Intensification of sonochemical degradation of malachite green by bromide ions

Abstract

Sonochemical oxidation has been investigated as a viable advanced oxidation process (AOP) for the destruction of various pollutants in water. Ultrasonic irradiation generates OH radicals that can recombine, react with other gaseous species present in the cavity, or diffuse out of the bubble into the bulk liquid medium where they are able to react with solute molecules. The extent of degradation of an organic dye such as malachite green (MG) is limited by the quantity of hydroxyl radicals diffused from cavitation bubbles. In this work, the effect of bromide ions on sonolytic degradation of MG was investigated. The obtained results clearly demonstrated the considerable enhancement of sonochemical destruction of MG in the presence of bromide. No significant differences were observed in the presence of chloride and sulfate, excluding the salting-out effect. Positive effect of bromide ions, which increases with increasing bromide level and decreasing MG concentration, is due to the generation of dibromine radical anion ( Br 2 - ) formed by reaction of Br − with OH radicals followed by rapid complexation with another anion. The generated Br 2 - radicals, reactive but less than OH, are likely able to migrate far from the cavitation bubbles towards the solution bulk and are suitable for degradation of an organic dye such as MG. Additionally, Br 2 - radicals undergo radical–radical recombination at a lesser extent than hydroxyl radicals and would be more available than OH for substrate degradation, both at the bubble surface and in the solution bulk. This effect compensates for the lower reactivity of Br 2 - compared to OH toward organic substrate. Addition of bromide to natural and sea waters induces a slight positive effect on MG degradation. In the absence of bromide, ultrasonic treatment for the removal of MG was promoted in complex matrices such as natural and sea waters.