Advanced photocatalysis as a viable and sustainable wastewater treatment process: A comprehensive review

In our era, water pollution not only poses a serious threat to human, animal, and biotic life but also causes serious damage to infrastructure and the ecosystem. A set of physical, chemical, and biological technologies have been exploited to decontaminate and/or disinfect water pollutants, toxins, microbes, and contaminants, but none of these could be ranked as sustainable and scalable wastewater technology. The photocatalytic process can harmonize the sunlight to degrade certain toxins, chemicals, microbes, and antibiotics, present in water. For example, transition metal oxides (ZnO, SnO2, TiO2, etc.), when integrated into an organic framework of graphene or nitrides, can bring about more than 90% removal of dyes, microbial load, pesticides, and antibiotics. Similarly, a modified network of graphitic carbon nitride can completely decontaminate petrochemicals. The present review will primarily highlight the mechanistic aspects for the removal and/or degradation of highly concerned contaminants, factors affecting photocatalysis, engineering designs of photoreactors, and pros and cons of various wastewater treatment technologies already in practice. The photocatalytic reactor can be a more viable and sustainable wastewater treatment opportunity. We hope the researcher will find a handful of information regarding the advanced oxidation process accomplished via photocatalysis and the benefits associated with the photocatalytic-type degradation of water pollutants and contaminants.

TiO2 based Photocatalysis membranes: An efficient strategy for pharmaceutical mineralization

Among the various emerging contaminants, pharmaceuticals (PhACs) seem to have adverse effects on the quality of water. Even the smallest concentration of PhACs in ground water and drinking water is harmful to humans and aquatic species. Among all the deaths reported due to COVID-19, the mortality rate was higher for those patients who consumed antibiotics. Consequently, PhAC in water is a serious concern and their removal needs immediate attention. This study has focused on the PhACs' degradation by collaborating photocatalysis with membrane filtration. TiO₂-based photocatalytic membrane is an innovative strategy which demonstrates mineralization of PhACs as a safer option. To highlight the same, an emphasis on the preparation and reinforcing properties of TiO₂-based nanomembranes has been elaborated in this review. Further, mineralization of antibiotics or cytostatic compounds and their degradation mechanisms is also highlighted using TiO₂ assisted membrane photocatalysis. Experimental reactor configurations have been discussed for commercial implementation of photoreactors for PhAC degradation anchored photocatalytic nanomembranes. Challenges and future perspectives are emphasized in order to design a nanomembrane based prototype in future for wastewater management.

Recent advances, influencing factors, and future research prospects using photocatalytic process for produced water treatment

Oilfield-produced water is the primary by-product generated during oil and gas extraction operations. Oilfield-produced water is often severely toxic and poses substantial health, safety, and environmental issues; adequate treatment technologies can bring these streams to a quality level. Photocatalysis is a photochemical catalytic reaction that is a highly promising tool for environmental remediation due to its efficiency in mineralizing persistent and potentially toxic contaminants. However, there is limited understanding of its application to treat oilfield-produced water with a complex and highly variable water composition. This review article discusses the mechanisms and current state of heterogeneous photocatalytic systems for oilfield-produced water treatment, highlighting impediments to knowledge transfer, including the feasibility of practical applications and the identification of essential research requirements. Additionally, the effects of significant variables such as catalyst quantity, pH, organic compound concentration, light intensity, and wavelength are discussed in detail. Some solutions are proposed for scientists and engineers interested in advancing the development of industrial-scale photocatalytic water treatment technologies.

Radiative transfer in a Solar CPC Photoreactor using the First-Order Scattering Method

This manuscript analyzes the suitability of a recently proposed numerical method, the First-Order Scattering Method (FOS), to describe radiation transfer in a Solar Compound Parabolic Collector Photoreactor (CPCP). The study considers five different irradiance conditions ranging from fully diffuse to fully direct solar radiation, with 90 and 45° angled rays. Three photocatalysts at different loadings were considered: Evonik P25, Graphene Oxide, and Goethite, selected due to (1) their relevance in photocatalytic applications and (2) the availability of optical transport properties in the open literature. The study shows that the method is efficient and free of statistical noise, while its accuracy is not affected by the boundary condition’s complexity. The method’s accuracy is very high for photocatalysts with low to moderate albedos, such as Goethite and Graphene Oxide, displaying Normalized Absoluted Mean Error below 3%, i.e., comparable to the Monte Carlo (MC) Method’s statistical fluctuations.

Computer Simulations of Photocatalytic Reactors

Photocatalysis has been considered future technology for green energy conversion and environmental purification, including carbon dioxide reduction, water splitting, air/water treatment, and antimicrobial purposes. Although various photocatalysts with high activity and stability have already been found, the commercialization of photocatalytic processes seems to be slow; it is thought that the difficulty in scaling up photocatalytic processes might be responsible. Research on the design of photocatalytic reactors using computer simulations has been recently intensive. The computer simulations involve various methods of hydrodynamics, radiation, and mass transport analysis, including the Monte Carlo method, the approximation approach–P1 model, and computational fluid dynamics as a complex simulation tool. This review presents all of these models, which might be efficiently used for the scaling-up of photocatalytic reactors. The challenging aspects and perspectives of computer simulation are also addressed for the future development of applied photocatalysis.

Photocatalytic degradation efficiency of hazardous macrolide compounds using an external UV-light irradiation slurry reactor

The current work investigates the removal of two hazardous macrolide molecules, spiramycin and tylosin, by photodegradation under external UV-light irradiation conditions in a slurry photoreactor using titanium dioxide as a catalyst. The kinetics of degradation and effects of main process parameters such as catalyst dosage, initial macrolide concentration, light intensity and stirring rate on the degradation rate of pollutants have been examined in detail in order to obtain the optimum operational conditions. It was found that the process followed a pseudo first-order kinetics according to the Langmuir-Hinshelwood model. The optimum conditions for the degradation of spiramycin and tylosin were low compound concentration, 1 g L^-1 of catalyst dosage, 100 W m^-2 light intensity and 560 rpm stirring rate. Then, a maximum removal (more than 90%) was obtained after 300 min of irradiation time. Furthermore, results show that the selection of optimized operational parameters leads to satisfactory total organic carbon removal rate (up to 51%) and biochemical oxygen demand to chemical oxygen demand ratio (∼1) confirming the good potential of this technique to remove complex macrolides from aqueous solutions.

Semiconductor Electrode Materials Applied in Photoelectrocatalytic Wastewater Treatment—an Overview

Industrial sources of environmental pollution generate huge amounts of industrial wastewater containing various recalcitrant organic and inorganic pollutants that are hazardous to the environment. On the other hand, industrial wastewater can be regarded as a prospective source of fresh water, energy, and valuable raw materials. Conventional sewage treatment systems are often not efficient enough for the complete degradation of pollutants and they are characterized by high energy consumption. Moreover, the chemical energy that is stored in the wastewater is wasted. A solution to these problems is an application of photoelectrocatalytic treatment methods, especially when they are coupled with energy generation. The paper presents a general overview of the semiconductor materials applied as photoelectrodes in the treatment of various pollutants. The fundamentals of photoelectrocatalytic reactions and the mechanism of pollutants treatment as well as parameters affecting the treatment process are presented. Examples of different semiconductor photoelectrodes that are applied in treatment processes are described in order to present the strengths and weaknesses of the photoelectrocatalytic treatment of industrial wastewater. This overview is an addition to the existing knowledge with a particular focus on the main experimental conditions employed in the photoelectrocatalytic degradation of various pollutants with the application of semiconductor photoelectrodes.

Novel photobioreactor design for the culture of Dunaliella tertiolecta - Impact of color in the growth of microalgae

Microalgae are affected by the amount of light received. This parameter can be controlled by changing the light source and altering the reactor used for their growth. In this study, the effect of different colors of light was analyzed in the growth of Dunaliella tertiolecta, observing that blue lighting systems reached a biomass 10 times superior to the one generated by orange lightning systems. This growth effect was seen in a novel tubular internally illuminated photobioreactor. In this photobioreactor, the blue reactor produced 1.7 times the biomass of the red reactor, with the particularity that the latter showed an oscillating behavior in its growth. From irradiance models, the light dispersion coefficient is higher than the absorption coefficient when using red light. In contrast, with blue light, the value of the scattering coefficient is almost null.

A review on the design and development of photocatalyst synthesis and application in microfluidic reactors: challenges and opportunities

Microfluidics is an emerging branch of science that has significant applications in various fields. In this review paper, after a brief introduction to the concept of photocatalysis, nanoparticle preparation methods and film formation techniques have been studied. Nanoparticle synthesis in microfluidic systems and microreactor types for on-chip photocatalyst synthesis and challenges of nanoparticles handling in microsystems have been reviewed. To resolve particle polydispersity and microchannel clogging, a good suggestion can be the use of droplet-based microreactors. The configurative designs for the microfluidic reactor with immobilized photocatalysts, their applications, and their challenges have been comprehensively addressed. The three main challenges ahead the immobilized photocatalytic microfluidic reactors are optimal light distribution, prevention of the recombination of photogenerated electrons and holes, and improved mass transfer. Internal light-emitting diodes with a waveguide can resolve the number one challenge of photocatalysis application in optofluidic reactors, that is, light distribution.

Solar photocatalytic reclamation of agro-waste water polluted with twelve pesticides for agricultural reuse

This study aims to demonstrate a technically feasible alternative to remove pesticide residues from agro-waste water produced in farms from remnants in containers and treatment tanks, rinse in tanks after treatments, and cleaning of machines and equipment. For this, the photocatalyzed degradation of 12 pesticides commonly used on vegetables, vines, citrus and fruit crops was investigated in aqueous suspensions of TiO₂ in tandem with Na₂S₂O₈ at pilot plant scale under natural sunlight in Murcia (SE of Spain) during summer and winter seasons. Previously, preliminary experiments were carried out at laboratory scale using a photoreactor to optimize the photocatalyst (200 mg L^-1) and oxidant (250 mg L^-1) concentrations on the rate constants of the studied pesticides. The photodegradation of all pesticides can be modelled assuming a pseudo-first-order kinetics. The time needed for disappearance of 90% (DT₉₀) of the studied pesticides, was lower than 4 h in summer in all cases with the exception of cyproconazole (4.9 h), while, cyproconazole (8.9 h), metalaxil (6.1 h) and propyzamide (7.9 h) showed DT₉₀ higher than 6 h in winter. The reaction rate was enhanced 3-fold in summer season, which is directly correlated to the higher accumulated fluence per time received during this season (about a factor of 2.9 higher than in winter). In both cases, the higher and lower degradation rates were obtained for cyprodinil and cyproconazole, respectively. The total fluence to get a 90% reduction (H₉₀) ranged from 4.6 to 5.2 J cm^-2 (cyprodinil) to 71.5-76 J cm^-2 (cyproconazole).

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.

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.

Applications of Continuous-Flow Photochemistry in Organic Synthesis, Material Science, and Water Treatment

Continuous-flow photochemistry in microreactors receives a lot of attention from researchers in academia and industry as this technology provides reduced reaction times, higher selectivities, straightforward scalability, and the possibility to safely use hazardous intermediates and gaseous reactants. In this review, an up-to-date overview is given of photochemical transformations in continuous-flow reactors, including applications in organic synthesis, material science, and water treatment. In addition, the advantages of continuous-flow photochemistry are pointed out and a thorough comparison with batch processing is presented.

Nanoparticle scattering characterization and mechanistic modelling of UV-TiO2 photocatalytic reactors using computational fluid dynamics

A computational fluid dynamic (CFD) model was developed to describe the process performance of a semi-batch annular TiO2-UV photoreactor in an Eulerian framework. The model accounted for the optical behaviour of titanium dioxide (TiO2) suspensions, the flow distribution and the oxalic acid degradation in the reactor. The scattering component of the optical model, explicitly included in the CFD simulations using a TiO2-specific scattering phase function integrated in the radiative transfer equation, was calibrated using an optical goniometer by comparing simulated scattering light profiles against irradiance measurements collected for various TiO2 concentrations and UV wavelengths and subsequently solved by the discrete ordinate (DO) radiation model. Several scattering phase functions were tested against the goniometric measurements confirming that the Henyey-Greenstein (HG) equation was the most appropriate angular distribution function at 254 and 355 nm, irrespective of the TiO2 concentration. Using the calibrated HG function, a new approach for quantifying the absolute values of absorption and scattering coefficients in TiO2 suspensions was proposed. It consists of iteratively solving, using the DO model, the radiative transfer equation for various combinations of absorption and scattering coefficients until the error between observed and predicted angular irradiance measurements is minimized. The accuracy of the optical parameters was verified with independent CFD simulations carried out for an annular photoreactor and already available in the literature. Predicted and simulated irradiance and oxalic acid degradation data were found to be in excellent agreement, confirming the considerable potential of the integrated modelling approach presented in this paper for the design, optimization and scale-up of photocatalytic technologies for water and wastewater treatment applications.

Model of Reactive Transport within a Light Photocatalytic Textile

This paper deals with the 3D-modeling of the reactive transport within a light photocatalytic textile used to decontaminate industrial effluents. The model consists of the coupling of fluid flow governing equations, species convection diffusion equations and a heterogeneous reaction equation. It is solved numerically on a Representative Volume Element (RVE) of the textile, i.e. at the microscopic scale regarding the industrial photocatalytic reactor using Comsol Multiphysics software. In a preliminary approach, the reactive transport model was first applied in a 2D simple geometry to verify its accuracy in terms of mass balance of the species. Then successive simulations using pseudo-periodic boundary conditions were performed in the RVE and the depollution efficiency along the textile length is analysed in terms of pollutant concentration. A sensitivity analysis was done to reveal the relative importance of the kinetic and hydrodynamic parameters in prediction of pollutant concentration fields in the RVE. It was found that a high adsorption rate associated with a low permeable fabric maximizes the amount of treated fluid. Finally the performances of a typical reactor composed of a stack of textiles were investigated. Results show a significant improvement of depollution efficiency of this particular configuration compared to single textiles in parallel.

High-Throughput Synthesis and Screening of Titania-Based Photocatalysts

Titanium dioxide is widely known as a prominent photocatalyst material and research in this area has increased substantially over the last decades. However, the photoactivity of TiO2 is hindered by several factors, such as a relatively high photogenerated electron-hole recombination rate and a wide bandgap of ∼ 3.2 eV, rendering it inactive under visible light. Approaches to optimize the TiO2 photocatalyst, either by altering its morphological or chemical properties, have been conducted for many years, yet further modification of this semiconductor has the potential to yield photocatalysts with excellent properties and higher photocatalytic activity. This could be effectively explored using combinatorial synthesis coupled with high-throughput characterization approaches. Such an approach has been widely applied for the discovery of new functional materials, including photocatalysts. By using high-throughput synthesis and characterization technology, preparation and screening of materials on small sample scales can be accelerated; hence, new TiO2-based photocatalysts with enhanced photocatalytic activity can be acquired more rapidly. Additionally, the large database of materials being systematically examined will greatly build our fundamental understanding of the relation between materials structure/composition and photocatalytic activity. This review details various high-throughput syntheses and characterization techniques applied to improve the photocatalytic properties of TiO2 materials and discuss several challenges that have been raised or may be encountered in the future when using this approach.

Optical fluence modelling for ultraviolet light emitting diode-based water treatment systems

This work presents a validated optical fluence rate model optimised for ultraviolet light-emitting diodes (UV-LEDs), which allow a very wide range of emission wavelengths and source geometries to be used in water treatment units. The model is based on a Monte Carlo approach, in which an incremental ray-tracing algorithm is used to calculate the local volumetric rate of energy absorption and subsequently convert it to the local fluence rate distribution for an UV-LED water treatment chamber of arbitrary design. The model includes contributions from optical reflections and scattering by treatment chamber walls and from scattering due to particulates and/or microorganisms. The model successfully predicts optical fluence rates in point-of-use water treatment units, as verified using biodosimetry with MS-2 bacteriophage at a UV-LED emission wavelength of 254 nm. The effects of chamber geometry are also modelled effectively and are consistent with the inactivation data for E. coli at 254 nm. The data indicate that this model is suitable for application in the design and optimisation of UV-LED-based water treatment systems.

Photochemical transformations accelerated in continuous-flow reactors: basic concepts and applications

Continuous-flow photochemistry is used increasingly by researchers in academia and industry to facilitate photochemical processes and their subsequent scale-up. However, without detailed knowledge concerning the engineering aspects of photochemistry, it can be quite challenging to develop a suitable photochemical microreactor for a given reaction. In this review, we provide an up-to-date overview of both technological and chemical aspects associated with photochemical processes in microreactors. Important design considerations, such as light sources, material selection, and solvent constraints are discussed. In addition, a detailed description of photon and mass-transfer phenomena in microreactors is made and fundamental principles are deduced for making a judicious choice for a suitable photomicroreactor. The advantages of microreactor technology for photochemistry are described for UV and visible-light driven photochemical processes and are compared with their batch counterparts. In addition, different scale-up strategies and limitations of continuous-flow microreactors are discussed.