Book Description

In this novel MD system, called photothermal membrane distillation (PMD), the membrane is embedded with light-absorbing photothermal materials that harvest solar energy and generate localized heat at the water-membrane interface to drive the MD process. To develop several PMD membranes with high solar conversion efficiency, polydopamine (PDA), which possesses the advantages of easy synthesis, good biocompatibility, and excellent light-to-heat conversion, was used as the photothermal material. First, a simple, stable, and scalable PDA-coated polyvinylidene fluoride (PVDF) membrane was synthesized for PMD. In a direct contact membrane distillation (DCMD) system under 0.75 kW/m2 solar irradiation, the membrane showed a high solar energy conversion efficiency (45%) and a high water flux (0.49 kg/m2·h) This performance was facilitated by the PDA coating, whose broad light absorption and outstanding photothermal conversion properties enabled a higher transmembrane temperature difference and increased the driving force for vapor transport. In addition, the excellent hydrophobicity achieved by fluoro-silanization gave the membrane great wetting resistance and high salt rejection. More importantly, the robustness of the membrane, stemming from the excellent underwater adhesion of the PDA, made it an outstanding candidate for real-world applications. Further, to increase the solar energy conversion efficiency, bacterial nanocellulose (BNC) was utilized to replace commercial PVDF membranes to decrease heat conductive loss from the photothermal layer to the cold distillate. A new photothermal membrane was thermally-engineered to incorporate a bilayered structure composed of two environmentally sustainable materials, PDA particles and BNC. The size-optimized PDA particles on the top layer maximized sunlight absorption and sunlight-to-heat conversion, and the bottom BNC aerogel insulating layer achieved high vapor permeability and low conductive heat loss. This thermally engineered design enabled a permeate flux of 1.0 kg/m2·h under 1 sun irradiation, and a record high solar energy-to-collected water efficiency of 68%, without ancillary heat or heat recovery systems. Moreover, the membrane showed effective bactericidal activity and was easily cleaned, increasing its lifespan. This study provides a new paradigm for using photothermal material incorporated in an aerogel to sustainably purify water. Using renewable solar energy, the PMD system can also provide decentralized desalination for remote or underdeveloped areas and can support resilient community development.In summary, the work described in this dissertation offers an in-depth and mechanistic understanding of the fate of nanoscale solids (e.g., engineered nanomaterials and naturally occurring nanoparticles) in SWM systems in the presence of different water constituents (e.g., anions, reactive radical species, and organic matter). It also provides insights for designing more stable, scalable, and sustainable nanomaterial-based membranes for water treatment and desalination. Ultimately, this research will better define the chemistry of nanoscale solids and organic matter in water management systems, benefiting the design of next-generation water treatment systems that are environmentally safer and more sustainable.