The support of Ferdowsi University of Mashhad – Iran (Research and Technology) for this work (code 3/38606, date 13/10/2015) is appreciated.
Textile industry wastewaters have large amounts of diverse dyes, which are generally bio-resistant and consequently conventional biological methods are not effective for their treatment . Moreover, other physical and chemical processes such as coagulation and adsorption merely transfer pollutants to secondary phases, that require more remediation . Hence, utilization of advanced oxidation processes (AOPs) are more appropriate not only to degrade, but also to mineralize various contaminants with no further waste . Among AOPs, the Fenton and ultrasonic processes are easy and efficient treatment techniques that are widely used for the degradation of different contaminates in polluted water sources . Hydroxyl radicals (OH) as a reactive oxygen species (ROS) have the substantial role in AOPs owing to unselective attack to organic pollutants and their degradation intermediates to convert them to harmless compounds like carbon dioxide, water and inorganic mineral salts . They can be produced by the heterogeneous Fenton reaction (Eq. (1)) or from water cleavage under ultrasonic waves (Eq. (2)) via cavitation phenomenon .
After directing ultrasonic irradiation into a liquid phase, the cavitation results in generation, growth, and eventually collapse of microbubbles forming high localized pressures and temperatures based on hot spot approach . However, ultrasonic process consumes more gdc-0980 and time in comparison of other methods in AOPs owing to its low degradation rate; thus it can be coupled with other processes such as Fenton process to enhance its performance for water treatment . Moreover, it should be mentioned that catalyst recycling and also its separation from the treated water restrict the application of homogeneous Fenton process, which can be carried out in acidic medium (pH 3) to prevent from the precipitation of iron. Besides, usage of heterogeneous Fenton process with no requirement for separation of catalyst, performing at milder pHs and low leached iron is another way to reduce these drawbacks .
Magnetite , goethite  and pyrite  are used as heterogeneous catalysts in Fenton process in which superficial Fe ions catalyze the generation of OH. The pyrite (FeS2) is the most plentiful and nontoxic metal sulfide in the earth . The application of synthesized pyrite has been studied in water treatment processes including the Fenton and adsorption methods [14,15]. It should be noticed that the heterogeneous Fenton process has some confines compared to the homogeneous one like mass transfer resistance and limited active reaction sites. The effective solutions to overcome these obstacles are usage of nanostructured catalysts and sonication . However, formation of nano-sized compounds by synthesis methods needs expensive and toxic reactants .
Plasma is an ionized gas composing of positive and negative ions, electrons and uncharged species, considered as forth state of matter. It is an environmentally-friendly way for formation of various nanostructures for different applications ; silent discharge, radio frequency and glow discharge techniques as non-thermal plasma method have been used for development of modified catalysts [19,20]. For instance, the surface area and activity of natural clinoptilolite and synthesized zeolites have been enhanced using plasma treatment . The catalytic performance and stability of the Pd/HZSM-5 catalyst have been improved after plasma modification . The hydrogenation selectivity of acetylene increases by using of the Ar, H2 and O2 atmosphere plasma for treated Pd/TiO2 catalyst . Modified magnetite by oxygen and argon glow discharge plasmas was utilized for degradation of an oxazine dye through catalytic ozonation . Hydrogenation of carbon monoxide has been carried out using plasma-treated Fe2O3/ZSM-5 catalyst with high activity and selectivity prepared by the oxygen and argon glow discharge plasmas .