Heterogeneous Photocatalysis Scalability for Environmental Remediation: Opportunities and Challenges

Heterogeneous photocatalysis is an ecofriendly technique for purifying organic pollutants in environmental systems. While pilot-scale photoreactors have explored photocatalytic system upscalibility, their practical implementation is restricted for various reasons. These include feed composition alteration, complicated photoreactor designs, high operation and synthesis costs, photocatalyst poisoning, low quantum yield under solar irradiation, fast exciton recombination, and low reuse or regeneration capabilities. In this paper, we highlight the photocatalyst scalability challenges for real-world applications. We also provide an in-depth discussion on photocatalyst opportunities for effective air and water pollution control. Lastly, we offer a contemporary perspective on photocatalysis scale-up for the real environmental treatment.

The HPC process is an advanced, ecofriendly approach to purify a wide range of gaseous and refractory pollutants in both air and water environmental systems [2,8]. Upon light irradiation, the HPC process harnesses photon energy when it is equal to or exceeds the bandgap energy (E g ). The absorbed photon energy induces electron (e − ) photoexcitation, followed by interfacial valence band (VB "hole (h + ) creation") charge transfer to the conduction band (CB; i.e., e − /h + pairs separation) and activation of surface redox reactions [1]. The photogenerated e − /h + pairs possess a high redox potential that can convert oxygen/water (O 2 /H 2 O) molecules adsorbed on the hydrated catalyst surface to a reactive oxidative species (ROS) such as free excitons (e − ), superoxide radical anions ( • O 2 − ), and/or hydroxyl/hydroperoxyl radicals ( • OH/HO 2 • ) [9]. ROS radicals play a vital role in accelerating the oxidative/reduction reactions on the surface of the photocatalysts, such as converting adsorbed organic/inorganic pollutants into less harmful compounds [10]. However, the rapid reverse reaction, fast e − /h + pair recombination, is a major limitation on scaling HPC technology to real-world conditions. Further, most of the newly developed photocatalysts, such as metal-organic frameworks (MOFs), metal-oxides, and nanocomposites, exhibit low quantum efficiency (<1%) and poor photonic stability under UV-visible/solar lights [1,11]. In this respect, (MOFs), metal-oxides, and nanocomposites, exhibit low quantum efficiency (<1%) and poor photonic stability under UV-visible/solar lights [1,11]. In this respect, MOF-based photocatalysts exhibited low photonic stability under normal environmental conditions due to the potential destruction of the coordination connectivity between a ligand and metal nodes in the presence of water molecules [1]. Such deformation in the MOF molecular structures can lead to a decrease in the photocatalytic activity with the disruption of the photogenerated charge mobility/separation mechanism (between ligand and metal nodes). Depending on the solution pH, ZnO photocatalyst can also be subjected to photodissolution during HPC reaction in the aqueous media (i.e., direct attack of the photon on the surface of Zn-O bonds, followed by the release of Zn ions (at pH = 3-5) or Zn hydroxide (at pH = 10)) [12]. Likewise, the photonic stability of ATP/TiO2/Ag3PO4 composite could be deteriorated as a result of the photoreduction of Ag + (in the Ag3PO4) into metallic Ag (Ag + + e − +hv→Ag) during the HPC reaction [13]. In addition, the use of HPC technology for air purification (e.g., ventilation and air conditioning (HVAC)) faces challenges of (i) poor effectiveness removing low (sub-ppm)concentration hazardous pollutants (e.g., volatile organic compounds (VOCs)), (ii) high energy consumption (UV-A light use), (iii) long reaction duration, and (iv) difficult photocatalytic reactor optimization (photon and mass transfer control) [14]. In this work, we highlight the main technical challenges restricting photocatalysis applications in environmental fields and address solutions based on up-to-date advances. We also discuss HPCbased photoreactor scalability in water and air purification systems.

Photocatalysis: Challenges vs. Opportunities
Many photocatalysts have been proposed for water and air decontamination applications, with ZnO and TiO2 nanoparticles being the preferred options. These include MOFs, carbon or organicbased materials, nanocomposites, and metal oxide semiconductors and are based on criteria such as commercial availability, cost-effectiveness, conditional stability, high catalytic activity, and nontoxicity [15,16]. Nonetheless, the scalability of ZnO and TiO2-based HPC systems is debatable due to In this work, we highlight the main technical challenges restricting photocatalysis applications in environmental fields and address solutions based on up-to-date advances. We also discuss HPC-based photoreactor scalability in water and air purification systems.

Photocatalysis: Challenges vs. Opportunities
Many photocatalysts have been proposed for water and air decontamination applications, with ZnO and TiO 2 nanoparticles being the preferred options. These include MOFs, carbon or organic-based materials, nanocomposites, and metal oxide semiconductors and are based on criteria such as commercial availability, cost-effectiveness, conditional stability, high catalytic activity, and non-toxicity [15,16]. Nonetheless, the scalability of ZnO and TiO 2 -based HPC systems is debatable due to the shortcomings of (i) a large bandgap energy (TiO 2 and ZnO have an E g of 3.1-3.3 eV) reduces quantum efficiency under sunlight, (ii) high energy consumption (high photon energy below 400 nm (UV-A) wavelength) induces e − /h + pair generation, (iii) ROS reduction due to rapid e − /h + Catalysts 2020, 10, 1109 3 of 8 pair recombination on active surface sites decreases photocatalytic activities, (iv) limited ability to scale up due to a complicated modification process required to improve solar-driven photocatalysis, (v) fast active surface site deactivation/poisoning via interaction with intermediated byproducts that lowers the kinetic reaction rate, (vi) natural scavenger presence (e.g., antioxidant systems including halides and polyphenols) that can impair the photocatalytic process via photogenerated ROS radical interactions, (vii) low photocatalyst chemical stability that shortens its reuse-cycle service lifetime, (viii) spent solid photocatalyst disposal as secondary solid waste, and (ix) low photocatalytic/photonic efficiency against high concentrations of mixed pollutants at high air and water flow rates.
When using HPC technology, photon absorption on photocatalyst surfaces may diminish due to catalysts aggregation and/or light reflection/scattering, especially in high-salinity water with solid suspension or color products [1]. The spent photocatalysts remain suspended in the mobilized photoreactors unless there is a suitable recovery system such as filtration. There are many diverse photoreactor types, including immobilized photoreactors, photocatalytic membrane reactors (PMRs), and hybrid membrane slurry photoreactors. These were designed to help resolve problems mentioned above, but they suffer from technical drawbacks such as catalyst efficiency and membrane fouling/scaling [16]. The photon and mass transfer limitations are also other engineering problems that restrict the implementation of HPC-based photoreactors in the industrial scale. In most photoreactors, it is not easy to achieve uniform irradiance over the entire photocatalyst surface, especially when loading large amounts of a catalyst. Further, part of the emitted photon cannot reach the photocatalyst surface due to simultaneous light absorption/diffraction during traveling through the fluid/gas media containing target reactants (or reagent species)). Such a consumption of photon light could require an increase in photon energy level for the activation of the photocatalyst. The limited mass transfer of the pollutant molecules over the restrained photocatalyst layer (due to the alteration of flow rates, amount of slurry photocatalyst, and total suspension volume) is also recognized as major impediments for the industrialization of fluidized bed photoreactors. An overview of the photoreactors design and their challenges (in terms of photon and mass transfer limitations) has been given elsewhere [17][18][19][20]. Moreover, HPC technology has been used to address diverse outdoor and indoor air VOC pollutants via painting buildings, roofs, bridges, pavement, and ceramic surfaces with TiO 2 photocatalysts. A recent study also proposed using three flat/tubular UV-based TiO 2 photoreactor configurations: bench-scale, semi-pilot, and full pilot (Figure 2a) for indoor air treatment [21]. However, the reaction rate of the photocatalytic system for outdoor/indoor air purification is affected by the combined effects of various factors, including substrate or platform configuration used for coating the semiconductors, light irradiance, VOC pollutants (types and concentration), temperature, photoreactor design, mass/air velocity, and moisture level [22].
Since 2016, 1500 studies have focused on the technical defects mentioned, with various prospects for enhancing environmental photocatalysis use. For example, many pre-/post-synthesis modification strategies have been utilized to improve the optical response and photoactivity of bulk photocatalysts in the visible/sunlight spectrum. Some strategies found that photocatalysts doped with heteroatoms like O, N, S, P, and B are more efficient due to a dipole moment increase. Such an increase in the dipole moment facilitates electron transfer kinetics from VB to CB and shifts the optical response to the visible spectrum [23]. Photocatalyst properties such as bandgap, Fermi level, charge transport, spin density, localized electronic state, and optical/magnetic characteristics can be tuned by doping the semiconductor crystal lattice with low-cost/abundant transition metals like Fe, Ni, Ti, Zn, and Co or alkali metals such as Li, Na, and K [24]. Metal-oxide photocatalyst defect formation (oxygen vacancies) is another effective option for enhancing photocatalyst activity [25]. Based on the bandgap alignment concept, the design of semiconductor coupling (heterojunction or Z-scheme photocatalyst) can be chosen to increase photogenerated charge lifetimes for photocatalytic performance enhancements in the visible light spectra [9]. At the heterojunction interface, photogenerated electrons in the CB can migrate from more to less negative Fermi energy (EF), whereas the VB demonstrates a hole transfer from more to less positive EF (Figure 2b) [16,26]. This charge transfer mechanism decreases the charge recombination rate so that the redox potential of the constructed heterojunction system can promote catalytic activities under solar irradiation. For example, AgFeO2-graphene/Cu2(BTC)3 MOF, as a ternary heterojunction photocatalyst (with good reusability cycles of up to 4 runs), has recently been developed to enhance sunlight-driven photocatalytic degradation of pharmaceutical pollutants [27]. Due to the low EF (−0.08 V vs. SHE) of graphene, the photogenerated electrons in the CB of AgFeO2 (−0.50 eV) and Cu2(BTC)3 MOF (−0.18 eV) migrated to the graphene layer (i.e., acting as a trapping site to increase the lifetime of e − /h + pairs). Likewise, several types of TiO2-based Z-scheme photocatalysts (e.g., Au@CdS/IO-TiO2, CdS-Ag-TiO2, TiO2/WO3/Au, TiO2/g-C3N4, and Zn0.5Cd0.5S-MWCNT-TiO2 systems) have been synthesized [28]. As mentioned above, these TiO2-based Z-scheme photocatalysts possess multiple merits such as: (a) low bandgap energy, (b) spatial separation of redox-active sites, (c) effective migration/separation rate for the e − /h + pairs, and (d) high redox ability to thermodynamically enhance visible-driven HPC reactions towards CO2 reduction, H2 production (e.g., water splitting), and Based on the bandgap alignment concept, the design of semiconductor coupling (heterojunction or Z-scheme photocatalyst) can be chosen to increase photogenerated charge lifetimes for photocatalytic performance enhancements in the visible light spectra [9]. At the heterojunction interface, photogenerated electrons in the CB can migrate from more to less negative Fermi energy (E F ), whereas the VB demonstrates a hole transfer from more to less positive E F (Figure 2b) [16,26]. This charge transfer mechanism decreases the charge recombination rate so that the redox potential of the constructed heterojunction system can promote catalytic activities under solar irradiation. For example, AgFeO 2 -graphene/Cu 2 (BTC) 3 MOF, as a ternary heterojunction photocatalyst (with good reusability cycles of up to 4 runs), has recently been developed to enhance sunlight-driven photocatalytic degradation of pharmaceutical pollutants [27]. Due to the low E F (−0.08 V vs. SHE) of graphene, the photogenerated electrons in the CB of AgFeO 2 (−0.50 eV) and Cu 2 (BTC) 3 MOF (−0.18 eV) migrated to the graphene layer (i.e., acting as a trapping site to increase the lifetime of e − /h + pairs). Likewise, several types of TiO 2 -based Z-scheme photocatalysts (e.g., Au@CdS/IO-TiO 2 , CdS-Ag-TiO 2 , TiO 2 /WO 3 /Au, TiO 2 /g-C 3 N 4 , and Zn 0.5 Cd 0.5 S-MWCNT-TiO 2 systems) have been synthesized [28]. As mentioned above, these TiO 2 -based Z-scheme photocatalysts possess multiple merits such as: (a) low bandgap energy, (b) spatial separation of redox-active sites, (c) effective migration/separation rate for the e − /h + pairs, and (d) high redox ability to thermodynamically enhance visible-driven HPC reactions towards CO 2 reduction, H 2 production (e.g., water splitting), and degradation of organic contaminants under visible irradiation. The detailed information on the basic principles of Z-scheme heterojunction photocatalysts, along with their types and synthesis strategies, can be found elsewhere [28,29].
Similar to a Z-scheme heterojunction photocatalyst (Type II, Figure 2b), plasmonic photocatalysts can enhance solar-driven HPC applications. The system's enhanced performance is due to plasmon resonance energy transfer from the plasmonic metal (e.g., Ag or Au) to the semiconductor through dipole-dipole interaction and/or hot electron injection [24]. Due to high noble metal costs (e.g., Ag and Au), more abundant plasmonic elements (e.g., Bi, Al, or Cu) are recommended. Enormous effort has been made to overcome single photoreactor obstacles by optimizing the operating parameters and integrating hybridized photoreactor systems through a combination of PMRs and immobilized/suspended photocatalysts (e.g., enhancing performance at reduced operation cost) [16,30]. To increase photocatalytic performance in the hybridized mode, the cross-flow mode for the feed stream is recommended for reducing catalyst/substrate accumulation on the membrane surface (cake formation). Both mechanochemical and hydrothermal synthesis methods are promising ecofriendly methods for reducing the cost of scaled-up production for solar photocatalysts. In particular, the mechanochemical synthesis has been proposed for the upscale production of various catalytically active materials (e.g., metal oxides, MOFs, and nanocomposites) due to its simple operation under milder conditions, short reaction period, solvent-free solid-state reaction (under dry milling), and high reproducibility. Based on the applied friction force during grinding, a variety of defects/changes can be introduced in the crystal lattice of the synthesized photocatalysts, leading to improved photoreactivity [31]. In the hydrothermal synthesis, an aqueous phase (e.g., ecofriendly, low-cost solvent) is used as the main reaction medium for catalyst synthesis under high temperature/pressure conditions. The hydrothermal approach has been adopted preferably for the sustainable production of zeolite catalysts (from kaolinite) in industry, with the high yields [32]. During the hydrothermal reaction, the properties of water (e.g., density, viscosity, dielectric/ionization constant, surface tension, and vapor pressure) are changed as a function of applied temperature/pressure. Accordingly, water can act as a solvent, chemical component, or a puffing accelerator for the rates of hydrolysis and ionic reactions as water properties change with the increases in applied temperature/pressure [33]. Synthesis and operation costs are, however, dependent on many other factors involved in the use of catalyst (e.g., individual component costs, energy consumption, regeneration process, and long-term stability).

Future Photocatalysis Perspective and Upscaling Recommendations
As discussed above, diverse approaches offer promising opportunities for increasing photocatalyst environmental applications. However, most HPC research was conducted using bench-scale setups under unrealistic controlled conditions that are far from real feed streams. Only limited efforts have assessed photocatalytic performance using real water and air feed streams [21,27,34]. However, it should be noted that there are many impractical/inconsistent data in the existing research regarding HPC application for environmental remediation. This is due to the frequent use of (i) deionized water (or pure air stream) as a reaction medium (e.g., ignoring the impact of background constituents on HPC system), (ii) longer reaction period (>3 h; i.e., unrealistic time), (iii) single pollutant system (i.e., far away from reality), and (iv) optimized batch reactor system (relative to practical continuous mode) to control the mass transfer process. Further, HPC technology scalability has barely been explored for industrial purposes like wastewater treatment or outdoor/indoor air purification. In addition, the photocatalytic water splitting to hydrogen (as renewable energy) is still suspected to be impractical in that it requires high-expensive sacrificial agents or electron donors (e.g., glucose, glycerol, triethanolamine, ethylene glycol, lactic acid, isopropanol, acetonitrile, and methanol) relative to the commercial route of H 2 generation [35]. Besides, it is crucial to design photoreactors capable of continuous evacuation/separation of coevolved explosive O 2 and H 2 gas mixture during upscaling of the HPC-based water splitting in order to (i) prevent photocatalyst inactivation by evolved O 2 , (ii) reduce operation cost, and (iii) avert the possible reverse reaction (i.e., recombination of H 2(g) and O 2(g) ) in the presence of active photocatalysts. As a fact, relatively little is known about catalyst deactivation mechanisms with respect to the application of HPC technologies under the real-world conditions. Most importantly, the performance of the integrated hybrid photoreactor is expected to be reduced if applied at the industrial scale due to many interfering variables (e.g., natural scavengers, sunlight intensity variation, feed quality, and side reactions (between mixed contaminants)). Further, the generation of the byproducts is generally ignored when assessing HPC performance (catalyst poisoning and quantum yield) under simulated lab-scale conditions.
In the upscaled fields, Purifics Photo-Cat is recognized as the only commercialized UV-driven HPC system for the real wastewater treatment although the many solar-driven photocatalysts have been proposed in the literature [24]. Photo-Cat is a closed-loop UV-driven TiO 2 slurry-based photocatalytic system with a high flux ceramic membrane for continuous water purification/decontamination from organic/biological pollutants, inorganic (or heavy metals), and submicron particulate matters (https: //www.purifics.com/photo-cat). A good number of HPC-based life projects(e.g., PhotoPAQ (LIFE08 ENV/F/000487: 2010-2014), MINOx-STREET (2013-2018), and EQUINOX (IFE12 ENV/ES/000749: 2019), co-funded by European Union's) have been carried out to confirm the feasibility of photocatalytic cementitious coating materials for outdoor air pollutant remediation (e.g., NO x , O 3 , VOCs, and airborne particles) [22]. Nonetheless, it is still challenging to predict HPC performance in the field due to the following possibilities: (i) photocatalyst aging processes when coated on roads and pavement for outdoor traffic, (ii) seasonal environmental parameter variations such as solar irradiation, wind, temperature, and humidity, and (iii) photocatalyst exposure conditions like air volume to photocatalytic surface ratio. There are additional challenges for solar-driven photocatalytic purification systems, which are required to remain in continuous, 24-h operation, at nighttime. These challenges require smart photocatalyst development, with operational flexibility under both solar and LED light, as well as self-cleaning behaviors to reduce surface deactivation. In this regard, TiO 2 has been utilized as a self-cleaning material in industrial projects such as coated liquid suspension for building materials (e.g., Hydrotech TM by TOTO Ltd., Beijing, China)) and in cementitious composites (e.g., TX Active TM , TX Aria TM , and TX Millennium TM by Italcementi SpA Company, Bergamo, Italy) [36]. This supports the premise that TiO 2 -based HPC technology is scalable and can be used to improve day-night cycle performance in real-world applications. Combining adsorption and photocatalysis is an alternative option for overcoming light source limitations and extends day-night cycle operational capabilities. A hybrid adsorbent-photocatalyst system will also reduce catalyst deactivation and extend service lifetime by adsorbing undesirable byproducts and accelerating the mineralization process (i.e., adsorbent surfaces can serve as an − OH species source). Future studies should focus on understanding hybridized photoreactor system efficacy and solar-driven photocatalyst performance and stability under real industrial conditions. Accurate cost estimates are also required for hybrid adsorbent-photocatalyst system production in scaled-up industrial operations.

Conclusions
We reviewed many reports on HPCs for water and air purification. Despite the number of publications, newly developed solar-driven photocatalysts such as MOF-based nanocomposites and plasmon semiconductors need to be validated for practical applications because of their high production costs and poor kinetic stability. TiO 2 /ZnO-based photocatalysis is the preferred real-world application option due to its cost, stability, self-cleaning behavior, and commercial availability. However, more efforts should aim to enhance an optical response and photocatalytic activity in visible light regions, using methods such as heteroatom or transition/alkali metal (as low-cost and abundant elements) doping, adsorption-photocatalysis combinations, and hybrid photoreactor tuning. Future investigations for implementing practical photocatalytic systems at the industrial scale are necessary to (i) select reproducible and easy synthesis protocols for photocatalyst, (ii) optimize the configuration of photoreactors, (iii) mitigate poor photocatalytic performance component materials such as suspended particles, colored components, and natural antioxidants, (iv) reduce photocatalyst deactivation phenomena by suppressing the generation of intermediate byproducts, and (v) reduce operational costs (e.g., relative to conventional treatment options).