# Packed Bed Photoreactor for the Removal of Water Pollutants Using Visible Light Emitting Diodes
## 2 Results and Discussion
The accuracy of the obtained value for \(k_{kinetic}\) was verified by performing another series of experiments in which the visible light intensity was varied from 17.2 up to 78 mW/cm\({}^{2}\) to verify the ability of the model to predict experimental behaviour. In this case, the photoreactor operated in in batch mode, according to the schematic picture reported in Figure 1. In fact, the packed bed photoreactor was coupled with a tank, and the aqueous solution is continuously recirculated by means of the peristaltic pump for feeding the polluted water into the photoreactor. In this way, the reactor can be modeled like a batch system, as also reported in the literature concerning different photoreactor configurations for water and wastewater treatment [43]. Therefore, the MB mass balance can be written as:  
\[\frac{dC}{dt}=-k_{kinetic}\cdot I\cdot\frac{W_{cat}}{V}\cdot C. \tag{4}\]  
The initial condition for Equation (4) is:  
\(t=0\); \(C=C_{0}\),  
where:  
\(C_{0}\) = MB initial concentration, mg/L;  
\(C\) = MB concentration at the generic irradiation time, mg/L;  
\(V\) = treated solution volume, L;  
\(W_{cat}\) = structured catalyst amount, g.  
Considering the mass balance on MB (Equation (4)) and integrating it between initial time (\(t=0\)) and generic irradiation time \(t\), it was obtained:  
\[\frac{C}{C_{o}}=e^{-(\frac{k_{kinetic}\cdot I\cdot W_{cat}}{V})\cdot t} \tag{5}\]  
The value of \(k_{kinetic}\), calculated from Equation (3) was used in Equation (5), and the calculated values of MB relative concentration as a function of irradiation time were compared with the experimental results collected at different light intensities (Figure 7). As expected, the photocatalytic performances increased with the visible light intensity and the results obtained by the mathematical model were in agreement with the experimental results, evidencing the ability of the mathematical model to be predictive. Degradation and Mineralization of Cefriaxone, Paracetamol, and Caffeine with the Packed Bed Photoreactor  
The efficiency of the packed bed photoreactor was also evaluated in the removal of water pollutants of emerging concern, like pharmaceuticals and personal care products (PPCPs) [44; 45; 46]. The molecules tested in the present study were selected among the emerging pollutants detected in some Italian rivers and lakes and to represent different classes of PPCPs [47]. More precisely, ceftriaxone (CFX), caffeine (CAF), and paracetamol (PC) were chosen for photocatalytic tests. CFX is a long-acting, broad-spectrum cephalospororin antibiotic for parenteral use [48]. CF is a well-known compound used in pharmaceutical formulations as a stimulant and, moreover, is the most widely consumed psychoactive drug [49]. Finally, PC, also known as acetaminophen, is the compound used in many analgesic and antipyretic medicines [49]. For the photocatalytic tests on CFX, CAF, and PC, the packed bed photoreactor was used operating in batch mode. The initial concentration value for all PPCPs was mg/L. The obtained results are depicted in Figure 8. It was possible to observe a progressive decrease of the pollutants concentration during the irradiation time, evidencing that the photocatalytic system was effective in the removal of all the tested PPCPs. In particular, for CFX, CAF, and PC, similar degradation degree (about 99% for CFX, 95% for CAF, and 99% for PC) and TOC removal (about 85% for CFX, 80% for CAF, and 83% for PC) were achieved after 180 min of visible light irradiation (Figure 9). Considering the literature concerning the degradation of CFX, PC, and CAF, it must be underlined that the most studied photocatalysts are used in powder form [49; 50], making their use on an industrial scale very difficult. However, some studies have also reported the immobilization of photocatalyst particles on different macroscopic supports [50; 51].