Optofluidics-Based Membrane Microreactor for Wastewater Treatment by Photocatalytic Ozonation

Microenvironment Engineering of Heterogeneous Catalysts for Liquid-Phase Environmental Catalysis

Environmental catalysis has emerged as a scientific frontier in mitigating water pollution and advancing circular chemistry and reaction microenvironment significantly influences the catalytic performance and efficiency. This review delves into microenvironment engineering within liquid-phase environmental catalysis, categorizing microenvironments into four scales: atom/molecule-level modulation, nano/microscale-confined structures, interface and surface regulation, and external field effects. Each category is analyzed for its unique characteristics and merits, emphasizing its potential to significantly enhance catalytic efficiency and selectivity. Following this overview, we introduced recent advancements in advanced material and system design to promote liquid-phase environmental catalysis (e.g., water purification, transformation to value-added products, and green synthesis), leveraging state-of-the-art microenvironment engineering technologies. These discussions showcase microenvironment engineering was applied in different reactions to fine-tune catalytic regimes and improve the efficiency from both thermodynamics and kinetics perspectives. Lastly, we discussed the challenges and future directions in microenvironment engineering. This review underscores the potential of microenvironment engineering in intelligent materials and system design to drive the development of more effective and sustainable catalytic solutions to environmental decontamination.

Solar Energy Storage in an All-Vanadium Photoelectrochemical Cell: Structural Effect of Titania Nanocatalyst in Photoanode

Solar energy storage in the form of chemical energy is considered a promising alternative for solar energy utilization. High-performance solar energy conversion and storage significantly rely on the sufficient active surface area and the efficient transport of both reactants and charge carriers. Herein, the structure evolution of titania nanotube photocatalyst during the photoanode fabrication and its effect on photoelectrochemical activity in a microfluidic all-vanadium photoelectrochemical cell was investigated. Experimental results have shown that there exist opposite variation trends for the pore structure and crystallinity of the photocatalyst. With the increase in calcination temperature, the active surface area and pore volume were gradually declined while the crystallinity was significantly improved. The trade-off between the gradually deteriorated sintering and optimized crystallinity of the photocatalyst then determined the photoelectrochemical reaction efficiency. The optimal average photocurrent density and vanadium ions conversion rate emerged at an appropriate calcination temperature, where both the plentiful pores and large active surface area, as well as good crystallinity, could be ensured to promote the photoelectrochemical activity. This work reveals the structure evolution of the nanostructured photocatalyst in influencing the solar energy conversion and storage, which is useful for the structural design of the photoelectrodes in real applications.

Advanced oxidation processes in microreactors for water and wastewater treatment: Development, challenges, and opportunities

The miniaturization of reaction processes by microreactors offers many significant advantages over the use of larger, conventional reactors. Microreactors' interior structures exhibit comparatively higher surface area-to-volume ratios, which reduce reactant diffusion distances, enable faster and more efficient heat and mass transfer, and better control over process conditions. These advantages can be exploited to significantly enhance the performance of advanced oxidation processes (AOPs) commonly used for the removal of water pollutants. This comprehensive review of the rapidly emerging area of environmental microfluidics describes recent advances in the development and application of microreactors to AOPs for water and wastewater treatment. Consideration is given to the hydrodynamic properties, construction materials, fabrication techniques, designs, process features, and upscaling of microreactors used for AOPs. The use of microreactors for various AOP types, including photocatalytic, electrochemical, Fenton, ozonation, and plasma-phase processes, showcases how microfluidic technology enhances mass transfer, improves treatment efficiency, and decreases the consumption of energy and chemicals. Despite significant advancements of microreactor technology, organic pollutant degradation mechanisms that operate during microscale AOPs remain poorly understood. Moreover, limited throughput capacity of microreactor systems significantly restrains their industrial-scale applicability. Since large microreactor-inspired AOP systems are needed to meet the high-throughput requirements of the water treatment sector, scale-up strategies and recommendations are suggested as priority research opportunities. While microstructured reactor technology remains in an early stage of development, this work offers valuable insight for future research and development of AOPs in microreactors for environmental purposes.

Rapid degradation of refractory organic pollutants by continuous ozonation in a micro-packed bed reactor

Recently microreactor technology attracts attention due to the excellent multiphase mixing and enhanced mass transfer. Herein, a continuous ozonation system based on a micro-packed bed reactor (μPBR) was used to improve the dissolution rate of ozone and achieved a rapid and efficient degradation of refractory organic pollutants. The effects of liquid flow rate, gas flow rate, initial pH, initial O₃ concentration and initial phenol concentration on the phenol and chemical oxygen demand (COD) removal efficiencies were also investigated. Experimental results showed that phenol and COD removal efficiencies under optimal conditions achieved 100.0% and 86.4%, respectively. Compared with large-scale reactors, the apparent reaction rate constant in μPBR increased by 1-2 orders of magnitude. In addition, some typical organic pollutants (including phenols, antibiotics and dyes) were treated by ozonation in μPBR. The removal efficiencies of these organic pollutants and COD achieved 100.0% and 70.2%-80.5% within 71 s, respectively. In this continuous treatment system, 100% of the unreacted ozone was converted to oxygen, which promoted the healthy development of aquatic ecosystems. Thus, this continuous system based on μPBR is a promising method in rapid and efficient treating refractory organic pollutants.

Kinetic Modeling of Advanced Oxidation Processes Using Microreactors: Challenges and Opportunities for Scale-Up

With the increasing number of recalcitrant pollutants in wastewater treatment plants, there will be a stringent need for rapid and convenient development of tertiary treatment processes such as advanced oxidation processes (AOPs). Microreactors offer a great opportunity for ultrafast and safe intrinsic kinetic parameters determination, by-products identification, and ecotoxicity assessment. Despite the considerable potential of these devices, they have been mostly used for catalyst screening or pseudo-first order kinetics determination, not allowing for knowledge transfer across scales. This work offers an overview of the adoption of micro- and photo-microreactors for intrinsic kinetics investigations in the field of AOPs to guide future research efforts.

A new type of microphotoreactor with integrated optofluidic waveguide based on solid-air nanoporous aerogels

In this study, we developed a new type of microphotoreactor based on an optofluidic waveguide with aqueous liquid core fabricated inside a nanoporous aerogel. To this end, we synthesized a hydrophobic silica aerogel monolith with a density of 0.22 g cm^-3 and a low refractive index of 1.06 that-from the optical point of view-effectively behaves like solid air. Subsequently, we drilled an L-shaped channel within the monolith that confined both the aqueous core liquid and the guided light, the latter property arising due to total internal reflection of light from the liquid-aerogel interface. We characterized the efficiency of light guiding in liquid-filled channel and-using the light delivered by waveguiding-we carried out photochemical reactions in the channel filled with aqueous solutions of methylene blue dye. We demonstrated that methylene blue could be efficiently degraded in the optofluidic photoreactor, with conversion increasing with increasing power of the incident light. The presented optofluidic microphotoreactor represents a versatile platform employing light guiding concept of conventional optical fibres for performing photochemical reactions.

A solar responsive photocatalytic fuel cell with the membrane electrode assembly design for simultaneous wastewater treatment and electricity generation

In this work, a photocatalytic fuel cell (PFC) with membrane electrode assembly (MEA) structure was designed for simultaneous organic compounds degradation and electricity generation. For the photoanode, the TiO₂ with the quantum-dot sensitization by CdS-ZnS was used to broaden the absorption spectrum to visible light. For the cathode, an air-breathing mode was utilized to enhance the oxygen transport. The performance of the developed PFC was evaluated under different operation conditions, including the light intensity, liquid flow rate, concentrations of electrolyte and organics. Results indicated that the designed PFC could yield good performance. The increase of the light intensity and electrolyte concentration could improve the PFC performance. It is also found that when the flow rate was increased, the PFC performance dropped down in the testing range. Too high organics concentration led to the decrease of the PFC performance.