MnO₂ efficiently removes indigo carmine dyes from polluted water

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 S₂O₈ ^2- 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' (PO₄ ^3-, CO₃ ^2-, HCO₃ ^-) and 'no effect' (SO₄ ^2-, 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.