Decolorization of C.I. Reactive Red 2 by catalytic ozonation processes

This study adopted O3, UV/TiO2/O3, O3/Mn(II) and O3/MnO2 systems to assess the decolorization efficiency of C.I. Reactive Red 2 (RR2). The decolorization rate increased with concentrations of Mn(II) and MnO2 in the ranges 0.05-0.1 and 0.05-0.8 g/l, respectively. However, when 0.5-3 g/l TiO2 was added, the effect of TiO2 dosage for RR2 decolorization was insignificant in the UV/TiO2/O3 system. At pH 2, the decolorization rate constants of O3, O3/Mn(II) (0.05 g/l), O3/Mn(II) (0.1 g/l), O3/Mn(II) (0.15 g/l), O3/MnO2 (0.05 g/l) and O3/MnO2 (0.8 g/l) were 0.816, 2.001, 3.173, 3.087, 1.040 and 1.648 min−1, respectively. After 5 min of reaction, the decolorization rates followed the order O3/Mn(II) > O3/MnO2 > O3 > UV/TiO2/O3; however, the TOC removal did not vary among these systems. Adding ethanol reduced the decolorization rate of the UV/TiO2/O3 and O3/MnO2 systems and did not affect the decolorization rate of O3/Mn(II). Decolorization in UV/TiO2/O3, O3/Mn(II) and O3/MnO2 systems is suggested to proceed by mainly radical-, surface- and radical-type mechanisms, respectively. Additionally, direct ozonation cannot be ignored in O3/Mn(II) and O3/MnO2 systems.