## 3 Results and Discussion
### Photocatalytic Performance
The enhanced degradation of MB in the river water matrix may be due to a small amount of ions that can affect the photodegradation of MB dye [43; 45]. It was found that degradation of MB dye in lake and milli-Q water samples is very similar. The lowest photodegradation of MB was obtained in the seawater sample. Seawater usually contains a large amount of inorganic salts (especially NaCl) which can have a negative impact on the photocatalyst activity. The deactivation of the photocatalyst by inorganic species in the seawater matrix has been connected with free radicals and the active sites on the catalyst surface are thus terminated [42; 46]. The reaction rate constant (\(k\)) was calculated again from the slope of a plot between ln(C/C\({}_{00}\)) and reaction time (\(t\)) as shown in Figure 5C,D. The obtained results for different water matrices were as follows: 0.0216 min\({}^{-1}\), 0.02732 min\({}^{-1}\), 0.02232 min\({}^{-1}\)and 0.01601 min\({}^{-1}\) for milli-Q, river, lake, and seawater matrices, respectively (Figure 5D). It was observed that immobilised rGO/TiO\({}_{2}\) nanocomposite can play an important role in the efficient and fast photocatalytic degradation of MB dye in several aquatic matrices. The improved photocatalytic activity of the immobilised rGO/TiO\({}_{2}\) photocatalyst may be described by an increased establishment of radicals leading to a better separation of charge carriers. These conditions can be created by the presence of rGO material repressing the recombination of the charge carrier. The recombination may be achieved over the graphene surface due to the high mobility of electrons. Furthermore, freed oxidative holes can directly react with the organic pollutant compounds or indirectly through the OH radicals, which are strong oxidants [39, 47]. Active components such as hydroxyl radical (\({}^{\bullet}\)OH), hole (h\({}^{+}\)), and electron (e\({}^{-}\)) play different roles in the photodegradation of organic dyes. For example, the reduction of e\({}^{-}\) may demolish the double bond in the structure of organic dyes [33, 48]. To investigate the role of different hydroxyl radical in the photocatalytic degradation of MB in an aqueous medium (milli-Q water), identical catalytic experiments have been carried out in the presence of isopropanol (IPA), AgNO\({}_{3}\), and EDTA-2 Na, as shown in Figure 6. IPA rapidly reacts with hydroxyl radical and because these radicals are terminated, we can determine the role of hydroxyl radical in photocatalytic reactions [47]. In this case, it was observed that the photodegradation efficiency increases with the presence of isopropanol; to be precise, with 0.01 mol L\({}^{-1}\) of isopropanol, the process efficiency  
Figure 5: Effect of different water matrices (milli-Q water, river water, seawater, and lake water) on the (**A**) adsorption (stirring in the dark) and photodegradation efficiency of MB dye using rGO/TiO\({}_{2}\) nanocomposite under solar-like irradiation, (**B**) degradation efficiency of MB, (**C**) the \(-\ln\) [MB]/[MB] versus reaction time, and (**D**) kinetic rate constants of photocatalytic reaction in different environmental water matrices. increases by 4.5% after reaction duration of 120 min compared to the MB degradation without the presence of active species. If hydroxyl radicals were the primary species leading to the degradation of MB during immobilised rGO/TiO\({}_{2}\) photocatalyst, the addition of isopropanol should inhibit the photodegradation. The minimal level of increase observed upon addition of hydroxyl radical indicates that the hydroxyl-radical-based oxidation pathway does not play a significant role in the degradation of MB with immobilised rGO/TiO\({}_{2}\) nanocomposite photocatalyst under simulated solar irradiation [49; 50; 51; 52]. To determine the presence of h\({}^{+}\) and e\({}^{-}\) in photodegradation of MB, EDTA-2 Na and AgNO\({}_{3}\) were used as an active species.