Optofluidic reactors for photocatalytic water purification

Abstract

Optofluidics is a new technology that enables simultaneous delivery of light and fluids with microscopic precision. This study aims to explore the opportunities of applying the optofluidics technology to the photocatalytic systems for water purification. In this study, three types of optofluidic reactors for photocatalysis water purification are designed, fabricated and characterized to overcome the fundamental limitations of current bulk reactors. The first design is a planar reactor that attempts to overcome the mass transfer limit and the photon transfer limit in the bulk reactors. It has exhibited promising features such as small sample volume, short reaction time and easy flow control. The degradation percentage reaches 94% at the effective residence time of 36 s and the degradation rate gets up to 8%/s at the effective residence time of 6 s. Its success has encouraged the proposal of the second design, which uses BiVO4 as the visible photocatalyst and mounts a blue-light LED panel as the integrated light source. The mounted LED provides uniform irradiation of light and enables to utilize the heat of light source to assist the photodegradation. The degradation efficiency was increased by 4 times and the heat contribution to degradation was about 4~6%. The third design is a novel photoelectrocatalytic microreactor, which aims to eliminate a fundamental limit of photocatalysis the recombination of photo-excited electrons and holes by applying an external electric field. In the experiment, positive and negative bias potentials are applied across the reaction chamber to suppress the e-/h+ recombination and to select either the hole-driven or electron-driven oxidation pathway. Another important feature is that the degradation percentage increases linearly with the residence time. It is 5.2% s⁻¹ for the negative bias state and 4.7% s⁻¹ for the positive bias state. In summary, the optofluidic microreactors have been developed to help overcome different problems in the bulk reactors such as photon transfer limitation, mass transfer limitation, oxygen deficiency, and lack of reaction pathway control. These reactors may find niche applications in rapid screening and standardized tests of photocatalysts and may also be scaled up for large-throughput industrial applications of water process.