## 3 Results and Discussion
### Photocatalytic Performance
irradiation were as follows: 0.005 min\({}^{-1}\) and 0.0171 min\({}^{-1}\) for pure TiO\({}_{2}\) and rGO/TiO\({}_{2}\) nanocomposite, respectively (Figure 4D). The obtained linear correlation coefficient (\(R^{2}\)) was 0.9788 and 0.9859 for pure TiO\({}_{2}\) and rGO/TiO\({}_{2}\) nanocomposite, respectively. The photodegradation rate constant of all tested materials follows the first-order kinetic rate. The obtained values of the first-order rate constant and the photocatalytic degradation efficiency decrease in the following order: rGO/TiO\({}_{2}\)\(>\)TiO\({}_{2}\) for both irradiation sources (ultraviolet and simulated solar light). The enhanced photocatalytic degradation of MB dye from an aqueous medium (milli-Q water) at natural pH may be due to the high light intensity of simulated solar irradiation where the intensity of the UV was 4.90 \(\pm\) 0.1 mW cm\({}^{-2}\) at a distance of 15 cm from the light source. On the other hand, the intensity of the UV-A lamp was significantly lower, 0.98 \(\pm\) 0.05 mW cm\({}^{-2}\) for the same experimental setup. Several authors have reported the photocatalytic capabilities of rGO/TiO\({}_{2}\) nanocomposite in the form of suspension for the degradation of persistent organic pollutants such as dyes under different types of irradiation sources. Furthermore, they reported the high photocatalytic effectiveness of the rGO/TiO\({}_{2}\) nanocomposite in several studied systems [1; 14; 20; 33; 39; 40]. [PERSON] et al. [10] studied the photocatalytic degradation of 4-chlorophenol while using immobilised TiO\({}_{2}\)/rGO nanocomposite under UV irradiation. The authors reported degradation around 80% for 4-chlorophenol after 300 min irradiation, respectively. In another study, [PERSON] et al. [41] investigated the capability of immobilised rGO/TiO\({}_{2}\) for degradation of MB under UV light irradiation. It was found that according to optimised experimental parameters, the photodegradation of MB was complete after 120 min. Thus considered, our results for the immobilised rGO/TiO\({}_{2}\) nanocomposite are in good agreement with the reported studies. Moreover, immobilised rGO/TiO\({}_{2}\) nanocomposite in this study shows a high potential for photocatalytic application due to the possible use of natural sunlight irradiation. Furthermore, the use of immobilised rGO/TiO\({}_{2}\) photocatalyst is more cost-effective than in the form of suspension due to the high cost of separation. Advanced oxidation processes are effective for the removal of organic pollutants, but the oxidation rates of the compounds may be mainly affected by the composition of the water matrix. The water matrix can play a crucial role in the rate of photodegradation reactions. Dissolved inorganic and organic species can act as promoters and/or inhibitors in the photocatalytic processes [42; 43; 44]. Adsorption and photocatalytic efficiency of MB dye in several water environments were investigated. The obtained results for MB dye degradation using immobilised rGO/TiO\({}_{2}\) nanocomposite, under solar-like irradiation, in milli-Q water, river water, lake water, and seawater, are shown in Figure 5. It can be observed that the adsorption efficiency of MB on the surface of immobilised rGO/TiO\({}_{2}\) nanocomposite was higher in all monitored water matrices (river, sea, and lake) than in milli-Q water. The obtained adsorption of MB was \(\sim\)10%, 20%, 21%, and 24% for milli-Q water, lake water, river water, and sea water, respectively (Figure 5A). In all the samples, almost complete photodegradation of MB dye was observed after 120 min. At the end of the photocatalytic reaction, 92.7%, 96.1%, 93.3%, and 84.9% of MB dye concentrations were removed from milli-Q water, river water, lake water, and sea water, respectively, as shown in Figure 5B. As noted, a significant increase of MB dye degradation was observed in the sample of river water using immobilised rGO/TiO\({}_{2}\) nanocomposite under simulated solar irradiation.