Advanced Nanomaterials and Module Designs for Efficient Photothermal Water Treatment

Water scarcity is one of the most serious challenges that human beings face, and the demand for clean water will continuously increase as populations grow. Creating potable water from saline water is an important way to meet global freshwater demands. However, current desalination techniques require high energy consumption and a large scale to be efficient, restricting their utilization in remote and underdeveloped areas. These constraints motivate the development of sustainable water treatment methods that can be used with less energy input, in small modular configurations, and in off-grid water treatment systems. As an abundant and renewable energy source, sunlight has been considered for water treatment. However, due to its broadband spectrum and relatively low irradiation per unit area, harvesting solar energy efficiently poses challenges. To address these challenges, this dissertation has three objectives. The first is to advance solar-based desalination by engineering nanomaterials to more efficiently harvest sunlight. The second is to investigate the photo-induced transformation of nanomaterials to better understand their long-term reliability during solar-based desalination processes. The third objective is to design innovative desalination modules with improved performance for maximizing solar-based clean water generation.Photothermal evaporation (PE) is an emerging solar desalination technique that uses interfacial heating from incident sunlight to achieve efficient evaporation. Ideally, the photothermal materials in a photothermal evaporator will efficiently convert incident sunlight to heat, be scalably synthesized, and be environmentally benign. Pursuing the first objective of this dissertation, after considering several photothermal materials, we focused on chemically exfoliated (ce) MoS2, one type of two-dimensional (2D) transition metal dichalcogenides (TMD). Here, we demonstrated that MoS2 can be a promising photothermal material that meets all the requirements for a photothermal evaporator to create fresh water. Notably, the phase transition of MoS2 from 2H (trigonal prismatic coordination) to 1T (octahedral coordination) during the exfoliation process enhanced the light absorption of the ce-MoS2, generating heat more effectively. Owing to the efficient photothermal conversion of ce-MoS2 nanosheets and to heat localization from using bacterial nanocellulose foam as a support, high solar evaporation efficiencies were achieved. In addition, the cytotoxicity of ce-MoS2 nanosheets was lower than that of graphene oxide (GO) nanosheets with a similar size, a commonly suggested material for solar thermal evaporation, which can alleviate a potential environmental risk. The second objective of this dissertation recognizes that the distinctive properties of 2D nanomaterials suggest their use in photo-enabled water treatment. Because 2D nanomaterials undergo different physicochemical processes under sunlight irradiation, understanding their photostability and photo-induced transformation is important to determining their reliability and longevity in photothermal desalination. To elucidate their photostability, we selected MoS2, a promising 2D nanomaterial for photothermal desalination, and investigated the effects of its nanostructures and water’s ionic contents on its photostability. Particularly, because the band structure and electrical properties of MoS2 nanosheets depend on their thickness, we investigated the thickness-dependent dissolution of 18 nm thick, 46 nm thick, and bulk MoS2. Under simulated sunlight irradiation, MoS2 dissolution was accelerated, the Mo6+ composition increased, and the solution pH decreased. These results suggested that light exposure promoted the oxidation of MoS2, causing faster dissolution. Importantly, 18 nm thick MoS2 exhibited faster dissolution than either 46 nm or bulk MoS2, driven by the superoxide radical (O2•−) generation promoted by its relative thinness. In addition, photo- and non-photo induced redox pathways, driven by MoS2 nanosheets, can facilitate redox cycling of ferric/ferrous ions. Because iron ions are common in natural water and widely used in Fe-catalyzed water treatment, the effects of aqueous iron on MoS2’s photostability deserve careful consideration. In this regard, we investigated the effects of aqueous iron on the morphologies, chemical compositions, and photocatalytic activities of MoS2 nanosheets. Light exposure and the presence of iron ions synergistically accelerated MoS2 oxidative dissolution through photo-Fenton reactions. Moreover, circular-shaped nanopits were observed to form on the basal planes of MoS2 nanosheets during their dissolution, and we demonstrated that the pits’ formation was mainly caused by the photocatalytic activity of the MoS2 nanosheet. Because of the severe oxidation driven by photo-Fenton reactions, the photocatalytic performance of a 1hr-reacted MoS2 nanosheet decreased by ~37 % compared to that of pristine MoS2. These findings provided important information on the effects of nanostructures and water constituents on 2D nanomaterials’ photostability and the alteration of their photochemical activity, which will be helpful in predicting their long-term reliability in photothermal desalination. In pursuing the dissertation’s third objective, we examined photothermal membrane distillation (PMD), which combines interfacial heating by photothermal conversion with membrane distillation. PMD efficiently utilizes photothermally generated heat to produce clean water. Because PE is challenged to efficiently collect the vaporized water, PMD appears as a sustainable and superior alternative solar desalination technique for producing drinking water However, despite their efficient vapor collection, most PMD designs suffer from relatively low solar efficiency because of the large fractional heat loss to the surroundings. To improve the performance of PMD, we designed a multi-layer stacked membrane module with an optimized module airgap thickness, which can maximize latent heat recovery and reduce conductive heat loss. With an efficient photothermal membrane, an optimized airgap (2 mm), and four stacked heat recovery layers, we achieved 105 % solar efficiency (1.17 kg/m2/hr), the result of heat re-use. Owing to thermal bridging between condensed produced water and the photothermal membrane, in many cases, PMD is often not conducted with the theoretically optimum airgap thickness, reducing its performance. To maintain a thin airgap in a PMD module without thermal bridging, super-hydrophilic aluminum (Al) plates were employed in our new module. The thin airgap in the module provided low resistance to vapor transfer, and a super-hydrophilic Al plate provided facile water transport. The combination increased the solar efficiency by ~12 % and shortened the induction time for a producing purified water by 41%. In comparison to a direct contact membrane distillation (DCMD) configuration, the PMD module a super-hydrophilic plate and an optimized airgap membrane distillation (AMD) configuration exhibited ~34% higher solar efficiency. This dissertation provides useful guidelines for advancing solar-based water treatment through engineering nanomaterials, understanding their photo-induced transformation, and designing and engineering the components and configuration of modules for maximizing the use of sunlight. In particular, the findings of this research offer an in-depth understanding of how the crystal phase and structure of nanomaterials and the aquatic chemistry influence their photochemical properties and photostability in aqueous media. It also provides insights into the importance of thermal engineering and heat recovery in solar-based desalination, advancing it as a sustainable and off-grid desalination technique