# Solar Concentration for Wastewaters Remediation: A Review of Materials and Technologies
## 1 Introduction  
The demand for clean water sources has been rapidly increasing in recent decades, led by industrialization, the expansion of agriculture and, especially, population growth. Access to safe water supplies has thus become an issue of global significance [1]. Moreover, the health risk associated with polluted water resources is projected to become a major global issue within the next few decades [2]. Among the various practical strategies and solutions proposed for more sustainable water management, wastewater reuse and recycling stands out as the most economically-viable and environmentally-friendly [3]. Presently, the most common wastewater treatments are based upon a combination of mechanical, biological, physical and chemical processes such as filtration, flocculation, chemical or biological oxidation of organic pollutants. A common problem for current technologies is poorly biodegradable organic pollutants, the so-called bio-recalcitrant organic compounds (BROCs). A class of treatments capable of tackling BROCs is known as advanced oxidation processes (AOPs), relying on the formation of highly-reactive transient (e.g., superoxide, peroxide, hydroxyl radical) chemical species that can convert BROCs into more biodegradable compounds or, ideally, into inorganic carbon [4]. Among the most efficient AOPs are those based on hydroxyl radicals OH\({}^{\bullet}\), a powerful oxidant. These methods are generally based on the dissociation of hydrogen peroxide in water, either by direct absorption of ultraviolet (UV) photons or by mediation with metal ions (Fe and Co are among the most studied) through Fenton and photo-Fenton reactions [5; 6; 7]. In addition, and equally or even more important, is the possibility ofgenerating OH\({}^{\bullet}\) radicals by the interaction, in water, of artificial or natural light with semi-conductors. This process is commonly described as photocatalysis [8]. In recent decades, the photocatalytic degradation of several organic compounds has received growing attention as a water purification process. Irradiating semi-conductors, for example TiO\({}_{2}\), in the form of micro- or nano-sized particles, on fixed supports or in aqueous suspensions, creates a redox environment that is reactive towards most organic species [9]. A number of reports have shown that surfactants, pesticides and dyes can be effectively converted into less dangerous products like carbon dioxide and hydrochloric acid. In 1998, the United States Environmental Protecting Agency (EPA) published a detailed list of molecules capable of being degraded by AOPs [10]. In photocatalysis, photons can be seen as reactants and/or co-catalysts, hence playing a critical role in the process. On this basis, considerable research effort has been made since the 90s to employ solar radiation as an abundant, renewable and potentially zero-cost light source. In this approach, solar photons are collected and directed into a photoreactor where they power catalytic reactions. Traditionally, solar collector systems are broadly classified as concentrating or non-concentrating, according to their concentration factor or to the temperature which can be reached by the system [11]. Combining wide versatility, potentially very low cost and the capability of total conversion to non-toxic products, the use of solar-powered AOPs for the treatment of urban and industrial wastewaters is likely the most promising application of AOP technology [12; 13]. The application of solar disinfection (SODIS) has also been recently demonstrated on real wastewater samples with the possibility of integrating SODIS technology to an urban wastewater treatment plant (WWTP) [14; 15]. Nanofiltration in combination with tertiary processes, including solar photocatalysis, is also the object of considerable research interest; however, it has presented uncertain results [16; 17]. Another more material-oriented approach, albeit one that is still at an early stage in terms of literature reports, suggests the use of a combination between a photocatalyst and adsorbents.