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This technology could fall into two primary subgroups: homogeneous and heterogeneous photocatalytic systems. Homogeneous systems are broadly employed due to their high reaction rate, short process time, and high accessibility, yet they present some critical drawbacks such as high cost, cumbersome extraction, complicated recycling procedures, and reactor corrosion (Ali et al., 2014; Cui et al., 2018). Moreover, implementing process-engineering strategies in photocatalytic processes favors heterogeneous photocatalysis due to the higher stability and ease of recovering heterogeneous photocatalysts.

Generally, photocatalysts are semiconductor (SC) materials that are photoactive. When an SC is irradiated by a light source of photonic energy (hv) larger than that of its bandgap (E_bg), an excited electron shifts from the valence band (VB) to the vacant conduction band (CB), and an electron (e⁻)–hole (h⁺) pair is created (Zhang et al., 2018). Consequently, the nature of the light source critically affects the concentration of (e⁻)–(h⁺) pairs and therefore the effectiveness of the system. Regardless of the type of pollutants, molecules, or MPs particles, the principle of photocatalytic degradation is quite similar and mainly grounded on the oxidation by ROS to transform the toxic product into less harmful components (Kim et al., 2022). The steps of such a mechanism are explained in Figure 3. As seen, while a part of the photo-generated electron–holes (1) recombine within 10 to 100 ns through radiative (photoluminescence) or non-radiative (energy release) processes (5), the rest generate a charge carrier capable of spatially separating on the catalytic surface (2) (Alfaifi et al., 2018; Hoffmann et al., 1995 Habib et al., 2020; Danish et al., 2021).

Upon reaching the surface, these electron–hole pairs could either recombine again (6) or undergo electronic transfers with the localized states of the adsorbed molecules (3),(4). In this regard, the protons’ reduction [E_NHE (H⁺/H₂) = 0.0 eV] and water oxidation [E_NHE (O₂/H₂O) = 1.23 eV] are the main reference energy levels that acquire a particular relevance concerning most SCs (as seen in Figure S1) (Tong et al., 2012).

Generally, metal oxide SCs have their valence bands with the main contribution from oxygen 2p orbitals, at 1–3 eV below the O₂/H₂O redox couple, while transition metal oxides with conduction bands mainly constituted by d orbitals of the metal have the position of their s orbital close to, or below the H₂O reduction potential (Coronado, 2013). Thus, in aqueous media, a photo-activated SC enables the generation of reactive oxygen species (ROS) such as hydroxyl radical (^•OH) and superoxide radical ( O 2 · − ) , which exhibit strong oxidation of organic matters, leading to the total mineralization of recalcitrant organic particles into less harmful products such as H₂O and CO₂ (Ganguly et al., 2019; Habib et al., 2020; Saravanan et al., 2020).

SC photocatalysts can be classified into three main categories: single, double (binary), and three (ternary) components (Figure 4). The usage of classical single components shows certain limits related to the large amount of electron–hole recombination, low absorption of the visible region, and poor conduction of the charge carrier (Pawar and Lee, 2015). Although binary and ternary photocatalysts have been showing a better performance compared to the single components, they present many downsides when used alone (Xu et al., 2019).

For instance, SCs with sulfides or nitrides (binary SC) possess a narrow bandgap but are unstable in the aqueous medium (Mishra and Chun, 2015). Moreover, using metal oxides in their pure form presents limited adsorption capacity, lower recovery, reusability, and higher recombination rates (Mohamed et al., 2020). For example, TiO₂-based SCs have been studied, and widely used for photocatalytic applications, yet, they present poor quantum yield due to the fast recombination rate of photogenerated electron–hole pairs, poor and selective adsorption, and short-lived photo-generated carriers (Adekoya et al., 2017; Lum et al., 2019). Another broadly used metal oxide, zinc oxide (ZnO), is prone to be soluble in both alkaline and acidic medium (Saravanan et al., 2020). In addition, certain ternary SCs have an enough narrow E_bg that renders them active under visible light, yet they do not possess appropriate band edge potentials to perform diverse reactions (Xu et al., 2019). For this reason, optimum formulations of performant photocatalytic systems can be achieved by either coupling large bandgap binary/ternary SCs with narrower bandgap binary/ternary ones, or coupling modified/unmodified SCs into nanocomposites, or even by adopting the two-step photoexcitation system known as Z-scheme (1st, 2nd, and 3rd generations) (Yadav et al., 2018; Xu et al., 2019; Chung et al., 2019).

The generation of stable hydroxo-iron(III) complexes such as [Fe(OOH)]²⁺ and [Fe(OH)]²⁺ by the interaction of Fe³⁺ ions with H₂O₂ and OH⁻ results in a poor regeneration of iron(III) ions (Rn. 3,4) (Ahmed et al., 2011):

Herein, this rate could be increased by the introduction of a UV-vis (<580 nm) irradiation source, thus reducing sludge formation and maintaining the oxidation/reduction cycle in a step that produces greater amounts of ^•OH radicals (Rn. 5) (Babuponnusami and Muthukumar, 2014; Lee et al., 2014).

Nevertheless, several key parameters have to be considered including (i) the initial concentration of recalcitrant molecules, (ii) the initial concentration of Fenton reagents, (iii) the operating temperature, and, most critically, (iv) the pH (Hamd and Dutta, 2020). For instance, at pH > 3.5, various aspects are noticed:

The advantage of the heterogeneous photo-Fenton processes relies upon extending the medium pH towards more practical ranges because catalytic reactions take place at the active region on the solid catalyst’s surface, therefore reducing the leaching of iron ions, decreasing the production of the iron sludge, and permitting the reusability of the catalyst (Liu et al., 2017; Hamd and Dutta, 2020).

Basically, two main interfacial mechanisms were proposed based on the two-stage degradation kinetic process (Figure S2) (He et al., 2015; Hamd and Dutta, 2020):

In heterogeneous Fenton systems, hydroxyl radical ^•OH (the main oxidant species) is commonly produced either from heterogeneous catalytic reactions between the surficial Fe^II (≡Fe^II) and H₂O₂ (Rn. 8) (Lin and Gurol, 1998) or from the outer-sphere reaction of uncomplexed Fe²⁺ ions with H₂O₂ (Rn. 10) (Remucal and Sedlak, 2011):

As for other oxidants such as hydroperoxyl radical ( HO 2 · ) and superoxide anion ( O 2 · − ) , they are produced by reactions involving surface-complexed Fe^III (≡Fe^III − OH) and H₂O₂, ^•OH and H₂O₂, in addition to reactions involving carbon-centered R^• and oxygen (Rn. 9-11) (Santos-Juanes et al., 2011; He et al., 2015).

Owing to the proportionality of the catalytic decomposition rate of hydrogen peroxide to its own concentration and the surface area of the goethite as well (Huang et al., 2001; Kwan and Voelker, 2003), the proposed mechanism of H₂O₂ decomposition by Lin and Gurol seems to be the most reasonable (Rn. 12-18) (Lin and Gurol, 1998):

Besides these reactions, coupling the system with solar irradiation, photocatalysis could occur owing to the narrow bandgaps (2.0–2.3 eV) of iron oxide (Gulshan et al., 2010). The introduction of sunlight to heterogeneous photo-Fenton processes was found to be beneficial compared to UV light sources due to the noticeable decrease in operating expenses and the photolysis of ferric complexes, which takes place at wavelengths of approximately 600 nm and provides the catalytic medium with a higher number of ^•OH radicals (Hamd and Dutta, 2020).

Elemental composition is an analytic tool for identifying the composition of copolymers, plastic blends, and molecular weight. Each plastic material has a characteristic molecular weight, chemical composition, and binding state that may alter after photocatalytic degradation.

Many studies have determined molecular weight through tools like gel permeation chromatography (GPC), nuclear magnetic resonance (NMR), viscosimetry, and osmometry, which have limited application to only soluble plastics. GPC, also known as size exclusion chromatography, is a fast, efficient, and most commonly used technique that measures a wide range of molecular weight and polydispersity index (De los Santos-Villarreal and Elizalde, 2013). Liang et al. (2013) conducted a high-temperature GPC analysis to determine the fluctuations in the molecular weight of the LDPE film before and after UV irradiation in which the temperature of the GPC injector, column, and the detector was kept at 150°C (Liang et al., 2013). 1,2,4-trichlorobenzene was employed in the mobile phase with an elution rate of 1.00 ml/min. The photo-irradiation of LDPE composites decreased the molecular weight to smaller values due to the production of low-molecular-weight compounds.

NMR spectroscopy is comparatively expensive but accurate, providing information (both qualitative and quantitative) on molecular structure, molecular weights, molecular weight distribution, chain orientation, and molecular motion of any unknown MP that is incomparable to other methods (De los Santos-Villarreal and Elizalde, 2013; Carraher, 2018). In a work carried out by Izunobi and Higginbotham et al. (2011), R-methoxy-ω-aminopolyethylene glycol and R-methoxy-polyethylene glycol-block-poly-ϵ-(benzyloxycarbonyl)-L-lysine were dissolved in N-deuterated dimethyl sulfoxide (DMSO-d6); the solution was observed on a spectrometer (499.8 MHz) at 60°C to obtain NMR spectra (Izunobi and Higginbotham, 2011). They mentioned the inability of determining weight-average molecular weight (M_w) unlike the GPC method; therefore, its potential in molecular weight determination is not appreciated.

In contrast, osmometry and viscosimetry are relatively cheaper, less efficient, and suitable for MPs having high- and low-molecular-weight determination, respectively (De los Santos-Villarreal and Elizalde, 2013; Carraher, 2018).

Tools like x-ray photoelectron spectroscopy (XPS) has a broad application in detecting relative composition, depth profiling, and chemical state from the surface of MPs. Such a system is expensive, requires high vacuum conditions, and provides only surface information. Yet, XPS provides straightforward and non-destructive surface characterization information of chemical bonding, and the total elemental composition of any MP. Each element demonstrates characteristic peaks and relative abundance when exposed to x-rays (Andrade, 1985). Celasco et al. (2014) used XPS for measuring surface texture and copolymer thickness in polyethylene glycol (PEG) nanocapsules prepared by solvent displacement method (Celasco et al., 2014). Miao et al. (2020) analyzed the elemental composition of PVC MPs before and after electrocatalytic oxidation using high-resolution XPS (Miao et al., 2020). The PVC surface morphology transformed significantly as identified by the change in the number of C, O, and Cl atoms.

The photodegradation of MPs generally starts at the external bulk surface and gradually goes deep (Tofa et al., 2019). Therefore, it is of immense importance to investigate the surface condition to understand the effects of degradation on the mechanical and physical properties of MPs. There are numerous techniques and tools for a surface analysis whose usability depends on different purposes. Here, we discuss a few conventional optical techniques and their serviceability in characterizing synthetic plastic materials.

A simple optical microscope has been a common technique to study the surface topography of a bulk structure in a reflected light mode. Tofa et al. (2019) (Tofa et al., 2019), Llorente-García et al. (2020) (Llorente-García et al., 2020), Uheida et al. (2020) (Uheida et al., 2020), and Vital-Grappin et al. (2021) (Vital-Grappin et al., 2021) used optical microscopes to observe the surface texture after the exposure. An optical microscope has underlying limitations of high magnification and restricted sample thickness between 5 and 40 mm. Digital imaging with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning probe microscopies are often used for image analysis and image processing (Galloway et al., 2002; Bonnet, 2004).

SEM offers both higher focus and larger depth of field and is a widely applicable technique for determining surface morphology, texture, and composition. A focused beam containing electrons is exerted to reach nano-scale imaging. MPs’ surfaces must be coated with conductive materials to obtain any acceptable image (Carraher, 2018). SEM has been used to determine wrinkles, cracks, and cavities of varying sizes on commercial grade LDPE films as a result of photo-oxidation (Tofa et al., 2019). Liang et al. (2013) used SEM to display surface morphology and the chalking phenomenon of LDPE composite films after UV irradiation for 230 h (Liang et al., 2013). The image analysis found 1- to 2-µm-wide and 0.5- to 1-mm-deep cavities all over the LDPE/TiO₂ and LDPE/MPS-TiO₂ film surfaces.

Another useful, versatile, and high-resolution microscopy technique for different imaging competencies is TEM. It enables observing detailed structural information at an atomic level; magnifications may reach up to one million under good conditions. Images are usually detailed, offer bright field and dark field, and can capture chemical information of the sample. The imaging and the analytical capabilities improve when coupled with a spectrometer (Burge et al., 1982; Bonnet, 2004; Carraher, 2018). In a study by Motaung et al. (2012), TEM was used for imaging the TiO₂-polymethylmethacrylate (TiO_2-PMMA) nanocomposites (Motaung et al., 2012).

Common instrumentation for monitoring thermal properties includes thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC). DSC is a commonly used thermal analysis that is applied to determine the melting point, heat of fusion, crystallinity percentage, crystallization kinetics, phase transitions, and oxidative stability. The heat absorbed or radiated by any MP as a function of temperature against time is measured with DSC (Gill et al., 2010). The thermal analysis provides phase transition information, e.g., melting and glass-to-rubber transition to the crystalline and amorphous phases, respectively (Aguilar-Vega, 2013). Uheida et al. (2020) carried out DSC measurements on visible light-exposed PP MPs irradiated at different lengths to investigate the change in the crystallization behavior (Uheida et al., 2020). They observed a shift in the melting point to lower temperatures due to the degradation of the polymeric chain.

While DSC measures the heat quantity, TGA measures the rate of change of weight as a function of temperature in a controlled environment. It has a wide application in determining the thermal and oxidative stabilities, and in the identification and composition of plastic material. Many studies have used TGA to confirm the change in the thermal properties and determined kinetic parameters after photocatalytic degradation of MPs by monitoring the weight loss (Liang et al., 2013; Uheida et al., 2020; Zhao and Zhang, 2021).

Tensile tests are used to determine nominal tensile strength, elongation, and elastic modulus (Aguilar-Vega, 2013). Usually, tensile testing, a destructive method, is conducted following ASTM 882-85, the universal testing machine (UTM). Analysis of several studies shows that photocatalytic degradation has an adverse effect on the tensile strength of films. The samples become brittle and fragile. Also, percent elongation decreases as carbonyl group formation increases (Zhao et al., 2007; Liang et al., 2013; Ali et al., 2016).

A dynamic mechanical analyzer (DMA) is extensively used for measuring viscoelastic properties of MPs at a molecular level where a minor deformation is applied to a given sample. This permits the materials to undergo cyclic loading, which causes both elastic and viscous deformation. The amorphous and crystalline parts of the material cause elastic component deformation, while the viscous part is induced by the movement of polymer segments (Menard and Menard, 2020). In the work of Tofa et al. (2019) photo-irradiated LDPE films were exposed to a DMA that resulted in the strain as a function of temperature at a frequency of 1 Hz (Tofa et al., 2019). The result elucidated the loss of elasticity after photocatalytic activities due to chain scission of LDPE films.

Recently, Luo et al. (2021) used a Lorentz contact resonance (LCR) technique for determining the viscoelastic properties of nanoscale TiO₂-coated MPs by measuring the contact stiffness between the AFM probes and MPs (Luo et al., 2021). It provides a high-resolution map of different phases on extremely thin films. A greater frequency was associated with a stiffer MPs material, while a lower frequency was associated with a smoother material. The MPs became stiffer with increasing irradiation, forming wrinkles, cracks, and fragmentation.

The determination of kinetics is of immense importance to understanding the degradation mechanism and predicting the stability and life span. Here, we discuss a few simple techniques for determining the degradation kinetics of MPs.

The utilization of weighing balance is a simple, low-cost technique to study weight loss during photocatalytic degradation (Liang et al., 2013; Ali et al., 2016; Ariza-Tarazona et al., 2019). For Lee et al. (2020), weight loss was determined to evaluate the photocatalytic degradation kinetics of the PA66 microfibers using different doses of TiO₂ and by changing UV wavelength (Lee et al., 2020). It was elucidated that the shorter UV wavelengths were more efficient in damaging PA66 microfibers in comparison with the longer ones. They discovered that the initial kinetic rate is precisely proportional to the catalyst dose.

Turbidity is a measure of the degree of losing transparency due to the presence of suspended particulates in the water. A turbidimeter, a cheap, widely used tool for monitoring drinking water quality, measures this haziness in water (Gregory, 1998). Ma et al. (2019) used a turbidimeter to observe the elimination of PE using coagulant AlCl₃.6H₂O and FeCl₃.6H₂O at different doses. Under low pH, a 5 mM dose AlCl₃.6H₂O ensured higher removal efficiency of PE particles (Ma et al., 2019). A pseudo-first-order kinetics of PS after in an aqueous environment was calculated using turbidity data (Domínguez-Jaimes et al., 2021).

Chemical oxygen demand (COD) in addition to total organic carbon (TOC) could be employed to evaluate the mineralization of MPs. COD is a measure of the requirement of oxygen for the oxidation of both organic and inorganic matters present in water, by a strong chemical oxidant. COD can evaluate the degree of mineralization that had generated during the photocatalytic process of MPs (Rajamanickam and Shanthi, 2016; Lee et al., 2020). On the other side, TOC has been in practice for exploring the concentration of total organic compounds (TOCs) in aqueous solution before and after photooxidative degradation (Giroto et al., 2010). For instance, polymethylmethacrylate and PS nanobead solutions were degraded using UV irradiation of TiO₂/β−SiC alveolar foams, wherein reaction kinetics and TOC value were found to increase with the shorter wavelength. The TOC value ensures the breakdown of PS MP in suspension (Allé et al., 2021). Another research group used a TOC analyzer to investigate the degree of mineralization of PVC as a function of temperature using an electro-Fenton technology. Under the electrocatalysis process combined with high temperature (100°C), TOC can greatly increase. They also suggested that TOC values can be relied on reflecting the decomposition of PVC MPs (Miao et al., 2020).

GC, GC-MS, and FTIR are the most commonly used tools for assessing MP degradation and by-products. The chromatograph is a destructive technique. In contrast, FTIR is a non-invasive method to identify and characterize transformation in chemical structure.

GC is the most popular analytical tool for monitoring by-products after degradation. GC is not suitable for the analysis of organic compounds having high molecular weight or low vaporization temperatures (Hakkarainen and Karlsson, 2006). Zhao et al. (2007) utilized GC to determine the concentration of CO₂ and volatile organics generated during photocatalytic degradation of PE under solar and UV light. They proposed that CO₂ is the key element produced from the photocatalytic degradation of PE plastic (Zhao et al., 2007). Other volatile organics like methane, ethene, ethane, propane, acetaldehyde, formaldehyde, and acetone were also detected, albeit of a lesser amount. GC was also used to study the photocatalytic degradation of PS plastic exposed to fluorescent light, and generation of volatile organic compounds followed by CO₂ evolution was confirmed (Shang et al., 2003).

Polymer degradation constituents have been investigated by many scientists using GC-MS (Barlow et al., 1961). It permits both qualitative and quantitative evaluations (Medeiros, 2018). GC-MS analyzed obtained by-products from photocatalytic degradation of PP MPs over ZnO nanorods. It has been found that ethynyloxy/acetyl radicals, hydroxypropyl, butyraldehyde, acetone, propenal, and the pentyl group are the most abundant degraded by-products (Uheida et al., 2020). During the investigation of degraded species after oxidation of PVC MPs via an electro-Fenton system, short-chained organics like alkenes, alcohols, monocarboxylic acids, dicarboxylic acids, and esters were detected (Miao et al., 2020).

LC-MS is a highly sensitive tool, mostly suitable for MP identification, sensing lower volatile compounds of high polarity, determining molecular weight, and conducting end group analysis of polymers. Sample preparation is comparatively easy, and it has the ability to recognize and measure a wider range of compounds (Perez et al., 2016). Tapia et al. (2019) recently used LC-MS and developed a method for identifying the degradation compounds of a cross-linked polyester for biomedical relevance (Tapia et al., 2019). They used both LC-MS and LC-MS/MS for characterizing structural isomers and their corresponding structures.

ATR-FTIR spectroscopy is a widely used non-destructive technology for measuring infrared spectra for a variety of materials, especially for indicating the structural transformation in MP structure after degradation. This is an inexpensive, sensitive, and time-saving analytical method that provides an indirect result of the extent of oxidation of polymeric materials. The functional group within polymeric compounds vibrates at its own characteristic frequency due to the absorption of IR light. FTIR spectroscopy is applicable for the identification and detection of structural transformation in plastic materials (Tofa et al., 2019; Almond et al., 2020). Numerous studies on chemical transformation within polymeric chains using FTIR have been reported (Cho and Choi, 2001; Shang et al., 2003; Tofa et al., 2019).

A parabolic trough reactor (PTR) focuses solar energy on a transparent tube located along the parabolic focal line and whereby the reactant fluid flows using a long, reflective parabolic surface (see Figure 7). Concentration factors of 5 to 30 suns are commonly utilized for photocatalytic applications, while a concentration factor of 70 suns has been observed utilizing a parabolic dish reflector (Alfano et al., 2000; Malato et al., 2002; Oyama et al., 2004).

Increasing the intensity of the incoming light allows the use of a reduced quantity of photocatalyst leading to a reduction of the operating costs, as well as simplified catalyst separation and recycling (Goswami et al., 2004). This also enables the use of a smaller diameter absorber tube, allowing for higher operational pressures, which is beneficial in industrial-scale plants where hundreds of meters of absorber tubing and associated components can result in considerable pressure losses (Alfano et al., 2000).

PTR configuration was applied in Sandia National Laboratory, New Mexico (SNL) for the treatment of 1,100 L of polluted water by salicylic acid using TiO₂ slurry. The employed PTR was 218 m long and 2.1 m wide, with a 38-mm-diameter borosilicate glass pipe. With a total aperture size of 465 m² and a concentrating ratio of 50, the device was able to achieve 96% degradation in just 40 min. The same system was used for treating other molecules such as trichloroethylene, tetrachloroethylene, chloroform, trichloroethane, carbon tetrachloride, and methylene chloride (Prairie et al., 1991). CIEMAT71, a Spanish corporation, and an American partnership consisting of the National Renewable Energy Laboratory (NREL), Lawrence Livermore National Laboratory (LLNL), and Sandia National Laboratories (SNL) built similar systems (Mehos et al., 1992). The CIEMAT development consisted of 12 “Helioman” units that were placed in sequence at the Plataforma Solar de Almeria (PSA) in Spain. Four parabolic trough aluminized reflectors with an overall collection area of 29.2 m² and two-axis tracking constituted each Helioman. The concentration factor of this configuration is approximately 6 suns, and the photonic efficiency (expressed as moles of degraded contaminant per incident mole of photons inside the reactor) was registered as ∼1% (Malato et al., 2002). This PTR configuration turned out to be efficient for the photocatalytic degradation of pentachlorophenol (Minero et al., 1993), oxalic acid (Bandala et al., 2004), and 2,4-chlorophenol (Malato et al., 1997) contaminants in water. In addition, for the treatment of trichloroethylene by a suspended photocatalyst supplied at 0.8 g/L, the NREL consortium used single-axis tracking PTRs stacked in series over two drive trains with a collection area of 78 m² each and a concentration factor of ∼20 suns (Mehos et al., 1992). A significant decrease in trichloroethylene concentration in contaminated groundwater beneath the level considered drinkable (from 106 ppb to <0.5 ppb) was observed in this installation.

However, there are various drawbacks associated with the use of PTR reactors. PTR can only collect and benefit from direct sunlight, which reduces the overall quantity of collectible solar radiation and makes the use of a solar tracking system inevitable, making their use quite expensive and inefficient on cloudy days. Overheating of the reaction solution is another key flaw of PTR (Blanco-Galvez et al., 2007). While heating the reaction fluid is normally good for enhancing the reaction rate (Falconer and Magrini-Bair, 1998), it may become unpleasant at higher concentration factors because of the possible formation of unwanted products through direct photolysis. Overheating can also lead to leaks and corrosion (Adesina, 2004), and can affect treatment efficiency. Moreover, an increase in the temperature leads to a decrease in the solubility of oxygen in the water and therefore bubble generation. Because O₂ is required for reaction completion (Prairie et al., 1991), the endless contribution of oxygen to the reaction or addition of scavengers such as hydrogen peroxide/sodium persulfate becomes essential for the process (Minero et al., 1993), which will complicate the water purification procedure and deteriorate its practicality. Because of the elevated operation cost related to water cooling, the existence of moving parts, and solar tracking devices, PTRs are not generally considered for scaling-up and commercialization.

Finally, the National Solar Thermal Test Facility at Sandia Laboratories in Albuquerque, New Mexico (USA) constructed the first outdoor engineering-scale reactor in 1989 (Figure 7, left), and the PSA in 1990 designed and built the very first PTC engineering scaled-up solar photochemical facility designated for the detoxification of water in Europe (Figure 7, right) (Minero et al., 1993; Malato et al., 2007).

The reflector geometry in CPC grants reflection of indirect light onto the absorber tube surface leading to an increase in the total quantity of photons and allowing the photoreactor to operate on overcast days when direct beams are unavailable (Bockelmann et al., 1995). As a result, by employing a CPC setup, many of the issues related to PTR could be avoided. Therefore, according to the following equation, the concentration factor and the acceptance angle of a CPC are directly associated:

where C_CPC is the concentration factor, θ_a is the acceptance half-angle, α is the width of the reflector aperture, and r is the radius of the absorber tube. As a result, a CPC with a concentration factor equal to 1 sun (no concentration) will possess an acceptance half-angle equal to ~90°C, which means that all received light beams by the aperture (direct and diffuse) would be reflected in the absorber tube. The enhancement in the CPC performance over that of the PTRs was shown to range between 30% and 200% (Parra et al., 2001; McLoughlin et al., 2004a) owing to the ability of CPCs to harvest diffused beams and to usethem more efficiently (as a consequence of homogeneously illuminating the absorber tube) (McLoughlin et al., 2004b; Malato et al., 2007). Likewise, no sun tracking technology is required, decreasing the system’s complexity and cost dramatically.

However, CPC reactors use a greater quantity of catalyst and wide absorber tubes compared to PTRs, making it necessary to work at low operating pressures (Fernández et al., 2005; Tanveer and Tezcanli Guyer, 2013). A schematic illustration and photo of a compound parabolic collector (CPC) are shown in Figure 8 (Fendrich et al., 2018).

The CPC design was employed in several pilot-scale assemblies. The PSA in Spain built a modular array of six CPC modules for the degradation of dichlorophenol solutions with a maximum concentration of 200 mg/L. Not more than 100 min was enough to remove the pollutant when using an overall collection area equal to 8.9 m (2216). Such CPC configurations were effectively employed for the degradation of a wide spectrum of water contaminants, such as bacteria (Fernández et al., 2005) pharmaceuticals (Augugliaro et al., 2005), municipal wastewater (Kositzi et al., 2004), pesticides (Oller et al., 2006), and other resistant compounds (Sarria et al., 2004).

A non-concentrating CPC design using suspended photocatalytic technology was also developed in the European “Solardetox” project for the treatment of commercial wastewater by building a modular system having each module made up of 16 parallel, 1.5-m tubes backed by CPC reflectors (Blanco et al., 1999; Malato et al., 2002). A full-size demo plant was built in HIDROCEN, Madrid (Blanco-Galvez et al., 2007). In a recirculating batch mode, the facility is completely automated and capable of treating 1,000 L of persistent non-biodegradable chlorinated industrial water pollutants. The plant has an overall aperture area of 100 m² and is made up of two rows of 21 CPC collectors connected in series (each one is 1.5 m × 1.5 m and contains 16 parallel glass tubes with an inner diameter of 29.2 mm) (Blanco et al., 1999). Similar collectors were utilized for treating paper mill effluents in Brazil and Germany (Machado et al., 2003; Sattler et al., 2004; Eduardo da Hora Machado et al., 2004) and paper mill effluents (Amat et al., 2005), surfactants (Amat et al., 2004), and textile dyes (Prieto et al., 2005) in Spain.

A new CPC-based plant was constructed in 2004 to treat pesticides remaining in water after the recycling of plastics. Recycling of plastic containers was followed by industrial cleansing of the shredded plastic, resulting in water polluted with very harmful persistent chemicals (pesticides). In a batch process, this photo-Fenton-based treatment was able to mineralize up to 80% of the TOC in the washing water (Fallmann et al., 1999; Sagawe et al., 2001; Rossetti et al., 2004). Photocatalysis mineralizes approximately 95% of the pollutants, while the activated carbon filter eliminates the remaining 5%. The plant includes four parallel rows consisting of 14 photocatalytic reactor modules built with 20 tubes/module, 2.7 m²/module, and installed on a 37°C inclined platform, based on CPC solar collectors (local latitude). The overall collector surface area is 150 m², and the combined volume of the photoreactor is 1,061 L (Blanco-Galvez et al., 2007).

In 2006, a new initiative was taken by effectively treating saline industrial wastewater with approximately 0.6 g/L of non-biodegradable α-methylphenylglycine (MPG) and 0.4–0.6 g/L TOC utilizing a combination of solar photo-Fenton and aerobic biological processes. It was made up of a solar CPCs photo-Fenton reactor (100 m²) and an aerobic biological treatment plant with a 1-m³ capacity fixed biomass activated sludge reactor. The combined system reached approximately 95% mineralization, wherein the photo-Fenton pre-treatment degraded 50% of the initial TOC, and the aerobic biological treatment eliminated 45% (Malato et al., 2007).

Zapata et al. (2010) established an approach for the development of a large-scale combination of solar photo-Fenton and aerobic biological systems for the remediation of commercially available pesticides using CPC reactors (Zapata et al., 2010). The industrial plant was made of a scaled-up biological treatment plant mainly consisting of two 1,230-L IBRs, a total collector with a 150-m² surface, and an overall photoreactor volume of 1,060 L. The combined industrial-scale system (photo-Fenton/bio) has an efficiency of 84%, with 35% belonging to the photo-Fenton treatment and 49% related to the biological process.

Solwater is an alternative project that disinfected bacterially contaminated water using CPC photoreactor technology with a supported photocatalyst and photosensitive dyes. This project is funded by the European Union and conducted in South America. Following 3 months of deployment, the removal efficiency was diminished due to the deposition of calcium carbonate over the photocatalyst and the photosensitizer (Navntoft et al., 2007). This decrease in TiO₂ efficiency was similarly observed when CPC packed with supported TiO₂ on glass beads was employed for remediating of resulting BTEX spiked molecules from secondary wastewater treatment (Miranda-García et al., 2001). Extending the efficiency of the reactor was achieved by using a filter of ~0.35 μm, and an anionic and cationic ion-exchange resin as a pretreating stage. Catalyst fouling problem was also seen in tubular type reactors packed with supported TiO₂ on silica spheres for urban wastewater treatment. Following a single month of treatment, iron sludge fouled the catalyst and dramatically reduced its efficiency (Nakano et al., 2004).

A CPC pilot-scale packed bed reactor made up of 5 Pyrex tubes (1.9 m length and 22.2 mm outer diameter) joined successively was employed for handling 21 L of triclosan solution over supported TiO₂ on volcanic mesoporous stones (Martínez et al., 2014). Finally, Silva et al. propose and tested a new approach for the remediation of landfill leachates following an aerobic treatment on a pilot scale (Silva et al., 2017). This approach consists of an aerobic activated sludge biological pre-oxidation (ASBO), coagulation, and sedimentation steps, and photo-oxidation through a CPC (10 modules, 20.8 m²) photo-Fenton (PF) reactor. The optimal option for treating 100 m³/day of leachate with a COD value less than 150 ppm and 1,000 mg O₂/L was a blend of solar and artificial radiation, considering the energetic needs across the year.

Finally, a variation of CPC called tubular reactor with an aperture and gross areas of 10 m² and 37.2 m² was respectively installed in a field test facility at Tyndall Air Force Base, Florida to tackle spiked water with BTEX in a recirculating batch mode (Vargas and Núñez, 2010). A total of 530 L of sieved groundwater was recycled with TiO₂ slurry and 100 mg/L of H₂O₂. The removal of 75% of BTEX was achieved after 3 h. Recently, Vargas et al. (2010), reported a tubular pilot plant (TPP) using slurry TiO₂ for the treatment of 15 L of contaminated water with industrial oil hydrocarbons (Vargas and Núñez, 2010). Dibenzothiophene (DBT) and naphthalene (NP) attained ~90% mineralization in 60 min when all pH values were tested; meanwhile, p-nitrophenol (PNP) reached ~40% in 180 min when pH = 6 and 10 were employed using slurry TiO₂.

Although CPC suffers from optical losses and tube aging (Malato et al., 2002; De Lasa et al., 2005), reactor clogging, catalyst fouling, the need for oxygen injection, and high pumping costs, it is still a fitting technology for pilot-scale and larger-scale operations. Extensive testing of photocatalysis and solar thermal engineering led to tremendous knowledge and understanding in the design and operation of photocatalytic systems incorporating CPC photoreactors, resulting in an attainable design and scaling-up of a plant based on such technology.

Inclined plate collectors (IPC) are flattened or curved panels that maintain a thin (about 1 mm) laminar flow of wastewater (normally 0.15–1.0 L/min). It offers the advantage of ensuring great mass transfer across various segments of the reactor throughout the large surface supporting the photocatalyst, in a structure known as a “thin-film fixed bed reactor” (TFFBR) (Ollis and Turchi, 1990; Fendrich et al., 2018). This configuration allows the non-concentrated photons to first travel throughout the reactant fluid before attaining the photocatalyst. Therefore, water overheating heating is not an issue (Tian et al., 2017). IPCs are perceived to be highly well suited to small-scale applications with limited amounts of fluid to be treated (Zhang et al., 2018). A schematic illustration of an IPC is shown in Figure 9 (Fendrich et al., 2018).

Furthermore, the IPC may be closed or kept exposed to the atmosphere. When closed (flat plate reactor), they require purging with O₂ or supplying with oxidants, in addition to glazing. When exposed to the atmosphere (failing film reactor), purging reaction solution with air and oxygen becomes unnecessary, as further increased efficiency is realized by excluding reactor covering from absorbing light beams, and therefore eradicating the likely formation of a photocatalyst opaque film on the inner surface (Braham and Harris, 2009). However, during severe gusts, falling film reactors suffer from a loss of homogeneity and dryness of the catalysts (Bockelmann et al., 1995; Gernjak et al., 2004).

A TFFBR pilot plant was installed in a textile factory in Menzel Temime, Tunisia for the treatment of two commercial textile azo dyes using supported TiO₂ in a recirculating batch configuration. The pilot plant was composed of 2 reactors, each having a total illuminated area of 50 m² capable to treat a volume of 730 L. Seventy percent of degradation was attained in 8 h with an initial TOC in the water of 44.8 mg/L (Zayani et al., 2009).

Originally reported by van Well et al. (1997), the engineering of a double-skin sheet reactor (DSSR) is straightforward (Van Well et al., 1997). DSSR is a non-concentrating reactor composed of commercial Plexiglas (transmission of light above 320 nm) double-skin sheet. It benefits from the employment of direct/diffuse radiation, as well as the maintenance of a turbulent flow regime that bypasses mass transfer boundaries between the photocatalyst and pollutants. However, it may be solely utilized in a low-throughput slurry mode, and it has low optical efficiency and bubble entrapment (De Lasa et al., 2005). Moreover, being a closed reactor, it needs purging of reaction solution with O₂ or supplementing with additional oxidants (Bahnemann, 1999), which makes them unsuitable for commercialization. It was demonstrated that the present photocatalytic process using DSSR is inapt for tackling a large volume of effluent water. For instance, the projected areas necessary to decrease TOCs to 5% out of their initial concentration in 100 m³/day of effluents were extremely large and unviable, spanning from 5,350 m² for lubricating oil effluent to a huge 106,100 m² for chlorinated organics effluent (Gulyas et al., 2005).

Nonetheless, a pilot plant composed of 12 DSSRs (2.45 m × 0.94 m) working in a recirculating batch mode with an overall surface area of 27.6 m² and loaded with 5 g/L of TiO2 was installed in 1998 at the Volkswagen plant in Wolfsburg to handle 500 L/day of treated wastewater via biological approaches. The reduction of TOC reached 40% after 5.5 h beneath UV-solar irradiation (13.3 W/m²) (Dillert et al., 1999).

The cascade falling film principle is used in the step photoreactor to ensure adequate sunlight exposure and oxygenation of the effluent to be treated. The STEP solar reactor (Figure 10) is composed of a rectangular (2 m × 0.5 m = 1 m²) 70-mm-high stainless-steel staircase vessel with 21 steps (Malato et al., 2007). To prevent water evaporation, it has a Pyrex glass (UV-transparent) lid. For the degradation of 4-chlorophenol, formetanate, and dyes, such reactors are set on a stationary rack inclined at 37°C, Almeria’s latitude. The photocatalyst-support sheet was 1.36 m² in size. Twenty-five liters of DI water with the specified organic compounds was cycled in the dark at 7.5 L min⁻¹ at the start of the experiment. For the entire degradation of 4-chlorophenol and formetanate, the STEP solar reactor was shown to be as competent as the CPC. In STEP reactor trials, however, both dyes took more time to be treated. The reactor was used to treat 25 L of water in a recirculating batch configuration. This setup was utilized to treat 25 L of an aqueous pesticide mixture with an initial TOC concentration of 8 mg/L in a recirculating batch mode. Exposed to a UV-A intensity of 30 W/m², 80% degradation was obtained in 4.5 h (Chan et al., 2001). However, owing to high capital costs caused primarily by the cover material, in addition to film problems, the need for reaction solution purging with O₂ or oxidant supplementation, and regular cleaning requirements, the STEP reactor became problematic for scale-up and commercialization. An example of a STEP reactor constructed at the PSA is shown in Figure 10. The major types of pilot-scale photoreactors are listed in Table 5.

To our knowledge, a pilot-scale photocatalytic reactor for the degradation of MPs does not exist yet. However, we took a big step in this framework during the CLAIM H2020 EU project where we successfully implemented a lab-scale photocatalytic reactor for the degradation of invisible MPs such as PP, PE, PVC, and nylon. For instance, 2 weeks of visible light irradiation of PP MPs resulted in a 65% decrease in average particle volume. The successful outcomes of the experimental work prove the efficiency of the designed reactors and suggest their prospects for employment in scaled-up water and wastewater treatment.

Finally, it should be noted that other reactor designs are proposed in the literature to treat wastewater but are not attractive for scale-up and commercialization due to the above-mentioned disadvantages: the use of the laminar flow regime, the need to purge with oxygen or to supply oxidants, the requirement of additional support for glazing, high cost, low optical efficiency, the need for a tracking sunlight system, mechanical aspects, and climatic conditions. These reactors include the slurry bubble column reactor (used to treat nitrobenzene, chlorobenzene, and phenol) (Kamble et al., 2004), the flat plate column reactor (used for tackling aqueous solution of methylene blue) (Vaiano et al., 2015b), the pebble bed photoreactor (used for tackling water containing dye molecules) (Rao et al., 2012), the flat-packed bed reactor (used for the remediation of DI water inoculated with Escherichia coli) ( Hanaor and Sorrell, 2014), the optical fiber photoreactors (used to treat 4-chlorophenol) (Peill and Hoffmann, 1997), and the fountain/water bell photocatalytic reactor and the rotating disk reactor (Braham and Harris, 2009).