UV-LEDs assisted preparation of silver deposited TiO2 catalyst bed inside microchannels as a high efficiency microphotoreactor for cleaning polluted water

LED-irradiated photo-Fenton process on pollutant removal: outcomes, trends, and limitations

This manuscript critically reviews the state of the art on the application of photo-Fenton processes irradiated by light-emitting diode arrays (LED) with a focus on the removal of contaminants of emerging concern (CEC) from aqueous matrices. LEDs are clean, low-cost radiation sources with longer lifespan compared to mercury lamps. This study covers the influence of LED sources, wavelengths, and dose upon CEC removal, and the potential for disinfection, abatement of antibiotic-resistant bacteria (ARB), and genes (ARG). The bibliographic search was performed in Scopus database using keyword combinations and resulted in a portfolio containing 52 relevant articles published between 2010-2023. According to reviewed papers, LED photoreactor design has evolved in the past decade aiming to improve CEC degradation in aqueous matrices while reducing construction and operation costs, and energy consumption. Among several reactors (annular, fluidized bed, parallel plate, wireless, pathway systems, and microreactor) surveyed for their performance and scalability, LED chips and strips are particularly suitable for application due to their wide emission angle (≈120°) and small size (mm2), which allow for, respectively, efficient illumination coverage and flexible arrangement and design. LED microreactors are very efficient in the degradation of contaminants and scalable with reduced area requirements. Although most studies were performed in synthetic solutions and at laboratory scale, the externally LED irradiated cylindrical reactor was successful for application in full-scale municipal water treatment plants. Future studies should focus on evaluating CEC removal in wastewater using scalable devices for continuous operation of solar photo-Fenton at night.

A high-performance microreactor integrated with chitosan/ Bi2WO6/CNT/TiO2 nanofibers for adsorptive/photocatalytic removal of cephalexin from aqueous solution

A Z-scheme Bi₂WO₆/CNT/TiO₂ photocatalyst was synthesized hydrothermally and loaded on chitosan nanofibers with different mass percentages using the electrospinning process. The batch adsorption experiments for chitosan nanofibrous samples containing Bi₂WO₆/CNT/TiO₂ revealed that the adsorption process and its kinetic followed the Langmuir isotherm and pseudo-second-order model, respectively. A planar microreactor with a reusable plate-type configuration was fabricated employing an inexpensive micromachining technique and integrated with chitosan/Bi₂WO₆/CNT/TiO₂ nanofibers. The synergistic effect of the adsorption and photocatalysis was assessed for removing cephalexin under simulated sunlight irradiation in a continuous flow microreactor. The nanofibers containing 15 wt% of Bi₂WO₆/CNT/TiO₂ exhibited the most removal efficiency. The effects of operational variables were investigated in the microreactor and optimized using response surface methodology as light intensity = 17.45 W/m², retention time = 256 s, pH = 4.8, and initial cephalexin concentration = 29 mg/L. At this condition, cephalexin and TOC removal efficiencies reached 99.2% and 92.4%, respectively. The kinetic of disappearance of cephalexin under optimal conditions followed the Langmuir-Hinshelwood model. The adsorption equilibrium constant deduced from this model was similar to that one calculated from the Langmuir isotherm model. At the optimum condition, cephalexin removal efficiency reduced to 80% after 1500 min of microreactor operation and the nanofibers revealed appropriate stability and reusability.

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.

Synthesis of modified ZnO nanorods and investigation of its application for removal of phthalate from landfill leachate: A case study in Aradkouh landfill site

In this study, zinc oxide nanorods, co-doped with iron and silver, were synthesized in a co-precipitation method. Its properties were determined using X-ray diffraction (XRD), Transmission electron microscopy (TEM), Field Emission Scanning Electron Microscopy (FE-SEM), Brunauer-Emmett-Teller (BET), Dynamic light scattering (DLS) and X-ray photoelectron spectroscopy (XPS) analysis. The results of FE-SEM and TEM showed that zinc oxide nanoparticles synthesized and co-doped with iron and silver were formed as separate nanorods. Also, the average values of DBP and DEHP amount of phthalates in the leachate from the landfill site of Aradkouh were obtained 52.5 and 94.69 mg/L, respectively. The highest removal efficiency in real samples for phthalates was found to be 52%. The highest removal efficiency of TOC were was 61%. The synthesized nanostructure could have proper efficiency in removal of phthalates from water sources under the visible light of LED lamp.

Facile fabrication of hybrid titanium(IV) isopropoxide/pozzolan nanosheets (TnS-Pz) of high photocatalytic activity: characterization and application for Cr(VI) reduction in an aqueous solution

This paper presents the synthesis of a hybrid material through the use of natural pozzolan and titanium(IV) isopropoxide using the sol-gel method and its application in the photocatalytic hexavalent chromium reduction. The characterization data indicated a mesoporous material possessing a surface area of 271.7 m² g^-1. The morphology studies (SEM and TEM) showed nanosheet hybrid structures. The analysis of DRUV, FTIR, XRD, and Mössbauer spectroscopy provides a different electronic structure of the synthetized material when compared with the originals, proving the hybridization process between pozzolan and titanium(IV) isopropoxide. The photocatalytic reduction of Cr(VI) to Cr(III) using the hybrid material showed a better performance than conventional photocatalysts (precursor and TiO₂-P25). Operational conditions such as chromium initial concentration (0.02-0.20 mM), solution pH (3-6), and type of scavenger (citric or tartaric acid) were evaluated in order to determine the best experimental conditions for the Cr(VI) photoreduction. At their optimum (catalyst load of 15 mg L^-1, tartaric acid as scavenger, [scavenger]₀/[Cr(VI)]₀ M ratio = 3:1, pH 3, and 25 °C), the total photoreduction of 0.20 mM Cr(VI) was achieved in 180 min. The novel hybrid materials synthesized from pozzolan and titanium(IV) isopropoxide showed to be a potential catalyst for the Cr(VI) reduction in aqueous solution. Graphical abstract.

An investigation of scale effect and applicability of compact light sources in microchannel reactors applied to pollutant abatement

In this work, the performance of microreactors irradiated with conventional (fluorescent) and UV-LED light was evaluated. For this purpose, a microfluidic reactor with an equivalent diameter of 133.5 μm was used. In addition, the effect of scale variation on the performance of photochemical reactors was assessed using reactors with three internal diameters (600, 1200, and 2300 μm), 2 residence times (30 and 60 s), and two sources of UVA radiation (A with irradiance of 115 W m-2 and B with irradiance of 44 W m-2). Also, the relationship between the configuration of the photocatalyst film and the effect of the scale on the performance of photochemical reactors was experimentally and theoretically investigated. For both cases, methylene blue dye was used as a model pollutant and titanium dioxide (TiO2) as a photocatalyst deposited on the inner wall of the photocatalytic reactors. For the residence time of 30 s, the smaller the reactor diameter, the greater was the degradation (22, 18, and 6%, respectively, for lamp A and 17, 16, and 8 %, respectively, for lamp B). The influence of the diameter of the reactor was also observed for the residence time of 60 s, but only for the reactor with a 2300-μm internal diameter. The reactors with diameters 600 and 1200 μm only showed different results when illuminated with lamp B (33 and 28% of methylene blue conversion, respectively). Moreover, computational simulation results suggested higher efficiency as the reactor's diameter is decreased and an optimum thickness of photocatalyst film to maximize the performance of devices in which photocatalytic reactions are carried out.

Seaweed bio-inspired ZnO piezoelectric cilia array applied in microreactors for enhanced photocatalytic performance

A piezoelectric built-in electric field is believed to be an effective way to separate photoinduced carriers and enhance photocatalytic performance.

Designing Microflowreactors for Photocatalysis Using Sonochemistry: A Systematic Review Article

Use of sonication for designing and fabricating reactors, especially the deposition of catalysts inside a microreactor, is a modern approach. There are many reports that prove that a microreactor is a better setup compared with batch reactors for carrying out catalytic reactions. Microreactors have better energy efficiency, reaction rate, safety, a much finer degree of process control, better molecular diffusion, and heat-transfer properties compared with the conventional batch reactor. The use of microreactors for photocatalytic reactions is also being considered to be the appropriate reactor configuration because of its improved irradiation profile, better light penetration through the entire reactor depth, and higher spatial illumination homogeneity. Ultrasound has been used efficiently for the synthesis of materials, degradation of organic compounds, and fuel production, among other applications. The recent increase in energy demands, as well as the stringent environmental stress due to pollution, have resulted in the need to develop green chemistry-based processes to generate and remove contaminants in a more environmentally friendly and cost-effective manner. It is possible to carry out the synthesis and deposition of catalysts inside the reactor using the ultrasound-promoted method in the microfluidic system. In addition, the synergistic effect generated by photocatalysis and sonochemistry in a microreactor can be used for the production of different chemicals, which have high value in the pharmaceutical and chemical industries. The current review highlights the use of both photocatalysis and sonochemistry for developing microreactors and their applications.

Intensification of heterogeneous TiO2 photocatalysis using the NETmix mili-photoreactor under microscale illumination for oxytetracycline oxidation

This study focuses on the intensification of heterogeneous TiO₂ photocatalysis for the removal of a contaminant of emerging concern (CEC), oxytetracycline (OTC), as a polishing step of urban wastewaters, using an innovative NETmix mili-photoreactor under UVA-LEDs illumination. The effect of catalyst coated surface per reactor volume and the illumination mechanism, back-side (BSI) or front-side (FSI) irradiation, on OTC oxidation were evaluated. For that, a thin film of photocatalyst was uniformly deposited on the front borosilicate slab (BS) (BSI mechanism; 333 m²_catalyst m^-3_reactor) or on the network of channels and chambers imprinted in the back stainless-steel slab (SSS) (FSI mechanism; 989 m²_catalyst m^-3_reactor) using a spray system. OTC removal was also assessed as a function of TiO₂ film thickness immobilized on both slabs. The photocatalyst reactivity in combination with photoreactor was significantly enhanced (3.4 times) from 0.64 to 2.19 mmol_OTC m^-3_{illuminated reactor volume} s^-1, when considering the BSI and FSI mechanisms, respectively. In addition, the influence of UVA-LEDs intensity on OTC oxidation rate was investigated. UVA-LEDs plates were placed on the top of the NETmix borosilicate window. Moreover, the effect of water matrix was assessed using a secondary effluent from an urban wastewater treatment plant fortified with OTC. OTC oxidation rate was only inhibited in about 1.3 times in the presence of the real matrix, showing the ability of the NETmix to overcome matrix effects due to its unique characteristics. Catalyst film stability over four consecutive reaction cycles was evaluated using synthetic and real matrices fortified with OTC.

Removal of dibutyl phthalate from aqueous environments using a nanophotocatalytic Fe, Ag-ZnO/VIS-LED system: modeling and optimization

An (Fe, Ag) co-doped ZnO nanostructure was synthesized by a simple chemical co-precipitation method and used for the degradation of dibutyl phthalate (DBP) in aqueous solution under visible light-emitting diode (LED) irradiation. (Fe, Ag) co-doped ZnO nanorods were characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy, UV-VIS diffuse reflectance spectroscopy, elemental mapping, Field emission scanning electron microscopy, transmission electron microscope and Brunauer-Emmett-Teller surface area analysis. A Central Composite Design was used to optimize the reaction parameters for the removal of DBP by the (Fe, Ag) co-doped ZnO nanorods. The four main reaction parameters optimized in this study were the following: pH, time of radiation, concentration of the nanorods and initial DBP concentration. The interaction between the four parameters was studied and modeled using the Design Expert 10 software. A maximum reduction of 95% of DBP was achieved at a pH of 3, a photocatalyst concentration of 150 mg L^-1 and a DBP initial DBP concentration of 15 mg L^-1. The results showed that the (Fe, Ag) co-doped ZnO nanorods under low power LED irradiation can be used as an effective photocatalyst for the removal of DBP from aqueous solutions.

Horizontally rotating disc recirculated photoreactor with TiO2-P25 nanoparticles immobilized onto a HDPE plate for photocatalytic removal of p-nitrophenol

In this study, a horizontally rotating disc recirculated (HRDR) photoreactor equipped with two UV lamps (6 W) was designed and fabricated for photocatalytic removal of p-nitrophenol (PNP). Photocatalyst (TiO₂) nanoparticles were immobilized onto a high-density polyethylene (HDPE) disc, and PNP containing solution was allowed to flow (flow rate of 310 mL min^-1) in radial direction along the surface of the rotating disc illuminated with UV light. The efficiency of direct photolysis and photocatalysis and the effect of rotating speed on the removal of PNP were studied in the HRDR photoreactor. It was found that TiO₂-P25 nanoparticles are needed for the effective removal of PNP and there was an optimum rotating speed (450 rpm) for the efficient performance of the HRDR photoreactor. Then effects of operational variables on the removal efficiency were optimized using response surface methodology. The results showed that the predicted values of removal efficiency are consistent with experimental results with an R₂ of 0.9656. Optimization results showed that maximum removal percent (82.6%) was achieved in the HRDR photoreactor at the optimum operational conditions. Finally, the reusability of the HRDR photoreactor was evaluated and the results showed high reusability and stability without any significant decrease in the photocatalytic removal efficiency.

Oxidation of tartrazine with ultraviolet light emitting diodes: pH and duty cycles effects

The presence of tartrazine (TAR) in the water cycle poses serious threats to human health. This study investigated the used of light emitting diodes (LEDs) in the advanced oxidation of TAR under different pH and duty cycle (DC) conditions. The first order reaction rate constant for TAR oxidation was positively correlated with DC, negatively correlated with pH, and typically greatest at pH 6. Chemical byproduct analysis indicated that OH addition, H abstraction, and electron transfer without molecule transfer were among the relevant reaction mechanisms for TAR degradation. Six byproducts were identified, four were reported for the first time, and two demonstrated that TAR rings were cleaved. This research is the first to determine the optimal pH for UVLED-driven oxidation of TAR and the first to identify new TAR-related byproducts from UVLED-based water treatment.

A calibrated UV-LED based light source for water purification and characterisation of photocatalysis

Photocatalysis can become a cost effective industrial process for water cleaning. This paper describes the design and performance of a novel LED-based light engine for this purpose.

Ultrasonic-assisted synthesis of Ce doped cubic-hexagonal ZnTiO3 with highly efficient sonocatalytic activity

Ce doped ZnTiO3 as a novel catalyst with highly efficient and stable sonocatalytic activity was synthesized via an ultrasound-assisted sol-gel method using non-ionic surfactant Pluronic F127 as structure directing agent. Synthesized samples were characterized by using various techniques, such as XRD, TEM, SEM, EDX, ​XRF, BET, DRS, and PL, and their sonocatalytic activity studied toward degradation of p-Nitrophenol as a model organic compound. The synthesized mesoporous Ce/ZnTiO3 had mixed cubic-hexagonal phase with large surface area (118.2 m(2) g(-1)) and narrow pore size distribution (4.9 nm). The effects of cerium concentration, calcination temperature, and calcination time on the structure and the sonocatalytic activity of Ce/ZnTiO3 were studied in detail. XRD results were suggested that the relation between the phase structure and the catalytic activity is considerable. Significant decrease in band-gap and PL intensity was observed with increasing the cerium concentration in the ZnTiO3. It became clear that the Ce/ZnTiO3 (0.81 mol%) shows high sonocatalytic activity compared with pure ZnTiO3 and other Ce/ZnTiO3 samples as well as commercial TiO2-P25. The possible mechanism for the enhanced sonocatalytic activity of Ce/ZnTiO3 was discussed in details. The electrical energy consumption was also considered during sonocatalytic experiments.

Microfluidic-based photocatalytic microreactor for environmental application: a review of fabrication substrates and techniques, and operating parameters

This article gives an overview of photocatalytic microreactors with an application in environmental science, in particular, the degradation of different toxic dyes within microchannels.