Environmentally Friendly Catalysts for Energy and Water Treatment Applications

Water and energy are two essential factors for the sustainable development of human activities, and are increasingly driven by continuous industrial growth and societal development. Water resources and their quality, which are increasingly threatened by the overexploitation of the planet’s natural reserves, require appropriate protection to ensure their universal availability for current and future generations. Only a very small fraction (<0.03%) of the Earth’s water mass is potable. The lack or inefficiency of water treatment processes make the development of novel technologies, in many cases catalytic, necessary for the treatment of wastewater and drinking water.

While catalysts are useful for many environmental applications, some of them may pose negative environmental or health impacts. Therefore, ensuring their safety and sustainability should be a primary consideration in catalyst research and design. In this Special Issue (SI), priority has been given to the development and application of environmentally friendly catalysts (EFCs). As a result, a wide range of materials have been proposed, including single-metal catalysts and mixed metal oxides supported on alumina, titania, activated carbon, zeolites, or immobilized directly in the reactor. This approach facilitates controlled usage and enables the efficient recycling of the catalysts at the end of their service life.

First, the publications included in this SI can be broadly classified into two main thematic areas:

Water treatment as target. The primary objective of all the publications is to present the latest trends in the design and application of environmentally friendly catalysts (EFCs) for the development of advanced oxidation processes (AOPs). In water treatment, it is important to distinguish between processes aimed at wastewater and those focused on the production of drinking water. For wastewater treatment, there is growing concern over the removal of so-called emerging contaminants before the treated water is discharged back into the environment. Several contributions in this SI address this challenge, including a review article (Contribution 1) that explores various treatment strategies. Specific studies report the degradation of pharmaceuticals and their mixtures, such as dexamethasone, naproxen, and ketorolac (Contribution 2), diclofenac (Contribution 3), paracetamol (Contribution 4), and the antibiotic levofloxacin (Contribution 5). The removal of other emerging pollutants, such as polystyrene nanoplastics (Contribution 6), is also emphasized in response to the increasing demand for water reuse systems. In the context of drinking water production, notable examples include the use of photocatalytic materials for pathogen removal (Contribution 7) and the degradation of trace organic matter (Contribution 8), with the latter also being addressed using non-photocatalytic graphene-based materials (Contribution 9).

Energy concern. The energy requirements associated with treatment processes can be a determining factor in assessing their viability and overall efficiency. In this context, Contribution 2 stands out for presenting the development of an iron-based ligand derived from metallurgical slag, which demonstrates high efficiency in harnessing solar energy. Contribution 1 further explores the remarkable potential of graphitic carbon in the design of highly effective photocatalytic materials, with applications extending to solar energy conversion and hydrogen production. Additionally, Contributions 4 and 9 highlight other advanced catalytic systems designed for efficient energy capture and utilization. These include materials capable of storing energy to enhance the activation of oxidants, such as peroxymonosulfate (Contribution 9), or generating singlet oxygen (Contribution 4), both of which are essential for advancing modern non-photolytic catalytic processes.

Second, the publications in this SI have been classified according to the type of catalytic system employed, with an additional category created to highlight the most relevant contributions focused on the treatment of microcontaminants, as detailed below.

Homogeneous Catalysis. This section refers to Contributions 2 and 3, both of which employ the photo-Fenton process using iron catalysts in solution for the removal of pharmaceutical contaminants. In both studies, elimination efficiency serves as the basis for selecting the most suitable operational strategy. Contribution 3 investigates the degradation of diclofenac using the conventional photo-Fenton process with iron salts. The study places particular emphasis on the degradation pathway, identifying intermediates and by-products, and assessing water quality indicators such as turbidity, color, and aromaticity. Interestingly, the findings reveal that UV irradiation is not always beneficial. In the case of diclofenac, UV exposure appears to promote the formation of aromatic compounds during degradation, ultimately reducing the overall removal efficiency compared to other pharmaceuticals. In Contribution 2, the authors propose a novel circular economy approach by using metallurgical slag as a source of iron to form an iron–citrate complex, which then serves as a homogeneous catalyst in the photo-Fenton process. This strategy integrates waste valorization into water treatment. The study demonstrates the feasibility of this approach for the degradation of a mixture of three pharmaceuticals and shows the system’s resilience to variations in both liquid depth and radiation intensity. Furthermore, the investigation offers valuable insight into the influence of key operational parameters, contributing to the process’s potential for scale-up.

Heterogeneous Catalysis. It is worth highlighting Contributions 4, 7 and 5, which present materials that promote electron transfer from pollutant to oxidant without the need for light irradiation. In Contribution 4, the Fe-N-C_1.5 catalyst was synthesized by pyrolysis in a mixed solvent, generating Fe–N_x active sites, graphitic nitrogen, and carbonyl (C=O) groups. This Fe-N-C_1.5 catalyst activates peroxymonosulphate (PMS) through non-radical pathways, leading to the production of singlet oxygen (¹O₂). Notably, it achieves high efficiency (100% degradation of paracetamol in 7 min) and stable reusability (94% degradation) after several cycles. This makes it highly recommended for treating water contaminated with persistent organic microcontaminants, where high selectivity is required. With a similar reactive behavior, Contribution 5 reports the synthesis of a MnO₂/CN@SiO₂ catalyst, an amorphous compound containing Si–OH groups that enhance the rapid adsorption of PMS, thereby improving its reactivity. Its main advantages include a high degradation efficiency (80.8% removal of levofloxacin) and excellent stability after multiple cycles, achieved through more selective, non-radical pathways based on electron transfer and the generation of ¹O₂, maintaining effectiveness even in complex water matrices. Contribution 7 presents a catalyst based on ZnO nanoparticles (~20 nm) with a wurtzite structure, synthesized via a green method using leaf extracts. Its advantages include a higher photocatalytic efficiency (up to 74% degradation of methylene blue), notable antibacterial activity against both Gram-positive and Gram-negative bacteria, and an environmentally friendly synthesis process. This makes it especially suitable for sustainable disinfection and the efficient degradation of contaminants.

Heterogeneous Photocatalysis. In this field, Contribution 1 highlights the potential of graphitic carbon nitride (gCN) as a promising, metal-free photocatalyst for sustainable water decontamination. Key strategies to enhance its photocatalytic performance include morphological modifications (e.g., 0D–2D hybrid structures), improved visible-light absorption (via doping), and better charge separation (through defects and functional groups). These approaches significantly boost gCN’s ability to degrade emerging pollutants in a cost-effective and environmentally friendly manner, supporting its scalability for large-scale photocatalytic applications. Contribution 10 investigates the interaction of different strontium titanate photocatalysts with alkaline (solonetz) soil solutions. After exposure, most samples showed improved phenol degradation activity despite surface changes and the accumulation of organic residues. Although the crystalline structures remained unchanged, the samples exhibited alterations in the bandgap and crystallite size. The findings underline the importance of assessing the environmental interactions of photocatalysts, especially for solar-driven water purification applications.

Microcontaminants. This section includes Contributions 8, 6, and 9, which describe the use of highly adsorptive catalytic materials capable of interacting with low concentrations of microcontaminants, thus enabling their removal. Contribution 8 reports the synthesis of a catalyst consisting of carbon nanotubes supporting silica–alumina nanoparticles, forming mesoporous 3D structures with a high surface area and a notable bandgap reduction to 1.65 eV. This promotes light absorption and the generation of reactive species that accelerate photodegradation. The catalyst shows excellent performance in the complete removal of persistent dyes at low concentrations under UV/visible light, with much faster degradation rates than conventional TiO₂ and ZnO catalysts. Contribution 6 is based on natural pyrolusite (n-MnO₂) combined with oxalic acid, which enhances the generation of reactive oxygen species (ROS) during ozonation, facilitating both chemical degradation and the physical fragmentation of polystyrene nanoparticles (PSNPs). The material exhibits a stable, reusable structure without Mn leaching, achieving up to 75% removal of total organic carbon and a reduction in turbidity within just 30 min. Contribution 9 presents the synthesis of the β/N-rGO catalyst (nitrogen-doped graphene combined with β-cyclodextrin), which features a high specific surface area and good dispersion. This design enhances PMS reactivity and the generation of oxidizing species derived from ¹O₂. Its high stability across a wide pH range makes it particularly suitable for the rapid removal of trace antibiotics in water.