Heterogeneous Photocatalytic Reduction of Iron(VI): Effect of Ammonia and Formic Acid

Ammonia is a potential pollutant that can contribute to eutrophication of rivers and lakes and its removal is thus becoming an important issue. Formic acid is a byproduct of many industrial processes and its removal from wastewater is of great interest. The removal of ammonia and formic acid was sought by studying the heterogeneous photocatalytic oxidation of ammonia and formic acid in UV-irradiated TiO 2 suspensions with and without Fe(VI) (Fe VI O 4 2- ) at pH 9.0. The kinetics of the reactions was determined by monitoring both reduction of Fe(VI) and oxidation of ammonia/formic acid. The initial rate of Fe(VI) reduction (R) can be expressed as R = k Fe(VI) [Fe(VI)] m where m = 1.25 ± 0.03 and 0.70 ± 0.06 for ammonia and formic acid, respectively. The rate constant, k Fe(VI), depends on the concentration of ammonia and formic acid. The values of k Fe(VI) for the oxidation of ammonia was determined as k Fe(VI) = [Ammonia]/(a[Ammonia]+b), a = 6.0 × 10 3 µM 0.25 s and b = 4.1 × 10 6 µM -1.25 s -1 . The k Fe(VI) for the photocatalytic oxidation of formic acid showed a positive linear relationship, which can be written as k Fe(VI) = 2.41 × 10 -3 + 1.58 × 10 -7 ([Formic Acid]). The rates of oxidation of ammonia and formic acid in TiO 2 /UV suspensions were enhanced in the presence of Fe(VI). Results suggest the photocatalytic production of a highly reactive species, Fe(V), a powerful oxidant, to oxidize ammonia and formic acid. A combination of Fe(VI) and the TiO 2 photocatalyst has the potential to enhance the oxidation of pollutants in the aquatic environment.