Heterogeneous photocatalysis using UV/VIS light or natural solar radiation as the energy source is one of the most efficient advanced oxidation processes (AOP) for the removal of chemical and bacterial pollutants from water. One of the least investigated oxides in this context, i.e. MnO2 and its combination MnO2/TiO2 are examined as potential photocatalysts for the removal of Indigo carmine (IC) dye pollutant from the water. The catalysts are characterized by XRD, FTIR, SEM, TEM, adsorption and surface area measurements. While MnO2 is very efficient for the decolorization of the dye, it is not effective enough for mineralization. MnO2 /TiO2 as the photocatalyst at the optimized ratio of 9:1 combines the advantages of both the oxides, i.e., rapid decolorization and efficient mineralization. Persulphate (S2O82-) enhances the degradation while H2O2 inhibits the same. The degradation is dependent on pH with higher degradation under extremely acidic conditions. The influence of dissolved salts/anions in the water on the degradation varies from ‘moderate inhibition' to ‘no effect' or even ‘enhancement' depending on the chemistry of the anion and reaction conditions. Effect of various parameters such as reaction time, substrate concentration, catalyst dosage, the presence of O2, recycling of the catalyst, etc. on the efficiency of degradation is investigated. The results are critically analyzed, and a tentative mechanism is proposed.
Key words: anion effect; Indigo carmine dye; manganese dioxide; photocatalysis; titanium dioxide; persulphate.
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