Effect of initial concentration of dye and UV irradiation time
The effect of the initial concentration of RB-194 and irradiation time on photodegradation was examined by varying the concentration from 50 ppm to 150 ppm and time from 30-90 minutes while maintaining other parameters, such as 0.6 mL H2O2, 144 W UV intensity, and pH 3. The % degradation decreased from 80% to 65% when the initial concentration of RB-194 was increased from 50 ppm to 150 ppm. The results are illustrated in Fig. 3a. The decrease in the % age degradation at increasing dye concentrations is attributed to a relatively lower no of °OH radicals attacking dye molecules as dye concentration increases competition between °OH radicals and dye molecules also increases (Bibi et al. 2017a, Bibi et al. 2017b). These results are consistent with earlier research that the initial concentration of dye has a significant effect on dye degradation, for example, Shahab-ud-Din et al. (Shahab-ud-Din et al. 2018) investigated the photodegradation of Coralene Red F3BS under UV irradiation and found that dye degradation considerably depends on the initial concentration of dye. The degradation was reduced from 95% to 77% when the initial concentration of dye was increased from 10 ppm to 30 ppm (Jamil et al. 2020b). The photocatalytic activity of RB-194 was examined at different times (30-90min). Fig. 3b displays the rate of RB-194 degradation as a function of irradiation time. It was observed that dye degradation increases with increasing irradiation time. The effectiveness of the advanced oxidation process was intriguing since it allowed up to 66% degradation to occur within 30 minutes of irradiation. As the exposure time increased, 87% degradation was observed after 90 minutes of irradiation.
Effect of H2O2 concentration
The influence of H2O2 concentration on RB-194 degradation was studied by changing the concentration of H2O2 from 0.2 to 0.6 mL. The addition of H2O2 was thought to be a useful parameter for improving photocatalytic degradation. In comparison to UV irradiation alone, the findings in Figs. 4a and 4b reveal that the UV/H2O2 had increased the photocatalytic degradation of RB-194 dye. A degradation of 81% was obtained using 0.6 mL H2O2 for 90 minutes of irradiation and without the addition of H2O2, 55% degradation of RB-194 dye was obtained. The degradation efficiency was improved by increasing the amount of H2O2. This is attributed to the photoirradiation of hydrogen peroxide to produce hydroxyl radicals (ºOH) and the tendency of H2O2 to capture electrons. The formation of hydroxyl radicals (ºOH) during radiolysis promotes the oxidative degradation of dyes, and this process is accelerated in the presence of H2O2.
H2O2 2 ºOH
The findings clearly illustrate that in the absence of H2O2 the degradation of reactive blue 194 reduces as the concentration of dye increases, demonstrating the explanation for the intense color by increasing concentration. The degradation profile changed dramatically after introducing H2O2 (Jamil et al. 2020c).
Effect of ZnO, TiO2, and SnO2 photocatalysts dose
The effect of catalyst concentrations on the photocatalytic degradation of reactive blue 194 dye was investigated by varying the concentration of photocatalysts (ZnO, SnO2, and TiO2) from 0.3-0.9 g while maintaining all other variables. The UV/ZnO/H2O2 process has the highest degradation efficiency (90%) relative to UV/TiO2/H2O2 and UV/SnO2/H2O2, which were 85% and 83% respectively. The results are presented in Fig. 5. Many other researchers have conducted comparative investigations on ZnO and TiO2 photocatalysts, with ZnO outperforming TiO2 (Qamar &Muneer 2009). (Han et al. 2012) attributed the improved photocatalytic activity of ZnO to its large absorption of radiations in the UV region than TiO2. One of the important elements affecting photocatalytic activity is the surface area of a photocatalyst. ZnO had the lowest specific surface area (10.18 m2/g) among all the catalysts used in this research, whereas TiO2 had the highest specific surface area (42.54 m2/g) due to its low crystal density and tiny particle size. SnO2 also has a comparable specific surface area of 12.91 m2/g. The findings showed that, despite a less specific surface area, ZnO had better photocatalytic activity than TiO2 and SnO2. It is due to changes in the fundamental properties of SnO2, ZnO, and TiO2 catalysts. The variations in photocatalytic activity can be explained by quantum efficiency; ZnO has a higher quantum efficiency than SnO2 and TiO2 (Kansal et al. 2007). The band gap is another important parameter influencing photocatalytic activity. Since a catalyst with a large band gap requires a considerable quantity of UV rays to excite electrons and produce electron-hole pairs in the catalyst (Abo et al. 2016, Sakthivel et al. 2003). SnO2 showed comparatively modest activity in the current investigation due to its large band gap (3.64 eV). Band gaps in TiO2 and ZnO were comparable (3.20 and 3.24 eV respectively). TiO2 has the smallest band gap (3.20 eV) and was predicted to have greater catalytic activity than ZnO. However, the results clearly show that ZnO exhibited better photocatalytic activity than TiO2. The photocatalytic activities provide strong evidence that ZnO absorbs large amounts of photon energy more effectively than TiO2 within the UV range (Pardeshi &Patil 2008). This behavior might be explained by the inherent defects in ZnO crystals. Oxygen vacancies and positively charged zinc interstitials are the most common defects in ZnO. These defects promote redox reaction by trapping photogenerated electrons, decreasing electron and hole recombination, and making them available for the degradation of dyes during the formation of electron-hole pairs (Han et al. 2012, Shinde et al. 2017).
Effect of pH
The influence of pH on the degradation of various concentrations of RB-194 (50-150 ppm) was investigated at different pH values ranging from 3-9. The results are shown in Fig. 6. The degradation of RB-194 by UV/H2O2 with different photo-catalysts (ZnO, SnO2, and TiO2) depends significantly on pH. In this investigation, ZnO displayed more catalytic efficiency under UV irradiation than TiO2 and SnO2 indicating that ZnO/UV/H2O2 plays an important role in photocatalytic degradation. The discoloration and degradation processes are more successful in an acidic media. The degradation of RB-194 significantly decreased from 90% to 64% as the pH was increased from 3 to 9 because a considerable portion of H2O2 is used for the degradation of alkalis, forming water and oxygen instead of producing ºOH radicals under UV irradiations. The UV/ZnO/H2O2 process is highly susceptible to the carbonate scavenging effect at high pH. Thus, the ºOH concentration immediately drops, which lowers the efficiency of the process (Jamil et al. 2020c). The degradation of RB-194 as a function of various process variables is given in Table 2.
Identifications of degradation by-products
FTIR analysis was used to determine the different functional groups. The FTIR spectra of RB-194 before and after irradiation are shown in Figs. 7 and 8. The FTIR spectrum of RB-194 indicated a wide peak at 3386 cm-1 that represents a broad band of the OH group. The many sharp peaks at 1545 cm-1, 1466 cm-1, 1135 cm-1, 1038 cm-1, and 997 cm-1 belonged to N=N azo group, CH bending, C-O group, S=O, and C=C stretching vibrations (Li et al. 2016). The FTIR spectrum after treatment only shows one peak at 1636 cm-1, indicating the destruction of several chromophoric groups due to the degradation of RB-194 by UV irradiations.
LCMS is a powerful tool for analyzing the intermediates produced during dye degradation. LCMS was used to examine the degradation by-products of RB-194. The degraded samples were scanned in positive mode with an m/z of 200-1200 and a retention period of 1.28 minutes for higher resolution. The peaks of degradation products were examined with m/z values of 147.08, 247.25, and 453.25. The LCMS spectrum of RB-194 treated with UV radiation is shown in Fig. 9.
Proposed degradation pathway of reactive blue 194
The primary by-products of reactive blue 194 were identified through LCMS to study the degradation mechanism. The mass spectral information of by-products is given in Table 3. The primary degradation products were discovered to be 6-chloro-1,3,5-triazine-2,4-diamine, sodium 5-hydroxy-3-((4-sulfonatophenyl)diazenyl)naphthalene-2-sulfonate, and sodium 4-hydroxynaphthalene-2-sulfonate. The proposed degradation pathway is given in Fig. 10. The C-N and azo bonds between the benzene, triazine, and naphthalene rings, were broken in the initial step. This step resulted in the formation of triazine, aniline, and naphthalene derivatives. The phenolic derivatives of aniline may be oxidized to benzoquinone, which might be oxidized further to produce oxalic acid and maleic acid. The subsequent oxidation of oxalic acid and maleic acid resulted in the production of water and carbon dioxide. The derivatives of naphthalene are decomposed to 2,5-hydroxynaphthoquinone and 4-(phenyldiazenyl) phthalic acid and undergo further oxidation to produce maleic acid and oxalic acid. The further oxidation of triazine derivatives resulted in 6-chloro-1,3,5-triazine-2,4-diol by the conversion of the amino group into a hydroxyl group. The naphthalene and benzene rings were produced during the aforementioned process by the breakage of the sulfonic groups attached to the sulfate ions. The oxidation of nitrogen present in dye molecules produced nitrate ions (Liu et al. 2014).