Development and assessment of the performance of a light-sensitive and zwitterionic membrane for water reclamation

Abstract

Water molecules are an optimal solvent for carbon-based life forms due to their hydrogen bonding, high heat capacity, surface tension, and dielectric constant, which are essential characteristics of life. Diverse marine ecosystems are sustained by the preponderance of the Earth's water, which is approximately 1.386 billion cubic kilometers of salt water.<br /> <br /> Water contamination is a substantial public health concern on a global scale, necessitating water consumers to improve their own water management or remediation. In order to prevent the diffusion of contaminants from a variety of sources for consumption, treatment water mechanisms are indispensable. Filtration is a multi-barrier operational mechanism responsible for removing residual material, such as bacteria, viruses, and other dissolved metals. Granular filtration improves the efficacy of disinfection by removing suspended solids and particulate matter from water. In this research project, we concentrated on the design and fabrication of a multifunctional responsive membrane that would be responsive to light exposure and capable of eliminating organic pollutants from water. This was achieved by modifying a commercial polymer to add hydrophilic properties, incorporating the zwitterionic group diethylamine N-oxide, and incorporating the semiconductor tungsten disulfide in a nanosheet morphology.<br /> <br /> The modification of membranes and their precursors has emerged as the preferred alternative in water treatment using membrane technology as the search for innovative materials with distinctive properties to assist in the remediation of polluted water effluents becomes more urgent. In <strong>Chapter 3</strong>, we initiated an investigation into the feasibility of modifying the molecular structure of polysulfone (PSF) to incorporate the zwitterionic group diethyl amine N-oxide (DEAO) and tungsten disulfide (WS₂) in nanosheet form as a photo-responsive material for water purification applications. The primary characteristics of the membranes, including selectivity, pollutant rejection, and water flux, can be enhanced by these materials.<br /> <br /> The molecular structure of the PSF backbone was modified to incorporate the DEAO group, and the resulting polymer (PSF-DEAO) was characterized using FT-IR and 1H-NMR. When in nanosheet form, WS₂ is a semiconductor that can interact with UV/Vis light, generate reactive oxygen species (ROS), and degrade organic pollutants in the presence of oxygen. The WS₂ nanosheets were characterized using UV/Vis, FT-IR, Raman spectrometry, X-ray diffraction (XRD) diffractometry, and transmission electron microscopy (TEM) after being obtained through the liquid-exfoliation method. Membranes with high water fluxes, high selectivity to organic pollutants, and low reverse salt fluxes have the potential to be produced by these materials. This could potentially prevent bacterial growth and further degrade contaminants.<br /> <br /> The fabrication and performance of the Janus membrane in water flux, reverse solute flux, and photocatalytic degradation of contaminants were examined in <strong>Chapter 4</strong> following the addition of the DEAO group and exfoliation of the WS₂. This type of membrane is capable of converting into a multifunctional membrane for the photodegradation of organic contaminants, as it possesses two functionalities, one for each face. The membranes that were prepared underwent a comprehensive characterization process that included scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), surface Z-potential, and water contact angle (CA).<br /> <br /> <strong>Chapter 5</strong> provides insights regarding the development of a bioinspired membrane enhanced with WS₂ nanosheets for operation in forward osmosis mode, thereby integrating the benefits of polymeric and semiconducting materials in the repulsion of model pollutants and avoiding the formation of biofouling, which is becoming increasingly essential. This membrane was synthesized by modifying the polysulfone (PSF) polymer backbone with the zwitterionic group diethyl amine N-oxide (DEAO) to increase the hydrophilic properties of the final material. Subsequently, tungsten disulfide (WS₂) nanosheets were embedded into the polymeric solution to integrate its photosensitive characteristics into the resulting membrane. The resulting membrane was able to maintain a good water flux, degrade organic pollutants tested, and inhibit the formation of biofilms on its surface.<br /> <br /> The results indicate that the photodegradation of the RhB is greater than 92% and 98% after 8 and 12 hours, respectively. The degradation performances of these membranes are nearly 20% greater than those of the control membranes. The WS₂ nanosheets are likely to generate reactive oxygen species (ROS), which expedites the degradation process. These results are encouraging, as this method has the potential to simplify the production of membranes with dual functionalities.<br /> <br /> <strong>Chapter 6</strong> compares the removal efficacy of the Janus and PSF-DEAO@WS₂ membranes in removing RhB and their ability to prevent the formation of biofilms on the membrane surface. In comparison to the Janus membrane, the PSF-DEAO@WS₂ membrane demonstrated exceptional water flux (12.4 L/m²h), dye rejection (99.4%), and dye removal (61.5%) under light conditions. The formation of biofilms was prevented by the ability of both Janus and PSF-DEAO@WS₂ membranes to impede the growth of bacteria on their surfaces. These findings indicate that the membrane's performance is enhanced when the nanosheets are embedded over the narrow layer of the nanosheets on the active side of the membrane, thereby enhancing the essential features required for forward osmosis systems.<br /> <br /> The selectivity, responsiveness to external stimuli, high permeability, and versatility of multifunctional and responsive membranes for water remediation applications have garnered attention. This study highlighted the benefits of modifying the backbone of a commercial polymer to enhance its hydrophilic properties, as well as the antifouling properties and the ability to degrade organic pollutants, which are ascribed to the addition of photo-responsive components. Additionally, we reported that the separation process was improved through the interaction with light by the use of a highly responsive membrane. Consequently, the FO separation process's feasibility for wastewater reclamation was confirmed.