Advances in Photoassisted and Photocatalytic Processes for Water Remediation

Since heterogeneous photocatalysis emerged as a promising approach able to harness light energy—ideally solar radiation—to drive oxidation and reduction as effective technologies for water remediation under mild conditions [1], research has increasingly focused on bridging the gap between laboratory-scale studies and real-world applications. This includes addressing the challenges posed by complex water matrices, developing visible-light-active and stable photocatalysts, and integrating photocatalytic processes with complementary technologies. In this context, this Special Issue entitled “Advances in Photoassisted and Photocatalytic Processes for Water Remediation” presents a collection of contributions that address key aspects such as emerging pollutant removal, real wastewater treatment, catalyst design, and the scale-up of photoassisted AOPs.

The presence of pharmaceuticals and other contaminants in real water matrices, such as greywater, represents a major challenge for photocatalytic processes, as background constituents can significantly affect treatment efficiency. Inorganic ions, natural organic matter, and surfactants may act as radical scavengers or interfere with light penetration, thereby limiting degradation performance. Understanding these matrix effects is therefore essential for advancing photocatalysis toward realistic and scalable applications [2,3].

In this context, (Contribution 1) evaluate the photocatalytic degradation of pharmaceuticals—naproxen, metformin, and sulfamethoxazole—in membrane bioreactor-treated greywater, demonstrating that efficient removal can be achieved even in complex matrices. The study highlights the influence of background constituents on degradation kinetics and supports the feasibility of photocatalysis as a post-treatment step for water reuse. Complementarily, (Contribution 2) investigate photocatalytic treatment of greywater under realistic conditions, emphasizing the role of operational parameters and matrix composition on process performance. Together, these contributions provide valuable insight into the challenges and opportunities associated with real wastewater applications.

The study of (Contribution 3) demonstrates an effective and sustainable approach for naphthalene (NAP) removal from water using persulfate activated by Fe(III)–goethite and LED irradiation at natural pH. Optimal conditions were identified, achieving over 92% NAP removal while reducing toxicity. The use of a solid iron source, mild operating conditions, and energy-efficient LED light highlights the environmental sustainability of this treatment for water remediation as previously described by García-Muñoz et al. [4].

The scale-up of heterogeneous photocatalysis also remains a critical challenge, particularly due to issues related to catalyst recovery and reactor design. Immobilized photocatalytic systems have emerged as a promising alternative to slurry reactors, as they enable continuous operation and facilitate integration into treatment systems. However, their practical implementation depends on the development of scalable fabrication methods that ensure both activity and durability [5,6].

(Contribution 4) report the fabrication of titania–siloxane photocatalytic coatings using low-temperature UV and plasma curing methods, enabling deposition on temperature-sensitive substrates. The resulting films exhibit effective photocatalytic activity while maintaining structural integrity, and their compatibility with roll-to-roll manufacturing provides a clear pathway toward large-scale production. This work directly addresses key engineering challenges associated with the practical deployment of photocatalytic technologies.

One of the main limitations of conventional photocatalysts such as TiO₂ is their restricted activity under UV irradiation. To overcome this limitation, band-gap engineering strategies, particularly anionic doping, have been widely explored to extend light absorption into the visible region, with nitrogen doping being the most studied anion to effectively modify the electronic structure and enhance photocatalytic activity [7,8].

Another key and extended strategy involves modifying titania to increase its activity and to avoid the recombination of photogenerated charges. In this sense, (Contribution 5) hydrothermally synthesized Sr-doped TiO₂ for the electrocatalytic application of water splitting; 1% Sr-doped TiO₂ nanoparticles showed photocatalytic activity, and 1% and 5% Sr-doped TiO₂ depicted maximum current density for both HER and OER.

The contribution of (Contribution 6) showed the process to obtain a ZnIn₂S₄ photocatalyst with the aim of Cr(VI) photoreduction in water, another typical use of photocatalysis. The employed solvents in the synthesis protocol and the methods were critical for the removal yields of Cr(VI), not just because of the band-gap-associated changes but also because of the resulting surface morphology.

In another work, a zirconia-based photocatalyst was synthesized by (Contribution 7) via a simple thermal decomposition of UiO-66, with Ti(IV) and Hf(IV) cations used to tailor its photocatalytic performance. Among all samples, tetragonal ZrO₂ showed the highest activity, achieving 89% methyl orange degradation under UV irradiation in 300 min, outperforming monoclinic and mixed oxides and comparable to commercial anatase TiO₂.

Other strategies to overcome intrinsic limitations such as wide band gaps and rapid charge recombination include composite formation and the use of alternative semiconductors, which have been widely explored to enhance photocatalytic performance under visible light [3,9].

Another contribution to this Special Issue is a study carried out by (Contribution 8), who obtained Bi₂O₃/CoFe₂O₄ nanocomposites. The heterojunction was applied for the photocatalytic degradation of methyl orange in the water phase. The Bi₂O₃/CoFe₂O₄ exhibited narrowed band gaps (1.58–1.62 eV) and enhanced activity compared to the individual oxides. An equimolar nanocomposite showed the best performance due to a synergistic type-II heterojunction effect.

In this context, (Contribution 9) investigate N-doped photocatalysts, demonstrating that nitrogen incorporation enhances light absorption and improves photocatalytic performance compared to undoped materials. The study highlights the relationship between electronic structure modification and degradation efficiency, reinforcing the importance of rational catalyst design for the development of visible-light-driven systems in water remediation. For their part, (Contribution 10) present a comprehensive review of photocatalytic systems for CEC removal, focusing on catalyst modification strategies and process intensification. The authors discuss alternative materials such as g-C₃N₄ and WO₃, as well as the benefits of combining photocatalysis with ozonation to enhance degradation efficiency. This contribution provides a critical overview of current strategies to improve photocatalytic performance and supports the development of more efficient and scalable treatment systems.

Photocatalytic performance is fundamentally governed by material properties such as band-gap energy, band-edge positions, and charge carrier dynamics. These factors determine light absorption, redox capability, and recombination rates, and are therefore critical for both water splitting and pollutant degradation processes. Transition metal oxides remain among the most studied materials, although challenges related to efficiency and stability persist [3,10]. In this regard, (Contribution 11) provide a detailed analysis of photocatalytic water splitting based on transition metal oxides, focusing on the relationship between physicochemical properties and performance. The work identifies key limitations such as recombination losses and highlights design criteria for improving efficiency. These insights are directly relevant to the development of advanced photocatalysts for water remediation.

On the other hand, an increasing trend consists of modeling the newly developed processes to validate the technology. In this sense, (Contribution 12) developed a model for simulating the degradation of pollutants via a combined bio-chemical treatment with good fittings (R² = 0.998).

Finally, the increasing complexity of wastewater treatment has driven the development of hybrid systems combining photocatalysis with complementary technologies. Such integrated approaches can overcome the limitations of individual processes and enhance overall efficiency, particularly in real water matrices [2,6]. In this framework, (Contribution 13) present a comprehensive review of photoassisted AOPs for micropollutant removal, with particular emphasis on their integration with other treatment technologies. The authors critically analyze combined systems, discussing their mechanisms, advantages, and limitations, and identify key factors for optimization and scale-up. This contribution provides a valuable overview of current trends and future directions in hybrid photocatalytic systems.

The contributions collected in this Special Issue highlight the rapid progress and increasing maturity of photoassisted and photocatalytic processes for water remediation. From fundamental material design to process integration and real wastewater applications, these works collectively address key challenges associated with efficiency, scalability, and practical implementation. In particular, the importance of understanding real matrix effects, developing visible-light-active materials, and integrating photocatalysis with complementary technologies emerges as a unifying theme. These advances contribute to paving the way toward sustainable and efficient water treatment solutions.