Physical Mechanism of Sono-Fenton Process Sankar Chakma and Vijayanand S. Moholkar Dept. of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati 781 039, Assam, India DOI 10.1002/aic.14150 Published online in Wiley Online Library (wileyonlinelibrary.com) Hybrid advanced oxidation processes (AOPs), where two or more AOPs are applied simultaneously, are known to give effective degradation of recalcitrant organic pollutants. This article attempts to discern the physical mechanism of the hybrid sono-Fenton process with identification of links between individual mechanism of the sonolysis and Fenton pro- cess. An approach of coupling experimental results with simulations of cavitation bubble dynamics has been adopted for two textile dyes as model pollutants. Fenton process is revealed to have greater contribution than sonolysis in the over- all decolorization of both dyes. H 2 O 2 added to the liquid medium as a Fenton reagent scavenges • OH radicals produced by cavitation bubbles. Addition of only H 2 O 2 to the medium during sonolysis does not yield marked difference in decol- orization. Elimination of transient cavitation with application of elevated static pressure to the medium does not alter the extent of decolorization. The synergy between sonolysis and Fenton process is, thus, revealed to be negative. The dissolved oxygen in the medium is found to play an important role in decolorization through conservation of oxidizing radicals. V C 2013 American Institute of Chemical Engineers AIChE J, 00: 000–000, 2013 Keywords: fenton process, advanced oxidation process, cavitation, sonolysis, decolorization Introduction Use of synthetic organic dyes by the textile industries has increased significantly in recent years and many of these dyes are toxic organic pollutants. Based on their chemical structure, the synthetic dyes are categorized into various classes like acidic, basic, azo, nonazo etc. Among these classes, use of azo dyes has been the maximum (70%). 1 These dyes appear in the wastewater discharge from textile industries. The degradation (or decolorization) of these dyes using conventional biological techniques is very difficult due to their complex structure. Globally, approximately 10,000 types of dyes and pigments are produced annually, and about 20% of these are discharged as textile industrial effluents without any pretreatment. For the effective decolorization of the synthetic dyes, advanced oxidation processes (AOPs) have been widely employed as alternative methods. These techniques include Fenton, ozonation, photocatalytic techni- ques, peroxide treatment, sonolysis, enzyme treatments, etc. Literature published in the last decade includes more than 100 papers that report the use of these techniques for decol- orization of textile wastewater, containing a wide range of dyes belonging to the different classes mentioned previously. For further boosting, the efficiency of AOPs, hybrid methods have also been widely used that simultaneously employ two or more AOPs mentioned previously and voluminous litera- ture has also been published in this area in the past decade. The combinations of AOPs that have been used includes sono-Fenton, sono-photocatalysis, sono-enzymatic treatments, photo-Fenton techniques, UV/H 2 O 2 , UV/ozone, ozone/H 2 O 2 , etc. 2–5 An excellent review of the individual as well as hybrid AOP techniques for treatment of wastewater has been given by Gogate and Pandit. 6,7 The hybrid techniques have been reported to enhance the decolorization kinetics as well as yield. Most of the literature in the area of hybrid AOPs is focused on results than rationale. Although in many articles an attempt is made to quantify the enhancement factor for the decolorization (or degradation) with reference to one of the AOPs, only a few studies have attempted to deduce the exact mechanism of the synergism between the individual AOP. In this article, we have addressed this basic issue in the context of a widely used hybrid AOP, i.e., sono-Fenton process. Several recent articles report employment of sono- Fenton processes for degradation of different organic pollu- tants. 8–19 We have used two dyes, viz. Acid red B (an azo dye, abbreviated hereafter as ARB) and Blue HE2R (a non- azo dye, abbreviated hereafter as BLH), as model pollutants in this study in view of the large pollution generated by effluent from textile industry, as mentioned earlier. Our approach for identifying the mechanism of the synergism between sonolysis and Fenton process is twofold, i.e., to couple experiments with simulations of cavitation bubble dynamics. In the next section, we present our contemplations and conjectures, on which the methodology and protocols of this study are based. Contemplations and Conjectures The degradation (or oxidation or decolorization) efficiency of an advanced oxidation process such as sonolysis or Additional Supporting Information may be found in the online version of this article. Correspondence concerning this article should be addressed to V. S. Moholkar at vmoholkar@iitg.ac.in. V C 2013 American Institute of Chemical Engineers AIChE Journal 1 2013 Vol. 00 No. 00