## 7 Challenges and Limitations of Heterogeneous Catalysis  
Despite the significant progress and promise of heterogeneous photocatalysis in water treatment, several scientific, technical, and practical challenges continue to hinder its large-scale and commercial implementation. While heterogeneous photocatalysis offers many advantages, its large-scale deployment remains limited by a range of technical, environmental and economic challenges, which are reviewed in detail in this section. These limitations include sensitivity to water matrix composition, difficulties in catalyst recovery, and limited efficiency under natural sunlight conditions. Understanding these limitations is essential to guide future research and promote the transition from laboratory-scale studies to real-world applications. * Limited photocatalytic efficiency under natural light  
Many widely studied photocatalysts, such as TiO2 and ZnO, require ultraviolet (UV) light for activation due to their wide band gaps. However, UV represents less than 5% of the solar spectrum, which limits the efficiency of solar-driven photocatalytic systems. Although visible-light-responsive materials have been developed, their activity is often lower, less stable, or poorly characterized under realistic conditions. * Fast recombination of charge carriers  
One of the major intrinsic limitations of semiconductor photocatalysis is the rapid recombination of photogenerated electron-hole pairs. This reduces the number of reactive oxygen species (ROS) available for pollutant degradation. Although heterojunction formation, doping, and co-catalyst strategies have improved charge separation, recombination remains a key bottleneck, especially in scaled-up systems. * Catalyst stability and deactivation  
Photocatalysts may suffer from loss of activity over time due to fouling, surface poisoning by reaction intermediates or structural changes under prolonged irradiation. In real water matrices, the presence of natural organic matter (NOM), inorganic ions, or biofilms can contribute to catalyst deactivation and require periodic regeneration, which complicates operation and increases costs. * Recovery and reuse of catalysts  
Slurry reactors, which typically offer higher activity due to greater surface area, require post-treatment separation of the catalyst particles. Recovery of nanoparticles is particularly challenging, leading to secondary contamination risks and resource losses. Immobilized systems offer easier reuse but often at the cost of reduced photocatalytic efficiency and mass transfer limitations. * Variability in real water matrices  
Most studies are performed using synthetic water with single contaminants, which do not reflect the complexity of real water matrices. In actual applications, competing substances (e.g., anions, cations, dissolved organics) can interfere with catalyst performance, ROS availability, and light penetration. The presence of turbidity and suspended solids also limits photon transfer and contaminant-catalyst interactions. * Energy consumption and cost  
Photocatalytic systems relying on artificial UV light are energy-intensive, limiting their sustainability and cost-effectiveness. Even with solar-driven systems, achieving adequate performance under fluctuating light conditions remains difficult. Moreover, the high cost of advanced photocatalysts, especially those using noble metals or rare elements raises concerns for large-scale use. * Environmental and health risks  
The potential release of engineered nanomaterials into the environment during operation or disposal raises questions about long-term safety. The ecotoxicological effects of these materials, particularly under light exposure, are not yet fully understood. Regulatory guidelines and standardized testing protocols are lacking for the safe use of photocatalytic materials in water treatment. Table 9 summarizes the main scientific and practical challenges currently limiting the large-scale application of heterogeneous photocatalysis in water treatment.