approaches [11,12]. For the first one, Figure 1(a) shows the detail of the parabolic concentrator for 5 cm diameter glass/catalyst tubes containing contaminated water [6]. Its efficiency for the photocatalytic decontamination of water polluted with phenol was evaluated on different catalyst [6] and presented in Fig. 1(b). The developed system was evaluated over long periods of use; the reduction in its efficiency after 250 hours is due to the accumulation of calcium carbonate from bacteria. The accumulation of calcium carbonate was removed using a citric acid solution (lemon juice), which acidified the surface and dissolved the carbonate scale, restoring 99% efficiency [7] as it is shown in Figure 1(c). Figure 1(d) illustrates the disinfection of water in an autonomous recirculating cylindric parabolic compound (CPC) system, for which in the inset is presented the reflective CPC geometry, used catalyst were based on cellulose/TiO₂ and ruthenium, which was evaluated in Cusco city [8]. The photocatalytic disinfection of E. coli bacteria, based on the number of catalysts uses at specified treatment times, was evaluated and is presented in Figure 1(e) [8] and in Figure 1(f) the CPC system based on Cobalt ferrites [9,10], for which disinfection was observed even in the dark as it is seen in Figure 1(g) [9], addressing a great opportunity to avoid bacteria regrowing under dark conditions. On the other hand, regarding the second topic, Figure 1(h) shows the capacitive water desalination (CDI) prototype [11,12] and the developed characterization system, with its deionization dynamics shown in Figure 1(i) [11], this technology has shown to need just half of the energy than reverse osmosis to do the same work [11]. Composite materials combining adsorbents and photocatalysts (e.g., activated carbon with titanium dioxide) were also explored [13], along with photocatalysts supported on polymers [14]. These efforts led to an innovative prototype for disinfection and decontamination designed for disaster zones [15] Figure 2, which can be easily deployed and, using solar energy, convert unsafe water into drinking water. The non-recirculating photocatalytic water decontamination system is shown in Figure 2(a), using a PET bottle with contaminated water and pellets [13]. A cross section of the activated carbon and TiO₂ pellet is shown in Figure 2(b), the efficiency for photocatalytic decontamination of phenol-contaminated water using solar energy [13] over two consecutive days is shown in Figure 2(c). Figures 2(d) and 2(he) illustrate a household-scale procedure for arsenic removal from water: Figure 2(d) shows the state before treatment, and Figure 2(e) the situation after treatment, with the precipitate containing the contaminant separated from the water [16,17].
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