## 3 Results and Discussion  
### Intensification by Reactor Design  
Our recent studies showed that the incident photon flux at the surface of the reactor or photocatalytic film facing the lamps in FPCR can be calculated with the Extensive Source Superficial Diffuse Emission (ESSDE) model [49]. Due to the angular position of the panel with respect to the cascade surface, variations along the cascade axial direction (H) are expected, while the incident irradiation on a single point of the FPCR surface varies also in respect to the distance from the lamp center. In the current study, average values of the incident photon flux at the surface of the photocatalytic film (\(I_{0}\), mW/cm\({}^{2}\)) were determined using radiometric readings before and during experiments at different positions along the photocatalyst surface. As shown in Figure 2, the mean \(I_{0}\) intensities in FPCR (Figure 2a) are a result of a combination of irradiation emissions from used lamps (Figure 2b) and correspond well to the irradiation conditions during cloudy to partially sunny days in winter and early spring in Zagreb, Croatia (45\({}^{\circ}\)N). The UVB and UVA irradiance levels at the given location were obtained from the radiometric readings on horizontal surface. The use of those lamps allowed different UVA and UVB intensities in reaction space (Figure 2a,b), which is a close simulation of natural sunlight, whereby intensity levels depend not only on the time of the day and a season but also on the cloudiness. As shown in Figure 2a, the irradiated surface was divided into areas in i.e., distinct UV regions. The exact UVB and UVA intensities were used in the kinetic models, which resulted in reliable parameters that were confirmed in all experiments. In CPC, total irradiation in the quartz tubes is the sum of the incident irradiation--i.e., direct and diffuse irradiation coming from the lamps--and reflected radiation coming from the parabolic mirrors. [PERSON]'s law of specular materials can be used to estimate the flux loss of the reflected radiation reaching the reactor, Equation (1)  
\[I_{\text{refl}}=I_{0,\text{total}}\Psi^{\text{g}} \tag{1}\]  
where \(I_{0}\) is the incident solar radiation flux reaching the reactor walls, \(\Psi\) is the material reflectivity and g is the number of consecutive reflections of the ray on the involute surface [45]. In the current study, the main hypothesis is the intensification of the photocatalytic process due to the CPC design, caused by the addition of reflected radiation and higher UVB and UVA intensities inside the quartz tubes. In that manner, the reflected irradiation was not calculated, but the same incident solar irradiation flux (\(I_{0}\)) was used for FPCR and CPC reactors. The difference in actual incident irradiation inside the CPC and used \(I_{0}\) is proportional to intensification index, since only better light capture is responsible for intensification. The corresponding intensification index will be calculated later on. The photocatalytic degradation of CT, HQ and RS and binary mixtures was performed in both the FPCR and CPC reactor, and the results are given in Figure 3a-c. In both reactors, direct photolysis of CT, HQ and RS or the respective adsorption on TiO\({}_{2}\)-GF in the dark was negligible during 180 min of the control experiment. Results from control experiments also excluded any effect of dark catalytic reactions on pollutant degradation during the solar photocatalytic process under given circumstances (data not shown). As shown in Figure 3, the obtained results show the similar kinetic trends, with faster degradation in CPC. The extent of CT degradation in FPCR over irradiated TiO\({}_{2}\)-GF film reached 46%, while degradation in CPC reached 63% after 180 min of treatment. The degradation extents of HQ were 58% and 78% in FPCR and CPC, respectively. The degradation extents of RS were 56% and 71% in FPCR and CPC, respectively. Similar observations were made for CT, HQ and RS degradation in binary mixtures.