## 8 Future Perspectives  
The application of heterogeneous photocatalysis in advanced water treatment holds considerable potential for addressing pressing global challenges related to water quality, sustainability, and public health. To fully realize its potential, future research and development efforts must focus on improving efficiency, scalability, and environmental safety, while aligning with the principles of green chemistry and sustainable engineering. 1. Development of efficient visible-light photocatalysts  
A key priority remains the design and synthesis of photocatalysts capable of efficiently harvesting visible light, which dominates the solar spectrum. Future work should emphasize the following:  
* Low-cost, non-toxic materials with stable photocatalytic performance under natural light. * Tailored band structures for optimized redox potentials and selective pollutant degradation. * Multifunctional composites that combine photocatalysis with adsorption or antimicrobial activity. The exploration of bio-inspired, earth-abundant, and metal-free materials is also expected to grow, helping reduce environmental and economic barriers. Figure 14 provides a conceptual overview of the fundamental directions in the design and development of visible-light-responsive photocatalysts, which are essential for advancing solar-driven water treatment technologies. As visible light comprises the majority of the solar spectrum, the ability to efficiently harvest this energy source is key to reducing reliance on artificial UV irradiation and enabling cost-effective, decentralized treatment systems. Figure 14 highlights three major research priorities. First, it underscores the need for low-cost, non-toxic materials that maintain stable photocatalytic performance under natural sunlight, addressing both environmental and economic sustainability. Second, the figure emphasizes the engineering of tailored band structures to fine-tune redox potentials, thereby enhancing the selectivity in pollutant degradation and improving overall efficiency. Third, it features multifunctional composite materials that not only drive photocatalysis, but also incorporate complementary functionalities such as adsorption or antimicrobial activity, broadening the applicability of these systems. Together, these strategies form the foundation for the next generation of high-performance photocatalysts that align with global sustainability goals and the transition toward circular and carbon-neutral water treatment solutions. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Challenge Area** & **Key Implications** & **Research Needs** \\ \hline Real water matrix variability & Experimental results may not be representative; scalability becomes uncertain. & Standardize testing in complex water matrices and assess real-world performance. \\ \hline Energy and cost concerns & High operational costs and dependence on noble materials limit affordability and adoption. & Optimize solar use, reduce reliance on rare materials, and assess life-cycle costs. \\ \hline Environmental and health risks & Uncertainties in ecotoxicity and lack of regulation hinder responsible implementation. & Conduct long-term safety studies and establish regulatory frameworks for nanomaterials. \\ \hline \hline \end{tabular}
\end{table} 
Table 9: _Cont._(b) Scalable and modular reactor designs  
The next generation of photocatalytic reactors must be designed with scalability and field deployment in mind. Important directions include the following:  
* Modular solar reactors suited for decentralized or rural water treatment. * Immobilized systems with optimized surface area and durability. * Integration with existing infrastructure, such as in hybrid membrane-photocatalysis systems or in tertiary treatment steps in water reuse schemes. Process intensification and automation could further support continuous operation and real-time performance monitoring.