Gas responsive microgels as novel draw agents for forward osmosis desalination

Forward Osmosis (FO) process is a low-energy membrane separation technique, which has attracted increasing attention recently for desalination applications. Unlike Reverse Osmosis, which needs a high-pressure pump; FO works via natural osmotic pressure provided by a draw solution. Therefore, development of efficient draw solutions is quite important. Polymeric stimuli-responsive microgels/hydrogels are promising options as they can be recovered by applying the proper stimulus heating or gassing processes. The temperature-responsive microgels/hydrogels have been developed for FO application in recent years. This thesis study was aimed to the development of gas-responsive microgels as draw solutions for FO desalination. Two main series of microgels: CO₂-responsive and O₂-responsive microgels are for the first time fabricated and evaluated for FO desalination throughout the thesis. The feed saline water used here is 2000 ppm NaCl, which is considered as brackish water. A few of polymer monomers with tertiary amine moieties are selected for synthesizing CO₂-responsive microgels. Water flux of the microgels was measured by monitoring conductivity of the saline feed water and interpreting it to the water flux through the membrane. The microgels are active and protonated as a draw solution after CO₂ purging, and can be recovered after CO₂ stripping by N₂ purging. Microgels synthesised with diethylaminoethyl methacrylate (DEAEMA) can provide water flux as high as 56 LMH. Characterization tests are carried out to explore the most-effective microgels with respect to cationic monomers: DEAEMA and dimethylamino ethyl methacrylate (DMAEMA), and the type and concentration of crosslinkers: poly (ethylene glycol diacrylate) (PEGDA), N,N′-methylene-bisacrylamide (BIS) and ethylene glycol dimethacrylate (EGDMA). The microgels are recovered at their isoelectric point, where microgels are not charged and release water easily. O₂-responsive microgels are synthesised and their FO desalination performance is studied systematically. Two Fluoro-containing monomers (2,3,4,5,6 pentafluorostyrene (FS), 2,2,2-trifluoroethyl methacrylate (FM)), which are responsive to oxygen, are selected to copolymerize with four suitable ionic and non-ionic monomers: DEAEMA, Hydroxyethyl methacrylate (HEMA), DMAEMA and N-isopropylacrylamide (NIPAM). The results show that the water recovery ratio can be enhanced if a proper non-ionic monomer like NIPAM is used. The O₂-responsive microgels synthesised by DMAEMA and 5wt% FM monomer can perform the highest water flux up to 29 LMH. The experimental data reveal that HEMA is not a suitable non-ionic monomer to synthesise O₂-responsive microgels as HEMA has –OH groups, which lead to high negative surface charges and affect the water recovery. FO desalination data show that O₂-responsive microgels perform comparable water flux and water recovery capability. Dynamic light scattering (DLS) as the main characterization test for microgels is done. The microgels show larger hydrodynamic diameter after CO₂ or O₂ purging and they become smaller after removing these gases via N₂ purging. The swelling ratio for the microgels is up to 14 and 6.5 for CO₂ responsive and O₂-responsive microgels, respectively. As new polymer draw agents, CO₂- and O₂-responsive microgels demonstrate high water flux and water recovery capabilities as promising draw solutes for energy-effective FO desalination. CO₂-responsive DEAEMA microgels with 1wt% PEGDA crosslinker performed water flux of 56 LMH with 50 % water recovery ratio. DMAEMA CO₂-responsive microgels perform smaller water flux due to lower pKₐ of DMAEMA than DEAEMA. O₂-responsive microgels show relatively lower water flux than CO₂-responsive microgels. The best water flux performance is observed for DEAEMA/DMAEMA-5wt% FM microgels with 26-29 LMH, while the highest water recovery is given by NIPAM-5wt% FM microgels with 56%.Thesis (M.Phil.) (Research by Publication) -- University of Adelaide, School of Chemical Engineering, 2018

Gas transport properties of reverse-selective poly(ether-b-amide6)/[Emim][BF4] gel membranes for CO2/light gases separation

The present research investigates deeply the effect of 1-ethyl-3 methylimidazolium tetrafluoroborate ([Emim][BF]) ionic liquid on separation performance and transport properties of poly(ether-b-amide6) (Pebax1657) at different operating pressures from 2 to 20bar and temperatures from 25 to 65°C. [Emim][BF] showed interesting separation factor for CO/light gases as a solvent and it was expected that its addition to Pebax1657 leads to more amorphous structure, thereby increasing diffusion and permeability of gases. [Emim][BF] was added to the polymer solution up to 100wt% of Pebax1657 weight and permeation coefficients of CO, H, CH and N through the prepared membranes were measured. The results showed remarkable increment in permeation of all the tested gases, particularly CO and ideal selectivity of CO/H enhanced significantly due to high solubility selectivity of the added compound. Effect of operating conditions on solubility coefficients was also investigated; thus sorption isotherms and activation energies of permeability, solubility and diffusion were calculated. In addition, the membranes were characterized by SEM, DSC, FT-IR spectroscopy and Tensile analysis to inspect changes in their physical and thermal properties, precisely

Gas-responsive cationic microgels for forward osmosis desalination

Polymers as draw materials for forward osmosis (FO) desalination have attracted increasing attention in recent years, where the water adsorption and dewatering abilities of draw materials are crucial to overall desalination performance. Here, we developed gas-responsive microgels as new draw materials for FO desalination in which water adsorption and dewatering are driven by sweeping CO2 and N2. Cationic microgels of 2-(diethylamino) ethyl methacrylate (DEAEMA) and 2-(dimethylamino) ethyl methacrylate microgels (DMAEMA) were synthesised. The gas-responsivity of these microgels on water-flux and water recovery was systematically examined in a laboratory FO desalination system. CO2 is able to protonate DEAEMA microgels to enhance water adsorption. The DEAEMA microgels with 1 wt% polyethylene glycol diacrylate crosslinker show the highest water flux of 56 LMH. At the isoelectric points, the adsorbed water can be released by purging N2 gas at room temperature due to the hydrophobic characteristics of deprotonated DEAEMA microgels. Water recovery by 50% can be achieved for these microgels. Comparing with more hydrophilic DMAEMA microgels, the gas-responsive DEAEMA microgels perform higher swelling ratio, water-flux and recovery capability. Our results reveal that these gas-responsive microgels can be used as promising draw materials for future FO process with high water permeability and low-operation cost

Improvements in permeation and fouling resistance of PVC ultrafiltration membranes via addition of Tetronic-1107 and Triton X-100 as two non-ionic and hydrophilic surfactants

Two non-ionic and hydrophilic surfactant additives, Tetronic-1107 and Triton X-100, were added to poly(vinyl chloride)/NMP polymeric solution to prepare ultrafiltration membranes via immersion precipitation. Surfactants at three different weight percentages up to 6 wt% were added, and the fabricated membranes were characterized and their performance for water treatment in the presence of bovine serum albumin (BSA) as a foulant was assessed. The scanning electron microscopy images indicated remarkable changes in morphology due to higher thermodynamic instability after surfactant addition. The membranes are more porous with more macro-voids in the sub-layer. Plus, the membranes become more hydrophilic. Water flux increases for the modified membranes by nearly two times and the ability of membranes for flux recovery increases from 66% to over 83%. BSA rejection reduces slightly with the addition of surfactants, however this parameter is still almost over 90% for the membranes with the highest amount of surfactants

Microtubular electrodes: An emerging electrode configuration for electrocatalysis, bioelectrochemical and water treatment applications

Electrochemical processes have attracted much attention as they can be empowered by renewable electricity for zero-emission processes under ambient conditions. Applications of electrochemistry in various areas such as electrocatalysis (e.g., water electrolysis, CO2 reduction), (waste)water treatment, fuel cells, and microbial processes have been recently emerging. Electrode design is a crucial feature in electrochemical systems. In some applications, porous electrodes are required to maximize the reaction sites and participate in reactants delivery, such as gas-diffusion electrodes (GDEs) for gas-phase electrolysis or membrane electrodes for water treatment. Planar shape porous electrodes are the conventional configuration with some drawbacks; for example, planar GDEs are made in multiple layers and are relatively complex to manufacture on large scales. Recently, microtubular (or hollow fiber) shape electrodes have been introduced as an alternative due to several advantages such as a higher active surface area to the volume ratio, small electrolyzer footprint, ease of processability, etc. This review presents a critical overview of the design and fabrication of microtubular electrodes and the structure-performance relationship. After that, the recent advances of microtubular electrodes in three main categories, including gas-phase electrocatalysis, (waste)water treatment, and bioelectrochemical systems, are discussed, with more focus on gas electrolysis wherein microtubular electrodes act as GDEs. GDEs for gas electrocatalysis are of great significance as they effectively boost reaction rate by continuously delivering reactant feeds to the reaction sites, resolving the issue of mass transport resistance, and microtubular GDEs can address several issues of planar GDEs. In the last section, future research opportunities are suggested to showcase the promises of microtubular electrodes as a versatile electrode configuration for electrochemical applications

Geometric restriction of microporous supports on gas permeance efficiency of thin film composite membranes

Gas permeance of thin film composite (TFC) membranes may be restricted by the porosity and pore size of microporous supports, resulting in a reduction in permeance. This work extends a theoretical expression presented in the literature to describe the geometric restriction of the microporous supports on gas permeance efficiency in the TFC membranes. For this purpose, the permeance efficiency of penetrants through forty-eight different TFC membranes is systematically investigated. The experimental results are modelled using two equations developed by Wijmans and Hao, and Ramon, Wong and Hoek. The overall relative error for the values predicted by the Wijmans-Hao and Ramon-Wong-Hoek equations are 31% and 38%, respectively. The WijmansHao equation was developed based on computational fluid dynamics (CFD) studies. We demonstrate that a modification of the Wijmans-Hao equation by introducing the number of pores per unit surface area of the microporous support layer, Np, in the definition of the scaled thickness (S = l/rp, where l and rp are respectively selective layer thickness and mean pore size of support layer) improves its estimation precision. Therefore, the definition of scaled thickness is modified to the following equation: Sm = l/rp/sδ with δ = a(πNp)b where a and b are adjustable parameters. The deviation of the modified Wijmans-Hao equation from the experimental values reduces to 9.4%

Fouling reduction of emulsion polyvinylchloride ultrafiltration membranes blended by PEG: the effect of additive concentration and coagulation bath temperature

In the present work, ultrafiltration membranes were prepared using emulsion polyvinyl chloride (EPVC) with the addition of various concentrations of polyethylene glycol (PEG) to investigate the morphological structure and separation properties. The effects of polymer concentration, coagulation bath temperature (CBT), and PEG (6\ua0kDa) concentrations—a pore former hydrophilic additive—were studied. Through the phase inversion, the membranes—which were induced by immersion precipitation in a water coagulation bath—were fabricated through dissolving EPVC in N-methyl-pyrrolidinone, a polymer solvent. Morphological features of the membranes were characterized through scanning electron microscopy, pore size and porosity, and contact angle measurements. Water and bovine serum albumin (BSA) were used in order to study the separation and permeation performance of the fabricated membranes at 3\ua0bar, which is operating pressure. The results which were obtained from contact angle test indicated an increment in the membranes hydrophilicity with an increase in PEG concentrations, and then it decreased again. Increasing the CBT led to macrovoid formation in the membrane structure and the appreciation of both membrane permeability and BSA rejection. The addition of PEG resulted in a more porous structure and a higher water flux for those membranes, which were prepared with 13\ua0wt.% EPVC; while, for those which were fabricated with 15\ua0wt.% polymer, an opposite trend was observed

Improvement in flux and antifouling properties of PVC ultrafiltration membranes by incorporation of zinc oxide (ZnO) nanoparticles

In this study, modification of polyvinyl chloride (PVC) ultrafiltration membranes with zinc oxide (ZnO) nanoparticle addition was taken into consideration. The ZnO at five different weights was added to the polymeric solution, and the membranes were fabricated by the phase inversion method using water as a nonsolvent and PEG 6 kDa as a pore former additive. The results showed that the pure water flux of the modified membranes increased up to 3 wt% ZnO addition, which was the optimized amount of the nanoparticle addition in this study. Also, at 3 wt% ZnO addition, flux recovery ratio reached from 69% to above 90%, indicated that the nanocomposite membranes were less susceptible to be fouled. BSA rejection of the membranes also enhanced up to 97% by 3 wt% ZnO addition. The membranes were further characterized by SEM images and remarkable changes in their morphologies were observed, and they became more porous with higher interconnectivity between the pores. Furthermore, EDAX analysis was applied to study ZnO dispersion in the membrane structure and except for 4 wt% ZnO addition which particles aggregation was noticeable, ZnO was dispersed finely in the membrane structure. In addition, the modified membranes had higher hydrophilicity and lower contact angle that was effective to obtain higher water flux

Preparation and characterization of PVC/PAN blend ultrafiltration membranes: effect of PAN concentration and PEG with different molecular weight

The current study investigates the effect of polyacrylonitrile (PAN) addition on morphology and antifouling properties of poly(vinyl chloride) (PVC) asymmetric flat ultrafiltration (UF) membranes. The membranes are prepared via phase inversion method induced by immersion precipitation at different PVC/PAN blending ratio up to 40 wt% PAN. Also, membranes with blending ratio of PVC/ PAN:70/30, which showed the highest water flux and flux recovery ratio, were used for membrane preparation with 4 wt% of Polyethylene glycol (PEG) addition in four different molecular weight, 600 Da, 1,000 Da, 6,000 Da and 20,000 Da, which was used as pore former and hydrophilic polymeric additive. The performance of the membranes was studied by using pure water and bovine serum albumin (BSA) as feed at operating pressure of 3 bar. The cross-sections of the fabricated membranes were studied using SEM, and the images showed remarkable changes in morphology and structure of the prepared membranes after PAN and PEG addition. PAN addition led to increment in water flux up to 30 wt% and then decreased. The similar trend was observed in the case of flux recovery ratio. Also, viscosity of polymeric solution, contact angle and porosity of the membranes, antifouling and flux recovery of the membranes were studied

Enhancement in permeation and antifouling properties of PVC ultrafiltration membranes with addition of hydrophilic surfactant additives: Tween-20 and Tween-80

The effects of Tween-20 and Tween-80 as hydrophilic and non-ionic surfactants on morphology, mechanical and separation efficiency of PVC ultrafiltration membranes was studied widely. The membranes were fabricated via phase inversion method and different percentages of Tween-20 and Tween-80 from 1 wt% to 7 wt% was added to casting polymeric solution. Ultrafiltration of pure water and BSA, as the foulant, was done at the operating pressure of 2 bar. The results indicated great changes in morphology, and the membrane became more porous with macro-voids in the sub-layer, due to miscibility of surfactants with non-solvent and thermodynamic instability of the casting solution. The fabricated membranes have more hydrophilic surface and water contact angle has decreased from 70° for PVC membrane to 55° to 58° for the ones with 7 wt% Tweens. Pure water flux increased continuously with Tween addition because of higher surface hydrophilicity and more porosity. In addition, the membranes showed much better fouling resistance and were able to recover their water flux after three filtration cycles acceptably. PVC/7 wt% Tween-20 and PVC/7 wt% Tween-80 membranes showed 33% and 28% better flux recovery ratio compared to bare PVC membrane, respectively. Slight reduction in BSA rejection was observed, which was not very significant to affect the whole efficiency of the prepared membranes. The membranes have lower mechanical resistance due to more porous structure. Water flux and recovery ratio of the PVC/Tween-20 membranes were slightly higher than that of the PVC/Tween-80 membranes which can be due to more instability in thermodynamic of casting solution