MnO2 is identified as a highly efficient sonocatalyst and sonophotocatalyst for the complete removal of even very small concentration of Indigo carmine (IC) dye pollutant from water. The effect of various reaction parameters, viz. dosage of the catalyst, concentration of pollutant, volume of reaction system, pH, dissolved gases, presence of anions/salts and oxidants etc. on the rate of degradation is evaluated and optimum parameters are identified. The degradation follows variable kinetics depending on the concentration of the substrate. The rate of degradation is facilitated by acidic pH. Classic oxidants H2O2 and S2O82− behave differently, with the former inhibiting and the latter enhancing the degradation. The effect of anions/salts on the degradation is complex and ranges from ‘inhibition’ (PO43−, CO32−, HCO3−) and ‘no effect’ (SO42−, Cl−) to ‘enhancement’ (NO3−, CH3COO−). The high affinity of MnO2 for O2 and its extremely efficient adsorption of H2O2 and the substrate play key roles in the efficiency of the process. Participation of lattice oxygen from MnO2 in the reaction, whenever the dissolved or adsorbed oxygen is deficient, is an important highlight of the process. Major transient intermediates formed during the process are identified by LC–MS. Combination of sonocatalysis with UV photolysis (sonophotocatalysis) enhances the efficiency of degradation and mineralization of IC.

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 (S2O8 2-) 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.