Photocatalytic mineralization of commercial herbicides in a pilot-scale solar CPC reactor: photoreactor modeling and reaction kinetics constants independent of radiation field

Hydrogen peroxide electrogeneration from O2 electroreduction: A review focusing on carbon electrocatalysts and environmental applications

Hydrogen peroxide (H2O2) stands as one of the foremost utilized oxidizing agents in modern times. The established method for its production involves the intricate and costly anthraquinone process. However, a promising alternative pathway is the electrochemical hydrogen peroxide production, accomplished through the oxygen reduction reaction via a 2-electron pathway. This method not only simplifies the production process but also upholds environmental sustainability, especially when compared to the conventional anthraquinone method. In this review paper, recent works from the literature focusing on the 2-electron oxygen reduction reaction promoted by carbon electrocatalysts are summarized. The practical applications of these materials in the treatment of effluents contaminated with different pollutants (drugs, dyes, pesticides, and herbicides) are presented. Water treatment aiming to address these issues can be achieved through advanced oxidation electrochemical processes such as electro-Fenton, solar-electro-Fenton, and photo-electro-Fenton. These processes are discussed in detail in this work and the possible radicals that degrade the pollutants in each case are highlighted. The review broadens its scope to encompass contemporary computational simulations focused on the 2-electron oxygen reduction reaction, employing different models to describe carbon-based electrocatalysts. Finally, perspectives and future challenges in the area of carbon-based electrocatalysts for H2O2 electrogeneration are discussed. This review paper presents a forward-oriented viewpoint of present innovations and pragmatic implementations, delineating forthcoming challenges and prospects of this ever-evolving field.

Investigation of Photocatalyst Composites for Pollutant Degradation in a Microslit Reactor Utilizing High Throughput Screening Techniques

The high-throughput screening investigations on TiO₂ based photocatalyst composites presented here have been carried out in a 60-fold parallel photoreactor. Additional catalyst testing was performed in a microslit reactor system with immobilized catalysts. For further enhancing the photocatalytic activity of TiO₂ (P25), composites of P25 and, for example, Bi₂ O₃ , CeO₂ , g-C₃ N₄ , WO₃ or ZnO were formulated in different nominal molar ratios. The catalysts' performances were assessed by their conversion of 17α-ethinyl estradiol (EE2) in aqueous solutions, determined by LC-MS. Findings show rapid EE2 conversions in short residence times. The extensive testing of catalysts led to the conclusion that the photocatalytic conversion is rather a function of residence time than a function of the materials utilized. This makes adequate process development seem more important than material development. The novelty of this contribution lies in the unique combination of testing a wide range of composite catalysts in a unique microreactor geometry.

Engineering and modeling perspectives on photocatalytic reactors for water treatment

The debate on whether photocatalysis can reach full maturity at commercial level as an effective and economical process for treatment and purification of water and wastewater has recently intensified. Despite a bloom of scientific investigations in the last 30 years, particularly with regards to innovative photocatalytic materials, photocatalysis has so far seen a few industrial applications. Regardless of the points of view, it has been realized that research on reactor design and modeling are now equally urgent to match the extensive research carried out on innovative photocatalytic materials. In reality, the development of photocatalytic reactors has advanced steadily in terms of modeling and reactor design over the last two decades, though this topic has captured a smaller specialized audience. In this critical review, we introduce the latest developments on photocatalytic reactors for water treatment from an engineering perspective. The focus is on the modeling and design of photocatalytic reactors for water treatment at pilot- or at greater scale. Photocatalytic reactors utilizing both natural sunlight and UV irradiation sources are comprehensively discussed. The most promising photoreactor designs and models are examined giving key design guidelines. Other engineering considerations, such as operation, cost analysis, patents, and several industrial applications of photocatalytic reactors for water treatment are also presented. The dissemination of key photocatalytic reactor design principles among the scientific community and the water industry is currently one of the greatest obstacles in translating PWT research into widespread real-world application.

New approach for the dimensionless analysis of a unidirectional flow solar reactor based on Damköhler's number profiles

A methodology for the analysis of the behavior of complex reactors based on the construction of profiles of a dimensionless number (Damköhler) for each main chemical species (Dai) was proposed. A 4-chlorophenol mineralization reaction in a heterogeneous solar reactor with suspended TiO₂ and addition of H₂O₂ with tubular geometry and radiation collectors, fluid flow and a recirculation system was selected as a complex model system in order to validate the approach. The dynamic behavior of the reactor in dimensionless variables was modeled as a function of Dai. Where Dai(z,t) is a local property and grouped the optical and surface's properties of the catalyst, catalyst load, radiation intensity, the photon absorption rate, rate of non-photochemical reactions, the H₂O₂ effect, the reaction rate of different stages like adsorption, attack of radicals, surface reactions, plus design and operation variables like reactor volume and volumetric flow.

A coupling of orthogonal collocation and Runge-Kutta methods were used to solve the PDEs and carry out the simulations to the different experimental conditions, resulting in profiles of Dai, Ci, and conversion in function of time and space. The Dai profiles proposed in the new methodology are capable of describing the disturbances in solar reactors, to indicate consumption and generation rates, instantaneous changes of reaction rate, to describe competitive reactions and quenching effects and to determine equilibrium concentrations, all of the above at each time and space. Therefore, this approach is a analysis tool of reactors which complements the concentration profile. This methodology can be extended to other reactive systems, adapting the intrinsic reaction rates.

3D printed photoreactor with immobilized graphitic carbon nitride: A sustainable platform for solar water purification

Solar-energy-enabled photocatalysis is promising for sustainable water purification. However, photoreactor design, especially immobilizing nano-sized photocatalysts, remains a major barrier preventing industrial-scale application of photocatalysis. In this study, we immobilized photocatalytic graphitic carbon nitride on chitosan to produce g-C₃N₄/chitosan hydrogel beads (GCHBs), and evaluated GCHB photoreactivity for degrading phenol and emerging persistent micropollutants in a 3D printed compound parabolic collector (CPC) reactor. The CPC photocatalytic system showed comparable performance with slurry reactors for sulfamethoxazole and carbamazepine degradation under simulated sunlight, and it maintained the performance for contaminant removal in real water samples collected from water/wastewater treatment plants or under outdoor sunlight irradiation. Global drinking water production was estimated for the CPC system, and it holds promise for small-scale sustainable water treatment, including, but not limited to, the production of high-quality potable water for single houses, small communities, rural areas, and areas impacted by natural disasters in both developed and developing countries.

A MATLAB-Based Application for Modeling and Simulation of Solar Slurry Photocatalytic Reactors for Environmental Applications

Because of the complexity caused by photochemical reactions and radiation transport, accomplishing photoreactor modeling usually poses a barrier for young researchers or research works that focus on experimental developments, although it may be a crucial tool for reducing experimental efforts and carrying out a more comprehensive analysis of the results. This work presents PHOTOREAC, an open-access application developed in the graphical user interface of Matlab, which allows a user-friendly evaluation of the solar photoreactors operation. The app includes several solar photoreactor configurations and kinetics models as well as two variants of a radiation absorption-scattering model. Moreover, PHOTOREAC incorporates a database of 26 of experimental solar photodegradation datasets with a variety of operational conditions (model pollutants, photocatalyst concentrations, initial pollutant concentrations); additionally, users can introduce their new experimental data. The implementation of PHOTOREAC is presented using three example cases of solar photoreactor operation in which the impact of the operational parameters is explored, kinetic constants are estimated according to experimental data, and comparisons are made between the available models. Finally, the impact of the application on young researchers’ projects in photocatalysis at the University of Cartagena was investigated. PHOTOREAC is available upon request from Professor Miguel Mueses.

Reclamation of grey water for non-potable purposes using pilot-scale solar photocatalytic tubular reactors

Application of pilot-scale slurry-type tubular photocatalytic reactor was tested for the decentralized treatment of actual grey water. The reactors were fabricated by reusing the locally available materials at low cost, operated in batch recycle mode with 25 L of grey water. The influence of operational parameters such as catalysts' concentration, initial slurry pH and addition of H₂O₂ on COD abatement were optimized. The results show that Ag-decorated TiO₂ showed a two-fold increase in COD abatement than did pure TiO₂. Better COD abatement was observed under acidic conditions, and addition of H₂O₂ significantly increases the rate of COD abatement. Within 2 h, 99% COD abatement was observed when the reactor was operated with optimum operational conditions. Silver ion lixiviate was also monitored during the experiment and is five times less than the permissible limits. The catalyst shows good stability even after five cycles without much loss in its photocatalytic activity. The results clearly reveal that pilot-scale slurry tubular solar photocatalytic reactors could be used as a cost-effective method to treat grey water and the resulting clean water could be reused for various non-potable purposes, thus conserving precious water resource. This study favours decentralized grey water treatment and possible scaling up of solar photocatalytic reactor using locally available materials for the potential reuse of treated water.

The Technology Horizon for Photocatalytic Water Treatment: Sunrise or Sunset?

Advanced oxidation processes via semiconductor photocatalysis for water treatment have been the subject of extensive research over the past three decades, producing many scientific reports focused on elucidating mechanisms and enhancing kinetics for the treatment of contaminants in water. Many of these reports imply that the ultimate goal of the research is to apply photocatalysis in municipal water treatment operations. However, this ignores immense technology transfer problems, perpetuating a widening gap between academic advocation and industrial application. In this Feature, we undertake a critical examination of the trajectory of photocatalytic water treatment research, assessing the viability of proposed applications and identifying those with the most promising future. Several strategies are proposed for scientists and engineers who aim to support research efforts to bring industrially relevant photocatalytic water treatment processes to fruition. Although the reassessed potential may not live up to initial academic hype, an unfavorable assessment in some areas does not preclude the transfer of photocatalysis for water treatment to other niche applications as the technology retains substantive and unique benefits.

Identification of intermediates, acute toxicity removal, and kinetics investigation to the Ametryn treatment by direct photolysis (UV254), UV254/H2O2, Fenton, and photo-Fenton processes

This paper reports the degradation of 10 mg L^-1 Ametryn solution with different advanced oxidation processes and by ultraviolet (UV₂₅₄) irradiation alone with the main objective of reducing acute toxicity and increase biodegradability. The investigated factors included Fe²⁺ and H₂O₂ concentrations. The effectiveness of the UV₂₅₄ and UV₂₅₄/H₂O₂ processes were investigated using a low-pressure mercury UV lamp (254 nm). Photo-Fenton process was explored using a blacklight blue lamp (BLB, λ = 365 nm). The UV₂₅₄ irradiation process achieved complete degradation of Ametryn solution after 60 min. The degradation time of Ametryn was greatly improved by the addition of H₂O₂. It is worth pointing out that a high rate of Ametryn removal was attained even at low concentrations of H₂O₂. The kinetic constant of the reaction between Ametryn and HO^● for UV₂₅₄/H₂O₂ was 3.53 × 10⁸ L mol^-1 s^-1. The complete Ametryn degradation by the Fenton and photo-Fenton processes was observed following 10 min of reaction for various combinations of Fe²⁺ and H₂O₂ under investigation. Working with the highest concentration (150 mg L^-1 H₂O₂ and 10 mg L^-1 Fe²⁺), around 30 and 70% of TOC removal were reached within 120 min of treatment by Fenton and photo-Fenton processes, respectively. Although it did not obtain complete mineralization, the intermediates formed in the degradation processes were hydroxylated and did not promote acute toxicity of Vibrio fischeri. Furthermore, a substantial improvement of biodegradability was obtained for all studied processes.

Braiding kinetics and spectroscopy in photo-catalysis: the spectro-kinetic approach

The combination of kinetic and spectroscopic tools has become a key scientific methodology for the understanding of catalytic behavior but its application in photocatalysis has inherent difficulties due to the nature of the energy source of the reaction. This review article provides an overview of its use by, first, presenting mechanistically derived kinetic formulations and spectroscopic data handling methods including intrinsic expressions for light and, second, highlighting representative examples of application. To do it we consider universal catalytic systems, particularly (although not exclusively) titania-based materials, and the most frequent hole and/or electron triggered reaction schemes. This review also provides a general framework to pave the way for the future progress of the spectro-kinetic approach in the photocatalysis area.

Trace Organic Pollutant Removal by VUV/UV/chlorine Process: Feasibility Investigation for Drinking Water Treatment on a Mini-Fluidic VUV/UV Photoreaction System and a Pilot Photoreactor

The vacuum-ultraviolet/ultraviolet/chlorine (VUV/UV/chlorine) process, with a VUV/UV mercury lamp used as the light source, was found to be a highly efficient advanced oxidation process (AOP) in a previous study. Hence, its application feasibility for trace organic pollutant removal from drinking water becomes attractive. In this work, a bench-scale mini-fluidic VUV/UV photoreaction system was used to determine the degradation kinetics of sulfamethazine (SMN), a model sulfonamide antibiotic frequently detected with trace levels in aquatic environments. Results indicated that SMN (0.1 mg L^-1) could be degraded rapidly by VUV/UV/chlorine, and a synergism was observed between the VUV/UV and UV/chlorine processes. Photon-fluence based rate constants of SMN degradation were determined to be 6.76 × 10³ and 8.51 × 10³ m² einstein^-1 at chlorine doses of 0.05 and 0.5 mg L^-1, respectively. The presence of natural organic matter in real waters significantly inhibited SMN degradation. In addition, pilot tests were conducted to explore the practical performance of the VUV/UV/chlorine process, thereby allowing electrical energy per order to be calculated for cost evaluation. The effect of flow pattern on photoreactor efficiency was also analyzed by computational fluid dynamics simulations. Both bench- and pilot-scale tests have demonstrated that the VUV/UV/chlorine process, as a new AOP, has potential applications to trace organic pollutant removal in small-scale water treatment.

Photodegradation Efficiencies in a Photo-CREC Water-II Reactor Using Several TiO2 Based Catalysts

This study reports phenol degradation using several TiO2 photocatalysts (DP25, Anatase 1, Hombikat UV-100, Anatase 2) in a Photo-CREC Water-II Reactor. The physicochemical properties of the photocatalysts used, such as crystallinity, superficial area, and pore size distribution are reported. Reactor efficiencies are calculated using both Quantum Yields (QYs) and Photochemical and Thermodynamic Efficiency Factors (PTEFs). This is accomplished using phenol and phenol intermediate photoconversion rates. This allows the determination of hydroxyl radical consumption rates, at every step of the photodegradation process. With these data, and with the absorbed photon rates, energy efficiencies are calculated. It is shown that for the best performing photo catalysts the maximum QYs reach 50 % levels. These favourable photoconversion efficiencies confirm the critical importance of having available highly performing photocatalysts and photoreactors, such is the case of Photo-CREC Water-II Reactor unit.

Modeling the photocatalytic mineralization in water of commercial formulation of estrogens 17-β estradiol (E2) and nomegestrol acetate in contraceptive pills in a solar powered compound parabolic collector

Endocrine disruptors in water are contaminants of emerging concern due to the potential risks they pose to the environment and to the aquatic ecosystems. In this study, a solar photocatalytic treatment process in a pilot-scale compound parabolic collector (CPC) was used to remove commercial estradiol formulations (17-β estradiol and nomegestrol acetate) from water. Photolysis alone degraded up to 50% of estradiol and removed 11% of the total organic carbon (TOC). In contrast, solar photocatalysis degraded up to 57% of estrogens and the TOC removal was 31%, with 0.6 g/L of catalyst load (TiO₂ Aeroxide P-25) and 213.6 ppm of TOC as initial concentration of the commercial estradiols formulation. The adsorption of estrogens over the catalyst was insignificant and was modeled by the Langmuir isotherm. The TOC removal via photocatalysis in the photoreactor was modeled considering the reactor fluid-dynamics, the radiation field, the estrogens mass balance, and a modified Langmuir–Hinshelwood rate law, that was expressed in terms of the rate of photon adsorption. The optimum removal of the estrogens and TOC was achieved at a catalyst concentration of 0.4 g/L in 29 mm diameter tubular CPC reactors which approached the optimum catalyst concentration and optical thickness determined from the modeling of the absorption of solar radiation in the CPC, by the six-flux absorption-scattering model (SFM).

Optimization and application of TiO₂/Ti-Pt photo fuel cell (PFC) to effectively generate electricity and degrade organic pollutants simultaneously

A TiO2/Ti-Pt photo fuel cell (PFC) was established to generate electricity and degrade organic pollutants simultaneously. The electricity generation was optimized through investigation the influences of photoanode calcination temperature and dissolve oxygen on the resistances existing in PFC. TiO2 light quantum yield was also improved in PFC which resulted in a higher PC degradation efficiency. Two kinds of real textile wastewaters were also employed in this PFC system, 62.4% and 50.0% Coulombic efficiency were obtained for 8 h treatment. These refractory wastewaters with high salinity may become good fuels in PFC because a) TiO2 has no selectivity and can degrade nearly any organic substance, b) no more electrolyte is needed due to the high salinity, c) the energy in wastes can be recovered to generate electricity. The electricity generated by the PFC was further applied on a TiO2/Ti rotating disk photoelectrocatalytic reactor. A bias voltage between 0.6 and 0.75 V could be applied and the PC degradation efficiency was significantly improved. This result was similar with that obtained by a 0.7 V DC power.

Solar photocatalytic activity of TiO2 modified with WO3 on the degradation of an organophosphorus pesticide

In this study, the solar photocatalytic activity (SPA) of WO3/TiO2 photocatalysts synthesized by the sol-gel method with two different percentages of WO3 (2 and 5%wt) was evaluated using malathion as a model contaminant. For comparative purpose bare TiO2 was also prepared by sol-gel process. The powders were characterized by X-ray diffraction (XRD), Raman spectroscopy, diffuse reflectance UV-vis spectroscopy (DRUV-vis), specific surface area by the BET method (SSABET), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy with a high annular angle dark field detector (STEM-HAADF). The XRD, Raman, HRTEM and STEM-HAADF analyses indicated that WO3 was present as a monoclinic crystalline phase with nanometric cluster sizes (1.1 ± 0.1 nm for 2% WO3/TiO2 and 1.35 ± 0.3 nm for 5% WO3/TiO2) and uniformly dispersed on the surface of TiO2. The particle size of the materials was 19.4 ± 3.3 nm and 25.6 ± 3 nm for 2% and 5% WO3/TiO2, respectively. The SPA was evaluated on the degradation of commercial malathion pesticide using natural solar light. The 2% WO3/TiO2 photocatalyst exhibited the best photocatalytic activity achieving 76% of total organic carbon (TOC) abatement after 300 min compared to the 5% WO3/TiO2 and bare TiO2 photocatalysts, which achieved 28 and 47% mineralization, respectively. Finally, experiments were performed to assess 2% WO3/TiO2 catalyst activity on repeated uses; after several successive cycles its photocatalytic activity was retained showing long-term stability.

Photocatalytic degradation of water contaminants in multiple photoreactors and evaluation of reaction kinetic constants independent of photon absorption, irradiance, reactor geometry, and hydrodynamics

The literature on photocatalytic oxidation of water pollutants often reports reaction kinetic constants, which cannot be unraveled from photoreactor type and experimental conditions. This study addresses this challenging aspect by presenting a general and simple methodology for the evaluation of fundamental "intrinsic" reaction kinetic constants of photocatalytic degradation of water contaminants, which are independent of photoreactor type, catalyst concentration, irradiance levels, and hydrodynamics. The degradation of the model contaminant, oxalic acid (OA) on titanium dioxide (TiO2) aqueous suspensions, was monitored in two annular photoreactors (PR1 and PR2). The photoreactors with significantly different geometries were operated under different hydrodynamic regimes (turbulent batch mode and laminar flow-through recirculation mode), optical thicknesses, catalyst and OA concentrations, and photon irradiances. The local volumetric rate of photon absorption (LVRPA) was evaluated by the six-flux radiation absorption-scattering model (SFM). The SFM was further combined with a comprehensive kinetic model for the adsorption and photodecomposition of OA on TiO2 to determine local reaction rates and, after integration over the reactor volume, the intrinsic reaction kinetic constants. The model could determine the oxidation of OA in both PR1 and PR2 under a wide range of experimental conditions. This study demonstrates a more meaningful way for determining reaction kinetic constants of photocatalytic degradation of water contaminants.

Two-Dimensional Modeling of an Externally Irradiated Slurry Photoreactor

A batch cylindrical photocatalytic reactor, externally irradiated by 1–6 UV fluorescent lamps and containing a stirred slurry of polycrystalline TiO2, was modeled by coupling a modified Langmuir–Hinshelwood kinetics together with a two-dimensional light intensity field. The radiation field has been determined on the main assumptions of diffuse radiation, isotropic scattering and negligible backward reflected photon flow. The model has been applied to the photocatalytic oxidation of organic substrates which do not undergo homogeneous photochemical degradation. The model is characterized by the following four parameters: the kinetic constants of substrate adsorption, desorption and degradation and the exponent of the power law expressing the kinetics dependence on the light intensity. The model constants may be determined by applying a simple least-squares best fitting procedure.

Molecularly imprinted photocatalyst with a structural analogue of template and its application

To realize selective mineralization of low-level chlorophenols (CPs) in the presence of high-level ordinary pollutants, molecularly imprinted polymers (MIPs) coated photocatalyst was prepared using substrate analog as template. The pseudo-template imprinted photocatalysts showed rapid decomposition ability toward a group of CPs. Based on the complete dechlorination and spectrophotometry, a new method was proposed to detect the total organochlorine on CPs in water samples. The method showed good linearity when the concentrations of the total organochlorine on CPs is in the range of 12.0-200.0μmolL(-1). The detection limit is 1μmolL(-1) for this method. When this method was applied to measure the total organochlorine of the CPs in both tap water and river water samples, an average recovery ranged from 96.3% to 105.1% was obtained with RSD values less than 5%. In this green and simple method, the common inorganic ions in water showed no interference for the detection. The determination of the total organochlorine on the CPs might be used for estimation of the toxicity and the persistence of the water samples.

Highly efficient visible light TiO2 photocatalyst prepared by sol-gel method at temperatures lower than 300°C

Highly efficient visible light TiO(2) photocatalyst was prepared by the sol-gel method at lower temperature (≤ 300°C), and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), UV-vis diffuse reflectance spectroscopy (UV-vis DRS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and differential scanning calorimetry-thermogravimetric analysis (DSC-TGA). The effects of the heat treatment temperature and time of the as-prepared TiO(2) on its visible light photocatalytic activity were investigated by monitoring the degradation of methyl orange solution under visible light irradiation (wavelength ≥ 400 nm). Results show that the as-prepared TiO(2) nanoparticles possess an anatase phase and mesoporous structure with carbon self-doping and visible photosensitive organic groups. The visible light photocatalytic activity of the as-prepared TiO(2) is greatly higher than those of the commercial TiO(2) (P-25) and other visible photocatalysts reported in literature (such as PPy/TiO(2), P3HT/TiO(2), PANI/TiO(2), N-TiO(2) and Fe(3+)-TiO(2)) and its photocatalytic stability is excellent. The reasons for improving the visible light photocatalytic activity of the as-prepared TiO(2) can be explained by carbon self-doping and a large amount of visible photosensitive groups existing in the as-prepared TiO(2). The apparent optical thickness (τ(app)), local volumetric rate of photo absorption (LVRPA) and kinetic constant (k(T)) of the photodegradation system were calculated.

Radiation absorption and optimization of solar photocatalytic reactors for environmental applications

This study provides a systematic and quantitative approach to the analysis and optimization of solar photocatalytic reactors utilized in environmental applications such as pollutant remediation and conversion of biomass (waste) to hydrogen. Ray tracing technique was coupled with the six-flux absorption scattering model (SFM) to analyze the complex radiation field in solar compound parabolic collectors (CPC) and tubular photoreactors. The absorption of solar radiation represented by the spatial distribution of the local volumetric rate of photon absorption (LVRPA) depends strongly on catalyst loading and geometry. The total radiation absorbed in the reactors, the volumetric rate of absorption (VRPA), was analyzed as a function of the optical properties (scattering albedo) of the photocatalyst. The VRPA reached maxima at specific catalyst concentrations in close agreement with literature experimental studies. The CPC has on average 70% higher photon absorption efficiency than a tubular reactor and requires 39% less catalyst to operate under optimum conditions. The "apparent optical thickness" is proposed as a new dimensionless parameter for optimization of CPC and tubular reactors. It removes the dependence of the optimum catalyst concentration on tube diameter and photocatalyst scattering albedo. For titanium dioxide (TiO(2)) Degussa P25, maximum photon absorption occurs at apparent optical thicknesses of 7.78 for CPC and 12.97 for tubular reactors.