Acid-treated clay catalysts for organic dye ozonation - Thorough mineralization through optimum catalyst basicity and hydrophilic character.

Catalytic ozonation of Methylene Blue, Methyl Green, Methyl Orange and Methyl-thymol Blue was investigated in the presence of ion-exchanged montmorillonite (NaMt and Fe(II)Mt), crude bentonite and acid-activated counterparts. An original approach never tackled so far consisted in correlating the basicity and hydrophilic character to the dye-catalyst interactions occurring on the catalyst surface. This was achieved through CO2 and water thermal programmed desorption. Kinetics study revealed that ozonation starts in the bulk solution, and dye adsorption turns out to be an essential requirement for high catalytic effectiveness. On NaMt, dye molecules appear to adsorb mainly via hydrophobic interaction. On Fe(II)Mt, the contributions of hydrophobic interaction, cation-exchange and Fe2+ mobility to the catalytic activity prevail. Acid activated clay catalysts exhibited lowest hydrophilic character favoring adsorption through organophilic interaction and affording thorough and fast dye mineralization. This was explained in terms of increased number of silanols and -Si-O-Si- groups. For all catalysts, short ozonation of all dye molecules resulted in similar end-chain products, which were totally eliminated after prolonged reaction times. This result is of great importance because it provides valuable theoretical findings that allow envisaging total mineralization of organic molecules by recyclable metal-free clay catalysts.