Towards Flow Heterogeneous Photocatalysis as a Practical Approach to Point-of-Use Water Remediation Strategies

Abstract

The United Nations and the World Health Organization provide clear guidelines to ensure water security for urban and rural populations. Common contaminants include bacteria and a variety of organic contaminants, such as medications and agricultural runoff. The rapid advancement of point-of-use water treatment is crucial to align with these international recommendations. While some problems are chronic and require long-term solutions, others are transient contamination issues that occur without warning and frequently lead to boil water advisories that can last for extended periods. In these cases, providing reliable water security requires solutions that can be deployed rapidly, are affordable, and can be implemented at the point of use with minimal operator training. Our research explores the state of the art in photocatalysis as a method for purifying water from organic contaminants and bacteria. We present a comparative analysis of various catalysts, supports, and light sources, along with our perspective on the benefits of flow systems. Practical solutions require flow techniques that are portable and can address at least the recommended survival requirements of ~7.5 L per capita per day for small communities, schools, or small hospitals. In this perspective, we propose that flow-compatible modified TiO₂ catalysts can offer practical solutions implemented with either solar light or LED sources in the UVA or visible region.

1. Introduction

Many communities worldwide face unsafe water sources that pose a threat to their health [1,2]. Boil water advisories provide a temporary solution, but they are not a reliable remedy for the uncertainty of drinking water quality due to bacterial or organic contamination. Given the persistence of this contamination, substantial research has been directed toward developing photocatalytic methods that offer both efficacy and economic viability. Research often focuses on proof-of-concept strategies using powder-based catalysts that can destroy small amounts of contaminants. Sometimes, these catalysts use surrogate contaminants like dyes that can be photobleached when exposed to light in the presence of the catalyst. However, these solutions are far from practical and do not meet the recommendations of the World Health Organization (WHO) regarding the per capita water requirements for daily consumption. Practical solutions must operate in flow systems, preferably using fixed-bed photocatalysts that can utilize solar light or readily available LED light sources, preferably in the visible region.

Water contaminants can be categorized into three distinct groups: bacterial and organic contaminants, as previously mentioned, and transition metals, as illustrated in Figure 1. Photocatalytic treatment is an effective approach to addressing bacterial and organic contaminants, but its effectiveness is limited in the case of transition metal pollutants. While light can easily alter the oxidation state of metal ions, it does not eliminate them. Further, techniques that involve their precipitation would likely require the addition of other reactants. In contrast, alternative strategies based on adsorption may be more promising in this scenario [3], but they will not be explored in this perspective.

We aim to map the road towards easily deployable and accessible water treatment alternatives that are crucial for some populations, particularly in remote locations facing water security challenges. Flow-compatible solutions are essential because water treatment methods are more scalable when utilizing flow strategies. In this context, light-based alternatives, such as inorganic photocatalysts, emerge as excellent candidates. These solutions can utilize both artificial and natural light sources.

While we may mention other catalytic systems, we will emphasize solutions that involve titanium dioxide, reflecting its excellent catalytic properties, the tunability of its absorption properties, its low cost and low toxicity, and naturally, our own interests.

2. Defining the Problem

Long-term purification requirements are currently met in industrial facilities. However, there are instances where unexpected releases of materials, whether accidental or due to natural events, create a sudden demand for a point-of-use (POU) water treatment. In these situations, portable and easily deployable equipment would be highly beneficial.

This contribution focuses on treating water contaminated with bacteria or organics, such as pharmaceuticals commonly found in regional populations (e.g., estrogens, ibuprofen, diuretics). It also addresses agricultural contaminants like pesticides, which are often a concern for rural communities. This approach is not intended for producing drinking water from sewage or highly contaminated sources but rather for water that has a treatable level of contamination An adapted diagram from a recent publication [4] is reproduced in Figure 2 and illustrates the problems that concerns us in this perspective well.

A recent report analyzes the water quality in 163 sites across southern Canada. Of these, only about half rate the water quality as fair to excellent, and 17% are rated as poor or marginal [5].

2.1. Organic and Bacterial Contamination

Common organic contaminants, which include both naturally occurring and man-made chemicals, can pose chronic health risks from long-term exposure, even at low concentrations [6]. Man-made chemicals are often categorized as Synthetic Organic Compounds (SOCs) or Volatile Organic Compounds (VOCs). Regulated values known as the Maximum Contaminant Levels (MCLs) from the World Health Organization (WHO) [7], the US Environmental Protection Agency (EPA) [8], and the European Union (EU) [9] can be found in

Table 1. MCL ^a of common SOCs and VOCs in drinking water.

Contaminant	Type	Primary Source/Use	WHO Guidelines [7]	US EPA Limits [8]	EU Limits [9]
Benzene	VOC	Fuel/Gasoline, Solvents	10	5	1.0
Carbon Tetrachloride	VOC	Industrial Solvent, Refrigerant	4	5	0.5
Vinyl Chloride	VOC	Plastic Manufacturing (PVC)	0.3	2	0.5
1,2-Dichloroethane	VOC	Vinyl Chloride Production, Solvents	30	5	3.0
p-Dichlorobenzene	VOC	Mothballs, Air Fresheners	300	75	0.5
Trichloroethylene (TCE)	VOC	Industrial Solvent, Degreaser	70	5	10 (for tetrachloroethene and trichloroethene sum)
Atrazine	SOC	Herbicide (Agricultural Runoff)	100	3	0.1 (for individual pesticides)
Alachlor	SOC	Herbicide (Agricultural Runoff)	20	2	0.1 (for individual pesticides)
Total Trihalomethanes (TTHMs)	VOC	Chlorination Byproduct	100	80	100

^a Units of MCL are expressed in µg/mL.

.

Other water contaminants that raise concern are pharmaceuticals such as ibuprofen and other pain medications, estrogens, and diuretics and their metabolic subproducts [4,10,11]. Unexpected releases of these materials, whether accidental or due to natural events, create a sudden and urgent need for effective water treatment solutions. Given the wide array of organic contaminants, the term “Total Organic Carbon” (TOC) was introduced as a more practical way to refer to their collective sum [12]. While source water can naturally contain levels up to 10 mg/L or more, treated water must usually show a high percentage of TOC removal to limit not only direct consequences but also undesired microorganism growth. Indeed, TOC can serve as a nutrient for microbes; this fraction is referred to as Assimilable Organic Carbon (AOC) [13]. Therefore, if treatment fails to remove sufficient natural organic matter, the overall bacterial population in the water will multiply. An AOC value for outgoing drinking water around 10 µg acetate-C/L (C=Carbon) is not considered to pose a risk, but the higher the value above that, the greater the risk of unwanted bacterial growth. However, the primary concern is the presence of pathogenic microorganisms that can cause acute illness. Regulatory agencies around the world set stringent standards (MCL: 0 CFU/mL) to ensure these are absent. In light of practical constraints that hinder the rapid identification of specific pathogenic bacterial strains, routine measurements of indicator bacteria are used to assess potential fecal contamination [14].

2.2. Contaminant Degradation vs. Full Mineralization

The ideal endpoint for removing organic contaminants from water is full mineralization, which is the destruction of the compound into simple, harmless inorganic substances like carbon dioxide and water [15]. This offers the highest assurance of safety by eliminating all residual toxicity. However, achieving full mineralization is often costly and involves highly energy-intensive technologies such as reverse osmosis, electrochemical treatments, or the full oxidation of pollutants [16]. Alternatively, contaminant degradation (structural change) is faster and cheaper, involving the partial breakdown of the contaminant into a new, smaller chemical [17]. This approach is favored for its efficiency and ability to meet the MCLs for the parent compound. Its major drawback is the risk of incomplete degradation, where the new chemical product (a transformation product) might be more toxic or carcinogenic than the original. For example, breaking azo dyes into smaller chemical compounds releases highly toxic substances, such as aromatic amines (like benzidine), which are known carcinogens [18].

Therefore, degradation is acceptable only if all resulting byproducts are proven non-toxic. In fact, many photocatalysts have proven to efficiently break contaminant molecules into smaller, safer compounds [19]. The advantage of this approach is the easiest translation into flow systems for POU devices.

2.3. International Guidelines for Drinking Water Security

The United Nations (UN) recognizes access to safe and clean drinking water and sanitation as a human right essential for life and the enjoyment of all rights [20]. The resolution calls on states and international organizations to mobilize finance, build capacity, and transfer technology to secure universal access. In 2015, the UN General Assembly reaffirmed and further clarified these as distinct but interrelated rights (to safe drinking water and to sanitation) setting expectations for national policies, plans, and budgets that prioritize universal, sustainable access and address structural inequalities [20]. The right to water specifies that water must be sufficient and continuous (availability), safe, acceptable, physically accessible, and affordable for personal and domestic uses.

The World Health Organization (WHO) guidance links how much water people can actually use to how close, reliable, and affordable that water is, and it shows that health gains follow step-changes in access, not just liters supplied. The updated second edition of Domestic water quantity, service level and health (2020) reviews the evidence and recommends targets that combine quantity, accessibility, reliability/continuity, and price to secure public health benefits [21].

At the survival/consumption end, the WHO’s foundational analysis by Howard and Bartram estimates that ~7.5 L per capita per day (lpcd) covers typical drinking and food preparation needs for most people under most conditions. Crucially, this does not include water for hygiene. In humanitarian operations, the WHO advises rapidly achieving ≥15 lpcd, with ~20 lpcd preferred to enable basic handwashing and food hygiene; higher volumes are needed for laundry/bathing unless performed at the source [22]. It is unrealistic to meet these recommendation with anything other than flow technologies.

For technologies and utility upgrades purporting to enhance drinking water security, the WHO’s benchmarks translate into design and monitoring targets: (1) guarantee the consumption minimum (≥7.5 lpcd per person equivalent) in all contexts; (2) reach ≥ 15–20 lpcd rapidly in emergencies and step up toward ~50–100+ lpcd by prioritizing on-plot access; (3) engineer for reliability/continuity (pressure management, storage, backup power) and affordability by design so households can actually use enough water; and (4) report outcomes using service levels, not liters in isolation, to capture real health protection. Together, these principles convert volume targets into rights-compatible, health-protective services suitable for policy and investment decisions.

3. Choosing a Photocatalyst

Eventually, a photocatalyst must be flow-compatible. However, on the journey to such a catalyst, the use of heterogeneous materials, particularly powders, can be a stepping stone in the learning process until the perfect catalyst is invented. We begin this section by reporting on studies of pristine titanium dioxide, predominantly the anatase form, which is abundant, inexpensive, and widely studied.

3.1. Titanium Dioxide Powders

Titanium dioxide is a semiconductor which has been extensively studied for its use as a photocatalyst [23,24]. In nature, this metal oxide can exist in three polymorphs: anatase, rutile, and brookite. Of these, rutile is the thermodynamically stable crystalline phase of TiO₂, with a tetragonal crystal structure containing TiO₆ octahedra with a slight distortion. Anatase and brookite are metastable [25], with the former detaining a tetragonal crystal structure with a greater distortion in the TiO₆ octahedron. The least naturally abundant of these, brookite, is difficult to synthesize and therefore has not been the centerpiece of as many studies relative to rutile and anatase. Additionally, the more abundant metastable phases generally display higher photocatalytic activity [26] when compared with brookite. This is reflected in the wide commercial availability of rutile and anatase, which can be acquired as pristine white powders.

Pristine TiO₂ is a suitable starting point when selecting an optimal catalytic candidate for pollutant degradation because, relative to other semiconductors, it has demonstrated long-term stability, strong oxidizing ability, and non-toxicity. Moreover, the ubiquitous use of this catalyst for environmental applications is directly tied to its bandgap being approximately 3.2 eV, making it photoactive under UVA light irradiation [27]. This results in TiO₂ being capable of generating photo-induced electrons which move into the conduction band to perform reductive processes, while holes in the valence band are key to the catalyst’s oxidative ability. A classic representation is shown in Figure 3; much of this perspective is concerned with ways to improve this process to develop effective water remediation strategies.

3.2. Improvements to Titanium Dioxide Powders

While photoexcited pristine TiO₂ is known to degrade organic matter either by trapping holes via organics or through ROS reactions following the trapping of conduction band electrons by oxygen, the process is inefficient due to rapid and facile electron–hole recombination and TiO₂’s inability to absorb visible light photons. To address this issue, two strategies are employed: reductive treatments can produce black TiO₂, which exhibits excellent visible light absorption, and doping or decorating with metal ions or their oxides.

3.2.1. Reduction of Titanium Dioxide

Typically, introducing Ti³⁺ centers into the crystalline structure of titanium dioxide leads to the achievement of a narrower bandgap [28,29]. In recent years, the reduction of pristine TiO₂ has emerged as an effective pathway to generating Ti³⁺ sites within the photocatalyst’s structure. Chen et al. successfully obtained “black TiO₂” through a hydrogenation process, resulting in a reduced bandgap of 1.7 eV [28]. This dark powder becomes photoactive in the visible region. The key factor contributing to increased visible light absorption in blackened TiO₂ is the enhanced hydrogen mobility. The defects introduced into the lattice facilitate rapid hydrogen diffusion and exchange, leading to a modified electronic structure and enhanced visible light absorption [30]. Within a semiconductor, the valence band primarily comprises oxygen 2p states, while the conduction band comprises titanium 3d states. Consequently, a smaller bandgap arises from midgap levels that overlap with the O 2p and Ti 3d orbitals [31].

When examining pristine and blackened TiO₂, the reduced form exhibits greater photocatalytic activity as a result of its enhanced light absorption. Black TiO₂ possesses a bandgap as low as 1.54 eV due to the presence of lattice defects originating from the Ti³⁺ impurities and subsequent oxygen vacancies. A variety of reports have developed methods for introducing Ti³⁺ centers or oxygen vacancies to distort the electronic structure and improve the visible light absorption of TiO₂. Amongst these techniques, high-pressure hydrogenation [32], plasma treatment [33], chemical reduction via NaBH₄ [34], and laser ablation [35] have been successful in achieving black TiO₂. Despite obtaining the desired material, these methods are costly in terms of energy or relatively expensive due to high temperatures, pressures, or vacuums being needed. An alternative route using ethanol as reducing agent has been developed in our laboratory [19], which presented a greener approach to fabricating black TiO₂.

3.2.2. Doping and Decorating TiO₂ to Improve Its Performance

With the goal of reducing the bandgap of TiO₂, decorating and doping this semiconductor with metals and non-metals have been shown to help in attaining this goal [36]. A smaller bandgap is obtained by forming an acceptor level near the valence band (VB) or donor levels near the conduction band (CB) of TiO₂.

In the context of this perspective, it is important to note the use of the terms “doping” and “decorating”. The former refers to impurities that are incorporated in the bulk of the material as is frequently performed in the semiconductor industry, for example, by adding boron or arsenic to silicon. The term “decorating” is used for discrete particles, frequently nanoparticles that are incorporated on the surface of other materials, as in AuNP@TiO₂, where gold nanoparticles are on the surface of TiO₂ particles. These concepts are illustrated in Figure 4.

Some illustrative examples of methods for introducing donor states are nitrogen, carbon, and boron doping. In the case of nitrogen, when doped into TiO₂, the N 2p levels pair with the O 2p states which generate acceptor levels and effectively lower the bandgap of the catalyst [37]. Similarly, carbon-doped TiO₂ possesses a smaller bandgap than the pristine metal oxide due to the additional states generated after C-Ti-O is formed [38]. In the case of boron doping, B replaces O in the TiO₂ structure, and consequently, Ti⁴⁺ sites are converted into Ti³⁺. This modification, which echoes the properties observed in blackened TiO₂, provides a shortened bandgap by the creation of half-filled states under the CB and expands the photoactive range of the catalyst into the visible region. Similarly, doping using transition metals has been shown to shorten the bandgap of TiO₂ by providing mid-level bands to the initial bandgap of the catalyst [39]. The use of transition metals allows d states to help modify the spectral properties of the initial material. An example of this was shown by Hou et al. who found that Ag-doped TiO₂ had an extended absorption edge into the visible region because of hybridized Ag 4d and Ti 3d states [40]. Guo and Du demonstrated that metal doping using Cu, Ag, or Au resulted in the addition of new states near the valence band of TiO₂ [41]. A list of bandgap values of doped TiO₂ is shown in

Table 2. A list of bandgap values for doped TiO₂ from the literature.

Doped TiO2	Bandgap (eV)	Reference
Pristine TiO2	3.2	[27]
N2-doped TiO2	2.30	[42]
C-doped TiO2	2.75	[43]
B-doped TiO2	2.85	[44]
Ag-doped TiO2	2.60	[45]
Au-doped TiO2	2.61	[46]
Pd-doped TiO2	3.06	[47]
Co-doped TiO2	2.24	[48]
Cu-doped TiO2	2.80	[49]
Zn-doped TiO2	2.83	[50]
Ni-doped TiO2	2.90	[51]
Fe-doped TiO2	2.73	[51]

.

Despite the advantage of generating a smaller bandgap, doping and creating defects in the material can result in additional recombination sites for the photogenerated electrons and holes [52]. The recombination of these charge carriers limits the photocatalytic ability of the semiconductor, so techniques such as the metal decoration of TiO₂ have been employed to minimize charge carrier recombination [53]. The addition of metal nanoparticles on the surface of TiO₂ allows photoexcited electrons to migrate from the conduction band semiconductor to the metal surface where photodegradation can take place through reduction processes. In tandem, this leaves holes in the valence band, which provide sites for oxidation reactions to occur; see “decorating” in Figure 4.

It is important to note that, although metal decoration can provide the desired optical and catalytic properties, there is a concern regarding metal leaching into the waters being treated and thus becoming an additional source of contamination. It is therefore imperative to test the synthesized catalysts to ensure there is no metal ion leaching, especially for materials used under flow conditions, which must be robust and reusable. Moreover, the implementation of toxic metals into our photocatalytic materials such as lead, chromium, arsenic, and cadmium, to name a few, is strongly discouraged for remediation applications [54]. These metals have been shown to be harmful to aquatic wildlife and humans when consumed at the microgram scale. With this in mind, our work in this field has focused on utilizing catalysts decorated with metals deemed safe, a few examples being iron, copper, and silver. An examination of preferred materials is presented in Section 3.4 [54].

3.3. Beyond Titanium Dioxide: Other Possible Photocatalysts

Following the discovery of TiO₂’s photocatalytic activity, a widespread effort began to identify alternative materials [55]. This research is driven by the need for catalysts with enhanced properties, such as a narrower bandgap to maximize solar light absorption, high charge carrier mobility to suppress recombination, and superior chemical stability [56]. This has expanded the classes of functional photocatalysts to include metal oxides, chalcogenides, carbon-based materials, nitrides, and perovskites [57]. The primary goal is to develop materials that are intrinsically robust under solar irradiation, eliminating the need for sacrificial scavengers while maximizing solar energy harvesting.

Because achieving all desired characteristics in a single material is challenging, significant research has focused on synthesizing heterostructure photocatalysts, including dual, tertiary, and quaternary systems. The design of Type II and Z-scheme heterojunctions is a popular strategy to promote efficient charge separation across component interfaces [58].

Beyond their performance, the practical applicability of these materials for water treatment hinges on several other factors. Cost-effective synthesis, environmental friendliness, and a low risk of material leaching and toxicity must be assessed. These constraints frequently limit the viability of certain highly performing photocatalysts.

While TiO₂ remains the most extensively studied photocatalyst for water remediation, alternatives like ZnO, g-C₃N₄, BiVO₄, WO₃, Ag₃PO₄, Fe₂O₃, Cu₂O/CuO, and CdS offer distinct advantages, especially for flow-based and POU systems. ZnO, with a similar bandgap to TiO₂, exhibits superior electron mobility and UV stability. It generates ROS, effectively degrading dyes, pharmaceuticals, and pesticides with minimal secondary pollution. Doping and surface modifications further enhance its photocatalytic efficiency [56].

Other materials of interest are copper-based catalysts. Copper oxides (Cu₂O, CuO) are low-cost, visible light-active semiconductors capable of generating ROS for pollutant degradation. Despite challenges with charge recombination and photodegradation, their performance is enhanced through heterojunctions and surface engineering [59,60,61].

Table 3. Summary of some selected photocatalysts capable of being used for water remediation applications.

Photo-Catalyst	BG (eV)	Limitations	Advantages	Enhancement Strategies	Applications	Ref
TiO2	~3.2	Limited visible light activity	Widely studied, stable, effective under UV light	Doping, surface modification, heterojunctions	Water remediation and bacterial inactivation	[19,55,62,63]
ZnO	~3.2	Similar UV-only limitation as TiO2, photocorrosion under acidic conditions	High electron mobility, UV stability, ROS generation	Doping, surface modification, heterojunctions	Dye, pharmaceutical, pesticide degradation	[56,64]
g-C3N4	~2.7	Low quantum efficiency, fast recombination	Visible light-active, metal-free, stable, basic sites	Bandgap engineering, heterojunctions, oxidant coupling (e.g., H2O2, persulfate)	Water splitting, CO2 reduction, organic degradation	[65,66]
BiVO4	~2.4	Poor charge mobility, recombination	Visible light-active, solar-driven PEC applications	Doping, heterojunctions, MXene composites	Pollutant degradation, water splitting	[67]
WO3	~2.6	Fast electron–hole recombination, limited conduction band position	Stable, Earth-abundant, visible light-active	Doping, heterojunctions, oxygen vacancy engineering	Dye and pharmaceutical degradation	[68,69]
Ag3PO4	~2.4	Photocorrosion, poor recyclability	High quantum efficiency, strong oxidant	Doping, noble metal deposition, magnetic supports	Pollutant degradation, water oxidation	[70]
α-Fe2O3	~2.1	Poor conductivity, recombination	Non-toxic, inexpensive, abundant	Doping, morphology control, composites (e.g., Fe2O3–g-C3N4)	Dye degradation, hydrogen production	[71]
Cu2O/CuO	~2.0–2.2	Charge recombination, photodegradation	Low-cost, visible light-active, ROS generation	Heterojunctions, graphene oxide decoration, surface engineering	Decentralized water treatment, dye and drug degradation	[59,60,61]
CdS	~2.4	Cadmium toxicity, photocorrosion	Excellent solar-driven activity	Biofunctionalization, composites to reduce Cd2+ leaching	Water splitting, pollutant degradation	[72]

summarizes several other materials that hold promise for applications in the field of environmental remediation.

Beyond performance metrics, the practical applicability of these materials for water treatment depends on additional factors: cost-effective synthesis, environmental safety, and a low risk of leaching or toxicity. These constraints often limit the viability of certain high-performing photocatalysts [73]. For example, CdS-based systems, despite their visible light activity, pose toxicity concerns due to cadmium leaching, while complex perovskites and transition metal oxides may involve costly or resource-intensive synthesis routes [74]. TiO₂ is the preferred catalyst for scalable photocatalysis primarily due to its advantages in cost, toxicity, and industrial scalability [74]. Derived from abundant mineral ores, TiO₂ offers a low production cost that far surpasses that of alternatives containing rare or noble metals. Its established non-toxic and biocompatible profile enables its safe use in food, cosmetics, and medical applications, eliminating environmental and health risks associated with heavy-metal-based catalysts such as CdS or high copper concentrations [75]. Furthermore, TiO₂ benefits from numerous synthesis methods that yield large quantities of material with robust physical properties, ensuring reliable integration into existing industrial processes. This scalability contrasts sharply with many novel photocatalysts that, while promising in laboratory settings, remain synthetically challenging and economically impractical for large-scale deployment [76].

3.4. Looking for the Perfect Catalytic Couple: Different Supports

For the practical remediation of aqueous contaminants using continuous flow chemistry, the immobilization of TiO₂ is a fundamental requirement. Various solid-state supports are employed, such as silica, glass wool, glass fiber, polymeric matrices, and carbon-based materials. In general, the support material is not an inert base, and its role goes beyond catalyst immobilization; its physical and chemical properties inherently influence the overall photocatalytic performance. A comprehensive understanding of the support–interfacial interaction is essential for optimizing the desired degradation pathways.

Further, when incident light is the only source of photocatalytic activation, the geometry and opacity of the chosen support must be carefully considered. It is essential that the immobilized system facilitates maximal light penetration and efficient photon harvesting to sustain high levels of activity throughout the flow reactor. Research is actively exploring methods for improving the activity and reusability of immobilized TiO₂. A study demonstrated that the mesoporous silica structure SBA-15 can enhance photocatalytic activity by up to eight times. Furthermore, efforts to immobilize TiO₂ within polymeric matrices (in shapes of membranes or monoliths) have shown significant promise. Specifically, polymeric membranes exhibited excellent reusability and stability, maintaining performance even after 20 reaction cycles [77,78,79].

A comparative study investigated various immobilization supports for TiO₂, specifically examining glass filters, glass wool, and titanium nanofibers has been reported [80]. This research conclusively demonstrated that, within a continuous flow system, glass wool exhibited superior overall photocatalytic performance and convenience.

The final selection of a support material needs a comprehensive evaluation of practical factors beyond flow compatibility and pure reaction kinetics. When assessing critical metrics like cost-effectiveness, synthetic convenience, and mechanical durability, glass fiber was shown to possess advantages over glass wool in these categories. This distinction highlights an important trade-off: researchers must often balance the maximum catalytic efficiency with the requirements for scalable, economically viable, and mechanically robust industrial applications [80]. Figure 5 shows a radar plot where each of the important parameters received a score from 1 to 5, with 5 being the most desirable score, so for toxicity, a score of 5 corresponds to essentially non-toxic.

Table 4. Properties of different materials that could be used as supports for TiO₂.

	Reuse	Toxicity	Ease/Cost	Light	Mechanical	Flow
Fe3O4 NPs	4 Magnetically separable. Surface can oxidize to Fe(III) after 3–10 cycles.	4 Can produce ROS. Oxidizes to inert and biocompatible Fe(II) [81].	3 Safe, eco-friendly, and low-cost [82].	2 Absorbs visible light. Can compete with photocatalysts.	5 Magnetic. Easy-to-decorate surface.	2 Not flow-compatible, unless anchored by magnets.
Glass wool	5 Easily separable. Washing surface can remove active sites (5–7 cycles).	3 Microfibers can detach and potentially be ingested (high aspect ratio) [83].	4 Low-cost, widely available, easy to modify [84].	4 Highly scattering.	4 Easy to decorate. Silanization-compatible. Shape-adaptable to setup.	3 High flow rate challenge.
Glass microfiber filter (GMF)	4 Separable from most matrices. Small fibers can be released (5–7 cycles).	3 Microfibers can detach and potentially be ingested. Inert [85].	5 Low-cost and stable under irradiation [86].	3 Opaque, might affect light absorbance.	3 Can accept large loads. High abrasion risk.	4 Highly flexible, light-weight, thermally stable [86].
Silica	3 Catalyst loss possible during recycling. Efficacy can decrease after ~3 cycles.	5 Inert and non-toxic at bulk or nanoscale [87].	5 Low cost for commercial silica.	4 Good UV scatterer.	2 Large surface area, inert. High abrasion risk.	1 Powder.
Carbon nanofibers	5 Electrospinning TiO2, decreases catalyst loss and increases reusability to >10 cycles [88].	2 High aspect ratio causes pulmonary issues. Can produce harmful ROS [89].	2 High energy requirements (electrospinning/CVD) [90].	2 Broad light absorption.	4 Withstands flow stress. Flexible; adapts to reaction setups.	1 Powder.
Carbon membranes	5 Low separation from support. May lose active sites after multiple cycles (>10 cycles) [91].	4 Thin films; low risk of dissolution. May release microplastics. Inert [92].	2 High cost—electrospinning.	2 Broad light absorption.	5 Withstands high pressure gradients. Resistant to abrasion.	5 Flexible [93].

offers an explanation of the various scores and the rationalization of the values selected.

The criteria used to assign scores to different materials and properties are summarized in Table 4. The criteria chosen to assign scores were developed by our group for this specific case but can be adapted and widely used by others. These factors are ranked based on priority from our point of view from high to low. “Reuse” is defined as ease of recyclability and how many full cycles it remains effective for. “Toxicity” includes hazards to human health if the material was ingested and environmental concern if released into an ecosystem. “Ease/cost” analyzes commercial availability, ease of synthesis, the cost of synthesis (electricity, precursors) or purchase, and the safety of synthesis (if required). “Light” refers to the interaction of the support: transparency, optical inertness in the active region for the catalyst, its ability to help the photocatalyst as a photosensitizer, and photostability under irradiation. “Mechanical” ranks how the supports’ mechanical properties improved their capabilities as a heterogeneous support for ease of modification, loading capability, and fragility. “Flow” categorizes the immobilization ability of the supports in the reactor and their compatibility with different flows (fragility in aqueous solution under high flow rates), as well as flexibility.

3.5. Developing Flow-Compatible Photocatalysts

As previously mentioned, any practical solution that can contribute to water security for communities affected by water contamination issues must employ flow techniques, be easy to deploy, and operate at the point of use. In our own research, we highlighted materials that meet these criteria, and this section focuses on our own experience in developing novel flow-compatible photocatalysts.

As previously mentioned, a study was conducted by our research group examining various supports and showed that glass fiber was an optimal choice for flow compatibility. With this knowledge, studies by Yaghmaei et al. looked at immobilizing TiO₂ on glass fiber for in-flow contaminant degradation [19]. The use of titanium isopropoxide as a liquid precursor allowed TiO₂ to directly be deposited onto the glass fiber paper when submerged in water. Following a heat treatment, the catalyst was directly attached to the support, which allowed a strip of the glass fiber paper to be cut and shaped appropriately for utilization in a flow system. Moreover, the immobilized TiO₂ could then be reduced using volatile ethanol under inert conditions to obtain strips of black TiO₂ that were flow-compatible and photoactive in the visible region. This direct deposition strategy is advantageous for the fabrication of supported catalysts and can be expanded to be applied to TiO₂ that is doped or decorated with metals or non-metals.

In cases where the powder catalyst is synthesized first and then immobilized, an anchoring agent is used to secure the powder and prevent leaching under flow. Work by Wang et al. made use of 3-Aminopropyltriethoxysilane (APTES) to anchor Pd onto glass wool during a nitro-to-amine reduction reaction [94]. Their studies showed that APTES was successful in increasing the stability of the catalyst by having minimal Pd migration across several hours of flow. Work with this anchoring agent has proven to be reliable in immobilizing various catalysts including pristine TiO₂ on glass wool for water remediation [80]; Ag nanoparticles on rice-husk-derived silica for antimicrobial studies [95]; and decatungstate on silica, alumina, titania, and glass wool for oxidative photocatalysis [96]. These works demonstrate that APTES can be a powerful tool in the design of a flow-compatible material which can combine a robust support with an optimized photocatalyst.

Flow systems, utilizing the optimized catalyst/support combinations discussed previously, represent a promising approach for point-of-use applications. Prototype testing demonstrates that the WHO standard of 20 lpcd can be achieved within hours using a device with a compact footprint. However, upscaling this technology for industrial water treatment presents distinct challenges, specifically regarding the uniform irradiation of large water volumes and the mitigation of catalyst fouling caused by high organic loads.

4. Choosing Light and Light Sources

In the pure form, the bandgap of TiO₂ enables excitation with wavelengths as low-energy as those in the UVA region, spanning from 320 to 400 nanometers. This, compared to higher-energy UVB and UVC light, is relatively accessible and safe to work with. Common fluorescent UVA lamps emit light around 365 nm, and newer, more affordable UV-LED alternatives operate within the same spectral range. In recent years, additional powerful ultraviolet LED sources have emerged, offering shorter wavelengths in the UVB and UVC regions. Some of these LEDs possess the potential to kill bacteria or degrade organic matter even without a catalyst, although their efficiency is limited, their cost is increased, and additional training is required for point-of-use operators. Consequently, the use of UVA or visible light remains the more appealing option. This principle extends to solar energy, as sunlight contains about ten times more photons in the visible light spectrum compared to the ultraviolet region. Nonetheless, there is a growing preference in research to move farther along the spectrum into the visible region. Along with being the predominant portion of solar emission, visible light sources are also typically safer, more energy-efficient, and more affordable than UVA sources. The selection of light sources can thus be complicated at times, as many factors come into play.

With a typical bandgap of the anatase form of approximately 3.2 eV [27] and photocatalytic activity as reported by Fukishima and Honda [55], TiO₂ has since become a material that is ubiquitous in terms of photocatalysis. From a purely research standpoint, the focus often settles on abiding by the first law of photochemistry [97]—that is, the selected wavelength of emission must match well with the absorbance of the photoactive species to encourage the desired reaction. In these bench-scale cases, the other factors mentioned earlier are often not as strongly considered, and more specialized, high-intensity lamps are employed to maximize reaction efficiency. However, this creates a barrier when trying to move technologies from the laboratory setting into applications, especially for those in environmental remediation wherein the application may be as a POU in a community or residential area, rather than in an industrial setting. Here, these other considerations become prominent, and it is imperative that the light source and the setup as a whole are simple to operate, do not require significant specialized safety measures, and are affordable and robust overall, bearing in mind that on-the-ground technologies will likely see continuous use compared to working-hour use in the laboratory. It is here, therefore, that the appeal for visible light becomes more apparent. Both broad-spectrum LEDs (white lights) and narrow-emission LEDs of specific colors are widely available in a variety of sizes and intensities, offering flexibility and tailoring to suit the catalyst and reactor components; some examples are provided in Section 5.2. More appealing yet is the harnessing of visible light from the sun, intrinsically affordable and simple, though the location of the application may dictate the extent to which this can be feasible—for instance, for point-of-use water purification sources for a community in northern Canada, sunlight may work for select parts of the year, but come winter, daylight hours are extremely limited, and thus an alternative white lamp should also be included in the reactor setup to facilitate year-round use.

5. Dye Bleaching as a Quick Screening Test

While dye bleaching studies may not provide a long-term solution to address water security issues, they serve as the fastest screening method for identifying potential catalysts. It is crucial to recognize that a key characteristic of common dyes is their strong light-absorbing properties, which could aid in their own degradation through various mechanisms, such as Type II photooxidation [98,99], involving singlet oxygen mechanisms. Additionally, dye bleaching simply involves the loss of conjugation in the dye structure, not necessarily on the pathway to mineralization. Despite these shortcomings, the strategy remains a valuable screening method. Furthermore, there are several reports suggesting that the dye contamination of rivers may actually contribute to water insecurity in certain locations [100,101].

The subsequent subsections present a curated selection of reported studies, along with our own research endeavors aimed at assessing dye bleaching under standardized and reproducible illumination conditions.

5.1. Photocatalytic Dye Bleaching in the Literature

In evaluating the photocatalytic performance of a given photocatalyst (PC) using dye degradation, many studies often overlook critical control experiments that are essential for reliably attributing dye removal to true photocatalytic activity. A systematic evaluation should include three key control experiments:

• Photolysis control: The dye solution is exposed to the same light source for a defined period (t) in the absence of the photocatalyst, to assess any direct photodegradation.
• Dark adsorption control: The dye is mixed with the photocatalyst and kept in the dark for the same duration (t) to evaluate dye adsorption on the catalyst surface.
• Thermal control: The system temperature is monitored and maintained constant—using a fan or cooling setup, for instance—to ensure that any observed dye degradation is not thermally induced.

It is also essential to record the UV–Vis absorption or diffuse reflectance spectra of the photocatalyst before and after the experiment to detect possible dye adsorption or surface changes. The choice of dye plays a crucial role as well; dyes susceptible to self-sensitized degradation or singlet oxygen generation [102] under illumination should be avoided. Robust and well-characterized dyes such as Rhodamine B, Methylene Blue, Methyl Orange, and Congo Red are generally preferred for evaluating photocatalytic activity.

Furthermore, the accurate reporting of the light source irradiance (in W/m²) and its spectral distribution is critical for a meaningful comparison of the photocatalytic results across studies. Unfortunately, these details are frequently omitted, compromising reproducibility. Including such parameters would ensure transparent and comparable assessments of photocatalytic performance.

A summary of selected dye photodegradation data is presented in

Table 5. List of selected TiO₂-based photocatalysts and evaluation of their photocatalytic performance by monitoring photobleaching of dyes.

PC (Specific Sample)	BG (eV)	Dye	Degradation Efficiency	PC (mg/mL)	Light Source	Irradiance W-m−2	Ref
Colored rutile TiO2 (rR3)	2.97	MB	68% in 90 min	0.6	White LED (400–700 nm)	150	[103]
TiO2 doped with Al3+/Al2+ and S6+ ions (X4)	1.98	MB	96.4% in 150 min	0.4	Halogen lamp, 200 W	NR	[104]
Ternary NiO/Ag/TiO2 composite	2.5	MB	93.2% in 60 min	1	Halogen lamp, 400 W (λf ≥ 400 nm)	170	[105]
Oxygen vacancy-rich nano-TiO2 (T150)	2.65	MB	93.8% in 180 min	0.2	Direct sunlight	104.5 Klux	[106]
Ce-doped TiO2 (7% of Ce doping of TiO2)	2.42	RhB	70% in 150 min	1.3	420 nm LED (7.5 W)	20	[107]
Hollow hierarchical porous TiO-Ag composite (HHPA6 (10:0.5))	3.08	MO	98.4% in 125 min	1.25	15W lamp (λmax 395 nm)	NR	[108]
Sol–gel-derived TiO2 (T2)	2.97	MB	99% in 75 min	0.6	Direct sunlight	500–800	[109]
Pd-doped TiO2 (0.5% Pd-TiO2)	3.12	MB, MO	99.4% in 120 min (MB); 92.6% in 120 min (MO)	1.0	100 W Hg lamp	65	[110]
Ag-TiO2	2.78	MO	86% in 180 min	1.0	Solar simulator 1.5 G	NR	[111]
Co3O4/TiO2/GO (2 wt% Co3O4/TiO2/GO-1)	3.04	CR	91% in 90 min	0.25	300 W Xe lamp (λf > 400 nm)	1000	[112]
Black TiO2	1.3	TC	66.6% in 240 min	0.20	1000 W Xe lamp (λf > 400 nm)	400	[113]
Black TiO2/SnO2 (BTS3)	2.55	RhB	96.6% in 90 min	0.62	150 W Xe lamp (λf > 420 nm)	NR	[114]
Black TiO2	NR	RhB, MB	>90% in 120 min (RhB); 70% in 220 min (MB)	0.5	800 W Xe lamp (λf > 420 nm)	NR	[115]
Black TiO2	1.5	Rh6G	49.2% in 240 min	0.3	100 W white LED 6500K	9000 lumen	[116]
Au nanocluster-decorated TiO2 thin film	NR	MB	90% in 120 min	1 cm wafer in 6.5 mL solution	UV lamp (λpeak 365 nm)	45	[117]
Fe2O3-TiO2 (TiNP-Fe2O3)	2.0	RhB	48.4% in 120 min	1.41 mg/mL	Visible light (no details)	202	[118]
CdS/TiO2 nanocomposite	3.5	AB	84% in 90 min	1.0	Halogen lamp (500 W)	9500 lum	[119]
Ni-TiOx	2.68	RhB, MO, TC	98.2% (RhB); 99.5% (MO); 93.5% (TC) in 120 min	0.15	Solar simulator	1000	[120]
TiO2-doped CoFe2O4	2.88	CR	99.9% in 250 min	0.8	150 W metal halide lamp; λ > 400 nm	NR	[121]
Cu-ZnO/TiO2 nanocomp. (CZT-2)	2.68	CR	100% in 20 min	0.5	Direct sunlight	NR	[122]
TiO2-SiO2 nanospheres	NR	RhB	100% in 110 min	0.8	Xe lamp (300 W, λ < 390 nm)	NR	[123]
Fe−TiO2 hollow nanospheres (2% Fe−TiO2)	3.04	RhB	95% in 115 min	1.0	Hg Lamp, XPA-Photoreactor 500 W	NR	[124]

Abbreviations: λ_f = cut-off filter; BG = bandgap; MB = methylene blue; MO = methyl orange; Rh6G = rhodamine 6G; RhB = rhodamine B; CR = Congo red; AB = acid blue; TC = tetracycline;

.

5.2. Photocatalytic Dye Bleaching from Our Laboratory

Work from our laboratory has included dye photobleaching in several cases [125], including Crocin [19,80] (the dye in saffron) [126], a convenient one, which, while absorbing in the visible region, is not a sensitizer for singlet oxygen.

For this contribution, we selected gold on niobium oxide, Au@Nb₂O₅, as the catalyst and Cresyl Blue (CrB), Rhodamine 6G (Rh6G), and Azure Blue (AzB) as test dyes (Scheme 1) and illuminated them with white and green LED lights.

The catalyst, Au@Nb₂O₅, contained 32 ppm gold, and each sample of 5 mL contained 20 mg of this catalyst and 10 μg/mL of the selected dye. The samples were irradiated under air in sealed, crimped, fused silica vials that were rotated at 6 rpm while exposed to either green or white LED light. The irradiance was 88.3 W/m² for the green LEDs and 1420 W/m² for the white LEDs. The spectra of these light sources are the shaded regions in Figure 6.

The rates of degradation are illustrated in Figure 7, and the data for each dye and conditions and initial rates are presented in

Table 6. Kinetic and spectroscopic data for aqueous dyes photobleached in presence of Au@Nb₂O₅ catalyst.

Dye	Conc., µM	λmax (nm)	Rate, µM/min	LED	Irradiance W/m2
Rh6G	21.0	527	(0.62) ‡	Green	88.3
Rh6G	21.0	527	0.21	White	1420
AzB	32.7	646	0.23	Green	88.3
AzB	32.7	646	0.61	White	1420
CrB	26.0	626	0.030	Green	88.3
CrB	26.0	626	0.36	White	1420

^‡ Fast consumption, but detailed data is compromised by high levels of light scattering when the excitation (green) overlaps extensively with the dye absorption, as shown in Figure 7.

.

Table 6 and Figure 6 illustrate the type of data that is required for a catalyst screening test to serve as a practical approach for preliminary catalyst evaluation. Unfortunately, this level of information is frequently not available for the data collected in Table 5. In general, green light is remarkably efficient, even if the catalyst’s AuNPs have a very broad absorption (see orange line in Figure 6); this suggests that irradiating monomeric AuNP is particularly effective, as long wavelength absorptions are usually due to nanoparticle aggregates.

6. Batch Photocatalytic Degradation of Drug Water Contaminants

6.1. Work Involving TiO₂ and Related Catalysts

The most influential early work on the batch photocatalytic degradation of estrogens using TiO₂ is characterized by two essential studies. The first was focused on natural estrogen, demonstrating that suspensions under UV light not only achieved complete degradation but, crucially, caused the immediate loss of estrogenic activity [128]. This was attributed to the rapid oxidative attack on the phenol moiety, the site responsible for biological activity. Following this, the research scope expanded to include more persistent synthetic hormones. For instance, in 2012, Frontistis et al. established that the problematic contraceptive ingredient, 17-alpha-Ethinylestradiol (EE2), also exhibited rapid, high-efficiency degradation kinetics in TiO₂ batch systems [129]. Collectively, these two lines of inquiry showed that TiO₂-based photocatalysis was a highly promising advanced oxidation technology for effectively detoxifying wastewater from both natural and synthetic estrogenic drug contaminants. Several studies followed and expanded on this [24,130].

6.2. Work from Our Laboratories with TiO₂ Materials

Our group has explored many applications of TiO₂ in its pure and modified forms. In an earlier contribution, we demonstrated the efficacy of TiO₂ fibers decorated with Pd, Co, and zinc phthalocyanine nanoparticles; such additions enable the use of visible light as the irradiation source. In the absence of oxygen, these decorated and/or blackened TiO₂ materials may be used for hydrogen generation with the help of an organic sacrificial electron donor (SED) [131]. This idea has been carried forward in recent work to generate hydrogen while simultaneously degrading pharmaceutical pollutants commonly found in water, as they may act as the needed SEDs [132].

Our group has been focusing on developing a highly efficient and sustainable method for water contaminant degradation using modified blackened TiO₂ as a photocatalyst. This change allowed us to overcome a critical limitation of conventional TiO₂ which requires high-energy ultraviolet (UV) light for activation. This black TiO₂ catalyst operates effectively under visible light illumination, such as sunlight or white LEDs. To ensure practical utility in water treatment, black TiO₂ is deposited onto a glass fiber support, resulting in a robust, flow-compatible photocatalyst. This fixed-bed configuration is essential for large-scale applications, as it eliminates the need for post-treatment filtration to recover catalyst powders. We demonstrated that, under visible light, this supported black TiO₂ degrades water contaminants with greater efficiency than its conventional counterpart [130,133]. Specifically, its effectiveness was validated against persistent pollutants, including hormonal disruptors such as 17-beta-estradiol, a chemical of significant environmental and health concern. We envision that this catalyst would be suitable for producing inexpensive, durable, and easily deployable solar or LED-driven water decontamination systems.

7. Batch Antibacterial Studies Using Powder Photocatalysts

7.1. Antibacterial Studies Using TiO₂ in Any Form

The ROS production triggered by light allows TiO₂ to be effective against a broad spectrum of microorganisms, including various Gram-positive bacteria (e.g., Staphylococcus aureus, Streptococcus pneumonia, Bacillus subtilis), Gram-negative bacteria (e.g., Proteus vulgaris, Pseudomonas aeruginosa, Escherichia coli), fungi like Candida albicans, and even viruses like SARS-CoV-2 [134].

Studies have explored various forms of TiO₂ and methods to enhance its antimicrobial activity. For instance, the antimicrobial activity of TiO₂ also tends to increase with decreasing nanoparticle size [135]; sadly, its toxicity increases too. Alternatively, doping TiO₂ with other elements, such as silver, has been shown to significantly enhance its antibacterial properties under visible light irradiation while maintaining its biocompatibility. The phase structure, crystallite size, and crystallinity of TiO₂ directly affect reactive oxygen species (ROS) generation, which is ultimately responsible for damaging bacterial cell walls, disrupting enzyme activity, and causing genotoxicity [136].

Beyond pristine TiO₂, various modifications and composites have been investigated. Cellulose/TiO₂ nanocomposites, for example, have been synthesized and characterized for their antibacterial properties against common pathogenic bacteria, showing promise for applications in food packaging and biomedical devices. Despite extensive research, there is an ongoing effort to optimize factors such as particle size, shape, composition, and synthesis parameters to further enhance the antimicrobial efficacy of TiO₂ nanoparticles [63].

Despite the promising photoactivity of TiO₂, early batch setup systems for water decontamination using a form of blackened TiO₂ were deemed inadequate for comprehensively testing catalyst bactericidal properties. The short lifespan and limited travel distance of reactive oxygen species (ROS) meant that only bacteria immediately near the catalyst were affected. Other limitations of the batch system include sample inhomogeneity due to limited stirring and the risk of bacteria settling and forming biofilms that could deactivate the TiO₂ catalyst [133].

7.2. Approaching Antibacterial Studies with TiO₂ Forms Adaptable to Flow Catalysis

Challenges in the batch setup motivated a shift to flow systems to prevent bacterial settling. Strips were made from glass fiber functionalized with black TiO₂ and set up inside a glass tube to be irradiated with white light. While water flowed through this strip, the increased surface contact resulted in a slightly higher inactivation compared to batch tests [80,137].

It was only in our latest flow setup (vertical tube), when gas (air or oxygen) was added through the rubber septum at the bottom and flowed in small bubbles up the catalyst, that we noticed considerably more bacterial inactivation. This added more turbulence to the flow and provided more oxygen for ROS creation by the black TiO₂. Details on the flow setups can be found in the published master thesis [137]. The system was the most successful to date, achieving a bacterial inactivation of up to two log units within 30 min of exposure. There is still room for improvement as the times of action need to be quicker. Zigzag, or spiral shapes, instead of planar strips, could help in this sense. Our latest vertical tube flow setup, incorporating gas (air or oxygen) introduced via a bottom rubber septum, demonstrated significantly enhanced bacteria inactivation.

8. Flow Photocatalysis for the Treatment of Drug and Bacterial Contamination: A Promising Future for Point-of-Use Applications

Our lab is not the only one exploring the transition from batch to flow. Several examples of flow-supported catalysts have been noted for the purpose of removing bacteria and organic contaminants—namely pharmaceuticals—for water decontamination. Both bacteria and pharmaceutical contaminants can be removed by an advanced oxidation process (AOP) initiated by ROS formation—something at which TiO₂ excels. TiO₂’s photocatalytic properties make it an excellent candidate to remove pharmaceuticals from water. TiO₂ has prior success in laboratory flow setups to rapidly remove pharmaceuticals from secondary sewage effluent [138]. TiO₂ is also an antimicrobial agent, which can successfully remove E. Coli, S. Aureus, and other bacteria in flow [137].

In point-of-use applications, an integrated system capable of removing several different types of contaminants is the most logical strategy. To this effect, studies [139,140] have begun to present options which have several targets, such as combined bacteria and organic contaminant removal. While some materials, such as ZnO and TiO₂, have been reported to be effective in removing both these contaminants, another option is to decorate such materials with noble metals [141]. Silver, for instance, has had longstanding use as an antimicrobial material, although other metals, such as gold, have also been reported to have strong light-activated antibacterial properties due to strong surface plasmon resonance [142,143]. Further, there are many ways that a photocatalyst can be modified to maximize its output specifically in a flow setup, as have been discussed.

Continuous flow is generally considered a more feasible method compared to batch setup to meet the high-throughput requirements of a water source for remote communities. It requires photocatalysts capable of generating enough ROS to degrade bacterial and pharmaceutical products in water over a short period of time. Even in flow development, however, bench-scale flow rates can be quite low, generating sub-liter quantities hourly, though some examples offer larger volumes suitable for applications. Indeed, at elevated volume outputs, factors such as pump capacities, tube diameters, and the relative amounts and surface area of catalysts must be considered and adjusted in tandem in order to achieve the desired efficacy. Thus, much remains to be performed in efforts for multi-target systems at scales beyond the lab bench.

9. Conclusions and Outlook

With increasing environmental pollution and the longstanding need for clean water in remote communities around the world, point-of-use water treatment is becoming indispensable. Heterogenous flow photocatalysis holds significant promise for developing compact, reusable devices capable of purifying water of various pollutants. Moreover, flexibility in scale, light source, and bandgap tuning enables systems which are energy-efficient, accessible, and readily deployed in remote areas. In the future, we may see artificial intelligence and machine learning become true contributors to the evaluation and development of new catalysis tools and materials [144].

Our research indicates that a flow system illuminated by LEDs offers several advantages which meet these criteria, with catalysts like black TiO₂ having the crucial visible light activity needed for such applications. Further work should focus on enhancing reusability and ensuring flexibility towards a range of contaminants, ideally with continued scale-up to enable a portable and comprehensive water purification system.