Rotating Disk Photocatalytic Reactor Research Articles

Recent Developments of Light-Harvesting Excitation, Macroscope Transfer and Multi-Stage Utilization of Photogenerated Electrons in Rotating Disk Photocatalytic Reactor

The rotating disk photocatalytic reactor is a kind of photocatalytic wastewater treatment technique with a high application potential, but the light energy utilization rate and photo quantum efficiency still need to be improved. Taking photogenerated electrons as the starting point, the following contents are reviewed in this work: (1) Light-harvesting excitation of photogenerated electrons. Based on the rotating disk thin solution film photocatalytic reactor, the photoanodes with light capture structures are reviewed from the macro perspective, and the research progress of light capture structure catalysts based on BiOCl is also reviewed from the micro perspective. (2) Macroscope transfer of photogenerated electrons. The research progress of photo fuel cell based on rotating disk reactors is reviewed. The system can effectively convert the chemical energy in organic pollutants into electrical energy through the macroscopic transfer of photogenerated electrons. (3) Multi-level utilization of photogenerated electrons. The photogenerated electrons transferred to the cathode can also generate H2O2 with oxygen or H2 with H+, and the reduction products can also be further utilized to deeply mineralize organic pollutants or reduce the nitrate in water. This short review will provide theoretical guidance for the further application of photocatalytic techniques in wastewater treatment.

Novel wedge structured rotating disk photocatalytic reactor for post-treatment of actual textile wastewater

A wedge structured rotating disk photocatalytic (PC) reactor was used for post-treatment of actual textile wastewater collected after activated sludge treatment. Compared with a conventional planar disk, the wedge structured disk had larger surface area and exhibited better light utilization efficiency. Treatment conditions such as pH and rotating speed were optimized. The COD content in effluent was reduced from 380mgO2L−1 to 109 and 56mgO2L−1 respectively after 3h and 5h treatment at pH 6.0 and rotating speed of 20rpm. The effect of anions (NO3−, SO42− and Cl−) in actual textile wastewater on treatment was investigated. Benefit from high light utilization efficiency, the wedge rotating disk reactor could compensate COD removal inhibition caused by anions. A 1.5V voltage was also applied to perform photoelectrocatalysis process to further improve the performance, and no additional electrolyte was needed. COD in effluent was reduced to as low as 75mgO2L−1 after 3h treatment. In addition, this reactor was used to treat other water samples including the actual textile wastewaters collected before activated sludge treatment (initial COD of 880mgO2L−1) and after activated carbon adsorption post-treatment (initial COD of 108mgO2L−1). Experimental results suggested that wedge structured rotating disk photocatalysis process was stable, efficient and worth further scale-up investigation.

Photocatalytic degradation and electricity generation in a rotating disk photoelectrochemical cell over hierarchical structured BiOBr film

A double hierarchical structured BiOBr film was prepared on Ti substrate through hydrolysis of Bi2O3 film in the presence of Br− and H+. The prepared BiOBr film exhibited much better photocatalytic (PC) activity on Rhodamine B (RhB) degradation than Bi2O3 film in a rotating disk PC reactor. The double hierarchical structure could not only enhance the surface area, but also exhibited strong RhB adsorption capacity and high light utilization efficiency due to multiple reflections of incident light. Due to similar hierarchical structure, BiOCl film was also used to compare the PC activity of BiOBr film. Because the band gap energy of BiOBr (2.5eV) was narrower than that of BiOCl (3.2eV), weaker PC activity was obtained for BiOBr film under UV light. However, besides RhB photosensitization degradation, BiOBr could also be excited by part of visible light to generate •OH, resulting in a better RhB removal under visible light. A Pt cathode was further connected to BiOBr/Ti photoanode to establish a rotating disk BiOBr/Ti–Pt photoelectrochemical cell to generate electricity with the degradation of pollutants simultaneously. 0.77 and 0.74V open-circuit voltage, 0.019 and 0.010mAcm−2 short-circuit current density were obtained under UV and visible light irradiation, respectively, with 50mL5mgL−1 RhB as “fuel”. The PC activity was also improved due to the efficient separation of electrons and holes. The total current generated was stable but gradually decreased with treatment time due to the consumption of “fuel”.

The combination of rotating disk photocatalytic reactor and TiO2 nanotube arrays for environmental pollutants removal

A combined photocatalytic system on one single TiO2-nanotube (TNT)/Ti photocatalyst, which was indeed the functional combination of photon-efficient thin-film and conventional bulk-phase photocatalysis processes, was effectively developed in rotating disk photocatalytic reactor for environmental purification applications. The TNT/Ti rotating disk, of uniform size and well-aligned, was successfully prepared by direct anodic oxidation on a dominantly large surface area of 38cm2, compared to the typical 1cm2 in available literature. To estimate the potentials of combined photocatalytic system for environmental applications, the degradation of rhodamine B was carried out under the optimized conditions, a substrate removal efficiency of nearly 90% and a mineralization efficiency of 56% were observed for initial 20mg/L solution after 3h treatment. Compared with the combined photocatalytic system on TiO2 nanoparticle disk, a significant improvement in substrate removal efficiency of about 25–40% was observed on TNT/Ti disk. It was confirmed that the main degradation of rhodamine B occurred on the upper half of TNT/Ti disk above the heavily colorized sample solution, which was attributed to the superior UV utilization efficiency and the resultant high interfacial photoactivity.

Cu−TiO2/Ti Dual Rotating Disk Photocatalytic (PC) Reactor: Dual Electrode Degradation Facilitated by Spontaneous Electron Transfer

A Cu-TiO2/Ti dual rotating disk photocatalytic (PC) reactor has been developed based on our single rotating disk photoelectrocatalytic (PEC) reactor (Y. Xu, et al, Environ. Sci. Technol. 2008, 42, 2612-2617), and successfully applied to the treatment of laboratory and industrial dye wastewater. Round TiO2/Ti and Cu disks of the same size are connected by a Cu wire and fixed parallel on an axis continually rotating at 90 rpm. High treatment efficiency is obtained due to direct photooxidation on the TiO2/Ti photoanode as well as additional degradation on the Cu cathode, which is speculated via indirect hydrogen peroxide (H2O2) oxidation and direct electro-reduction of dye on cathode. The mechanism of the Cu-TiO2/Ti dual rotating disk PC reactor was investigated. In a 20 mg L(-1) Rhodamine B (RB) solution, approximately 100 mV of potential and 10 microA of current were measured between the Cu and TiO2/Ti electrode during PC treatment. Such phenomenon was explained by spontaneous electron transfer based on the same principle of establishing a Schottky barrier. On the Cu electrode surface, the photoelectrons either reduced dye molecules directly or reacted with dissolved oxygen (DO) to form H2O2. Rotation of electrodes out of the solution enhanced the mass transfer of target compound and kept the aqueous film fresh. The Cu-TiO2/ Ti dual rotating disk PC reactor is a simple and effective device for the treatment of RB dye wastewater.

Effect of hydrogen peroxide on the destruction of organic contaminants-synergism and inhibition in a continuous-mode photocatalytic reactor

The effect of hydrogen peroxide on the photocatalytic degradation of organic contaminants in water was investigated using a TiO 2 -rotating disk photocatalytic reactor (RDPR) operated in a continuous-mode and at steady state. The experiments were performed at pH 3.0, in the presence of near-UV radiation, and using 4-chlorobenzoic acid (4-CBA) as a model non-volatile organic contaminant at influent concentration of 300 μmol l −1 . Experiments were performed at concentrations of hydrogen peroxide in the range 0–10.74 mmol l −1 . Addition of hydrogen peroxide at small concentrations (<2 mmol l −1 ) had a synergistic effect and increased considerably the rates of photocatalytic reactions. An optimum influent hydrogen peroxide concentration was observed at 1.6 mmol l −1 , which caused an increased in the rates of 4-CBA degradation and total organic carbon (TOC) mineralization by 1.72 and 2.13 times, respectively. This corresponded to an optimum oxidant to contaminant molar ratio of 5.33. At higher concentrations, hydrogen peroxide was found to cause an inhibiting effect on the photocatalytic reactions. The synergistic and inhibiting effects of hydrogen peroxide were rationalized based on the reaction rate constants between relevant radical species.

Effect of oxygen in a thin-film rotating disk photocatalytic reactor.

A novel experimental procedure was developed to measure oxygen mass transfer during the oxygenation of water in a thin film of a rotating disk photocatalytic reactor (RDPR). The increase in dissolved oxygen (DO) of initially deaerated water was monitored with time in the reactor vessel at different disk angular velocities after exposure of the reactor to the atmosphere. Oxygenation was predominantly achieved by oxygen mass transport through the thin liquid film carried by the disk and to a much lesser extent by direct oxygenation of the water in the reactor vessel via a surface renewal mechanism. A mathematical model was developed to simulate the phenomenon considering both cases of presence and absence of oxygen mass transport limitations. In the latter case, the model considered that the amount of liquid carried by the disk was saturated with oxygen when returning to the reactor vessel. On the basis of the model and the experimental data, it was proven that mass-transfer limitations existed until the water in the reactor vessel became saturated with oxygen. Results obtained from the model were validated by an alternative analysis using dimensionless groups characteristic to the system. The study revealed that the mass-transfer coefficient increased linearly with disk angular velocity and thus disk Reynolds number. The results showed that oxygen mass-transfer limitations decreased with increasing disk angular velocity, mainly due to an increase in the overall mass-transfer coefficient. In the presence of UV radiation, the influence of oxygen on the photocatalytic oxidation of 4-chlorobenzoic acid was investigated in the RDPR operated in batch and continuous mode. The photocatalytic reactions occurred in a thin film of liquid carried by the disk in the presence of UV radiation and ST-B01 composite spherical ceramic (SiO2/Al2O3) balls coated with anatase TiO2 catalyst. It was found that the initial degradation rate followed Langmuir kinetics with respect to oxygen concentration in the gas phase. When the oxygen concentration in the gas phase surpassed that in air, the degradation rates did not improve significantly, suggesting that operation with air instead of oxygen is most probably a more realistic practical choice. Measurements of DO during the presence and absence of UV radiation suggested that the photocatalytic reactions were mainly oxygen concentration-limited rather than oxygen mass-transfer-limited.

Oxidation of organic contaminants in a rotating disk photocatalytic reactor: reaction kinetics in the liquid phase and the role of mass transfer based on the dimensionless Damköhler number

The photocatalytic degradation of a model non-volatile chlorinated aromatic compound 4-chlorobenzoic acid (4-CBA) was investigated as a function of disk angular velocity, contaminant concentration, and incident light intensity using a rotating disk photocatalytic reactor (RDPR). The study was designed to investigate the effect of all three parameters on the degradation rate as well as their importance on unveiling the existence of mass transfer limitations in the liquid phase under specific conditions. The results showed that the reaction rate increased with disk angular velocity in accordance with a saturation-type dependency. In the range of 2–6rpm the degradation rate increased almost linearly with disk angular velocity. Above 6rpm, however, the influence of disk angular velocity was not significant. The initial increase in the reaction rate with disk angular velocity was attributed to the longer time available per rotation resulting in higher down flow of liquid carried by the disk and to the increase in the overall mass transfer coefficient. The rates of 4-CBA acid decomposition and Cl− mineralization at 6rpm as a function of initial 4-CBA concentration followed Langmuir–Hinshelwood kinetics. At 6rpm, the rates of 4-CBA degradation followed a linear dependency with incident light intensity. This was attributed to the existence of low local values of incident light intensity on the illuminated disk. With respect to the effect of these three parameters on the degradation rate, the obtained results suggested the absence of significant mass transfer limitations at disk angular velocities higher than 6rpm. The latter was verified by additional calculations of the Damköhler (Da) number based on dimensionless analysis. The Da number was found to decrease significantly with disk angular velocity and at high disk angular velocities (ω>15rpm), Da was much lower than 0.1, even when the concentration of the contaminant in the bulk was extremely small (i.e. 1μmol/l).

Rotating disk photocatalytic reactor: development, characterization, and evaluation for the destruction of organic pollutants in water

This work focuses on the development, characterization and evaluation of the TiO 2 Rotating Disk Photocatalytic Reactor (RDPR) for the treatment of organic pollutants in water. A commercial TiO 2-based catalyst in the form of composite ceramic balls was used as the immobilized photocatalyst on the rotating disk. LiCl tracer studies conducted at different disk angular velocities, ranging from 20 to 5 rpm, proved that the mixing in the RDPR is close to that of an ideal CSTR. Two different techniques, an overflow method and the potassium ferrioxalate actinometry method were employed for determining the liquid holdup and turnover time of the rotating disk with glass beads substituted for TiO 2 coated beads. The two methods were in agreement and only the overflow method was employed for determining the same parameters for the rotating disk when loaded with TiO 2 composite ceramic balls. Power law correlations were obtained in all cases. TiO 2-assisted photocatalytic degradation of 4-chlorobenzoic acid (4-CBA) was investigated in the RDPR at a specified angular velocity of the rotating disk (4 rpm), initial pH=3.0, and room temperature using near-UV radiation.

Continuous-mode photocatalytic degradation of chlorinated phenols and pesticides in water using a bench-scale TiO2 rotating disk reactor

Photocatalytic degradation of phenol, chlorinated phenols, and lindane was evaluated in a continuous flow TiO2 rotating disk photocatalytic reactor (RDPR). The RDPR operated at a hydraulic residence time of 0.25 day and at a disk angular velocity of 12rpm. At low molar feed concentrations (0.038mmol/l), the removal efficiencies for phenol and chlorinated phenols were in the order of 86% or higher, whereas the removal efficiency for lindane at a feed concentration of 0.016mmol/l was in the order of 63%.For 2,4,6-trichlorophenol (TCP), an increase in the molar influent concentration resulted in a decrease in removal efficiency but in an increase in removal rate. The degradation rate of 2,4,6-TCP followed a saturation type dependancy with the effluent concentration, suggesting a Langmuir–Hinshelwood (L–H) reaction rate equation. A L–H equation was employed to determine the reaction rate constant and the adsorption coefficient for 2,4,6-TCP. The photonic efficiency increased from 0.68% at an influent concentration of 0.13mmol/l to 2.06% at an influent concentration of 1.0mmol/l.