In other words, the solution pH affected the way the TiO2 surface was ionized and caused amphoteric behavior of TiO2 nanoparticles at different conditions (Papadam et al. 2007). This behavior could change the process oxidation power. Results showed that decomposition of MB followed first order reaction in both reactions. R2 values reported in Table 5 also confirm this. Concerning the slope of diagram in terms of contact time, apparent speed constants () could be easily achieved at any level of initial concentration. Comparing apparent speed constants in both reactors showed that the efficiency of the batch reactor was higher in degrading MB than the tubular reactor. The effect of pH on the photocatalytic process efficiency with various catalyst nanoparticles is related to the point of zero charge (pHzpc) and structures of catalyst. The isoelectric point of TiO2 was reported to be in the range of 5.9–7.5 (Bahnemann et al. 2007). Also, the effects of pH on the efficiency of the photocatalytic process of TiO2 nanoparticles were as a result of TiO2 amphoteric behavior and weakness of the oxide power of the holes produced in basic conditions (Hoffman et al. 1995). Titanium atoms are functionalized by primary hydrates and then placed on the surface of titanium dioxide nanoparticles (as >Ti-OH). Moreover, solution pH affects the surface ionization in the titanium dioxide nanoparticles according to Equations (1) and (2) (Papadam et al. 2007):
1
2
Table 5

The apparent rate constant of decomposition of MB in tubular reactors and batch (T: 25 °C, I: 15 W/m2)

Batch reactor
Tubular reactor
MB initial concentration (ppm)kP(min−1)R2MB initial concentration (ppm)kP(min−1)R2
15 0.0058 0.948 15 0.002 0.948
30 0.0045 0.952 30 0.002 0.928
60 0.004 0.981 60 0.0017 0.948
Batch reactor
Tubular reactor
MB initial concentration (ppm)kP(min−1)R2MB initial concentration (ppm)kP(min−1)R2
15 0.0058 0.948 15 0.002 0.948
30 0.0045 0.952 30 0.002 0.928
60 0.004 0.981 60 0.0017 0.948
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