The authors wish to thank, in order of acquisition, the Mediterranean Office for Youth Program (MOY, call 2011-2014), by means of which Stefanos Giannakis has received a PhD mobility grant (MOY Grant No. 2010/044/01) in the joint Environmental Engineering Doctoral Program. Also, the Swiss Government for the Swiss Government Excellence Scholarship, by means of which Stefanos Giannakis has received a Research Visit fellowship (No. 2012.0499). Finally, Stefanos Papoutsakis was funded by the Swiss-Hungarian Co-operation Program “Sustainable fine chemical, pharmaceutical industry: screening and utilization of liquid wastes – Innovative approaches for the abatement of industrial/toxic waste in aqueous effluents”.
Advanced oxidation processes (AOPs) are a set of technologies which lead to the oxidation of pollutants by provoking the formation of highly reactive oxygen species, especially OH radicals. Among the most well known AOPs is the photo-Fenton process, a non-selective photocatalytic process in which OH is generated during the oxidation of Fe2+ to Fe3+ by H2O2. Application of ultraviolet light at frequencies below 400nm (present within the spectrum of solar radiation) can regenerate the Fe2+, greatly increasing the efficiency of the process [1–3]. It has been studied extensively for the treatment of many types of contaminants [4,5] at relatively low cost . As such, the photo-Fenton process can provide the industry with a powerful tool to treat wastewater with clean Lomefloxacin HCl cost from the sun . However, the process is not a universal remedy as it can demand large amounts of H2O2 and its dependence on light, can be a limiting factor in sunshine deprived regions. Iron also tends to form insoluble aqua complexes at pH above 4, sometimes necessitating either pH adjustment or the addition of iron-chelating agents for increasing its solubility in neutral pH . In order to minimize treatment time, reagent consumption and costs, significant investigative efforts have been made towards the development of hybrid processes such as biological treatment/photo-Fenton [9,10], electro-and photoelectro-Fenton , and ultrasound/photo-Fenton . It is this latter system that will be the focus of this study.
Ultrasonic (US) treatment is another AOP that has been gaining interest in the last years. When ultrasound is applied to a liquid medium, cavitation bubbles are formed. After a series of expansion and compression cycles, these bubbles violently collapse to generate very high temperatures and pressures concentrated in one localized ‘hot spot’ .The surrounding liquid will consequently quench it within a millisecond timeframe (cooling rates in excess of 1012K/s ), generating extreme gradients of both pressure and temperature. This quasi-adiabatic energy process has profound effects on the surrounding liquid  as well as the chemical species found in its vicinity. It has been reported that reactive radicals such as OH  can be generated via the thermal dissociation of water  during bubble collapse. As the OH radical has a short lifetime , hydrophobic compounds are expected to be preferentially oxidized near the bubble/bulk interface due to their close proximity to the cavitation bubble [18–20]. Additionally, if their vapor pressure is high, they may enter the bubble and be directly pyrolyzed during its implosion. It has also been shown at laboratory scale that ultrasound can promote the generation of H2O2 which could be utilized during the photo-Fenton reaction . It is however unverified whether this generation remains significant at pilot-scale. H2O2 maintains a complex role in pure ultrasonic processes as well, acting both as a source of free radicals by a dissociation process, as well as a radical scavenger .
High ultrasound frequencies are widely regarded as more efficient in the destruction of organic pollutants, with several studies pointing towards an optimal frequency region between 350 and 500kHz, with representative reported values at 358kHz , 506kHz  and 582kHz . Due to the high cost implicit to the use of pure ultrasound treatment, there is a strong interest in combining it with other AOPs and taking advantage of beneficial synergistic effects for lowering treatment costs.