# Packed Bed Photoreactor for the Removal of Water Pollutants Using Visible Light Emitting Diodes
## 2 Results and Discussion  
Disoloration and Mineralization of Methylene Blue with the Packed Bed Photoreactor Operating in Batch Mode  
Preliminary experiments were carried out while operating in batch mode (Figure 1) in order to verify that methylene blue (MB) was degraded by the heterogeneous photocatalytic process. The obtained results are reported in Figure 2. In the absence of a structured catalyst, no significant decrease in MB concentration was observed during the overall irradiation time, evidencing that the visible light was not able to degrade the MB molecule. A completely different behaviour was observed  
Figure 1: Schematic picture of the packed bed photoreactor working in batch mode. when N-TiO\({}_{2}\)/PS structured photocatalyst was placed into the photoreactor. In fact, the MB relative concentration gradually decreased in the run time and almost complete decolorization was achieved after 120 min of visible light irradiation. Several published papers have reported the removal of MB with fixed bed photoreactors irradiated by artificial light sources, as well as with solar photocatalytic reactors using structured photocatalysts (Table 1) [34; 35; 36; 37; 38]. For example, almost total MB decolorization was achieved after 48 h in the solar photoreactor proposed by [PERSON] et al. [34], in which TiO\({}_{2}\) nanoparticles were placed as coating on transparent granules and used in the prototype of a flat-panel photoreactor. [PERSON] et al. [35] observed the complete decolorization after 5 h under the sunlight irradiation using a solar concentrator photoreactor prototype with TiO\({}_{2}\) deposited on polyethylene terephthalate (PET) textile substrate sheets. In some cases, it was reported that MB degraded more than 96% in up to 90 min of reaction using TiO\({}_{2}\) immobilized on glass spheres [36]. However, the same authors used a lab scale compound parabolic collector reactor irradiated by lamp at high electrical power (300 W) simulating the solar spectrum. As observed in our previous work [33], the performances of the photocatalytic system were not influenced by mass transfer limitation phenomena of MB dye from the aqueous phase to the photoactive surface, evidencing that the MB photocatalytic degradation rate is controlled by kinetics. In addition, after 300 min of visible light irradiation, the system allowed to achieve a total organic carbon (TOC) removal of 85\(\pm\)2% for all the liquid flow rates (Figure 3). Similar behaviours were observed for other photocatalytic systems in which the photoactive phases were immobilized on macroscopic supports [38; 39]. In order to verify the stability of the structured photocatalyst, the photocatalytic degradation of MB was replicated up to four cycles. Before each reuse cycle, any cleaning or regenerating step of the structured photocatalyst was performed. Figure 2: Behavior of methylene blue (MB) relative concentration as function of irradiation time for N-TiO2/polystyrene (PS) and photolysis test. Light sources: visible light-emitting diodes (LEDs); light intensity: 78 mW/cm\({}_{2}\); initial MB concentration: 7 mg/L; liquid flow rate: 50 mL/min; photoreceptor configuration: batch mode. The treated solution was removed from the photoreactor, and a fresh solution with the same initial MB concentration was fed for a new test. The MB decolorization and total organic carbon (TOC) removal was analyzed after 300 min visible light irradiation and the obtained results are shown in Figure 4.  
From the comparison between the first cycle (when N-TiO\({}_{2}\)/PS was fresh) and the fourth cycle, it was possible to observe that the photocatalytic activity was similar. This result indicated that N-TiO\({}_{2}\)/PS did not deactivate after its reuse and no loss of photocatalyst particles from the polymer surface occurred, in agreement with previous results observed on structured photocatalysts prepared through solvent-casting method [32; 33; 40].