(607d) A Novel Solar Energy Driven Photoelectrodialysis Approach for Brackish Water Desalination

Although water and energy are the two most essential elements for life, the availability of clean water and energy are among the most important crises of our time. In recent years, a significant effort has been made to find energy efficient alternative water technologies. Among them, electrodialysis stands out with significant promise because of its tunability to varying degrees of salt removal, high water recovery and better fouling control for low salinity water. In this research, we have developed a novel energy efficient photoelectrodialysis process for desalination of brackish water using solar energy. A visible light active, nanostructured high surface area titania film, a photocatalyst that is inexpensive and highly stable in aqueous media is utilized to harness the sunlight, which functions as a photoanode. Ordered mesoporous titania films were synthesized using a scalable surfactant templated sol-gel process also known as evaporation induced self-assembly. To increase the light absorption efficiency and to increase the desalination capacity, a thicker film (1000 nm thickness) was fabricated using a layer-by-layer deposition technique.

To overcome its innate inability to absorb visible light due to its wide band gap, the visible light absorption of mesoporous TiO₂ films was enhanced by nitrogen doping using a plasma-based approach. The operating conditions of the plasma reactor including pressure, power, gas flow rate, and reaction time, are optimized to reduce the band gap of the mesoporous titania films from 3.5 eV to 2.88 eV while maintaining its pore structure. The nanostructure formation, pore accessibility, and visible light absorption, are some of the key features of titania films investigated by various advanced tools such as scanning electron microscopy, X-ray diffraction, UV-visible spectroscopy, X-ray photoelectron spectroscopy, ultra-fast transient absorption spectroscopy, and impedance spectroscopy.

These visible light active titania films are used in the photoelectrodialysis cell that is comprised of three compartments for photoanode, saline water and cathode. The saline water and photoanode are separated by an anion exchange membrane, and the saline water and the cathode are separated by a cation exchange membrane. Desalination of brackish water are performed using the photoelectrodialysis cell under the illumination of a solar simulator and LED lights. The effect of salt concentration in the brackish water from 3,000 to 15,000 ppm on the salt rejection and water recovery are studied. Our results suggest solar energy driven process can be energy efficient for brackish water desalination using solar energy.