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Ion-exchanged montmorillonite-rich materials (ca. 96% purity) like NaMt, Fe(II)Mt, Co(II)Mt, Ni(II)Mt and Cu(II)Mt showed catalytic activity in the ozonation of oxalic acid in water at room temperature, in the pH range 3.4–6.0. The conversion of oxalic acid exceeds 95% after 180 min of ozone bubbling in the presence of Fe(II)Mt. The oxalic acid removal efficiency was found to increase swiftly with the acid character of the clay surface up to a certain level, but decreases gently with excessive surface acidity. The pH exerts a strong influence on the catalyst efficiency, because it induces changes in the composition of both the liquid media and catalyst. The synergic action of ozone and clay catalysts at acidic pH seems to involve ozone adsorption and interaction between cation and adsorbed oxalate. The negative effect of increasing pH between 3.44 and 6.0 is discussed in terms of a decrease in the amount and mobility of the cation in the vicinity of the clay surface, and of a decay in the clay surface area available to ozonation.

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