Possibilities and Limitations of the Sono-Fenton Process Using Mid-High-Frequency Ultrasound for the Degradation of Organic Pollutants

Mid-high-frequency ultrasound (200-1000 kHz) eliminates organic pollutants and also generates H₂O₂. To take advantage of H₂O₂, iron species can be added, generating a hybrid sono-Fenton process (sF). This paper presents the possibilities and limitations of sF. Heterogeneous (a natural mineral) and homogeneous (Fe²⁺ and Fe³⁺ ions) iron sources were considered. Acetaminophen, ciprofloxacin, and methyl orange were the target organic pollutants. Ultrasound alone induced the pollutants degradation, and the dual competing role of the natural mineral (0.02-0.20 g L^-1) meant that it had no significant effects on the elimination of pollutants. In contrast, both Fe²⁺ and Fe³⁺ ions enhanced the pollutants' degradation, and the elimination using Fe²⁺ was better because of its higher reactivity toward H₂O₂. However, the enhancement decreased at high Fe²⁺ concentrations (e.g., 5 mg L^-1) because of scavenger effects. The Fe²⁺ addition significantly accelerated the elimination of acetaminophen and methyl orange. For ciprofloxacin, at short treatment times, the degradation was enhanced, but the pollutant complexation with Fe³⁺ that came from the Fenton reaction caused degradation to stop. Additionally, sF did not decrease the antimicrobial activity associated with ciprofloxacin, whereas ultrasound alone did. Therefore, the chemical structure of the pollutant plays a crucial role in the feasibility of the sF process.