Membrane distillation (MD) is a promising technology to treat high salinity wastewater, but the high operational cost, lack of comprehensive understanding in its heat and mass transfer, and limited ability in treating various wastewater (e.g., low surface tension wastewater, volatile contaminants rich wastewater, etc.) have impeded its translation from bench scale to field scale. Nanophotonic-Enabled Solar Membrane Distillation (NESMD) utilizes free sunlight to produce localized heat on the membrane surface, which can make MD economically viable. However, the current membranes for NESMD exhibit either an increased mass transfer resistance to water vapor or a decreased photothermal properties after immobilized on the substrate membrane surface. Plus, unlike conventional MD whose performance dependency on the operational and environmental conditions are well understood, the operational and environmental effects on NESMD have not been fully understood. In addition, none of the existing NESMD membrane is anti-wetting, which limits its application only in dealing high surface tension wastewaters. Lastly, all current MD and NESMD membranes suffer from the problem of poor rejection against volatile contaminants (VC). In this dissertation, core-shell structure hydrophilic photothermal nanofiber is used to solve the tradeoff between membrane permeability and solar absorptivity. With good water stability, high solar absorbance, fast heating and heat dissipation abilities, and no additional vapor mass transfer resistance, it can be used as a coating material to convert a commercial membrane to a photothermal active membrane. Compared to existing solar MD coating materials, the novel core-shell structure coating shows a better solar MD performance. To understand the impact of operation and environmental factors on NESMD, the response of NESMD to the environmental (i.e., solar irradiance, and feed water temperature and salinity) and operating conditions (e.g., feed flowrate) were systematically investigated. The results show that NESMD perform better under higher solar irradiance and feed/permeate inlet temperature, and lower IR portion light source, feed salinity, and feed flowrate.
To enable NESMD in treating low surface tension wastewater, a dual functional, omniphobic−photothermal nanocomposite membrane was developed to achieve wetting resistance and low energy consumption. The membrane was prepared by forming a hierarchical structure of 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FAS17) modified carbon black (CB) nanoparticles (NPs) on a polyvinylidene fluoride (PVDF) membrane surface. The fluorinated CB NPs absorbed sun light to provide localized heating for NESMD, which increased membrane flux by 25% upon simulated solar irradiation at one sun unit. The utilization efficiency of solar energy in the NESMD process, 75.9%, is more than one order of magnitude higher than the energy efficiency of the conventional direct contact membrane distillation process. Furthermore, the re-entrant structure formed by the CB NPs together with the hydrophobic FAS17 coating led to low surface energy and hence omniphobicity, increasing the contact angle of the 80 vol% ethanol-in-water from 0 to 94.2°. As a result, the dual functional membrane exhibited much higher resistance to wetting by surfactants. Whereas the pristine PVDF membrane was wetted by 0.2 mM SDS, SDS had no effect on the dual function membrane over the whole SDS concentration range tested (0.1 – 0.4 mM). The photothermal activity, improved thermal efficiency, and strong wetting resistance make the dual functional omniphobic−photothermal membrane an excellent membrane material for the NESMD process. Lastly, to improve the VC rejection ability in MD, Graphene oxide (GO) based membranes were fabricated by sandwiching GO and ethylene diamine crosslinked GO (GO-EDA) between a commercial polyvinylidene difluoride (PVDF) membrane and electrospun PVDF nanofiber, and tested their volatile contaminant rejection under different feed temperature, feed pH, and durations by using NH3 as a model volatile contaminant. For the first time, a volatile-contaminants-rejective MD membrane was reported, and the rejection mechanism of NH3 by GO membrane was revealed. Compared to commercial MD membrane, under different experimental conditions, our GO-based membranes always show two orders of magnitude better NH3 rejection with only one third drop in water vapor mass transfer resistance. The NH3 rejection of GO membrane is as high as 97.8%, which is 2.7 times better than the state of art RO membrane. The high volatile contaminants rejection makes our GO membrane good candidates in treating volatile contaminants rich wastewater.