Quantum Efficiencies of 4-Chlorophenol Photocatalytic Degradation and Mineralization in a Well-Mixed Slurry Reactor

Kinetic modeling of the photocatalytic degradation of acetaminophen and its main transformation products

In this study, a kinetic model of the heterogeneous photocatalytic degradation of acetaminophen and its main transformation products is presented. Kinetic photocatalytic modeling and photon absorption rate modeling were included. Monte Carlo method was used to model the photon absorption process. Experiments were carried out in a reactor operated in batch mode and TiO2 nanotubes were used as photocatalyst irradiated with 254 nm UVC. Kinetic parameters were estimated from the experiments data by applying a non-linear regression procedure. Intrinsic expressions to the kinetics of acetaminophen degradation and its main transformation products were derived. Model, kinetics and photon absorption formulations and parameters proved to be affordable for describing the photocatalytic degradation of acetaminophen, but improvements should be done for better description of formation and oxidation kinetics of main transformation products. The model should be tested with other pharmaceuticals and emergent pollutants to calibrate it and evaluate its applicability in a wide range of compounds.

Protecting against micropollutants in water storage tanks using in-situ TiO2 coated quartz optical fibers

Photocatalyst-coated optical fibers (P-OFs) using UV-A LEDs offer a highly promising solution for the degradation of micropollutants within municipal, reuse, industrial or home distribution systems, by integrating P-OFs into water storage tanks. P-OFs have photocatalysts attached to bundles of optical fibers, enabling their direct deployment within tanks. This eliminates the necessity for photocatalyst slurries, which would require additional membrane or separation systems. However, a current limitation of P-OFs is light management, specifically light oversaturation of the coated photocatalysts and short light transmission distances along fibers. This study overcomes this limitation and reveals strategies to improve the light dissipation uniformity along P-OFs, and demonstrates the performance of P-OFs on degrading a model micropollutant, carbamazepine (CBZ). Key tunable variables of fibers and light emission conditions, including photocatalyst coating patchiness (p), minimum light incident angles (θm), radiant flux launched to fibers (Φi), and fiber diameters (D), were modeled to establish their relationships with the light dissipation uniformity in TiO2-coated quartz optical fibers (TiO2-QOFs). We then validated modeling insights by conducting experiments to examine how these variables influence the generation of evanescent waves which are localized energy on fiber surfaces, leading to either photocatalyst activation or the recapture of unused light back into fibers. We observed substantial enhancements in evanescent waves generation by decreasing p and increasing θm, resulting in uniform light dissipation which reduces light oversaturation and improves light transmission distances. Moreover, these optimizations led to a remarkable three-fold improvement in CBZ degradation rates and a 65% reduction in energy consumption. Such improvement substantially reduces the capital and operational cost and enhances practicality of energy-efficient photocatalysis without additional chemical oxidants for micropollutant degradation in water storage tanks.

Evanescent waves modulate energy efficiency of photocatalysis within TiO2 coated optical fibers illuminated using LEDs

Coupling photocatalyst-coated optical fibers (P-OFs) with LEDs shows potential in environmental applications. Here we report a strategy to maximize P-OF light usage and quantify interactions between two forms of light energy (refracted light and evanescent waves) and surface-coated photocatalysts. Different TiO₂-coated quartz optical fibers (TiO₂-QOFs) are synthesized and characterized. An energy balance model is then developed by correlating different nano-size TiO₂ coating structures with light propagation modes in TiO₂-QOFs. By reducing TiO₂ patchiness on optical fibers to 0.034 cm²/cm² and increasing the average interspace distance between fiber surfaces and TiO₂ coating layers to 114.3 nm, refraction is largely reduced when light is launched into TiO₂-QOFs, and 91% of light propagated on the fiber surface is evanescent waves. 24% of the generated evanescent waves are not absorbed by nano-TiO₂ and returned to optical fibers, thus increasing the quantum yield during degradation of a refractory pollutant (carbamazepine) in water by 32%. Our model also predicts that extending the TiO₂-QOF length could fully use the returned light to double the carbamazepine degradation and quantum yield. Therefore, maximizing evanescent waves to activate photocatalysts by controlling photocatalyst coating structures emerges as an effective strategy to improve light usage in photocatalysis.

Design and performance evaluation of a photocatalytic reactor for indoor air disinfection

Since COVID-19 pandemic, indoor air quality control has become a priority, and the development of air purification devices effective for disinfecting airborne viruses and bacteria is of outmost relevance. In this work, a photocatalytic device for the removal of airborne microorganisms is presented. It is an annular reactor filled with TiO₂-coated glass rings and irradiated internally and externally by UV-A lamps. B. subtilis spores and vegetative cells have been employed as model biological pollutants. Three types of assays with aerosolized bacterial suspensions were performed to evaluate distinct purification processes: filtration, photocatalytic inactivation in the air phase, and photocatalytic inactivation over the TiO₂-coated rings. The radiation distribution inside the reactor was analysed by performing Monte Carlo simulations of photon absorption in the photocatalytic bed. Complete removal of a high load of microorganisms in the air stream could be achieved in 1 h. Nevertheless, inactivation of retained bacteria in the reactor bed required longer irradiation periods: after 8 h under internal and external irradiation, the initial concentration of retained spores and vegetative cells was reduced by 68% and 99%, respectively. Efficiency parameters were also calculated to evaluate the influence of the irradiation conditions on the photocatalytic inactivation of bacteria attached at the coated rings.

Heterogeneous photo-Fenton process using iron-modified regional clays as catalysts: photonic and quantum efficiencies

A regional raw clay was used as the starting material to prepare iron-pillared clays with different iron contents. The catalytic activity of these materials was tested in the heterogeneous photo-Fenton process, applied to the degradation of 2-chlorophenol chosen as the model pollutant. Different catalyst loads between 0.2 and 1.0 g L^-1 and pH values between 3.0 and 7.0 were studied. The local volumetric rate of photon absorption (LVRPA) in the reactor was evaluated solving the radiative transfer equation applying the discrete ordinate method and using the optical properties of the catalyst suspensions. The photonic and quantum efficiencies of the 2-chlorophenol degradation depend on both the catalyst load and the iron content of the catalyst. The higher values for these parameters, 0.080 mol Einstein^-1 and 0.152 mol Einstein^-1, respectively, were obtained with 1.0 g L^-1 of the catalyst with the higher iron content (17.6%). For the mineralization process, photonic and quantum efficiencies depend mainly on the catalyst load. Therefore, it was possible to employ a natural and cheap resource from the region to obtain pillared clay-based catalysts to degrade organic pollutants in water.

Photocatalytic degradation of an emerging pollutant by TiO2-coated glass rings: a kinetic study

This work presents the photocatalytic degradation of the pharmaceutical drug clofibric acid in a fixed-bed reactor filled with TiO₂-coated glass rings. Experiments were carried out under UV radiation. A kinetic model that takes into account radiation absorption by means of the local surface rate of photon absorption (LSRPA) has been developed. The LSRPA was obtained from the results of a radiation model. The Monte Carlo method was employed to solve the radiation model, where the interaction between photons and TiO₂-coated rings was considered. Data from experiments carried out with rings with different numbers of catalyst coatings and different irradiation levels were used to estimate the parameters of the kinetic model. A satisfactory agreement was obtained between model simulations and experimental results.

Experimental measurement and modelling of reactive species generation in TiO2 nanoparticle photocatalysis

The generation of reactive species in titanium dioxide (TiO₂) nanoparticle photocatalysis was assessed in a laboratory scale setup, in which P25 Aeroxide TiO₂ suspensions were photoactivated by means of UV-A radiation. Photogenerated holes and hydroxyl radicals were monitored over time by observing their selective reaction with probe compounds, iodide and terephthalic acid, respectively. TiO₂ aggregate size and structure were characterized over the reaction time. Reactive species quenching was then described by a model, accounting for radiative phenomena, TiO₂ nanoparticle aggregation and kinetic reactions. The interaction between iodide and photogenerated holes was influenced by iodide adsorption on TiO₂ surface, described by a Langmuir-Hinshelwood mechanism, whose parameters were studied as a function of TiO₂ concentration and irradiation time. Iodide oxidation was effectively simulated by modelling the reaction volume as a completely stirred two-dimensional domain, in which irradiation phenomena were described by a two-flux model and the steady state for reactive species was assumed. The kinetic parameters for iodide adsorption and oxidation were estimated and successfully validated in a different experimental setup. The same model was adapted to describe the oxidation of terephthalic acid by hydroxyl radicals. The kinetic parameters for terephthalic acid oxidation were estimated and validated, while the issues in investigating the interaction mechanisms among the involved species have been discussed. The sensitivity of operating parameters on model response was assessed and the most relevant parameters were highlighted.

Kinetic modeling of the photocatalytic degradation of clofibric acid in a slurry reactor

A kinetic study of the photocatalytic degradation of the pharmaceutical clofibric acid is presented. Experiments were carried out under UV radiation employing titanium dioxide in water suspension. The main reaction intermediates were identified and quantified. Intrinsic expressions to represent the kinetics of clofibric acid and the main intermediates were derived. The modeling of the radiation field in the reactor was carried out by Monte Carlo simulation. Experimental runs were performed by varying the catalyst concentration and the incident radiation. Kinetic parameters were estimated from the experiments by applying a non-linear regression procedure. Good agreement was obtained between model predictions and experimental data, with an error of 5.9 % in the estimations of the primary pollutant concentration.

Semiconductor photocatalysis--mechanistic and synthetic aspects

Preceding work on photoelectrochemistry at semiconductor single-crystal electrodes has formed the basis for the tremendous growth in the three last decades in the field of photocatalysis at semiconductor powders. The reason for this is the unique ability of inorganic semiconductor surfaces to photocatalyze concerted reduction and oxidation reactions of a large variety of electron-donor and -acceptor substrates. Whereas great attention was paid to water splitting and the exhaustive aerobic degradation of pollutants, only a small amount of research also explored synthetic aspects. After introducing the basic mechanistic principles, standard experiments for the preparation and characterization of visible light active photocatalysts as well as the investigation of reaction mechanisms are discussed. Novel atom-economic C-C and C-N coupling reactions illustrate the relevance of semiconductor photocatalysis for organic synthesis, and demonstrate that the multidisciplinary field combines classical photochemistry with electrochemistry, solid-state chemistry, and heterogeneous catalysis.

Combination of Advanced Oxidation Processes and biological treatments for wastewater decontamination--a review

Nowadays there is a continuously increasing worldwide concern for development of alternative water reuse technologies, mainly focused on agriculture and industry. In this context, Advanced Oxidation Processes (AOPs) are considered a highly competitive water treatment technology for the removal of those organic pollutants not treatable by conventional techniques due to their high chemical stability and/or low biodegradability. Although chemical oxidation for complete mineralization is usually expensive, its combination with a biological treatment is widely reported to reduce operating costs. This paper reviews recent research combining AOPs (as a pre-treatment or post-treatment stage) and bioremediation technologies for the decontamination of a wide range of synthetic and real industrial wastewater. Special emphasis is also placed on recent studies and large-scale combination schemes developed in Mediterranean countries for non-biodegradable wastewater treatment and reuse. The main conclusions arrived at from the overall assessment of the literature are that more work needs to be done on degradation kinetics and reactor modeling of the combined process, and also dynamics of the initial attack on primary contaminants and intermediate species generation. Furthermore, better economic models must be developed to estimate how the cost of this combined process varies with specific industrial wastewater characteristics, the overall decontamination efficiency and the relative cost of the AOP versus biological treatment.

Degradation of methyl orange using short-wavelength UV irradiation with oxygen microbubbles

A novel wastewater treatment technique using 8 W low-pressure mercury lamps in the presence of uniform-sized microbubbles (diameter = 5.79 microm) was investigated for the decomposition of methyl orange as a model compound in aqueous solution. Photodegradation experiments were conducted with a BLB black light blue lamp (365 nm), a UV-C germicidal lamp (254 nm) and an ozone lamp (185 nm+254 nm) both with and without oxygen microbubbles. The results show that the oxygen microbubbles accelerated the decolorization rate of methyl orange under 185+254 nm irradiation. In contrast, the microbubbles under 365 and 254 nm irradiation were unaffected on the decolorization of methyl orange. It was found that the pseudo-zero order decolorization reaction constant in microbubble system is 2.1 times higher than that in conventional large bubble system. Total organic carbon (TOC) reduction rate of methyl orange was greatly enhanced by oxygen microbubble under 185+254 nm irradiation, however, TOC reduction rate by nitrogen microbubble was much slower than that with 185+254 nm irradiation only. Possible reaction mechanisms for the decolorization and mineralization of methyl orange both with oxygen and nitrogen mirobubbles were proposed in this study.

Comparison of H2O2/UV and heterogeneous photocatalytic processes for the degradation of dichloroacetic acid in water

A comparative study between two advanced oxidation technologies for pollutant degradation has been made. With the use of dichloroacetic acid (DCA) as the model pollutant, the reactions with hydrogen peroxide and UV radiation (H2O2/UV, 253.7 nm) and photocatalysis with titanium dioxide (TiO2/UV, 300-400 nm) are analyzed. Three criteria have been selected to compare the performances of both processes: (i) the percentage conversion of DCA and TOC (total organic carbon) at a fixed reaction time; (ii) the quantum efficiency, employing the true radiation absorption rates for both activated species (H2O2 and TiO2); (iii) the specific energy consumption to degrade 50% of the initial TOC. The optimal molar concentration ratio of H2O2/DCA and the optimal catalyst concentration have been employed in the experiments. The results indicate that, under the optimal operating conditions, the H2O2/UV process exhibits, by a large difference, the best performance taking into account the above-mentioned criteria. Nevertheless, both systems show similar values of specific energy consumption when a thinner reactor is employed. These results cannot be safely extrapolated to other contexts if (i) other compounds of different structure are degraded and (ii) a different catalyst is used. Moreover, they were obtained under optimized conditions, and typical, real-life situations may render quite different results due to the robustness of the titanium dioxide operation. They should serve as an indication that, under the studied conditions, a much-improved catalyst performance must be achieved to parallel, with a heterogeneous process, a yield similar to the one obtained with the homogeneous system.