Guest Editorial

This special issue of the Journal of Solar Energy Engineering is dedicated to the solar detoxification of water. Solar photocatalytic treatment has become an important field of application, in which the sun, as a versatile nonpolluting energy source, provides a significant environmental value for sustainable water and hazardous waste management strategies. The main photocatalytic techniques for the treatment of recalcitrant or refractory pollutants—the so-called advanced oxidation processes (AOP)—are the heterogeneous $TiO2$ photocatalysis and the homogeneous photo-Fenton photocatalysis. Both involve the production of hydroxyl radicals responsible for the oxidation processes. Ongoing international research and development is covered in seventeen papers from Argentina, Australia, Austria, Brazil, Colombia, Germany, Ireland, Italy, Mexico, Peru, Portugal, Spain, Switzerland, Turkey, and the United States. The topics presented include solar wastewater treatment by $TiO2$ and by photo-Fenton in field applications (pesticides, surfactants, dyes, and olive-mill wastewater), solar disinfection, use of immobilized photo-sensitizers, and solar collector modeling, design, and optimization. Among the accomplishments reported in these refereed articles are novel methodologies for calculating radiation absorption and for scaling-up AOPs, improvement of the apparent radiative flux on $TiO2$ submerged in the liquid phase by gas bubbling, the addition of activated carbon in $TiO2$ to favor oxidation, the control of $H2O2$ concentration in a solar photo-Fenton system, and the application of spray-pyrolysis and sol-gel techniques for depositing $TiO2$ films inside glass tubing. A review article summarizes the pertinent research and indicates the viability of the associated technology. Although wastewater treatment processes have been studied in the lab during the last three decades, commercial applications and engineering design methodologies have only been developed recently, as attested by the material presented in this special issue.