and wetting was also monitored for both PVDF and PVDF-PAA membranes during MD by time-resolved in-situ electrochemical impedance spectroscopy (EIS), which has offered great capability for understanding fouling in reverse osmosis, but limited studies so far in MD. This study uniquely adopted a wide frequency sweep between 1 Hz and 500 kHz. Our results showed, in contrast to prior EIS studies in MD, the decrease in the impedance at high frequencies might not be a direct indication of wetting, which was consistently observed for both PVDF and PVDF-PAA membranes. However, only the pristine membrane showed evidence of wetting as indicated by the traditional measurements of permeate electrical conductivity (EC) which underwent a sudden rise after 6 hours. The novel approach and analysis led to the third major scientific contribution of this study, providing a holistic analysis of the EIS results showing the largest changes in the impedance for both membranes occurred at low frequencies, and these changes were more significant for the PVDF membrane that was wetted during the process. Equivalent circuit analysis including the resistance and capacitance elements represented the membrane, and the component values changed during the first 6 hours of the MD process for the PVDF membrane until wetting occurred, but remained almost constant for the PVDF-PAA membrane that did not wet. In summary, this thesis found the innovative scalable oxygen tolerant UV-ATRP method is a practical surface modification method for functional membranes. However, further investigation is needed to address its remaining limitations such as controllability and facile initiator grafting to fully realise its potential for growing different types of polymer brushes on various membranes. In addition, the novel models proposed for the surface structure of the PVDF-PAA membranes proposed in the current study can be employed for a rational design of Janus membranes with advanced functionalities that are resistant to the adhesion of both hydrophilic and hydrophobic compounds. Finally, the results of the in-situ EIS analyses in the current study has created a better understanding of MD wetting phenomena, and can be extended to in-situ studies of other membrane systems.

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