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Summary: This reprint focuses on new trends in photo-electrocatalysis, specifically addressed to the remediation of wastewater and energy production. The remediation of wastewater, up to a level that is acceptable for discharge into receiving waterbodies, involves an ever-growing demand of energy, so effective and low-energy treatment processes are highly desirable. Among the other treatments, photo- and photo-electrochemical treatment processes may be considered as advanced oxidation processes (AOP), which are based on the generation of OH radicals, strong oxidizing agents able to indiscriminately degrade even the most persistent organic compounds. Photocatalysis and photo-electrocatalysis can be considered as effective methods for organic degradation, especially when the semiconductor is active in the range of visible light. Several results are presented on new morphologies and structures, which allow more photoactive, visibly responsive, and stable materials, as well as studies on combined processes in which photo- or photo-electrochemistry contribute to an increase in the sustainability of the whole process, lowering costs and achieving the most valuable final products. In view of the circular economy concept, microbial fuel cell systems are also considered as possible way to recover energy from organic pollutants contained in wastewater.

Summary: In recent years, inorganic oxidic nanomaterials have been extensively investigated for their outstanding properties that allow their use within a large variety of raising fields of interest, ranging from (photo)catalysis to the development of functional nanocomposites. Nanoscopic metal (eventually mixed) oxides are often fabricated following soft-chemistry approaches, characterized by the possibility of favouring specific morphologies, particles dimensions, and surface porosities through different synthetic methods and templating processes. In this context, surface functionalities and reactivity play a major role in the determination of nanomaterials final properties. For this reason, further surface functionalization with specific chemical moieties is often recommended to extend their field of application. This Special Issue, now edited as a book, aimed at extending the comprehension of the mechanisms involved in the synthesis and templating of inorganic oxidic nanomaterials, as well as in their surface functionalization and reactivity. Additionally, this Special Issue aimed at increasing the knowledge on the latest advances of these systems in (photo)catalysis, environmental clean-up processes, energy storage, controlled transport and/or release, biomedicine, sensing, development of smart-materials, stimuli-responsive materials, and nanocomposites. Nanomaterials of interest included: silica, alumina, titania, zirconia, zinc oxide, aluminosilicates, iron oxides, perovskites and titanates, rare earth oxides, and composites. Furthermore, particular attention was dedicated toward studies describing alternative ecofriendly preparation methods.

Summary: This reprint is the compilation of the articles submitted in the Special Issue entitled "Metallic or Metallic Oxide (Photo)catalysts for Environmental Applications" from the journal Catalysts, which gives an overview of the latest advances in the development of metallic or metallic oxide (photo)catalytic materials, with environmental applications for the elimination of organic pollutants or the valorization of biomass.

Summary: Population growth and climate change are leading to global water scarcity. Water shortages are thus hindering rural, urban and industrial development. These days, approximately half of the world's population is affected temporarily by water scarcity. To enable a secure water supply, alternative water sources must be generated to tackle the challenge of water scarcity. An important alternative resource is the reuse of treated wastewater. Water reuse processes are rarely considered and implemented. In contrast to the storage and use of rainwater, treated wastewater is a valuable resource, as it is available daily. Certain wastewater treatment processes are required to produce the new resource "reused water". The treatment processes depend on the quality of the wastewater since industrial and municipal wastewater flows are characterized by different concentrations. Moreover, water reuse methods must be developed in order to use the treated wastewater as efficiently as possible. Ideally, the reused water can be provided according to the "fit for purpose" principle and applied directly in areas such as irrigation, street cleaning, toilet flushing or make-up water for cooling systems.The Special Issue brings together new wastewater treatment technologies and water reuse concepts to tackle the challenges of climate change with the aim of bringing the resource "reused water" according to the "fit for purpose" principle to the user. This issue aims to draw on global experiences, approaches and solutions.

Summary: This reprint is a compilation of the articles submitted in the Special Issue entitled, "Heterogeneous Photocatalysis: A Solution for a Greener Earth", from the journal Catalysts, which presents an overview of the latest advances in the development of innovative photocatalytic processes.