## 5 Reactor Systems and Process Parameters
### Light Sources  
The choice of light source significantly impacts system efficiency, energy consumption, and operating costs [65].  
* UV lamps: Conventional low- and medium-pressure mercury lamps are effective, but energy-intensive and limited to UV-active photocatalysts.
* Visible light and solar illumination: Increasing research efforts focus on visible-light-responsive photocatalysts compatible with LEDs or sunlight, offering more sustainable and cost-effective operation.
* Light emitting diodes (LEDs): These are increasingly preferred due to their energy efficiency, long lifespan, and tunable wavelength output.  
Figure 10 presents a comparison of three primary light sources used in photocatalytic water treatment: UV lamps, visible light (including solar illumination), and light-emitting  
diodes (LEDs). It highlights key distinctions in their operational characteristics, including energy efficiency, wavelength compatibility, and suitability for different photocatalysts. The layout supports a clear understanding of how light source selection influences the photocatalytic performance and sustainability.  
A study conducted by [PERSON] et al. (2021) examined the degradation of methylene blue (MB) dye using a CFA-ZnFe\({}_{2}\)O\({}_{4}\) photocatalyst under three distinct lighting conditions: white LEDs, natural sunlight, and ultraviolet (UV) light at 254 nm [29]. Among these, UV light (144 W, 254 nm) exhibited the highest degradation efficiency, which is attributed to its shorter wavelength and higher photon energy. In a separate investigation, [PERSON] et al. (2018) explored how varying the number of LED lamps influences the photocatalytic degradation of ibuprofen (IBU) using TiO\({}_{2}\) at a concentration of 1.0 g/L [144]. Their results demonstrated a positive correlation between the number of LEDs and the degradation rate of IBU, suggesting that enhanced light intensity leads to increased production of hydroxyl radicals, thereby boosting photocatalytic activity. Additional research findings on the influence of different light sources on the photocatalytic removal of organic pollutants are compiled in Table 8.  
\begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline \multicolumn{1}{c}{**Photocatalyst**} & \multicolumn{1}{c}{**Target Pollutant**} & \multicolumn{1}{c}{**Light Source**} & \multicolumn{1}{c}{**Photocatalytic Performance**} & \multicolumn{1}{c}{**References**} \\ \hline \multicolumn{1}{c}{} & & & Under UV light, 100\% of 4-acetylphenol was degraded in 150 & \\ \multicolumn{1}{c}{} & & UV radiation, sunlight & & \\ \multicolumn{1}{c}{} & & & Under sunlight, 100\% of 4-acetylphenol was degraded in 120 & \\ \multicolumn{1}{c}{} & & & min. & \\ \hline \hline \end{tabular}
\end{table}
Table 8: The photocatalytic destruction performance of different organic compounds is affected by different light sources (UV radiation, sunlight, and visible light) (reused from [PERSON] et al., 2022 [102], under Elsevier License 6000951320125, 2 April 2025).  
Figure 10: Comparative overview of common light sources used in photocatalytic water treatment, including UV lamps (100–400 nm), visible light (400–700 nm), and LEDs. The diagram summarizes their typical wavelength ranges, energy efficiency, and compatibility with different photocatalyst types.