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The aim of this work was to apply the many various oxidation agents namely H 2 O 2 , K 2 S 2 O 8 and Fe 2+ on the aqueous solution of vitamin B 12 with the presence of ZnO. The results indicated that the use of the mixture of H 2 O 2 and K 2 S 2 O 8 in suspension of vitamin B 12 and ZnO gave a maximum efficiency percentage about 95.85% in 12 min; it was higher than that without the addition of oxidation reagent 79.33% in the same time. The obtained results demonstrated that the activation energy for this photoreaction without the addition of oxidation agent was more than that the apparent activation energy value with the used mixture of H 2 O 2 and K 2 S 2 O 8 . The thermodynamic study showed that both reactions were endothermic, less-random and non-spontaneous. The ΔH # with the oxidant agents (31.43 kJ mol -1 ) is less than that without using oxidant agents (35.81 kJ mol -1 ). Moreover, the addition of series of oxidant agent solution to vitamin B 12 solution led to change the photocatalytic activity for decolorization of vitamin B 12 in suspension solution of ZnO, and was found the activity sequence as follows: (H 2 O 2 + K 2 S 2 O 8 ) > (H 2 O 2 + Fe(II)) (Fenton reaction) > (H 2 O 2 ) > (K 2 S 2 O 8 ) > (without the oxidation agents) > (K 2 S 2 O 8 + Fe(II)) > Fe(II).

Effect of Oxidation Agents on Photo-Decolorization of Vitamin B<sub>12</sub> in the Presence of ZnO/UV-A System